Cold-Finished Steel Bars

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Cold-Finished Steel Bars

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Cold-Finished Steel Bars

Messaggioda Aldebaran » 02/07/2010, 20:54

Introduction
COLD-FINISHED STEEL BARS are carbon and alloy steel bar products (round, square, hexagonal, flat, or special
shapes) that are produced by cold finishing previous hot-wrought bars by means of cold drawing, cold forming, turning,
grinding, or polishing (singly or in combination) to yield straight lengths or coils that are uniform throughout their length.
Not covered in this article are flat-rolled products such as sheet, strip, or plate, which are normally cold finished by cold
rolling, or cold-drawn tubular products.
Cold-finished bars fall into five classifications:
• Cold-drawn bars
• Turned and polished (after cold drawn or hot roll) bars
• Cold-drawn, ground, and polished (after cold draw) bars
• Turned, ground, and polished bars
• Cold-drawn, turned, ground, and polished bars
Cold-drawn bars represent the largest tonnage production and are widely used in the mass production of machined and
other parts. They have attractive combinations of mechanical and dimensional properties.
Turned and polished bars have the mechanical properties of hot-rolled products but have greatly improved surface finish
and dimensional accuracy. These bars are available in sizes lager than those that can be cold drawn. Turned bars are
defect and decarb free.
Cold-drawn, ground, and polished bars have the increased machinability, tensile strength, and yield strength of cold-
drawn bars together with very close size tolerances. However, cold-drawn, ground, and polished bars are not guaranteed
to be defect free.
Turned, ground, and polished bars have superior surface finish, dimensional accuracy, and straightness. These bars find
application in precision shafting and in plating, where such factors are of primary importance.
Cold-drawn, turned, ground, and polished bars have improved mechanical properties, close size tolerances, and a surface
free of imperfections.
Note
* K. M. Shupe, Bliss & Laughlin Steel Company; Richard B. Smith, Stanadyne Western Steel; Steve Slavonic,
Teledyne Columbia-Summerill; B. F. Leighton, Canadian Drawn Steel Company; W. Gismondi, Union
Drawn Steel Company, Ltd.; John R Stubbles, LTV Steel Company; Kurt W. Boehm, Nucor Steel; Donald
M. Keane, LaSalle Steel Company
Bar Sizes
Cold-finished steel bars are available in a wide variety of sizes and cross-sectional shapes. Normally, they are furnished in
straight lengths, but in some sizes and cross sections they may be furnished in coils. Cold-finished steel bars are available
with nominal dimensions designated in either inches or millimeters. Cold-finished product is available in standard size
increments, which vary by size range. Special sizes can be negotiated depending on hot mill increments and cold-finish
tooling.
References cited in this section
1. J.G. Bralla, Handbook of Product Design for Manufacturing, McGraw-Hill, 1986
2. Alloy, Carbon, and High Strength Low Alloy Steels, Semifinished for Forging; Hot Rolled Bars; Cold
Finished Steel Bars; Hot Rolled Deformed and Plain Concrete Reinforcing Bars, AISI Steel Products
Manual, American Iron and Steel Institute, 1986
Product Types
In the manufacture of cold-finished bars, the steel is first hot rolled oversize to appropriate shape and is then subjected to
mechanical operations (other than those intended primarily for scale removal) that affect is machinability, straightness,
and end-cut properties. The two common methods of cold finishing bars are:
• Removal of surface material by turning or grinding, singly or in combination
• Drawing the material through a die of suitable configuration
Pickling or blasting to remove scale may precede turning or grinding and must always precede drawing. For bar products,
cold rolling has been almost superseded by cold drawing. Nevertheless, cold-finished bars and special shapes are
sometimes incorrectly described as cold rolled.
Commercial Grades. Any grade of carbon or alloy steel that can be hot rolled can also be cold finished. The choice of
grade is based on the attainable cold-finished and/or hardenability and tempering characteristics necessary to obtain the
required mechanical properties.
Production methods vary widely among cold-finished cold-drawn suppliers. For example, one supplier currently anneals
and cold draws grades 1070, 1090, and 5160, and in the future plans to do the same with grade 9254. Grade 1070 is a
high-volume item, and cold drawing is required for precision sizing and subsequent nondestructive testing of the bar,
using a rotating-probe eddy current device (see the articles "Eddy Current Inspection," "Remote-Field Eddy Current
Inspection," and "Steel Bar, Wire, and Billets" in Nondestructive Evaluation and Quality Control, Volume 17 of ASM
Handbook, formerly 9th Edition Metals Handbook) for detecting surface seams. Cold drawing is also necessary because
the smallest hot-rolled size typically available for some applications is not small enough for customer use. Thus, a
supplier whose smallest hot-rolled bar size is 11.1 mm (0.437 in.) cold draws this diameter to as small as 9.98 mm (0.393
in.).
Carbon steels containing more than 0.55% C must be annealed prior to being cold drawn so that the hardness will be
sufficiently low to facilitate the cold-drawing operation. For carbon steels containing up to 0.65% C, this will normally be
a lamellar pearlitic anneal; for carbon steels containing more than 0.65% C, a spheroidize anneal is required. The type of
structure required is normally reached by agreement between the steel producer and the customer.
Alloy steels containing more than 0.38% C are usually annealed before cold drawing.
Machined Bars. Bar products that are cold finished by stock removal can be:
• Turned and polished
• Turned, ground, and polished
• Cold drawn, ground, and polished
• Cold drawn, turned, and polished
• Cold drawn, turned, ground, and polished
Turning is done in special machines with cutting tools mounted in rotating heads, thus eliminating the problem of having
to support long bars as in a lathe. Grinding is done in centerless machines. Polishing can be done in a roll straightener of
the crossed-axis (Medart) type with polished rolls to provide a smooth finish. Polishing by grinding with an organic wheel
or with a belt is of increasing interest (see the article "Grinding Equipment and Processes" in Machining, Volume 16 of
ASM Handbook, formerly 9th Edition Metals Handbook) because it is cost effective to grind and polish the bars on the
same machine simply by using grinding wheels or belts of different grit size. Grinding produces a smoother finish than
turning; polishing improves the surface produced by either technique. Turned, ground, and polished rounds represent the
highest degree of overall accuracy, concentricity, straightness, and surface perfection attainable in commercial practice
(Ref 3).
