Altan T. Metal Forming Handbook

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content, for example from 0.45 to 1.0%, can be easily formed, i.e.

without costly intermediate heat treatment. Inductive hardening steels

such as Cf 53, Cf 60 and the ball bearing steels 100 Cr 6 are classical

examples of application for warm forming. For practice-oriented calcu-

lation of force and energy requirements, a good approximation for the

flow stress curve for hot forging can be obtained by multiplying the cold

forging flow curve values with a factor of 0.3 to 0.5.Figure 6.3.1indicates

the temperature-dependent variation of the flow stress curves for Ck 15

and Cf 53.

Table 6.3.1 provides a summary of the most important steel material

groups. It lists not only the DIN and Eurostandard designations but also

the abbreviations for similar materials in accordance with the American

AISI and A.S.T.M standards as well as with the Japanese JIS standard.

The last column provides information on whether the steel is suitable

for hot forming (H), warm forming (W) or cold forming (C). For more

information on workpiece materials, please refer to DIN worksheets

17006, 17100, 17200, 17210, 17240, 17440. Flow stress curves for cold

forming are given in the VDI Guidelines 3134, 3200 sheet 2 (ferrous

metals) and sheet 3 (non-ferrous metals).

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Materials, billet production and surface treatment

Fig. 6.3.1 Temperature-dependent flow curves Ck15 and Cf 53

truestrain [-]

flow stress k [N/mm ]

200

400

600

800

1000

materialCk15

materialCf53

0,2 0,4 0,6 0,8 1,0

20°C

500°C

600°C

700°C

800°C

20°C

500°C

600°C

700°C

800°C

=40s

-1

=40s

-1

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

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Solid forming (Forging)

Table 6.3.1: Selection of steels for solid forming

Material group No. DIN 1654 Eurostandard American Japanese Suitable for cold (C),

standard AISI standard warm (W)

