Strengthening Mechanisms in Creep of Advanced Ferritic Power Plant Steels Based on Creep Deformation Analysis

Strengthening mechanisms in creep of 9% Cr steel are examined in terms of solute hardening, precipitation or dispersion hardening, dislocation hardening and boundary or sub-boundary hardening at 550-650 C. The creep strengthening is mainly caused by the retardation of onset of acceleration creep, which effectively decreases the minimum creep rate and increases the creep life. The sub-boundary hardening enhanced by fine distributions of precipitates along boundaries gives the most important strengthening way in creep of tempered martensitic 9Cr steel. A dispersion of nanometer size MX nitrides along boundaries and the addition of boron significantly improve long-term creep strength. Excess addition of boron and nitrogen causes the formation of large particles of boron nitrides during normalizing heat treatment at high temperature, which offsets the benefits due to boron and nitrogen. Newly alloy-designed 9Cr-3W-3Co-0.2V-0.05Nb steel with 130-160 ppm boron and 70-90 ppm nitrogen exhibits excellent creep strength of base metal and no degradation in welded joints at 650 C.