1. Structural Changes in Steel during Hot Rolling
Structural Changes during Reheating
Kinds of Grain Restoration Process
Dynamic Restoration Process
Static Restoration Process
Effect of Initial Grain Size of Static Recrystallization
Effects of Temperature and Microalloying
Effect of Amount of Deformation
Factors Affecting Critical Reduction for
Recrystallization
Grain Growth after Deformation
Structural Changes in Steel during Cooling
Effect of Steel Structure on Flow Stress
2. Fundamental Principles of the Metal Rolling
Process
3. Steels for Magnetic Applications
Electrical Steels-Metallurgy and Properties
Introduction
Utilization and Property Requirements
Optimization of Magnetic Properties
Type of Electrical Steel
Classification
Steel Grades
Market Segmentation
Conclusions
4. Preparing and Heating the Initial Materials
Preparations for Rolling
Heating before Rolling Operations
5. Hot Seamless Tube Rolling Processes
Elements of Skew Rolling Theory
Tube Rolling in Plug Mill Type Seamless Tube Mills
Tube Rolling in Continuous Seamless Tube Mills
Tube Rolling in Three-Roll Mills
Tube Rolling in Pilger Mills
Seamless Tube Production by the Extrusion Process
Seamless Tube Finishing Operations
6. Bolt and Nut Manufacturing Technology
Introduction
Fundamentals of Mechanically Working and Cutting Metals
(a) Cold Forming
(b) Hot Forging
(c) Metal Cutting
Manufacturing Technologies
(a) Cold Forming of Bolts
(b) Cold Forming of Nuts
(c) Hot Forging of Bolts
(d) Hot Forging of Nuts
(e) Machining of Bolts and Nuts from Hexagon Bar
Steel Pre-Processing
(a) Steel Making
(b) Surface Treatments and Wire Drawing
Fastener Steels and Heat Treatments
(a) Alloying Elements
(b) Heat Treatments
Finishing Operations
7. Casting of Steel for Flat Products
Type of Cast Products
Casting of Ingot
Types of Ingots
Methods of Continuous Casting of Thick Slabs
Continuous Casting of Thick Slabs
Slab Width Control
Continuous Casting of Thin Slabs and Strip
Requirements for Continuously Cast Steels
Oxide Inclusions in Concast Steel
Formation of Oxide Phases
Influence of Caster Type on Steel Quality
8. The Rolling of Rails, Wheels and Rings
Introduction
Early Types of Rails and their Production
The Evolution for the Rail Mill
Modern Rail Mills
Rail Joints and their Manufacture
The Forging and Rolling of Wheels
Ring Rolling
9. Mill Automation for the Rolling of Flat Products
Automation of Flying Shear Operation in a Continuous
Hot-Rolling Mill
Automation of Coiler Operation for Hot Strip
Automation of Strip Measuring Gauges for Hot Rolling
Automation of Continuous Pickle Line Operation
Automation of Strip Thickness Gauges for Cold Reduction
Automation of Strip Thickness Control by the Screw-
Down Gear
10. General Steelmaking Processes
Welding Material for Super Low Temperature Steels
Refining Steel by Blowing Oxygen Beneath the Surface
Cold Reduced Aluminum Stabilized Steel having High
Drawability
Sulfide Modification of Steel
Steel Sheets having Excellent Enamelability
Liquid Sintering with Titanium Alloys
Liquid-Solid Alloys for Casting
Rimmed Unkilled Enamelling Steel
Producing an Enamelling Steel Sheet
Deep Drawable Deoxidized Steel
Alloying Steel with Highly Reactive Materials
Prevention of Surface Cracking during Steel Reheating
Prestrengthened Stress Relieved Elongated Steel
Vacuum Treated Steel for Forging Ingots
Metallurgical Addition Product
Uncropped, Unworked, Elongated Leaded Steel Casting
Adding Alloys to Steel
Production of Low Carbon Ferroalloys
Forging Powder Alloy Steels
Production of Leaded Steel
Low Carbon Ferrochromium
High Explosive Fragmentation Munition
11. Varnishing and Printing of Packaging Steels
Introduction
General Aspects of Organic Coatings used for
Varnishing and Printing
Definition
Types of Organic Coating
Organic Coating Constituents
Application and Curing of Organic Coatings
Application with Roller Varnishing Machines
Curing
Other Application Techniques
Inspection Methods
Printing and Decoration of Metallic Packaging
Conclusions
12. Phase Transformation in Steel
Phase Diagram
Constituents in Steels
Austenite
Ferrite
Graphite
Cementite
Eutectoid
Pearlite
Eutectic
Ledeburite
Transformation Temperature
Phases in Hypoeutectoid Steel
Phases in Eutectoid Steel
Phases in Hypereutectoid Steel
Phase Transformation Hysteresis
Supercooling or Austenite
Bainite
Martensite
Isothermal Transformation Diagram
Continuous-Cooling Transformation Diagram
13. Optimization and Modernization of Hot Strip
Mills
Main Strategy in Optimization of Rolling Process
Metallurgical Requirements
Energy Consumption Requirements
Yield Requirements
Product Quality Requirements
Analysis of Temperature Conditions in Hot Strip Mill
Methods of Optimizing Temperature Conditions
Optimizing Operating Parameters
Close Coupling of Continuous Rougher with Finishing
Mill
Close Coupling of a Reversing Rougher with Finishing
Mill
Combining a Reversing Rougher with Finishing Mill
Coilbox
Intermediate Steckel Mill
Reradiating Thermal Cover System
Main Objectives in Modernization of Hot Strip Mill
Requirements for the Evaluation Models
Evaluation of the Solutions for Mill Modernization
14. Low Carbon Constructional Alloy Steels
Low Temperature High Strength Tough Steel
Alloy Steel for Arctic Service
High Strength Cold Rolled Steel with High Press
Formability
Production of High Strength Cold Rolled Steel Sheet
Low Alloy Steel for Low Temperature Services
Full Continuous Annealing Process
High Strength Killed Wire Rods and Bars
High Formability High Strength Steel
High-Strength Cold-Workable TI Added AL Killed Steel
Improving Strength and Toughness by Controlled
Cooling
High Strength Notch Tough Steel
Hot Rolled High Strength Low Alloy Steel
Preparation of Hot Rolled Niobium Structural Steel
Hot Rolled Flat Steel for Cryogenic Service
High Strength Structural Steel with Good Weldability
15. Hot Rolling of Plate, Sheet and Strip
Types and Sizes
Initial Materials and Reheating them for Rolling
Hot Rolling Sheet and Plate Mills
Hot Rolling Processes for Plate and Sheet in Various
Types of Mills
Rolling Steel Strip in a Planetary Mill
Finishing of Hot-Rolled Flat Products
16. Rolling of Section Steel, Bars and Rods
Types and Sizes
Initial Materials and Reheating them for Rolling
Section Mills
Rod Mills
Strip Mills
Roll Pass Design for the Rolling of Rounds
Roll Pass Design for the Rolling of Squares
Roll Pass Design for the Rolling of Flats and Strip
Roll Pass Design for the Rolling of Angles
Finishing Operations on Bars and Rods
17. Modern Rolling Plant
Mills for the Continuous Rolling of Wide Strip
Modern Plant for the Rolling of Non-ferrous Material
Copper and Copper Alloys
Nickel and Nickel Alloys
Aluminium and Aluminium Alloys
18. Metal Fasteners
Machine Bolts
Cap Screws
Machine Screws
Set Screws
Thread-forming Screws
Stove Bolts
Carriage Bolts
Stud Bolts
Nuts
Castle Nuts
Jam Nuts
Cap or Acorn Nuts
Wing Nut
Washers
Rivets
Machine Pins
19. Production of Welded Pipe
Continuous Furnace Butt-Welded Pipe Manufacturing
Processes
Electric Resistance Welded Pipe and Tubing Production
High Frequency Electric Resistance Welding in Pipe
and Tubing Production
Submerged-Arc Welded Pipe and Tubing Production
Production of Submerged-Arc Welded Straight-Seam
Pipe
Production of Submerged-Arc Welded Helical-Seam
Pipe
Other Welded Pipe Production Methods
Inert-Gas Metal-Arc Welding of Pipe
Induction Welding of Pipe and Tubing
20. Sheet Forming for Packaging Applications
Drawing of Packaging Steels
Specific Aspects of Packaging Steels
Characterization of Packaging Steels
Parameters Affecting Drawing Behavior
Example Applications
Drawing and Wall Ironing of Packaging Steels
Preliminary Drawing
Wall Ironing
Necking and Flanging
Full Operture Easy-Open Can Ends
Score Line Profile (Tool Geometry and Residual
Thickness)
Score Line Shape in the Plane of the LID
End Profiles
Steel Grades
Can End Seaming
Principle of Double Seaming
Seaming Tools
21. Mill Automation for Pipe and Tubing Production
22. Steels for Small Gage Welded Tubes
The Small Gage Welded Tube Market
Manufacturing Processes
Steel Products used in the Manufacture of SWT’s
Major Property Requirements
Conditions to be Met in SWT Manufacture
Geometry Control
Principal Grades Employed
23. Steel Products for the Building Trade
Statutory Requirements
Building Steels and their Coatings
Steel Selection
Galvanized Steels
Coil Coated Steels
The New Solissime Range
Coating Selection Guide
Utilization and Maintenance Precautions
Additional Products
“Condensation-proof” Coatings
Acoustic Insulation
Thermal Insulation
Solconfort Sandwich Sheets
Isofran Sandwich Sheets
Typical Applications
Walling and Roofing
Facing Systems
Flooring
Conclusions

^ Top

Structural Changes in Steel During Hot Rolling

Structural Changes During Reheating

One of the consequences of reheating process is grain coarsing. The control of grain coarsing behavior of steels is an important step in the design of thermomechemical process striving to achieve fine-grained products.

