Influence of high pressure normalizing heat treatment on microstructure and creep strength of high Cr steels

Chromium is an effective element for improvement in the oxidation resistance of steels. However, chromium addition to ferritic creep resistant steel is restricted to around 9–10 wt.%, since excess addition of chromium results in formation of delta-ferrite which decreases creep strength and toughness. In this study, influence of normalizing heat treatment under high pressure on microstructure and creep strength has been investigated in a steel containing 15 wt.% of chromium, since austenite single phase region is extended up to about 20 wt.% of chromium under high pressure of about 4 GPa. In contrast to a ferritic microstructure of the steel normalized under atmospheric pressure of 100 kPa, the martensitic one was found in steel normalized under high pressure of 4 GPa. Hardness in the as tempered condition of the steels subjected to normalizing heat treatment at 1273 and 1173 K under 4 GPa was 290 and 289 HV, respectively. This is significantly higher than the value of 182 HV for the steel normalized at 1273 K under atmospheric pressure of 100 kPa. Creep rate of the steel normalized under high pressure of 4 GPa was significantly smaller than that of the steel normalized under pressure of 100 kPa. It has been concluded that high pressure normalizing heat treatment is effective to suppress a formation of delta-ferrite and to improve both creep strength and oxidation resistance.