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

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298 / Cold and Hot Forging: Fundamentals and Applications

Fig. 22.4 Fatigue wear mechanism [Bay, 2002]

Fig. 22.5 Corrosive wear mechanism [Bay, 2002]

Table 22.1 Summary of wear coefﬁcient, K

adh

for adhesive wear [Bay, 2002]

Surface condition Like metallic pairs Unlike metallic pairs

Clean 5 ⳯ 10

ⳮ3

2 ⳯ 10

ⳮ4

Poor lubrication 2 ⳯ 10

ⳮ4

2 ⳯ 10

ⳮ4

Average lubrication 2 ⳯ 10

ⳮ5

2 ⳯ 10

ⳮ5

Excellent lubrication 2 ⳯ 10

ⳮ6

to 10

ⳮ7

2 ⳯ 10

ⳮ6

to 10

ⳮ7

Billet Geometry. The geometry of the billet

inﬂuences the amount of sliding that will take

place during the forging process. Because abra-

sive and adhesive wear are proportional to slid-

ing length, the use of preforms where die wear

is a concern will increase die life. A billet or

preform that requires a small amount of defor-

mation can be forged with a lower load (pres-

sure) and a shorter contact time. In large-volume

production, most of the forging deformation is

completed in preform or blocker dies. Thus,

there is little metal ﬂow in the ﬁnisher die, which

is used for coining in order to reduce the die

wear and maintain part tolerances [Dahl et al.,

1998, and Schey, 1983].

Billet Weight and Weight Tolerances. Die

wear increases with increasing billet weight.

This is likely the effect of increased interface

pressure and contact time. In addition, if the bil-

let weight exceeds the weight tolerances (i.e.,

the volume of the billet is larger than the volume

of the die cavity), the die will ﬁll prematurely,

causing increased interface pressures and thus

increased wear and decreased fatigue life [Dahl

et al., 1998].

Die Failures in Cold and Hot Forging / 299

Fig. 22.6 Process parameters leading to die failure [Artinger, 1992]

22.6.2 Billet Separation

Separation Method. The separation method

inﬂuences the obtainable weight tolerances. The

effect of exceeding the weight (volume) toler-

ance was discussed previously. In general, crop-

ping and sawing are the primary methods by

which billets are separated. Cropping is known

to produce a geometrically inferior billet to saw-

ing. This method results in more surface and

edge defects (i.e., cracks and burrs) than sawing

[Dahl et al., 1998].

Sorting of billets either by weight or size can

help to extend the useful life of the dies. Billets

can be sorted into two or more groups based on

weight or size. The smaller billets should be

forged ﬁrst. As the dies begin to wear and the

cavities become larger, larger billets may be in-

troduced. The largest billets are forged at the end

of the die life when the cavity is the largest. This

technique helps to reduce the risk of excessive

interface pressures generated as a result of pre-

mature die ﬁll [Dahl et al., 1998].

Edge Quality. Angularity and other geomet-

rical imperfections that result from billet sepa-

ration can cause uneven loading of the dies. If

the imperfections are consistent in the billet sep-

aration process, the uneven loading becomes a

repeated problem occurring at every stroke of

the press. This may cause high local wear [Dahl

et al., 1998].

22.6.3 Billet Heating

The billet heating process must be closely

controlled in order to keep the billets in the op-

timum forging temperature range and to reduce

scale formation. As discussed previously, hard,

abrasive scale can be a major source of die wear.

Scale may accumulate on the billet surface as a

result of burning during the heating process and

excessive transportation times between heating

and forging. Coatings may be applied to the bil-

lets before heating in order to reduce scale for-

mation [Dahl et al., 1998].

22.6.4 Forging Equipment

The press or hammer type inﬂuences the

length of the contact time. Increased contact

times result in increased wear. The reason for

this is that the longer the die is in contact with

the hot billet, the more the temperature of the

dies increases and the temperature of the billet

decreases. The increased die temperatures cause

decreased die hardness due to thermal softening.

