Cold resistance of new casting Cr – Mn – Ni – Mo – N steel. Part 2. Investigation of the factor of non-metallic inclusions particles under static and impact loading at low temperatures

The new casting austenitic Cr – Ni – Mn steel with 0.5 % N (grade 05Kh21AG15N8MFL) surpasses the casting steel of the 18Cr – 10 Ni type used for comparison in the entire range of climatic temperatures. In this part of the article, attention is paid to the particles of non-metallic inclusions (NMI) in the cast metal of nitrous steel as a factor that can affect the mechanical properties under static and impact loading at low temperatures. NMI in laboratory metal are 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. It has been established that when testing casting steel for impact bending at –160 °C, these NMI do not serve as a source of crack initiation, do not contribute to their propagation and are in a fracture surface 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, and even when NMI reaches the surface of the sample in working part in the neck zone, they do not serve as a source of crack initiation. Cracks at the boundary “NMI/ deforming metal” 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 and merge into a crack nucleus. The results obtained correlate with the literature data that NMI can act as stress relaxers in ductile steels.