Study on the microstructure and hardness of in-service welded joint of X70 pipeline steel

摘要

Hydrogen induced cracking (HIC) is one of the main problems of in-service welding onto active pipeline. Microstructure and hardness of welded joint have a vital effect on hydrogen induced cracking. The microstructure and hardness of welded joint of X70 pipeline steel were studied using simulation in-service welding device. The results show that the main microstructures of in-service welded seam are grain boundary ferrite, intracrystalline acicular ferrite, as well as small amount of widmanstatten structure. The main microstructures of coarse grain heat-effected zone ( CGHAZ) are coarse granular bainite, lath ferrite and martensite. Metastable phases such as martensite and lath ferrite are found in CGHAZ because of the too quick cooling velocity and the hardness of the CGHAZ is high.