ASM Metals HandBook Vol. 14 - Forming and Forging

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Radial forging is sometimes confused in the literature with rotary (orbital) forging. In the rotary forging process, the axis

of the upper die is tilted at a slight angle with respect to the axis of the lower die, and one or both dies rotate. Additional

information is available in the article "Rotary Forging" in this Volume.

Radial forging was initially used for the hot forging of small parts and for the cold forging of tubes over mandrels.

Current applications include:

• Bars with round, square, or rectangular cross section starting from ingots or blooms

• Stepped solid shafts and axles for locomotives, railroad cars, and trucks

• Stepped hollow shafts for components in the automotive and aircraft industries

• Preforms for turbine shafts or for subsequent closed-die forging

• Thick-wall tubes forged over a water-cooled mandrel

• Necks and bottoms of steel bottles

• Couplings and tool joints

Figure 1 illustrates typical parts formed by radial forging.

Fig. 1 Typical parts formed by radial forging.

Radial Forging

Hans Hojas, Gesellschaft für Fertigungstechnik und Maschinenbau mbH

Equipment and Process

The four-hammer radial forging machine (Fig. 2) is basically a short-stroke mechanical press. The stroke of the forging

connecting rods is initiated through eccentric shafts. The eccentric shafts are supported in housings that allow adjustment

of the stroke position of the four forging connecting rods. One or two electric motors drive the eccentric shafts through a

drive gear, which simultaneously controls the synchronization of the four eccentric shafts. The forging connecting rods

can be changed in their stroke position either in unison or in pairs so that round, square, or rectangular cross sections can

be forged.

Fig. 2 Schematic of four-

hammer radial forging machine with mechanical drive. (a) Cross section through

forging box. (b) Longitudinal section through forging box. 1, eccentric shaft; 2, sliding

block; 3, connecting rod;

4, adjustment housing; 5, adjusting screw; 6, hydraulic overload protection; 7, hammer adjustment drive

shafts; 8, chuckhead; 9, centering arms; 10, clutch; 11, clutch disk.

Depending on its application, the part-handling system of the machine can be equipped with either one or two workpiece

manipulators, which differ widely from conventional forging manipulators. In contrast to press or hammer forging, the

workpiece axis in radial forging is always maintained on the forging machine centerline, regardless of the diameter. The

manipulator moves only in the longitudinal direction. In order to achieve exact guidance, the chuckhead slides on a

machine bed. During the forging of round cross sections, the chuckhead rotates the workpiece in cycle with the forging

hammers; that is, the rotary movement will be stopped during the time the hammers are in contact with the workpiece.

The rotary movement of the chuckhead spindle is synchronized with the hammer blows; therefore, twisting of the

workpiece is eliminated. The indexing positions of the chuckhead spindle required for forging squares, rectangles, or

hexagons can be set automatically.

In radial forging, the entire forging process, including loading and unloading, can be performed automatically by

computer numerical control (CNC). The forging process is no longer dependent on the discretion of the operator, and an

optimal forging program is maintained in an unchanged manner. This guarantees the manufacture of uniform forged

pieces, which are trimmed to optimal machining allowances. These workpieces are well suited to subsequent machining

on CNC lathes because of their consistent dimensional accuracy.

The technology of the four-hammer forging machine differs from that of all other hot-forming methods. Conventional

presses and hammers, or even rolling mills, use only two tools per forming operation. In the radial forging machine,

however, a workpiece is formed at the same time by four hammers arranged in one plane (Fig. 3). The free spreading that

occurs between the two contacting tools in all conventional forging methods is eliminated. A radial press contacts the

circumference of the workpiece equally and puts the entire surface of the workpiece under compressive stresses. These

compressive stresses prevent the formation of surface cracks during the forging process and improve existing defects.

Fig. 3 Arrangement of hammers in a four-

hammer radial forging machine. The workpiece rotates

intermittently, and the diameter of the forged part is determined by the stroke position of the tools.

(a) Front

(end) view. (b) Side view

In forging between four hammers, temperature increases will occur in the work material that depend on the deformation

rate and the forming resistance of the material. The higher the forming resistance, the higher the temperature increase at

each pass. Therefore, the temperature loss of the workpiece (because of heat radiation) can be compensated for by

preselecting the correct deformation rate, and forming of the workpiece can take place in temperature ranges with the

highest material ductility. In practical terms, this means that all forming can be done in one heat from the ingot to finished

bar steel, regardless of the alloy. Chamber furnaces, pit furnaces, and hearth-type furnaces can therefore be replaced by

continuously operating furnaces. Material can be transported to and from the machine on roller conveyors, and the entire

manufacturing process--heating the ingot, radial forging, dividing and cutting the ends of formed parts, and cooling or

annealing--can be done continuously and automatically.

