Guirong Yang

Microstructure characteristics of Ni/WC composite cladding coatings

A multilayer tungsten carbide particle (WCp)-reinforced Ni-based alloy coating was fabricated on a steel substrate using vacuum cladding technology. The morphology, microstructure, and formation mechanism of the coating were studied and discussed in different zones. The microstructure morphology and phase composition were investigated by scanning electron microscopy, optical microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. In the results, the coating presents a dense and homogeneous microstructure with few pores and is free from cracks. The whole coating shows a multilayer structure, including composite, transition, fusion, and diffusion-affected layers. Metallurgical bonding was achieved between the coating and substrate because of the formation of the fusion and diffusion-affected layers. The Ni-based alloy is mainly composed of γ-Ni solid solution with finely dispersed Cr7C3/Cr23C6, CrB, and Ni+Ni3Si. WC particles in the composite layer distribute evenly in areas among initial Ni-based alloying particles, forming a special three-dimensional reticular microstructure. The macrohardness of the coating is HRC 55, which is remarkably improved compared to that of the substrate. The microhardness increases gradually from the substrate to the composite zone, whereas the microhardness remains almost unchanged in the transition and composite zones.

Wear behavior of Ni/WC surface-infiltrated composite coating on copper substrate

Abstract Ni/WC surface-infiltrated composite coating was fabricated on copper alloy substrate through vacuum infiltration casting using Ni-based alloying powder and with different WC particle contents as raw materials. The wear behavior of Ni/WC surface-infiltrated composite coating was investigated using a block-on-ring tester at different loads and sliding speeds at room temperature. Results show that the wear rate of Ni/WC surface-infiltrated composite coating decreased to approximately one-sixth of the wear rate of the Ni-based alloy infiltrated coating. This phenomenon resulted from the supporting function of WC particles under varying loads applied on the specimen surface and the antifriction effect of the transformation layer. Wear rate was reduced by the Ni/WC-infiltrated composite coating with increasing load, especially when the load exceeded 100 N. The friction coefficient decreased with increasing sliding speed for all infiltrated coatings at any load condition. The reduction in the friction coefficient at high sliding speed was larger than that at low sliding speed with increasing load. The wear mechanism was dominated by oxidation under all experimental conditions and accompanied by adhesion and abrasion mechanisms at high load and high sliding speed.

Effect of Cl− Concentration on the Corrosion Behavior of 16Mn Steel in Saturated H2S/CO2 Solution

The corrosion behavior of 16Mn steel was studied in saturated H2S or H2S/CO2 solutions containing different Cl− concentrations at 80 °C. The microstructure and chemical composition of the corrosion products were investigated through scanning electron microscopy, energy-dispersive X-ray spectroscopy, EPMA, and X-ray diffraction. Results showed that the corrosion rate decreased with increasing Cl− concentration in saturated H2S or H2S/CO2 solution at pH 4. Conversely, the corrosion rate increased with increasing Cl− concentration in saturated H2S solution at pH 6. The relative H+ concentration decreased because of the increase of Cl− concentration at pH 4, and Cl− acted as a catalyst in the corrosive medium at pH 6 because the net H+ concentration decreased obviously compared with the condition at pH4. Cl− promoted the formation of Fe-deficient iron sulfide at pH 4, and the opposite effect was observed in the nearly neutral solution. The corrosion rate increased firstly with increasing Cl− concentration and then decreased in the saturated H2S/CO2 solution at pH 6. The corrosion products were mainly composed of two kinds of iron sulfide. Sulfide FeS1−x was a kind of tetragonal crystal, whereas the other was the hexagonal/monoclinic iron sulfide Fe1−xS. The corrosion film that was mainly composed of FeS1−x did not confer a protective effect on the base metal. The atomic ratio of Fe/S was more than 1 for FeS1−x. The appearance of sulfide FeS1−x resembled a square block or small, needle-like, flocculent particles. The atomic ratio of Fe/S was less than 1 for Fe1−xS, and the corrosion film mainly composed of Fe1−xS conferred some protective property on the base metal. The sulfide FeS1−x exhibited a long claviform morphology with a hexagonal or quadrilateral cross-sectional shape.

Wear performance of Ni/ZrO2 infiltrated composite layer

The Ni/ZrO2 was used as raw materials to fabricate the surface infiltrated composite layer with 1–4 mm thickness on cast steel substrate through vacuum infiltrated casting technology. The microstructure indicated that the infiltrated composite layer included surface composite layer and transition layer. Wear property was investigated under room temperature and 450°C. The results indicated that the abrasion volume of substrate was 8 times that of the infiltrated composite layer at room temperature. The friction coefficient of infiltrated composite layer decreased with the increasing load. The wear resistance of infiltrated composite layer with different ZrO2 contents had been improved obviously under high temperature. The friction coefficient of infiltrated composite layer was decreased comparing with that at room temperature. The oxidation, abrasive and fatigue abrasion was the main wear mechanism at room temperature. Oxidation abrasion, fatigue wear and adhesive wear dominated the wearing process under elevated temperature.