Altan T. Metal Forming Handbook

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6.7.2Die and punch design

Dies are subjected to high degrees of internal pressure which can quick-

ly lead to die failure using single ring extrusion containers manufac-

tured with die steels available today. The extremely high stresses, pre-

sent on the internal die wall of the extrusion container (cf. Sect. 6.5.1)

can be reduced by pre-stressing (shrink-fit) the container with one or

more shrink rings. Punches are subjected initially to pressure and then

additionally, depending on the process, to bending stress as a result of

off-center loads. Less frequently, the possibility of punch buckling must

be taken into consideration. The calculation methods outlined here for

dies and punches are described in detail in VDI Guidelines 3176, VDI

3186 and ICFG Doc. No. 5/82.

491

Die design

Fig. 6.7.5 Shearing die with wedge-action pusher, holding sleeve hydraulically clamped

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Die design

In Fig.6.7.6,various shrink ring designs are illustrated. Cylindrical

shrink fits are simpler to produce, but can only be used up to interfer-

ences which can be achieved by warm shrink fitting, in order to avoid

cold welds during assembly.

Tapered shrink fits permit simpler, scratch-free disassembly. A draw-

back of this method are the high production costs. The tapered shrink

fit exhibits different levels of radial pre-stress, which can lead in case of

very long containers to dimensional differences on the forged part.

Usual taper angles are 0.5 and 1°; if the height to diameter ratio of the

container is below 0.8, taper angles of 2 to 3°are used in order to avoid

disassembly of the shrink rings during part ejection. Analytic approxi-

mate operations are based on the method by LAMÉ and were published

by Adler/Walter. These assume an ideal loading situation where an infi-

nitely long thick-walled cylinder is subjected to a constant hydrostatic

pressure over its entire length. Using Tresca’s yield condition, which can

be applied in this case, the stress distribution can be calculated. It is

found that the maximum values are at the internal wall of the contain-

er and correspond approximately to the value of the internal pressure p

492

Solid forming (Forging)

Fig. 6.7.6 Shrink fits with single and double rings – each with cylindrical and tapered fits

cylindrical shrink

fit surface

cylindrical shrink

fit surface

tapered shrink

fit surface

tapered shrink

fit surface

single shrink ring double shrink ring

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

for radial stress s

and tangential stress s

(Fig. 6.7.7, left). The internal

pressure can therefore, due to

amount to no more than , for the effective stress s

= R

p0.2

. Thus,

for material values of R

p0.2

= 2,000N/mm

, maximum pressure levels

of 1,000 N/mm

are permissible. If, in the case of thick-walled pipes

subjected to internal pressure, the effective stress exceeds the yield

strength of a material with sufficient toughness, then plastic flow

occurs at the inside wall of the container. If the ultimate strength is

reached in containers or inserts from brittle materials, cracks occur.

As a result of adding compressive stress, the tangential stress and

thus also the effective stress status at the inside wall of the extrusion

container (or insert) are reduced (Fig. 6.7.7center, right).

The radial pre-stress is generated by means of shrink rings. A shrink

ring has an internal diameter that is smaller than the outer diameter of

the corresponding inner ring by a selected dimension (interference). By

maintaining the outer diameter unchanged, the permissible internal

pressure can be increased by up to 100% as a result of the shrink ring

compared to a container without a shrink ring. For a given permissible

internal pressure, the outside diameter of the container, using shrink

493

Die design

Fig. 6.7.7 Theoretical stress distribution in a thick-walled single-piece hollow cylinder

(left),

a two-part

(center)

and three-part

(right)

shrink fit design (red: without internal

pressure, blue: with internal pressure)

2.5

2.0

1.5

1.0

0.5

-0.5

-1.0

σσσ

vrti

p=+=2

p02

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

rings, can be reduced by about 60% (Table 6.7.2). In multiple-station

forming machines this results in a substantial reduction of die diame-

ters, the distances between stations, the dimensions of the die holder

and thus also the machine bed size.

