Change in carbide morphology and microstructure during recovery in 0.2%C lath martensite

The changes in carbide morphology and matrix microstructure that occur during 973 K annealing have been investigated for a low alloy 0.2%C lath martensitic steel by optical and transmission electron microscopy. From optical metallographic observation, striking changes 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 are gradually changed during the annealing. Stick type carbides which formed along lath boundaries in the early stage of annealing play an important role in suppressing the recovery of dislocations within the lathes. The dislocation density of the matrix decreases when the morphology of lath boundary carbides changes from stick to oval or sphere, leading to a discontinuous change in tensile properties. It was also confirmed that deformation to a pre-annealed specimen containing stick type lath boundary carbides does not greatly affect recovery during the following annealing. The growth of lath boundary carbides controls the recovery of the lath martensite matrix.