Tendency of low-alloy thermally improved steel 12GN2MFAYu to thermal embrittlement

Abstract

1. Long-term (500–10,000 h) holding at elevated temperatures (340–500°C) exerts a minor influence on the level of the strength characteristics (σ0.2 and σu) of steel 12GN2MFAYu. Some hardening and softening of the steel in individual stages, which is associated with the processes of aging and the additional segregation and coagulation of particles of the disperse phase, partially or completely compensate one another. 2. An increase in the TKDS level during the thermal embrittlement of steel 12GN2MFAYu is accompanied by an increase in the portion of the intersubgranular-intergranular component in the brittle zone of specimen fractures. The apparent activation energy of the embrittlement process with ΔTKDS=50°C approaches values characteristic for the process of reversible temper brittleness. 3. Thermal embrittlement of thermally improved steel 12GN2MFAYu at 340–550°C is governed by the formation of elongated boundaries (chiefly former packets of martensite and austenitic grains) on and near which the increased (critical) density of vanadium carbonitride V(C, N) and cementite particles is attained. By additionally weakening the cohesive strength along the subgrain and grain boundaries, adsorption phosphorus enrichment of the near boundary regions of the solid solution contributes to growth of brittle cracks in accordance with the intersubgranular-intergranular mechanism and to a further rise in the TKDS level.