The Microstructure And Mechanical Properties Research Of Flow Forming T-250 Co-free Maraging Steel

The influence of different heat treatment process on microstructure and mechanical properties of flow forming T-250 Co-free maraging steel was investigated in this paper. The α'→γ transformation temperature of solution treated and flow forming steel was measured by thermal dilamation device and the recrystallization temperature of deforming martensite was examined by optical microscope(OM). Hardness and strength changing regulations under various heat treatment conditions was also studied. The microstructure of the alloy in different heat treatment condition was observed using transmission electronic microscope(TEM), and the α'→γ cyclic transformation grain refinement process was applied to T-250 Co-free maraging steel.Results showed that the reversed austenite transformation starting temperature As of four pass flow forming T-250 Co-free maraging steel after 815℃ solid solution treating and quenching was 557℃, and the finishing temperature Af was 718℃. Cold flow forming make As temperature increased and Af decreased. So the austenite transformation range become smaller.The recrystallization of deformation martensite of four pass flow forming T-250 Co free maraging steel was 530℃ and finished at 600℃. The recrystallization temperature of one pass flow forming samples increased due to the smaller deformation. The deforming microstructure can be eliminished when the temperature reached 815℃.Mechanical and hardness tests presented that ultimate strength of three pass flow forming T-250 steel, after solid solution treating between 815°℃-920℃ and aging at 510℃ for 180min, would decrease and plasticity would increase with the solid solution temperature raising.Three pass flow forming samples, after solid solution hardened at 815℃, aged between 450℃-600℃, the hardening rate in the early stage become faster and peak hardness decreased with the temperature increasing. The tensile test results showed that changing the aging time from 180min to 60min at 510℃ can improve the strength and the elongation percentage remain above 5%.TEM observations indicate that a great deal of small and disperse precipitates formed in lath martensite matrix, which result in the ultimate high strength of T-250...