Heat Treatment and Fatigue Behavior of PM Steels: Guidelines from MPIF Standard # 35

摘要

MPIF (Metal Powder Industries Federation) Standards are the most complete and duly updated reference for design engineers involved in material selection and part design. The data collected by MPIF are based on the results from independent test programs and are here analyzed to obtain characteristic ratios between properties of PM steels, with special attention to those between bending fatigue limit (FL) and UTS or be-tween FL and TRS. These ratios enable to rate the different possibilities of plastic yielding, before failure, under external actions. The ratios FL/UTS allow to assimilate most PM steels to grey cast iron: pore effects recall those of graphite flakes; only few PM steels behave in fatigue in a manner recalling that of nodular cast irons. The possible effects of porosity on notch sensitivity are discussed. The porosity gradient from part sur-face to part core appears another reason to explain the low notch sensitivity, which is a positive characteristic of PM steels. The correlation, proposed by Murakami, between HV hardness and FL emerging from the data (only for homogeneous materials) differs from that one characterizing fully-dense quenched and tem-pered steels, but the slopes are equal in both cases. The possible influence of density via Young' modulus, following an approach recently proposed by Bergmark is investigated. The different effects of heat treatment on typical families of PM steels (grouped by the MPIF coding system) are compared and the possible varying role of alloy additions - and dependence on sintering conditions as well - are discussed. Not-completely dif-fused nickel confirms its positive influence even after heat treatment. Some data summarize the indication from MPIF Standard and the results of the study to draw guidelines useful to help design engineers towards reliable and sound choices of PM steels for demanding applications, at different levels of fatigue stress.