Therefore, abrasive wear increases. In addition,

the decreased billet temperatures cause in-

creased ﬂow stress, which leads to increased in-

terface pressures. This also leads to increased

wear. If the die experiences large temperature

oscillations due to long contact times and sub-

300 / Cold and Hot Forging: Fundamentals and Applications

Fig. 22.8

Heat-transfer coefﬁcient and temperature changes

in a typical hot forging operation (A, heated billet

resting on lower die; B, contact time under pressure; C, forging

removed from lower die; D, lubrication of die; E, dwell time with

no billet on lower die before next cycle begins) [Knoerr et al.,

1989]

Fig. 22.7

Contact time components in forging processes

[Dahl et al., 1998]

sequent lubrication/cooling, thermal fatigue

wear may also be a concern [Dahl et al., 1998,

and Schey, 1983].

In general, hydraulic presses transfer energy

from the ram to the workpiece and die slower

than other press types. Thus, the contact time is

longer. Hammers typically transfer energy very

quickly. Thus, the contact time is shorter; how-

ever, fatigue fracture is a major concern for dies

with low toughness. In addition, the stiffness of

the press inﬂuences the contact time. If a press

has low stiffness, there is more elastic deﬂection

during loading. As a result, the contact time is

longer [Kesavapandian et al., 2001].

The press (ram) speed inﬂuences the relative

sliding velocity between the billet and the dies.

Increased sliding velocity results in increased

strain rates, which cause increased ﬂow stress

and thus increased wear and decreased fatigue

life. However, if the press speed it too slow, long

contact times may become important [Dahl et

al., 1998].

The effect of the press (ram) speed and the

contact time is extremely important for die life.

The longer the contact time between the billet

and the dies, especially under pressure, the

shorter is the die life. It is useful to divide the

total contact time, t

, in precision forging into

its various components, as follows (using forg-

ing in a vertical press as an example) (Fig. 22.7)

[Dahl et al., 1998]:

t ⳱ t Ⳮ t Ⳮ t Ⳮ t Ⳮ t (Eq 22.2)

T12345

where:

●

⳱ rest time. (The heated billet or preform

is placed on the lower die. The pressure at

the die/workpiece interface is due to the

workpiece weight only.)

●

⳱ initial dwell time under pressure. (The

upper die touches the workpiece. The ram

stops for a very short time while the pressure

builds up. This is the case in some hydraulic

presses, or when a mechanical press ram has

some excessive clearances in the eccentric

bearings, or the lift-up cylinders are not

functioning properly.)

●

⳱ contact time under pressure. (Defor-

mation occurs during this time. This time is

inﬂuenced by the press stiffness. The effect

of elastic deﬂection, i.e., press stiffness, is

especially important in mechanical and

screw presses.)

●

⳱ ﬁnal dwell time under pressure. (The

forging stroke is completed. The ram is at

bottom dead center (BDC). The ram must be

lifted upward. This dwell time is due to elas-

tic deﬂection of the press in mechanical and

Fig. 22.9 Temperature-hardness curve [Dahl et al., 1999]

Die Failures in Cold and Hot Forging / 301

screw presses and the time necessary to re-

verse the hydraulic pressure to lift up the ram

in some hydraulic presses.)

●

⳱ ﬁnal dwell time without pressure. (The

upper ram has lifted, but the forging is still

in the die before it is removed manually or

lifted out by a knockout mechanism.)

As shown in Fig. 22.8, the heat-transfer co-

efﬁcient at the die/workpiece interface is differ-

ent for the different contact time components.

22.6.5 Forging Dies

Die Material and Heat Treatment. The

strength (i.e., ﬂow stress), toughness, resistance

to thermal softening, and hot hardness of the hot

forging die material inﬂuence its wear resis-

tance. Recall that abrasive and adhesive wear are

inversely proportional to the strength/hot hard-

ness of the die material. In addition, good tough-

ness is also important for resistance to fatigue

wear/fatigue fracture. Because toughness is a

Fig. 22.10 Fatigue analysis method [Knoerr et al., 1994]

302 / Cold and Hot Forging: Fundamentals and Applications

function of strength and ductility, it usually in-

creases with increasing temperature [Altan et al.,

1983, and Dahl et al., 1998]. A typical tempera-

ture-hardness curve is shown in Fig. 22.9. Ad-

ditional information on die materials is given in

Chapter 21, “Die Materials and Die Manufac-

turing,” in this book.

