Fatigue behavior of binder-treated P/M steels

Abstract

The microstructure, tensile, and axial behavior of Fe-Mo-Cu-Ni alloys made by binder-treated processing were investigated and the attendant mechanical behavior compared to that of an anologous processed by diffusion alloying. Binder tretatment can provide a variety of advantages in manufacturing over diffusion alloyed powders, including faster and more consistent flow into the die cavity, increased green strenght, and reduction of fine 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 particles. The heterogenous microstructure in both alloys was typical of P/M alloy steels, consisting of areas of divorced pearlite, martensite, and nickel-rich ferrite. Tensile and fatigue resistance were enhanced by an increase in the molybdenum content in the alloys. The tensile strenght of both types of alloys was similar and fatigue life was essentially identical for the two systems. Fractographic observations showed that fracture initiated primarly at pore clusters in the surface region. Investigation of small cracks by a surface replication technique showed that fatigue cracks by a surface replication technique showed that fatigue cracks nucleated at pores or pore clusters, and that crack propagation exhibited a significant amount of deflextion and branching, attributed to local obstacles in the interparticle bridge regions, cleavage facets in pearlitic regions, and striations due to cyclic loding.