Effect of Laves phase on the creep rupture properties of P92 steel

Stress rupture tests of normalized and tempered P92 (9Cr-0.5Mo-1.8 W) steel were performed in the range of 135-215 MPa at 650 °C. Effect of tempering temperature in the range of 740-780 °C on the creep rupture life was investigated. Resulting rupture times varied from 100 to 3000 h, and creep rate by one order of magnitude. In the high stress regime, lower tempering temperature resulted in the highest rupture time due to initial high dislocation density and fine laths. However, at lower stresses, highest rupture time was observed for highest tempering temperature. Formation of Laves phase (Fe2Mo, Fe2W) adjacent to M23C6 carbides was responsible for increase in rupture time. Back scattered electron imaging (BSE) in scanning electron microscopy (SEM) was used to identify Laves phases, and study their distribution. Reduction in dislocation density and coarsening of laves phase precipitates result in decrease in stress exponent value 'n' at higher test temperatures of 650 °C.