Microstructure and properties of HAZ for X100 pipeline steel

In general, the weld thermal cycle results in significant changes in microstructure and mechanical properties of the weld heat affected zone （ HAZ）. The microstructure, microhardness and low temperature impact toughness of HAZfor X100 pipeline steel were studied by means of welding thermal simulation. Influence of cooling time on the microstructure and properties in coarse-grained heat affected zone （CGHAZ） was investigated. The results illustrated that polygonal ferrite and a small amount of granular bainite were obtained when the cooling time ts/5 is larger than 1 500 s. Mainly granular bainite was formed when the cooling time t8/5 is in the range of 1500 s to 100 s. Bainite ferrite was observed when the cooling time is smaller than 60 s. Martensite appeared in the CGHAZ with the 20 s cooling time. The value of microhardness in the CGHAZ was higher than that of base metal （ BM） when the cooling time ts/s is smaller than 100 s. The CVN absorbed energy in the CGHAZ was higher than the value of BM when the cooling time t8/5 is smaller than 30 s.