Influence of nitrogen on mechanical properties and technological plasticity of austenitic steel

The effect of nitrogen (up to 0.30 wt. %) on the macrostructure, mechanical properties and technological plasticity of chromium-nickel-manganese steel in comparison with low carbon austenitic nickel-chromium steel 03CH18N11 (AISI 304L) was studied. The study of these steels ingots macrostructure in the longitudinal and transverse direction showed a change in the metal crystallization mechanism. The addition of nitrogen leads to a decrease in the width of the columnar crystals and increase the zone of equiaxed crystals. Also nitrogen causes the grain grinding and reduce shrinkage under the same conditions of crystallization and cooling of the ingot. Increase the concentration of nitrogen in steel leads to a significant steel strength properties (by 30-60 %) increase with practically unchanged ductility of the metal under the test conditions at room temperature. With an increase in the test temperature the strength properties a gradual alignment of nitrogen steel and AISI 304L occurs. Along with this, a significant decrease in technological plasticity (~ 1.5-2.0 times) of nitrogen steel was found at hot deformation temperatures of austenitic steels. This circumstance indicates the need to study the issue of microalloying with boron or rare earth metals in order to exclude large losses for stripping due to surface defects in the case of commerical development of nitrogenous chromium-nickel-manganese austenitic steel. Additionally, the cold shortness of steel with nitrogen was evaluated at negative temperatures down to -196 °C. It was established that prior to testing, the alpha-phase content in the metal was less than 1.0 %, and when examining destroyed impact specimens after testing for cold shortness, a magnetic phase was detected in the amount of 1.4% at a test temperature of -100 °C and 2,2% at the test temperature of -196 °C. These indicates that deformation nature of their occurrence and explains the decrease in the toughness of the steel with a decrease in the test temperature.