Altan T. Metal Forming Handbook

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6Solid forming (Forging)

6.2Benefits of solid forming

6.2.1 Economic aspects

The benefits offered by solid forming, compared with other production

processes, can be summarized as superior quality combined with lower

manufacturing costs. The standard of quality is due to the favorable

mechanical properties of cold and warm forged parts, such as high strength

and toughness, an uninterrupted fiber flow, close tolerances and a good sur-

face quality. The cost benefit achieved using forging techniques can be con-

siderable, but does depend on the specific part considered and on the pre-

viously used production method. Individual cost factors include:

–Low material input:Almost the complete initial volume of the billet is

processed into the finished part. Compared to machining, savings

can be as great as 75% (Fig.6.2.1).Where high-alloyed materials are

used, the benefit of low material input becomes increasingly signifi-

cant in relation to overall manufacturing costs.

–Use of low-cost raw materials:Strain hardening that occurs during warm

and cold forming results on the one hand in higher levels of flow

stress, but also in higher ultimate and fatigue strength. Therefore, it is

possible to use lower-cost steel grades with lower initial strength char-

acteristics in order to achieve the same mechanical properties obtain-

able in machined parts. A workpiece with, for example, a hexagonal

geometry is produced where machining methods are used from costly

extruded material with a hexagonal cross section. By forging instead of

machining, this type of part can be formed using substantially less

expensive round bar as starting material.

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

–Reduction/elimination of subsequent machining:Subsequent metal re-

moval processes are frequently only necessary in the case of geome-

tries which present a particular problem for forming processes, for

example recesses, undercuts or threads. By using cold forging tech-

niques, substantial savings can be achieved by reducing investment

in machine tools for metal-cutting and in metal-operating staff.

–Minimal process, logistic and transport costs:This benefit is created as a

result of the low cost of automation where transfer systems are used.

–Generally high productivity:Small workpieces produced from wire on

horizontal forging machines can be manufactured at stroking rates of

up to 200/min. For larger parts manufactured by forging from billet in

vertical presses from billet, production speeds of up to 50 parts/min

are possible.

–Facility for integration of several functions/geometries in a single com-

ponent:Cold extruded parts often provide an opportunity for “re-engi-

neering” to create lower-cost designs. The aluminium oil filter hous-

ing illustrated on the left in Fig. 6.2.2,for example, is cold extruded in

a single piece. This allowed the production of three individual com-

ponents – flange, drawn cup and connecting piece. Thus, the com-

plex welding operation required previously (Fig. 6.2.2, right)could be

reduced to a single extrusion operation.

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Solid forming (Forging)

Fig. 6.2.1 Comparison of the input weight and achievable geometry for machining and

forming processes

Ø53

billet weight

1.275kg

Ø50

Ø45

Ø52

billet weight

0.398kg

starting workpiece and fiber flow

in a machined part

starting workpiece and fiber flow

in a forged part

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

6.2.2Workpiece properties

Due to the high degree of geometrical accuracy, surface quality and

highly favorable mechanical properties, the workpiece characteristics

of cold forged components are generally very good. The mechanical

characteristics depend on physical factors and can only be influenced

to a limited degree. The factors influencing operating accuracy in the

production of precise geometries and surface properties, which are list-

ed in Table 6.2.1, must be taken into consideration in the forge plant.

In principle, the primary requirement is for a highly precise and rela-

tively rigid forging press. If such a press is not available, even full com-

pliance with all the other factors such as excellent die design cannot

compensate for this deficiency. Next, in order of importance are the

volume control of the billet, the process concept, etc. Compliance with

and monitoring of all the process parameters thus ensure to achieve the

required service characteristics of a component. However, it is neces-

sary to consider that at the same time production costs also increase.

Despite the high degree of cost reduction achieved in automated

forging lines, it is only with well trained staff, who are capable of ensur-

ing correct operation of the equipment, that it is possible to achieve a

satisfactory return on such a large capital investment. To highlight the

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Benefits of solid forming

Fig. 6.2.2 Oil filter housing – extruded single component compared to three-part welded

configuration

extruded welded

flange

cup

connecting piece

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

importance of the human factor here, this has been included as a sepa-

rate item in Table 6.2.1. The skill of the press operator is paramount in

the successful operation of the production system. He/she takes the

influencing factors into account and evaluates them.

