Structure and phase formation in austenitic steel with 0.5 – 0.6 % N during casting and its influence on mechanical properties and cold resistance of steel

The authors assessed the influence of structure and phase formation during casting, and subsequently during heat treatment, on the strength properties and cold resistance of corrosion-resistant austenitic steel 05Kh21AG15N8MFL, depending on its nitrogen content of 0.47 % (steel 1 ) and 0.58 % (steel 2 ), and the presence/absence of a niobium microadditive (steel 2 , 0.07 % Nb). Centrifugally cast steel 12Kh18N10T-CC was used as a comparison material. Both nitrogen-alloyed steels exhibit twice the impact toughness of the reference steel at 20 °C and exhibit higher values ​​over a wide range of cooling temperatures. The ductile-to-brittle transition temperature ( T DBT ) for steels 1 and 2 is –75 and –138 °C, respectively, while the impact toughness corresponding to the T DBT values ​​for steels 1 and 2 is KCV = 125 ± 10 J/cm 2 , respectively. This means that steel 2 surpasses steel 1 in cold resistance. It was shown that increasing the nitrogen significantly stabilizes the austenite of cast steel 2 with respect to the formation of the σ-phase as a result of crystallization: this phase is practically absent in steel with 0.58 % N, while in steel 1 its amount is ~12 vol. %. Both steels contain a phase with a bcc crystal lattice: in steel 1 , after heat treatment, this phase is δ-ferrite (formed during heating by the reaction σ + γ 1 → δ + γ 2 ), while in cast steel 2 , it is a phase enriched in Nb, V, and Mo. In both cases, the presence of this phase negatively impacts cold resistance. In steel 1 , δ-ferrite becomes embrittled at cryogenic temperatures: during impact bending tests, it is the crystals of this phase, which are below the cold brittleness threshold, that serve as the source of crack nucleation. In steel 2 , particles of the niobium-containing phase serve as nucleation sites for cracks and chips during low-temperature testing. In both cases, heat treatment facilitates the elimination of the ferrite phase from the steel structure. Based on the research results, austenitic corrosion-resistant steel 2 is recommended as a high-strength, cold-resistant casting material for cryogenic equipment.