Cold resistance of new cast Cr – Mn – Ni – Mo – N steel. Part 2. Studying non-metallic inclusion particles under static and impact loading at low temperatures

New cast austenitic Cr – Ni – Mn steel with 0.5 % N (grade 05Kh21AG15N8MF) surpasses cast steel of 18 Cr – 10 Ni type used for comparison in terms of the impact strength in the entire range of climatic temperatures. This part of the paper will pay attention to particles of non-metallic inclusions (NMI) in cast nitrogen-containing steel as a factor which affects mechanical properties under static and impact loading at low temperatures. NMI in laboratory metal consist of globular oxysulfides, with SiO₂ oxides in the central part and an outer layer formed by manganese sulfide MnS, with an average particle size of ~75 % up to 4 μm. During the steel impact bend test at –160 °C, these NMI do not initiate cracking and do not contribute to crack propagation as a fracture in isolated pits. Under tensile conditions at –110 °C, the yield strength of nitrogen-containing steel increases by more than 1.7 times in comparison with the properties at +20 °C. Ductility does not decrease when cooled to –110 °C. In this case, NMI particles are strongly deformed due to the development of cracks in their oxide part. Even when NMI reach the surface of a sample in the working part in the neck zone, they do not initiate cracking. Cracks at the “NMI/deforming metal” interface are not formed. Even with a random arrangement of particles in the form of chains along the axis of application of the tensile load, at a distance of 5 – 20 μm from each other, pores do not form around the particles or merge into a crack nucleus. The results obtained correlate with the literature data that NMI can act as stress relaxers in ductile steels.

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