Effect of C and Si contents on microstructure and impact toughness in CGHAZ of offshore engineering steel

The combined effect of the C and Si contents on the microstructure and low-temperature impact toughness of heat-affected zone (HAZ) of the offshore engineering steel. As the C and Si content is decreased from 0.09 to 0.07 wt.% and 0.12 to 0.03 wt.%, the HAZ toughness at −40 °C is increased from 36 to 180 J, and the hardness in the coarse-grained heat-affected zone (CGHAZ) is decreased from 325 to 297 HV 0.05 . The higher C and Si content promote the precipitation of M 23 C 6 and cementite, but restrain the formation of MC and M 2 C carbide in the present developed steels. The microstructures in the CGHAZ are comprised of a mixture of martensite and bainite after the welding with the heat input of 50 kJ/cm and are similar with the different C and Si contents. As the C and Si contents are decreased, the density of the geometrically necessary dislocation (GND) is increased. The fracture is changed from brittle to ductile manner, and the cleavage facet size is decreased from 23 to 11 µm. Reducing the C and Si contents significantly decreases the packet size from 25 to 17 µm, thereby leading to the improvement of HAZ toughness.