Cold Resistance of New Casting Cr–Mn–Ni–Co Steel with 0.5% of N. Part One

Аннотация

This study focuses on the laboratory metal’s cold resistance of a new austenitic nitrogen-containing (21–22) Cr–15Mn–8Ni–1.5Mo–V casting steel (Russian grade 05Kh21AG15N8MFL) with a nitrogen content of 0.5% and a yield strength of ~400 MPa. The temperature relation of impact viscosity was constructed for this steel in a range from +20 to –160°C, the steel appeared to have a broad range of ductile-brittle transition temperatures with commercial, centrifugally cast 18Cr–10Ni steel (grade 12Kh18N10-CC), and it reached this KCV at 20°C. It is not prone to ductile-brittle transition, its impact strength decreases more smoothly at T < –80°C, and its KCV is higher than that of nitrous steel. However, its impact viscosity exceeds in the entire climatic temperature range by nitrous cast steel with 0.5% of N. The tested steels have residual δ-ferrite in the cast structure—up to 10% in Cr–Ni industrial steel and a smaller amount in laboratory nitrous steel. This ferrite is enriched with chromium up to 26 and 34 wt %, respectively, and contains ~14% of Mn in nitrogen steel. The Mn presence does not affect its type of fracture at climatic temperatures. However, at ‒160°C, the δ-ferrite of nitrous steel is beyond the cold brittle threshold. Therefore, its fracture obtained at this temperature contains numerous cracks in δ-ferrite crystals. When critical brittleness temperature is T_cr ≈ –110°C, this material is not recommended for use, as recommended by the criterion technique. It corresponds to KCV of 68 to 83 J/cm², higher than KCV allowed by Russian standards for austenitic steel casts at 20°C (up to 59 J/cm²). According to current literature and new research, the corrosion-resistant steel existing with nickel cannot have high cold-resistance and, at the same time, high strength by alloying it with 0.5 to 0.6% of nitrogen.