Abstract
Microstructural observations were carried out on modified high chromium steels which were subjected to tempering at various temperatures. The kind of identified fine precipitates was mainly influenced by tempering temperature. Large amount of M2X was recognized in the steels tempered at below 993K regardless of its chemical composition. M2X was scarcely identified in the steels tempered at above 1023K. Many MX, however, were recognized instead of M2X in the steel tempered at above 1023K. The composition of M2X was changed by tempering temperature. The amount of vanadium in the M2X increased as tempering temperature was raised. A little amount of fine NbX was identified except for insoluble NbX. During high temperature exposure, the amount of extracted residue increased with time. One of the features of its tendency was that the amount of vanadium gradually increased in the steel tempered at below 933K. Precipitation of fine VX and Z phase were recognized in the steels aged or crept for more than 10000 h at 873K. Fine M2X remained at least for 10000h at around 873K.
