ASM Metals HandBook Vol. 14 - Forming and Forging

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1095

. . . 1175 2150

Alloy steels

4130

Chromium, molybdenum 1205 2200

4140

Chromium, molybdenum 1230 2250

4320

Nickel, chromium, molybdenum

1230 2250

4340

Nickel, chromium, molybdenum

1290 2350

4615

Nickel, molybdenum 1205 2200

5160

Chromium 1205 2200

6150

Chromium, vanadium 1215 2220

8620

Nickel, chromium, molybdenum

1230 2250

9310

Nickel, chromium, molybdenum

1230 2250

Source: Ref 1

Steels have been forged in quantity since near the beginning of the Industrial Revolution. Despite (or perhaps because of)

this long history, the forging of steels is an intuitive, empirical process, and literature on the subject is relatively scarce.

This article will attempt to present forgeability data for carbon and alloy steels whenever possible, and to provide some

general guidelines for the forging of these materials. The thermomechanical processing of high-strength low-alloy

(microalloyed) forging steels also will be discussed.

Reference

J.T. Winship, Fundamentals of Forging, Am. Mach., July 1978, p 99-122

Forging of Carbon and Alloy Steels

Hot Forging Behavior

The hot forging of carbon and alloy steels into intricate shapes is rarely limited by forgeability aspects with the exception

of the free-machining grades mentioned earlier. Section thickness, shape complexity, and forging size are limited

primarily by the cooling that occurs when the heated workpiece comes into contact with the cold dies. For this reason

equipment that has relatively short die contact times, such as hammers, is often preferred for forging intricate shapes in

steel.

Forgeability

Hot-Twist Testing. One common means of measuring the forgeability of steels is the hot-twist test. As the name

implies, this test involves twisting of heated bar specimens to fracture at a number of different temperatures selected to

cover the possible hot working temperature range of the test material. The number of twists to fracture, as well as the

torque required to maintain twisting at a constant rate, are reported. The temperature at which the number of twists is the

greatest, if such a maximum exists, is assumed to be the optimal hot working temperature of the test material. Figure 1

shows forgeabilities of several carbon steels as determined by hot-twist testing. More information on the hot-twist test is

available in Ref 2, 3, and 4.

Fig. 1 Forgeabilities of various carbon steels as determined using hot-twist testing. Source: Ref 2.

Other Forgeability Tests. Numerous other tests are used to evaluate the forgeability of steels, including:

• The wedge-forging test, in which a wedge-

shape specimen is forged between flat dies and the vertical

deformation that causes cracking is established

• The side-pressing test, which con

sists of compressing a cylindrical bar specimen between flat, parallel

dies with the axis of the cylinder parallel to the dies. The ends of the cylinder are unconstrained, and

forgeability is measured by the amount of deformation obtained before cracking

• The upset test,

in which a cylinder is compressed between flat dies and the surface strains at fracture at

the equator of the cylinder are measured

• The notched-bar upset test,

which is similar to the upset test except that axial notches are machined into

the test specimen to introduce high local stress levels. These higher stresses may be more indicative of

the stresses experienced during actual forging operations than those produced in the standard upset test

• The hot tensile test, which often uses a spec

ial test apparatus to vary both strain rates and temperatures

over a wide range

More detailed information on these test procedures, as well as other techniques used to evaluate the bulk workability of

materials, is available in the articles in the Section "Evaluation of Workability" in this Volume and in Ref 5 and 6.

Effect of Strain Rate on Forgeability. As previously stated, the forgeability of steels generally increases with

increasing strain rate. This effect has been shown for low-carbon steel in hot-twist testing (Fig. 2), where the number of

twists to failure increases with increasing twisting rate. It is believed that this improvement in forgeability at higher strain

rates is due to the increased heat of deformation produced at higher strain rates. Excessive temperature increases from

heat of deformation, however, may lead to incipient melting, which can lower forgeability and mechanical properties.

Fig. 2 Influence of deformation rate on hot-twist characteristics of low-

carbon steels at 1095 °C (2000 °F).

Source: Ref 7.

Flow Stress and Forging Pressure

Flow stresses and forging pressures can be obtained from torque curves generated in hot-twist tests or from hot-

compression or tension testing. Figure 3 shows torque versus temperature curves for several carbon and alloy steels

obtained from hot-twist testing. These data show that the relative forging pressure requirements for this group of alloys do

not vary widely at normal hot-forging temperatures. A curve for AISI type 304 stainless steel is included to illustrate the

effect of higher-alloy content on flow strength.

Fig. 3 Deformation resistance versus temperature for various carbon and alloy steels. Source: Ref 7.

Figure 4 shows actual forging pressure measurements for 1020 and 4340 steels and AISI A6 tool steel for reductions of

10 and 50%. Forging pressures for 1020 and 4340 vary only slightly at identical temperatures and strain rates.

Considerably greater pressures are required for the more highly-alloyed A6 material, and this alloy also exhibits a more

significant increase in forging pressure with increasing reduction.

Fig. 4 Forging pressure versus temperature for three steels. Data are shown for reductions of 10 and 50%

Strain rate was constant at 0.7 s

-1

. Source: Ref 9.

Effect of Strain Rate on Forging Pressure. Forging pressures required for a given steel increase with increasing

strain rate. Studies of low-carbon steel (Ref 8) indicate that the influence of strain rate is more pronounced at higher

forging temperatures. This effect is illustrated in Fig. 5, which gives stress-strain curves for a low-carbon steel forged at

various temperatures and strain rates.