A.S.T.M. JIS and hot (H) forming

general 1.0303 QSt 32-3 C4C Armco C

structural steel 1.0213 QSt 34-3 C7C SS34 C

1.0204 UQSt 36 C11G1C C

1.0160 UPSt37-2 H

1.0224 UQSt-38 C14G1C SS41 C

1.0538 PSt50-2 H

case-hardening 1.0301 C10 C10 C1008 S10C W, H

steels 1.1121 Ck10 C10E C1010 S 9CK C, W, H

1.1122 Cq10 C10C C1010 S 9CK C, H

1.0401 C15 C15 S15C W, H

1.1141 Ck15 C15E C1015 S15CK C, W, H

1.1142 Cq15 C15C C1015 S15Ck C

1.5919 15CrNi6 C

1.7016 17Cr3 C

1.7131 16MnCr5 SCr22 C, W, H

1.7321 20MoCr4 C

heat-treatable 1.6523 21NiCrMo2 C

steels 1.1151 Ck22 C22E C1020 S20C C

1.1152 Cq22 C22C C1020 C

1.0501 C35 S30C W, H

1.1181 Ck35 C35E C1035 S35K C, W

1.1172 Cq35 C35C S35K C, W, H

1.0503 C45 S40C W, H

1.1191 Ck45 C45E C1045 S45C C, W, H

1.1192 Cq45 C45C C

1.1193 Cf45 C45G W

1.1213 Cf53 C53G W

– Cf60 C60G W

alloyed 1.5508 22B2 C

heat-treatable 1.5510 28B2 C

steels 1.5511 35B2 C

1.5523 19MnB4 C

1.7076 32CrB4 C

1.7033 34Cr4 5130/-40 SCr1 C, W, H

1.7035 41Cr4 5130/-40 SCr4 C, W, H

1.7218 25CrMo4 4120/32 C, W, H

1.7220 34CrMo4 4135 SCM3 C, W, H

1.7225 42CrMo4 4140/-42 SCM4 C, W, H

1.6582 34CrNiMo6 C, H

stainless steels 1.4016 X6Cr17 C, ferritic

1.4006 X10Cr13 X10Cr13 C, martensitic

1.4024 X15Cr13 X15Cr13 C, H, austenitic

1.4303 X5CrNi18-12 X4CrNi18-12 C, austenitic

1.4567 X3CrNiCu18-9 X3CrNiCu18-9-4 C, austenitic

roller bearing steel 1.3505 100Cr6 52100 SUJ1/2 C, W

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

Aluminium alloys

Wrought aluminium materials which have been manufactured by form-

ing, not casting, are used for forming processes. Aluminium with a fine-

grained microstructure offers a number of advantages over coarser-

grained aluminium: Improved formability, superior surface quality of

components and greater strength. Ultra-pure and pure aluminium (Table

6.3.2) permit an extrusion ratio (cross-sectional reduction) of up to 95%

(tube production). With an increasing magnesium content, it is possible

to set higher strength characteristics. Age-hardenable, corrosion-resistant

aluminium alloys such as A1MgSi1 exhibit high strength characteristics

as a result of heat ageing which exceed the values achieved when in a

strain-hardened state. Flow curves for aluminium materials are also giv-

en in the VDI directives 3134 and 3200.

Copper alloys

Like aluminium, copper and its alloys offer outstanding extrusion capa-

bility. The yield stress increases with increasing alloying elements,

whereby forming should take place while the material is in a soft-

annealed state. In the case of technically pure copper, mainly E-Cu and

SE-Cu are used, for example, in the production of terminals. Among the

variety of different bronzes used, the tin bronze with 1 to 2 % Sn and sil-

icon bronze is most widely formed. If alloy elements such as chrome, zir-

conium, berylium or silicon (CuNi2Si) are present, many copper materi-

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Materials, billet production and surface treatment

Material group No. DIN 1712/25/45 ISO AA JIS

pure/ultra-pure aluminium 3.0285 Al 99,8 Al 99,8 1080A A1080

3.0275 Al 99,7 Al 99,7 1070A A1070

3.0255 Al 99,5 Al 99,5 1050A A1050

non age-hardenable alloys 3.0515 Al Mn1 AlMn1 3103 A3003

3.3315 Al Mg1 AlMg1 5005A

3.3535 Al Mg3 AlMg2,5 5754

3.3555 Al Mg5 AlMg5 5056A

hardenable alloys 3.3206 Al Mg Si 0,5 6060 (6063) A6063

3.2315 AlMgSi 1 6082 (6061) A6082

3.1325 AlCuMg 1 2017A A2017

3.4365 AlZnMgCu 1,5 7075 A7075

Table 6.3.2: Selection of aluminium materials for cold and hot extrusion

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

Material group Abbreviation No. DIN US designation / UNS JIS

pure copper E-Cu F20 2.0060 1787 ETP 99,95 C0-0,04O / C11000 OFCu

SF-Cu 2.0090 1787 DHP Copper / C12200 DCu

copper alloys CuZn10 2.0230.10 17660 Commercial copper / C22000 BS( ), RBS ( )

CuZn28 2.0261.10 17660 Cartridge brass 70 % / C26000 RBS ( )

CuZn40 2.0360 17660 Muntz Metal / C28000 RBS ( )

CuNi2Si 2.0855 17666 Silicon bronze / C64700 CN ( ) 1

CuNi12Zn24 2.0730.10 17663 65Ni-12Ag / C75700

CuNi20Fe 2.0878.10 17664 Cu-20Ni / C71000

CuAl11Ni 2.0978 17665 Aluminium bronze / C63200

CuSn2 2.1010 17662 Phosphor bronze / C50500

als are age-hardenable through heat treatment. The most important cop-

per alloys are the brass family. The formability of the material improves

as the copper content increases and the Zn content decreases. With a Zn

content above 36%, the materials become brittle and their cold forma-

bility decreases rapidly. As a result, hot forming methods should prefer-

ably be used. Flow stress curves for copper alloys are given in the VDI

Guidelines 3134 and 3200.

Other materials such as lead, zinc, tin, titanium or zirconium and

their alloys can also be successfully formed. However, in terms of quan-

tity and economic significance, these play only a minor role.

6.3.2 Billet or slug preparation

The workpiece materials are specified by the user: Any modifications to

the material, to exploit the benefits offered by cold forming (harden-

ing) or to simplify the forming process, must be agreed upon with the

die maker and the production engineer. In addition to the workpiece

material itself, the nature of the semi-finished product, its separation to

prepare individual billets, and the heat and surface treatment are all

important points for consideration.

