Wear and corrosion are two important causes of surface failure of materials. However, surface strengthening techniques such as chemical heat treatment, laser cladding, vapor deposition, electroplating and spraying can effectively improve the wear resistance of metal surfaces. At present, nitriding treatment technology and laser cladding technology are common strengthening means, and have been widely used in offshore industry, aviation, nuclear power and other fields. The FeCrNiMo coating was prepared by laser cladding technology. It was found that in the form of ring-block friction wear, the main mechanism of the cladding layer was abrasive wear and oxidation wear. In the form of ball and disc reciprocating friction and wear, the cladding layer is dominated by oxidation wear and fatigue wear mechanism.
The binding force of Ti and N is very strong. Adding appropriate Ti can increase the surface hardness and depth of nitriding layer, and improve the nitriding efficiency. At the same time, Ti has the effect of refining grains, which can improve the toughness of high nitriding layer. Therefore, in this paper, different Ti content (NiCr) 92-x Mo8Tix (x=2, The effects of Ti content on the microstructure, hardness, wear resistance and corrosion resistance of Ni-Cr-Mo-Ti laser cladding and plasma nitriding composite layer were studied. The possible wear and corrosion mechanisms were discussed to provide theoretical and experimental basis for improving the wear and corrosion resistance of the coating.
1. Experimental materials and methods
304SS was selected as the base material, Ni, Cr, Mo, Ti metal powder with purity higher than 99.95 mass% and particle size of 48 ~ 74 μm was selected. Weighing was performed according to molar ratio (NiCr) 92-x Mo8Tix (x=2 and 4 at%). In order to simplify the description, the prepared coatings were named S1 and S2 respectively, and the body composition was shown in Table 1. The powder is put into a vacuum stainless steel container, the stainless steel ball is used as the grinding ball for 6 h, and the ground powder is dried in a vacuum drying oven at 60 degree for 24 h. The alloy powder with a thickness of 2 mm was coated on the surface of the matrix by the preset powder method, and the optical fiber semiconductor excizer (LSJG-BGQ-2000) with a maximum output power of 2 kW was used for cladding. The power is 2.0 kW, the scanning rate is 30 mm/min, the overlap rate of multi-channel cladding is 40% ~ 50%, and the Ar gas is passed into the AR gas at a rate of 15 L/min. The samples were nitrided by a vertical auxiliary heating ion nitriding furnace (FD-WR60/80) with operating voltage of 720 V, vacuum degree of (350 ± 10) Pa, nitriding temperature of 540 degree , and holding time of 8 h. The sample names were 304-N, S1-N and S2-N after nitriding, when N2 and H2 were injected in the ratio of 1:5.
Table 1 Chemical compositions of the ( NiCr ) 92-x Mo8 Tix coatings ( at%)
|
S1 |
45 |
45 |
8 |
2 |
|
S2 |
44 |
44 |
8 |
4 |
The sample was cut into 10 mm× 5 mm test blocks by wire cutting machine, polished and polished to the gold phase standard, and corroded with king water (HCl ∶ HNO3=3 ∶ 1). The phase composition of the sample was analyzed by D/MAX-2500PC X-ray diffractometer (XRD). The Cu K target (λ=0.15405 nm) is used as the radiation source, the tube voltage is 40 kV, the tube current is 100 mA, and the scanning Angle is 20 degree ~ 100 degree . Scanning electron microscopy (SEM, FEI Nova NanoSEM 450) and energy dispersive spectroscopy (EDS) were used to analyze the microstructure, chemical composition and nitriding thickness of the coating. A Vickers hardness tester (HVS-1000) was used to measure the microhardness of the coating surface and from the top of the coating to the substrate with a load of 100 g and a loading time of 15 s. A reciprocating ball and plate friction and wear test machine (Rect MFT-5000) was used to test the wear line of the coating as follows: the loading load was 20 N, the wear time was 10 min, and the grinding material was Al2 O3 ball with a straight diameter of 9.8 mm. The friction coefficient (COF) was recorded at the same time, and the abrasion morphology was analyzed by BRUKER Contour GT-K1 and SEM. The corrosion behavior of the cladding coating and nitriding surface was tested using the traditional three-electrode system mode. The test equipment was Gmary Reference 3000 electrochemical workstation. The test surface was used as the working electrode, the saturated calomel electrode (SCE) was used as the reference electrode, and the platinum electrode was used as the counter electrode. The electrolyte is 3.5 mass% NaCl solution. The sample was soaked in 1 mol/L HCl solution for 24 h, washed slightly with anhydrous alcohol and dried, and the corrosion morphology was observed by SEM.
2. Conclusion
1) (NiCr) 92-x Mo8 Tix cladding layer is mainly composed of FCC phase, σ-CrMo phase and a small amount of Cr2Ti phase. The formation of (Cr,Ti) N phase after nitriding treatment. With the increase of Ti content, the content of (Cr,Ti) N phase increases, and the thickness of nitriding layer increases.
2) With the increase of Ti content, the coating hardness increases, up to 531 HV0.1. After nitriding treatment, the hardness of the coating is greatly increased, and the highest is 1258 HV0.1. The friction coefficient, width and depth of wear marks and wear volume of 304SS are much smaller than those of the non-nitriding coating and the nitriding treatment. The wear mechanism changes from adhesive wear to abrasive wear, and the wear resistance is significantly improved.
3) The corrosion current density (Icorr) of the nitriding coating is much lower than that of the non-nitriding coating and 304SS after nitriding, and no pitting phenomenon occurs. Among them, S1-N has better corrosion resistance in 3.5 mass% NaCl solution. The results of immersion corrosion test show that there are only slight corrosion marks on the surface of the coating after nitriding, and the corrosion resistance is improved.
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