Investigation and improvement on structural stability and stress rupture properties of a Ni-Fe based alloy

The structural stability and stress rupture properties of a Ni-Fe based alloy, considered as boiler materials in 700 degrees C advanced ultra-supercritical (A-USC) coal-fired power plants, was studied. Investigation on the structural stability of the existing alloy GH984 shows that the most important changes in the alloys are gamma' coarsening, the gamma' to eta transformation and the coarsening and agglomeration of grain boundary M23C6 during thermal exposure. The stress rupture strength was found to be slightly lower than the requirement of 700 degrees C A-USC. The fracture mode of creep tested specimens was intergranular fracture. Detailed analysis revealed that eta phase precipitation is sensitive to Ti/Al ratio and can be suppressed by decreasing Ti/Al ratio. The coarsening behavior of gamma' phase is related to Fe content. Adding B and P was suggested to stabilize M23C6 and increase grain boundary strength. Based on the research presented and analysis of the data, a modified alloy was developed through changes in composition. For the modified alloy, eta phase is not observed and M23C6 is still blocky and discretely distributes along grain boundary after thermal exposure at 700 degrees C for 20,000 h. Moreover, the creep strength is comparable to the levels of Ni-based candidate alloys for 700 degrees C A-USC. (C) 2015 Elsevier Ltd. All rights reserved.