Effect of carbon equivalent on microstructure and impact toughness of high grade pipeline steel at ICCGHAZ

Microstructure and impact toughness of intercritically reheated coarse-grained heat affected zone (ICCGHAZ) of two high-grade pipeline steels with different carbon equivalents (Ceq = 0.46 and 0.54) were studied. Gleeble−3500 thermal simulation machine was used to simulate different partitions of the heat-affected zone (HAZ) of pipeline steel welding through thermal simulation experiments, and the microstructure of the ICCGHAZ was characterized via optical metallography (OM), scanning electron microscopy (SEM), electron back scatterd diffraction (EBSD) and transmission electron microscopy (TEM). The research showed that, in this region, the martensite-ausenite (M-A) component with a large island chain at the grain boundary preferentially becomes the crack nucleation site, and M-A component with a long rod in the grain easily initiates microcracks. When Ceq = 0.54 in ICCGHAZ, the M-A components in the microstructure are small in size and dispersed in the matrix, so the impact toughness is good, and the toughness embrittlement zone of the HAZ is narrow.