Preparation And First Principles Study The Properties Of "expanded" ? Phase Layer On Martensitic Stainless Steel

Martensitic stainless steel is widely used in the key equipment parts of aviation,aerospace,and other areas.With the development of science and technology,the requirements for the properties of martensitic stainless steel are higher and higher.The properties of martensitic stainless steel after normal heat treatments cannot meet the increasingly stringent requirements.In this project,1Cr17Ni2 and 0Cr17Ni4Cu4Nb martensitic stainless steels were used as the research objects,and their surface modifications were carried out by the new low-temperature gas nitriding technology.The microstructure and properties of low temperature nitrided layers were analyzed and tested,and the effect of annealing treatment(560 ^oC service temperature)on the microstructure and properties of the low temperature nitriding layer was studied.Based on the first-principles calculation of the electronic structure and properties of typical low temperature nitriding phases,the formation of low temperature nitrided layer and its strengthening and toughening mechanism are discussed.The experimental results show that the low temperature nitrided layer with the thickness of 7-32?m can be formed on the surfaces of 1Cr17Ni2 and 0Cr17Ni4Cu4Nb stainless steels after gas nitriding at 390-450 ^oC.With the increase of the nitriding temperature and time,the thickness of nitrided layer increases.The thickness of the nitrided layer on 0Cr17Ni4Cu4Nb stainless steel surface is greater than that of on 1Cr17Ni2 Stainless steel surface.The nitrided layers are mainly composed of nitrogen-containing"expanded"?phase and Fe_2-3N phase.The existence of alloying elements Cu and Nb benefits the formation of the"expanded"?phase.With the increase of nitriding temperature and nitriding time,the content of"expanded"?phase increases,and the content of Fe_2-3N phase decreases.The surface hardness of 1Cr17Ni2 and0Cr17Ni4Cu4Nb stainless steels can reach 1440 HV_0.1 and 1420 HV_0.1 respectively,and the hardness increases with the increase of the nitriding time.After low temperature nitriding treatment,the surface wear resistance of 1Cr17Ni2 and 0Cr17Ni4Cu4Nb stainless steels are obviously improved,and the wear resistance is improved with the increase of nitriding temperature and nitriding time.First-principles calculation results show that Fe₂N and Fe₃N phases are energetic stable,and their theoretical hardness can reach 5.96 GPa and 6.64 GPa,respectively.For the"expanded"?phase structure of?-Fe,it is stable when the nitrogen content is less than 20 at.%,and when the nitrogen content is less than 5.9 at.%,it is easy to form and meets the energetic stability.The calculated hardness increases first and then decreases with the increase of nitrogen content.The"expanded"?phase structures of Fe-Cr system,Fe-Cr-Ni system,and Fe-Cr-Ni-Cu system are not easy to be formed,but they are stable once being formed,and the stability increases with the increase of nitrogen content.The hardnesses of"expanded"?phase structures of Fe-Cr system are higher than those of"expanded"?phase structure of Fe-Cr-Ni system and Fe-Cr-Ni-Cu system.The calculated results of the electronic structure and properties of the low temperature nitriding phase can provide a theoretical basis for analyzing the phase structure and properties of the low temperature nitrided layer.