Intensive Interstitial Strengthening of Stainless Steels

AbstractThe high chromium content of stainless steel impairs the interstitial solubility of carbon in austenite at solution annealing or hardening temperature. Replacing carbon by nitrogen improves the interstitial solubility which is raised most, if carbon and nitrogen are added jointly. Respective thermodynamic equilibrium calculations have led to a new group of austenitic and martensitic steels as well as to a thermochemical surface treatment, that make use of the C + N concept to intensively strengthen stainless steels with about 0.5 to 1 mass% of these interstitials. Pressurized electro slag remelting (PESR) is required to raise the N content in the melt and to avoid degassing during solidification of martensitic stainless steels with C + N known as CRONIDUR®. For austenitic CrMn steels the C + N concept affords to dissolve about 1 mass% of these elements in the melt at atmospheric pressure and keep them in solid solution during solidification of the new CARNIT® steels. Case hardening of stainless martensitic steels with nitrogen instead of carbon, called SolNit®, combines the effect of C in the steel and N dissolved in a surface zone via heat treatment in N2 gas of controlled pressure. The key properties and respective microstructures of these three versions of the C + N concept are discussed along with some applications.