Effects of Boron on Increasing Toughness of High Strength High Manganese Non-Magnetic Steels

Abstract

High Mn non-magnetic steels, which contain above 30% Mn, are liable to embrittle at low temperatures when cooled at a slow rate from annealing temperature or reheated at about 873K due to intergranular precipitation of carbide or nitride. Therefore, these steels are generally recommended to be cooled at a fast rate from annealing temperature. Shibata et al.1, however, observed that cryogenic intergranular fracture was enhanced by water-quenching in a 32Mn-7Cr-0.3N steel. They also observed that the steel was toughened by reheating1 at around 773K after water-quenching or by cooling2 at an intermediate rate from annealing temperature through suppression of the intergranular fracture. These two heat treatments which improve the cryogenic toughness will be referred in the present paper to post-annealing heat treatments. These phenomena, that is to say, the high susceptibility to cryogenic intergranular embrittlement and cryogenic toughening through the post-annealing heat treatments, were also revealed by Shibata et al.1, 2 in other high Mn non-magnetic steels, which had different content of Mn, Cr, C and N. The detailed mechanism and reason for these two phenomena are discussed in the previous papers by Shibata et al.3, 4