Cold and Hot Forging: Fundamentals and Applications / Edited by Taylan Altan, Gracious Ngaile, Gangshu Shen

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Die Materials and Die Manufacturing / 287

Table 21.16 Relative performance of tool steels [ICFG, 1992]

Tool steels Designation (AISI) Hardness, HRC Wear-resistance index Toughness index Machinability index Grindability index

Cold worked (CW)

Plain carbon W1 60 7 5 9 10

W2 60 7 ... 9 10

Medium/high-alloy CW A2 60 7 6 8 7

O1 60 7 7 8 9

S1 55 5 8 7 8

6F7 55 4 10 6 8

High-carbon chromium D2 60 8 4 3 4

D3 60 8 3 6 3

Hot worked (HW)

Medium-alloy HW 6F2 45 2.5 10 6 8

High-alloyed HW H13 45 4 9 8 8

H10 45 4 8 8 8

H19 45 4 8 5 7

High-speed steel M2 60 9 4 4 5

M3 64 10 2 3 2

T15 64 10 2 1 1

T42 64 10 2 1 1

Note: 10 ⳱ best; 1 ⳱ poor

has grown drastically. Today, the material is

used for more complex shapes and many forms

of extrusion tooling. Carbides are characterized

by high hardness, high modulus of elasticity, and

great compressive strength, while the tensile

strength is limited. The hardness is typically

measured using the Vickers or Rockwell A

scale. It is important to note that Rockwell C is

completely inadequate for measuring carbide

hardness, and an attempt to do so could likely

damage the diamond tip on the hardness ma-

chine.

The International Cold Forging Group has

broken cemented carbides into six groups: A, B,

C, D, E, and F. These classiﬁcations are based

on composition as relevant to cold forging ap-

plications [ICFG, 1992]. Table 21.17 displays

typical ranges for the mechanical properties of

each carbide class.

21.4.2 Material Selection for

Cold Forging Tools

The material used to produce each tool com-

ponent can be selected once the dimensions of

the tool have been selected. Important factors to

consider are the forming process, press param-

eters, batch size, and forming load. Table 21.18

displays common tool steels for each component

in the two most common processes. Table 21.19

indicates which carbide groups are appropriate

for the individual components.

When choosing a die or insert material, the

ﬁrst decision is whether to use tool steel or car-

bide. When deciding between the two, it is es-

pecially important to consider the size of the die

insert, the geometry of the die proﬁle, the tol-

erances of the component to be manufactured,

the batch size, and the operating conditions. Car-

bide inserts must be under compressive stresses,

since carbide cannot sustain tensile stresses.

In general, if the die insert is 1.2 in. (30 mm)

in diameter or less, carbide can be used. Pro-

cesses such as free extrusion permit die of

greater size to be made from cemented carbide.

Also, carbide is used almost exclusively for dies

in high production.

With respect to geometry, all corners of a car-

bide die must have generous radii. In general,

the shapes must be simple in order to use car-

bide. The ratio D/d

that compares the overall

diameter of the die, D, and bore diameter of the

die insert, d

, must be greater than 4.5 for car-

bide to be applicable. In the case of free extru-

sion, the ratio can be reduced to 3. If the ratio is

less than the critical value or the geometry is too

complex for carbide, tool steel should be used.

Considerations for selecting the punch mate-

rial are very similar to those accounted for in die

material selection. Since cemented carbide has a

very high compressive strength, it should be

used if the maximum compressive stress is

greater than 365 ksi (2500 MPa). If the punch

will be used for can extrusion, cemented carbide

may be used if the H/D ratio is greater than 2.5,

where H is the depth of the can bore hole, and

D is the diameter of the punch. When carbide is

used for backward extrusion, the length of the

punch should be kept as short as possible [ICFG,

1992].

When selecting a material for warm forging

dies, the preliminary concerns must be the ther-

288 / Cold and Hot Forging: Fundamentals and Applications

Table 21.17 Cemented carbide properties [ICFG, 1992]

Hardness Transverse rupture strength Compressive strength

Composition, wt%

grain

size, Density,

Room temp.

570 F

(300 C) Room temp.