Carbide cannot be subject to tensile stresses. Therefore, when using

dies or inserts from carbide, the extrusion container must be pre-

stressed in such a way that no tangential tensile stress occurs under

internal pressure. Accordingly, the interference dimensions are consid-

erably higher than those used for tool steels. Guidelines also have been

established for the calculation of axial pre-stress levels; in the thin, ring-

shaped lateral split, surface pressures of 700 to 1,000 N/mm

are as-

sumed as guideline values.

In extrusion dies, the actual stress conditions differ from the ideal

assumptions made here, however. The extrusion containers have a

finite length, a limited and possibly eccentric pressure area, no uniform

internal pressure exists, and the containers have often off-center die

openings. Realistic conditions in the configuration of shrink fit assem-

blies can be determined by using discrete approximation techniques,

for example the finite element method (FEM). For practice-oriented

application, various calculations have been conducted by making a

parametric study using FEM. These results have been made accessible

for practical application in the form of Nomograms (VDI 3176, ICFG

Doc. 5/82) and corresponding computer programs.

Punch design

Extrusion punches are subjected to an axial force with an average com-

pressive stress. They are calculated as follows:

494

Solid forming (Forging)

Table 6.7.2: Increase of the allowable internal pressure p

i zul.

or reduction of the outside diameter d

through single and double shrink rings

Characteristic value Press container

without shrink ring with single shrink ring with double shrink ring

i zul.

[N/mm

] 690 1,160 1,395

[mm] with d

= 20 mm 100 40 36.4

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

The off-center applied force causes additional bending stresses. The

overall stress in the punch then amounts to:

whereby d [mm] is the punch diameter and e [mm] the off-center posi-

tion of force application. Values for critical buckling stress are provided

in Fig.6.7.8. With backward cup extrusion, the eccentricity generally

amounts to e/d = 0.01 to 0.02. The calculated punch stress is compared

to the compressive yield strength of the punch material in its hardened

state.

The counterpunch is subjected to an average compressive stress of

during backward cup extrusion.

495

Die design

Fig. 6.7.8Critical buckling stress in function of the slenderness ratio (l/d) of a punch made of

high-speed steel: 1 forward rod extrusion; 2 backward cup extrusion; d punch dia-

meter;e off-center position of force application; l (free) punch length

1,000

2,000

3,000

024681012

0.1

0.05

0.01

e/d=0

forward

rodextrusion

backward

cupextrusion

=3,000N/mm

E=225,000N/mm

Stges St

ped=+⋅

()

18/

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

6.7.3Die and punch materials

Materials used for cold and warm forming are required to have high

strength, high toughness and high hardness. For an economical die life,

extremely good wear and tempering resistance are necessary. Die mate-

rials should be chosen, where possible, to ensure good machinability,

and as a result, low manufacturing costs (cf. Sect. 4.1.3).

All these requirements cannot be fulfilled simultaneously by each

type of tool steel. The characteristics wear resistance and toughness, for

example, show opposing tendencies. The materials 55NiCrMoV6

(1.2713), 57NiCrMoV7 (1.2714) and X3NiCoMoTi1895 (1.2709) are

extremely tough and are used for shrink rings and inserts which are

subjected to high levels of elongation. Their wear characteristics, in

496

Solid forming (Forging)

Table 6.7.3: Survey of the most frequently used cold, warm and hot forging tool steels

