Effect of Heat Treatment on Microstructure and Hardness of Internal Crack Healing in a Low Carbon Steel

The effect of heat treatment on microstructure and hardness of internal crack healing in a low carbon steel was studied. The internal cracks were produced into the samples by a drilling and compression method. The microstructure of crack healing zone was examined using optical microscopy (OM) and scanning electron microscopy (SEM). The hardness of crack healing zone was measured using a Vickers micro-hardness testing machine (FM-800). The results show that healing temperature plays a more significant role in internal crack healing than holding time. Compared as-quenched samples with as-normalized samples under the same healing parameters, it is found that cooling speed is also an important factor for internal crack healing. The migration and enrichment of iron atoms provide material source for recrystallization and grain growth of crack healing zone. The existence of micro-voids leads to the hardness of the ferrite in the crack healing zone lower than that in the matrix.