Ductility dip cracking (DDC) is known to occur in highly restrained welds and structural overlays made using high chromium (Cr) nickel (Ni) based filler metals in the nuclear power generation industry, resulting in costly repairs and reworks. Previous work explored the role of mechanical energy imposed by the thermo-mechanical cycle of multipass welding on DDC formation in a highly restrained Alloy 52 filler metal weld. It was hypothesized that imposed mechanical energy (IME) in the recrystallization temperature range would induce dynamic recrystallization (DRX), which is known to mitigate DDC formation. It was not shown however that IME in the recrystallization temperature range (IMERT) induced DRX. The objective of the work is to discern if a relationship between IMERT and DRX exists and quantify the amount of DRX observed in a filler metal 52 (FM-52) groove weld. DRX was analyzed and quantified using electron beam scattered diffraction (EBSD) generated inverse poll figures (IPF), grain surface area and grain aspect ratio distribution, grain orientation spread (GOS), kernel average misorientation (KAM), and grain boundary (GB) length density. From the analysis, GOS was determined to be an unsuitable criterion for quantifying DRX in multipass Ni-Cr fusion welds. Based on the observed criteria, higher IMERT regions correlate to smaller grain surface area, larger grain boundary density, and higher grain aspect ratio, which are all symptoms of DRX. High IMERT has a strong correlation with the symptoms DRX, but due to the lack of observable DRX, creating a threshold for DRX grain size, grain aspect ratio, and GB density is not possible. Future work will aim to optimize characterization criteria based on a Ni-Cr weld with large presence of DRX.

Grade 91 steel has been found to be susceptible to Type IV cracking in the base metal heat affected zone (HAZ). In order to better understand this type of failure, a study on the metallurgical reactions occuring within the HAZ was conducted, particularly within the fine grained (FG) and intercritical (IC) regions where Type IV cracking is most commonly found to occur. The course grained (CG), FG and IC regions of the HAZ in Grade 91 steel were simulated using a Gleeble 3800 Thermo-Mechanical Simulator. A dilatometer was used to determine the phase transformations occuring during simulation of weld thermal histories. For the first time, it was shown that ferrite can form in the IC HAZ of Grade 91 steel welds. The magnitude of the ferrite transformation was observed to decrease with faster cooling rates. The presence of ferrite in the simulated IC HAZ microstructure was shown to decrease the high temperature tensile strength and increase the high temperature elongation compared to HAZ regions that did not undergo ferrite transformation. Welding parameters such as heat input, preheat and interpass temperature can be selected to ensure faster cooling rates and reduce or potentially avoid formation of ferrite in the IC HAZ.

Unpredictable failures near the phase boundary in Grade 91 dissimilar metal welds (DMW) with nickel based filler metals represent a significant problem for the power generation industry. In order to determine the root cause for these failures, it is necessary to understand the formation of the microstructure in the weld regions around the site of failure. Thermal histories were therefore measured inside the Grade 91 steel heat affected zone (HAZ) of an autogenous weld and of a DMW in the form of bead on plate with Alloy 625 to study the effect of the weld thermal cycle on microstructural formation. It was found that the HAZ in the DMW experienced longer dwell time at high temperatures because of the latent heat of fusion released during Alloy 625 solidification (1350 - 1125 °C). This allowed longer time for carbide dissolution and phase transformations in the DMW than in the autogenous weld. Additionally, the 625 filler metal created a large chemical potential gradient for carbon, which when combined with longer dwell times, yielded carbon depletion in the heat affected zone. Retention of δ ferrite in the coarse grained HAZ (CGHAZ) of DMWs was found to be an indicator for these mechanisms.