Relation between Mechanical Properties and Microstructures of 9Cr-W-Mo Steel

The effects of normalizing and tempering conditions on creep rupture properties at 650°C and hardness of 9 Cr-W-Mo steel were investigated in conjunction with changes in microstructres. The microstructures were examined with an optical and an electron microscopy and the precipitated elements were assesed by analyzing extracted residues. Creep rupture strength and hardness of tempered steels increased with an increase in normalizing temperature from 950°C to 1200°C, while they decreased with an increase in tempering temperature from 700°C to 780°C and the time from 1h to 100h. The amount of undesolved MX particles, which were observed in mormalized steels, decreased with an increase in normalizing temperature. A number of very fine MX and rod-like particles precipitated during tempering. The analysis of extracted residues showed that the amount of precipitated Nb and V in normalized steels decreased with an increase in norma1izing temperature. Fe, Cr, Mo, and W were desolved in matrix at 950°C. All of the alloying Nb and half of the V precipitated after tempering, while most of Mo and W were still in solution. Since the amounts of precipitated Nb and V after tempering were independent on normalizing temperature, the number of very fine MX particles increased with an increase in normalizing temperature. On the other hand, tempering temperature and time had no influence on precipitation behavior of very fine MX partic1es. These results suggest that mormalizing temperature dependence of hardness and creep ruptures trength is related to precipitation hardening by fine MX particles and tempering temperature and time dependence of them is related to recovery of the martensite structure.