Abstract: The relationship between microstructure and toughness of the welding heat-affected zone (HAZ) of a pipeline steel is investigated by means of thermal simulation test and transmission electron microscope (TEM). It is found that the inter-critically reheated coarse-grained heat-affected zone (IRCGHAZ) of experimental steel has the lowest toughness when the second peak temperature is at (α+γ) region during multi-pass welding. This local embrittlement is mainly attributed to the morphology, amount and size of M-A constituent. It is also found that the microstructure inhabitance at IRCGAHZAZ has a deleterious effect on the toughness. On the basis of above experimental results, it is suggested that the local embrittlement should be prevented by using pre-heating cycle, which could eliminate the microstructure inhabitance, and using temper-heating cycle which could improve the morphology, amount and size of M-A constituent.

Key words: Embrittlement, HAZ, Pipeline steel, Prevention