ASM Metals HandBook Vol. 14 - Forming and Forging
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Fig. 10 Typical influence of upset reduction on axial and radial ductility of forged steels.
References cited in this section
8. J.F. Alder and V.A. Phillips, The Effect of Strain Rate and Temperature on the Resistance o
f Al, Cu, and
Steel to Compression, J. Inst. Met., Vol 83, 1954-1955, p 80-86
10.
F.W. Boulger et al.,
"A Study on Possible Methods for Improving Forging and Extruding Process for
Ferrous and Nonferrous Materials," Final Engineering Report, Contract AF 33(600)-
26272, Battelle
Memorial Institute, 1957
11.
L.E. Sprague, "The Effects of Vacuum Melting on the Fabrication and Mechanical Properties of Forging,"
Steel Improvement and Forge Company, 1960
12.
H. Voss, Relations Between Primary Structure, Reductio
n in Forging, and Mechanical Properties of Two
Structural Steels, Arch. Eisenhüttenwes., Vol 7, 1933-1934, p 403-406
13.
R.T. Rolfe, Steels for the User, 3rd ed., Philosophical Library, 1956
Forging of Carbon and Alloy Steels
Forging Lubricants (Ref 14)
For many years, oil-graphite mixtures were the most commonly used lubricants for forging carbon and alloy steels.
Recent advances in lubricant technology, however, have resulted in new types of lubricants, including water/graphite
mixtures and water-base synthetic lubricants. Each of the commonly used lubricants has advantages and limitations
(Table 3) that must be balanced against process requirements.
Table 3 Advantages and limitations of the principal lubricants used in the hot forging of steels
Type of
lubricant
Advantages
Limitations
Water-base
micro-
graphite
Eliminates smoke and fire; provides die cooling; is easily extended
with water
Must be applied by spraying for best results
Water-base
synthetic
Eliminates smoke and fire; is cleaner than oils or water-base
graphite; aids die cooling; is easily diluted, and needs no agitation
after initial mixing; reduces clogging of spray equipment; does not
transfer dark pigment to part
Must be sprayed; lacks the lubricity of
graphite for severe forging operations
Oil-base
graphite
Fluid film lends itself to either spray or swab application; has
good performance over a wide temperature range (up to 540
°C, or 1000 °F)
Generates smoke, fire, and noxious odors;
explosive nature may shorten die life; has
potentially serious health and safety
implications for workers
Source: Ref 14
Selection Criteria. Lubricant selection for forging is based on several factors, including forging temperature, die
temperature, forging equipment, method of lubricant application, complexity of the part being forged, and environmental
and safety considerations. At normal hot-forging temperatures for carbon and alloy steels, water-base graphite lubricants
are used almost exclusively, although some hammer shops may still employ oil-base graphite.
The most common warm-forming temperature range for carbon and alloy steels is 540 to 870 °C (1000 to 1500 °F).
Because of the severity of forging conditions at these temperatures, billet coatings are often used in conjunction with die
lubricants. The billet coatings used include graphite in a fluid carrier or water-base coatings used in conjunction with
phosphate conversion coating of the workpiece.
For still lower forging temperatures (less than about 400 °C, or 750 °F), molybdenum disulfide has a greater load-carrying
capacity than does graphite. Molybdenum disulfide can either be applied in solid form or dispersed in a fluid carrier.
More information on lubricant chemistry, application, and selection is available in Ref 14.
Reference cited in this section
14.
D.W. Hutchinson, "The Function and Proper Selection of Forging Lubricants," Acheson Colloids Company,
1984
Forging of Carbon and Alloy Steels
Steels for Forging
Carbon and alloy steel ingots, blooms, billets, and slabs for forging are hot rolled or cast to approximate cross sectional
dimensions; therefore, straightness, camber, twist, and flatness tolerances do not apply. Semifinished steel products for
forging are produced to either specified piece weights or specified lengths.
Surface Conditioning. Semifinished steel products for forging can be conditioned by scarfing, chipping, or grinding to
remove or minimize surface imperfections. It should be kept in mind that, regardless of surface conditioning, the product
is still likely to contain some surface imperfections.
