Improvement of tractor crankshaft properties by rapid high temperature salt bath nitriding

Abstract: Salt bath nitriding is one of the most popular surface modification technologies to treat tractor's crankshaft for obtaining the required properties. Generally, several hours is needed to acquire the required layer depth in normal salt bath nitriding, and especially, it is almost impossible to get the required qualified deep layer depth due to the loose outermost surface layer formed at longer duration. Therefore, it is of significant value to improve its efficiency and get the required layer depth with good quality. In order to enhance the nitriding efficiency of tractor's crankshaft, AISI 1045 carbon steel, which is a type of common material used to produce tractor's crankshaft in industrial production, was selected as the testing material. Rapid salt bath nitriding was conducted at higher temperature of 660℃ instead of normally used 560℃. Then we compared normal salt bath nitriding with higher temperature salt bath nitriding for different nitriding time. Optical microscopy (OM), X-ray diffraction (XRD), micro-hardness testing and electrochemical workstation were employed for analyzing the microstructure, phase, micro-hardness and corrosion resistance of the tested material, respectively. The results show that compound layer (also called white layer) is formed at the outermost surface after salt bath nitriding under all conditions. The thickness of compound layer is only 3.8 μm nitrided at 560℃ for 20 min, which is too thin to effectively improve the surface hardness and corrosion resistance of AISI 1045 steel, and it is increased to 17.1 μm treated at higher temperature of 660℃ for 20 min, which is the same thickness as that nitrided at 560℃ for 140 min. Therefore, it can be concluded that higher temperature salt bath nitriding is much more efficient than normal salt bath nitriding. Meanwhile the main phases of compound layer are composed of ε-Fe2-3N and γ'-Fe4N nitrided under different conditions. But the relative contents of ε-Fe2-3N and γ'-Fe4N are different, which can be obtained by calculating the ratio of the strongest peak of γ'-Fe4N to the strongest peak of ε-Fe2-3N. The ratio of γ'-Fe4N to ε-Fe2-3N is 0.61 and 0.46 for higher temperature salt bath nitriding and normal temperature salt bath nitriding, respectively, which clearly shows that the relative content of γ'-Fe4N is more at higher temperature salt bath nitriding. Higher surface hardness and modestly higher sub-surface hardness are obtained with the same compound layer thickness after higher temperature salt bath nitriding. The maximum cross-sectional micro hardness of the AISI 1045 steel is increased from 702 to 784 HV0.01, which implies that the tractor's crankshaft made of AISI 1045 steel treated by higher temperature salt bath nitriding may have better wear resistance. Moreover, AISI 1045 steel treated by higher temperature salt bath nitriding shows higher corrosion potential of -993 mV and lower corrosion current of 0.302l A/cm2 compared with that treated by normal temperature salt bath nitriding, which indicates that AISI 1045 steel treated by higher temperature salt bath nitriding has better corrosion resistance. The mechanism of high temperature salt bath nitriding process is that the decomposition rate of CNO? is greatly accelerated, and thus much more active nitrogen atoms can be produced in the salt bath media, plus the vacancy concentration in AISI 1045 steel is significantly increased at higher temperature. Therefore, a conclusion can be drawn that high temperature salt bath nitriding is possible to replace normal salt bath nitriding to further enhance the tractor's crankshaft made of AISI 1045 steel, since high temperature salt bath nitriding can not only shorten salt bath nitriding time, but also enhance the combined properties of tractor's crankshafts.