Investigation and improvement on structural stability and stress rupture properties of a Ni–Fe based alloy. (25th December 2015)

• Record Type: Journal Article
• Authors: Wang, Changshuai; Guo, YongAn; Guo, Jianting; Zhou, Lanzhang
• Is Part Of: Materials & design. Volume 88(2015:Dec.)
• Page Start: 790
• Page End: 798
• Publication Date: 2015-12-25
• Subjects: Ni–Fe based alloy -- Creep strength -- Structural stability -- 700 °C A-USC; Materials -- Periodicals; Engineering design -- Periodicals; Matériaux -- Périodiques; Conception technique -- Périodiques; Electronic journals; 620.11
• DOI: 10.1016/j.matdes.2015.09.098
• Languages: English
• ISSNs: 0264-1275

Abstract

Abstract: The structural stability and stress rupture properties of a Ni–Fe based alloy, considered as boiler materials in 700 °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 γʹ coarsening, the γʹ to η transformation and the coarsening and agglomeration of grain boundary M23 C6 during thermal exposure. The stress rupture strength was found to be slightly lower than the requirement of 700 °C A-USC. The fracture mode of creep tested specimens was intergranular fracture. Detailed analysis revealed that η phase precipitation is sensitive to Ti/Al ratio and can be suppressed by decreasing Ti/Al ratio. The coarsening behavior of γʹ phase is related to Fe content. Adding B and P was suggested to stabilize M23 C6 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, η phase is not observed and M23 C6 is still blocky and discretely distributes along grain boundary after thermal exposure at 700 °C for 20, 000 h. Moreover, the creep strength is comparable to the levels of Ni-based candidate alloys for 700 °C A-USC. Graphical abstract: SEM images and LMP curve of GH984 and the modified alloy. The modified alloy exhibits more structural stability and stress rupture properties at 700 °C. Highlights: The most important structural instabilities are the precipitation of η phase and the growth of γʹ and M23 C6 ; γʹ to η transformation can be suppressed by decreasing Ti/Al ratio; Low Fe content decreases γʹ growth rate; Adding B and P can stabilize M23 C6 and increase grain boundary strength; The stress rupture properties can be improved by increasing grain boundary strength and structural stability.