Fig. 5 Forging pressure for low-carbon steel upset at various temperatures and two strain rates. Source: Ref 8.

Similar effects have been observed in alloy steels. Figure 6 shows the forging pressures required upset 4340 steel at

several temperatures and strain rates.

Fig. 6 Forging pressure for AISI 4340 steel upset at various temperatures and two strain rates. Source: Ref 9.

References cited in this section

Evaluating the Forgeability of Steel, 4th ed., The Timken Company, 1974

H.K. Ihrig, The Effect of Various Elements on the Hot Workability of Steel, Trans. AIME,

Vol 167, 1946, p

749-777

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

G.E. Dieter, Mechanical Metallurgy, 2nd ed., McGraw-Hill, 1976

G.E. Dieter, Ed., Workability Testing Techniques, American Society for Metals, 1984

C.T. Anderson, R.W. Kimball, and F.R. Cattoir, Effect of Various Elements on the Hot Working

Characteristics and Physical Properties of Fe-C Alloys, J. Met., Vol 5 (No. 4), April 1953, p 525-529

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

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

Forging of Carbon and Alloy Steels

Effects of Forging on Properties

The shaping of a complex configuration from a carbon or alloy steel bar or billet requires first that the steel be "arranged"

into a suitable starting shape (preformed) and then that it be caused to flow into the final part configuration. This

rearrangement of the metal has little effect on hardness and strength of the steel, but certain mechanical properties, such

as ductility, impact strength, and fatigue strength, are enhanced. This improvement in properties is thought to take place

because forging:

• Breaks up segregation, heals porosity, and aids homogenization

• Produces a fibrous grain structure (Fig. 7) that enhances mechanical properties parallel to the grain flow

• Reduces as-cast grain size

Fig. 7 4140 steel forged hook showing fibrous structure (flow lines) resulting from hot forging.

Etched using

50% hot aqueous HCl. 0.5×

Typical improvements in ductility and impact strength of heat-treated steels as a function of forging reduction are shown

in Fig. 8 and 9. These data illustrate that maximum improvement in each case occurs in the direction of maximum

elongation.Toughness and ductility reach maximums after a certain amount of reduction, after which further reduction is

of little value.

Fig. 8 Effect of forging ratio on reduction of area of heat-

treated steels. (a) 4340 steel at two sulfur levels. (b)

Manganese steel. (c) Vacuum melted 4340 with ultimate tensile str

ength of 2000 MPa (290 ksi). Forging ratio is

ratio of final cross-sectional area to initial cross-sectional area. Source: Ref 8, 10, and 11.

Fig. 9 Effect of hot-working reduction on impact strength of heat-treated nickel-

chromium steel. Forging ratio

is the ratio of initial cross-sectional area to final cross-sectional area. Source: Ref 12.

The typical longitudinal mechanical properties of low- and medium-carbon steel forgings in the annealed, normalized,

and quenched and tempered conditions are listed in Table 2. As might be expected, strength increases with increasing

carbon content, while ductility decreases.

Table 2 Longitudinal properties of carbon steel forgings at four carbon contents

Ultimate

tensile strength

Yield strength,

0.2% offset

Fatigue strength

(a)

Carbon

content,

MPa ksi MPa ksi

Elongation,

Reduction of

area, %

MPa ksi

Hardness, HB

Annealed

0.24 438 63.5 201 29.1 39.0 59 185 26.9

122

0.30 483 70.0 245 35.6 31.5 58 193 28.0

134

0.35 555 80.5 279 40.5 24.5 39 224 32.5

157

0.45 634 92.0 348 50.5 24.0 42 248 35.9

180

Normalized

0.24 483 70.0 247 35.8 34.0 56.5 193 28.0

134

0.30 521 75.5 276 40.0 28.0 44 209 30.3

148

0.35 579 84.0 303 44.0 23.0 36 232 33.6

164

0.45 690 100.0 355 51.5 22.0 36 255 37.0

196

Oil quenched and tempered at 595 °C (1100 °F)

0.24 500 72.5 305 44.2 35.5 62 193 28.0

144

0.30 552 80.0 301 43.7 27.0 52 224 32.5

157

0.35 669 97.0 414 60.0 26.5 49 247 35.8

190

0.45 724 105.0 386 56.0 19.0 31 277 40.2 206

Source: Ref 13

(a)

Rotating beam test at 10

endurance limit.

It should be recognized that closed-die forgings for the most part are made from wrought billets that have received

considerable prior working. Open-die forgings, however, may be made from either wrought billets or as-cast ingots.

Metal flows in various directions during closed-die forging. For example, in the forging of a rib and web shape such as an

airframe component, nearly all metal flow is in the transverse direction. Such transverse flow improves ductility in that

direction with little or no reduction in longitudinal ductility. Transverse ductility could conceivably equal or surpass

longitudinal ductility if forging reductions were large enough and if metal flow were primarily in the transverse direction.

Similar effects are observed in the upsetting of wrought billets. In this case, however, the original longitudinal axis of the

material is shortened by upsetting, and lateral displacement of metal is in the radial direction. When upset reductions

exceed about 50%, ductility in the radial direction usually exceeds that in the axial direction (Fig. 10).