The development and application of creep damage constitutive equations for high Cr steels over a wide range of stress

The increasing thermal efficiency while at the same time keeping safe production are two vital targets that are required to be achieved to high productivity from power plants. The development and application of high creep resistant chromium (Cr)steel is becoming increasingly critical over a wide range of stress at high temperatures for power plant components. Knowledge of creep behaviour processes such as creep strain, creep damage, and rupture time can aid in the design and development of components. During the past three decades, a series of creep damage constitutive equations have been developed and applied to describe the creep behaviour for high Cr steel. The Continuum Damage Mechanics (CDM)model describing tertiary creep damages such as cavitation damage mechanism that is a dominant factor in the process of creep fracture. However, they are phenomenologically based. In addition, the most developed equations have only focused on middle-high stress levels; they were not developed for the low stress level and found invalid when compared with experimental data.

This thesis describes the development of new creep damage constitutive equations for high Cr steel over a wide range of stress conditions. It also reports its additional application to 316H stainless steel. This broadening of application means that the novel equation is suitable for more stress levels than traditional equations, especially under low stress situations.

The research undertaken can be summarised in the following four aspects. Firstly, the previously developed “novel hyperbolic sine law” is applied over a wide range of stress for P91 and P92steels. Its adaptability has been shown to be better than traditional methods by experimental data under the widest range of stress. Secondly, a novel creep cavitation damage equation is successfully applied and calculated for E911 steel at rupture time which can build a good foundation to apply the equation at different stages of creep lifetime and therefore achieve the predicted lifetime for components. Thirdly, the creep cavitation rupture modelling is developed and applied at 600°C and 650°C for P92 steel. This includes developing and confirming the “novel hyperbolic sine law” to discuss the relationship between the creep rupture time coefficient U’ and a wide range of stress, and as a result, accurately predict lifetime model for different stages of creep lifetime is developed at 550°C and 675°C for 316Hsteel. This achieves a further application of the novel creep cavitation equation and confirms that the creep cavitation damage equation is not only applied at creep rupture time but is also suitable for a creep at any time period. This thesis contributes to the creep damage modelling methodology and specific knowledge.