ASM Metals HandBook Vol. 14 - Forming and Forging

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Fig. 5 Planes (a) and directions (b) of metal flow during the forging of a relatively complex shape.

The finished

forging is shown in (c).

In designing the cross sections of a blocker (preform) die impression, three basic rules must be followed:

• The area of each cross section along the length

of the preform must be equal to the area of the finish

cross section augmented by the area necessary for flash. Therefore, the initial stock distribution is

obtained by determining the areas of cross sections along the main axis of the forging

• All the co

ncave radii (including fillet radii) of the preform should be larger than the radii of the forged

part

•

When practical, the dimensions of the preform should be greater than those of the finished part in the

forging direction so that metal flow is mostly of

the upsetting type rather than the extrusion type. During

the finishing operation, the material will then be squeezed laterally toward the die cavity without

additional shear at the die/material interface. Such conditions minimize friction and forging loa

d and

reduce wear along the die surfaces

Application of these three principles to steel forgings is illustrated in Fig. 6 for some solid cross sections. The qualitative

principles of preform design are well known, but quantitative information is rarely available.

Fig. 6 Suggested blocker cross sections for steel forgings. B, blocker; F, finished forging.

For the forging of complex parts, empirical guidelines may not be sufficient, and trial-and-error procedures may be time

consuming and costly. A more systematic and well-proven method for developing preform shapes is physical modeling,

using a soft material such as lead, plasticine, or wax as a model forging material and hard plastic or low-carbon steel dies

as tooling. Therefore, with relatively low-cost tooling and with some experimentation, preform shapes can be determined.

Detailed information on physical modeling and the use of computer-aided design and manufacturing (CAD/CAM) for

forging design is available in the Section "Computer-Aided Process Design for Bulk Forming" in this Volume. The use of

CAD/CAM in die design is also discussed in the section "CAD/CAM of Forging Dies" in this article.

Closed-Die Forging in Hammers and Presses

Flash Design

The influences of flash thickness and flash land width on forging pressure are reasonably well understood from a

qualitative viewpoint (Fig. 7). Essentially, forging pressure increases with decreasing flash thickness and with increasing

flash land width because of combinations of increasing restriction, increasing frictional forces, and decreasing metal

temperatures at the flash gap.

Fig. 7 Metal flow (a to c) and load-stroke curve (d) in closed-die forging. (a) Upsetting. (b) Filling. (c) End.

A typical load-versus-stroke curve for a closed-die forging is shown in Fig. 8. Loads are relatively low until the more

difficult details are partly filled and the metal reaches the flash opening (Fig. 7). This stage corresponds to point P

in Fig.

8. For successful forging, two conditions must be fulfilled when this point is reached. First, a sufficient volume of metal

must be trapped within the confines of the die to fill the remaining cavities, and second, extrusion of metal through the

narrowing gap of the flash opening must be more difficult than filling the more intricate detail in the die.

Fig. 8 Typical load-stroke curve for a closed-die forging showing three distinct stages.

As the dies continue to close, the load increases sharply to a point P

, the stage at which the die cavity is filled

completely. Ideally, at this point, the cavity pressure provided by the flash geometry should be just sufficient to fill the

entire cavity, and the forging should be completed. However, P

represents the final load reached in normal practice for

ensuring that the cavity is completely filled and that the forging has the proper dimensions. During the stroke from P

, all metal flow occurs near or in the flash gap, which in turn becomes more restrictive as the dies close. In this respect,

the detail most difficult to fill determines the minimum load for producing a fully filled forging. Therefore, the

dimensions of the flash determine the final load required for closing the dies. Formation of the flash, however, is greatly

influenced by the amount of excess material available in the cavity, because this amount determines the instantaneous

height of the extruded flash and therefore the die stresses.

A cavity can be filled with various flash geometries if there is always sufficient material in the die. Therefore, is it

possible to fill the same cavity by using a less restrictive (thicker) flash and to do this at a lower total forging load if the

necessary excess material is available (in this case, the advantages of lower forging load and lower cavity stress are offset

by increased scrap loss) or if the workpiece is properly preformed (in which case low stresses and material losses are

obtained by additional preforming).

