Applicability of C* parameter in assessing Type IV creep cracking in Mod. 9Cr–1Mo steel welded joint

A welded joint of Mod. 9Cr–1Mo steel, whose Type IV cracking behavior is an important issue to be assessed, was subjected to a series of creep crack propagation experiments in order to clarify the applicability of existing standard ASTM E1457-98. Standard 1T-C(T) specimens made of base metal (BM) and welded joint (WJ), in which the heat affected zone (HAZ) was set to be the crack plane, were subjected to the experiments under 600, 650, and 700 °C, and with a few load level conditions. While the crack planes of BM specimens were fairly flat, those in WJ specimens showed bumpy surfaces following the shape of multi-path weld beads. The cracks in WJ specimens were of typical Type IV cracking, and their crack passed through nearby the interface of BM and weld metal. There are the fairly good relationships between the creep crack propagation rate (da/dt) and C* parameter. All the BM and WJ data fallen in each one C*–da/dt relationship for BM and WJ, respectively, regardless of the temperature and load level. The C* parameter used here is defined for the homogeneous material and does not give a physically correct C* for WJ, nevertheless all the WJ data still tends to gather each other on single C*–da/dt relationship. This fact suggests that the geometrical limitations of E1457-98 standard also can act well as the limitation for the inhomogeneity of weld structure and may eliminate the effect of large scale inhomogeneity due to the combination of BM and weld metal. The da/dt of WJ were about 3–10 times faster than that of BM for the same C* value. This difference can be attributed as the effect of difference in triaxiality, the relative constraint between the weld metal and the base metal, or the difference in resistance for creep crack propagation in HAZ material.