Effect of Carbide Morphology on the Susceptibility to Type IV Cracking of a 1.25Cr-0.5Mo Steel

Abstract

Type IV cracking is considered to be the likely failure mode of ferritic steel welds when operated for long duration. The carbide morphology of the service-exposed 1.25Cr-0.5Mo steel weldment, which is composed of a forged flange and pipe fabricated from plates, has been examined before and after a creep test. Higher susceptibility to Type IV cracking was observed at the Intercritical HAZ (ICZ) on the flange side despite higher creep resistance of the parent material compared with a pipe. The change in carbide morphology during the creep exposure was the most pronounced at the flange ICZ. The coarse bainitic carbide originally existing depleted the intragranular carbides and significant variation in carbide density inside the ICZ was generated. In contrast, carbides at the pipe ICZ were more uniformly distributed. It was interpreted that higher susceptibility of the flange ICZ was accelerated by the heterogeneous distribution of carbide density and the resultant variation of creep strength and would enhance grain boundary sliding associated with creep strain accumulation.
It was proved that the susceptibility to Type IV cracking was highly dependent upon the characteristics of a parent material by experiments using simulated ICZ specimens. Significant difference in the effect of heat treatment to simulate the microstructure at the ICZ upon the creep strength was observed between a flange and pipe. The simulated ICZ specimen generated by a pipe parent showed no apparent change due to the heat treatment compared with a parent material. On the contrary, it reduced the time to rupture and changed the fracture mode from transgranular to intergranular for a flange material. The feature of grain boundary cracking was similar to that of actual weldment which took place preferentially at the inclined grain boundaries to the tensile direction, that was to be considered the evidence of grain boundary sliding.