Semi-finished products

Wires, bars and tubes are used as semi-finished products for the pro-

duction of billets or slug. Wire is delivered in coil form and generally

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Solid forming (Forging)

Table 6.3.3: Selection of copper materials for cold extrusion

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

ranges from 5 to 40 mm in diameter. Wire diameters of up to 50 mm are

processed in individual cases, although the handling capability of the

wire bundle becomes increasingly difficult as the coils increase in weight.

Low-cost rolled wire in accordance with DIN 59 110 and 59 115 has

a wide diameter tolerance and a rough surface finish. It therefore

requires preliminary drawing on a wire breakdown machine. The

reduction in cross section should be between 5 and 8%. The wire must

be annealed, de-scaled and phosphated.

Drawn wire after DIN 668 K (K = cold drawn) is used to produce parts

with a lower true strain (small reduction in area), as the material has

already undergone a certain degree of preliminary strain hardening

during the drawing process. This material also has to be descaled, phos-

phated and possibly also lubricated (stearate drawn). When a higher

degree of true strain is involved (e.g. cup backward extrusion), after pre-

liminary drawing, annealing (G) or spheroidized annealing (GKZ) must

be performed in order to restore the original material formability. The

form of delivery is specified with the designations K+G or K+GKZ. Light

oiling helps prevent rust formation on the wire. In the case of high-

alloy steel qualities, a further finish drawing process takes place after

annealing with cross section reductions of less than 6%. The form of

delivery is then specified as K+G+K or K+GKZ+K.

In practice, bar stock can also be used economically in the larger

diameter ranges. With diameters of up to 70 mm, bars of between 6 and

12 m in length are produced. Hot-rolled round steel to DIN 1013 and

DIN 59130 is the most favorable selection in terms of cost.

The allowable diameter tolerances cannot be reduced by drawing,

leading to volume variations. These must be fully compensated for by

excess material during the forming process. For a component weighing

around 1.3 kg with a diameter of 46 mm, a weight tolerance of 88 g

(average deviation) or 45g (precision deviation) must be integrated into

the die if the admissible tolerances are fully utilized. The transforma-

tion clearance in the first stage has to take account both, the diameter

tolerance and also the ovality of the billet caused by the shearing

process. For a material with a diameter of 46 mm, the first transforma-

tion clearance will be approx. 2.5 mm.

Where round steel is being cold forged, the descaling, phosphatizing

and lubricating operations take place prior to forging. In the case of

warm forging, the billets are fed directly to the heating line.

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Materials, billet production and surface treatment

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

Diameter Tolerances DIN 668 DIN 59110/59115 DIN 1013

approx. in mm DIN 970/971 DIN 59130 Precision

h9/h11 DIN 1013 deviation

Usual deviation

15 – 0.04 / – 0.10 +/– 0.5 +/– 0.20

20 – 0.05 / – 0.13 +/– 0.5 +/– 0.20

25 – 0.05 / – 0.13 +/– 0.6 +/– 0.25

30 – 0.05 / – 0.13 +/– 0.6 +/– 0.25

35 – 0.06 / – 0.16 +/– 0.8 +/– 0.30

40 – 0.06 / – 0.16 +/– 0.8 +/– 0.35

45 – 0.06 / – 0.16 +/– 0.8 +/– 0.40

50 – 0.075 / – 0.19 +/– 1.0 (+/– 0.50)

80 – 0.09 / – 0.19 +/– 1.3 (+/– 0.60)

Bright round steel after DIN 970 and 971 is drawn or peeled following

hot rolling, to tolerances h9/h11. The weight tolerance for the above

given example is reduced to 5 and 9 g respectively, which, including an

admissible length tolerance of 0.2 mm, corresponds to the customary

weight tolerance of between 0.5 and 1%. By weighing the billets and

adjusting the cut-off length, it is possible to achieve weight tolerances of

< + 1%. Peeling of hot-rolled bar allows the elimination of surface inclu-

sions and seams as well as any surface decarburization. Table 6.3.4 gives

the diameter tolerances for different round materials.

