Study On Microstructure And Properties Of Haz Of X70 Pipeline Steel With Excellent Deformability

Pipeline steel with excellent deformability is a kind of pipeline steel based on strain design and can stand up lager deformability generated during geological disasters. In order to understand the microstructure and mechanical properties of pipeline steel after welding, the grain growth kinetics and phase transformation kinetics of coarse grained heat affected zone(CGHAZ) were investigated using a Gleeble-3500 thermal-mechanical simulator and a dilatometer. Optical microscope(OM), scanning electron microscopy(SEM) and transmission electron microscopy(TEM) were used to investigate the change of microstructure. The results of microstructure indicate that, with the increasing of heat input, the grains of CGHAZ grow very quickly, the volume of lath bainite decreases while the volume of granular bainite increases. In the intercritical heat affected zone(ICHAZ), the austenitizing volume increases with increasing heat input which leads to the change of microstructure. The amount of M/A constituent of the intercritical coarse grained heat affected zone(ICCGHAZ) decreases with increasing heat input. The subcritical heat affected zone(SCHAZ) is the special sub-region which has no phase transformation. However, with the increasing of peak temperature, blocky Fe3 C begins to precipitate and grows from the interface of M/A constituent and ferrite. With the increasing of heat input, the size of blocky Fe3 C decreases following increasing and acicular Fe3 C begins to precipitate and grow in the M/A constituent. The dislocation density decreases with increasing peak temperature or increasing heat input. The results of mechanical properties show that the sub-regions have been soften differently except CGHAZ and ICCGHAZ. The impact energy of CGHAZ and ICCGHAZ dropped a lot and became the most deteriorated sub-regions.