570 F

(300 C)

Weibull

Room temp.

570 F

(300 C)

0.2%

proof

Young’s modulus

Poisson’s

Coefﬁcient

of thermal

Group Co C TiC lm g/cm

HV HRA HV TRE ksi MPa ksi MPa exponent ksi MPa ksi MPa stress ksi MPa ratio expansion

A 5–7 0–2.5 1 1–5 14.7–15.1 1450–1550 90.5–91 1200 0.5 260 1800 230 1600 9 595 4100 450 3100 70 91,000 630,000 0.22 4.9 ⳯ 10

ⳮ6

B 8–10 0–2.5 1 1–5 14.4–14.7 1300–1400 89–90 1050 0.7 290 2000 260 1800 9 550 3800 420 2900 65 88,000 610,000 0.22 5.0 ⳯ 10

ⳮ6

C 11–13 0–2.5 1 1–5 14.1–14.4 1200–1250 88–88.5 1000 1.0 320 2200 290 2000 10 520 3600 405 2800 60 83,000 570,000 0.22 5.4 ⳯ 10

ⳮ6

D 14–17 0–2.5 1 1–5 13.8–14.2 1100–1150 87–87.5 900 1.2 360 2500 335 2300 10 480 3300 390 2700 55 78,000 540,000 0.22 5.6 ⳯ 10

ⳮ6

E 18–22 0–2.5 1 1–5 13.3–13.8 950–1000 85.5–86 750 1.5 360 2500 320 2200 11 450 3100 360 2500 45 73,000 500,000 0.23 6.3 ⳯ 10

ⳮ6

F 23–30 0–2.5 1 1–5 12.6–13 800–850 83.5–84 600 1.9 335 2300 275 1900 11 390 2700 335 2300 35 65,000 450,000 0.24 7.0 ⳯ 10

ⳮ6

Die Materials and Die Manufacturing / 289

Table 21.19 Recommended carbide groups

[ICFG, 1992]

Carbide group

Tool part A B C D E F

Extrusion punch X X X X . . . . . .

Die insert . . . . . . . . . X X X

Heading punch . . . . . . X X X X

Heading die insert . . . . . . X X X X

Reducing die insert . . . . . . X X X X

Ironing die insert X X X X . . . . . .

Ironing punch X X X . . . . . . . . .

Table 21.18 Tool component materials [ICFG,

1992]

Forward rod extrusion

Backward extrusion

Punch Container Die insert Punch

Container or

die insert Counterpunch

6F7 O1 S1 A2 O1 O1

D2 6F7 6F7 D3 S1 D2

D3 D2 D2 H13 6F7 D3

M2 D3 6F2 M2 D3 H13

M3 M2 H13 M3 H13 M2

T42 M3 M2 T42 M2 M3

M3 M3

mal conditions. When considering the material

properties, the same approach can be applied as

in cold forging. Due to the importance of the

thermal properties, minimal applications exist

for carbide dies in warm forging; thus, tool steels

are the most applicable material.

21.5 Die Manufacture

Forging dies or inserts are machined from

solid blocks or forged die steels. By using stan-

dard support components such as die holders

and guide pins, which assure the overall func-

tionality of tooling assembly, the time necessary

for manufacturing a die set is reduced, and ma-

chining is mainly devoted to producing the cav-

ities or punches.

The information ﬂow and processing steps

used in die manufacturing may be divided into

die design (including geometry transfer and

modiﬁcation), heat treatment, tool path genera-

tion, rough machining (of die block and/or elec-

trical discharge machining, or EDM, electrode),

ﬁnish machining (including semiﬁnishing where

necessary), manual ﬁnishing, or benching (in-

cluding manual or automated polishing), and

tryout. An information ﬂow model (including

the use of a coordinate measuring machine but

neglecting heat treatment and coating) is pre-

sented in Fig. 21.9. As seen in this ﬁgure, forg-

ing dies are primarily manufactured by three- or

four-axis computer numerical controlled (CNC)

milling, EDM, or a combination of both.

21.5.1 High-Speed and Hard Machining

In a typical conventional die-making opera-

tion, the die cavity is usually rough machined to

about 0.01 in. (0.3 mm) oversize dimensions.

