Influence of Plastic Deformation on Thermal Stability of Low Carbon Bainitic Steel

Bainite is metastable due to its high dislocation density, and consequently bainitic steel structures have the problem of thermal stability. Plastic deformation of bainite can further increase dislocation density and change dislocation configuration at the same time. The influence of plastic deformation on thermal stability of low carbon bainitic steel during isothermal holding at 650 degrees C was investigated with hardness analysis, in-situ tracing metallographic analysis and transmission electron microscopy. Bainite in the low carbon steel evolves into polygonal ferrite via recovery and recrystallization during isothermal holding at 650 degrees C. There is a considerably long period (about 20 h) between end of recovery and commencement of recrystallization of undeformed bainite, in which the hardness of the sample maintains a constant value slightly lower than that before reheating. Slight plastic deformation of bainite induces rearrangement of pre-existing dislocations and forming of low-energy dislocation cells inside bainite laths, which has little influence on thermal stability of bainite, whereas heavy plastic deformation results in obvious dislocation multiplication and accelerates recrystallization of bainite. Recrystallization of heavily-deformed bainite occurs preferentially at prior austenite grains boundaries. The samples subjected to heavy torsion exhibit obviously higher thermal stability than the samples subjected to heavy compression despite their same initial hardness, Which can be attributed to different influences of torsion and compression on dislocations and boundaries of bainite.