Diffusional creep contribution to the type IV fracture mechanism

Type IV fracture is the typical failure mechanism of welded joints which operate under creep conditions. This mechanism is the main cause of lacking performance of 9 and 12 wt-% chromium steels for high temperature applications [1]. The fine-grained heat affected zone (FGHAZ) is the region most sensitive to the premature cracking generally occurring with a limited overall failure strain. Detailed evaluation of the strain developed in uniaxial cross-weld creep specimens [2] has shown that deformation can be heterogeneous, i.e. type IV fracture develop due to relatively large strain over a very local region. In recent years, a number of experimental studies has investigated the occurrence of type IV failure in laboratory test pieces. It has been demonstrated that reducing the stress level, the failure mode changes from failures in the parent material to failures in the HAZ [3], furthermore the suppression of grain refinement increases the creep strength of crosswelds [4,5]. Several attempt to predict the creep joint behavior are available in literature [6,7]. Usually each of the distinct material regions of the weldments is assumed to behave differently by using different creep model parameters. This approach requires an expensive and time consuming characterization for the parent metal, weld metal and HAZ regions. In the present work the type IV fracture is investigated by a mechanism-based creep model [8,9] that account for both diffusional and dislocational contribution to the overall creep behavior. In this work, a physically-based creep model is used to predict the occurrence of type IV fracture in 9%Cr welded steel joint providing the variation of the microstructure as a result of the weld process. In the proposed model the creep rate is obtained as the sum of two terms: the first due to the lattice diffusion and the latter due to dislocation creep.