Microstructure and Properties of Weld CGHAZ under Different Heat Input for X90 Pipeline Steel

Two kinds of industry trial X90 pipeline steels which have different chemical composition were chosen as test objects, and the grain coarsening, microstructural characteristics and the variation rules of low-temperature impact toughness in weld CGHAZ of this two steel under different welding heat input were studied by physical thermal simulation technology, SEM, optical microscope and Charpy impact tests. The results showed that the microstructure in weld CGHAZ of 1# steel was mainly bainite ferrite (BF) and most of the M/A constituents were blocky or short rod-like; the grains of 2# steel were coarse and there was much granular bainite (GB). Meanwhile M/A constituents became coarser and their morphology changed from block to long bar; alloy content of X90 pipeline steel under different weld heat input had great effect on the grain size of original austenite. When heat input was lower than 20KJ/cm, the impact toughness in CGHAZ of lower alloy content pipeline steel was good; as heat input increased, impact toughness in CGHAZ of 1# steel increased to the values between 260J and 300J when heat input was between 20KJ/cm and 25KJ/cm and the dispersion of impact energy was small. The impact toughness of 2# steel decreased gradually and the impact energy had the obvious dispersion.