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 so-called Type IV failure in the heat affected zone (HAZ). To study the creep behavior of the susceptible HAZ region responsible for the reduction in cross-weld creep behavior, phase transformation analysis and microstructural characterization techniques are being investigated as part of an overall effort to develop a standardized procedure for creating representative and relevant synthetic HAZ microstructures and samples. The current study aims to analyze the effects of heating rate and peak temperature in simulated samples of the completely transformed and partially transformed zones (CTZ and PTZ, respectively) in Grade 92.
Grade 92 HAZ samples were thermally cycled using a Gleeble™ 3800 Thermomechanical simulator. The 0.25 inch diameter samples were loaded with a free span of 1.0 inch to ensure consistent free cooling. Heating rates of 0.1, 1, 10, 100, 300, and 600 °C/s were chosen for the HAZ simulations to bound potential heating rates representing the commonly used arc welding processes for component fabrication. Peak temperatures range from 880°C (representative of the partially transformed zone [PTZ]) to 1250°C (representative of the completely transformed zone [CTZ]). All samples were prepared using standard techniques, evaluated using Vickers macrohardness (10kg load, 5 indents per sample), and etched with Vilella’s for light optical microscopy.
The document temperatures and hardness values follow a sequential format linking to heating rate values of 0.1, 1, 10, 100, 300, and 600°C /s. The Ac1 temperatures for the CTZ samples were 843, 852, 881, 912, 940, and 942°C. Ac3 temperatures for the samples were 914, 915, 955, 988, 1015, and 1021°C. The peak temperatures for the PTZ tests correspond to the midpoint of the Ac1 and Ac3 temperatures determined from the CTZ transformation temperatures for their respective heating rates: 880, 885, 918, 950, 980, and 982 C. The resulting Ac1 temperatures from the PTZ tests were 833, 847, 878, 911, 928, and 924 C. Ac1 and Ac3 temperatures were also determined for the variable peak temperature with constant heating rate and found to average 906 C and 975°C, respectively. The average hardness for the CTZ samples were 397, 437, 454, 455, 455, and 459 HV10. Average hardness for PTZ samples were 389, 369, 375, 400, 427, and 434 HV10. For the variable peak temperature tests with constant heating rate, hardness was reported as 400, 425, 435, 449, 455, 443, and 455 HV10 for 950, 1000, 1050, 1100, 1150, 1200, and 1250°C peak temperatures, respectively. For PTZ and CTZ tests, it was found the Ac1 and Ac3 temperatures increased with increasing heating rates and the martensite start and finish temperatures decreased with increasing heating rates. For the investigation into the effect of peak temperature using a constant heating rate, the Ac1 and Ac3 temperatures showed no significant change. Martensite transformation temperatures were, however, found to decrease with increased peak temperature. Hardness was found to increase with both an increased heating rate and peak temperature. LOM of all samples found that, qualitatively, grain size decreased with increasing heating rate.
Understanding the effect of heating rate and peak temperature in the HAZ during representative welding thermal cycles is essential to develop relevant microstructure(s). 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 this could not be conclusively assessed and will require more advanced electron microscopy to confirm. The findings presented in this research are being utilized to further discussion towards standardizing a procedure for the development of relevant and representative simulated HAZ microstructures in 9%Cr CSEF steels.