The die is then hardened, which may cause some

distortion, and then EDMed to ﬁnal dimensions.

The trend in die manufacturing today is toward

hard machining, both in roughing and ﬁnishing,

and in replacing EDM whenever possible. The

term hard refers to the hardness of the die ma-

terial, which is usually in the range of 45 to 62

HRC. By using hard machining, the number of

necessary machine setups is reduced and

throughput is increased. Recent technological

advances in high-speed machining of hardened

die steels make this trend feasible and econom-

ical [Altan et al., 2001].

The main object of high-speed machining of

hardened dies is to reduce benching by improv-

ing the surface ﬁnish and distortion; thus, quality

is increased and costs are reduced. High-speed

machining of hardened dies (40 to 62 HRC) has,

within an approximate range, the following re-

quirements and characteristics [Altan et al.,

2001]:

●

Feed rates: 50 ft/min (15 m/min) or higher

when appropriate-pressured air or coolant

mist is provided, usually through the spindle

●

Spindle rpm: 10,000 to 50,000, depending

on tool diameter

●

Surface cutting speeds: 985 to 3280 ft/min

(300 to 1000 m/min), depending on the hard-

ness of the die/mold steel and the chip load

●

High-speed control with high-speed data and

look-forward capability to avoid data star-

vation. The look-forward capability tracks

surface geometry, allowing the machine to

accelerate and decelerate effectively for

maintaining the prescribed surface contour.

●

High acceleration and deceleration capabil-

ities of the machine tool in the range of 2.6

to 3.9 ft/s

(0.8 to 1.2 m/s

)

Hard machining requires cutting tools that can

withstand very high temperatures and provide

long tool life. Most commonly used tools have

indexable cutting inserts from cemented carbide

coated with TiN, TiCN, and TiAlN; polycrys-

talline cubic boron nitride (PCBN); or cubic bo-

ron nitride.

290 / Cold and Hot Forging: Fundamentals and Applications

Fig. 21.9

Information ﬂow and processing steps in die manufacturing. CMM, coordinate measuring machine; CNC, computer

numerical control; EDM, electrical discharge machining

Ball-nosed end mills, from solid-coated car-

bide or with inserts, are commonly used in the

production of die cavities. The ball nose allows

the machining of complex curves and surfaces

in the die cavity. Using an appropriate step-over

distance and computer-aided tool-path genera-

tion, most cavities can be machined with ac-

ceptable surface ﬁnish.

In manufacturing cold forging dies from tool

steels, hard-turning PCBN tools are used. How-

Die Materials and Die Manufacturing / 291

Fig. 21.10

Schematic of electrodischarge machining [Altan

et al., 1983]

ever, in manufacturing carbide inserts, EDM

must be utilized.

21.5.2 Electrodischarge Machining (EDM)

The EDM process is a versatile process for

die manufacturing. The process consists of a

power supply that passes a current through an

electrode, creating a voltage potential between

it and a conductive workpiece. As the electrode

moves closer to the workpiece, the gap between

the two will become sufﬁciently small, and a

spark will pass from one material to the next.

The spark vaporizes portions of both the elec-

trode and the workpiece, and the removed ma-

terial is washed away by a dielectric ﬂushing

ﬂuid. There are two types of electrodischarge

machines: sink and wire.

The sink EDM (SEDM), as shown in Fig.

21.10, generates internal cavities by lowering a

graphite or copper electrode into the die block.

As the spark removes material, the workpiece

begins to take the form of the electrode. The

electrodes are most frequently created by CNC

milling. Advantages of the SEDM are that it cre-

ates a good surface ﬁnish; accuracy and repeat-

ability are high; and the hardness of the material

does not inﬂuence the efﬁciency of the process.

Therefore, the majority of die makers use EDM

mainly for ﬁnishing of dies from already rough-

machined and hardened die steels and for manu-

facturing of carbide die inserts.

