Thermomechanical fatigue behaviour of aged heat resistant austenitic alloys

Abstract

The increasing demands for efficiency and flexibility result in more severe operating conditions for the materials used in critical components of biomass power plants. These operating conditions involve higher temperature ranges, more pronounced environmental effects and cyclic operations. Austenitic stainless steels have shown to possess promising high temperature properties which makes them suitable as candidates for critical components in biomass power plant. However, their behaviour under such conditions is not yet fully understood. This work investigates three commercial austenitic alloys: Esshete 1250, Sanicro 25 and Sanicro 31HT. The alloys were subjected to in-phase (IP) thermomechanical fatigue (TMF) testing under strain-control in the temperature range of 100 • C to 800 • C. Both virgin and pre-aged TMF specimens were tested in order to simulate service degradation resulting from long-term usage. The results show that the pre-aged specimens suffered shorter TMF-life compared to the virgin specimens. The scanning electron microscopy methods electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) were used to analyse and discuss active failure and deformation mechanisms. The difference in TMF-life produced by the two testing conditions was attributed to an embrittling effect by precipitation, reduced creep properties and oxidation assisted cracking.

The effect of dwell on thermomechanical fatigue in superaustenitic steel Sanicro 25

“…The initial microstructure of this Sanicro 25 alloy (i.e. after annealing at 1200°C for 1 h) has been studied earlier by Heczko et al 10,11,26 and Warner et al 27 . The average grain size of 60 μm was determined by the intercept method; however, large grains of the size up to 200 μm were present as well.…”

“…Laves phases were also found in the microstructural study of the specimens Sanicro 25 exposed to 700°C for 3000 hours reported by Chai et al 2 . Warner et al 27 investigated thermomechanical behaviour of virgin and pre-aged specimens subjected to IP-TMF loading in the temperature range from 100°C to 800°C. Microstructural investigation revealed the formation of Nb(C,N), Z-phase, Cr 2 N, M 23 C 6 and μ-phase in virgin specimens while strained pre-aged specimens resulted in the extra formation of Laves phase (Fe,Cr) 2 W. Precipitates having larger dimensions and lower density like tungsten carbides or Z-phase do not affect substantially the material strength.…”

The effect of dwell on thermomechanical fatigue in superaustenitic steel Sanicro 25

“…The initial microstructure of Sanicro 25 alloy (i.e., after annealing at 1200 C for 1 h) has been studied earlier by Heczko et al 35,36,46 and Warner et al 25 The average grain size of 60 μm was determined by the intercept method; however, large grains of the size up to 200 μm were present as well. The characteristic attribute of the dislocation structure before applying cyclic loading is planar dislocation arrangement consisting of individual dislocations, stacking faults, pile-ups, and so forth.…”

“…The most severe loading conditions are those where stress or strain and temperature vary simultaneously, that is, in the case of thermomechanical fatigue. Numbers of thermomechanical studies were devoted to superalloy single and polycrystals 5–13 and to austenitic, low alloyed ferritic and dual hardening steels 14–28 …”

“…Numbers of thermomechanical studies were devoted to superalloy single and polycrystals [5][6][7][8][9][10][11][12][13] and to austenitic, low alloyed ferritic and dual hardening steels. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] From the newly developed austenitic steels, the highest interest was devoted to advanced austenitic stainless steels such as NF709, H3RC and Sanicro 25. Originally, the creep resistance was investigated.…”