Alloying elements such as chromium, tung-

sten, vanadium, and molybdenum are typically

employed in hot forging dies in order to improve

the wear resistance of the die. In particular, mo-

lybdenum is responsible for resistance to ther-

mal softening at hot forging temperatures. Va-

nadium improves the resistance to abrasion and

thermal fatigue. Tungsten increases toughness

(resistance to mechanical fatigue) and resistance

to thermal softening [Dahl et al., 1998, and Tul-

syan et al., 1993].

In addition, the microstructure resulting from

heat treatment inﬂuences die wear. It has been

shown that microstructure type, grain size, uni-

formity, and the number of microcracks affect

die wear [Shivpuri et al., 1988].

The thermal properties of the die material also

inﬂuence its wear resistance. High thermal con-

ductivities eliminate the large thermal gradients

that lead to thermal fatigue wear by carrying the

Fig. 22.11

Fatigue fracture in forward extrusion die [Lange

et al., 1992a]

Fig. 22.12 Forward extrusion die cross section used by Hettig for fatigue failure investigation [Hettig, 1990]

Table 22.2 Test conditions and results of Hettig’s fatigue failure investigation along with fatigue

analysis results [Knoerr et al., 1994]

Transition

radius (R)

Case Insert material Hardness, HRC

Die opening angle (2␣),

degrees in. mm

Experimental die life,

parts

Predicted die life,

parts

I AISI M2 61 120 0.04 1 50–400 280

II AISI D2 60 120 0.04 1 65–200 210

III AISI D2 60 90 0.04 1 900–1000 950

IV AISI D2 60 90 0.10 2.5 10,000–11,000 10,500

Die Failures in Cold and Hot Forging / 303

heat away from the die surface more quickly.

Low thermal expansion rates reduce stresses in-

duced by dimensional changes in the die at high

temperatures [Dahl et al., 1998].

Surface Treatments. Because dies simulta-

neously require high hardness to prevent wear

and high toughness to prevent fatigue fracture,

many dies incorporate a surface treatment, such

as nitriding or boriding in hot forging or a coat-

ing such as TiN or TiAlN in cold forging, that

improves the hardness of the surface while leav-

ing the bulk of the die relatively soft [Dahl et

al., 1998].

Die Design. In hot forging, die design param-

eters, such as ﬂash geometry, ﬁllet radii, draft

angles, and die face contact area, inﬂuence die

wear and fatigue life. It has been found that die

wear decreases and fatigue life increases with an

increase in ﬂash thickness, because the contact

stresses between the die and the ﬂash decrease.

In addition, die wear increases and fatigue life

decreases with an increase in ﬂash-metal escape,

because higher contact stresses are produced by

the higher loads required to deliver higher ﬂash-

metal escape rates [Aston et al., 1969].

As ﬁllet radii increase, die wear decreases and

fatigue life increases, because small radii intro-

duce stress concentrations. Increasing draft an-

gle decreases die life, because higher draft an-

gles require higher loads to ﬁll the die cavity

[Knoerr et al., 1989].

Die Manufacturing. The method used to

manufacture the die inﬂuences the surface char-

acteristics and thus the wear of the die. For ex-

ample, electrical discharge machining typically

produces a very hard and brittle surface that has

been shown to be more wear resistant than those

produced with other manufacturing methods

[Knoerr et al., 1989].

