Microstructure investigation on inner crack thermal healing in Q235 steel with La addition

Thermal healing experiment to inner crack made by compressing a drilled hole on samples of Q235 steel with various La addition was carried out. The effects of various heating temperature on the appearance and morphology evolution of inner crack healing in Q235 steel were investigated. Optical microscope and SEM observation indicate that artificial inner crack samples of Q235 steel with La addition can occur healing phenomenon at 650°C, while inner crack samples have apparent healing at 800°C and the microstructure in healing area is mainly ferrite. La in the solute solution decreases the critical healing temperature, makes the microstructure in host appear obvious variety during crack healing, not only decreases the quantity of pearlite and its shape becomes like-pearlite or granular but also increases the quantity of ferrite and the concentration of carbon in ferrite. The healing process was controlled by the diffusion and migration of Fe atoms from steel host to crack zone.