Grade 91 (9Cr-1Mo-V) steel is a common structural material for fossil-fuel fired power plants because of its creep strength and oxidation resistance at high temperatures. To remedy degradation caused by exposure to harsh service conditions, Grade 91 steel components are commonly subjected to onsite welding repairs. To ensure the welding process with/without post-weld heat treatment worked properly in field, there is a need for an effective field-deployable non-destructive evaluation (NDE) method that can detect and quantify the deleterious microstructure such as, untempered martensite in a repair weld.
In this research, we studied the feasibility of correlating the high-resolution images obtained from the Immersion Ultrasonic Testing technique to microstructure regions in Grade 91 steel plate weldments. Specifically, with this NDE method, the Fusion Zone (FZ), Heat-Affected Zone (HAZ), and Base Metal (BM) regions of two (2) samples, machined from two Grade 91 steel plate butt weldments, were examined. One weldment was fabricated using conventional flux cored arc welding (FCAW), and the other using low heat-input cold metal transfer (CMT) welding. Both weldments were post weld heat treated (PWHT). The samples were analyzed for microstructural differences including grain size and phases. Hardness profiles showed that the CMT weld joint contained less softening in the fine-grained HAZ (FGHAZ) and intercritical HAZ (ICHAZ) compared to the same regions in the FCAW weld joint.
The high-resolution ultrasonic images were made using 20MHz transducer with a 19 degrees longitudinal wave through transmission, and a 38-40 degrees Shear wave signal immersed in water. The ultrasonic testing results were analyzed using texture, beam propagation amplitudes and homogeneity, which in turn were used to relate to the different regions of the samples welded using FCAW and CMT processes.