Fatigue behavior of binder-treated P/M steels

The microstructure, tensile, and axial fatigue behavior of Fe-Mo-Cu-Ni allo ys made by binder-treated processing were investigated and the attendant me chanical behavior compared to that of an analogous alloy processed by diffu sion alloying. Binder treatment can provide a variety of advantages in manu facturing over diffusion alloyed powder, including faster and more consiste nt flow into the die cavity, increased green strength, and reduction of fin e particle dusting. In addition to conventional porosity, smaller, "copper diffusion" pores were observed where copper particles were present prior to forming a liquid phase during sintering and diffusion into the iron partic les. The heterogeneous microstructure in both alloys was typical of P/M all oy steels, consisting of areas of "divorced pearlite," martensite, and nick el-rich ferrite. Tensile and fatigue resistance were enhanced by an increas e in the molybdenum content in the alloys. The tensile strength of both typ es of alloys was similar and fatigue life was essentially identical for the two systems. Fractographic observations showed that fracture initiated pri marily at pore clusters in the surface region. Investigation of small crack s by a surface replication technigue showed that fatigue cracks nucleated a t pores or pore clusters, and that crack propagation exhibited a significan t amount of deflection and branching, attributed to local obstacles in the microstructure, such as Ni-rich areas. Fracture surfaces showed ductile fra cture in the interparticle bridge regions, cleavage facets in pearlitic reg ions, and striations due to cyclic loading.