Tolerances

Achievable ISO tolerances for different types of cold forging and com-

plementary processes are summarized in Table 6.2.2. The “variation

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Solid forming (Forging)

Table 6.2.1: Classification of process parameters that affect the geometrical accuracy and surface

quality of a forged part

Line: Machine kinematics, stiffness, off-center load

capacity, gib precision, heating behavior

(warm), natural frequency

Automation weight control, careful feed and discharge,

positioning accuracy, transfer, press force

control

Heating type, temperature, temperature control

Cooling temperature control

Billet: Starting material analysis, strength, microstructure

Material form

wire, bar, thick sheet; rolled, peeled or drawn

Manufacturing process shearing, sawing, blanking

Pre-treatment possibly soft-annealing, pickling,

phosphating, coating

Process: Processing sequence

No. of steps single-step, multiple-step, process

combination

Temperature hot (h), cold (c), warm (w)

Intermediate treatment recrystallization, normalization (warm),

pickling, phosphating, coating

Lubrication and cooling additional lubrication, spraying, flooding

(warm), lubricant and coolant (warm)

Clearance for part transport

Die: Precision die concept, details, production process,

adjustment of upper and lower die,

mechanical die deflection, thermal die

expansion

Wear die material, surface, coating, change

interval

Human element: Die designer,

die manufacturer,

operating staff,

training and instruction specialized skills

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

Table 6.2.2: ISO tolerances for the cold extrusion and complementary processes (VDI 3138)

range” of a forging process indicated here should be interpreted so that

smaller, lighter-weight components fall in general to the left-hand side

of the bar (lower tolerance fields) and larger, heavy-duty components

fall mainly to the right-hand side (wider tolerance fields). The red sec-

tors are tolerances which can be achieved under normal conditions.

The orange areas are ISO tolerances which can be achieved in special

cases but with larger effort and cost. This information also indicates

that the ironing and reducing processes permit the improvement of

accuracy levels by as much as two quality levels. The high degree of

dimensional accuracy is not achieved equally for all the dimensions of

a component, but should be restricted to dimensions that are critical

for functional or material removal considerations.

The achievable dimensional tolerances (normal, narrow) for cold

extrusion or the process combination warm extrusion and subsequent

cold sizing are indicated for the three most frequent forming processes,

solid forward extrusion, cup backward extrusion and ironing (Tables

6.2.3 to 6.2.5). This information is given relative to the main dimen-

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Benefits of solid forming

formingprocess

hotextrusion

warmextrusion

warm/coldextrusion

coldextrusion

ironing

upsetting

opendieforwardextrusion

ITspecificationasperDINISOquality

5 6 7 8 11 12910 15161413

achievablewithspecialmeasures

achievablewithoutspecialmeasures

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

Table 6.2.4: Diameter, concentricity, wall and web thickness tolerance of backward extruded cups,

in mm

tolerances

up to

diameter run-out

normal narrow normal narrow

DdDd

25 0.20 0.15 0.10 0.03 0.25 0.10

50 0.30 0.20 0.125 0.035 0.30 0.15

75 0.35 0.30 0.15 0.04 0.35 0.20

100 0.40 0.50 0.175 0.06 0.40 0.25

s, h

tolerances

up to

wall thickness s bottom thickness h

normal narrow normal narrow

2 0.10 0.075 0.25 0.15

5 0.15 0.10 0.35 0.20

10 0.25 0.15 0.45 0.30

15 0.35 0.25 0.55 0.40

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Solid forming (Forging)

Table 6.2.3: Diameter, length and deflection tolerance of solid forward extruded parts, in mm

tolerances

up to

diameter D

up to

length deflection B

normal narrow narrow narrow

normal normal

25 0.10 0.02 150 0.25 0.20 0.15 0.03

50 0.15 0.08 200 0.75 0.50 0.25 0.05

75 0.20 0.15 300 1.50 1.00 0.35 0.07

100 0.25 0.18 450 2.00 1.50 0.50 0.10

Table 6.2.5: Diameter, concentricity, wall thickness tolerance in mm of tubular components

produced by ironing

up to

diameter

run-out

up to

wall thickness s

normal

narrow normal

25 0.15 0.10 0.075 0.025 0.25 0.10 1 0.075

50 0.25 0.20 0.10 0.03 0.30 0.15 2 0.075

75 0.30 0.25 0.125 0.35 0.035 0.20 4 0.10

100 0.35 0.30 0.15 0.45 0.045 0.25 6 0.10

0.035

0.04

0.05

0.07

narrow

normal

narrow

tolerances

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

sions from Fig.6.2.3.As already indicated, closer tolerances can be

achieved as a result of special measures such as centering of upper and

lower dies.

