METHOD FOR CHEMICAL-THERMAL TREATMENT OF STEEL ARTICLES Russian patent published in 1995 - IPC

FIELD: machine building, superficial hardening machine parts of carburizing structural steels, particularly pinions of chromium-nickle alloys. SUBSTANCE: articles of chromium- nickle steel are subject to carburization in atmosphere of endothermic gas, natural gas and ammonia; interim cooling; reheating for hardening; and hardening. Carburization is carried out in two stages, namely active saturation stage, and diffusion leveling stage. Ammonia percentage in saturating atmosphere is 0.05 - 0.3. Active saturation stage is conducted at 1.2 per cent of carbon potential. Diffusion leveling stage is conducted at carbon potential corresponding to eutectoid composition of steel grade in working during 1/4 - 1/3 of total carburization time. Interim cooling down to (A₁- 40^°C)-(A₁+80^°C). temperature is carried out after reheating for hardening. EFFECT: increased fatigue resistance due to increased hardenability and hardening capacity of hardened layer, and increased content of products of martensite conversion, allowing one to increase bending strength by 27 - 30 per cent, and resistance to contact fatigue by a factor of 1.3 - 2. 3 tbl