The surface finish desired is specified by using the process names given above because the industry has not developed
standard numerical values for roughness, such as microinch or root mean square (rms) numbers. However, surface finish
with respect to rms (root mean square deviation from the mean surface) as determined with a profilometer can be
negotiated between the producer and a customer. This could be done for such critical-finish applications as turned and
polished bars used to produce shafting as well as stock used to produce machined parts of which a superior finish is
required on surfaces not machined.
1
The published range of diameters both for turned and for turned and ground bars is 13 to 229 mm ( to 9 in.) inclusive;
2
1
for cold-drawn and ground bars, it is 3.2 to 102 mm ( to 4 in.) inclusive. These are composites of size ranges throughout
8
the industry; an individual producer may be unable to furnish a full range of sizes.
1
to 9 in.), another from 29 to 203
For example, one well-known producer supplies turned rounds from 13 to 229 mm (
2
1
mm (1 to 8 in.)--all finished sizes. Yet another producer supplies sizes up to and including 152 mm (6 in.) that are
8
turned on special turning machines and ground on centerless grinders; larger sizes are lathe turned and ground on centers.
Because turning and grinding do not alter the mechanical properties of the hot-rolled bar, this product can be machined
asymmetrically with practically no danger of warpage (Ref 3).
Stock removal is usually dependent on American Iron and Steel Institute (AISI) seam allowances (Ref 2). Stock removal
1 1
in turning, or turning and grinding, measured on the diameter, is normally 1.6 mm ( in.) for sizes up to 38 mm (1
16 2
1 1 3
in.), 3.2 mm ( in.) for the 38 to 76 mm (1 to 3 in.) range, 4.8 mm ( in.) for the 76 to 127 mm (3 to 5 in.) range, and
8 2 16
1
6.4 mm ( in.) for 127 mm (5 in.) diameter and larger.
4
1
Cold-drawn round bars are available in a range of diameters from 3.2 to 152 mm ( to 6 in.). The maximum
8
diameters available from individual producers, however, may vary from 76 to 152 mm (3 to 6 in.). The reduction in
1 3
diameter in cold drawing, called draft, is commonly 0.79 mm ( in.) for finished sizes up to 9.5 mm ( in.) and 1.6 mm
32 8
1 3
( in.) for sizes over 9.5 mm ( in.). Some special processes use heavier drafts followed by stress relieving. One
16 8
producer employs heavy drafting at elevated temperature. With this exception, drawing operations are begun with the
material at room temperature to start, and the only elevated temperature involved is that developed in the bar as a result of
drawing; this temperature rise is small and of little significance.
Originally, cold finishing, whether by turning or by cold rolling, was employed only for sizing to produce a bar with
closer dimensional tolerances and a smoother surface. As cold-finished bar products were developed and improved,
increased attention was paid to the substantial enhancement of mechanical properties that could be obtained by cold
working. This additional advantage is now more fully appreciated, as evidenced by the fact that increased mechanical
properties are an important consideration in about 40% of the applications. In approximately half of these applications, or
20% of the total, cold drawing is used only to increase strength; in the other 20%, close tolerances and better surface
finish are desired in addition to increased strength.
As-rolled microalloyed high-strength low-alloy (HSLA) steels or microalloyed HSLA steels in various combinations of
controlled drafting and furnace treatment provide an extension of property attainment. A high percentage of free-
machining steels are cold drawn for the combination of size accuracy and improved machinability. Recent developments
in microalloyed steels provide hot-rolled turned bars, under certain circumstances, having mechanical properties similar
to cold-drawn nonmicroalloyed steels.
An appreciable fraction of all applications of cold finishing to carbon steel bars utilizes cold drawing to improve
mechanical properties. For alloy steel, however, cold finishing is commonly used to improve surface finish and
dimensional accuracy, and not for additional mechanical strength. When additional mechanical strength is desired, alloy
steel bars may be heat treated (quenched and tempered) and then cold drawn and stress relieved. Elevated-temperature or
warm-drawn steels are also available with increased mechanical strength and improved machinability.
Heavily drafted and strain-tempered carbon and alloy steels subjected to induction hardening of the surface provide many
additional property combinations. The extra cost of using alloy steel in cold-finished bars can be justified only when heat
treatment (quenching and tempering) is necessary for meeting the required strength level. Because work-hardening effects
are removed during heating prior to quenching, the benefit of increased mechanical strength due to cold finishing is
eliminated from the finished product.
Turning Versus Cold Drawing. Basic differences exist between bars finished by turning and those finished by cold
drawing. First, it is obvious that turning and centerless grinding are applicable only to round bars, while drawing can be
applied to a variety of shapes. Drawing, therefore, is more versatile than turning.
Second, there is a difference in the number and severity of the surface imperfections that may be present. Because stock is
removed in turning and grinding, shallow surface imperfections and decarburization may be completely eliminated. When
material is drawn, stock is only displaced, and surface imperfections are only reduced in depth (in the ratio of the change
in bar diameter or section thickness). The length of these imperfections may be slightly increased because in the drawing
operation an increase in length accompanies the reduction in cross section.
Cold-drawn bars can approach the freedom from surface imperfections obtained in turned or turned and ground bars if the
hot-rolled bars from which they are produced are rolled from specially conditioned billets. Quality conditions such as
cold-working quality are available from producers of hot-rolled bars. The depth limits of the surface imperfections are as
agreed to between the producer and the customer. However, if maximum freedom from surface imperfections is the
controlling factor, turned bars have an advantage.
Different size tolerances are applicable to cold-finished products, depending on shape, carbon content, and heat treatment.