Alternatively to bar material, tubular or ring-shaped formed parts can

also be produced using tube sections as billets or preforms. Criteria to

be considered here include wall thickness tolerance, concentricity and

economy in comparison to producing these preforms from sheared bil-

lets through upsetting, backward cup extrusion, piercing, ironing and

by using various intermediate heat treatments.

Where height-to-diameter ratios are h/d #0.2, production using

sheared billets or blanks cut out of hot-rolled coil stock (DIN 1016), flat

steel (DIN 1017), thicker sheet metal (DIN 1542/1543) is beneficial.

This is the case particularly when irregular starting shapes (preforms)

are required.

When processing wire, it is useful to have the preliminary treatment

performed by the semi-finished product supplier, as a large investment

is required for this type of equipment and the spectrum of required

treatment types varies greatly.

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Solid forming (Forging)

Table 6.3.4: Diameter tolerances for round materials with selected diameters in mm

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

When using bar and flat material, the billets are generally sheared by

the forger and subjected to heat and surface treatment. Strain-harden-

ing effects which occur partially at the flat surfaces of the billet due to

the shearing process are thus eliminated prior to forging.

Billet separation

The most commonly used billet separation process is shearing. Slicing

on the trimming lathe is used only rarely, for example for test purpos-

es. Blanking and fine blanking are explained in Sects. 4.5 and 4.7.

Shearing is characterized by practically loss-free separation at an

extremely high level of output, in terms of quantity. The shear blade

plastically deforms the material until its deformation limit in the shear-

ing zone has been exhausted, shearing cracks appear and fracture

occurs. With all four shearing principles – without bar and cut-off hold-

er, with bar holder, with bar and cut-off holder and with axial pressure

application – plastic flow lateral to the shearing direction is increasing-

ly prevented, while compressive stress increases during the shearing

operation. Both tendencies exercise a positive influence over the geom-

etry (ovality, tolerance) of the sheared surface. The most accurate billets

are produced using the shearing principle with bar and cut-off holder.

For cold forging, the sheared billets should have the greatest possible

rectangularity, volume control and little plastic deformation. The

sheared surfaces should be free of shearing defects and exhibit only a

moderate amount of strain-hardening. The appearance of the sheared

surfaces is the result of interactions between workpiece characteristics,

tool, machine and friction. The shearing clearance exercises a major

influence here. The greater the strength of the steel, the smaller is

the shearing clearance (cf.Fig.4.5.12).However, aluminium and lead

always require a small blanking clearance. The following values may be

taken as a guideline for the shearing clearance of steel:

soft steel types 5 – 10 %

(of the starting material diameter in mm)

hard steel types 3 – 5 %

brittle steel types 1 – 3 %

Rough fractured surfaces, tears and seams indicate an excessively wide

shearing tool clearance. Cross fractured surfaces and material tongues

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Materials, billet production and surface treatment

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

indicate an insufficient tool clearance. With increasing shearing veloc-

ity, the deformation zone reduces, the hardness distribution becomes

more uniform and the hardness increase in the sheared surface

becomes less pronounced, i.e. the material characteristics become more

“brittle” (exception: austenitic chrome-nickel and chrome-manganese

steels).

Open shears and closed shearing guides are used as tools (Fig. 6.3.2).

Shearing guides produce very high quality blanks, but involve a num-

ber of drawbacks: Sheared billets are more difficult to eject, stops have

to be used and demands on the diameter tolerance of the starting mate-

rial are more stringent. For round materials, the relative shearing clear-

ance for each billet thickness to be sheared can be kept constant by

grinding an elliptical relief cut on the guides. The parts sheared in this

way demonstrate improved angularity and reduced burr formation at

the edge of the fractured surface. Moreover, the sheared billet quality is

less dependent on variations in material properties, which in turn

reduces variations in volume.

The shearing force F

and shearing work W

can be calculated

approximately when seperating round material with the diameter d

using the following formula:

= A

· k

= x · F

· s,

whereby A

is the sectional surface to be sheared, k

the shearing resis-

tance of the billet material and s approx. 20% (hard, brittle materials)

to 40% (soft, tenacious materials) of the shearing stroke, i.e. the diam-

eter d. The correction factor x indicates the extent to which the increase

in force deviates from a rectangular force-stroke curve. In general, x is

taken to be between 0.4 and 0.7. The shearing resistance k

amounts to

approximately 0.7 to 0.8 · R

Heat treatment of steel

Soft-annealing (spheroidized annealing) plays a particularly important

role in the heat treatment of steel materials used for billet production.

