Setsuo Takaki1, Shunji Iizuka2, Kouki Tomimura1 and Youichi Tokunaga1
1Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka
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. For optical metallographic observation, striking changes were not found out 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. The dislocation density of matrix pronouncedly 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 the deformation to a pre-annealed specimen containing the stick type lath boundary carbides does not greatly affect the recovery behavior during the following annealing. The result also reveals the fact that the growth of lath boundary carbides controls the recovery of lath martensite matrix.
lath martensite, annealing, recovery, carbide morphology, preferential precipitation, dislocation density, mechanical property, transmission electron microscopy
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