High Nitrogen Steels. Dissolution and Reprecipitation of Nitrides in an Austenitic Stainless Steel Produced by Powder Metallurgy.

Production of nitrogen-alloyed AISI 316 steel containing titanium has been studied using a powder metallurgy route. The material is prepared by nitriding titanium-free 316 powder in an ammonia/nitrogen gas mixture at temperatures below 900°C and then blending with the titanium alloyed steel to give the required total nitrogen content in the alloy. Alloys with sufficient nitrogen for precipitation of titanium nitride (TiN) and an excess of 0.2%N in solid solution were prepared. The blend powders were cold isostatically pressed into cylindrical samples and sintered at temperatures between 1100 and 1350°C. Samples were also extruded at 1200°C without prior sintering. The results show that the titanium alloyed powder is not fully austenitic in the as-sprayed form but the austenite phase is stabilised by the presence of nitrogen. The as-nitrided powder contains a considerable amount of chromium nitride and the microstructure of the sintered or extruded steel is controlled by the behaviour of that phase. The best conditions for densification are found to be at 1350°C in a vacuum furnace with a partial pressure of nitrogen gas applied as the temperature approaches the sintering maximum, followed by a hold for one hour. This gives a material of relatively high density with a controlled nitrogen distribution, high hardness and a fully austenitic structure. During the heating cycle and while the pore structure remains open the vacuum results in the decomposition of some chromium nitride and redistribution of nitrogen into solid solution, but as this is above the atmospheric solubility limit some of it is lost to the atmosphere. The process is limited by the onset of pore closure at maximum sintering temperature. The rate of decomposition of the nitride in the solid state is slow and if the sintering temperature is low then the process of decomposition of the nitrides is incomplete, residual (coarse) chromium nitride remains and the properties are poor. This is also the case in extruded samples which contain remains of the original nitrided particles even after heat treatment. However, when the sintering regime involves liquid phase sintering, the process of dissolution of the original chromium nitride goes to completion, fine titanium nitride remains and a high-density strengthened component is achieved.