Microstructure and Properties of Heat-Affected Zones in X100 Steel Grade Line Pipes

The relationship between microstructure and properties of weld heat-affected zones in X100 grade pipeline steels was studied. It was found that the intercritically reheated coarse-grained heat-affected zone (IRCGHAZ) of experimental steels has the lowest toughness values when the second peak temperature is at the intercritical (α + γ) region during multi-pass welding. The local embrittlement is attributed to the morphology, amount, and size of the M-A constituent. It is also found that the microstructural inheritance at IRCGHAZ has a deleterious effect on toughness. On the basis of the experimental results, it is suggested that the local embrittlement could be prevented by using pre-heating or post-heating thermal cycle. Pre-heating thermal cycle would eliminate the microstructural inheritance and meliorate M-A constituent. Furthermore, the use of a post-heating thermal cycle will improve the morphology, amount and size of the M-A constituent, and improve the conformation of ferrite matrix.