Weight tolerances for billets, blooms, and slabs are often ±5% for individual pieces or for lots weighing less than 18
Mg (20 tons). Lots weighing more than that are frequently subject to weight tolerances of ±2.5%.
Cutting. Semifinished steel products for forging are generally cut to length by hot shearing. Depending on the steel
composition, hot sawing or flame cutting may also be used.
Quality, as the term is applied to semifinished steel products for forging, is dependent on many different factors,
including the degree of internal soundness, relative uniformity of chemical composition, and relative freedom from
surface imperfections.
Forging quality semifinished steel is used in hot forging applications that may involve subsequent heat treatment or
machining operations. Such applications require relatively close control of chemical composition and steel manufacture.
Forging-quality carbon and alloy steel products are produced to the guidelines described in Ref 15.
Powder metallurgy (P/M) steels are also forged from both sintered preforms and green (unsintered) preforms.
Detailed information on the forging of P/M steels and the properties of the resulting products is available in the article
"Powder Forging" in this Volume.
Reference cited in this section
15.
Alloy, Carbon, and High Strength Low Alloy Steels: Semifinished for Forging; Hot Rolled Bars, Cold
Finished Bars, American Iron and Steel Institute, Mar 1986
Forging of Carbon and Alloy Steels
Heat Treatment of Carbon and Alloy Steel Forgings (Ref 16)
Usually steel forgings are specified by the purchaser in one of four principal conditions: as forged with no further thermal
processing; heat treated for machinability; heat treated for final mechanical/physical properties; or specially heat treated
to enhance dimensional stability, particularly in more complex part configurations.
As Forged. Although the vast majority of steel forgings are heat treated before use, a large tonnage of low-carbon steel
(0.10 to 0.25% C) is used in the as-forged condition. In such forgings, machinability is good, and little is gained in terms
of strength by heat treatment. In fact, a number of widely used ASTM and federal specifications permit this economic
option. It is also interesting to note that, compared to the properties produced by normalizing, strength and machinability
are slightly better, which is most likely attributable to the fact that grain size is somewhat coarser than in the normalized
condition.
Heat Treated for Machinability. When a finished machined component must be produced from a roughly
dimensioned forging, machinability becomes a vital consideration to optimize tool life, increase productivity, or both. The
purchase specification or forging drawing may specify the heat treatment. However, when specifications give only
maximum hardness or microstructural specifications, the most economical and effective thermal cycle must be selected.
Available heat treatments include full anneal, spheroidize anneal, subcritical anneal, normalize, or normalize and temper.
The heat treatment chosen depends on the steel composition and the machine operations to be performed. Some steel
grades are inherently soft, others become quite hard in cooling from the finishing temperature after hot forging. Some
type of annealing is usually required or specified to improve machinability.
Heat Treated to Final Physical Properties. Normalizing or normalizing and tempering may produce the required
minimum hardness and minimum ultimate tensile strength. However, for most steels, a hardening (austenitize) and
quenching (in oil, water, or some other medium, depending on section size and hardenability) cycle is employed, followed
by tempering to produce the proper hardness, strength, ductility, and impact properties. For steel forgings to be heat
treated above the 1034 MPa (150 ksi) strength level and having section size variations, it is general practice to normalize
before austenitizing to produce a uniform grain size and minimize internal residual stresses. In some instances, it is
common practice to use the heat for forging as the austenitizing cycle and to quench at the forge unit. The forging is then
tempered to complete the heat treat cycle. Although there are obvious limitations to this procedure, definite economies are
possible when the procedure is applicable (usually for symmetrical shapes of carbon steels that require little final
machining).
Special heat treatments are sometimes used to control dimensional distortion, relieve residual stresses before or after
machining operations, avoid quench cracking, or prevent thermal shock or surface (case) hardening. Although most of the
heat-treating cycles discussed above can apply, very specific treatments may be required. Such treatments usually apply
to complex forging configurations with adjacent differences in section thickness, or to very high hardenability steels and
alloys. When stability of critically dimensioned finished parts permits only light machining of the forging after heat
treatment to final properties, special treatments are available, including marquenching (martempering), stress relieving,
and multiple tempering.
