Grain boundary character and precipitates in 15Cr-30Ni-2Ti-3Cu valve alloy after solution treatment

The microstructure, precipitates and grain boundary character of 15Cr-30Ni-2Ti-3Cu valve alloy after different solution treatments were investigated. The results show that the precipitates in the forged alloy are Laves phase, (Ti,Nb)C carbides and Sigma phase. The grain sizes of the alloy are 35.21 +/- 12.17 mu m (980 degrees C/1 hour) and 53.14 +/- 15.88 mu m (1015 degrees C/1 hour), respectively, both of which exhibit a single peak distribution, indicating that the austenite grains show normal growth mode. A two-stage grain growth model was established to predict austenite grain growth during solution. The average absolute relative error of the predicted values is 4.93 % and the correlation coefficient of linear fitting is 0.983. According to grain boundary engineering analysis, the ratio of special grain boundaries in the alloy after the solution treatment at 980 degrees C for 3 hours reaches the largest (63.1 %). The activation energy for (Ti,Nb)C carbide dissolution is 223.6 kJ/mol based on the modified JohnsonMehl-Avrami-Kolmogorov (JMAK) model calculation. The microhardness of the alloy is the highest in the forged state. The microhardness decreases first and then increases with the extension of the soaking time, and reaches the maximum value of 238.7 +/- 8.4 HV after the solution treatment at 1050 degrees C for 3 hours.