Study On Microstructure And Properties Of 30CrMo And 35Mn2V Of Salt Bath Nitriding

Sucker rod and tubing are prone to failure due to the long-term effects of wear and corrosion in the process of service,which will seriously affects the normal oil production and causes some unnecessary losses,and a composite layer can be formed on the surface of workpiece through salt bath nitriding,which combines nitridation and oxidation together,to achieve the effect of surface strengthening and modification.In this paper,the 30CrMo sucker rod and 35Mn2V tubing were subjected to different temperature and time of salt bath nitriding as the matrix.And then metallographic test,loose grade evaluation,XRD analysis,SEM observation,EDS analysis,brittleness rating,hardness test,tensile test,friction and wear test and corrosion wear test were done to study the influence of salt bath nitriding process on the microstructure and properties of the nitrided layer.Finally,the quality of the nitrided layer was evaluated comprehensively and the mechanism of wear resistance was discussed.The result shows that the thickness of the compound layer and the loose degree increase with the increase of the nitriding time and temperature.The thickness of the nitrided layer is related to the diffusion coefficient of nitrogen atom in the phase.The content of C,O,N and Fe elements from the surface to the core has changed,whose trends are not the same.The layer is mainly composed of?-Fe₃N,?-Fe₂N and Fe₃O₄.The surface brittleness of nitrided layer increases with the increasing nitriding temperature,which changed little with the change of nitriding time.The strength of the samples decreases with the increase of the nitriding temperature and time.For 35Mn2V specimen,the elongation decreases remarkably with the increase of the nitriding temperature and time.After the salt bath nitriding,the micro fracture morphology of 30CrMo specimen has not changed markedly,and for 35Mn2V specimen,with the increase of nitriding temperature and time,the depth of dimples in the micro fracture surface becomes shallower,the number decreases,or even disappears,and the toughness of35Mn2V specimen decreases.After the ring-block wear test of 30CrMo,the wear and friction coefficient decreases with the increase of nitriding temperature and time.The wear amount of nitriding at 630?for 150 minutes is about 81%lower than that of 580?for 30 minutes,and the friction coefficient is reduced by about 8%.When the specimen is subjected to corrosion wear test with small load,the friction coefficient of the oxide film layer is small,and then the friction coefficient obviously increases when the loose layer is worn,the friction coefficient is the smallest and relatively stable of the compound layer owing to the iron nitride.When the compound layer is worn out,the friction coefficient of the diffusion layer increases.