A discussion of the effects of composition and heat treatment on the toughness of a medium carbon secondary hardening steel

In our development of a lower cost, high toughness ultra-high strength steel we have examined the effect of nickel content on the strength and toughness of a base heat whose composition was similar to that of the hot work die steel H-11. The base heat was found to have exceptionally low toughness. Nickel additions improved the toughness significantly. Possible reasons for the low toughness of this base composition include: effect of secondary carbides precipitated with the grains; the decomposition of retained austenite to sheet like inter-lath cementite upon tempering; carbide precipitation on austenite grain boundaries during the air cool from the austenitizing temperature, and the formation of upper bainite during the air cool from the austenitizing temperature. The purpose of this work has been to examine these four hypotheses. The results suggest that the primary reason for the low toughness of the heat with no nickel was due to the formation of upper bainite during the air cool from the austenitizing temperature. It was found that nickel additions improve the hardenability and suppress the formation of bainite during the air cool. As a result, the toughnesses of heats modified with nickel additions were quite high.