Recrystallization annealing, normalization and under certain circum-

stances also recovery annealing are typical heat treatments used for

intermediate annealing of already formed workpieces whose formabili-

ty has been exhausted.

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Solid forming (Forging)

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

The soft annealingprocess spheroidizes any existing harder flaky ele-

ments of the microstructure. While the hardness is retained, the flow

stress (cf. Sect. 2.2.3) of the material is reduced.

Recrystallization annealingis advantageous for austenitic steels with a

low carbon content, as with this heat treatment the lowest flow stress

levels are obtained. However, if coarse grain occurs as a result of locally

low levels of true strain or deformation (cf. Sect. 2.2.2) during recrystal-

lization annealing, and if this type of coarse grain is not permissible, a

normalization process should be carried out.

As a result of two-fold recrystallization (perlite + ferrite <=> austenite)

an even, fine-grained and fine-lamellar perlite microstructure is achieved

through normalization.This can be re-annealed if required to create

grained perlite (soft-annealing). A marked difference in microstructure

(coarse and fine grains) can also result during warm forming as a result of

locally differing cooling conditions and degrees of true strain. These dif-

ferences can cause cracks during subsequent cold sizing operations

(reducing, ironing). This problem can be avoided by normal annealing.

Recovery annealingoffers a low-cost alternative to recrystallization or

soft annealing, provided the deformation in the subsequent forming

operation is minimal, i.e. the level of true strain is not too large.

6.3.3Surface treatment

A difference is drawn between abrading and depositing surface treat-

ments. Abrading processes include chemical and mechanical cleaning

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Materials, billet production and surface treatment

Fig. 6.3.2 Open and closed shearing knives and a closed shearing knife ground with elliptical

relief

shearingclearance

wedge

angle

wedgeangle

cutting

edge

pressure

surface

pressuresurface

flanksurface

depthofreliefgrind

cuttingedge

elliptical

reliefgrind

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

methods, degreasing and descaling. Depositing processes include phos-

phating, soaping, molycoting and the use of oils. The phosphate layer

serves as a carrier layer which facilitates the adherence of lubricants to

the surface. Soaping is used for simpler forming operations, molycoting

for extremely high degrees of deformation (cf.Sect.2.2.2) and for parts

such as gears. In the case of horizontal multi-station presses operating

with wire, oil is used for lubrication and cooling. In vertical multi-

station presses, oil is used as a supplementary lubricant in addition to

soap/molycote (spray or flood application) (cf. Sect. 6.8).

Cleaning, degreasing and descaling

Chemical cleaning and degreasing processes are performed either through

immersion or spraying of solvents, or through the condensation of sol-

vent vapor on the workpieces. Solvent types used include organic solvents

(hydrocarbons, petroleum ether, petroleum), chlorinated hydrocarbons

(trichloroethylene and perchloroethylene), and water-soluble cleaning

agents such as acids, acid saline solutions and alkaline solutions. The

mechanical methods used include sand blasting or tumbling using lime,

sand, steel shot and other materials. The choice of cleaning methods and

agents depends on the type and extent of the contamination, the required

degree of purity and the type, shape and quantity of the items to be

cleaned. Environmentally friendly water-soluble cleaning agents are fre-

quently used.

Chemical descaling (pickling) is performed using pickling baths con-

taining sulphuric acid or hydrochloric acid solutions. The pickling agents

serve to release hydrogen on the substrate metal, causing the oxide layers

to split away. As the absorption of hydrogen in the substrate metal leads

to embrittlement, as short a pickling time as possible should be used.

Crack formation during forging can, for example, be the result of exces-

sive pickling treatment. Sand blasting and tumbling are used for the

mechanical removal of thick scale layers. Here, the brittle scale layer is

parted off from the tough substrate material through partial application

of force.

Phosphating

Phosphating is taken to mean the generation of cohesive crystalline

phosphate layers which are firmly adhered to the substrate material.

For cold forming processes, mainly zinc phosphate and in some cases

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Solid forming (Forging)

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