The Development of Creep Damage Constitutive Equations for High Chromium Steel Based on the Mechanism of Cavitation Damage

Due to increasing electrical energy power supply, thermal efficiency and the desire to reduce CO2 emissions, creep-resistant high chromium steels are becoming widely developed and applied for components of electric power plants under high pressure at high temperature. The limited design factors such as strain histories, damage evolution and lifetime are important factors when creating the components of a power plant. Obtaining a long-term (100,000h, over 11 years) creep data is time consuming and costly, hence long-term creep data is very limited, and the extrapolation using the conventional empirical methods may not be reliable due to limited data (Chen et al., 2011; Shrestha et al., 2013; Ghosh et al., 2013). To design against failures, creep damage constitutive equations have the advantage of traceability from the physics based constitutive equation to the fundamental microstructural and damage behaviour. Thus, creep modelling constitutive equations for materials of the critical components of, for example, power plants and other safety critical systems, are a key issue in the research of materials.

In the past decade, a range of creep damage constitutive equations have been developed to describe creep damage behaviour for high chromium steel, however, some models are only based on creep deformation (creep microstructural degradation) and are not really concerned with cavitation damage, which is a dominant factor in creep rupture; most of them are proposed based on high stress levels of high chromium steel and extended to a low stress level, the modelling results fail to explore the phenomenon of stress breakdown. Besides, the cavitation damage equations were developed on experimental data of pure metal and super alloy, the fundamental nature of the evolution of creep cavitation damage is still unclear and necessary to solve for high chromium steel. Thus, the aim of this research project was to develop a novel creep damage constitutive equation for high chromium steel based on the mechanism of cavitation damage under a wide range of stress levels.

This research made contributions to the specialised knowledge on the following three aspects. Firstly, a modified hyperbolic sine law, which describes the relationship between minimum creep strain and applied stress, was applied to high chromium steel. Through which we found that the modelling results fitted better with published experimental data by NIMS in comparison with conventional functions such as power law, hyperbolic sine law and linear power law. The other two aspects of innovation in the development of creep damage constitutive equation had been achieved. Secondly, using the quantitatively analysed results of the cavity size distribution along grain boundary by the superior 3D technology of X-ray micro-tomography, a novel creep cavitation damage equation was developed and applied to describe the evolution of cavity along grain boundary in the creep process for high chromium steel. Thirdly, the novel creep damage constitutive equations, that coupled appropriate creep deformation mechanisms with the new cavitation damage equation, were successfully applied to high chromium steel under a wide range of stress level according to comparisons made between the modelling results of novel creep damage constitutive equations, classic uniaxial KRH constitutive equations and experimental data for P91 steel at 600℃ and also applied to P91 steel at 625℃.