Temperature Dependence of the Solid Solution Strengthening Effect of Group IVb, Vb and VIb Elements on the High Temperature Creep Behaviors of Carbon Free 25Cr-35Ni Steels

Abstract

Steady state creep rates and friction stresses were measured at 900°C with a series of carbon free 25Cr-35Ni steels, and the extent of the solid solution strengthening with the addition of the groups IVb, Vb and VIb elements to the steel was discussed in terms of effective stress (-applied stress minus friction stress). In comparison of these results with those obtained in previous data at 1000°C, temperature dependences of solid solution strengthening and friction stress were analyzed.
The steady state creep rate of the steel decreased remarkably with the increase in Mo, W, Nb, Ta, Zr and Hf contents, but slightly decreased with the addition of Cr, V and Ti. A linear relation was found to exist at 900°C between both logarithms of the steady state creep rates and the effective stresses of all the steels independent of the kinds and the amounts of solute elements, and to be parallel with that at 1000°C. These results conclusively demonstrated that the increase in the extent of the solid solution strengthening at 900°C due to the addition of solute elements was mainly attributed to the increase in the friction stress as well as in the case at 1000°C.
The more effective the solid solution strengthening was obtained, the larger the activation energy for creep at the constant applied stress became. However, the activation energies estimated at the constant effective stress were 60±5 kcal/mol in all the steels studied, being independent of the kinds and the amounts of solute elements. Moreover, temperature dependence of the friction stress was observed, and then, it was pointed out that temperature dependence of the solution strengthening was due to that of the friction stress.