Thermochemical treatments of austenitic stainless steel in fluidised bed furnace for improved mechanical and ribological properties

Austenitic stainless steels have been treated thermochemically in gaseous environments of ammonia, methane or a mixture of ammonia-methane in fluidised bed furnace at temperature 450°C for 8 h time period. The purpose of this investigation is to enhance the mechanical properties, especially the hardness and wear resistance of austenitic stainless steel without compromising its corrosion resistance. In general the thermochemical processes are carried out in such conditions so that formation of nitride and carbide precipitates is avoided because they reduce the corrosion resistance of the alloyed layers. In the present investigation, it was possible to produce hardened case layer by changing the process variables of the techniques. Analysis of the treated specimens using Optical microscope, Microhardness tester, Linear Taber Abraser, Field Emission Scanning Electron Microscope and X-ray diffraction show that low temperature treatments at 8 h can produced a hard layer without precipitation of nitride and carbides. This hardened layer consisted of expanded austenite phase which is known as S phase. The hardness development of the case is comparable to those achieved by other modern diffusion processes.