A Study On Rapid Salt Bath Nitriding For Carbon Steel

Carbon steel has been widely used in fabricating a variety of mechanical parts in industrial.But the carbon steel with low hardness,poorly corrosion and wear resistance should be carried on the surface treatment.Salt bath nitriding can significantly improve the carbon steel mechanical parts surface hardness,corrosion resistance,and wear resistance.But under the normal salt bath nitriding temperature conditions,the decomposition rate of salt bath medium to produce active nitrogen atoms is slow.In order to obtain the effective thickness of nitrided layer,the nitriding time must be greatly increased.Therefore,the efficient of normal salt bath nitriding is slow.The 35 steel and 45 steel were selected as the material in this paper.The enhancement effect of higher temperature salt bath nitriding,rare earth salt bath nitriding and salt bath pre-oxidation nitriding has been studied.Characterization of the modified surface layers was made by means of optical microscopy,X-ray diffraction,energy dispersive spectroscopy（EDS）,Vickers micro-hardness tester,XRD and electrochemical workstation.The results show that salt bath nitriding at higher temperature can significantly enhance the nitriding efficiency and thus reduce the duration.To obtain the same thickness of compound layer,the nitriding time was decreased from 140 min to 20 min when nitriding temperature was increased from 560 ℃ to 660 ℃.The austenite layer was formed in some region between surface and substrate when nitriding at higher temperature of 660 ℃,which can further improve the corrosion resistance.Meanwhile,γ’-Fe4 N appeared in the compound layer nitrided at higher temperature.Kinetics analysis shows that the diffusion coefficient of nitrogen atoms is increased more than 10 times when salt bath nitriding temperature is increased from normally used 560 ℃ to higher temperature of 660 ℃ and the activation energy of nitrogen atom diffusion（Q）was significantly decreased.The nitriding efficiency could be significantly enhanced assisted by RE La,and the enhancement effect was RE La concentration dependent.It was found that 5 wt.% was the optimal concentration in the present conditions.The thickness of compound layer was increased to 18.5 μm,more than 40% while treating at 560 ℃ for 120 min.The micro-hardness and corrosion resistance can be significantly improved with RE La addition.The XRD and EDS show the RE La atoms can diffuse into the nitrided layer.In addition,The EDS also shows the nitrogen concentration is increased at any corresponding depth with the enhancement effect of RE La.The salt bath pre-oxidation could significantly enhance the nitriding efficiency than conventional air pre-oxidation.Under the same salt bath nitriding parameters（560 ℃×120 min）,the thickness of compound layer is only 13.2 μm treated by air pre-oxidation,and it is increased to 20.8 μm treated by salt bath pre-oxidation,more than 60% increased.Meanwhile the hardness of sample treated by salt bath pre-oxidation nitriding is smaller than that treated by air pre-oxidation nitriding.But the effective hardened layer thickness and corrosion resistant are greater than that treated by air pre-oxidation nitriding.The sample treated by salt bath pre-oxidation has higher relative content of Fe3O4.Kinetics analysis shows that the nitrogen diffusion coefficients were increased significantly by salt bath pre-oxidation and the activation energy of nitrogen atom diffusion（Q）was decreased from 216 kJ·mol^-1 and 158 kJ·mol^-1.