Listed in Tables 2, 3, and 4 are the tolerances for cold-finished carbon and alloy steel bars published in ASTM A 29.
These tables include cold-drawn bars; turned and polished rounds; cold-drawn, ground, and polished rounds; and turned,
ground, and polished rounds. From the data in Tables 2, 3, and 4, certain generalizations can be stated. The tolerances for
cold-drawn and for turned and polished rounds, for example, are the same for sizes up to and including 102 mm (4 in.).
There are differences, however, between the tolerances that apply to carbon steel and those that apply to alloy steels.
Tolerances for several finishes also vary with certain levels of carbon content. Broader tolerances are applicable to bars
that have been heat treated before cold finishing. In contrast, tolerances are closer when bars are ground, and these
tolerances are independent of carbon content.
References cited in this section
2. Alloy, Carbon, and High Strength Low Alloy Steels, Semifinished for Forging; Hot Rolled Bars; Cold
Finished Steel Bars; Hot Rolled Deformed and Plain Concrete Reinforcing Bars, AISI Steel Products
Manual, American Iron and Steel Institute, 1986
3. Handbook of Machining Data for Cold Finished Steel Bars, LTV Steel Flat Rolled and Bar Company, 1985
4. Steel--Bars, Forgings, Bearing, Chain, Springs, Vol 1.05, Annual Book of ASTM Standards, American
Society for Testing and Materials, 1989
Product Quality Descriptors
The term quality relates to the suitability of a mill product to become an acceptable part. When used to identify cold-
finished steel bars, the various quality descriptors are indicative of many characteristics, such as degree of internal
soundness, relative uniformity of chemical composition, and relative freedom from detrimental surface imperfections.
Because of the characteristic surface finish of cold-drawn bars, close visual inspection cannot identify detrimental surface
imperfections. Therefore, for applications that do not allow surface imperfections on the finished surfaces of standard
quality cold-drawn carbon steel bars and regular quality cold-drawn alloy steel bars, the user should recognize that some
stock removal is necessary to eliminate such imperfections as seams. The recommended stock removal per side for all
1
nonresulfurized grades is 0.025 mm (0.001 in.) per 1.6 mm ( in.) of cross section, or 0.25 mm (0.010 in.), whichever is
16
greater. For example, for a 25 mm (1 in.) bar, recommended stock removal is 0.41 mm (0.016 in.) per side. For the
1
in.), or 0.38 mm (0.015 in.),
resulfurized grades, recommended stock removal is 0.038 mm (0.0015 in.) per 1.6 mm (
16
whichever is greater. Therefore, for a 25 mm (1 in.) bar, recommended stock removal is 0.61 mm (0.024 in.) per side.
Occasionally, some bars in a shipment may have imperfections that exceed the recommended stock removal limits.
Therefore, for critical applications, inspection of finished parts is recommended, or more restrictive quality and/or
additional inspection methods can be specified by agreement of both supplier and customer.
Carbon Steel Quality Descriptors
Standard quality is the descriptor applied to the basic quality level to which cold-finished carbon steel bars are produced.
Standard quality cold-finished bars are produced from hot-rolled carbon steel of special quality (the standard quality for
hot-rolled bars for cold finishing). Steel bars of standard quality must be free from visible pipe and excessive chemical
segregation. They may contain surface imperfections. In general, the size of surface imperfections increases with bar size.
Restrictive requirement quality A (RRA) incorporates all the features of standard quality carbon steel bars
described above, plus any one of the following restrictive requirements.
Special surface bars are produced with special surface preparation to minimize the frequency and size of seams and
other surface imperfections. These bars are used for applications in which machining allowances do not allow sufficient
surface removal to clean up the detrimental imperfections that occur in standard quality bars.
Special internal soundness bars have greater freedom from chemical segregation and porosity than standard quality
bars.
Special hardenability bars are produced to hardenability requirements other than those of standard H-steels.
Cold-finished carbon steel bars are also produced to inclusion ratings as determined by standard nonmetallic
inclusion testing.
Restrictive requirement quality B (RRB) incorporates all the features of standard quality carbon steel bars, plus
any one of the following.
Special discard is specified when minimized chemical segregation, special steel cleanliness, or internal soundness
requirements dictate that the product be selected from certain positions in the ingot.
Minimized decarburization is specified whenever decarburization is important, as in heat treating for surface
hardness requirements.
Single restrictions other than those noted above, such as special chemical limitations, special processing techniques,
and other special characteristics not previously anticipated, are also covered by this quality level.
Multiple restrictive requirement quality (MRR) applies when two or more of the above-described restrictive
requirements are involved.
Cold-forging quality A and cold-extrusion quality A apply to cold-finished carbon steel bars used in the
production of solid or hollow shapes by means of cold plastic deformation involving the movement of metal by
compression with no expansion of the surface and not requiring special inspection standards. For an individual
application, if the type of steel or chemical composition specified does not provide adequate cold-forming characteristics
in the as-drawn condition, a suitable heat treatment to provide proper hardness or microstructure may be necessary.
Cold-heading quality, cold-extrusion quality B, cold-upsetting quality, and cold-expansion quality
apply to cold-finished carbon steel bars used in production of solid or hollow shapes by means of severe cold plastic
deformation by cold heading, cold extrusion, cold upsetting, or cold expansion involving movement of metal by
expansion and/or compression. Such bars are obtained from steel produced by closely controlled steelmaking practices
and are subject to special inspection standards for internal soundness and surface quality and uniform chemical
composition. For grades of steel with a maximum specified carbon content of 0.30% or more, an anneal or spheroidize
anneal heat treatment may be required to obtain the proper hardness and microstructure for cold working.
Restrictive cold-working quality applies to cold-finished carbon steel bars used in the production of solid or hollow
shapes by means of very severe cold plastic deformation involving cold working by expansion and/or compression. This
degree of cold working normally involves restrictive inspection standards and requires steel that is exceptionally sound, of
uniform chemical composition, and virtually free of detrimental surface imperfections. Such severe cold-forming
operations normally require suitable heat treatment to obtain proper hardness and microstructure for cold working.
