Abstract
The effect of austenitizing conditions on creep rupture properties has been investigated for a 12%Cr heat resisting steel.
1) The creep rupture strength at 600°C was improved considerably by increasing austenitizing temperature from 950°C to 1100°C. Increasing austenitizing temperature further from 1100°C to 1150°C or higher has no distinct improving effect on creep rupture strength, but causes a drastic reduction in creep rupture ductility. Therefore, the optimum austenitizing temperature for the steel would be around 1050°C to 1100°C.
2) The increase in austenitizing temperature resulted in a decrease in the amount of undissolved carbides such as M23C6 and NbC. M23C6 dissolves completely at about 1 100°C and NbC does at about 1150°C for 1/2 h. For the decrease in the amount of undissolved carbides gives solution hardening effect and fine-precipitation hardening effect after tempering, it is considered that the change in the amount of residues with the change in austenitizing temperature is the most important factor affecting creep rupture strength.
3) The increase of prior austenite grain size with the increasing austenitizing temperature also contributes some what to the improvement of creep rupture strength, but may bring about higher concentration of S at the grain boundaries and thus, reduces creep rupture ductilities drastically.
4) Varying austenitizing time from 30min to 2 h at 1050°C and 1100°C has no distinct effect on creep rupture properties.