The disadvantages of the SEDM process be-

gin with the lead time required to design and

manufacture the electrodes. Process concerns in-

clude electrode wear, material removal rate, and

particle ﬂushing. Electrode wear is caused, be-

cause the process consumes it in addition to re-

moving the workpiece material. This limits the

repeatability of the process. The EDM technol-

ogy functions on a much smaller scale than con-

ventional machining, which produces inherently

lower material removal rates. This reduces the

efﬁciency of the process. Typically, the SEDM

is used because the desired geometry requires

the technology. As the workpiece material is re-

moved, a ﬂushing ﬂuid is passed through the gap

to remove the particles. The ﬂuid also cools the

workpiece, the electrode, and the removed par-

ticles. Particles resolidify to the workpiece sur-

face during cooling and form a martensitic layer

referred to as the white layer. This surface is

composed of high tensile residual stresses from

the thermal cycle and may have numerous sur-

face cracks, which severely reduce the fatigue

life of the die material. The dielectric ﬂuid used

in the EDM process is ﬁltered to collect the par-

ticles ﬂushed during the process. The cost of dis-

carding this residue is high, because it is envi-

ronmentally hazardous.

In the United States, the large majority of the

electrodes are made from graphite. Graphite is

used because it is a soft material that can easily

be polished with sandpaper. When a better sur-

face ﬁnish is required, copper is selected for the

electrode material. Copper is the second most

common material used in the United States, and

more common in Europe and Japan. In addition

to an improved surface ﬁnish, copper electrodes

can produce much tighter tolerances.

The wire EDM (WEDM) functions in much

the same way as the SEDM. The primary dif-

ference is that the electrode is a wire ranging in

diameter from 0.002 to 0.012 in. (0.05 to 0.30

mm). Today, almost all machines are four-axis

centers in which the upper and lower wire guides

can move independently of one another in both

the x- and y-directions. In addition, the table that

holds the workpiece can move in the same di-

rections. Movement of the guides and worktable

is controlled by numerical-controlled program-

ming. In forging, WEDM is mainly used for

manufacturing trimming dies.

21.5.3 Hobbing

In hobbing, a hardened (58 to 62 HRC) punch

is pressed into an annealed, soft die steel block,

using a hydraulic press. The process may be cold

or at elevated temperatures (warm or hot hob-

bing). A single hobbing punch can be used to

manufacture a large number of cavities. This

process is particularly attractive for making dies

with shallow cavities or dies that can be hard-

ened after the cavity has been produced. Major

292 / Cold and Hot Forging: Fundamentals and Applications

Table 21.20 Relative ranking of various surface treatments and coatings [Krishnadev et al., 1997]

Surface treatment/coating Impact toughness Maximum exposure temperature Thermal checking resistance Wear resistance

Ion nitriding Low Same as base Medium Medium

CrN High 1380 F (750 C) Very high High

TiN High 840 F (450 C) Very high High

TiAlN High 1470 F (800 C) Very high High

TD-VC High 1470 F (800 C) Very high Very high

Laser High Same as base High High

PFS High Tailorable Not applicable High to very high

examples are coining dies and dies for hot and

cold forging of knives, spoons, forks, handtools,

etc.

21.5.4 Other Die-Making

Methods [Altan et al., 1983]

In addition to the methods discussed above,

there are a few other methods commonly used

for die making. Cast dies, although not exten-

sively used in practice, have been used success-

fully in some applications. This alternative may

be attractive where many dies of the same ge-

ometry are to be made. Special cases in which

cast dies are made cost-effective is isothermal or

hot-die forging. In this application, the dies are

made from nickel- and cobalt-base high-tem-

perature alloys. Because these alloys cannot be

machined easily, it is best to cast these dies and

obtain the ﬁnished die cavity geometry by EDM.

Electrochemical machining is similar to EDM

but does not use sparks for material removal.

Only direct current between the metal electrode

and the die steel is used for material removal.

This method is more efﬁcient than EDM in

terms of metal removal rate; however, electrode

wear is also quite large and, more importantly,

difﬁcult to predict. As a result, this method is

used for die making only in selected applica-

tions.

