Influence of ageing on thermomechanical fatigue of austenitic stainless steels

Abstract

The increasing demands on efficiency and flexibility results in more detrimental operating conditions for the materials used in the boiler section of biomass power plants. Hence, the materials will be exposed to cyclic operation, increased temperature and higher pressure where austenitic stainless steels have shown to be promising candidates to cope with the increasing demands. However, their behaviour under these conditions are not yet fully understood. This work investigates the response of the commercial austenitic stainless steels Sanicro 25, Sanicro 31HT and Esshete 1250 and focus on the material degradation from cyclic operating condition and long-term usage. Pre-aged specimens (2000 hours at 800 °C) were exposed to thermomechanical fatigue (TMF) testing under strain-control and in the temperature range of 100 °C to 800 °C. The two TMF-test conditions IP- and OP-cycling performed on the investigated materials did not differ that much in hardening behaviour. However, the plastic strain range were in general greater for IP-cycle condition. An observed barrelling effect was simulated by a simplified finite element analysis (FEA), which showed influence of specimen geometry. The degree of deformation of the test specimens was measured by the use of low angle grain boundary (LAGB) density calculations and it showed no barrelling effect before 500 OP-cycles.