The Role of Precipitated Austenite in the Grain Refinement of Lath Martensitic Steel

摘要

Intercritically tempered (5-9)Ni steels are toughened by the introduction of islands of precipitated austenite along the boundaries of dislocated martensite laths. The primary toughening mechanism is the refinement of the effective grain size of the dislocated martensite packets that make up the microstructure. The interlath islands of precipitated austenite transform to martensite under the stress field of a cleavage crack that propagates along the common {001} cleavage plane of the martensite packet. However, the austenite does not transform to the martensite variant of the packet, but rather to a crystallographically different variant that represents a different Bain variant of the transformation. The result is to disrupt the cleavage plane at the fresh martensite boundary, efficiently refining the effective grain size for fracture. A straightforward calculation shows that this behavior is dictated by the need to minimize the elastic strain energy of the fresh martensite.