The shape classification (Fig. 4) has been used in the systematic evaluation of flash dimensions in steel forgings. The

results for shape group 224 are presented in Fig. 9 as an example. In general, the flash thickness is shown to increase with

forging weight, while the ratio of flash land width to flash thickness decreases to a limiting value.

Fig. 9 Variations in flash land-width-to-

thickness ratio (top) and in flash thickness (bottom) with forging weight

for carbon and alloy steel forgings in shape group 224 (see Fig. 4).

Closed-Die Forging in Hammers and Presses

Prediction of Forging Pressure

It is often necessary to predict forging pressure so that a suitable press can be selected and so that die stresses can be

prevented from exceeding allowable limits. In estimating the forging load empirically, the surface area of the forging,

including the flash zone, is multiplied by an average forging pressure known from experience. The forging pressures

encountered in practice vary from 56 to 98 kg/mm

(80 to 140 ksi), depending on the material and the geometrical

configuration of the part. Figure 10 shows forging pressures for parts made of various carbon (up to 0.6% C) and low-

alloy steels. In these trials, flash land-width-to-thickness ratios from 2 to 4 were used. The variable that most influences

forging pressure is the average height of the forging. The lower curve in Fig. 10 relates to relatively simple parts, and the

upper curve to more difficult-to-forge parts.

Fig. 10 Forging pressure versus average height of forging for carbon and low-

alloy steel forgings. Lower curve

is for relatively simple parts; upper curve relates to more difficult-to-

forge part geometries. Data are for flash

land-to-thickness ratios from 2 to 4.

Most empirical methods, summarized in terms of simple formulas or nomograms, are not sufficiently general for

predicting forging loads for a variety of parts and materials. Lacking a suitable empirical formula, one may use analytical

or computer-aided techniques for calculating forging loads and stresses.

Closed-Die Forging in Hammers and Presses

CAD/CAM of Forging Dies

During the last decade, computers have been used to an increasing extent for forging applications. The initial

developments focused on the numerically controlled (NC) machining of forging dies. In the mid-1970s, computer-aided

drafting and NC machining were also introduced for structural forgings and for forging steam turbine blades. During the

early 1980s several companies began to use stand-alone CAD/CAM systems--normally used for mechanical designs,

drafting, and NC machining--for the design and manufacture of forging dies.

Stand-alone CAD/CAM systems are commercially available and have the necessary software for computer-aided drafting

and NC machining. A typical CAD/CAM system consists of a microcomputer or minicomputer, a graphics display

terminal, a keyboard, a digitizer with menu for data entry, an automatic drafting machine, and hardware for information

storage and NC tape punching or floppy disk preparation. Such systems also allow, at various levels of automation, three-

dimensional representation of the forging and the possibility of zooming and rotating the forging-geometry display on the

graphics terminal screen for the purpose of visual inspection. These systems also allow sectioning of a given forging, that

is, the description, drawing, and display of desired forging cross sections for the purpose of die stress and metal flow

analyses. Therefore, the results can be displayed for easy interaction between the designer and the computer system,

modifications to die design can be easily made, and alternatives can be explored.

The ultimate advantage to computer-aided design in forging is achieved when reasonably accurate and inexpensive

computer software is available for simulating metal flow throughout a forging operation (Fig. 11). In this case, forging

experiments can be conducted on a computer by simulating the finish forging that would result from an assumed or

selected blocker design, and the results can be displayed on a graphics terminal. If the simulation indicates that the

selected blocker design would not fill the finisher die or that too much material would be wasted, another blocker design

can be selected and the computer simulation, or trial, can be repeated. Such computer-aided simulations reduce the

required number of expensive die tryouts. More information on CAD/CAM in forging design is available in the Section

"Computer-Aided Process Design for Bulk Forming" in this Volume.