Many applications, such as crankshafts, camshafts, gears, forged rolls, rings, certain bearings, and other machinery
components, require increased surface hardness for wear resistance. The important surfaces are usually hardened after
machining by flame or induction hardening, carburizing, carbonitriding, or nitriding. These processes are listed in the
approximate order of increasing cost and decreasing maximum temperature. The latter consideration is important in that
dimensional distortion usually decreases with decreasing temperature. This is particularly true of nitriding, which is
usually performed below the tempering temperature for the steel used in the forging. Detailed information on heat
treatment practices for carbon and alloy steels is available in Heat Treating, Volume 4 of the ASM Handbook.
Reference cited in this section
16.
R.T. Morelli and S.L. Semiatin, Heat Treatment Practices, in Forging Handbook, T.G. Byrer, S.L. Semia
tin,
and D.C. Vollmer, Ed., Forging Industry Association/American Society for Metals, 1985, p 228-257
Forging of Carbon and Alloy Steels
Microalloyed Forging Steels
Microalloying--the use of small amounts of elements such as vanadium and niobium to strengthen steels--has been in
practice since the 1960s to control the microstructure and properties of low-carbon steels (Ref 17). Most of the early
developments were related to plate and sheet products in which microalloy precipitation, controlled rolling, and modern
steelmaking technology combined to increase strength significantly relative to that of low-carbon steels.
The application of microalloying technology to forging steels has lagged behind that of flat-rolled products because of the
different property requirements and thermomechanical processing of forging steels. Forging steels are commonly used in
applications in which high strength, fatigue resistance, and wear resistance are required. These requirements are most
often filled by medium-carbon steels. Thus, the development of microalloyed forging steels has centered around grades
containing 0.30 to 0.50% C.
The driving force behind the development of microalloyed forging steels has been the need to reduce manufacturing
costs. This is accomplished in these materials by means of a simplified thermomechanical treatment (that is, a controlled
cooling following hot forging) that achieves the desired properties without the separate quenching and tempering
treatments required by conventional carbon and alloy steels. In Fig. 11 the processing sequence for conventional
(quenched and tempered) steels is compared with the microalloyed steel-forging process.
Fig. 11
Processing cycles for conventional (quenched and tempered; top) and microalloyed steels (bottom).
Source: Ref 26.
Effects of Microalloying Elements (Ref 18)
Carbon. Most of the microalloyed steels developed for forging have carbon contents ranging from 0.30 to 0.50%, which
is high enough to form a large amount of pearlite. The pearlite is responsible for substantial strengthening. This level of
carbon also decreases the solubility of the microalloying constituents in austenite.
Niobium, Vanadium, and Titanium. Formation of carbonitride precipitates is the other major strengthening
mechanism of microalloyed forging steels. Vanadium, in amounts ranging from 0.05 to 0.2%, is the most common
microalloying addition used in forging steels. Niobium and titanium enhance strength and toughness by providing control
of austenite grain size. Often niobium is used in combination with vanadium to obtain the benefits of austenite grain size
control (from niobium) and carbonitride precipitation (from vanadium).
Manganese is used in relatively large amounts (1.4 to 1.5%) in many microalloyed forging steels. It tends to reduce the
cementite plate thickness while maintaining the interlamellar spacing of pearlite developed (Ref 19); thus, high
manganese levels require lower carbon contents to retain the large amounts of pearlite required for high hardness.
Manganese also provides substantial solid solution strengthening, enhances the solubility of vanadium carbonitrides, and
lowers the solvus temperature for these phases.
The silicon content of most commercial microalloyed forging steels is about 0.30%; some grades contain up to 0.70%
(Ref 20). Higher silicon contents are associated with significantly higher toughness, apparently because of an increased
amount of ferrite relative to that formed in ferrite-pearlite steels with lower silicon contents.
Sulfur. Many microalloyed forging steels, particularly those destined for use in automotive forgings in which
machinability is critical, have relatively high sulfur contents. The higher sulfur contents contribute to their machinability,
which is comparable to that of quenched and tempered steels (Ref 21, 22).
