Long Term Microstructural Evolution of 9-12%Cr Steel Grades for Steam Power Generation Plants

The 9%Cr steel grades were developed in the last 25 year in order to favourite the target of the improvement of the steam temperature and pressure necessary to increase the efficiency of the modern coal fired power plant for electricity production. Now a relevant experience in testing and service conditions has been generated and it is possible to produce consistent assessments of the mechanical and creep behaviours for long term application. On the other hand the further trials to increase the Chromium content to improve the corrosion-oxidation behaviours of the 9%Cr grades, despite the very promising short term properties, generate unpredicted premature failure after long term exposure. This failure was identified in the instability of the compositions with the appearance of Z-phase, with consequent drop down of the creep behaviours below their parent 9%Cr grades. This paper summarizes the maturated knowledge in the field of the microstructural evolution of the 9-12%Cr steels, as well as, the current status of development of these steel grades.