Die materials I / Cold forging tool steels / Active components

Material No. DIN ANSI JIS Composition in %

to DIN Germany USA Japan C Si Mn P S Co Cr Mo Ni V W Ti

1.2363 X100CrMoV51 A2 SKD11 1.00 0.30 0.55 0.03 0.03 – 5.00 1.10 – 0.20 – –

1.2369 81M0CrV4216 0.81 0.25 0.35 – – – 4.00 4.20 – 1.00 – –

1.2379 X155CrVMo12 1 D2 SKD11 1.55 0.30 0.35 0.03 0.03 – 12 0.70 – 1.00 – –

1.2709 X3NiCoMoTi1895 0.03 0.10 0.15 0.01 0.01 9.25 0.25 5.00 18 – – –

1.27131. NiCrMo55NiCrMoV6 6F2 0.55 0.30 0.60 0.03 0.03 – 0.70 0.30 1.70 0,10 – –

1.2714 57NiCrMoV7 SKT4 0.58 0.30 0.70 0.03 0.03 – 1.00 0.50 1.70 0.10 – -

1.2767 X45NiCrMo4 6F7 0.45 0.25 0.30 0.03 0.03 – 1.35 0.25 4.00 – – –

1.3207HSS S10-4-3-10 T42 SKH57

1.3343 S-6-5-2 M2 SKH51 0.90 0.45 0.40 0.03 0.03 – 4.15 5.00 – 1.85 6.35 –

1.3344 S-6-10-2 M3/2 1.20 0.45 0.40 0.03 0.03 – 4.15 5.00 – 3.00 6.35 –

Die materials I / Hot-warm forging tool steels / Active components

Material No. DIN ANSI JIS Composition in %

to DIN Germany USA Japan C Si Mn P S Co Cr Mo Ni V W Ti

1.27131. NiCrMo55NiCrMoV6 6F2 SKT4 0.55 0.30 0.60 0.03 0.03 – 0.70 0.30 1.70 0.10 – –

1.2714 57NiCrMoV7 SKT4 0.58 0.30 0.70 0.03 0.03 – 1.00 0.50 1.70 0.10 – –

1.23432. CrNiMoVX32CrMoV51 0.38 1.00 0.40 0.03 0.03 – 5.30 1.10 – 0.40 – –

1.2344 X40CrMoV51 H13

H11 SKD6

SKD61 0.40 1.00 0.40 0.03 0.03 – 5.30 1.40 – 1.00 – –

1.2365 X32CrMoV33 H10 0.32 0.30 0.30 0.03 0.03 – 0.30 2.80 ––––

1.2367 X40CrMoV53 SKD7 0.30 0.20 0.30 0.03 0.03 – 2.40 – – – 4.30 –

1.26063. WCrV X40CrMoW51

1.2622 X60WCrCoV93 SKH51

SKD62

SKT4

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

contrast, are not quite as favorable. The high-speed steel S10-4-3-10

(1.3207) offers outstanding wear characteristics, but tends to be more

brittle. The materials X155CrVMo121 (1.2379) and S-6-5-2/S-6-10-2

(1.3343/44) represent a compromise concerning wear and toughness

characteristics: Their tempering resistance can also be considered to be

very good at 550°C. The most important criterion to be considered

when selecting a tool material is the type and extent of load, followed

by the layout and geometry of the die.

Table 6.7.3 provides a summary of the most commonly used tool

materials for cold and warm forming. The table also specifies the desig-

nations of comparable steels from the USA or Japan and their composi-

tions. Table 6.7.4 describes the heat treatment, i.e. the annealing tem-

497

Die design

Table 6.7.4: Heat treatment of cold, warm and hot forging tool steels

Die material II / Cold forging tool steels / Active components

No. Heat treatment (

°C) Hardness Cooling Application/hardness (HRC/N/mm

)

AnnealingHardening

Quenching

medium

Quenching

medium

Tempering (customary)Water

1.2363 800-840 930-970 Ö,WB 400 180-400 60 +/–1 blanking/punching dies

1.2369 800-840 1,070-1,100Ö,WB 450-550 550 61 +/–1 punches, blanking/punching dies