Aluminum and Nitrogen. As in hardenable fine-grain steels, aluminum is important for austenite grain size control in
microalloyed steels (Ref 19). The mechanism of aluminum grain size control is the formation of aluminum nitride
particles. It has been shown that nitrogen is the major interstitial component of vanadium carbonitride (Ref 23). For this
reason, moderate to high nitrogen contents are required in vanadium-containing microalloyed steels to promote effective
precipitate strengthening.
Controlled Forging (Ref 24)
The concept of grain size control has been used for many years in the production of flat-rolled products. Particularly in
plate rolling, the ability to increase austenite recrystallization temperature using small niobium additions is well known;
the process used to produce these steels is usually referred to as controlled rolling (see the article "Flat, Bar, and Shape
Rolling" in this Volume).
The benefits of austenite grain size control are not, of course, limited to flat-rolled products. Although the higher finishing
temperatures required for rolling of bars limit the usefulness of this approach to microstructural control, finishing
temperatures for microalloyed bar steels must nonetheless be controlled. It has been shown that, although strength is not
significantly affected by finishing temperature, toughness of vanadium-containing microalloyed steels decreases with
increasing finishing temperature (Ref 25, 26). This effect is shown in Fig. 12, which compares Charpy V-notch impact
strength for a microalloyed 1541 steel finished at three temperatures. This detrimental effect of a high finishing
temperature on impact toughness also carries over to forging operations, that is, the lower the finish temperature in
forging, the higher the resulting toughness, and vice versa. After extensive testing, the investigators in Ref 26
recommended that finishing temperature for forging be reduced to near 1000 °C (1800 °F). Such treatment resulted in
impact properties equal to or better than those of hot-rolled bar (Ref 26). The same investigators concluded that rapid
induction preheating was beneficial for microalloyed forging steels, and that cost savings of 10% (for standard
microalloyed forgings) to 20% (for resulfurized grades) were possible.
Fig. 12
Effect of hot finishing temperature on impact strength of microalloyed 1541 steel (AISI 1541 plus
0.10% V). Source: Ref 25.
Lower finishing temperatures, however, take their toll in terms of higher required forging pressures (and thus higher
machine capacities needed) and increased die wear. The improved toughness resulting from lower finishing temperatures,
as well as any cost savings that may be achieved as a result of the elimination of heat treatment, must be weighed against
the cost increases caused by these factors.
Microalloyed Cold Heading Steels
Steels used in the production of high-strength fasteners by cold heading were previously produced from quenched and
tempered alloy steels. To obtain sufficient strength with adequate ductility required six processing steps. Recent
developments have led to the use of microalloyed niobium-boron steels that require no heat treatment (Ref 27). These
steels make use of niobium and boron additions to develop bainitic structures with high work-hardening rates. In most
cases they use the deformation of cold heading to achieve the required strength levels without heat treatment. Table 4 lists
the compositions and selected properties of these materials.
Table 4 Compositions and selected properties of three microalloyed cold heading steels
Yield
strength
Ultimate
tensile
strength
Steel Nominal composition, %
MPa
ksi
MPa
ksi
Elongation,
%
Reduction
of area, %
Grade 1 Fe-0.20C-1.2Mn-25-50 ppm B 350 51 600 87 35
68
Grade 3
Fe-0.12C-1.6Mn-0.08Nb-25-50 ppm B
550 80 720 104
23 62
References cited in this section
17.
J.H. Woodhead and S.R. Keown, The History of Microalloyed Steels, in
HSLA Steels: Metallurgy and
Applications,
J.M. Gray, T. Ko, Z. Shouhua, W. Baorong, and X. Xishan, Ed., ASM INTERNATIONAL,
1986, p 15-28
18.
G. Krauss, "Microalloyed Bar and
Forging Steels," Paper presented at 29th Mechanical Working and Steel
Processing Conference, Iron and Steel Society of American Institute of Mining, Metallurgical, and
Petroleum Engineers, Toronto, Oct 1987
19.