Fig. 11 Computer simulation of def

ormation in the forging of an axisymmetric spike. (a) Undeformed grid. (b)

Deformation at a die stroke of one-half the initial billet height.

Closed-Die Forging in Hammers and Presses

Equipment for Closed-Die Forging

Hammers and Presses. The various types of hammers and presses used to provide the force for closed-die forging are

described in the article "Hammers and Presses for Forging" in this Volume. Capacities and ratings of each major type of

press or hammer are discussed in the article "Selection of Forging Equipment" in this Volume.

Dies for closed-die forging are discussed in detail in the article "Dies and Die Materials for Hot Forging" in this Volume.

Cutting of bar stock can be accomplished by cold or hot shearing, sawing, abrasive cutting, and thermal or electric arc

cutting. These operations, as well as equipment used for cutting, are described in the Section "Shearing, Slitting, and

Cutting" in this Volume.

Heating Equipment. There are wide variations in the forging temperature ranges for various materials (Table 1). These

differences, along with differences in stock and the availability of various fuels, have resulted in a wide variety of heating

equipment. Various types of electric and fuel-fired furnaces are used, as well as resistance and induction heating.

Regardless of the heating method used, temperature and atmospheric conditions within the heating unit must be controlled

to ensure that the forgings subsequently produced will develop the optimal microstructure and properties.

Closed-Die Forging in Hammers and Presses

Forging Temperatures for Steels

Maximum safe forging temperatures for carbon and alloy steels are given in Table 2, which indicates that forging

temperature decreases as carbon content increases. The higher the forging temperature, the greater the plasticity of the

steel, which results in easier forging and less die wear; however, the danger of overheating and excessive grain coarsening

is increased. If a steel that has been heated to its maximum safe temperature is forged rapidly and with large reduction, the

energy transferred to the steel during forging can substantially increase its temperature, thus causing overheating.

Table 2 Maximum safe forging temperatures for carbon and alloy stools of various carbon contents

Maximum safe forging temperature

Carbon steels

Alloy steels

Carbon

content, %

°C °F °C

°F

0.10 1290 2350 1260

2300

0.20 1275 2325 1245

2275

0.30 1260 2300 1230

2250

0.40 1245 2275 1230

2250

0.50 1230 2250 1230

2250

0.60 1205 2200 1205

2200

0.70 1190 2175 1175

2150

0.90 1150 2100 . . .

. . .

1.10 1110 2025 . . . . . .

The effect of carbon content on forging temperature is the same for most tool steels as for carbon and alloy steels.

However, the complex alloy compositions of some tool steels have different effects on forging temperature. Forging

temperatures for tool steels are listed in Table 3.

Table 3 Recommended forging temperature ranges for tool steels

Forging temperatures

Preheat slowly to:

Begin forging at

(a)

Do not forge below:

Steels

°C °F °C °F °C

°F

Water-hardening tool steels

W1-W5 790 1450 980-1095

(b)

1800-2000

(b)

