On aging a steel containing 17Cr-7Ni-1Al (weight percent) between 800 F and 1100 F , an increase in austenite content was observed precipitation reaction occurs in this steel. A maximum hardness of Rockwell C 43 was obtained by aging between 900 and 1000 F. On the other hand, it was found that a steel containing 17Cr-4Ni-2.75Mo appears to rely primarily upon the martensite-to-austenite transformation for its strength, even after aging.