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: pamax = sigmaf(1 + (m* d1)/(1.73 h1))

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

pa = sigmaf Qa = sigmaf(1 + (m* d1)/(3 x 1.73 h1))

Where Qa is a factor that includes the effects of friction and can be calculated, or found from charts.

.....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 ho/do 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:

flow stress = K (true strain)n is converted to:
log(flow stress) = n log(true strain) + log(K) which can be compared to the equation of a straight line:
y = mx + c

Hence the index n is equal to the slope of the log against log graph.
K is found from the intercept of the log against log graph line with the vertical axis: log(true strain) = 0, so true strain = 1 and at this point the flow stress = K.

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).

Although in general the forgeability of metals increases with increasing temperature, for certain metals there is a maximum temperature above which some undesirable phenomena occur, such as fast grain growth or melting of a phase.
Fine grain metals have better forgeability. Metals with insoluble inclusions tend to be brittle and have low forgeability.

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

Aluminium alloys
400 - 550
Magnesium alloys
250 - 350
Copper alloys
600 - 900
Carbon and alloy steels
850 - 1150
Martensitic stainless steels
1100 - 1250
Maraging steels
1100 - 1250
Austenitic stainless steels
1100 - 1250
Nickel alloys
1000 - 1150
Semi-austenitic PH stainless steels
1100 - 1250
Titanium alloys
700 - 950
Iron-base superalloys
1050 - 1180
Cobalt-base superalloys
1180 - 1250
Columbium alloys

Tantalum alloys
1050 - 1350
Molybdenum alloys
1150 - 1350
Nickel base super alloys
1050 - 1200
Tungsten alloys
1200 - 1300
Beryllium

References:
1. 'Introduction to Manufacturing Processes', J A Schey, McGraw-Hill International, 1987 - see chapter 4.
2. 'Metals Handbook', Volume 14, Forming and Forging, 9th Ed., ASM International, 1988.

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David J Grieve, 18th December 2008.