Other Carbon Steel Qualities. The quality descriptors listed below are some of those that apply to cold-finished
carbon steel bars intended for specific requirements and applications. They may have requirements for surface quality,
amount of discard, macroetch tests, mechanical properties, or chemical uniformity as indicated in product specifications:
• Axle shaft quality
• Shell steel quality A
• Shell steel quality C
• Rifle barrel quality
• Spark plug quality
Alloy Steel Quality Descriptors
Regular quality is the descriptor applied to the basic, or standard, quality level to which cold-finished alloy steel bars
are produced. Steels for this quality are killed and are usually produced to a fine grain size. They are melted to chemical
ranges and limits and are inspected and tested to meet normal requirements for regular constructional alloy steel
applications. Regular quality cold-finished alloy steel bars may contain surface imperfections to the depths mentioned in
the opening paragraphs of the section "Product Quality Descriptors" in this article. In general, the size of detrimental
surface imperfections increases with bar size.
Cold-heading quality applies to cold-finished alloy steel bars intended for applications involving cold plastic
deformation by such operations as upsetting, heading, or forging. Bars are supplied from steel produced by closely
controlled steelmaking practices and are subject to mill testing and inspection designed to ensure internal soundness,
uniformity of chemical composition, and freedom from detrimental surface imperfections. Proper control of hardness and
microstructure by heat treatment and cold working is important for cold forming. Most cold-heading quality alloy steels
are low- and medium-carbon grades. Typical low-carbon alloy steel parts, made by cold heading, include fasteners (cap
screws, bolts, eyebolts), studs, anchor pins, and rollers for bearings. Examples of medium-carbon alloy steel cold-headed
parts are bolts, studs, and hexagon-headed cap screws.
Special cold-heading quality applies to cold-finished alloy steel bars for applications involving severe cold plastic
deformation when slight surface imperfections may cause splitting of a part. Bars of this quality are produced by closely
controlled steelmaking practices to provide uniform chemical composition and internal soundness. Also, special
processing (such as grinding) is applied at intermediate stages to remove detrimental surface imperfections. Proper
control of hardness and microstructure by heat treatment and cold working is important for cold forming. Typical
applications of alloy steel bars of this quality are front suspension studs, socket screws, and some valves.
Axle shaft quality applies to cold-finished alloy steel bars intended for the manufacture of automotive or truck-type,
power-driven axle shafts, which by their design or method of manufacture are either not machined all over or undergo
less than the recommended amount of stock removal for proper cleanup of normal surface imperfections. Axle shaft
quality bars require special rolling practices, special billet and bar conditioning, and selective inspection techniques.
Ball and roller bearing quality and bearing quality apply to cold-finished alloy steel bars used for the
manufacture of antifriction bearings. Such bars are usually produced from alloy steels of the AISI-SAE standard alloy
carburizing grades and the AISI-SAE high-carbon chromium series. These steels can be produced in accordance with
ASTM A 534, A 295, and A 485. Bearing quality steels are subjected to restricted melting and special teeming, heating,
rolling, cooling, and conditioning practices to meet rigid quality requirements. The steelmaking operations may include
vacuum treatment. The foregoing requirements include thorough examination for internal imperfections by one or more
of the following methods: macroetch testing, microscopic or ultrasonic examination for nonmetallic inclusions, and
fracture testing.
Aircraft quality and magnaflux quality apply to cold-finished alloy steel bars for important or highly stressed parts
of aircraft and for other similar or corresponding purposes involving additional stringent requirements, such as magnetic
particle inspection, additional discard, macroetch tests, and hardenability control. The meet these requirements, exacting
steelmaking, rolling, and testing practices must be employed. These practices are designed to minimize detrimental
inclusions and porosity. Phosphorus and sulfur are usually limited to 0.025% maximum. There are many aircraft parts and
many parts for missiles and other rockets that require aircraft quality steel. The magnetic particle testing requirements
given in AMS 2301 are sometimes specified for such applications.
Other Alloy Steel Qualities. The quality descriptors listed below apply to cold-finished alloy steel bars intended for
rifles, guns, shell, shot, and similar applications. They may have requirements for amount of discard, macroetch testing,
surface requirements, or magnetic particle testing as indicated in the product specifications:
• Armor-piercing (AP) shot quality
• AP shot magnaflux quality
• Gun quality
• Rifle barrel quality
• Shell quality
• Shell magnaflux quality
Mechanical Properties
A major difference between machined and cold-drawn round bars is the improvement in tensile and yield strengths that
results from the cold work of drawing. Cold work also changes the shape of the stress-strain diagram, as shown in Fig. 1.
Within the range of commercial drafts, cold work markedly affects certain mechanical properties (Fig. 2). The variations
1
in percentage of reduction of cross section for bars drawn with normal commercial drafts of 0.8 and 1.6 mm ( and
32
1 1 3
in.) and with heavy drafts of 3.2 and 4.8 mm ( and in.) are shown in Fig. 3. Normal reductions seldom exceed
16 8 16
20% and are usually less than 12%. According to Fig. 2, the more pronounced changes in significant tensile properties
occur within this range of reductions (up to about 15%).
Impact Properties. Available data are limited on the effect of cold work on notched-bar impact properties. The results
of one of the more important studies are included in Fig. 5, 6, 7, and 8, which show the effect of cold work over a wide
range of drafts on three carbon steels with increasing carbon contents and the effect of cold work on 8630 alloy steel.
Within the range of commercial drafts, energy absorbed (breaking strength) falls rapidly for the 1016 steel and less
rapidly for 8630 steel. At any level of cold work, energy absorbed decreases with increased carbon content.
In the stress-relieved condition, the fracture transition temperature generally rises with increasing amounts of cold work
up to 20 to 30% reduction. Beyond this commercial range of reductions, the transition temperature falls. For 1016 steel,
extremely heavy drafts lower the transition temperature to below that of the original hot-rolled material. Increasing carbon
content raises the transition temperature.