21.6 Surface Treatments

Sometimes, after the die cavities of hot forg-

ing dies are formed using EDM or high-speed

machining (HSM), the surface of the die is

treated with special methods to increase hard-

ness, die life, or surface ﬁnish. Some common

methods of surface treatments include physical

vapor deposition (PVD) (CrN, TiAlN, FUTURA

multilayer) and duplex treatments, which in-

clude PVD and plasma nitride welding of the

surface [Navinsek et al., 2001]. Physical vapor

deposition is a process that coats the surface of

the die with a hard, abrasive-resistant coating

processed at a relatively low temperature

(around 930 F, or 500 C). Plasma nitride sur-

face welding is a process that adds nitrogen to

the die cavity surface to increase its wear capa-

bilities.

Because these surface treatments can signiﬁ-

cantly increase (50 to 100%) the tool life of hot

forging dies, there exist certain drawbacks as

well. One drawback is the added operation to

the die-making process, which can be costly in

terms of both money and time. Another draw-

back is that having a surface treatment on the

die decreases its potential to be remachined.

This is especially a drawback when it comes to

HSM, because the surface treatments can be det-

rimental to tool inserts. Electrodischarge ma-

chining has a signiﬁcant advantage to HSM in

that even after coatings have been applied, it has

been shown that the die cavity can be resunk

using EDM.

The simultaneous need for high hardness to

resist wear and high ductility to prevent fracture

can be achieved using surface treatments. The

common coatings used in forging are reactive

coatings (nitriding or nitrocarburizing, boriding,

vanadizing, carburizing, and ion implantation)

and deposited coatings (hard chromium coating;

nickel plating; hard facing; chemical vapor de-

position coating, i.e., VC, WC, TiC, TiN, TiCN,

TiC/TiN, Cr

; PVD coating, i.e., TiN, TiCN,

TiAlN, CrN) [Krisnadev et al., 1997]. The qual-

ities of various surface treatment techniques are

given in Table 21.20

REFERENCES

[Altan et al., 1983]: Altan, T., Oh, S., and

Geghel, H., “Metal Forming: Fundamentals

and Applications,” American Society for

Metals, 1983.

[Altan et al., 2001]: Altan, T., Lilly B., Yen,

Y.C., “Manufacturing of Dies and Molds,”

Keynote Paper, Annals of the CIRP, Vol 50,

Feb 2001.

Die Materials and Die Manufacturing / 293

[Blau, 1992]: Blau, P., Friction and Wear of Or-

dered Intermetalic Alloys of Ni3Al, ASM In-

ternational, 1992.

[Bramley et al., 1989]: Bramley, A., Lord, J.,

and Davies, B., “Determination of Wear Re-

sistance of Hot Work Die Materials,” Annals

of the CIRP, Vol 38/1, 1989, p 231–234.

[ICFG, 1992]: “Objectives, History and Pub-

lished Documents,” ISBN 3-87525-058-3, In-

ternational Cold Forging Group, 1992.

[Johansonn et al., 1985]: Johansson, B., Jons-

son, L., et al., “Some Aspect in the Properties

of QRO 80M and Die Casting Die Perfor-

mance,” presented at the 13th International

Die Casting Congress and Exposition (Mil-

waukee, WI), North American Die Casting

Association, 1985.

[Kesavapandian et al., 2001]: Kesavapandian,

G., Ngaile, G., Altan, T., “Evaluation of Die

Materials and Surface Treatments for Hot Pre-

cision Forging,” ERC/NSM Report No. B/

ERC/NSM-01-R-42A, The Ohio State Uni-

versity, 2001.

[Krauss et al., 1998]: Roberts, G., Krauss, G.,

et al., Tool Steels, ASM International, 1998.

[Krishnadev et al., 1997]: Krishnadev, and

Jain, C., “Enhancing Hot Forging Die Life,”

Forging, winter 1997, p 67–72.

[Lange, 1976]: Lange, K., “Massive Forming,”

Textbook of Metal Forming, Vol. 2, Springer-

Verlag, New York, 1976 (in German).

[Maddox et al., 1997]: Maddox, G., and Orth,

J., “Performance of Nickel Aluminide Forg-

ing Dies,” Forging, winter 1997, p 75–78.

[Nagpal, 1976a]: Nagpal, V., “Selection of

Steels for Forging Dies,” Battelle, Columbus,

OH, 1976.

