The Process And Microstructure Of High Pressure Ammonia Nitriding Of Stainless Steels

Stainless steels could obtain high strength and toughness, great wear-resistance andcorrosion resistance by nitrogen alloying. As solution nitriding plays an important role innitrogen alloying, the study of it has improved greatly at the moment. After propersolution nitriding, nitrogen was soluted in the stainless steels as a interstitial element, soexcellent properties can be obtained. At the same time, solution nitriding not only refinedproperties to stainless steels but also was a method to produce high nitrogen stainless steelas a new chemical heat treatment process..Two kinds of stainless steels (15Cr7.5Mn2.6Mo and0Cr18Ni9) were chosen in thisstudy. The pseudo-equilibrium phase diagrams of the two kinds of stainless steels, theequilibrium phase proportion of15Cr7.5Mn2.6Mo at different temperatures and themaximum equilibrium nitrogen content of0Cr18Ni9were computed with thethermodynamic calculation method. At the same time, the nitrogen potential and thediffusion coefficient in15Cr2.6Mo7.5Mn with different nitriding conditions werecalculated. Based on the Fick’s Law, the nitrogen concentration distribution of0Cr18Ni9was obtained. Then different solution nitriding plans were carried out to the two kinds ofstainless steels, temperature:950~1150℃, pressure:0.1~0.5MPa, time:5、10、15、20h. Thesurface layer microstructures of the tested steels were observed and investigated by usinga combined technique of optical microscope, X-ray diffractometer and micro-hardnessmeter, and the main conclusions were as follows:The diffusion coefficient increased as the temperature increased, and a method for thecalculation of diffusion coefficient was founded:D a D b D. The value7.772×10-6cm2/s was the maximum value under the nitriding condition. The solutionnitriding under NH3was very different from nitriding by N2. A formula was founded toexpress nitrogen potential After950、1050、1100、1150℃,0.1、0.25、0.5MPa,5、10、15、20h solution nitriding treatment,the suface layer of15Cr7.5Mn2.6Mo was turned out to be fully nitrogen-rich austenite. The test results showed that with the nitriding temperature, pressure and holding timegrowing up, the depth of nitriding layer increased. The hardness of surface layerimcreased to varying degrees after nitriding treatment. Under the condition of1050℃,0.5MPa, nitriding for10h, the depth of nitriding was over2mm, the whole specimen wasnitriding compeletly, and the nitrogen-rich stainless plate was obtained successfully.The nitrogen concentration distribution diagram in0Cr18Ni9steel was simulatedbased on the Fick’s Second Law and Gauss’s Solution. The result was expressed as anformula:2D. And a nitrogen-rich austenite layer in0Cr18Ni9steel could be obtained after solution nitriding treatment under the condition of950、1050、1150℃，0.1、0.25、0.5MPa，5、10、20h. The hardness of nitriding layer was higher thanunnitriding part of the sample. As the holding time got longer, the austenite grain sizegrew up in the sample.