815

1500

Shock-resisting tool steels

S1, S2, S4, S5 815 1500 1040-1150 1900-2100 870

1600

Oil-hardening cold-work tool steels

O1 815 1500 980-1065 1800-1950 845

1550

O2 815 1500 980-1040 1800-1900 845

1550

O7 815 1500 980-1095 1800-2000 870

1600

Medium-alloy air-hardening cold-work tool steels

A2, A4, A5, A6 870 1600 1010-1095 1850-2000 900

1650

High-carbon high-chromium cold-work tool steels

D1-D6 900 1650 980-1095 1800-2000 900

1650

Chromium hot-work tool steels

H11, H12, H13 900 1650 1065-1175 1950-2150 900

1650

H14, H16 900 1650 1065-1175 1950-2150 925

1700

H15 845 1550 1040-1150 1900-2100 900

1650

Tungsten hot-work tool steels

H20, H21, H22 870 1600 1095-1205 2000-2200 900

1650

H24, H25 900 1650 1095-1205 2000-2200 925

1700

H26 900 1650 1095-1205 2000-2200 955

1750

Molybdenum high-speed tool steels

M1, M10 815 1500 1040-1150 1900-2100 925

1700

M2 815 1500 1065-1175 1950-2150 925

1700

M4 815 1500 1095-1175 2000-2150 925

1700

M30, M34, M35, M36

815 1500 1065-1175 1950-2150 955

1750

Tungsten high-speed tool steels

T1 870 1600 1065-1205 1950-2200 955

1750

T2, T4, T8 870 1600 1095-1205 1950-2200 955

1750

T3 870 1600 1095-1230 2000-2250 955

1750

T5, T6 870 1600 1095-1205 2000-2200 980

1800

Low-alloy special-purpose tool steels

L1, L2, L6 815 1500 1040-1150 1900-2100 845

1550

L3 815 1500 980-1095 1800-2000 845

1550

Carbon-tungsten special-purpose tool steels

F2, F3 815 1500 980-1095 1800-2000 900

1650

Low-carbon mold steels

P1 . . . . . . 1205-1290 2200-2350 1040

1900

P3 . . . . . . 1040-1205 1900-2200 845

1550

P4 870 1600 1095-1230 2000-2250 900

1650

P20 815 1500 1065-1230 1950-2250 815 1500

(a)

The temperature at which to begin forging is given as a range; the higher side of the range should be used for large sections and heavy or rapid

reductions, and the lower side for smaller sections and lighter reductions. As the alloy content of the steel increases, the time of soaking at

forging temperature increases proportionately. Similarly, as the alloy content increases, it becomes more necessary to cool slowly from the

forging temperature. With very high alloy steels, such as high-speed steels and air-hardening steels, this slow cooling is imperative in order to

prevent cracking and to leave the steel in a semisoft condition. Either furnace cooling of the steel or burying it in an insulating medium (such

as lime, mica, or diatomaceous earth) is satisfactory.

(b)

Forging temperatures for water-hardening tool steels vary with carbon content. The following temperatures are recommended: for 0.60-1.25%

C, the range given; for 1.25 to 1.40% C, the low side of the range given.

Heating Time. For any steel, the heating time must be sufficient to bring the center of the forging stock to the forging

temperature. A longer heating time than necessary results in excessive decarburization, scale, and grain growth. For stock

measuring up to 75 mm (3 in.) in diameter, the heating time per inch of section thickness should be no more than 5 min

for low-carbon and medium-carbon steels or no more than 6 min for low-alloy steel. For stock 75 to 230 mm (3 to 9 in.)

in diameter, the heating time should be no more than 15 min per inch of thickness. For high-carbon steels (0.50% C and

higher) and for highly alloyed steels, slower heating rates are required, and preheating at temperatures from 650 to 760 °C

(1200 to 1400 °F) is sometimes necessary to prevent cracking.

Finishing temperature should always be well above the transformation temperature of the steel being forged in order

to prevent cracking of the steel and excessive wear of the dies, but should be low enough to prevent excessive grain

growth. For most carbon and alloy steels, 980 to 1095 °C (1800 to 2000 °F) is a suitable range for finish forging. More

information on forging parameters for ferrous alloys is available in the articles "Forging of Carbon and Alloy Steels" and

"Forging of Stainless Steel" in this Volume.

Closed-Die Forging in Hammers and Presses

Forging Temperatures for Steels

Maximum safe forging temperatures for carbon and alloy steels are given in Table 2, which indicates that forging

temperature decreases as carbon content increases. The higher the forging temperature, the greater the plasticity of the

steel, which results in easier forging and less die wear; however, the danger of overheating and excessive grain coarsening

is increased. If a steel that has been heated to its maximum safe temperature is forged rapidly and with large reduction, the

energy transferred to the steel during forging can substantially increase its temperature, thus causing overheating.

Table 2 Maximum safe forging temperatures for carbon and alloy stools of various carbon contents