EFFECT OF NIOBIUM AND HEAT TREATMENT ON THE CREEP PROPERTIES OF 15Cr-14Ni HEAT RESISTING STEELS.

Experimental data for creep at 650 C were obtained for one very low carbon steel with and without Nb, and for six nominally 0. 15C-15Cr-14NI steels with zero to 2% Nb. The data indicate that the increment of the creep rupture strength is proportional to the product (%C)**0**. **4 multiplied by (%Nb)**0**. **4 multiplied by (nI)**0**. **2 multiplied by (D gamma )**0**. **4, where (%C) and (%Nb) are the contents of carbon and niobium in solution (nI) is the distribution density of undissolved carbides and (D gamma ) is the mean austenite grain diameter after solution treatment. The product thus expresses the combined effects of solid solution hardening by carbon and niobium, dispersion strengthening by carbides, M//2//3C//6 and NbC, and the effect of grain size.