Forging Over a Mandrel. Equipment for mandrel forging (Fig. 4) is available in different designs for the hot and cold

forging of tubular workpieces. Figure 4 illustrates the forging of tubular parts over short and long mandrels.

Fig. 4 Forging of tubular parts over a short mandrel (a) and a long mandrel (b)

Long tubes with cylindrical bores are forged over a short mandrel. The short mandrel is held in position between the

forging tools with a mandrel rod while the chuckhead moves the workpiece through the forging plane. The mandrel is

slightly tapered, and this makes it possible to perform corrections on inside diameter dimensions by changing the position

of the mandrel between the hammers. During loading and unloading of the workpiece, the mandrel is automatically

retracted into the hollow spindle of the chuckhead.

Forging over a long mandrel is used for relatively short tubes with stepped bores and stepped, cylindrical, or conical

contours. The long mandrel is clamped by the chuckhead and moves together with the workpiece feed. After forging, the

mandrel is pulled out of the workpiece and retracted into the hollow spindle of the chuckhead.

During hot forging, the mandrel is water cooled while in contact with the workpiece. Tungsten carbide mandrels are often

used in cold forging for improved mandrel life.

Radial Forging

Hans Hojas, Gesellschaft für Fertigungstechnik und Maschinenbau mbH

Advantages of Radial Forging

Some of the most important advantages of radial forging are:

• High production. Production of low-alloy steel produc

ts using radial forging is approximately four times

greater than that of hammer or press forging; high-alloy steel production is six times higher (Fig. 5)

• Low energy consumption as a result of a single ingot heating and continuously operating furnaces

•

Close tolerances, which result in less wasted material in subsequent operations. Required machining

allowances are approximately 33% of the usual allowances on conventional forged products (Fig. 6)

Fig. 5

Comparison of production rates of radial forging and hammer and press forging in the production of alloy

steel bars. Starting diameter of the steel was 550 mm (22 in.).

Fig. 6

Typical machining allowance versus diameter for radial forging. Machining allowances are about 33% of

those allowed in a German standard (DIN 7527).

Radial Precision Forging. Full CNC radial precision forging machines are available for hot or cold forging and are

built in different capacities with appropriate special equipment as required. Radial precision forging offers the following

advantages:

• Forging to net or near-net shape

• Precise, repeatable forging operations

• Low tooling costs

• High flexibility

• Fully automatic operation

• Excellent workpiece surface finish, especially on tube inside diameters

• Forging of internally profiled components to finished inside dimensions

Radial Forging

Hans Hojas, Gesellschaft für Fertigungstechnik und Maschinenbau mbH

Examples of Applications

Example 1: Automotive Transmission Shaft.

Figure 7 shows a typical transmission shaft used in an automobile automatic transmission. With conventional

manufacturing, it is difficult to machine a stepped bore with a surface roughness in the range of 0.4 m (16 in.), which

is necessary on some areas of the inside diameter. An additional requirement is a maximum run-out of 0.05 mm (0.002

in.) on the inside diameters.

Fig. 7

5120 steel shaft for automobile automatic transmission produced by cold radial forging. The shaft inside

diameter is formed to net shape. Dimensions given in millimeters (1 in.

= 25.4 mm). (a) Blank. (b) Forged

shaft.

These requirements can be met if the shaft is radial forged over a short tungsten carbide mandrel. Inside diameter surface

quality is improved, and the blank can be kept shorter because the reduction in cross-sectional area creates an elongation

of the shaft. The proper reduction in area is between 28 and 40%.

The tolerance for the bores of this 5120 steel shaft is ±0.02 mm (±0.0008 in.). The cycle time of the shaft is

approximately 2.3 min. Typical cycle times depend on part length.

Example 2: Turbine Shaft Preform.

Figure 8 shows a radial forged turbine shaft preform with the proper volume distribution. The subsequent closed-die

forging operation results in an almost flashless workpiece. The material is either a titanium or a nickel-base alloy, both of

which have a narrow temperature range for forging. The correct forging temperature range is maintained by varying feed

rates (and therefore deformation rates) during the forging process. A higher feed rate creates more deformation per unit

time, which results in higher temperatures in the workpiece. Tolerances on the outside diameter are approximately 1% of

the diameter.

Fig. 8 Blank (a) and turbine shaft preform (b) produced by hot radial forging from titanium or nickel-

base

alloys. Dimensions given in millimeters (1 in. = 25.4 mm)

Example 3: Hollow Axle With Center Upset.