Surface Finish. Generally, the rougher the

surface, the more wear is produced. This results

because the number of asperities in contact is

less for a rough surface. Thus, greater loads per

asperity are generated on rough surfaces. How-

ever, in cases where the surfaces are very smooth

and, more importantly, very clean, adhesive

Fig. 22.13 (a) Plastic zone at the transition radius. (b) Tensile maximum principal stress at transition radius [Knoerr et al., 1994]

Fig. 22.14

Fatigue fracture in backward extrusion of con-

stant velocity joints [Nagoa et al., 1994]

304 / Cold and Hot Forging: Fundamentals and Applications

wear is likely to occur unless coatings are used

[Tulsyan et al., 1993].

22.6.6 Lubrication

Die Lubricant Type. The lubricant type in-

ﬂuences the interface pressure and the heat

transfer between the die and the billet. In gen-

eral, decreasing lubricity results in increased in-

terface pressure and thus increased die wear and

decreased fatigue life. Also, the lubricant acts as

an insulator in order to protect the die against

extreme temperature changes and thermal fa-

tigue wear [Schey, 1983, and Dahl et al., 1998].

Mode of Application. In hot forging, the lu-

bricant application parameters, i.e., spray time,

spray angle, spray distance, and ﬂow rate, inﬂu-

ence how the lubricant covers the die. Inade-

quate lubrication results in increased interface

pressure and reduced insulation against extreme

die-temperature changes. However, excessive

lubricant can result in the buildup of solid lu-

bricant particles in die cavities. This buildup

may cause premature die ﬁll and increased in-

terface pressure [Dahl et al., 1998].

22.6.7 Process Conditions

Die Temperature. The die temperature inﬂu-

ences the hardness of the die. In general, the die

hardness is inversely proportional to the die tem-

perature. Thus, because abrasive and adhesive

Fig. 22.15 (a) Plastic zone at the cup bottom. (b) Tensile maximum principal stress at cup bottom [Nagao et al., 1994]

Fig. 22.16 (a) Plastic zone at the ﬂange area. (b) Tensile maximum principal stress at ﬂange area [Nagao et al., 1994]

Die Failures in Cold and Hot Forging / 305

die wear are inversely proportional to die hard-

ness, die wear is proportional to die temperature,

which is signiﬁcantly affected by contact times

[Dahl et al., 1998, and Schey, 1983].

Billet Temperature. The billet temperature

inﬂuences the ﬂow stress of the material. As dis-

cussed previously, increased ﬂow stress results

in increased interface pressure and thus in-

creased wear and decreased fatigue life [Dahl et

al., 1998 and Schey, 1983]. In addition, the billet

temperature inﬂuences the amount of scale on

the workpiece surface, i.e., the amount of scale

typically increases with increasing temperature

[Tulsyan et al., 1993].

Sliding Velocity. Increasing the sliding ve-

locity between the die and the workpiece in-

creases die wear. In general, the relative sliding

between the die and workpiece creates heat. The

higher the sliding velocity, the more heat is pro-

duced. This heating lowers the hardness of the

die [Tulsyan et al., 1993].

Number of Forging Operations. While the

exact number of forging operations is not im-

portant, what is important is that there are

enough operations so that the severity of the de-

formation at one station is not excessive. Exces-

sive deformation in one die cavity causes large

sliding between the billet and the die, which re-

sults in increased die wear. Therefore, appropri-

ate and economic preforming operations should

Fig. 22.17

Deformation of the inserts under load for the straight container (original design) and the proﬁled container (optimum

design) [Nagao et al., 1994]

Fig. 22.18

Knockout pin design and insert failure point

[Hannan et al., 2001]

306 / Cold and Hot Forging: Fundamentals and Applications

be used, especially just prior to the ﬁnal forging

operation where wear rates must be kept low to

maintain desired tolerances [Dahl et al., 1998].

Transfer Time. The transfer time, i.e., amount

of time required to transfer the billet from the

furnace to the press, inﬂuences the temperature

of the billet. Cooling of the billet will raise the

ﬂow stress of the billet. As discussed previously,

increased ﬂow stress results in increased inter-

face pressure and thus increased wear and de-

creased fatigue life [Dahl et al., 1998].