For microalloyed steels, the reheating temperature should be high enough to provide solubility of stable particles. If the stable particles remain undissolved, the beneficial precipitation hardening effects cannot be obtained.

Addition of aluminum, niobium, vanium, titanium, etc., produces abnormal type of grain growth (Fig. 1) which involves the growth of very few grains in relatively unchanged fine-grain matrix. The abnormal grain growth occurs at the temperatures, which are significantly lower than the microalloying solution temperature. The temperature that corresponds to commencing of the abnormal grain growth is sometimes referred to as grain-coarsing temperature.

        The grain size distribution has a complicated dependence on the reheating temperature as depicted in Fig. 2 in application to Nb-V-microalloyed steel. When reheating temperature is equal to 1200ºC (2192ºF), the maximum area fraction of the steel microstructure corresponds to the grain size of approximately 0.12 mm (0.0048/in.). When the reheating temperature is lowered to 1150ºC (2102ºF), the grain size occupying the maximum area fraction is reduced to 0.06 mm (0.0024 in.). However, further decrease in reheating temperature to 1050ºC (1922ºF) produces two pronounced peaks in distribution of the grain size, one of each is at the grain size of about 0.18 mm (0.0072 in.) and the second one is at 0.022 mm (0.0009 in.).

Reheating temperature also affects a formation of so-called deformation bands which play an important role during subsequent grain restoration processes. As can be seen from Fig. 3, the higher reheating temperature the smaller amount of deformation bands will be formed and with less uniformity after the same reduction.

        While it does not appear that the final average austenite grain size after deformation is strongly dependent on the reheated grain size, it is likely that the distribution of the grain sizes above average is much smaller when the reheating temperature is kept below the grain-coarsing temperature.

Kinds of Grain Restoration Process

        Prior to the start of hot rolling, the steel microstructure consists of coarse equiaxed grains of austenite. During passing through the rolls, the austenite grains are getting flattened and elongated on the average; each austenite grain undergoes a dimensional change corresponding to that of the work piece as a whole. The deformation bands may also be induced within the grains as illustrated in Fig. 4.

The three following kinds of restoration process are associated with hot rolling:

1. Dynamic restoration process—This process starts and completes during deformation.

2. Metadynamic restoration process—This process starts during deformation and completes after deformation.

3. Static restoration process—This process starts and completes after deformation.

Dynamic Restoration Process

        When steel is deformed in the austenitic state at high temperature, the flow stress rises to a maximum and then falls to a steady-state as shown in Fig. 5a.

Dynamic restoration process includes dynamic recovery and dynamic recrystallization.

Dynamic recovery is a reduction of work-hardening effects without motion of large-angle grain boundaries. It occurs in a range of strain less than that for peak stress.

Dynamic recrystallization takes place in the range of strain that corresponds to steady state of flow stress.

Role of dynamic recrystallization of austenite in practical rolling of C-Mn steels is small. It is due to the fact that a critical strain required for achieving the steady state of the flow stress is very large, even at high temperatures. The grain refinement of these steels is usually achieved by static recrystallization.

Static Restoration Process

        The microstructures developed by dynamic restoration are not stable and at the elevated temperatures are modified by metadynamic and static restoration processes. The latter processes may include static recovery, static recrystallization and metadynamic recrystallization as shown in Fig. 5b.

In hot rolling, static recrystallization may start spontaneously. Nuclei of recrystallization take place preferentially at elongated grain boundaries and interfaces of deformation bands.

Softening by static recovery and recrystallization occurs at the rates which depend on the prior deformation conditions and the holding temperature. The recrystallization curves generally follow an Avrami equation of the form. 

Fundamental Principles of the Metal Rolling Process

        The chief departments of a metallurgical plant operating on a complete ore-to-finished product cycle are the blast furnace, steel making and rolling departments (Fig. 1).

Almost all the steel that is produced in the steel making department passes through the rolling department; only a small portion is used for making castings and forgings. The rolling process, in which the finished product is produced, is the concluding stage of metallurgical production.

The finished product of such a plant is rolled stock of various types, designed for various purposes, such as: rails; beams; channels; angles; round, square or strip steel; special-purpose shapes, plate the sheet; tubes, etc.

The initial material supplied to the rolling mill is the ingot, which may be either square or rectangular in cross-section. In certain cases round ingots are employed (in the production of tubes, wheels and types).

        The rolling process in a modern metallurgical plant comprises two stages: 1) rolling the ingot into the semi finished product and 2) rolling the semi finished into the finished product.

        It is not expedient to roll small blooms or billets in heavy blooming mills since this lowers the productive capacity and considerably increases power consumption of the mills.

        A blooming mill can operate efficiently if it rolls ingots into blooms of large cross-section, from 200 × 200 to 350 × 350 mm in size. These blooms are subsequently rolled into billets of various size (to suit the production schedules of the mills rolling the finished product) in billet mills.

Billet mills are usually located adjacent to the blooming mills. This arrangement enables small billets to be rolled from heavy ingots in a single heating. This is obviously good practice from the economical point of view.

The rolling of billets in two mills has proved to be highly efficient. The larger the final cross-section of the bloom, the higher the blooming mill output will be. On the other hand, the smaller the cross-section of the billet supplied to the mill rolling the finished product, the simpler the design of this mill will be and the higher its productive capacity. Another factor is the higher size accuracy and quality of a finished product rolled from small billets or blanks.

The breaking-down department, producing semi-finished products, may contain only a blooming mill or a blooming mill with a continuous billet mill. The preferable arrangement depends on the production facilities of the section-rolling department.

In modern metallurgical plants, the production of sheet and plate also comprises two stages: 1) rolling ingots into slabs and 2) rolling slabs into plate or sheet.

Advantages of this two-stage procedure over that practiced in old metallurgical plants, where sheet and plate were rolled directly from the ingot, are:

 (1) Output of sheet and plate mills is increased because the billet they roll is of comparatively small thickness and because the top and bottom of the ingot are cut off after slabbing;

(2) The quality of the rolled plate and sheet is improved since the slabbing mill reduces the ingot on all sides and the slabs may be inspected after rolling so that defects may be removed.

        Slabs may be produced either in blooming or in slabbing mills. The ever-increasing production of plate and sheet steel, in conjunction with the development of continuous sheet and plate mills, facilitated the widespread use of powerful slabbing mills, designed specially for this purpose. The chief advantage of the slabbing mill, in comparison with the blooming mill, is that the former has two vertical rolls in addition to its horizontal rolls. This enables the width to be rolled without turning the ingots on edge.

        The slabbing mill is the chief breaking-down or primary mill in plants designed for the large-volume production of sheet and plate. However, because of their narrower filed of application, slabbing mills are much more seldom installed than blooming mills. In the majority of cases, it is necessary for the primary mill to roll both blooms and slabs. Only a blooming mill will serve this purpose. 

The main requirements in rolling the finished product are:

 (1) To obtain a finished product of the specified size and shape at the highest possible rate of production and the lowest cost;

(2) To obtain a finished product of the highest feasible quality concerning, not only its physical and mechanical properties, but also its surface condition.

These requirements may be met only if the processing schedule for all operations in producing the given rolled product is strictly followed.

        The number of operations comprising the rolling process depends on the specifications stipulated for the shape accuracy, physical and mechanical properties, surface condition and macro-and microstructures of the rolled metal. The more exacting these specifications are, the more complicated the rolling procedure will be and the more operations it will comprise.

The chief operations in metal rolling production are:

(a)          Preparing the initial material for rolling;

(b)          Heating the initial material before rolling;

(c)          Rolling;

(d)          Finishing, including cutting, cooling, straightening, removing surface defects etc.

        The preparation of the initial material for rolling consists in the removal of various surface defects. This is a very important operation, especially in rolling high-quality carbon and alloy steels, as it ensures a high output of proper quality with minimum rejects.

        Strict observance of the prescribed conditions for heating the metal before rolling, proper determination of the temperatures at the beginning and end of the rolling process and determination of an optimum draughting schedule are of vital importance and directly influence the quality of the finished product.

        The prescribed procedure to be followed in cooling the metal after rolling may be quite significant in many cases. If it is not observed, the rolled product obtained may have defects such as flakes or cracks or it may have unsatisfactory properties.

        It is necessary as well to observe the prescribed conditions for all of the remaining finishing operations, which ensure a finished product of the specified quality.

        Fig. 2 shows a flow diagram of rolled stock production from the ingot to the finished product in modern rolling departments. It represents the production of ordinary and quality carbon steel and alloy steel stock.

        Proper control over the rolling process and quality control of the finished product are of primary importance.

        One quality control procedure practised in modern rolling mill departments is melt inspection on the basis of which the quality of the steel is determined and the melt is assigned for rolling. The scope of this inspection depends upon the requirements made to the grade of steel under consideration.

        Melt inspection begins in the steel making department where samples of the melt are taken to determine the average chemical composition of the steel in each melt. Usually two samples are taken from each ladle. The second sample serves to additionally check the chemical analysis. This is done in cases when it is necessary to check the content of certain elements or if such inspection is stipulated by special requirements of the customer. In certain cases, the samples for this analysis are taken in the rolling mill department from the billet or the finished product. In the last years, many elements are determined by spectrographic analysis, which has become one of the most widespread physical methods of determining the chemical composition of metals and alloys.

