BEHAVIOR OF CGHAZ TOUGHNESS AND PRECIPITATES OF UNDERWATER WELDED Ti-V-Nb MICROALLOYED PIPELINE STEELS

Microstructure, impact toughness and precipitation behaviors of Ti-V-Nb particles in both the underwater and in-air welded coarse-grained heat-affected zone(CGHAZ) of X60, X65 and X70 microalloyed pipeline steels were investigated.The precipitate composition was also examined with Energy Disparsive X-ray(EDX) unit.The results showed that grain size of high-Ti steels (X60 and X70) CGHAZ zone was smaller than that of low Ti steels(X65) and impact toughness higher than that of it.It is related to the stability of different microalloying precipitates(Ti-rich and V-rich) in the thermal weld cycles and their different size distributions in parent materials.Higher cooling speed of underwater wet welding resulted in the presence of greater quantity martensite in the CGHAZ so that their impact toughness was lower than that of in-air welding.