R. Lagneborg, O. Sandberg, and W. Roberts, Optimization of Microalloyed Ferrite-
Pearlite Forging Steels,
in Fundamentals of Microalloying Forging Steels,
G. Krauss and S.K. Banerji, Ed., The Metallurgical
Society, 1987, p 39-54
20.
S. Engineer, R. Huchtmann, and V. Schuler, A Review of the Develop
ment and Application of
Microalloyed Medium-Carbon Steels, in Fundamentals of Microalloying Forging Steels,
G. Krauss and S.
K. Banerji, Ed., The Metallurgical Society, 1987, p 19-38
21.
V. Ollilainen, I. Lahti, H. Potinen, and E. Heiskala, Machinability
Comparison When Substituting
Microalloyed Forging Steel for Quenched and Tempered Steel, in
Fundamentals of Microalloying Forging
Steels, G. Krauss and S.K. Banerji, Ed., The Metallurgical Society, 1987, p 461-474
22.
D. Bhattacharya, Machinability of a Medium-Carbon Microalloyed Bar Steel, in
Fundamentals of
Microalloying Forging Steels, G. Krauss and S.K. Banerji, Ed., The Metallurgical Society, 1987, p 475-490
23.
J.G. Speer, J.R. Michael, and S.S. Hansen, Carbonitride Precipitation in Nb/V Microalloyed Steels,
Metall.
Trans. A, Vol 18A, 1987, p 211-222
24.
B.L. Jones, A.J. DeArdo, C.I. Garcia, K. Hulka, and H. Luthy, Microalloyed Forging Steels--
A Worldwide
Assessment, in HSLA Steels: Metallurgy and Applications, J.M. Gray, T. Ko, Z. Shouhua, W. Baoro
ng, and
X. Xishan, Ed., ASM INTERNATIONAL, 1986, p 875-884
25.
J.F. Held, Some Factors Influencing the Mechanical Properties of Microalloyed Steel, in
Fundamentals of
Microalloying Forging Steels, G. Krauss and S.K. Banerji, Ed., The Metallurgical Society, 1987, p 175-188
26.
P.H. Wright, T.L. Harrington, W.A. Szilva, and T.R. White, What the Forger Should Know About
Microalloy Steels, in Fundamentals of Microalloying Forging Steels,
G. Krauss and S.K. Banerji, Ed., The
Metallurgical Society, 1987, p 541-566
27.
B. Serin, Y. Sesalos, P. Maitrepierre, and J. Rofes-Vernis, Mem. Sci. Rev. Met., Vol 75, 1978, p 355
28.
B. Heritier, P. Maitrepierre, J. Rofes-
Vernis, and A. Wyckaert, HSLA Steels in Wire Rod and Bar
Applications, in HSLA Steels: Technology and Applications,
M. Korchynksy, Ed., American Society for
Metals, 1984, p 981-990
Forging of Carbon and Alloy Steels
References
1. J.T. Winship, Fundamentals of Forging, Am. Mach., July 1978, p 99-122
2. Evaluating the Forgeability of Steel, 4th ed., The Timken Company, 1974
3. H.K. Ihrig, The Effect of Various Elements on the Hot Workability of Steel, Trans. AIME,
Vol 167, 1946, p
749-777
4. C.L. Clark and J.J. Russ, A Laboratory Evaluation of the Hot Working Characteristic of Metals,
Trans.
AIME, Vol 167, 1946, p 736-748
5. G.E. Dieter, Mechanical Metallurgy, 2nd ed., McGraw-Hill, 1976
6. G.E. Dieter, Ed., Workability Testing Techniques, American Society for Metals, 1984
7. C.T. Anderson, R.W. Kimball, and F.R. Cattoir, Effect of Various Elements on th
e Hot Working
Characteristics and Physical Properties of Fe-C Alloys, J. Met., Vol 5 (No. 4), April 1953, p 525-529
8.
J.F. Alder and V.A. Phillips, The Effect of Strain Rate and Temperature on the Resistance of Al, Cu, and
Steel to Compression, J. Inst. Met., Vol 83, 1954-1955, p 80-86
9.