        Further melt inspection may include: determining the quality of the melt by its macro-and/or microstructure and longitudinal fracture; determining the grain size of the steel; determining the mechanical properties and hard-enability and other tests. For this purpose one or two control ingots are selected. In the latter case, one ingot is selected from the first tap and the second from the last tap. Control ingots of high-quality steels are selected from each tap. These ingots are rolled into billets (and sometimes into finished products) either separately or together with the ingots of the whole melt.

        Samples for melt inspection tests are selected from bars rolled from the steel just under the top of the control ingot. In certain cases, samples are taken from bars rolled from definite sections of the ingot in height, for example, from the top, middle and bottom sections.

        Macrostructure inspection of steel enables gas holes (not permissible in killed steel) to be revealed as well as shrinkage cavities, porosity, segregations, hairline cracks, flakes and other defects. These defects are evaluated by standard scales.

        Longitudinal fracture enables the degree of slatiness and the grain size to be determined; it reveals such defects as seams, shrinkage cavities, porosity, inclusions, stone like appearance, naphthalene appearance and other defects that can be seen by visual inspection.

        The microstructure inspection, the amount of nonmetallic inclusions, grain size, depth of the decarburised layer and other factors are determined.

        The second type of control is over the rolling process. It should ensure proper heating procedure for the initial materials, proper observation of the pass schedule for rolling the given section within the specified tolerances and proper finishing of the rolled material.

        The marking on the initial material must be carefully checked in the storehouse before charging it into the heating facilities.

        The temperature of hot ingots should be measured before placing them into the soaking pits. This will prevent thermal cracks. Processing instructions usually list the minimum permissible temperature of the ingot surface and the maximum soaking pit temperature. The heating temperatures and flame temperatures are checked during heating by appropriate instruments.

        In rolling metal it is necessary, first of all, to check the initial and final rolling temperatures, as well as the pass schedule. The setting of the rolls is checked continuously by measurements of the rolled sections; the condition of the roll grooves and roll gear is also checked frequently. Lately much attention has been paid to determining the pressure on the rolls and the torques applied in rolling by means of load cells and other instruments. This enables the available power of the mills to be utilized more correctly and fully.

        Mechanization and automation of rolling mills allow the rolling speed to be considerably increased, particularly for continuous mills, and enable more attained in cold rolling sheet steel in continuous mills, for example, became possible only after instruments were developed for contactless continuous gauging of the strip. Such instruments include radiation strip thickness gauges based on the use of gamma or beta radiation. Radioactive isotopes are utilized as the source of energy. By means of a strip thickness control system, these instruments actuate the roll-adjusting and strip tension devices. In the very latest installations, computers and television have also been applied. 

Steels for Magnetic Applications

1. Electrical steels-metallurgy and properties

Introduction

        Together with nickel, cobalt and a few other elements, iron is one of the rare ferromagnetic metals. This property is intimately related to the electronic structure of the iron atom and a detailed explanation can only be given in terms of quantum mechanics. However, the consequence is that the spins of certain electrons are aligned, resulting in an overall magnetic moment, which is in the same direction throughout small areas of the crystal structure. Iron and the ferrite steels are, therefore, composed of small saturated magnetic domains, each corresponding to a microscopic magnet. The magnetization is not always obvious on a macroscopic scale, due to the fact that the magnetization directions of individual domains tend to compensate one another. The magnetization directions of adjacent magnetic (Weiss) domains are different, and often opposite, and they are separated by bound-aries called Block walls (Fig. 1). When an external magnetic field is applied, the magnetization directions tend to reorient and domains more favorably oriented tend to grow at the expense of the others by movement of the Block walls, so that the individual magnetic fields no longer cancel out. In magnetically soft materials, the microstructure is such that displacement of the Block walls is facilitated, enabling the metal to respond rapidly to external excitation, and to transmit the magnetic flux with minimum power losses. On the contrary, in magnetically hard materials (magnets), the aim is to conserve a strong macroscopic magnetization (remanent induction) with a high coercive force or coercivity (the external field necessary to overcome the remanent induction). Figure 2 shows the typical shape of a hysteresis curve, illustrating the fundamental parameters Bs (saturation induction). Br (remanent induction or remanence) and He (coercivity). The magnetic permeability, which reflects the ability of the metal to transmit the magnetic flux, corresponds to the slope of the initial magnetization curve. A good electrical steel sheet must have a high permeability and minimum coercivity, together with other properties, which will be described in detail later.

Utilization and Property Requirements

        Transformer sheets are sold in the finished or semi-finished conditions and are used in the form of lamination stacks, mainly in electric motors, alternators and compressors, depending on their properties. The stacks form the magnetic core of the apparatus concerned. The sheets must satisfy several, sometimes contradictory, requirements, whose priorities depend on the specific application, such as high magnetic permeability, low hysteresis losses (i.e. low power consumption) and ease of cutting to shape. Electrical steels come in two principal categories, the oriented and non-oriented grades.

        Grain-oriented sheets are obtained by a complex processing cycle and give excellent results in terms of permeability and core losses in certain conditions, particularly in unidirectional fields, such as in transformers. The particular feature of these materials is their microstructure consisting of very coarse grains, oriented with the cube edge parallel to the rolling direction (110)<001> ( the so-called Goss texture). The crystallograbphic direction <100> is a direction of easy magnetization, so that when the sheet is magnetized in the rolling direction, its permeability will be very high and its coercive force very low. Figure 3 shows the magnetic anisotropy of iron and illustrates the advantage of having a <100> direction parallel to the exciton field. In contrast, because of their planer anisotropy, grain-oriented sheets have much poorer performance in rotating machinery, such as motors, where the excitation field rotates in the plane of the sheet. They are difficult to blank because of their large grain size. In France, they are manufactured by the Ugine S.A. company.

        Non-oriented sheets are produced by a much more conventional process, and can be subdivided into two categories:

 (a)          "Fully-processed" grades are delivered in the finished condition and are alloyed with silicon. They are continuously annealed at high temperature, and sometimes varnished. Although their magnetic properties in the rolling direction are not as good as in grain-oriented sheets, they are much less textured, making them better suited for use in rotary fields. They have good blankability, which depends on the silicon content, the grain size, and possibly on the coating.

 (b)          "Semi-processed" grades are continuously annealed at a lower temperature and are delivered after a significant skin-pass reduction (several %). They have good blankability, but must be given a final annealing treatment by the user to develop their magnetic properties. This treatment has several effects. Firstly, it produces a coarse grain size, due to the critical strain (1-5%) imparted by the skin-pass, leading to relatively few recrystallization nuclei, enhancing the magnetic properties. Secondly, decarburization is promoted by the use of a controlled atmosphere. Thirdly slight oxidation, producing a bluish tinge, provides electrical insulation between the laminations in a stack, limiting the formation of eddy-currents in alternating current machines. The uses of semi-processed grades are similar to those for fully processed materials, even though the clients are often different. Today's top semi processed grades are roughly equivalent to mid-range fully-processed products.

Optimization of Magnetic Properties

        The remainder of this article will concentrate on semi- and fully-processed non-oriented sheet.

        Among other characteristics, a good electric motor must have a high maximum speed (or torque) and high efficiency. The magnetic circuit design and the choice of metal have a decisive influence on these properties. Power losses, which affect the efficiency, have two major sources, resistive losses in the copper windings, and core losses due to induced eddy-currents and magnetic hysteresis. The resistive losses are proportional to the square of the induction current, and can be decreased if the coupling between the stator and the rotor is increased, i.e. if he core has a high permeability.

Hysteresis losses

The applied fields associated with alternating currents are sinusoidal, so that the hysteresis cycle is repeated 50 times per second at the normal mains frequency. The energy consumed by this process alone is equal to the area inside the hysteresis loop. The hysteresis is caused by internal magnetic friction associated with the movement of the Block walls. To reduce the losses, it is necessary to lower the coercive force, which correlates to high domain wall mobility. This can be done by eliminating as far as possible all precipitate particles, particularly cementite, requiring extremely low carbon contents. The "fully-processed" grades are decarburized either in the melting shop or by strand annealing after cold rolling. The "semi-processed" grades are decarburized by the user during final annealing after cutting to shape.

The very low solubility of carbon in ferrite at room temperature makes these steels highly sensitive to magnetic aging, which can lead to increased power losses due to the natural precipitation of fine carbide particles. The final carbon content must therefore be less than or equal to 20 ppm. Figure 4 shows the percentage increase in power losses with carbon content after simulated aging in a fully-processed grade. Manganese sulfide and aluminum nitride particles are less detrimental, due to their large size (induced by appropriate metallurgical control).

Another way of reducing hysteresis losses is to decrease the coercive force by increasing the grain size, since grain boundaries are effective obstacles to the movement of domain walls. In non-oriented grades, cold rolling is performed in a single reduction of about 70%. Fully-processed steels are continuously annealed at high temperature to obtain coarse grain sizes of the order of 4 to 8 ASTM (20-80 um). Semi-processed sheets are given a slightly smaller cold reduction, followed by strand annealing at lower temperature and skin-pass following corresponding to a strain of less than 10%. Critical strain recrystallization during the final high temperature annealing treatment carried out by the customer gives typical grain sizes of about 0 to ASTM (80-300 um), strongly diminishing hysteresis losses. The skin-pass reduction enables the blankability to be optimized. The electric motor manufacturers or blanking shops usually prefer YS/UTS ratios greater than 0.85 in order to improve productivity and reduce tool wear. The amount of cold work required to obtain such ratios is always a few percent higher than the critical recrystallization strain. Figure 5 shows a typical example of the variation of YS/UTS ratio with skin-pass reduction. The accompanying micrographs illustrate the as-annealed grain sizes. For low strains, recrystallizaion does not occur and the grain size remains fine, while strains of a few percent lead to coarse recrystallized grains.

