Cold resistance of new casting Cr – Mn – Ni – Mo – N steel with 0.5 % of N. Part. 1

Abstract

The authors have studied cold resistance of thelaboratorymetal of a new austenitic grade of nitrogen-containing casting steel (21 – 22) Cr – 15Mn – 8Ni – 1.5Mo – V (Russian grade  5Kh21АG15N8МFL) with nitrogen content of 0.5 % and yield strength of ~400 MPa. The temperature dependence of impact toughness was constructed in the range +20 ... –160 °C and it was shown that the steel is characterized by a wide temperature range of the viscous-brittle transition with T DBT = –75 °C, at which KCV = 120 ± 10 J/cm2. Comparison material – industrial, centrifugally cast 18Cr – 10Ni steel (grade 12Kh18N10-CC) has such a KCV level at +20 °C. It is not prone to viscous-brittle transition, its impact strength decreases more gently and at temperatures lower than –80 °C and its KCV level is higher than that of nitrous steel. However, in the entire range of climatic temperatures, nitrous casting steel with 0.5 % of N exceeds its impact strength. The studied steels have residual δ-ferrite in the cast structure in an amount of up to ~10 % in Cr– Ni industrial steel and a smaller amount in laboratory nitrous steel. It is enriched by chromium, up to 26 and 34 wt. % respectively, and contains ~14 % of Mn in nitrogen steel. Presence of Mn does not affect the nature of fractures at climatic temperatures. However, δ-ferrite of nitrous steel at –160 °C is beyond the cold brittle threshold. Therefore, its fracture obtained at this temperature contains numerous cracks in δ-ferrite crystals. The critical fragility temperature below which this material is not recommended for use is Тк ≈ –110 °С; it was determined by the criterion method. It corresponds to a level of KCV of 68 – 83 J/cm2, higher than the level of KCU at +20 °C, allowed by the standard of the Russian Federation for castings from austenitic class of steels (up to 59 J/cm2 ). Based on a comparison of literature and our own data, it was concluded that it is impossible to ensure high cold resistance and, at the same time, high strength, due to alloying of economically alloyed nickel (up to 4 %) corrosion-resistant steels by 0.5 – 0.6 % of N.