References cited in this section
2. Alloy, Carbon, and High Strength Low Alloy Steels, Semifinished for Forging; Hot Rolled Bars; Cold
Finished Steel Bars; Hot Rolled Deformed and Plain Concrete Reinforcing Bars, AISI Steel Products
Manual, American Iron and Steel Institute, 1986
5. L.J. Ebert, Report WAL 310/90-85 to Watertown Arsenal, 1955
Residual Stresses
The stress pattern produced by cold drawing depends on the amount of reduction and the shape of the die, as well as the
microstructure, hardness, and grade of steel. Figure 10 illustrates the effect of reduction in area on the magnitude and
distribution of stresses in bars of 1050 steel reduced by the amounts shown. Cold drawing of the bars to 4.1% reduction
resulted in surface compressive stresses, while increasing the amount of cold drawing to 12.3% reduction resulted in a
change of the surface stresses from compressive to tensile. The variation in longitudinal stress over a much wider range of
reduction values is shown in Fig. 11 for steel wire (the effect is qualitatively similar for bars). The greatest effect on the
residual stress is caused by the first 10% reduction. The effect of a very light draft is to produce compressive stress at the
surface, which rapidly changes to tensile stress with a relatively small increase in reduction.
Straightening in a skewed-rolls (Medart) machine significantly reduces residual stress, particularly at the surface, as
shown in Fig. 12(b). It is of interest to compare the longitudinal stress curve shown in Fig. 12(b) with that in Fig. 12(a).
Figure 12(c) shows the effect of two stress-relieving temperatures (425 and 540 °C, or 800 and 1000 °F) on residual
longitudinal stress. Stress relieving at these temperatures is only slightly more effective than straightening in reducing the
residual stress level. This phenomenon may be accounted for by an analysis of the nature of the stresses that are
developed in cold drawing.
The stress applied in cold drawing is sufficient to deform the material both elastically and plastically. Because the
initiation of plastic strain depends on the development of maximum elastic strain, the ratio of these two strains after
release of the deforming stress may be highly variable. If the deformation caused by cold drawing were uniform across
the section, as in pure stretching, the elastic stress would be released by the release of the deforming stress. Because the
plastic strain is not uniform, as shown by the dishing of the ends of drawn bars, neither is the accompanying elastic strain.
When the deforming stress is removed from such a system, the remaining nonuniform elastic-strain energy cannot be
released completely, because the resistance of low-strain regions prevents the complete recovery of regions of high strain.
A pattern of residual stress results from this unequal adjustment.
Stress Relieving. The inevitable residual stresses in as-drawn bars can be relieved mechanically or thermally (Ref 7).
Mechanical relief may take two forms. One involves the introduction of stresses of opposite sign, which can be
accomplished by shot peening. A second approach is to plastically deform the material further, thus affording additional
opportunity for the relief of non-uniform residual stresses. The data on rotary straightening in Fig. 12(b) demonstrate this
effect.
The thermal stress relieving of cold-drawn bars--also known as strain drawing, strain annealing, strain relieving, preaging,
and stabilizing--is probably the most widely used thermal treatment applied to cold-drawn bars. Its purpose is to modify
the magnitude and distribution of residual stresses in the cold-finished bar and thus produce a product with the desired
combination of mechanical properties for field service. Thermal stress relief temporarily reduces the strength level of the
material (at the stress-relieving temperature) and enables the elastic-strain energy to find release in small but significant
amounts of plastic deformation. After stress relieving, the maximum residual stress that can remain is equal to the yield
strength of the material at the stress-relieving temperature.
Temperatures up to about 650 °C (1200 °F) are commonly used for the stress relieving of cold-drawn bars. The upper
limit for the stress-relief temperature for a particular cold-worked steel is the recrystallization or lower critical
temperature of that steel, because if this temperature is exceeded, the strengthening effect of cold work is lost. The
temperatures used in commercial practice frequently range from 370 to 480 °C (700 to 900 °F). When stress relieving is
performed at relatively low temperatures (for example, 290 °C, or 550 °F), yield strength of most cold-drawn steels is
increased. At higher temperatures, however, hardness, tensile strength, and yield strength are reduced, while elongation
and reduction in area are increased. The choice of a specific time and temperature is dependent on chemical composition,
cold-drawing practice, and the final properties required in the bar.
The various categories of stress relief can be divided into three groups:
• Group 1: Complete relief of all cold-working stresses
• Group 2: Relief of cold-working stresses to a limited degree to increase ductility and stability in the
material
• Group 3: Relief of stresses in heavily drafted steels to develop high yield strength
Group 1 treatment is conducted above 540 °C (1000 °F). It removes all residual stresses that otherwise would cause
objectionable distortion in machining.
Group 2 Treatment. With group 2 processing, lower temperatures in the range of 370 to 540 °C (700 to 1000 °F) are
used, and the stresses are partially relieved to bring the mechanical properties within the limits of individual
specifications. Applications falling into this class are those that may require ductility close to that of hot-rolled steel,
along with good surface finish and close control of dimensions and stability during machining.
Group 3 stress relief is used for bars with heavy drafts. These drafts raise the tensile and yield strengths to high
levels, but reduce elongation and reduction in area. Heating to 260 to 425 °C (500 to 800 °F) restores the ductility while
retaining or increasing the strength and hardness imparted by the cold work.
References cited in this section
6. H. Buhler and H. Bucholz, Influence of Cold Drawn Reduction Upon Stresses in Round Bars, Arch.
Eisenhüttenwes., Vol 7, 1934, p 427-430
7. E. Dieter, Mechanical Metallurgy, McGraw-Hill, 1976
Heat Treatment
Heat treatment by quenching and tempering, followed by scale removal and then cold drawing, can also be used as a
method of producing stronger cold-finished bars in those grades amenable to quench hardening. Heat treatment provides
the required increase in strength, and cold drawing provides the size and finish, with a minimal increase in the mechanical
properties obtained by quench hardening. Alternatively, quenched and tempered bars can be cold finished by turning and
polishing. When bars are cold finished by turning and polishing, there is no increase in the mechanical properties obtained
by quench hardening.