H.J. Henning, A.M. Sabroff, and F.W. Boulger, "A Study of Forging Variables," Technical Documentary
Report ML-TDR-64-95, Battelle Memorial Institute, March 1964
10.
F.W. Boulger et al., "A Study on Possible Methods for
Improving Forging and Extruding Process for
Ferrous and Nonferrous Materials," Final Engineering Report, Contract AF 33(600)-
26272, Battelle
Memorial Institute, 1957
11.
L.E. Sprague, "The Effects of Vacuum Melting on the Fabrication and Mechanical Prope
rties of Forging,"
Steel Improvement and Forge Company, 1960
12.
H. Voss, Relations Between Primary Structure, Reduction in Forging, and Mechanical Properties of Two
Structural Steels, Arch. Eisenhüttenwes., Vol 7, 1933-1934, p 403-406
13.
R.T. Rolfe, Steels for the User, 3rd ed., Philosophical Library, 1956
14.
D.W. Hutchinson, "The Function and Proper Selection of Forging Lubricants," Acheson Colloids
Company, 1984
15.
Alloy, Carbon, and High Strength Low Alloy Steels: Semifinished for Forging; Hot Ro
lled Bars, Cold
Finished Bars, American Iron and Steel Institute, Mar 1986
16.
R.T. Morelli and S.L. Semiatin, Heat Treatment Practices, in Forging Handbook,
T.G. Byrer, S.L.
Semiatin, and D.C. Vollmer, Ed., Forging Industry Association/American Society for Metals, 1985, p 228-
257
17.
J.H. Woodhead and S.R. Keown, The History of Microalloyed Steels, in
HSLA Steels: Metallurgy and
Applications,
J.M. Gray, T. Ko, Z. Shouhua, W. Baorong, and X. Xishan, Ed., ASM INTERNATIONAL,
1986, p 15-28
18.
G. Krauss, "M
icroalloyed Bar and Forging Steels," Paper presented at 29th Mechanical Working and Steel
Processing Conference, Iron and Steel Society of American Institute of Mining, Metallurgical, and
Petroleum Engineers, Toronto, Oct 1987
19.
R. Lagneborg, O. Sandberg, and W. Roberts, Optimization of Microalloyed Ferrite-
Pearlite Forging Steels,
in Fundamentals of Microalloying Forging Steels,
G. Krauss and S.K. Banerji, Ed., The Metallurgical
Society, 1987, p 39-54
20.
S. Engineer, R. Huchtmann, and V. Schuler, A Re
view of the Development and Application of
Microalloyed Medium-Carbon Steels, in Fundamentals of Microalloying Forging Steels,
G. Krauss and S.
K. Banerji, Ed., The Metallurgical Society, 1987, p 19-38
21.
V. Ollilainen, I. Lahti, H. Potinen, and E. Heisk
ala, Machinability Comparison When Substituting
Microalloyed Forging Steel for Quenched and Tempered Steel, in
Fundamentals of Microalloying Forging
Steels, G. Krauss and S.K. Banerji, Ed., The Metallurgical Society, 1987, p 461-474
22.
D. Bhattacharya, Machinability of a Medium-Carbon Microalloyed Bar Steel, in
Fundamentals of
Microalloying Forging Steels, G. Krauss and S.K. Banerji, Ed., The Metallurgical Society, 1987, p 475-490
23.
J.G. Speer, J.R. Michael, and S.S. Hansen, Carbonitride Precipitation in Nb/V Microalloyed Steels,
Metall.
Trans. A, Vol 18A, 1987, p 211-222
24.
B.L. Jones, A.J. DeArdo, C.I. Garcia, K. Hulka, and H. Luthy, Microalloyed Forging Steels--
A Worldwide
Assessment, in HSLA Steels: Metallurgy and Applications, J.M. Gray, T. Ko, Z
. Shouhua, W. Baorong, and
X. Xishan, Ed., ASM INTERNATIONAL, 1986, p 875-884
25.
J.F. Held, Some Factors Influencing the Mechanical Properties of Microalloyed Steel, in
Fundamentals of
Microalloying Forging Steels, G. Krauss and S.K. Banerji, Ed., The Metallurgical Society, 1987, p 175-188
26.
