THE EFFECT OF HEAT TREATMENT ON THE MICROSTRUCTURE AND CREEP RUPTURE PROPERTIES OF 9Cr-0.5Mo-1.8W STEEL

摘要

The effects of austenitizing at temperatures in the range 970 to 1145°C and of tempering at temperatures in the range 715 to 835°C on the microstructure and stress rupture strength of the 9Cr0.5Mol.8W steel NF616 have been investigated. After austenitization and tempering, the steel exhibited elongated martensite laths within the prior austenite grains, the boundaries of which were decorated with M{sub}23C{sub}6 precipitates. Two types of MX (where X is C and/or N) precipitate were detected; spheroidal Nb(C,N) particles and plate-like V-rich nitrides. The prior austenite grain size and the size and distribution of fine carbides depended on the heat treatment temperatures. For low tempering temperatures (715 and 745°C), the hardness was higher, the higher the austenitization temperature. For the higher tempering temperatures, there was no effect of the austenitization temperature on hardness. The results of the stress rupture tests showed that tempering at 835°C caused a considerable loss of creep rupture strength. Tempering should therefore be carried out at temperatures of 800°C or below.