1.2379 840-860 1,040-1,080Ö,L,WB 400 180-250 60 +/–1 punches, dies

1.2709 840 480 L – 55 shrink ring

1.2713 650-700 830-870 Ö 300-650 45 +/–1 P,Ö,(W) shrink/intermediate ring and

1.2714 650-700 860-900 L 300-650 45 +/–1 P,Ö,(W) pressure pin (53+/-1 HRC, 1,150 N/mm

)

1.2767 610-630 840-870 W,Ö,L 160-250 54 +/–1 punch, mandrel, one piece press container

1.3343 1,100-900 790-820 Ö,L,WB 550 540-560 62 +/–1 punch, die (insert), press container and

1.3344 1,100-900 770-820 Ö,L,WB 550 550-570 62 +/–1 counterpunch, mandrel

Die material II / Hot, warm forging tool steels / Active components

No. Heat treatment (

°C) Hardness Cooling Application/hardness

AnnealingHardening Tempering (customary)

HRC

1.2713 650-700 830-870 Ö 300-650 42 +2 P,Ö,(W) shrink/intermediate ring and

1.2714 650-700 860-900 L 300-650 42 +2 P,Ö,(W) pressure pin (52+2 HRC, 1,150 N/mm

)

1.2343 750-800 1,000-1,040L,Ö,WB 500-550 550-650 50 +2 W,P,Ö die (insert), shrink ring (45+/–1 HRC)

1.2344 750-800 1,020-1,060L,Ö,WB 500-550 550-650 50 +2 P,Ö mandrel and counterpunch

1.2365 750-800 1,020-1,060Ö,WB 500-550 500-670 50 +2 (W),L,P,Ö die (insert), punch and

1.2367 750-800 1,060-1,100L,Ö,WB 500-550 600-700 54 +2 (W),L,P,Ö counterpunch

1.2606 750-790 1,020-1,050Ö,L,WB 500-550 550-650 W,L,P,Ö

1.2622 760-800 1,150-1,200Ö,WB 500-550 500-650 56 +2 L,Ö blanking die

W = water WB = water bath

L = air

P = compressed air

Ö = oil

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

perature, the hardening temperature, the quenching medium and the

tempering temperatures. The levels of hardness generally required for

dies used in cold forming applications are also given. Table6.7.5

describes the characteristics of wear, toughness, machinability and

grinding using a 10-point scale, whereby a high number indicates espe-

cially good capability. The tables make no claim to completeness, but

present a substantially reduced number of actually required tool steels

in following the principles of optimised and reduced inventory.

The most frequent tool material used for cold forging is high-speed

steel, while for warm forging temperature-resistant tool steel is used.

498

Solid forming (Forging)

Table 6.7.5: Properties of cold, warm and hot forging tool steels

Die materials III / Cold forging tool steel / Active components

Characteristics Remark

VZBS

7687

7/8 5

8 4 4 4 12 % Cr-steel

5 9 5 7 special steel

21068

41068

9245

9445

10334

2 10 6 8 44 HRC 1,400 -1,480 N/mm

52 HRC 1,800 - 1,900 N/mm

5 8 50 HRC 1,700 - 1,800 N/mm

5 8 8 8 surface welding Capilla 521 for erosive machining

surface welding Capilla 5200 for machining

8 8 8 50 HRC 1,700 - 1,800 N/mm

5 7 8 8 54 HRC 1,925 - 2,050 N/mm

56 HRC 2,050 - 2,200 N/mm

V = wear resistance

Z = toughness

B = machinability

S = grindability

No.

1.2363

1.2369

1.2379

1.2709

1.2713

1.2714

1.2767

1.3343

1.3344

No.

