Manufacturing Processes  MFRG 315  3.4.1 Forging 

3.4.1.1 Introduction
3.4.1.2 Applications
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^{*} d_{1})/(1.73 h_{1})) To calculate the total tool force the average interface pressure is needed: Where Q_{a} 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 unlubricated, 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, reheating when hot working and annealing when cold working allow further deformation. 3.4.1.6 Experimental Determination of K and n in cold working
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
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 ^{o}C

References:
1. 'Introduction to Manufacturing Processes', J A Schey, McGrawHill International,
1987  see chapter 4.
2. 'Metals Handbook', Volume 14, Forming and Forging, 9th Ed., ASM International, 1988.
David J Grieve, 18th December 2008.