The length tolerances refer to the shoulder lengths within the upper

or lower die, not to the overall length of the workpiece (including

excess material).

Surface finish

On net shape and near net shape components which do not need rema-

chining, the surface roughness is an important quality characteristic.

The contact surfaces of the core of formed claw poles, for example,

must not exceed a surface roughness of R

= 10mm, as otherwise the

required magnetic flux characteristics cannot be achieved. By the same

token, a similar requirement applies to forged gears. Table 6.2.6 pro-

vides a summary of the achievable surface roughness properties R

and

for different forming processes. The orange areas are values which

are measured before, the red areas are values which are measured after

removal of the lubricant layer. Guideline values on the phosphate layer

are R

= 0.3 – 0.8mm and after removal approx. R

= 0.6 – 2.5mm.

The surface quality is positively influenced by a high specific pres-

sure, high relative velocities and surface quality prior to forging.

Forming processes with high specific pressures and relative velocities

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Benefits of solid forming

Fig. 6.2.3 Main dimensions in solid forward extrusion, backward cup extrusion and ironing

process

forward rod extrusion

backward cup extrusion

ironing

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

are open die extrusion, ironing and solid forward extrusion. The surface

properties are influenced here by the following factors:

starting material: hot rolled, sand blasted, peeled, cold drawn

interface: sheared, sawn

intermediate annealing: scale free: under inert gas

with scale: sand blasted, pickled

Mechanical properties

The hardness, tensile strength and yield strength are increased by plas-

tic flow. However, at the same time the values for notch impact

strength, elongation and necking are decreased. Depending on the

degree of true strain, in the case of steels with low and medium carbon

content, tensile strength can increase by up to 120%, yield strength by

between 100 and 300%, and hardness by between 60 and 150%.

448

Solid forming (Forging)

Table 6.2.6: Surface qualities R

/ R

achieved using different forging methods

forming process

hot extrusion

warm extrusion

warm/cold extrusion

cold extrusion

ironing

upsetting

open die extrusion

5 121518 2030405060708090100

0,5

1 3 4 6 8 10 12

15 20 25 30

before removal of the lubricant carrier layer

after removal of the lubricant carrier layer

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

Where no heat treatment is carried out, this effect can be utilized in

order to replace high-alloy steels by lower-alloy grades. During the

development of the process plan, it is possible within certain limits to

achieve specified strength values in certain sections of a component by

suitable choice of the billet diameter and the process sequence.

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Benefits of solid forming

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

6Solid forming (Forging)

6.3Materials, billet production and surface treatment

Materials used in solid forming are mainly unalloyed, low-alloy and

high-alloy carbon steels, non-ferrous light and heavy alloys such as alu-

minium, magnesium, titanium and copper and their alloys (cf. Sect.

4.2.2). Significant factors in the selection of materials are formability

(flow stress), the path of the flow curve (cf. Fig. 6.1.5), the permissible

fluctuations in material composition and issues related to billet prepara-

tion including preliminary (heat treatment, coating) and intermediate

treatment.

6.3.1Materials

Steels

Low-carbon and low-alloy steels are particularly suited forcold forming.

With a carbon content of up to 0.2%, the conditions for forming are very

favorable, up to 0.3% favorable and up to 0.45% difficult, which results in

low formability, true strain and higher press forces. Phosphorus and sul-

phur content should remain below 0.035%, as these components tend to

reduce formability. The content of nitrogen is restricted to less than 0.01%

for reasons of susceptibility to ageing. Pre-heat-treated steels can be used for

parts which are sensitive to warping (valve tappet rods) and whose high

strength (upto1,100N/mm

) permit only minimal deformation. In some

cases, micro-alloyed steels containing boron, which permit penetration

hardening, are replacing alloyed steels with the prescribed full heat treat-

ment.

All steels which can be cold formed, can also be warm formed. How-

ever, it should be emphasized that even steels containing a higher carbon

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