The density of dislocations and other mechanical defects is also important. Semi-annealed sheets are annealed after blanking and therefore have very low dislocation densities. In contrast, fully-processed sheets can contain local regions of high strain after blanking. This is particularly true at notches in the stator, where the deformation is most severe. Since the purpose of these notches is to transmit the magnetic flux from the stator to the rotor, certain users perform a stress relieving treatment at about 800ºC to produce local recovery or even recrystallization.

        Finally, another useful way to reduce hysteresis losses to modify the texture, enhancing the density of (100) and (110) planes, which significantly raises the low flux density permeability and sharply lowers the coercivity. The texture can be modified by adapting certain stages of processing, but the operations involved are relatively complex.

Eddy-current losses

The other important source of core losses is the resistive heating associated with induced eddy-currents. The latter are caused by interaction between the magnetic filed and the conduction electrons of the metal. The losses due to this phenomenon are proportional to the square of the frequency and the thickness of the sheet (the current loops appear in the sheet section perpendicular to the magnetic flux, and create a counter-field which opposes the induction field). Apart from reducing the working frequency, which is rarely possible, there are three ways of decreasing this effect.

• Decreasing the sheet thickness is common practice, since magnetic cores are generally composed of stacks of thin sheets, less than 1 mm thick. This approach is limited by productivity considerations.

• Efficient inter-lamellar electrical insulation, generally obtained by oxidation (bluing) in semi-processed grades, at the end of the final annealing treatment performed by the user, and by varnishing for fully-processed products. The latter technique is much more effective.

• The use of alloying elements such as silicon, phosphorus, aluminum and manganese have the advantage of considerably increasing the resistivity. This significantly decreases the intensity of the eddy-currents and the associated power losses. Silicon can be incorporated in the steel in appreciable amounts, up to 3.5%, with appropriately adapted processes. Beyond this level, excessive brittleness prevents cold rolling. Furthermore, silicon reduces the saturation induction of the steel, and hence its permeability at medium and high flux densities. Steels and hence its permeability at medium and high flux densities. Steels with more than 0.5% silicon have various technological disadvantages. After continuous casting, the slabs must be transferred hot to the rolling mills, due to their tendency to crack on cooling, and silicon-containing sheets are difficult to weld. However, in spite of these drawbacks, the functional benefits of silicon-containing electrical steels have led to their widespread use, due to the critical importance of low power consumption, especially in large motors.

2. Type of electrical steel.

Classification

Non-oriented electrical steel sheets are covered by three French standards, depending on whether they are semi-or fully-processed. Semiprocessed products are described by the C 28-925 and 28-926 standards, while NF C 28-900 treats fully-processed sheet. It all cases, only the magnetic properties are guaranteed, particularly the total specific losses at a given magnetization level and the induction level for a given applied field, in both the transverse and rolling directions. For semi-processed sheet, after a reference heat treatment, the metal must guarantee a power consumption less than a specified value, together with a minimum induction level for a given applied field. The reference heat treatment corresponds  to annealing for two hours at 790ºC in a decarburizing atmosphere. The apparatus used for the measurement is called in Epstein frame and is described by the NF C 28-911 standard.

The designations of different steel grades according to these standards reflect the maximum power losses in watts for one kilogram of the metal concerned and an induction of 1.5.T. Since these losses depend on the thickness, the designation includes the standard thicknesses (0.5 and 0.65 mm for the semi- and fully-processed produces. plus 0.35 mm for fully-processed materials). For example, the FeV 660-50 grade corresponds to a material with a thickness of 50 hundredths of a millimeter, for which the guaranteed total losses must not exceed 6.6 W/kg measured in a standard Epstein frame. The difference between the two semi-processed product standards NF C 28-925 and 28-926 concerns the amounts of alloying elements, the first applying to unalloyed steels (< 0.5 wt. % Si) and the second to alloyed grenades (according to the NF EN 10-020 standard). The designations are differentiated by two letters. HD for unalloyed semi-processed grades, HE for alloyed semi-processed materials and HA for the fully-processed sheets. The NF C 28-900 standard mentions certain requirements concerning magnetic aging, the packing factor (volume increase on stacking) and magnetic anisotropy.

Steel Grades

Usidecoupe

Although not really electrical steels, these XC grades are used for the manufacture of small motors for very infrequent utilization (e.g. kitchen mixers, electric automobile window drives, etc.), for which the efficiency is of little importance. Although their magnetic properties are never guaranteed, their mechanical characteristics are severely controlled. Their blanking capacity must be excellent and invariant.

Semi-processed grades

Four grades are presently available, in different thicknesses. They cover all the requirements of the domestic appliance and small industrial motor markets. The differences concern the guaranteed maximum power losses, and are essentially related to the amount of alloying additions in the steel. The latter include the four main elements which increase the resistively, i.e., silicon, aluminum, phosphorus and manganese. Table 1 gives the guaranteed magnetic properties of these grades, compared to those required by the standards, together with their typical mechanical characteristics.

        The trend in semi-processed steels is towards more efficient decarburization during melting. This improves their intrinsic quality, but most of all, simplifies the process for the end user, by eliminating the need for decarburizing, increasing the productivity of the grain coarsening furnace. Moreover, modern stacking techniques, such as the Fastec process, become accessible to the semi-processed grades. Since assembly is performed immediately after blanking, decarburizing is not possible. However, in these applications, varnishing of the coils becomes essential. This development enables the steelmaker to differentiate between semi- and fully-processed grades at a much later stage in the manufacturing cycle, significantly simplifying internal product management.

Fully-processed grades

A large number of fully processed grades are available on the market. For a given thickness, it is possible to choose from a wide range of different power loss levels. Various types of varnish can be applied to the sheets. Table 2. illustrates the extreme grades in terms of guaranteed maximum power losses for three standard thicknesses, together with their typical mechanical characteristics.

The choice of power loss level depends on the application concerned, varying greatly from a washing machine motor to a nuclear power station turbine. Metallurgically speaking, the variations are due mainly to differences in the silicon and aluminium contents and in the annealing cyles employed. 

Preparing and heating the initial materials

Preparations for rolling

The initial materials (ingots and billets) are prepared to be rolled by removing various surface flaws. This is called surface conditioning and is an important operation, especially in rolling quality carbon and alloy steels intended for the manufacture of vital components in various fields of industry. The cost of preparing the initial materials will be justified in this case by the possibility of obtaining a product of the specified quality and by the decrease in the amount of rejects.

As a rule, hot ingots are charged into the soaking pits and, therefore, surface conditioning operations are performed on the billets (however, flaws may also be removed from the surfaces of the hot ingots).

Especially exacting requirements are made to the surface of alloy and high-alloy steel ingots. Therefore, the cast ingots may be completely cooled, surface conditioned and then reheated for subsequent rolling (a softening heat treatment may be employed before removing the flaws). In this case, surface flaws are removed both from the ingots and from the billets.

Surface defects, subject to removal, include: scabs, hair lines, cracks, rolling laps, nonmetallic inclusions, scratches, etc. All of these flaws are revealed by inspection. If all surface defects are to be removed, the billet is first pickled. Defects, enclosed by scale and therefore hidden from ordinary inspection, may be detected by acid pickling.

        Chipping operations to remove surface flaws, performed by means of pneumatic hammers usually operating under an air pressure of 5 to 5.5 atm, are still in wide use. This method has a low production capacity, especially for alloy steels, and is a health hazard as far as the operations are concerned.

Chipping requires the highest labour input of all rolling operations and is therefore little suited to modern rates of production. It is chiefly employed in the surface conditioning of billets and sometimes as a supplementary operation for removing certain deep flaws on ingots. These are flaws such as may remain, for example, after rough-machining the whole surface of the ingot or after performing some other allover cleaning operation.

Defects are usually chipped in a direction along the length of the billet or ingot since transverse chipping may caulk such defects as cracks. The sides of the chipped groove should slope gently and its width-to-depth ratio must be such that no new cracks or other defects will be produced on the surface of the finished product in subsequent rolling.

Swing-frame grinders are also used for surface conditioning operations. They are mounted on a special suspension device allowing them to be easily swiveled about a vertical axis and titled from the vertical position. Due to the high grinding speeds, the abrasive wheels used are of aluminum oxide with a resinoid (bakelite) bond.

In contradiction to chipping with pneumatic hammers, conditioning with a grinder is carried out in the transverse direction (on the billet) since it is difficult to detect hair lines or fine cracks, usually running along the billet, in longitudinal grinding. 

Grinders have a low production capacity in surface conditioning operations and the cost is higher than for chipping. Consequently grinding finds application in removing a large amount of slight defects and chiefly for conditioning high-alloy steel billets. Chipping of the latter with pneumatic hammers is extremely difficult or even impossible. In removing flaws by grinding from hard steel billets, the intensive heating of the metal and subsequent cooling may lead to the formation of grinding cracks. This can be avoided by taking lighter cuts and by selecting the grain size, grade and peripheral speed of the grinding wheel to suit the steel being processed. Grinding cracks are more liable to appear in conditioning hard steels. That were rapidly cooled after rolling, producing high stresses. Air-hardening steels should be conditioned in the as-annealed state to prevent the formation of cracks.

A new surface conditioning method, called flame scarifing, has come into use in the last years. It consists in burning out the flaws in the surface by means of a torch with an oxyacetylene flame.

Flame scarfing is performed either by hand or in special machines.

