Surface microporous formation on AISI 4140 using combination of diffusion treatment after nitriding gas in muffle reactor

Steel with microporous formation on surface layers is very important as self-lubricant and damping material. In this new method, AISI 4140 steel was subjected in two step combination treatment. First step is boost nitriding gas in fluidised bed reactor for 4 hours at 550°C and the second step is diffusion nitriding gas for 2 h at 550°C in muffle reactors. The diffusion without protective gas where air was present was applied after nitriding treatment in muffle reactors to release excess nitrogen which leads to microporous formation. This mechanism significantly increases surface porosity. The microporous formation at ε layer during diffusion at nitriding process is controlled by reaction formation of N2, NO, Cr2O3 and Fe2O3 which depends on the atmosphere condition in the muffle reactor. The dynamics of atoms during these formations create tensile and compressive stress in the grain boundary that increases porosity formation.