Creep Strengthening Mechanisms in High Chromium Steels

摘要

The contributions of the various microstructural features (dislocation density, sub-grain size, carbide, carbonitride and intermetallic precipitates) to the creep rupture strength of chromium steels containing 9% Cr are presented, with the emphasis on the steel P92 (Fe/9Cr/ 0.5Mo/1.8W/0.2V/0.06Nb/0.1C/0.05N). Variations in the initial microstructure were produced by different heat treatments. Exposure at typical service temperatures also leads to microstructural changes, comprising reduction in dislocation density as martensite recovery occurs; the growth of the sub-grains; the coarsening of carbide and carbonitride precipitates; and the formation of intermetallic phases, such as Fe_2(Mo,W). Using transmission electron microscopy, the microstructural changes have been quantified. Long-term creep rupture testing has been carried out with durations of 50000h and more at 600°C and from the correlation of the creep strength results with the quantitative TEM measurements, the relative contributions of the microstructural features to the long-term creep strength have been derived. The potential for further increases in the creep rupture strength of the high chromium steels to enable higher service temperatures to be achieved is discussed.