In this paper we tried to model the creep-strength degradation of selected advanced creep resistant steels which occurs under operating conditions. In order to accelerate some microstructure changes and thus to simulate degradation processes in long-term service, isothermal ageing at 650°C for 10 000 h was applied to P91, P92 and P23 steels in their as- received states. The tensile creep tests were performed at temperature 600°C in argon atmosphere on all steels both in the as-received state and after isothermal ageing, in an effort to obtain a more complete description of the role of microstructure stability in high temperature creep of these steels. Creep tests were followed by microstructure investigations by means of transmission and scanning electron microscopy and by the thermodynamic calculations. The applicability of the creep tests was verified by the theoretical modelling of the phase equilibrium at different temperatures. It is suggested that under restricted oxidation due to argon atmosphere microstructure instability is the main detrimental process in the long-term degradation of the creep rupture strength of these steels.

Creep behavior and degradation of creep properties of advanced 9-12%Cr ferritic steels are phenomena of major practical relevance, often limiting the lives of power plant components and structures designed to operate for long periods under stress at elevated temperatures. Because life expectancy is, in reality, based on the ability of the material to retain its high-temperature creep strength for a period of at least twice the projected design life, methods of creep property assessment based on physical changes in the material that are likely to occur during service exposure rather than simple parametric extrapolation of the short-term data are necessary. This work attempts to highlight the problem areas just in this respect. The proposed approaches are illustrated by recent experimental results on advanced high creep strength 9-12%Cr ferritic- martensitic steels (P91 and P92).