ing load and stress distributions in the die at the instant of full die closure (assuming complete die fill). This interactive simulation of forging die performance gives the designer immediate feedback on the feasibility of his design, and allows him to modify his design and retest  it as  desired. The slab method is an approximate method of stress analy-

sis which assumes the stresses in the deforming material  to be constant in any plane through the thickness of the mate- rial. The effects of triction are conveniently restricted to the die-workpiece interface. This technique permits the pision of complex forgings into several building blocks which can be treated separately. These basic deformation units  fall under the categories of (a) plane strain, (b) axisymmetric double (inside and outside) flash, and (c)  axisymmetric single flash. For example, in the case of a connecting rod forging, the flow of metal at the two end bosses would be considered to be axisymmetric, with the smaller boss having a single flash (outside only) and the larger boss possibly having a double flash  owing to a partially  indented center. In contrast, the central web-rib section of the connecting rod is treated as a plane strain section because of the lateral flow constraint exercised by the die cavity. Estimation of the stresses and forging load also requires input of  flow stress and interfacial data under actual forging conditions.  The flow of metal at the interface with the die could be governed either by interfacial friction or by internal shear, and the Battelle computer program includes a surface flow model capable of predicting the transformation from friction  flow to shear flow in closed-die forging. The forging load predict- ed by computer simulation of a small connecting rod forging was validated through experimental trials which showed the predicted  load to be within  five pct  of  the measured   load.

P/M FORGING

R.M. Szary of Federal Mogul Corporation, in his presentation of the Sinta-Forge process, described it as Fed- eral Mogul’s patented commercial approach to powder met- allurgy (P/M) forging of steel parts. Commercial production began in 1972 and current production is at several million parts per month, according to Mr. Szary. For high fatigue stress components, Sinta-Forge begins with prealloyed, at- omized, irregularly shaped powders,  equivalent  to SAE 4600 steel in alloy content. These are blended with lubricant and a part of the required final carbon content in the form of graphite (the balance of the carbon content being made up during sintering). The powder blend is then cold-compacted into a preform shape which is designed  to be close  enough to the desired end shape that the subsequent forging opera- tion can be done in a single strike. The green comJ3aCt (preform) is sintered under a controlled atmosphere. Federal Mogul uses a patented  procedure  for this, whereby  cafbofi is infused  into the  preform  at controlled  rates,  resulting in

J. APPLIED METALWORKING

stratified layers with predictably different levels of carbon. This procedure eliminates separate carburizing treatments. The sintered preform is induction—heated and warm—forged to produce the high level of density  (7.8 g/cm’ or 99.6  pct of theoretical full density) necessary for good high-fatigue— stress performance. Parts presently range from pound to 20 pounds in weight, the largest being a bearing outer race. Dimensional precision of a 0.005 inch on inside and outside diameters, and f 0.004 inch on concentricity and roundness, can be met  in  small- to  medium-size parts.

Federal Mogul’s marketing success with Sinta—Forge has been in applications where high—cycle fatigue performance is critical. According to Mr. Szary, the improvement in high-cycle fatigue over conventional bearing steels stems from two reasons: one, the Sinta—Forge part has similar case and core microstructures as conventionally carburized material but with the much finer grain size characteristic of P/M; two, the high quality of the starting material  (Mr. Szary noted that there is currently only one United States supplier of forging—quality, prealloyed, atomized powder). Thus, any inclusions present are similar in size to very clean cast/wrought steels, but the uniform dispersion of inclusions in Sinta-Forge P/M parts  gives them  superior  fatigue life. On the subject of tooling and  equipment,  Federal Mogul uses H26 die steel and CBM 350 tool steel for their tooling, with a 20,000—part die life expectancy on core rods. Their cold compacting presses range up to 750 tons, and the upper limit on their forging press line is 2500 tons. Federal Mogul operates two manufacturing plants to produce P/M forgings, of which one has 130,000 square feet and  300 employees, and the  other  is expanding  to  a similar capacity.