Effects of immediate water cooling and normalization after welding on microstructure and hardness of heat affected zone of ultra-fine grain steels welded joint

To refine the grain size of the coarse grain heat affected zone (CGHAZ) in the gas tungsten arc welded joints of ultra-fine grain (UPG) steels of 400 MPa strength, the immediate water cooling before transformation completion and normalization after welding were performed. Their effects on the microstructure and hardness of the HAZ were investigated. The results show that although the grain size of CGHAZ does not decrease for the immediate water cooling, the width of CGHAZ decreases compared with the air cooling condition. The effective grain sizes in CGHAZ are significantly refined by first time normalization treatment after welding, while the grain sizes in the fine grain heat affected zone (FGHAZ) become coarser than those before normalization. The hardness of HAZ of the joint subjected to immediate water cooling is higher than that of the joint subjected to air cooling. The whole hardness of each joint after normalization decreases much. There is a favorable hardness match between the HAZ and the original base metal after first normalization treatment only for the immediate water cooling joint. Therefore, the immediate water cooling followed by one time normalization is recommended to refine the grain size in CGHAZ and to make the hardness of HAZ close to that of original parent materials.