For the cold drawing of quenched and tempered bars to be economically justifiable, the minimum strength level produced
must be above that obtainable by conventional cold-drawn practices. The upper strength limit is not clearly defined, but
for most applications it is the upper limit of machinability. The cold drawing of quenched and tempered bars is applicable
to both carbon and alloy steels; however, for the process to be economically justifiable, alloy steel is used only in those
sizes above which carbon steel will not respond satisfactorily to liquid quenching. Typically, quenched and tempered
product offers superior ductility and heat-resistant properties. Other heat treatments--principally normalizing, full
annealing, spheroidizing, and thermal stress relieving--can be applied to suitable grades of hot-rolled steel before or after
cold drawing or turning and polishing as required by the end product.
Control of microstructure is frequently important, a good example being annealing for machinability. A controlled rate of
continuous cooling through the pearlite transformation range (so-called cycle annealing) is employed. Isothermal cycles
are also used. In the cycle-annealing process, the rate of cooling of the furnace charge is adjusted so that the time required
to traverse the pearlite temperature interval is sufficient to allow completion of that transformation. By regulating the
dwell in the transformation temperature range, the carbide distribution in the product can be varied from partly spheroidal
to fully pearlitic, and the pearlite from coarse to fine. In this manner, the optimum machining structure can be obtained for
the grade and the machining practice being used.
Spheroidize annealing thermal treatment is given to cold-finished bars that are to be used for severe cold-forming
operations. The aim of this treatment is to develop a microstructure consisting of globular carbides in a ferrite matrix. The
rate of spheroidizing depends to some degree on the original microstructures. Prior cold work also increases the rate of
spheroidizing, particularly for subcritical spheroidizing treatments.
The spheroidized structure is desirable when minimum hardness and maximum ductility are important. Low-carbon steels
are seldom annealed for machining because, in the annealed condition, they are very soft and gummy, which tends to
produce long, stringy chips that cause handling problems at the machine tool and contribute to a rough surface finish on
the machined part. When such steels are spheroidized, it is usually to permit severe cold deformation.
Carbon Restoration. During the hot-working operations involved in the production of bar products--the reduction of
cast ingots, blooms, or billets and subsequent conversion in bar mills--decarburization of the bar surface takes place
because of exposure to ambient oxygen at high temperatures throughout these operations. A specialized variant of full
annealing, called carbon restoration or carbon correction, is utilized to compensate for the loss of carbon due to
decarburization.
Carbon restoration for alloy steels is limited because vanadium carbide and molybdenum are not recovered. By heating
the descaled hot-rolled bars to approximately 870 to 925 °C (1600 to 1700 °F) in a controlled atmosphere, it is possible to
restore surface carbon to the required level. A modern controlled-atmosphere furnace is used for this purpose. Methane or
other light hydrocarbons are burned with a controlled amount of air in an endothermic generator to produce a gas with a
mixed ratio of CO to CO2. By controlling the CO/CO2 ratio of the endothermic gas, an atmosphere can be generated that
will be in equilibrium with the carbon content of the steel to be treated. Low-ratio gas is in equilibrium with lower-carbon
steels, and high-ratio gas is in equilibrium with higher content steels. The actual ratio used depends on the type of anneal
and the grade of steel to be annealed. This ratio must be closely controlled, or the atmosphere will become decarburizing
or carburizing to the steel. Modern instruments, such as oxygen probes, are available to maintain this close control.
After carbon restoration, bars are cold drawn. Material processed in this manner is useful when parts must have full
hardness on the cold-drawn (unmachined) surface after heat treatment. Many induction-hardened parts make use of a
carbon-restored material as a means of eliminating machining.
Machinability
Cold drawing significantly improves the machinability of the steels discussed in this article. The increase in hardness due
to cold work causes the chips formed by a cutting tool to tear away from the workpiece more readily, and to be harder and
more brittle, so that they break up easily and are less likely to build up on the tool edge. Deformation extends a shorter
distance above the edge of the tool, giving a sharper cleavage at that point. These factors contribute to improvements in
power consumption, tool wear, and surface finish. They result from the addition of the major contributors to
improvements in machinability: phosphorus, sulfur, nitrogen (diatomic), lead, bismuth, tellurium, selenium, calcium, and
so on, in various combinations.
In addition, the accuracy of size and section of cold-finished bars minimizes collet troubles and requires less surface
removal to obtain concentricity. The freedom from scale on the cold-finished bar also improves tool life and may permit
the surface of the bar to be used as the finished surface of the completed part.
Cold drawing generally improves the machinability of low-carbon steels because the high ductility of these materials in
the hot-rolled condition can be lowered considerably without raising strength excessively. In contrast, a steel such as
1144, which is inherently low in ductility because of its higher carbon content, shows little improvement in machining
after cold drawing. The increased hardness that results from cold drawing can be deleterious to the machinability of the
higher-carbon steels; it may be helpful to stress relieve after cold drawing to reduce hardness.
Another approach to maximum machinability with the higher-carbon grades is to anneal before cold drawing. This puts
the carbide in a form that is less abrasive to the cutting tool. Lamellar anneal and spheroidize annealing are used
depending on carbon level, machinability requirements, and heat treat response requirements. The trade-off values must
be decided for each individual application. Compared with hot-rolled steel, uniformity of hardness and structure are
improved.
One of the conventional indexes of machinability is the ratio of tool life to that encountered with 1212 cold-drawn bars.
