Evaluation and establishment of short-term stress relaxation test methodology for predicting long-term creep life of steam turbine components

Abstract

The traditional approach to material development. design and life assessment for high temperature components, involving creep/ stress-rupture testing, needs not only very long time but also high cost in experimentation. A short-term high precision stress-relaxation testing technique is adopted here with a view to establish the long-term creep life prediction and identification of basic creep strength of a material having difference in mechanical properties. The core region. known to possess poor fracture toughness, of large rotor steel of ICrMo V grade is used for present investigation. The material was given two different heat treatment cycles so as to providing wide difference in basic strength. To validate the creep life prediction via stress-relaxation test methodology, a number of creep tests were also conducted. The creep life is derived from stress-relaxation data by using special software, which can evaluate the inelastic strain rates from raw stress-relaxation data. The predicted lives when compared with actual creep lives, obtained from conventional creep test, delivered a very close fitting mostly at higher initial pre-strain levels (>0.6%) in both the heat treatment conditions. Even. the stress vs.LMP plots, generally used for design purposes are also found to have close matching. Microstructural degradation derived form both the test.il]g techniques are comparable. However, the deterioration rate of structure is less in stress-relaxed material in comparison to creep-ruptured material. The creep ductility is found to be reducing at lower stress levels or longer lives in both the heat treatment conditions and the reason is attributed to the containment of inclusions in the material.