P.H. Wright, T.L. Harrington, W.A. Szilva, and T.R. White, What the Forger Should Know About
Microalloy Steels, in Fundamentals of Microalloying Forging Steels,
G. Krauss and S.K. Banerji, Ed., The
Metallurgical Society, 1987, p 541-566
27.
B. Serin, Y. Sesalos, P. Maitrepierre, and J. Rofes-Vernis, Mem. Sci. Rev. Met., Vol 75, 1978, p 355
28.
B. Heritier, P. Maitrepierre, J. Rofes-
Vernis, and A. Wyckaert, HSLA Steels in Wire Rod and Bar
Applications, in HSLA Steels: Technology and Applications,
M. Korchynksy, Ed., American Society for
Metals, 1984, p 981-990
Forging of Stainless Steel
Revised by Thomas Harris and Eugene Priebe, Armco Inc.
Introduction
STAINLESS STEELS, based on forging pressure and load requirements, are considerably more difficult to forge than
carbon or low-alloy steels, primarily because of the greater strength of stainless steels at elevated temperatures and the
limitations on the maximum temperatures at which stainless steels can be forged without incurring microstructural
damage. Forging load requirements and forgeability vary widely among stainless steels of different types and
compositions; the most difficult alloys to forge are those with the greatest strength at elevated temperatures.
Forging Methods
Open-die, closed-die, upset and roll forging, and ring rolling are among the methods used to forge stainless steel. As in
the forging of other metals, two of these methods are sometimes used in sequence to produce a desired shape.
Open-die forging (hand forging) is often used for smaller quantities for which the cost of closed dies cannot be
justified and in cases in which delivery requirements dictate shortened lead times. Generally, products include round bars,
blanks, hubs, disks, thick-wall rings, and square or rectangular blocks or slabs in virtually all stainless grades. Forged
stainless steel round bar can also be produced to close tolerances on radial forge machines.
Although massive forgings are normally associated with open-die forging, most stainless steel open-die forgings are
produced in the range of 10 to 900 kg (25 to 2000 lb). Additional information on product types is available in the article
"Open-Die Forging" in this Volume.
Closed-die forging is extensively applied to stainless steel in order to produce blocker-type, conventional, and close-
tolerance forgings. Selection from the above closed-die types invariably depends on quantity and the cost of the finished
part. Additional information on these types of products is available in the article "Closed-Die Forging in Hammers and
Presses" in this Volume.
Upset forging is sometimes the only suitable forging process when a large amount of stock is needed in a specific
location of the workpiece. For many applications, hot upset forging is used as a preforming operation to reduce the
number of operations, to save metal, or both when the forgings are to be completed in closed dies.
The rules that apply to the hot upset forging of carbon and alloy steels are also applicable to stainless steel; that is, the
unsupported length should never be more than 2 times the diameter (or, for a square, the distance across flats) for
single-blow upsetting. Beyond this length, the unsupported stock may buckle or bend, forcing metal to one side and
preventing the formation of a concentric forging. Exceeding this limitation also causes grain flow to be erratic and
nonuniform around the axis of the forging and encourages splitting of the upset on its outside edges. The size of an upset
produced in one blow also should not exceed 2 diameters (or, for a square, 2 times the distance across flats). This
varies to some extent, depending on the thickness of the upset. For extremely thin upsets, the maximum size may be only
two diameters, or even less. Without reheating and multiple blows, it is not possible to produce an upset in stainless steel
that is as thin or with corner radii as small as that which can be produced when a more forgeable metal such as carbon
steel is being upset (see the article "Hot Upset Forging" in this Volume).
Roll forging can be used to forge specific products, such as tapered shafts. It is also used as a stock-gathering operation
prior to forging in closed dies. Details on this process are available in the article "Roll Forging" in this Volume.