Figure 9 shows the production sequence for the radial forging of a hollow axle. The upset portion near the center of this

part is heated to a higher temperature than the other sections. During upsetting, the forging hammers are closed on the

outside diameter of the intermittently rotating workpiece. An axial force applied along with the radial forging force

ensures that the work material flows toward the center of the axle.

Fig. 9

Steps in the production of a hollow axle with a center upset by radial forging. (a) Tube blank before

forging. (b) After center upsetting. (c) Stepped inside diameter contour formed over a water-

cooled mandrel.

Dimensions given in millimeters (1 in. = 25.4 mm)

After upsetting of the center portion, the stepped contour of the axle end is formed over a water-cooled mandrel. The

inside diameter is controlled by the mandrel; normal tolerances on both inside and outside diameter are 1% of the

diameter. Total cycle time for one end is approximately 40 s.

Radial Forging

Hans Hojas, Gesellschaft für Fertigungstechnik und Maschinenbau mbH

Two-Hammer Radial Forging Machines

The two-hammer radial forging machine was developed for the forging of unalloyed or low-alloy structural steels. The

primary design feature of this machine is the two horizontally arranged, mechanically driven, high stroke rate press rams

(Fig. 10) that radially forge the workpiece while it is guided by two forging manipulators. During forging, the workpiece

rotates between the two forging tools, as in the four-hammer radial forging machine. The tool layout on the two-hammer

radial forging machine is such that the working surfaces of the forging tools are at an obtuse angle to each other; thus,

four forming surfaces contact the workpiece with each stroke. Control is numerical, as in the four-hammer precision

forging machine. Semiautomatic control is usually used for the forging of bars. Stepped shafts are forged automatically.

Fig. 10 Schematic showing the mechanical drive of a two-hammer radial forging machine

The stroke motion of the press rams in two-hammer machines is initiated by the rotation of a double eccentric shaft (Fig.

10). The stroke position of the press rams can be changed by means of control gears, making it possible to determine the

reduction per pass and therefore the final dimensions of the forged workpieces. The height of the forging tools can be

adjusted in a tool guide. It is possible, therefore, to accommodate two different tool impressions in one forging tool. The

hammer position can be changed automatically within a few seconds during the process.

Isothermal and Hot-Die Forging

Sanjay Shah, Wyman-Gordon Company

Introduction

HOT-DIE AND ISOTHERMAL FORGING are special categories of forging processes in which the die temperatures are

significantly higher than those used in conventional hot-forging processes. This has the advantage of reducing die chill

and results in a process capable of producing near-net and/or net shape parts. Therefore, these processes are also referred

to as near-net shape forging processes. These processing techniques are primarily used for manufacturing airframe

structures and jet-engine components made of titanium and nickel-base alloys, but they have also been used in steel

transmission gears and other components.

Isothermal and Hot-Die Forging

Sanjay Shah, Wyman-Gordon Company

Isothermal Forging

In the isothermal forging process, the dies are maintained at the same temperature as the forging stock. This eliminates the

die chill completely and maintains the stock at a constant temperature throughout the forging cycle. The process permits

the use of extremely slow strain rates, thus taking advantage of the strain rate sensitivity of flow stress for certain alloys.

The process is capable of producing net shape forgings that are ready to use without machining or near-net shape forgings

that require minimal secondary machining.

Hot-Die Forging

The hot-die forging process is characterized by die temperatures higher than those in conventional forging, but lower than

those in isothermal forging. Typical die temperatures in hot-die forging are 110 to 225 °C (200 to 400 °F) lower than the

temperature of the stock. When compared with isothermal forging, the lowering of die temperature allows wider selection

of die materials, but the ability to produce very thin and complex geometries is compromised.

Advantages

The principal criterion for selecting these processes in production is the economic advantage offered because of reduced

input material and/or reduced machining. Therefore, they are primarily used for expensive and difficult-to-machine alloys

such as titanium and nickel-base alloys. The main advantages of isothermal and hot-die forging are discussed below.

Reduced Material Costs. These near-net shape processes allow the forging to be designed with smaller corner and

fillet radii, a smaller draft angle, and a smaller forging envelope. These design features reduce the additional material

incorporated to protect the finished part geometry and therefore reduce the weight of the forging considerably. An

example of this weight reduction for the isothermal forging of a nickel-base alloy disk is shown in Fig. 1. A similar

comparison for the hot-die forging of a Ti-6Al-4V structural forging is shown in Fig. 2, in which a typical cross section is

shown for comparison between conventional and hot-die designs. At current material prices, the reduction in input weight

amounts to a significant cost savings.