As a rule, hand (torch) scarfing is a localised operation in which certain definite defects are removed from the surfaces of ingots and billets. Here the tip of the torch is directed to one end of the defect and the metal is preheated to a temperature of 950°-1,000°C. This takes several seconds. The torch is held at a angle of 75°-80° to the billet surface. When the metal is heated to the burning point, the amount of oxygen supplied to the torch is increased.

This oxygen removes a layer of metal by oxidizing or burning the metal. As soon as the metal begins to burn, the torch is titled to an angle of 25°-30° to the surface. Then the jet of oxygen not only burns the metal but also blows the slag and liquid metal from the surface being scarfed.

At present, almost all types of steel are torch scarfed. All carbon and low-alloy steels may be torch scarfed without difficulty. Stainless, heat-resistant and other steels, with high chromium content, require the application of special fluxes and coatings, which facilitate burning and form slag with a low melting point. Frequently a special torch is used to scarf these steels. It differs from the ordinary torch in having a supplementary injector, supplied through one hose with an oxygen-flux mixture, and with additional “cutting” oxygen through a second hose. 

First, the beginning of the cut is heated, then the oxygen and flux delivery are turned on. When a sufficient amount of molten slag has formed, the supplementary oxygen injector is turned on and torch is moved along the line of scarfing. 

Cracks, which may be formed in flame scarfing, are due to the thermal stresses resulting from large temperature differences along the cross-section of the billet and also to stresses associated with the formation of a martensite or troosto-martensite structure from the austenite. The lower the thermal conductivity and initial temperature of the billet being scarfed, the higher its coefficient of thermal expansion and the lower the ambient temperature, the higher the thermal stresses will be. The higher the austenite stability and carbon content of the steel and the higher the cooling rate after scarfing, the more susceptible the steel will be to crack formation. Stresses due to scarfing may be reduced by preheating the billet or by surface conditioning directly after rolling when the billet is still hot.

Torch scarfing is performed in a direction along the billet. The scarfed groove should have sloping sides and should be of a width at least five times its depth and of a length at least three times the width.

The production capacity of hand torch scarfing is several times higher than chipping with pneumatic hammers. Highest output is achieved, however, in flame scarfing machines where surface defects are removed by an allover burning of the surface layer with an oxyacetylene flame.

Flame scarfing machines are usually installed beyond the mill, for example, in the roller table line between the blooming mill and the shear. Here hot blooms and slabs are scarfed directly after they leave the mill. Here, the surface layer of metal is removed simultaneously from all four sides of the bloom.

The same type of mechanised allover surface conditioning is also practiced in the billet storage where billets are treated either cold or preheated. In these installations, the torch head is mounted on a truck traveling on rails along which the billets are arranged.

Other surface conditioning methods, used for billets and ingots, are: 1) milling, 2) planning and 3) turning.

Of these, turning is the most extensively used operation for roughing ingots and billets.

Turning is applied in roughing round ingots and billets of heat-resistant, stainless and other special grade steels for the production of high-quality seamless tubing. Special lathes are available at present for roughing such round ingots. These lathes are equipped with special fixtures for rapidly setting up and centre-drilling the ingots. The use of several tools clamped on a single carriage effectively reduces machining time.

Alloy and high-alloy steel ingots, intended for producing sections, sheet or plate, are also turned in special cases. Such ingots have a square or rectangular cross-section and are machined on special tracer-controlled lathes. These machine tools, called multiple-cornered lathes, can accommodate ingots and billets of any cross-section.

Before multiple-cornered lathes came into use, square and rectangular ingots were roughed all over on general-purpose and special planers. The low output and high cost have almost excluded this method from general practice.

The surfaces of ingots and billets are milled both for allover surface roughing and for removing separate defects. This operation is performed on special milling machines.

Large surface defects, of a width up to 50 mm and a depth up to 15 mm, difficult to cut out by pneumatic hammer chipping, are usually removed by a local milling operation.

Ingots and billets of mild and medium hard steels are machined without being previously annealed. Hard steels are annealed before machining. 

Bolt and NuT Manufacturing Technology

Introduction

        The metal-working industry employs a range of different technologies. These include the casting of molten metal into moulds, fusion by welding, cold and hot mechanical working, cutting and chemical machining. technologies consist of a variety of distinct ‘techniques’; for example, metal cutting technology includes amongst many other ‘techniques’ turning, milling and drilling. The manufacture of nearly all threaded fasteners require the employment of methods or ‘techniques’ from more than one technology.

        In later chapters, bolts and nuts are described as being manufactured using a particular technology. It does not necessarily follow that only one technology is employed in the full sequence of manufacturing operations. Rather, the characterization refers only to the most important technology that is used in the complete manufacturing sequence.

        The next section of this chapter provides an introduction to the principles involved in the manufacturing of threaded fasteners. It is followed by more detailed discussion of alternative, technologies, the ranges of alternative ‘techniques’ within each technology and the types of tools and machines that are used. The descriptions of the principles and technologies are not comprehensive; the purpose of this chapter is to give a reasonable idea about the nature of choice currently available and to provide sufficient information to enable the reader unfamiliar with fastener manufacturing to follow the discussion in later chapters of the study.

        The three main metal-working technologies used in the manufacture of threaded fasteners are cold mechanical working or cold forming, hot mechanical working or hot forging and metal; cutting. In mechanical working, the processes used to shape metal include rolling, drawing, extrusion, up-setting open die forging, closed die forging, and presswork. Although mechanical working changes the shape of work pieces it does not change their volumes substantially. The processes involve the plastic re-shaping or deformation of either cold or heated work pieces by the external action of special tools. Mechanical working strengthens components by drawing impurities and grains into bands along the direction of working and closing minute cavities.

Fundamentals of Mechanically working and cutting metals

(a) Cold Forming

        When metals are subjected to progressively larger loads they first deform elastically. If the load is increased beyond a certain point the metal becomes plastic. Curve A on the load extension diagram in figure 1 shows how the rate and degree of deformation can vary with the magnitude of the applied load. Stress, which is the load per unit area, is plotted on the vertical axis of the diagram while deformation, expressed in terms of strain is shown on the horizontal axis. Beyond the transition point on curve A between elastic and plastic deformation some deformation remains when the load is removed. If the load continues to be raised the metal eventually breaks.

        Cold forming metal within its plastic range decreases its plasticity and this phenomenon is called strain or work hardening. Curves A and B in Figure provide a comprison of how mild steel behaves before and after cold working. Work hardening is not only associated with an appreciable decrease in plasticity but also with an increase in strength. Advantage is sometimes taken of this to deliberately increase the strength of a piece of metal by cold working it, but since plasticity decreases with cold working there is a maximum limit to the amount of cold working that may be undertaken. Advantages of cold forming over hot forging are that parts are free of surface scale, may require less raw material since they can be formed closer to final size, do not need to be pre-heated and require little cool-downtime after working.

(b) Hot Forging

        For each metal there is a range of temperatures in which the plasticity and resistance to work hardening is greatly increased. The forging temperature range varies from metal to metal. For mild steel it is around 1,200°C while for lead it is room temperature. The forgability of a metal is its ability to flow into the required shape without cracking and offer low resistance to the forces shaping it. Forging a metal to a given shape requires considerably less power than cold working it, but the accuracy and finish of the work piece are generally inferior.

(c) Metal Cutting

There are several ways of cutting metal: sawing, abrasive cutting-off (grinding), shearing or cropping, and machining. These methods have much in common both with each other and with the mechanical working technologies; all involve working metal in the zone beyond the elastic limit.

Manufacturing Technologies

        In this section each of the main bolt and nut manufacturing technologies employing the principles described in Section 2 is discussed.

(a) Cold Forming of Bolts

        Four widely used methods of cold forming are shown in the top row of Figure 2. These are upsetting etc. which involves a reduction in length and increase in cross-sectional area of work pieces, forward and backward extrusion which both have the effect of reducing the area and increasing the length of work pieces and thread rolling. In thread rolling the dies penetrate the surface of the blank to form the thread root, the displaced material flowing outwards and upwards to form the crest of the thread. Cropping or shearing, trimming and piercing are often carried out in association with cold forming but they employ cutting rather than forming techniques, are shown in the lower half of Figure 2. Most work pieces or blanks to be cold formed are first cut off by cropping them from wire stock. Hexagonal bolt heads are usually formed by trimming the periphery of cylindrical upset heads with a hollow hexagonal punch. Trimming removes any cracks on the edge of the heads caused by upsetting. Piercing involves the removal of a slug from the work piece to form a hole and is used in the cold forming of nuts.

        One sequence of operations which can be used to produce a bolt to its final shape together with an outline of the necessary tooling is shown in the left-hand side of Figure 3. In this case the heading machine cuts off and upsets the blanks in two blows. The additional secondary operations required to finish the blanks are carried out on a single blow trimming machine and thread rolling machine.

        In heading machines, the end of a coil of cold drawn wire that has been precoated with a lubricant passes through straightening and feed rollers which push the wire through the cut-off quill until it comes up against a stop. The blank is sheared from the end of the wire and transferred to the heading position by the cut-off mechanism. During the firsts stroke in the heading station the punch pushes the blank into the die until it comes up against the ejector pin and then commences the shaping of the head by upsetting a cone. Between the first and second blow the first punch is moved aside and the second punch takes its place. The second punch forms the head into a cylindrical cheese and usually also embosses the top of the head of the blank with symbols to permit identification of the manufacture, tensile strength and sometimes of the tread forms. Once the second punch has withdrawn, the blank is forced from the die by the ejector pin and falls clear of the die before the next blank is inserted.

        Two blows are needed to form the cheese head because there is a limit to the amount of metal that can be upset in a single blow without buckling the portion of the blank that is not supported within the die. In a single blow, the maximum amount that can be cold upset under control is two and a quarter diameters but most single blow heading is within the range of one to one and a half diameters. In two blow heading four and a half diameters can be upset.

