Nitrogen and Ausforming to Improve Stainless Martensitic Steels

The chromium content of standard martensitic stainless steel X65Cr14 is raised to 17 mass% to enhance the solubility of nitrogen. Up to 4 mass% of nickel are added to suppress a partially ferritic solidification. This combination increases the nitrogen content soluble at 1 bar pressure from 0.14 to 0.24 mass%, which allows to reduce the carbon content to about 0.4 mass% without a loss of hardening capacity. The lower carbon level prevents the precipitation of coarse eutectic carbides in segregated areas encountered in X65Cr14. Instead of nickel pressure is applied to raise the nitrogen content up to 0.45 mass%. Calculations are verified by melts. As the hardening temperature is increased the CrMoN solute content of austenite is raised and so is the pitting resistance after hardening. However, retained austenite reduces the hardness and ausforming at 200°C is applied to transform it during subsequent deep freezing and raise the hardness. Nitrogen enhances the pitting resistance of stainless martensitic steels. The solubility may be raised by alloying and by pressure. In both cases, the stability of retained austenite (RA) will increase and may lower the achievable hardness. To overcome this predicament, ausforming is applied, which consists of cold working and subsequent transformation of RA to martensite during cooling.