Evolution Behaviors and Mechanisms of Internal Crack Healing in Steels at Elevated Temperatures

The evolution behaviors and mechanisms of internal crack healing in three different kinds of steels, including 20 high-quality carbon structural steel, 30Cr2Ni4MoV steel, and SUS304 stainless steel, at elevated temperatures were systematically investigated. The morphology of the crack healing zone was observed using an optical microscope (OM) and a scanning electron microscope (SEM). The grain boundary character distribution evolution of the crack healing zone was investigated using electron backscatter diffraction (EBSD). The results show that the existence of two crack healing mechanisms is confirmed in all three experimental steels. Crack healing is controlled by atomic diffusion at lower temperatures 1173 K (≤ 900 °C) and depends mainly on recrystallization and grain growth mechanisms at higher temperatures 1273 K (≥ 1000 °C). Atomic diffusion provides materials for recrystallization and grain growth in the crack healing zone. Recrystallization leads to rapid crack healing, and grain growth facilitates microstructural homogenization and elimination of the crack healing zone. The process of internal crack healing can be divided into five stages.