Abstract
Salt bath preoxidation was primarily conducted prior to salt bath nitriding, and the effect on salt bath nitriding was compared with that of conventional air preoxidation. Characterization of the modified surface layer was made by means of optical microscopy, scanning electron microscope (SEM), micro-hardness tester and x-ray diffraction (XRD). The results showed that the salt bath preoxidation could significantly enhance the nitriding efficiency. The thickness of compound layer was increased from 13.3μm to 20.8μm by salt bath preoxidation, more than 60% higher than that by conventional air preoxidation under the same salt bath nitriding parameters of 560℃ and 120min. Meanwhile, higher cross-section hardness and thicker effective hardening layer were obtained by salt bath preoxidation, and the enhancement mechanism of salt bath preoxidation was discussed.
Highlights
Salt bath nitriding is a kind of valuable chemical heat treatment widely used for steels to obtain the required properties [1,2,3]
It clearly shows that compound layer is formed at the outermost surface after salt bath nitriding under different conditions and the thickness of compound layer is significantly increased by salt bath preoxidation, the thickness of compound layer of 13.3μm by APON is increased to 20.8μm by SPON, more than 60% increased
It clearly shows that the cross-section micro-hardness can be significantly increased by salt bath nitriding with different preoxidation methods
Summary
Salt bath nitriding is a kind of valuable chemical heat treatment widely used for steels to obtain the required properties [1,2,3]. A compound layer is formed, which can significantly improve the performance of metal [7,8,9]. Preoxidation process is conducted in air furnace before salt bath nitriding [15, 16]. It is found that air preoxidation has little enhancement effect on salt bath nitriding [17, 18]. Based on the efficiency of salt bath nitriding is much higher than that of gas nitriding, it is valuable to study if salt bath oxidation could have much better enhancement effect on salt bath nitriding. The goal of this research is to investigate if salt bath oxidation can bring out much better enhancement effect on salt bath nitriding than that of conventional air preoxidation
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