Improved Creep Resistance of 20cr25ninb Heat Resistant Steels Through Grain Boundary Intermetallic Precipitation Strengthening

Abstract

The enhanced creep resistance of 20Cr25NiNb austenitic heat resistant steel intended for application in supercritical gas-cooled reactors was achieved by adding W, Mo and B via grain boundary (GB) precipitation strengthening. The creep fracture time of steels with W/Mo and W/Mo/ 20 ppm B increased from 1069 h of base alloy to 1885 h and amazing 4700 h, respectively, with the minimum creep rate decreasing significantly. This creep enhancement also existed after 5000 h thermal aging. Creep cracks mainly propagated along GBs and at primary NbCs. The weakening GBs of base alloy resulted in mainly wedge-shaped intergranular cracks due to GB sliding. Mo and W elements introduced the intermetallic Laves phase to inhibit GB sliding. Rather than in σ and G phases, B element was primarily enriched in Laves phase. The increased and refined Laves particles through B addition effectively improved precipitation strengthening, reduced the amount of σ phase and retarded its coarsening, leading to enhanced creep strengthening.

Keywords: 20Cr25NiNb, Grain boundary precipitation, Creep, Laves, Boron, Isothermal aging