        There are other limitations to the lengths of material that can be headed using the methods shown in Figure 3. Ejector pins buckle if they are unsupported over a length of more than eight times their diameter which limits the length diameter ratio of formed b1they anks contained within a die. One way round this problem is to provide support to the ejector pins by using a telescopic ejector mechanism. This makes it possible to remove formed blanks with lengths contained within the dies up to twelve times as long as their diameters. For even longer parts which are upset (but not extruded) split dies are used which are made in two halves. The two halves are held together during the upsetting operation and act in a similar way to normal closed dies but are forced apart before ejection. With this arrangement blanks are inserted from the back as opposed to the front of the dies and are cut off by the back of the split dies before they close for the upsetting operation. The length of the wire for the next blank to be upset ejects the previous part from the die. The head of very long bolts can be cold formed by holding pre-cut blanko in split dies during upsetting. The heads of most cold formed threaded fasteners are upset using the closed rather than the split die arrangement. The reason is that when bolts are made from wire the split die configuration requires a different set of dies for each blank length whereas only the ejector pin requires replacement when, blank lengths are altered using closed dies.

        After heading, the blanks are collected from under the heading machine and transported to the trimming machine. The trimming machine performs two distinct functions: first it gives the head a hexagonal shape by trimming off material and second, it straightens the body and forward extrudes the end of the body in preparation for thread rolling. The headed blanks are usually deposited into the hopper of a trimming machine where they are first automatically orientated correctly for delivery into a chute at the lower end of which they are picked up and transferred to a point over the trimming die centre by a pair of fingers. The punch pushes the blank through the extrusion die as depicted in the left hand column of Figure3. Towards the end of its forward stroke the punch forces the head of the blank through the hexagonal trimming die. Once the punch is on the return stroke the blank is ejected from the die by means of a spring behind the ejector pin.

        There are limitations to the amount of forward extrusion that can be carried out with a single blow. If buckling of the portion of the blank that is not supported in the die is to be avoided during extrusion, the reduction in cross-sectional area resulting from a single operation must not exceed 30 per cent.

        The diameter to be thread rolled could be reduced by machining rather than extrusion, but this would waste material. Another method of producing threads is to cut them but this also wastes material. Thread rolling has other advantages, however, over thread cutting. Firstly, the cold working action during rolling increases the strength of the threads since the grain of the material follows the thread contours and secondly, compressive stresses are imparted to the thread roots during rolling which offset the tensile stresses produced by tightening nuts into bolts. In flat die thread rolling, parts are threaded between a pair of flat dies, one reciprocating and one stationary. The thread-shaped ridges on the working faces of the dies are inclined at the angle necessary to produce a continuous thread on the blank. A thread is rolled on one blank at a time while it rotates about its own moving axis, during the forward stroke of the reciprocating die. Thread rolling can take other forms. In planetary thread rolling, blanks are rolled between a centrally located rotating die and a stationary concave die segment. The circular die in the middle rotates continuously with the result that two or three parts can be passing between the dies simultaneously. Such machines can only roll relatively small diameter blanks. In another arrangement the blank is squeezed between two cylindrical rotating dies, which have, thread form cut round their circumferences.

        Thread rolling machines, like trimming machines, are usually arranged so that the blanks are automatically fed down a chute from a hopper although, in the case of very long blanks, it is necessary to hand-feed both trimming and rolling machines. Some fastener manufacturers link their heading, trimming and rolling machines by providing conveyors, which collect the blanks from under one machine and deliver them to the hopper feed on the next machine in the line. Although conveyors can reduce the amount of materials handling required between the operations they have two disadvantages. First, all the machines in a line linked by conveyor must be working on the same size of blank (when the machines are not linked up this is unnecessary) and second, overall machine utilization in a linked system can be low because when one machine stops running for more than a short period the other machines linked to it must also halt.

        The heading, trimming, extrusion and rolling operations can be combined in a single specialized machine. The operations are illustrated diagramatically on the right hand side of Figure3. The drawing shows a machine having a bank of five stations, with a cut-off, four dies, and four punches. Transfer fingers move each blank along the row of dies one die position at a time between blows. Each blank receives a single blow from each punch. In the case shown in Figure 3, wire with a diameter greater than the finished shank diameter is used. By extruding the body if the bolt down to its finished shank diameter in the first die the head can be upset in only one blow by the second punch without buckling occurring. This is possible because the section of the blank forming the head is less than two and a quarter times the diameter of the original wire. compared to two blow heading there is less deformation in the head for a given bolt size. The result is a better balance of properties between head and shank with the result that normalizing of finished bolts may be necessary. The portion of the shank that is to be thread rolled is extruded in the third die and the head is trimmed in the fourth die. In some machines, the trimmed blanks pass through the centre of the trimming die and enter a tube through which they are delivered to either a pointing and then a thread rolling station or directly to a thread rolling station. The pointing operation cuts a chamfer round the tip of the body so that the start of the rolled thread is even. In other machines, a trimming punch is used and parts are allowed to drop into a collecting belt after ejection from the last die.

        There are several types of cold forming machines, which represent intermediate stages between two blow heading machines and fully integrated machines. Often when parts with large diameter heads and slender shanks are required, two dies, three blow headers are used. Machines with four or five stations in line but without pointing or thread rolling stations are called either progressive or transfer headers. These machines are most often used for cylindrical parts other than bolts which would otherwise have to be machined from solid bar.

        In addition to the precoated solid lubricants on the wire small quantity of oil is usually applied to the material stock to lubricate forming operations. An oily rag is often tied round the wire ahead of the feed rolls but oil can be sprayed into the tool zone where there is no risk of large quantities of oil entering the die and causing work pieces to be partially filled. The important characteristics of these oils are resistance to high pressure and temperature and the minimum emission of smoke and toxic vapours. Most cold forming wires are precoated with thin dry coatings.

(b) Cold Forming of Nuts

Figure 4. shows a cold nut forming sequence and an outline of the tooling. Cold nut forming machines are quite similar to short stroke progressive headers. The wire stock is fed through feed rollers, the cut off quill and the cut off bush up against the length stop. A cylindrical blank is cut off and inserted into a transfer gripper which presents it to the first forming station. After forming in the first die the blank is ejected and received by a second gripper for transfer to the next forming station. This sequence is repeated until the blank leaves the last die position finished formed. During two of the transfers in the sequence shown in Figure 4 the blank is turned end on end through 180 degrees to enable the punches to work on both ends of the blank. Because of the work hardening that occurs during forming, the slug is punched out cleanly at the last station.

Nuts can also be formed in four stations cold nut forming machines which accept wire with a hexagonal cross-section or alternatively by cutting off and performing cylindrical blanks on one machine, heat treating the blanks to remove the effect of the work hardening induced during the initial forming and then finish forming the blanks on a second machine.

After forming, nut blanks have their thread cut on tapping machines.

(c) Hot Forging of Bolts

        Figure 5. shows two alternative operation sequences for the production of hot forged bolts together with outlines of the necessary tooling. Bolts are forged from hot rolled round bar of a diameter equal to that of the finished bolt shanks.

        In the first process in Figure 5, a blank, or pin, of sufficient length to form one bolt is cut off. Sawing is rarely used for this operation since it takes longer than cropping. Equipment used for cropping ranges from manually operated shears intended for cutting reinforcing bar on building sites to fully automatic high speed cropping machines. Manually controlled purpose-made semi-automatic cropping machines are available but simple reciprocating mechanical presses can be equipped with cropping tools to do the job in the same way. In such machines the bar stock is supported on the lower shear knife and the head carrying the upper knife reciprocates continuously while an operator pushes the bar in by hand against a stop during the interval between strokes. Sometimes two pins can be cut per stroke by loading bars in parallel. There is usually provision for stopping the head in the raised position between strokes to allow time for the manipulation of heavy bars. Automatic cropping machines are fed from stocks of bars held in magazines, which can be replenished without interrupting production.

        Most hot forged bolts are thread cut because steel in the hot rolled state tends to have an uneven and scaly finish, which is unsuitable for extrusion and thread rolling. Slight chamfers are usually cut on the ends of the shanks; these serve to remove metal distorted during cropping which might prevent the shank entering the forging die, provide lead-ins for thread cutting tools and also help start threads when bolts and nuts are assembled. This pointing operation is similar to that carried out on some cold formed bolts. A variety of purpose built pointing machines are available ranging from magazine fed automatics to simple manually loaded machines. Sometimes old lathes can be modified to do the job.

        Before forging, the end of the pin, which is to form the bolt head, is heated. Simple pin heaters consist of open hearths burning solid fuel with ledges round the top edges on which pins rest. The tips of the pins are introduced through holes in the refectory bricks. More sophisticated furnaces using oil, gas or electric induction heating are available which automatically deliver heated pins at a predetermined rate. Induction heating equipment is expensive to purchase but results in less surface scale and allows precise control over temperature. Three unsupported diameters of bar can be hot upset in a single blow if the end of the bar is square to its longitudinal axis. However, because the ends of most cropped pins are not flat and may be deflected sideways it is usual to employ two blows even for heads requiring less than three diameters of material. The two blow hot heading method shown in the left hand column of Figure 5 is similar to two blow cold heading in that a cone and cheese are upset using two punches and a closed die which supports the shank of the blank. On most two blow headers the die is mounted on a saddle so that it can be moved clear of the punches for the insertion of long pins. The operator can push the saddle back and forward but more often this is done automatically. The operator uses tongs to load the heated pins into the die. After heading, the ejector pin pushes the blank out so that its shank can be grasped by tongs for complete removal. Due to the uneven surface of hot rolled bar more clearance is provided between the bores of hot heading dies and work pieces than in cold heading dies. Because of this clearance and because the ejector pin does not have to push the bolt right out of the die, the length of blank that can be hot headed in a closed die is not restricted as in cold forming and this is a major advantage of hot over cold solid die heading. Water is usually used to cool the hot heading tools.

