Manufacturing Processes - MFRG 315 - 3.4.1 Forging

3.4.1.1 Introduction
Forging may be carried out 'hot' or 'cold' depending upon the material to be processed. It can be divided into two types of process - open die forging, such as compression of a billet or cylinder between flat platens (tools) and closed die (or impression die) forging where the workpiece acquires the shape of the die cavities while being forged. For complex shapes, such as crank shafts, the process may be carried out in a number of stages, each stage requiring appropriate dies.

3.4.1.2 Applications
Hot forging is often used in the initial stages of manufacturing high quality steel crank shafts and connecting rods, where the forging process provides an excellent micro structure. Medium carbon steels and alloy steels (nickel - chromium and nickel - chromium - molybdenum) are often used for these applications. The high costs of dies needs to be spread over several thousands, or tens of thousands of components to make the process viable.

3.4.1.3 Axisymmetric Upsetting of a Cylinder with Sliding Friction - Requires the end faces of the cylinder to slide on the tool surface, this is opposed by friction that causes the periphery of the cylinder to barrel. This barrelling is ignored in calculating the new diameter an the mean diameter is used.

The maximum stress that the tool is exposed to is: p_amax = sigma_f(1 + (m^* d₁)/(1.73 h₁))

To calculate the total tool force the average interface pressure is needed:

.....Axisymmetric compression of a cylinder applet

3.4.1.4 Axisymmetric Upsetting With Sticking Friction - When the platen is rough or un-lubricated, the interface shear stress may exceed the shear flow stress. All deformation takes place by internal shear of the cylinder. Material adjacent to the platens forms a dead metal zone - it does not move. The sides of the cylinder barrel and may fold over and come into contact with the platens. This is inhomogeneous deformation, Q remains close to unity for d/h less than 2.

3.4.1.5 Limitations When Upsetting - A slender cylinder may buckle, it is not normally possible to upset a cylinder with h_o/d_o greater than 2.

Barrelling may lead to cracking of the periphery due to secondary tensile tresses.

Deformation is commonly limited during a single stroke, re-heating when hot working and annealing when cold working allow further deformation.

3.4.1.6 Experimental Determination of K and n in cold working
These constants can be determined by axisymmetric compression of a short cylinder between smooth lubricated platens, noting the height and load at appropriate steps, calculating then plotting log(true stress) against log(true strain) and fitting a trend line. Log(true strain) should be along the horizontal axis. Thus the equation:

This experiment can also be carried out in a plane strain configuration, where a wide strip if material is compressed between two narrow platens which extend beyond the full width of the strip. Here material outside the deformation zone prevents sideways spread of the strip and the yield stress obtained (and subsequently the flow stresses) is the plane strain yield stress or 'constrained' yield stress. Applying von Mises yield criterion, using Mohr circle to help visualise the stresses, gives the plane strain yield stress (sometimes called S) = 2 k = 1.155 Y

For plane strain deformation processes, the uniaxial (unconstrained) flow stress should be multiplied by 1.15 for use in plane strain configurations.

3.4.1.7 Forgeability of Metals
Forgeability is a combination of the following characteristics:

• i) The flow stress
• ii) The ability to fill a die
• iii) The degree of deformation that can be carried out without failure (due to surface or internal cracking).

Two popular tests for determining the forgeability of materials are the 'upset test' (where cylindrical specimens are upset in steps until they start cracking radially or circumferentially) and the 'hot twist test' where a round bar is heated in a tubular furnace then twisted. The number of twist turns to failure is a relative measure of forgeability. Testing can be carried out in a range of temperatures and strain rates to determine the best conditions for practical forging.

The table below ranks metals / alloys in order of decreasing forgeability (ref. 2) and approximate hot forging temperature range in ^oC