Study on the microstructure and properties of 35Mn2V steel used in oil pipe by salt bath nitriding

Oil pipe is easy to fail due to wear and corrosion during its service, which seriously affects the normal oil production and causes some unnecessary losses，while salt bath nitriding can form a composite infiltration layer through the composite technology on the surface of the workpiece, which can realize the combination of nitriding and oxidation, to achieve the effect of surface strengthening and modification. In this paper, salt bath nitriding is carried out on oil pipe (35Mn2V steel) with different temperature and time, and metallographic test, loose layer grade evaluation, XRD analysis, SEM observation, corrosion wear test, etc. are conducted to study the effect of salt bath nitriding process on the structure and properties of the permeating layer, comprehensively evaluate the quality of the permeating layer, and explore the wear resistance mechanism. The results show that when 35Mn2V is nitrided at 580℃, the infiltration layer mainly consists of oxide film, porous layer, compound layer and diffusion layer, while when it is nitrided at 630℃, the infiltration layer consists of an additional nitrogenous austenite layer. The thickness of 35Mn2V compound layer increases with the increase of nitriding temperature and the extension of nitriding time, and the influence of changing nitriding temperature on the thickness is greater than that of changing nitriding time. After a short time and low temperature nitriding, the phase composition of the infiltration layer is mainly ε-Fe3N and ε-Fe3O4, after a long time and high temperature nitriding, the phase composition is mainly ε-Fe2N and Fe3O4, in which Fe3O4 is formed in the subsequent oxidation process. The degree of loose layer of the infiltration layer increases with the increase of nitriding temperature and the extension of nitriding time. Most of the loose layer grades of the infiltration layer are level 3, and a few are level 2. The degree of loose layer of the infiltration layer is determined by the process of salt bath nitriding, and the subsequent oxidation, ion stabilization and other processes have little influence on it. When 35Mn2V is subjected to corrosion wear test under small load, the friction coefficient of oxide film is small. When the loose layer is worn, the friction coefficient increases obviously. The compound layer is mainly composed of iron-nitrogen compounds, and its friction coefficient is minimal and relatively stable. When the compound layer is worn through, the friction coefficient of the diffusion layer increases.