Sensitization behaviour of the nitrogen alloyed austenitic stainless steel X8CrMnMoN18-19-2

Austenitic stainless steels have been used in many different industrial branches. Nitrogen alloyed austenite´s with up to 20 wt. % manganese and 0.8 wt. % nitrogen have been available on the market as nickel-free grade for some years. In these steels, the austenitic matrix is stabilized without the addition of nickel, while the corrosive resistance and mechanical properties are significantly improved by addition of Mn and N. As with all stainless steels, the chemical composition and heat treatment have a decisive influence on structure, mechanical properties and corrosion resistance. Solution annealing, quenching and work hardening are applied to combine excellent mechanical properties (R m from 900 MPa to 2,000 MPa, A 5 > 50 %, A v > 350 J) with high corrosion resistance. Solution annealing is applied to eliminate undesired precipitation phases (Cr 2 N, M 23 C 6 and sigma phase), distribute the alloy elements homogeneously in the austenite and assure the high corrosion resistance. If the distribution of the alloy elements (Cr, Mo and N) is impaired by suboptimal heat treatment, processing or high temperature conditions, the corrosion resistance can be reduced. Precise knowledge of the sensitization behaviour of these high nitrogen alloyed steels is necessary. Therefore, the sensitization behaviour of the nitrogen alloyed austenitic steel X8CrMnMoN18-19-2 (1.4456) was investigated by thermal ageing in the temperature rage of 500 °C to 900 °C for up to 100 h. Thermodynamic calculations were used to predict the formation of different phases. The microstructure of the various sensitization states was evaluated by scanning electron microscopy (SEM) and compared with the corrosion resistance characterized by the electrochemical potentiodynamic reactivation (EPR) test and the KorroPad method. The reduction of corrosion resistance is correlated with the occurrence of M 2 N nitrides in the microstructure and two sensitization diagrams were created based on the experimental results.