Creep Behavior and Rupture Life of the Simulated Intercritical HAZ for 1.25Cr–0.5Mo Steel under a Multiaxial Stress State

Abstract

For the life assessment of an elevated temperature component under complex loading, the effect of a mutiaxial stress state upon creep behavior must be taken into account. From a practical point of view, quantifying the creep life of welds exposed to mutiaxial stresses should be the first priority since welds are often located at the change in geometry. For the components fabricated from low alloy ferritic steels, the most likely failure mode should be Type IV cracking taking place at Intercritical HAZ (ICZ).
Thus, the triaxiality effect upon the creep behavior of ICZ for 1.25Cr–0.5Mo steel was examined using semicircular notched specimens. Though grain boundary damage was accelerated when tested by notched specimens, all the notched specimens showed notch strengthening behavior. To quantify the effect of the equivalent stress and maximum principal stress, stresses given by a code of practice for notched bar creep rupture testing were correlated with rupture lives of notched specimens. It was found that the equivalent stress played a major role in determining the mutiaxial rupture life for ICZ.
The above finding suggests that the Monkman–Grant type relationship between rupture life and Θ₄ of the theta projection, which was found in the uniaxial creep tests, could be also applicable to a multaxially stressed component since creep deformation is uniquely determined by the equivalent stress. As a matter of fact, the rupture lives of notched specimens were also well correlated with Θ₄. As a new scheme for the nondestructive life assessment, measuring the value of Θ₄, which is equivalent to the slope of strain rate versus strain at the tertiary creep regime, is considered to be potential.