Strength and Tonghness of Tempered Martensite and Bainite in 5%Ni-0.5%Mo Steels

The relation between strength and toughness of tempered martensite and bainite was investigated for 5%Ni-0.5%Mo steels containing 0.15% or 0.30%C with various austenite grain size. When toughness of martensite is compared with that of bainite in the same strength level, former is more affected by austenite grain size and less affected by carbon content.The surface and cross sections of fractured specimens after impact tests were examined by scanning and ordinary electron microscopy in order to investigate the structural factors affecting toughness. These microscopic observation indicated that the fracture facets were formed at the large martensites or bainites and at the packets of relatively thick laths which were found when the austenite grains were coarse or the C content was low. Although the concept of the effective grain size seemed to be valid in the martensite and bainite structure as the fracture facet size coincided with those of packets in these structures, behavior of toughness was not completely explained by this factor only. The role of carbide to impair touhgness is emphasized from the point that the increase of C content decreases the toughness of the bainite, in spite of the structure becoming fine, where the carbides become coarse and abundant.