INFLUENCE OF CARBON AND NITROGEN ON SOLID-SOLUTION DECAY AND 475 DEGREES-C EMBRITTLEMENT OF HIGH-CHROMIUM FERRITIC STEELS

475-degrees-C embrittlement of high-chromium ferritic steels with Cr content from 15 to 35 wt pct and different commercial impurities (C, N) has been investigated. The influence of preliminary treatment (600-degrees-C to 1250-degrees-C) and chemical composition of the alloy (Cr, C, N, Mo, Ti, Nb) on kinetics has been established. Internal friction (IF) was used to determine the contribution of interstitial atoms to the formation of Cr-modulated structure during different stages of embrittlement of high-chromium steels. By use of IF, static and dynamic (impact) mechanical tests with different states of stress, transmission and scanning electron microscopy of structures and fracture surfaces, and the application of diffraction methods, thermodynamic diagrams of the aging of high-chromium steels have been obtained. In the temperature range of 475-degrees-C embrittlement, the following sequence of processes has been established: (1) decay of solid solution supersaturated with interstitial atoms by dislocation pinning, (2) formation of substitutional-interstitial (s-i) complexes of interstitial (C, N) and substitutional (Cr) atoms, and (3) formation of zones enriched in Cr.