[Nagpal, 1976b]: Nagpal, V., Selection of Tool

Steels for Forging Dies, Battelle, Columbus,

OH, 1976.

[Nagpal et al., 1980]: Nagpal, V., and Lahoti,

G.D., “Application of the Radial Forging Pro-

cess to Cold and Warm Forging of Common

Tubes, Vol 1, Selection of Die and Mandrel

Materials,” Final report, Contract No.

DAAA22-78-C-0109, prepared for Watervliet

Arsenal by Battelle-Columbus Laboratories,

Columbus, OH, May 1980.

[Navinsek et al., 2001]: Navinsek, B., et al.,

“Improvement of Hot Forging Manufacturing

with PVD and DUPLEX Coatings,” Surface

and Coatings Technology, Vol 137, 2001, p

255.

[Roberts et al., 1980]: Roberts, G.A., and Rob-

ert, A.C., Tool Steels, American Society for

Metals, 1980.

[Semiatin et al., 1981]: Semiatin, S.L., and La-

hoti, G.D., “Forging Die Materials, Coatings,

and Surface Treatments,” Battelle, Columbus,

OH, 1981.

[Thyssen]: Thyssen, B., series of brochures

from Thyssen Company.

SELECTED REFERENCES

●

[Aston et al., 1969]: Aston, J., and Barry,

E., “A Further Consideration of Factors Af-

fecting the Life of Drop Forging Dies,” J.

Iron Steel Inst., Vol 210 (No. 7), July 1969,

p 520–526.

●

[Blackwood, 1973]: Blackwood, T.B., “The

Role of Tool Steels in Extrusion Tooling,”

SME Technical Paper MF73-565, Society of

Manufacturing Engineers, 1973.

●

[Bramley et al., 1975]: Bramley, A., Lord,

J., Beeley, P., “The Friction and Lubrication

of Solids,” Vol I, Oxford University Press,

London, 1975, p 38.

●

[Cerwin, 1995]: Cerwin, N., “Finkl Die

Handbook,” Vol 1–3 A. Finkl and Sons Co.,

1995.

●

[Cser et al., 1993]: Cser, L., Geiger, M.,

Lange, K., Hansel, M., “Tool Life and Tool

Quality in Bulk Metal Forming,” Proc. Instn.

Mech. Engr., Vol 207, 1993.

●

[Guobin et al., 1998]: Guobin, L., Jianjun,

W., Xiangzhi, L., Guiyun, L., “The Proper-

ties and Application of Bi-Metal Hot-Forg-

ing Die,” J. Mat. Proc. Tech., Vol 75, 1998,

p 152–156.

●

[Horton et al.]: Horton, J., Liu, C., et al.,

“Microstructures and Mechanical Properties

of Ni3Al Alloyed with Iron Additions,”

Oak Ridge National Laboratory, Oak

Ridge, TN.

●

[Klocke, 2000]: Klocke, F., “Fine Machin-

ing of Cold Forging Tools,” Proceedings,

Tenth ICFG Congress (Tagung Fellbach,

Germany), Sept 3–15, 2000, International

Cold Forging Group, p 175.

CHAPTER 22

Die Failures in Cold and Hot Forging

Mark Gariety

22.1 Introduction

In forging, die failure can be a signiﬁcant por-

tion of the overall production cost. The dies that

fail must be either repaired or replaced. The cost

to replace a die includes the basic cost of the die,

which encompasses the costs of material, ma-

chining, coating, and surface treatment as well

as the cost of labor. In any case, the most im-

portant cost of die failure is related to the down-

time of the manufacturing system, which re-

duces the overall productivity [Liou et al.,

1988].

Precision forging processes are utilized to

produce complex-shaped parts that require little

or no ﬁnishing, thereby reducing manufacturing

costs. In order to achieve both accuracy and

complexity in a forged part, the dies must be

produced with higher accuracy and tighter tol-

erances than the part to be forged. As a conse-

quence, the manufacturing time for the dies in-

creases considerably, making the dies more

expensive than those needed to produce parts

with conventional tolerances [Dahl et al., 1998,

and Dahl et al., 1999].

