Exploitable results Solution nitriding of stainless steel at high temperature for applications in mechanical and chemical engineering The project contained the transformation of the solution nitriding process from lab scale to industrial conditions, the constitution of the achieved high nitrogen surface of various stainless steels and the characterisation of the properties resulting therefrom. During solution nitriding an equilibrium between the nitrogen activity of the gas atmosphere and the activity of nitrogen dissolved in the steel surface is established. The equilibrium content of nitrogen at the surface depends on process temperature, nitrogen pressure and alloy concentration. The diffusion of nitrogen into the steel follows the solution of Fick's second law for a semi-infinite body of constant surface nitrogen content furnished by the nitrogen atmosphere and a given core content of nitrogen. According to conducted thermodynamic calculations stainless steels of high Cr and low Ni content may require a nitrogen pressure of less than 1 bar, steels of lower alloy content demand a nitrogen pressure of up to 3 bar in a temperature range between 1100 C and 1150 C. The defined pressure and temperature can be handled in commercially available cold wall vacuum furnaces with a well sealed graphite chamber. A powerful gas quenching system is required to limit the cooling time. Nitrogen gas with a heat conductivity of 400-500 W/m2K (10 bar) has proved to be an appropriate quenching medium to achieve the required cooling speed. It has been demonstrated that solution nitriding is beneficial for various stainless steels. Depending on the case microstructures (austenitic, austenitic-ferritic, ferritic-martensitic) different material properties are enhanced. In the solution nitrided condition the investigated steels X2CrNiMo17-13-2 and X2CrNiMoN22-5-3 present a fully austenitic, a martensitic (X14CrMo16) or an austenitic microstructure with nitrides (G-X3CrNiMoCuN26-6-3-3) in the case. A total solution nitriding time of 18 h leads to a penetration depth of nitrogen of up to 2.3 mm. Solution nitriding results generally in an increased macrohardness at the surface. The achievable macrohardness is independent of the solution nitriding time and increases with increasing nitrogen pressure up to 60% compared to the conventionally treated condition (solution annealed or hardened and tempered) of the investigated steels. The hardness decreases from a maximum at the surface to the core hardness. The strength of the case is directly proportional to the measured hardness. Compared to solution annealed austenitic or austenitic-ferritic steels and hardened and tempered martensitic or ferritic-martensitic carbon steels, the corrosive behaviour of solution nitrided steels is improved by the addition of nitrogen, as long as no nitrides are present. However, under additional tribological load (fretting, erosion-corrosion) the presence of nitrides can be beneficial as shown on lab scale and in a field test with a solution nitrided impeller. Additionally it was found, that solution nitriding results in a strong improvement of the cavitation resistance. For austenitic cases this behaviour is related to the increased strengthening potential due to the solid solution of nitrogen, for the martensitic case it is related to the significant higher hardness of the case. Searching for OpenAIRE data... There was an error trying to search data from OpenAIRE No results available
