| Piston rod is one of the important work parts on automobile shock absorber. The surface quality and performance of piston rod on shock absorber determine the driving safety and comfort of the vehicle. The automobile shock absorber piston rod mainly use quenched and tempered 45 steel as its substrate material. Mostly, hard chromium plating technology is adopted to improve the surface performance on wear and corrosion resistance. Chromium plating technology has strong carcinogen hexavalent chromium pollution to human body. At present, the government has to limit application of chromium plating technology. Therefore, looking for alternative technologies for hard chromium plating process on piston rod surface treatment is particularly important. The automobile brake disc oil cylinder which was usually made of low carbon steel always was always surface treated to improve its anti-corrosion performance. QPQ process has been widely used instead of chrome plating for the surface treatment of car brake disc cylinder. As we know that post-oxidation process in QPQ can further improve the corrosion resistance. However, in the practical application, the post-oxidation treatment show little changes in anti-corrosion resistance improvement. Finding out the reason of this phenomenon to improve the QPQ process has a certain practical significance.QPQ technology has been trying to replace electroplating technology, such as white zinc plating. Because of QPQ technology not only has good corrosion resistance, but also has good wear resistance. So QPQ technology has the feasibility of the substitute for hard chromium plating. In this study, the QPQ technology is applied on the surface modification of automotive shock absorber piston rod. Its influences on the surface structure and performance of the piston rod are compared with hard chromium plating technology at the same time. Low carbon steel was surface modified by QPQ complex salt bath treatment, and then the modified surface was systematically characterized. Metallographic microscope, Vickers hardness tester, Scanning electron microscope (SEM) and X-ray diffraction (XRD) are applied for the analyses of microstructure of nitriding layer and chromium plating layer. The two kinds of processes of corrosion resistance and wear resistance performance were carefully discussed.The results suggest:1. After conducting QPQ treatment automobile shock absorber piston rod with nitriding temperature at 570℃ and nitriding time of 150 min, it forms a 22.5 μm nitride layer on the surface of specimen. The surface hardness is HV0.1 709. While the chromium plating layer thickness is about 33.5 microns and surface hardness is measured as HV0.1 945. The nitride layer hardness gradient changes slowly from the outer surface to substrate, while the hardness gradient of chromium plating layer is steep. The hardness decreased from HV0.1 945 to HV0.1 291 nearby the interface of chromium plating layer and substrate. This kind of mutation is unfavorable for the improvement of coating binding force. The corrosion resistance ability of QPQ treated automobile shock absorber piston rod is about 15 times of the hard chromium plating processing. While the wear-resisting performance of the QPQ treatment sample is 0.3 times higher than hard chromium plating sample.2. The results reveal that the nitrided sample surfaces consist of Fe3N/Fe24N10/FeN0.0939, while the post-oxidized sample surfaces consist of Fe3O4/Fe2N/α-Fe. The Fe3N in the outer-layer and Fe24N10/FeN0.0939 in the sub-layer are transformed as Fe3O4/Fe2N and α-Fe, respectively, by post-oxidation process. For post-oxidized sample, many cracks and pores are observed on the outer-layer and sub-layer, respectively. The surface hardness profile decreased about HV0.025 50～100 with comparison to nitrided sample. A decreased nitrogen concentration is recorded in the post-oxidized sample surface down to the substrate. The polarization curves results reveal that the corrosion current density(icorr) of post-oxidized sample decreased about 0.055 μA.cm-2 with comparison to nitrided sample, indicating an improvement of corrosion resistance. However, the corrosion potential (Ecorr) decreased about 79 mV, which may be caused by the cracks and pores. |
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