Ring rolling is used to produce some ringlike parts from stainless steel at lower cost than by closed-die forging. The
techniques used are essentially the same as those for the ring rolling of carbon or alloy steel (see the article "Ring
Rolling" in this Volume). More power is required to roll stainless steel, and it is more difficult to fill corners. A large ring
mill capable of rolling carbon steel rings with a face height of 2 m (80 in.) can roll stainless steel rings up to about 1.25 m
(50 in.) in height. Because stainless steel is more costly than carbon or alloy steel, the savings that result from using ring
rolling are proportionately greater for stainless steel.
Ingot Breakdown
In discussing the forgeability of the stainless steels, it is critical to understand the types of primary mill practices available
to the user of semifinished billet or bloom product.
Primary Forging and Ingot Breakdown. Most stainless steel ingots destined for the forge shop are melted by the
electric furnace argon oxygen decarburization process. They will usually weigh between 900 and 13,500 kg (2000 to
30,000 lb), depending on the shop and the size of the finished piece. Common ingot shapes are round, octagonal, or
fluted; less common ingot shapes include squares. Until recently, all of these ingots would have been top poured.
Increasing numbers of producers are switching to the bottom-poured ingot process. This process is slightly more
expensive to implement in the melt shop, but it more than pays for itself in extended mold life and greatly improved ingot
surface.
Some stainless steel grades used in the aircraft and aerospace industries are double melted. The first melt is done with the
electric furnace and argon oxygen decarburization, and these "electrodes" are then remelted by a vacuum arc remelting
(VAR) or electroslag remelting (ESR) process. This remelting under a vacuum (VAR) or a slag (ESR) tends to give a
much cleaner product with better hot workability. For severe forging applications, the use of remelt steels can sometimes
be a critical factor in producing acceptable parts. These double-melted ingots are round in shape and will vary in diameter
from 450 to 900 mm (18 to 36 in.), and in some cases, they weigh in excess of 11,000 kg (25,000 lb). The breakdown of
ingots is usually done on large hydraulic presses (13,500 kN, or 1500 tonf). A few shops, however, still use large
hammers, and the four-hammer radial forging machine is being increasingly used for ingot breakdown.
Heating is the single most critical step in the initial forging of ingots. The size of the ingot and the grade of the stainless
steel will dictate the practice necessary to reduce thermal shock and to avoid unacceptable segregation levels. It is
essential to have accurate and programmable control of the furnaces used to heat stainless steel ingots and large blooms.
Primary forging or breakdown of an ingot is usually achieved using flat dies. However, some forgers work the ingot down
as a round using "V" or swage dies. Because of the high hot hardness of stainless steel and the narrow range of working
temperatures for these alloys, light reductions, or saddening (an operation in which an ingot is given a succession of light
reductions in a press or rolling-mill or under a hammer in order to break down the skin and overcome the initial fragility
due to a coarse crystalline structure preparatory to reheating prior to heavier reductions), is the preferred initial step in the
forging of the entire surface of the ingot.
After the initial saddening of the ingot surface is complete, normal reductions of 50 to 100 mm (2 to 4 in.) can be taken. If
the chemistry of the heat is in accordance with specifications and if heating practices have been followed and minimum
forging temperatures observed, no problems should be encountered in making the bloom and other semifinished product.
If surface tears occur, the forging should be stopped, and the workpiece conditioned. Some forgers use hot powder
scarfing, but this presents environmental problems. The most common method is to grind out the defect. The ferritic,
austenitic, and nitrogen-strengthened austenitic stainless steels can be air cooled, ground, and reheated for reforging. The
martensitic and precipitation-hardening grades must be slow cooled and overaged before grinding and reheating. The
ingot surface is important, and many producers find it advantageous to grind the ingots before forging to ensure good
starting surfaces.
Billet and Bloom Product. Forgers buy bars, billets, or blooms of stainless steel for subsequent forging on hammers
and presses. Forged stainless steel billet and bloom products tend to have better internal integrity than rolled product,
especially with larger-diameter sections (>180 mm, or 7 in.). Correctly conditioned billet and bloom product should yield
acceptable finished forgings if good heating practices are followed and if attention is paid to the minimum temperature
requirements. Special consideration must be given to sharp corners and thin sections, because these tend to cool off very
rapidly. Precautions should be taken when forging precipitation-hardening or nitrogen-strengthened austenitic grades.