Relation Between Microstructure and Strength in Conventional and Powder-Processed High-Speed Steels

Abstract

Mechanical tests and micro structural examinations have been performed on various tool steels, mainly M2 grades, manufactured by conventional cast and wrought processes and by powder metallurgy routes. A recently developed square indentation test, used for comparing strength proved to be superior to bend tests in flexibility and in discriminating between the various steels. Most of the powder metallurgy products were as strong as the conventional steels but they displayed more variability, partly because pores and inclusions behaved as incipient cracks. Oxygen in amounts as high as 0·054% did not affect the strength of the hot-extruded powder steels significantly, except when it was combined with other impurities to form coarse inclusions. Generally, it appears that the strength of M2 grades is probably determined by a microstructural feature such as a weakness at grain boundaries or carbide/matrix interfaces, which is also related to the aspect ratio of the prior austenite grain structure. Further examples of the application of the square indentation test are described. PM/0194