Cycle Time/Production Rate. If the heated

billet is allowed to rest on the die for a long

period of time before deformation occurs, the

billet will cool and the die will heat. Thus, wear

will be increased, because the ﬂow stress of the

billet will be increased and the hardness of the

dies will be decreased [Dahl et al., 1998]. Large

production rates in hot forging will increase the

overall temperature of the dies, i.e., in high-

speed horizontal hot forging machines. In such

cases, large amounts of coolants and lubricants

Fig. 22.19 FEM model of knockout pin [Hannan et al., 2001]

Fig. 22.20 Tensile maximum principal stress in failure area [Hannan et al., 2001]

Die Failures in Cold and Hot Forging / 307

are needed in order to reduce die temperatures.

Alternatively, it is also possible to leave one

forging station intermittently empty in order to

provide time for the dies to cool.

In order to produce an economically sound

part with prolonged die life by either cold or hot

forging, a multitude of variables, as discussed

above, must be carefully considered. In partic-

ular, the effect and importance of each variable

based on the nature of the given component must

be assessed.

22.7 Prediction of Die Fatigue

Fracture and Enhancement of

Die Life in Cold Forging Using

Finite-Element Modeling (FEM)

A reliable method to analyze, predict, prevent,

and/or control die fatigue fracture has long been

one of the most important issues in cold forging.

Therefore, a fatigue analysis method that can be

utilized to estimate tool life has been developed

and can be summarized as follows (Fig. 22.10)

[Knoerr et al., 1994, and Matsuda, 2002]:

●

The forging process is simulated using FEM

in order to estimate the tool stresses.

●

The tool stress values are used to complete

an elastic-plastic stress-strain analysis of the

tooling using FEM.

●

The stress-strain analysis is used to perform

a damage analysis and estimate the number

of cycles until fatigue fracture.

If the predicted tool life is insufﬁcient,

changes in the process and tooling design must

be made in order to reduce the loading condi-

tions. A signiﬁcant increase in tool life can be

achieved by reducing the stresses in the highest

loaded zone below the yield strength of the tool

material. This may be achieved by the following

means [Knoerr et al., 1994]:

●

Change material ﬂow in the die to reduce the

contact stresses on the tool.

●

Redesign the process to avoid drastic

changes in the direction of the material ﬂow,

which usually leads to peaks in the contact

stress.

●

Increase the transition radii to reduce the

notch effect.

●

Split the tooling at the highest loaded zone.

●

Increase the interference of the stress ring.

●

Apply advanced stress ring techniques, such

as strip-wound containers or proﬁled stress

rings.

Several case studies where this fatigue anal-

ysis method was employed are presented as fol-

lows.

22.7.1 Forward Extrusion—A Case Study

In forward extrusion, fatigue cracks initiate at

the transition radius to the extrusion shoulder

and propagate in the radial direction (Fig.

22.11).

In order to verify the fatigue analysis method

for evaluating tool life, an experimental inves-

tigation of fatigue fracture of forward extrusion

dies, performed at the Institute for Metal Form-

ing (IFUM) at the University of Stuttgart, was

used [Hettig, 1990]. Figure 22.12 shows the

cross section of the forward extrusion die, and

Table 22.2 summarizes the test conditions and

the results obtained. In addition, Table 22.2 sum-

marizes the results obtained from the fatigue

analysis for the same test conditions. As the ta-

ble shows, the predicted die lives were in the

range of those found experimentally. Thus, the

fatigue analysis method was veriﬁed.

Table 22.2 illustrates that the die opening an-

gle and the transition radius have a large effect

on the die life. With FEM-based fatigue analy-

sis, this effect is noted quickly and economi-

cally. Also, the FEM-based fatigue analysis eas-

ily determines the location where fatigue

fracture will occur. Figure 22.13 shows the local

plastic zone at the transition radius found during

the FEM-based stress-strain analysis of the die.

Figure 22.13 also shows that the maximum prin-

cipal stresses in this same region are tensile,

Fig. 22.21

Split knockout pin insert design [Hannan et al.,

2001]