Richard H. Johnson (III)
Published: 2021
Total Pages: 0
Get eBook
Grade 92 is a creep-strength enhanced ferritic (CSEF) steel widely used in the power generation industry. This steel shows a clear reduction in the cross-weld creep performance resulting in Type IV failure in the heat affected zone (HAZ). To study the creep behavior of the susceptible HAZ region responsible for reduction in cross-weld creep behavior, phase transformation analysis and microstructural characterization techniques are being utilized as part of an overall effort to develop a standardized procedure for creating representative and relevant synthetic HAZ microstructures and samples. Simulated and real weld HAZ microstructures are characterized using optical and electron microscopy techniques. Simulated Grade 92 HAZ samples were prepared using a GleebleTM 3800 Thermomechanical simulator. Heating rates for the HAZ simulations represented furnace heating and commonly used arc welding processes for component fabrication. Peak temperatures range from 880°C to 1250°C, representative of the partially transformed zone (PTZ) and completely transformed zone (CTZ), respectively. All samples were prepared using standard techniques, etched with Vilella’s reagent for optical microscopy, analyzed using SEM imaging, EBSD, and carbon replica extraction in the TEM for carbide analysis. Simulated samples were then compared to bead-on-plate samples created using representative heat inputs. Dilatometry results from GleebleTM HAZ simulations confirmed Ac1 and Ac3 transformation temperatures for each heating rate used in this study. Simulated samples were then created in the CTZ well above the Ac3 temperature, PTZ between the Ac1 and Ac3 temperatures, and PTZ above the Ac3 temperature. Bead on plate tests were conducted on 1” Grade 92 plates using 20, 35, and 50 kJ/in heat inputs to represent SMAW, SAW, and GTAW processes. BOP tests were cut and measured for thermocouple placement for thermal history acquisition. Previous studies found that increasing the heating rate for the simulated HAZ was found to increase hardness and the austenite phase transformation temperatures and decrease the martensite transformation temperature during free cooling. It is suspected that carbide dissolution and precipitation behavior is affected but more advanced characterization techniques were required to confirm. Simulated samples were analyzed using SEM imaging and EBSD to characterize prior austenite grain size, grain misorientation, carbide size, and distribution. Carbon replica extraction techniques were used for TEM analysis to identify and characterize carbides found in the HAZ. Microhardness mapping was also conducted. Analysis performed on real and simulated samples enabled the complete characterization of the region of interest and will lead to development of a standardized procedure for replication of relevant simulated microstructures using resistive and furnace heating methods. The characterization techniques employed in this study were used to define microstructural characteristics in real Grade 92 weld heat affected zones and compared to simulated samples to determine their efficacy. Carbide analysis resulted in a model to predict carbide behavior as a function of peak temperature and heating rate. Using advanced electron microscopy characterization techniques lead to standardizing a procedure for the development of relevant and representative simulated HAZ microstructures in 9%Cr CSEF steels.