Complex precision-forged parts are almost

exclusively produced in closed dies. Therefore,

the dies have to withstand high contact pressures

well above the ﬂow stress of the billet material

at the forging temperature. Also, the amount of

surface generation and the sliding velocities at

which the billet material moves along the die

surface are very large. In addition, in hot forging

operations, the die surface is subjected to sudden

changes in temperature. These changes are

mainly due to contact with the hot workpiece

and the cooling and lubrication practices. The

high pressures and sliding velocities as well as

the sudden changes in temperature may induce

die failure due to several mechanisms. When a

failure occurs, the production must stop to re-

place or repair the die. Consequently, the pro-

duction cost will be signiﬁcantly reduced if the

die life can be increased.

22.2 Classiﬁcation of Die Failures

In forging, die failures may be classiﬁed by

one of the following three failure modes:

●

The most common cause of die failure is

wear. Die wear occurs as the result of the die

and the workpiece sliding relative to one an-

other while in contact. Typically, die wear is

thought of as removal of material from the

die surface, but it also may include buildup

of material on the die surface or damage to

the die surface. Die wear results in the grad-

ual loss of part tolerances. Eventually, the

part tolerances will not meet the customer’s

speciﬁcations, and a new die or die insert

will need to be manufactured.

●

The second most common cause of die fail-

ure is fatigue fracture. Fatigue occurs as the

result of the continual stress cycles that the

dies are subjected to. The stress cycles are

attributed to both mechanical and thermal

loading and unloading of the dies. Fatigue is

accelerated in the vicinity of stress concen-

trations, such as small radii.

●

The third cause of die failure is plastic de-

formation. Plastic deformation results from

forming pressures that exceed the yield

strength of the die material.

Cold and Hot Forging Fundamentals and Applications

Taylan Altan, Gracious Ngaile, Gangshu Shen, editors, p295-317

DOI:10.1361/chff2005p295

www.asminternational.org

296 / Cold and Hot Forging: Fundamentals and Applications

Fig. 22.2 Adhesive wear mechanism [Bay, 2002]

Fig. 22.1

Typical failure modes and locations in precision-

forging die [Schey, 1983]

Figure 22.1 illustrates a typical precision-

forging die and the locations where these various

failures may occur. In hot forging, die wear is

responsible for nearly 70% of die failures

[Schey, 1983]. In cold forging, die failure is pri-

marily the result of fatigue fracture. Thus, the

remainder of this chapter focuses solely on wear

and fatigue fracture, which are the primary fail-

ure modes in hot and cold forging, respectively.

22.3 Fracture Mechanisms

In order to maximize die life, it is important

to understand the primary mechanisms of fa-

tigue fracture, summarized as follows:

●

Mechanically induced fatigue fracture oc-

curs when a crack is induced and propagates

through a die due to mechanical loading.

Crack initiation occurs if the tool load ex-

ceeds the yield strength of the tool material

in the area of a stress concentration, and a

localized plastic zone forms. This zone gen-

erally forms during the ﬁrst loading cycle

and undergoes plastic cycling during subse-

quent unloading and reloading. The plastic

cycling leads to the initiation of microscopic

cracks. These microscopic cracks may grow

and lead to crack propagation into the cross

section of the tooling if they are subjected to

tensile stresses during loading [Knoerr et al.,

1994].

●

Thermally induced fatigue fracture occurs

when a crack is induced and propagates

through a die due to thermal loading. Ther-

mal-induced fatigue fracture is not as com-

mon as mechanically induced fatigue frac-

ture, because the bulk die temperature does

not ﬂuctuate drastically during typical metal

forming operations.

22.4 Wear Mechanisms

In order to maximize die life, it is also im-

portant to understand the primary mechanisms

of wear. In metal forming, there are four primary

mechanisms by which wear may occur. These

mechanisms, summarized below, work simulta-

neously and may be difﬁcult to distinguish in an

actual die:

●

Adhesive wear occurs when two surfaces

slide relative to one another, and material

from one surface is sheared and remains ad-

hered to the other surface (Fig. 22.2). When

there is metal-to-metal contact between the

surface asperities of the workpiece and the

die, some small portion of the workpiece sur-

face may be sheared and remain attached to

the die surface [Schey, 1983].

