Influence of Extra Coarse Grains on the Creep Properties of 9 Percent CrMoV (P91) Steel Weldment

Abstract

With modern welding methods, satisfactory microstructures in 9%CrMoV (P91) steel can be obtained with a modest variation in hardness and prior austenite grain size. However, there is always a risk that significant deviations in the properties can be obtained, if the welding parameters are not optimized. In the present paper the role of extra coarse grains in the heat affected zone (HAZ) has been studied. Creep tests were carried out at 600°C for parent metal, weld metal, cross weld, simulated extra coarse grained HAZ, and simulated intercritical HAZ of a 9%CrMoV (P91) steel. The parent metal, the cross welds, the weld metal, and the simulated intercritical HAZ had about the same rupture strength except at long rupture times, where the values for the cross welds were considerably lower. In the cross welds, rupture took place in the intercritical HAZ at longer times (Type IV cracking). The simulated extra coarse grains gave considerably longer rupture times, lower strain rate and lower creep ductility than the parent metal and the weld metal. The creep strain behavior was successfully analyzed using the Omega model where the log creep strain rate is linear in the creep strain.

Creep Strain Modeling of 9 to 12 Pct Cr Steels Based on Microstructure Evolution

“…The dashed line corresponds to the numerical F model for primary creep and X model for tertiary creep:[26] …”

Creep in phosphorus alloyed copper during power-law breakdown

“…The form of Eq. (19) can be seen as a generalisation of (18) and these two equations are combined in a unified model…”

A phenomenological model for anisotropic creep in a multipass weld metal

“…It is well established that creep behavior is very sensitive to the type of microstructure and, in particular, to the grain size. Experimental data illustrating the significant influence of grain size on creep behavior are presented for copper in [22] and for various types of steel in [24,30,31]. The grain-size dependence is explained in materials science by two creep mechanisms: grain boundary sliding and grain boundary diffusion.…”

“…The results of creep testing of cross-weld specimens [19,20] and specimens with a simulated microstructure [24,25,30,31] show significant variation in creep properties in different material zones within the weld. Furthermore, they illustrate that the intercritical region of the heat-affected zone is the weakest part of the weld with respect to creep properties.…”