The crystallography of secondary carbide precipitation in high speed steel

Abstract

Precipitation of secondary carbides in a powder metallurgical high speed steel, ASP 23, has been investigated by transmission electron microscopy. Particular attention has been paid to the crystallographic orientation relationships between the precipitated phases and the ferrite matrix. During the hardening treatment prior to tempering, cementite precipitates on prior-austenite grain boundaries and twin boundaries within the martensite plates. The cementite is oriented with respect to one of the adjacent ferrite lattices according to the Bagaryatskii orientation relationship. During tempering at 560°C the cementite coarsens and fine dispersions of MC and M 2C precipitate within the martensite plates. MC obeys the Baker-Nutting orientation relationship while M 2C obeys both the Pitsch-Schrader orientation relationship and another relationship defined by: (0001) M 2C //(021) x; (11¯20) M 2C //(100) x; (¯1100) M 2C //(01¯2) x. The coexistence of both of these orientation relationships for M 2C precipitated in ferrite is explained in terms of the near-coincident site lattice model.