1.2713

1.2714

1.2343

1.2344

1.2365

1.2367

1.2606

1.2622

Die materials III / Hot, warm forging tool steel / Active components

VZBS

Characteristics Remark

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Powder metal-manufactured high-speed steels demonstrate very uni-

form carbide distribution and are segregation-free. This results in

improved toughness properties and extremely high compressive

strength. If a larger wear resistance is required, for example for forward

rod extrusion or ironing dies, for large-production series carbides are

also used. The carbide types used comprise tungsten carbide as a phase

in a cobalt matrix. The cobalt content which determines the character-

istics of the carbide, lies between 15 and 30%. With a rising tungsten

carbide content, hardness, compressive strength and wear resistance all

increase. However, at the same time toughness, notch impact strength,

bending and buckling resistance are reduced. The grain size exerts an

influence here: fine-grained carbides are unsuitable as a result of poor

toughness characteristics. Table 6.7.6 provides a summary of the

mechanical characteristics of carbides. The strength values apply to sta-

tic loads and must be reduced by 50% for stress cycles greater than 10

Tool steels for warm forming

The materials X40CrMoV53 (1.2367), X38CrMoV51 (1.2343), X40-

CrMoV51 (1.2344) and X60WCrCoV93 (1.2622) are currently used for

warm forming and in conjunction with the water cooling method gen-

erally applied today; In the case of shrink rings, 55NiCrMoV6

(1.2713)/57NiCrMoV7 (1.2714) or X38CrMoV51 (1.2343)/X40CrMoV-

51 (1.2344) can be used. Due to their low thermal shock resistance, car-

bides, high-speed steels and cold forming tool steel 81MoCrV4216

(1.2369) are only suitable when using water-free cooling methods

which were common in the past. On closer examination, it becomes

clear that materials for warm forging are generally those used in the

classical field of hot forging. Special materials for warm forging, which

are capable of withstanding relatively high pressure levels and relative-

ly high temperatures simultaneously, have not yet been developed for

implementation in series production. Developments of die materials

for warm forming are moving in the following directions:

Compared to hot forging tool steels, high-speed steels already exhib-

it good wear and heat resistance properties as well as improved temper-

ing resistance. By a gradual reduction of the carbon content, attempts

have been made to achieve a further improvement in thermal shock

resistance of high-speed steels. With simultaneous deterioration of the

wear and heat resistance, however, the thermal shock resistance level

499

Die design

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

was far below that of hot forming tool steels. In view of the successful

implementation of environmentally friendly water-soluble lubricants,

the factor thermal shock resistance is expected to continue to be a sub-

ject of particular interest.

Thus, investigations are concentrated on tool steels which are

derived from hot forming tool steels, i.e. those which have a relatively

low carbon content. These tool steels demonstrate superior wear and

heat resistance properties as well as very good thermal shock resistance.

A number of different die materials are being developed concurrently,

for example using nickel-based alloys (Inconell 718), which have

already produced good results in bar extrusion (temperatures 1,150 °C,

pressures of 900 to 1,100 N/mm

) and isotherm forging applications.

Die coatings (VDI 3198)

The cost-effectiveness of cold forging processes depends largely on the

tool costs and accordingly on the achievable service life of dies. Die

wear influences the dimensional stability and surface quality of the pro-

duced workpieces. There are two main types of wear resistant coatings

considered for use in forging technology. Coatings, for example using

adding-on processes such as CVD or PVD processes, and coatings

obtained through reaction layers such as nitriding and ion implanta-

tion. Processes which have proven successful for cold forming dies

include the CVD and PVD techniques, which permit service life to be

more than doubled (cf. Sect. 4.1.3).

An essential prerequisite for a successful coating is good adhesion

between the coating material and the substrate, whose hardness and

compressive strength must provide adequate support. High-speed steels,

500

Solid forming (Forging)

Table 6.7.6: Mechanical properties of carbides

Carbide types E-modulus Ultimate strength

Co content Density Hardness Compressive strength Bending strength

% by weight g/cm

HV N/mm

N/mm

15 14.0 1,150 530,000 4,000 2,400

20 13.5 1,050 480,000 3,800 2,600

25 13.1 900 450,000 3,300 2,600

30 12.7 800 420,000 3,100 2,700

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998