The average machinability ratings for cold-drawn carbon steel bars, nonresulfurized and resulfurized carbon steel bars,
and alloy steel bars, based on a value of 100% for 1212 bars, are given in Tables 8, 9, 10, and 11. The relative
machinability data listed in Tables 8, 9, 10, and 11 represent results obtained from experimental studies and actual shop
production information on the general run of parts. Any extraordinary features of the part to be machined or physical
conditions of the steel should be taken into consideration, and speeds and feeds altered accordingly. In addition,
machinability is influenced by various metallurgical factors, such as degree of cold reduction, mechanical properties,
grain size, and microstructure. Therefore, the data in Tables 8, 9, 10, and 11 are presented only as a starting point from
which proper speeds and feeds for specific parts can be determined. Further discussion of the machinability of cold-drawn
steel is included in the article "Machinability of Steels" in this Volume.
References cited in this section
2. Alloy, Carbon, and High Strength Low Alloy Steels, Semifinished for Forging; Hot Rolled Bars; Cold
Finished Steel Bars; Hot Rolled Deformed and Plain Concrete Reinforcing Bars, AISI Steel Products
Manual, American Iron and Steel Institute, 1986
8. Materials, Vol 1, 1989 SAE Handbook, Society of Automotive Engineers, 1989
9. "U.S. Air Force Machinability Report," Vol 2, Curtiss-Wright Corporation, 1951
Machinability
Cold drawing significantly improves the machinability of the steels discussed in this article. The increase in hardness due
to cold work causes the chips formed by a cutting tool to tear away from the workpiece more readily, and to be harder and
more brittle, so that they break up easily and are less likely to build up on the tool edge. Deformation extends a shorter
distance above the edge of the tool, giving a sharper cleavage at that point. These factors contribute to improvements in
power consumption, tool wear, and surface finish. They result from the addition of the major contributors to
improvements in machinability: phosphorus, sulfur, nitrogen (diatomic), lead, bismuth, tellurium, selenium, calcium, and
so on, in various combinations.
In addition, the accuracy of size and section of cold-finished bars minimizes collet troubles and requires less surface
removal to obtain concentricity. The freedom from scale on the cold-finished bar also improves tool life and may permit
the surface of the bar to be used as the finished surface of the completed part.
Cold drawing generally improves the machinability of low-carbon steels because the high ductility of these materials in
the hot-rolled condition can be lowered considerably without raising strength excessively. In contrast, a steel such as
1144, which is inherently low in ductility because of its higher carbon content, shows little improvement in machining
after cold drawing. The increased hardness that results from cold drawing can be deleterious to the machinability of the
higher-carbon steels; it may be helpful to stress relieve after cold drawing to reduce hardness.
Another approach to maximum machinability with the higher-carbon grades is to anneal before cold drawing. This puts
the carbide in a form that is less abrasive to the cutting tool. Lamellar anneal and spheroidize annealing are used
depending on carbon level, machinability requirements, and heat treat response requirements. The trade-off values must
be decided for each individual application. Compared with hot-rolled steel, uniformity of hardness and structure are
improved.
One of the conventional indexes of machinability is the ratio of tool life to that encountered with 1212 cold-drawn bars.
The average machinability ratings for cold-drawn carbon steel bars, nonresulfurized and resulfurized carbon steel bars,
and alloy steel bars, based on a value of 100% for 1212 bars, are given in Tables 8, 9, 10, and 11. The relative
machinability data listed in Tables 8, 9, 10, and 11 represent results obtained from experimental studies and actual shop
production information on the general run of parts. Any extraordinary features of the part to be machined or physical
conditions of the steel should be taken into consideration, and speeds and feeds altered accordingly. In addition,
machinability is influenced by various metallurgical factors, such as degree of cold reduction, mechanical properties,
grain size, and microstructure. Therefore, the data in Tables 8, 9, 10, and 11 are presented only as a starting point from
which proper speeds and feeds for specific parts can be determined. Further discussion of the machinability of cold-drawn
steel is included in the article "Machinability of Steels" in this Volume.
References cited in this section
2. Alloy, Carbon, and High Strength Low Alloy Steels, Semifinished for Forging; Hot Rolled Bars; Cold
Finished Steel Bars; Hot Rolled Deformed and Plain Concrete Reinforcing Bars, AISI Steel Products
Manual, American Iron and Steel Institute, 1986
8. Materials, Vol 1, 1989 SAE Handbook, Society of Automotive Engineers, 1989
9. "U.S. Air Force Machinability Report," Vol 2, Curtiss-Wright Corporation, 1951
Special Die Drawing
Two special die-drawing processes have been developed to give improved properties over those offered by standard
drawing practices. These processes are cold drawing using heavier-than-normal drafts, followed by stress relieving; and
drawing at elevated temperatures. In the production of steel bars by these special processes, drafts of approximately 10 to
35% reduction in cross-sectional area are employed at room or elevated temperature, depending on the practices and
facilities of the individual producer. Stress-relieving temperatures vary over a similarly wide range, depending on
producer facilities and end-product requirements.
Heavy Drafts. Because of the engineering and economic advantages of cold-finished steels, a considerable effort has
been made to improve the uniformity of mechanical properties after cold drawing. This has been accomplished by using
heavier-than-normal drafts (10 to 35% reduction) followed by subcritical annealing. Stepwise or tandem drawing has
been resorted to in order to avoid the formation of internal transverse fissures (cupping or bambooing) that may result
from heavy drafts taken in one pass.
Heavier drafts produce higher tensile and yield strengths. Elongation can be substantially improved by stress relieving at
510 °C (950 °F), and this treatment still provides tensile and yield strengths higher than those obtainable with normal
drafts. The combination of properties resulting from heavier drafts and higher stress-relieving temperatures is most
desirable from the design standpoint. Such processing is most effective when applied to medium-carbon steels of either
normal or higher manganese content. In the medium-manganese range, 1045 and 1050 respond most favorably; 1137,
1141, 1144, 1527, 1536, and 1541 show good response for the higher-manganese grades. The sulfur content of the 11xx
steels improves machinability without lowering mechanical properties. Typical tensile properties for 1144 bars subjected
to normal and heavy drafts are given in Table 13.
