Abstract:The solution treatment and thermal treatment (TT) processes of GH690 tube made in China were investigated with different processing parameters including temperature and time. The microstructural features, consisting of the grain size, carbide morphology along grain boundary and chromium-depleted zone of the investigated GH690 tube, were characterized through OM, SEM and TEM and evaluated based on Inconel 690 commercial tube. The results indicate that the proportion of large grains in the investigated GH690 tube increases gradually with increasing the solution temperature. The activation energy of grain growth is 265 kJ/mol. The influence of time on grain size becomes significant once the solution treatment is above 1100 °C. Thermal treatment processes for promoting the formation of fine semi-continuous carbides along grain boundaries are 680 °C/10~20 h, 715 °C/10~20 h and 750 °C/5~15 h. Similar microstructural features, including grain size, carbide morphology along grain boundaries and chromium depletion profile, are observed in both the investigated GH690 tube and the Inconel 690 commercial tube after solution treatment at 1090~1110 °C for 5 min followed by thermal treatment at 715 °C for 10 h or 15 h. Nonetheless, the size and density of TiN are lower and the minimum chromium content is higher in the investigated GH690 tube than those in the Inconel 690 commercial tube. Overall, microstructural features in GH690 tube made in China are superior to those in Inconel 690 commercial tube. The optimized heat treatment process is suggested based on microstructure analyses as well as the consideration of economic cost in production.