        After hot heading a separate machine is used to hot trim or strip the cheese to form a hexagonal head. The layout consists of an oil fired pin heater, a hot heading machine and a hot stripping machine. This layout enables a furnace man and one operator on each machine to head and strip bolts in one heat. The rate of output of such teams of three men is usually governed by the work pace of the heading machine operator.

        The sequence of operations on the right hand side of Figure 5 can be used to forge bolt heads without trimming waste. This process is called hot upsetting and is carried out on forging machines. Forging machines squeeze rather than hammer the work into the required shape. Because scale and surface defects are set cut away from round the head by stripping operation, this method is usually employed for heading large bolts with wider absolute tolerances than those on small bolts. The length of bar required to form the head is measured by the operator pushing the heated end of the pin against the first outside die. The gripper dies are closed to hold the shank before the heading stroke of the outside die commences. If the finish of the head is unsatisfactory, the bar can be rotated one sixth of a turn and the operation repeated. The head is finish formed at the second station.

        Forging machines can be equipped so that the work piece is manipulated mechanically, but on the smaller machines required for most sizes of bolt, the operator usually inserts and removes the work with tongs. Since the shanks of bolts protrude from the forging machine during upsetting the process does not restrict the length of bolt that can be headed.

        Screw cutting which is the final operation required to finish hot forged bolts is usually carried out on special purpose machines using rotating tangential cutting dies arranged as shown at the bottom of Figure 5. One advantage of this particular type of thread cutting is that the dies can be resharpened many times. The dies cut the thread in one pass and spring open before the return stroke. 

(d) Hot Forging of Nuts

        The material stock for hot forged steel nuts is normally hot rolled bar. Figure 6 shows alternative ways of hot forging nut blanks. In the sequence on the left of the figure the rectangular bar is progressively fed into the semi-automatic nut press by an operator. Once the bar has cooled below the forging temperature it is returned by the operator to the furnace. Forging is accomplished by first forging vees into the top and bottom faces of the bar to produce two sides of the hexagon of the first nut and two sides of the second nut and then shearing the blank of the first nut off from the parent bar. During shearing the cut off tool pushes the blank horizontally into an enclosed die where the hole is then pierced before the finished blank is ejected. Since forging in a hot nut press takes place to an enclosed die an automatic adjustment is provided to compensate for surface scale and variations in bar section. After hot pressing it is common for the end face of a nut to be lightly machined to remove punch burrs. This operation, called frazing, is most simply carried out using an end milling cutter mounted centrally in a vertical rotating spindle above a nut clamping fixture.

        The automatic hot nut forging sequence on the right hand side of Figure 6 has more in common with the cold nut forming process in Figure 4 than with the hot nut pressing method. The bar heated can be either fed from an automatic magazine or manually and the heat source can be a coal, oil or gas fired furnace or an electric induction coil. From the heater bars pass straight into the hot nut forging machine. Due to the ductility of the hot steel only three forming stations are required.

        A third method of upsetting a hexagon from round bar in a forging machine is not shown in Figure 6 since it is similar to that used for hot upsetting bolt heads shown on the right hand side of Figure 5. After upsetting a hexagonal nut blank on the end of a round bar, a hole is pierced through the upset hexagon, by pushing the bar back, this leaves the forged nut in the die. The metal pierced out of the nut remains attached to the end of the bar and is incorporated into the next nut. Several nuts can be made from a length of bar in one heat. Although this upsetting process wastes little or no metal, it is relatively slow with the result and it is usually only used for large nuts for which there is a relatively small demand.

(e) Machining of Bolts and Nuts from Hexagon Bar

        The bar stock for turned bolts and nuts is usually cold drawn to a hexagonal section and machining is usually on automatic lathes or operator controlled capstan or turret lathes. Both capstan and turret lathes grip one end of the work piece in a chuck or collet leaving the space around the other end of the work piece unrestricted so that a series of tools can be positioned there in an indexible turret. This arrangement is convenient for drilling and boring through the centre of a part and for facing its unsupported end. Turret and capstan lathes can have additional tools mounted on cross slides between the chuck and the turret.

        Figure 7 shows sequences of operations for turning bolts and nut blanks from hexagonal bar. The chuck which rotates the bar is opened when it is necessary to feed a new length of bar. The bar stock is fed against a job stop mounted in the first turret station. The second turret station holds a roller steady tool holder for turning the shank of the bolt. Roller steady tool holders are designed so that the cutting thrust, which tends to deflect the work piece away from the tool is balanced by rollers located on the opposite side of the work piece. During the roller supported cutting operation the turret is driven along the bed. The tool in the third turret station is a screw cutting die head, which springs open automatically at the end of the screw cutting run prior to a rapid return stroke. The back of the cross slide carries a parting off tool which is also used for cutting an annular groove prior to the head chamfering operation. A form cutter for chamfering is amounted on the front of the cross slide. This illustration is based on the use of three turret stations, but the turrets could be ‘double kitted’ by loading an identical set of tools into the remaining stations to halve the turret indexing time per piece.

        Nuts are turned using a similar tool set up to that just described for bolts. The place of the roller box tool holder is taken by a drill and a tool to chamfer the lip of the hole as the drill completes its cut.

        On manually operated lathes, the operator starts and stops the spindle, changes gear to alter the spindle speeds and tool feed rates between cuts, indexes the turret and feeds the bar stock. It may therefore not be possible to take two cuts simultaneously by, for example, cutting the chamfer on the corners of a nut while the hole is being drilled. But some automatic lathes can take more than one cut at a time, which can result in considerable savings in the time required to manufacture each piece. Another advantage of the automatic lathe is its reduced dependence on operator skills; this in turn reduces waste and inspection requirements. Multi-spindle automatics are available which can work on several parts simultaneously by passing each part to a succession of tools. The tool motions and feed rates on most automatics are controlled by rotating cams but increasingly electrical sequence controlled single spindle automatics are used which can be set up more quickly than can automatics and require less attention from operators than turret lathes.

        The internal threads on standard nuts are usually not cut before the nut is parted from the material stock because of the need to reverse the direction of spindle rotation to remove the thread cutting tap from the blind hole.

        The threads on most nuts are cut by specialized bent tap machines. Blanks are gravity fed down a channel from a magazine onto the nose of a continuously rotating tap. The cutting flutes of the tap are positioned in the middle of a stationary hollow hexagonal guide way, which prevents the nuts from rotating as they move along the tap. The shank of the tap beyond the threaded portion is bent so that it can be driven and the nuts are thrown off radially. Another type of nut tapper has a number of vertical spindles carrying straight taps. Nuts are placed by hand in a trough of lubricant under each spindle and foot pedals lower the spinning taps into the nuts. Threaded nuts collect on the taps and are periodically removed by hand. 

Casting of Steel for Flat Products

Type of Cast Products

Cast products utilized for flat rolling can be produced in the following forms:

1. Ingots—These are castings of simple shape. Slab ingots range in weight from 9 to 36 metric tons (10 to 40 net tons). In order to roll strip from the ingots, the latter are usually first rolled down to the size of a slab with thickness range from 150 to 350 mm (6 to 14 in.). Then they are further reduced in thickness at the roughing stands of hot strip mill down to 25-65 mm (1.0-2.5 in.) with subsequent reduction to the desired hot rolled thickness at the finishing mill. Final reduction in thickness may be done by rolling at the cold mill.

2. Thick cast slabs—These castings are usually from 150 to 350 mm (6 to 14 in.) in thickness. Utilization of the thick cast slabs allows one to eliminate reduction at the slabbing mill.

3. Thin cast slabs—These castings may be from 25 to 64 mm (1 to 2.5 in.) thick. Utilization of the thin slabs allows the elimination of both the slabbing mill and the roughing mill.

4. Cast strip—The thickness of the cast strip can be as thin as 1.3 mm (0.05 in.). It allows one to eliminate entirely the hot rolling process.

Casting of Ingot

        After the steel making operation is completed, the liquid steel is poured into a steel ladle. Additional alloying materials and deoxidizers may be added during the tapping of heat. The steel is then poured or teemed into a series of molds of the designed dimensions.

        The ingot molds are tall box-like containers made of cast iron with the internal cavity that is usually tapered from the top to the bottom of the mold. There are two principal types of molds:

          (a)          Big-end-down molds.

          (b)          Big-end-up molds.

        The inner wells of the molds may be plain sided, cambered, corrugated, or fluted. The last two shapes of the wall promote faster cooling and therefore minimize surface cracking during solidification.

        There are two methods of teeming the ingots:

          (a)          Top-pouring method,

          (b)          Bottom-pouring method.

        The use of the bottom-pouring method is found especially beneficial for high quality steels.

Types of Ingots

Molten steel solidifies first at the regions close to the mold walls, so the gases, chiefly oxygen, evolved from still-liquid portions may be trapped to produce blowholes.

Depending on the amount of gases released during solidification, the following types of ingots are known:

1. Fully-killed ingot—It is fully deoxidized and therefore it evolves no gas, its top is slightly concave, and below the top there is a shrinkage cavity that is commonly called pipe.

2. Semi-killed ingot—This ingot is deoxidized less than fully-killed. As a result, a small amount of carbon monoxide evolves producing a domed top. The blowhole formation in the lower half of the ingot is prevented due to ferrostatic pressure.

