Role of carbide in the recovery process of 0.2% carbon lath martensitic structure

The changes in carbide morphology and matrix microstructure that occur during 973 K annealing have been investigated for a low carbon lath martensitic steel by optical and transmission electron microscopy. For optical metallographic observation, striking changes in the lath martensitic structure were not found even in long-time annealed specimens because carbides preferentially precipitate on block and lath boundaries to maintain the structural character of lath martensite. The morphology of carbides and matrix microstructure, however, are gradually changed during the annealing. Stick type carbides, which were formed along lath boundaries in the early stage of annealing, play an important role in suppressing the recovery of dislocations within the lathes. Dislocation density within the matrix pronouncedly decreases when the morphology of lath boundary carbides changes from stick to oval or sphere, leading to an unusual change in tensile properties. It was also confirmed that the deformation to a pre-annealed specimen containing the stick type lath boundary carbides does not greatly affects on recovery behavior during the further annealing. The result also indicates the fact that the growth of lath boundary carbides controls the recovery of lath martensite matrix.