●

Abrasive wear, sometimes called “plowing,”

occurs when material is removed from a soft

surface due to interaction with a hard asper-

ity on another surface (two-body abrasion)

or due to interaction with hard, loose parti-

cles trapped between two surfaces (three-

body abrasion) (Fig. 22.3). A major concern

is the abrasive wear of the die by hard oxides

such as scale or intermetallics on the surface

of the workpiece.

●

Fatigue wear occurs when small cracks form

on the surface or subsurface of the die and

subsequently break off in the form of small

fragments, thus leaving small voids in the die

surface (Fig. 22.4). This phenomenon is a

result of the repeated mechanical loading

and unloading that dies are subjected to. In

hot forging, the repeated thermal loading and

unloading also contributes to fatigue wear

and is often called “heat checking.” In gen-

eral, fatigue wear is affected by surface de-

fects that act as stress-concentration points,

surface ﬁnish, residual stresses, and the lu-

bricant chemistry. For example, water-based

lubricants may accelerate fatigue wear due

to hydrogen embrittlement.

Die Failures in Cold and Hot Forging / 297

Fig. 22.3 Two-body and three-body abrasive wear mechanisms [Bay, 2002]

●

Corrosive/chemical wear occurs when sur-

face ﬁlms (i.e., oxidation) are removed as a

result of the relative sliding between the die

and the workpiece in a corrosive environ-

ment (Fig. 22.5). In general, corrosive wear

is accelerated by the use of lubricants. How-

ever, this level of wear is generally accepted

in order to prevent the more severe forms of

adhesive wear that would result if lubricants

were not used.

22.5 Analytical Wear Models

Some of the earliest analytical wear models

were developed by Holm (1940s), Archard

(1950s), and Rabinowicz (1960s). In general,

these models characterized adhesive and abra-

sive wear as follows:

K • P • S

V ⳱ (Eq 22.1)

where V is the wear volume, K is the experi-

mental wear coefﬁcient, P is the normal pres-

sure, S is the sliding length, and H is the hard-

ness of the softer material. Thus, it was

recognized very early that adhesive and abrasive

die wear could be minimized by minimizing the

wear coefﬁcient, the normal pressure, and the

sliding length and by maximizing the die hard-

ness.

Approximate values of the experimental wear

coefﬁcient, K, have been determined for many

die/workpiece material combinations. For ad-

hesive wear, K

adh

varies based on the material

pair and the lubricant used [Bay, 2002]. Table

22.1 summarizes these values. For abrasive

wear, K

abr

depends on the size, shape, and ori-

entation of the asperity/particle interacting with

the die surface and usually varies from 0.02 to

0.2 for two-body abrasive wear and 0.001 to

0.01 for three-body abrasive wear.

22.6 Parameters

Inﬂuencing Die Failure

There are numerous parameters that inﬂuence

die failure in cold and hot forging processes. The

interaction of these parameters is complex. They

may increase or decrease die failure, and the

trends are important to understand in order to

maximize die life. These parameters are dis-

cussed in the following sections and shown in

ﬂow chart form in Fig. 22.6.

22.6.1 Incoming bar

Billet Material. The ﬂow stress of the work-

piece material inﬂuences the normal pressure at

the die/workpiece interface. Considering that

wear is proportional to interface pressure, a ma-

terial with a lower ﬂow stress will produce less

pressure at the die/workpiece interface and thus

less die wear [Dahl et al., 1998].

In addition, adhesive wear is inversely pro-

portional to the workpiece material hardness.

Therefore, a material with a high hardness (but

less than die hardness) will produce less adhe-

sive die wear [Dahl et al., 1998].

Billet Surface. The scale present on the work-

piece surface inﬂuences the interface conditions.

It acts as an insulator and thus protects the die

against increased heating and thermal fatigue,

but if it is hard and brittle, it causes abrasive

wear. The properties of the scale are inﬂuenced

by billet heating time and temperature [Dahl et

al., 1998, and Schey, 1983].