The influence of the M23 (C,N)6 compound on the mechanical properties of type 422 stainless steel

Abstract

Ductility and toughness of Type 422 stainless steel was shown to be strongly influenced by either the presence or absence of large grain boundary precipitates of the M23(C,N)6 compound. A lengthy, isothermal solution heat treat cycle combined with a longer, lower temperature precipitation heat treatment was used to exaggerate the M23(C, N)6 compound along the boundaries of large austenite grains. After a subsequent conventional heat treatment, the presence of this compound along the prior austenite grain boundaries substantially reduced the ductility and the Charpy V-notch impact energy. The primary fracture mode was intergranular failure along these prior austenite grain boundaries. This is a reversible process and this compound can be completely dissolved. The M23 (C, N)6 compound was removed from these prior austenite grain boundaries by another lengthy, isothermal resolution heat treatment. After a subsequent conventional heat treatment, the ductility returned to values which are near those found in as-received and heat treated material; the Charpy V-notch impact energy increased to values above that found in commercially processed Type 422 stainless steel. The fracture appearance returned to the more normal transgranular ductile dimple mode.