3. Capped ingot—It is produced by pouring steel into big-end-down bottle-top-molds in which the constructed top or mouth of the mold facilitates the capping operation. The rimming action is allowed to begin normally but is then terminated at the end by sealing the mold with a cast-iron cap. In capped ingot, the gas bubbles in upper half are swept away due to the strong rimming action. An ingot of this type does not have the interiors of its blowholes exposed to oxidation during heating and soaking.

4. Rimmed ingot—This type of ingots is usually tapped without addition of deoxidizers to the steel in the furnace, and with only small additions to the molten steel in the ladle. The evolution of gas produces a boiling action that is commonly known as rimming action. Ingot No. 7 in Fig. 3 is a typical rimmed ingot in which gas evolution was so strong that the formation of blowholes was confined to only lower part of the ingot.

There are two types of design for the ingots.

          (a)          Hot-topped ingots

          (b)          Non hot-topped ingots.

The big-end-up, hot-topped killed steel ingots are used in order to provide a complete freedom from pipe.

Methods of Continuous Casting of Thick Slabs

        A number of methods have been proposed for continuous casting of steel. Below are some of the methods that have been practically implemented.

Vertical or “stick” casting—In this method, a straight mold, a vertical cooling chamber and a flame cut-off are used. A tilting receiving mechanism transfers the continuously cast slabs onto horizontal run-out table.

Vertical plus bending casting—In this method the casting direction is smoothly changed from vertical to horizontal as soon as the cast steel emerges from vertical cooling chamber.

Semi-horizontal or curved mold casting—This method allows one to simplify design and to substantially reduce dimensions of the continuous casting machines.

Horizontal continuous casting—Schematic representation of a typical horizontal casting machine is shown in Fig. 6. Some horizontal casting machines provide continuous movement of strand with oscillation of either both tundish and mold or mold only. However, the most reliable operation was achieved by providing an intermittent strand movement as shown Figs. 6c-6e.

Continuous Casting of Thick Slabs

        The most common method for continuous casting of thick slabs is vertical plus bending casting. Below is a brief description of the casting process that utilizes this method.

        In order to start the casting process, the dummy bar is inserted in the mold so that its top closes the bottom of the mold. The insertion of the dummy bar is made either from the top of the machine or through entire machine in the bottom of the mold. Liquid steel is then poured at a controlled rate from ladle into the tundish and then the metal flows through nozzles in the bottom of tundish and fills the mold.

There are two methods of pouring the steel from ladle to tundish and from tundish to mold:

(a)      Open stream casting

(b)      Close stream or shrouded casting.

        In an open stream casting the liquid metal flows through the air and therefore it picks up oxygen and some nitrogen from the air. It results in formation of undesired inclusion in the liquid steel. Shrouded casting allows to avoid this problem. In this method, steel is protected from contact with the air either by refractory tubes or by gas shrouding as shown in Fig. 9.

        After the mold is filled, withdrawal of the dummy bar is initiated. The gradually solidifying metal would follow the dummy bar head. At certain position, the dummy bar head is mechanically disassociated from solidified metal being cast and then the dummy bar is removed.

        Liquid steel starts to solidify in the water-cooled mold and the solidification of steel continues progressively along its path. The rate of solidification is controlled by secondary cooling water sprays. The distance from the meniscus level in the mold to the point of complete solidification is called metallurgical length. The point of complete solidification is usually ahead of straightener. Electromagnetic stirring of liquid steel during solidification may be implemented in order to improve steel quality and increase casting rate.

        The mold is oscillated in a vertical direction in order to prevent sticking of the solidified shell to the mold. Also, lubricants such as oils or fluxes are used to reduce friction. Support rolls are installed to guide the metal and to prevent bulging of the solidifying shell from internal ferrostatic pressure. Cutting of the cast section is done after straightening either by shears or by torches.

Table 1 shows main characteristics of one of the continuous casting machines installed at the Indiana Harbor Works for casting of thick slabs.

Slab Width Control

Desired slab width is usually achieved by using one of the following three methods.

1. Slab slitting—This method allows one to cast a small number of ‘master’ slab sizes with the slab product being slit longitudinally in a separate operation using either oxy-natural gas torches or rolling machines.

2. Adjustable mold width—This method allows one to minimize the time required to replace a mold. Various design for changing the mold widths are utilized. In the continuous casting machines of earlier designs, the mold width adjustment can be made while the previously cast slab is being removed from the machine. In the latest designs, the mold taper can be changed during the actual casting operation.

3. Divided molds—According to this method a divider installed in the mold permits the casting of two narrow slabs simultaneously in a single strand machine.

The Rolling of Rails, Wheels and Rings

Introduction

        By definition, the standard rail is a section symmetrical about its vertical axis, which consists of three areas : head, web and base. The term "tee" is used to designate the general class of rail designs which resemble an inverted letter T, and to distinguish those rails, which are generally used in open-track construction, from girder and girder-guard rails which are usually embedded in pavements. Crane rails differ from standard rails in that they feature shorter, thicker webs, larger heads and thicker bases to withstand heavy, concentrated loads. For railroad applications rails are rolled to sections up to 155 pounds per yard although most rails made today are 140 pounds per yard or less and are of the standard length of 39 feet.

        Normally, the rail section is formed from rectangular blooms by a series of 10 passes. Roll passes must be carefully designed and the rolling operation properly supervised in order to meet the stringent dimensional and quality specifications. After rolling, the rails, with their complete identification hot-stamped or rolled into them, are hotsawn so that they cool to within 3/8 inch of the desired cold length. They are then cambered (with the head on the convex side) so that they will be essentially straight at ambient temperature.

        Many rails are controlled cooled, being cooled normally on hot beds until their temperature falls to within the range 725 to 1000ºF. The rails are then charged into large insulated metal containers for a minimum of 10 hours.

           After cooling, rails are subjected to various finishing operations (straightening and drilling for joint bolts), inspected, the rail head chamfered in a grinding operations and the ends of the rail hardened.

        Many rails are now heat-treated by a full oil-quench and temper, which hardens the entire rail section, or subjected to an induction-heating operation, which provides a surface hardening of the wearing surface of the head. However, with the use of chromium and molybdenum as alloying elements, rails are being conventionally produced with yield strength of 200,000 psi and with a wear resistance equivalent to that of heat-treated rails.

        This chapter reviews the early types of rails and their production, examines the evolution of rail mills, describes some modern rail-making facilities and discusses rail joints and their manufacture. In addition, the production of railroad wheels and the hot rolling of rings are also reviewed.

Early Types of Rails and Their Production

        The earliest type of metal rail used in the eighteenth century consisted of a wooden base with flat strips of cast metal about 4 inches wide, 1¼ inches thick and 5 feet long nailed to wooden stringers, as illustrated in Figure 1 A. The cast metal straps were replaced by rolled iron straps about 1820 but this simple design soon proved to be inadequate. As a consequence, many improvements were soon developed, one of which is shown in Figure 1 B. This particular type of rail was rolled for the Amboy Division of the Pennsylvania Railroad as late as 1831.

        John Birkenshaw of the Bedligton Iron Works in England produced in 1820 rails consisting of a head and a web but no base. Rails such as these, laid on the ties in cast chairs, were used on the Stocketon Darlington Railway in 1825. They were produced with rolls contoured as illustrated in Figure 1 D. A more advanced design, shown in Figure 1 E, was used on the Boston and Lowell Railroad in 1830 as well as in England.

        Although the preference in England was for the bullhead rail shown in Figure 1 F, in the U.S.A., the tee rail soon became popular. R. L. Stevens of the Camden and Amboy Railroad designed the first tee rail in 1830, which was rolled in Great Britain in 1831.

        Another popular type of rail was the U-rail shown in Figure 1 H, rolled by the Mount Savage Rolling Mill Company of Allegheny County, Maryland, in 1844 and said to be the first shaped rail produced in the U.S.A. Roll passes used for a similar type of rail made in England about 1855 are shown in Figure 2.

        Of the many different rails produced during the last two hundred years, one interesting type was the hollow iron or closed-U rail rolled at the Camkbria Iron Works. However, the demand for more metal in the head of the rail for better wear resistance forced a final return in the period 1858 to 1868 to the tee shape with wide thin flanges.

        With respect to the early rolling of rails, it is probable that existing mills designed to roll bars were utilized with such alterations as were necessary. Credit for rolling the first steel rail in 1857 is given to the Dowlais Plant in Wales while credit for the first steel rail in the U.S.A. goes to Captain Ward's North Chicago Rolling Mills, where the first 50-pound Bessemer-steel rails were rolled experimentally in 1865 from blooms made of hammered ingots at Wyandotte, Michigan.

        Rail production was initiated on two-high mills with the bar pulled back over the top roll. To eliminate "idle" passes, various mills of unique design were tried. These included mills with oscillatory rolls, which provided rails of limited length, and the "Double-Duo" mill featuring two pairs of work rolls in the same mill stand, such as was used at the Dowlais Plant. Another mill, credited to Cabrols Colamineur, was developed about 1850. It consisted of two trains of rolls set almost side-by-side with each set rolling in opposite directions. After a bar had emerged from one pair of rolls, it was transferred laterally by a hand buggy for entry into the other pair.

        In 1866, a two-high mill utilizing, for the first time, a reversing steam engine was developed by the Ransbottom Crewe Works of the London and Northwestern Railway. About the same period, nonreversing engines were used with gearing and clutches being employed to reverse the mills.

The Evolution for the Rail Mill

        The successful development of the three-high mill in 1857 by John Fritz of the Cambria Iron Company of Johnstown, Pennsylvania, led to its general u