Steel bars that have been cold drawn using heavier-than-normal drafts and then stress relieved are often used in place of
quenched and tempered bars, and as already noted, several resulfurized grades (1137, 1141, and 1144) respond readily to
this process with resulting high strength values. Because the microstructures of these steels are still pearlitic, they
machine more easily than their quenched and tempered counterparts. Therefore, although these grades cost more than
nonresulfurized grades, they can provide significant savings in manufacturing costs, chiefly through the elimination of
heat treating. However, even through the strength of the special cold-drawn and stress-relieved bars may be equal to that
of quenched and tempered steel, it is not advisable to translate other properties from one condition to the other.
The torsional strength and endurance limits of these special-process bars are similar to those of quenched and tempered
bars at the same strength level. The same is true for the wear resistance of bars of the same surface hardness. The impact
test values of the process bars, as measured by Izod or Charpy notched-bar test, are lower than those of quenched and
tempered carbon steel bars and are significantly lower than those of quenched and tempered alloy steel bars. Failures of
machine components usually result from fatigue, corrosion, wear, or shock loading. With the possible exception of the
latter, there is no known correlation between instances of failure and the notched-bar impact test. When low temperatures
or high pressures are involved and where there is doubt as to the suitability of these special-process bars, the design of the
part should be reviewed.
Drawing at elevated temperatures between 95 and 540 °C (200 and 1000 °F), a special proprietary process, can,
under optimum conditions, produce steel bars that have higher tensile and yield strengths than those of bars cold drawn
with the same degree of reduction. The relative effects of cold drawing followed by stress relieving and of drawing at
elevated temperature can be seen in Fig. 21. Both processes were carried out with 20% draft on 25 mm (1 in.) diam bars
of 1144 steel. As shown in Fig. 21, elevated-temperature drawing affects tensile strength considerably, giving values
somewhat greater than those for cold-drawn stress-relieved bars. For yield strength, the same general effects are evident,
but the difference between the two processes is not as pronounced. The elongation values are quite similar for both
processes.
The pronounced effect of drawing at elevated temperatures changes the shape of the stress-strain curve from round to
sharp-kneed, as shown in Fig. 26. Hot drawing reverses the effect of cold work, that is, automatically stress relieves the
steel. This effect on the stress-strain curve is significant in applications in which minimum plastic deformation is
permissible, such as a stud that requires a proof load high in relation to its tensile strength.
References cited in this section
8. Materials, Vol 1, 1989 SAE Handbook, Society of Automotive Engineers, 1989
11. E.S. Nachtman and E.B. Moore, J. Met., April 1955
12. E.S. Nachtman and E.B. Moore, J. Met., April 1958
References
1. J.G. Bralla, Handbook of Product Design for Manufacturing, McGraw-Hill, 1986
2. Alloy, Carbon, and High Strength Low Alloy Steels, Semifinished for Forging; Hot Rolled Bars; Cold
Finished Steel Bars; Hot Rolled Deformed and Plain Concrete Reinforcing Bars, AISI Steel Products
Manual, American Iron and Steel Institute, 1986
3. Handbook of Machining Data for Cold Finished Steel Bars, LTV Steel Flat Rolled and Bar Company, 1985
4. Steel--Bars, Forgings, Bearing, Chain, Springs, Vol 1.05, Annual Book of ASTM Standards, American
Society for Testing and Materials, 1989
5. L.J. Ebert, Report WAL 310/90-85 to Watertown Arsenal, 1955
6. H. Buhler and H. Bucholz, Influence of Cold Drawn Reduction Upon Stresses in Round Bars, Arch.
Eisenhüttenwes., Vol 7, 1934, p 427-430
7. E. Dieter, Mechanical Metallurgy, McGraw-Hill, 1976
8. Materials, Vol 1, 1989 SAE Handbook, Society of Automotive Engineers, 1989
9. "U.S. Air Force Machinability Report," Vol 2, Curtiss-Wright Corporation, 1951
10. "Estimated Properties and Machinability of Plain Carbon and Re-sulfurized Plain Carbon Steel Bars," SAE
J414, SAE Handbook Supplement HS30, Society of Automotive Engineers, 1976, p 3.14
11. E.S. Nachtman and E.B. Moore, J. Met., April 1955
12. E.S. Nachtman and E.B. Moore, J. Met., April 1958
Introduction
WIRE ROD is a semifinished product rolled from billet on a rod mill and is used primarily for the manufacture of wire.
The steel for wire rod is produced by all the modern processes, including the basic oxygen, basic open hearth, and electric
furnace processes. Steel wire rod is usually cold drawn into wire suitable for further drawing; for cold rolling, cold
heading, cold upsetting, cold extrusion, or cold forging; or for hot forging.
Although wire rod may be produced in several regular shapes, most is round in cross section. Round rod is usually
7 47 1
in.).* As the
produced in nominal diameters of 5.6 to 18.7 mm ( to in.), advancing in increments of 0.4 mm (
32 64 64
rod comes off the rolling mill, it is formed into coils. These coils are usually about 760 mm (30 in.) in inside diameter and
weigh up to 2000 Kg (4400 lb). The dimensions and maximum weight of a single coil are determined by the capabilities
of the rolling mill. Coil weights that exceed the capabilities of the rolling mill sometimes can be obtained by welding two
1
or more coils together. The standard tolerances are ±0.4 mm (± in.) on the diameter and 0.64 mm (0.025 in.) maximum
64
out-of-roundness.
Producers of wire rod may market their product as rolled, as cleaned and coated, or as heat treated, although users
generally prefer to do such preparations themselves. These operations are explained in the following sections, along with
the several recognized quality and commodity classifications applicable to steel wire rods.
Acknowledgements
The helpful suggestions provided by Zeev Zimerman, Bethlehem Steel Corporation, and Bhaskar Yalamanchili, North
Star Steel Texas Company, are greatly appreciated.
Note
* Because steel wire rod manufactured in the United States is customarily produced to fractional-inch sizes,
rather than decimal-inch or millimeter sizes, the millimeter conversion for wire rod sizes may not be a
1
multiple of the 0.4 mm ( in.) increment size.
64
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