| Seawater hydraulic pump is the power source of hydraulic system. Its key friction pairs can easily get severe friction and corrosion when in poor lubricant seawater, and boundary friction and dry friction will happen when in a worse lubrication condition. Regarding this issue, 45# steel was treated by laser cladding-ion sulfurizing composite processing to obtain a composite coating with excellent wear-corrosion resistance and self-lubrication properties, meanwhile, prolong the life span of friction pairs of seawater hydraulic pump.The effects of laser power and scanning speed on cladding layer forming quality were systematically studied, and three measures of designing tooling, using auxiliary sulfurizing workpiece and controlling parameters were applied to improve the quality of sulfurizing. The paper presents the scheme of preparing sulfide layer with stable quality on Ni-based cladding layer and gets its rationality verified. Laser cladding power is 3.6kW, scanning speed is 150 mm/min and overlapping rate is 30%. Channel steel auxiliary cathode and 45# steel auxiliary sulfurizing workpiece are used during sulfurizing. Sulfurizing temperature and duration time respectively are 280℃ and 2h.The characterization results of surface morphologies, composition distribution, microstructure and mechanical properties show that sulfide layer possesses a loose structure, the average diameter of sulfate particles is 87 nm, roughness is 0.08μm while layer thickness is 3~4μm. Sulfide layer and cladding layer is chemically integrated in a sawtooth staggered way with 47.2N the binding force. Compound modified layer compose of matrix phases from cladding layer(Fe, Ni) solid solution, Cr7C3, Ni17 W and sulfide phases of FeS, FeS2 and WS2. XPS and AES analysis shows that there is an S element abundant layer on the surface of sulfide layer. Its main chemical form is simple substance Ni, sulfides, such as FeS and NiS, and little oxides of Fe and Cr. Both nano-hardness(2.072GPa) and elastic modulus(122.51GPa) of FeS film are very low, which means sulfide layer is easy to be deformed. The surface residual compressive stress increases from-63 MPa to-195 MPa after sulfurizing treatment, which is helpful to improve the bonding force and the fatigue performance of compound modified layer.A ball-on-disc wear tester was adopted to evaluate the influence of sulfurizing treatment on the tribological behaviors of cladding layer under dry friction condition. Results show that high performance compound modified layer which has better anti-friction, wear resistance and anti-scuffing properties, will make wear mechanism changed to mild abrasion from severe adhesive and abrasive wear, friction coefficient reduced to 1/2, wear losses dropped to 1/4, and anti-scuffing load increased to 1.5 times compare to untreated ones after sulfurizing treatment. What’s more, sulfurizing treatment could reduce the effect of load on friction coefficient. Under medium and low load, the friction-reducing, wear resistance and anti-scuffing performances of cladding layer can be significantly presented, while with the increase of speed and load, the superiority of sulfide layer’s lubrication performance will definitely decreasing. Component analysis indicates that part of sulfides transferred to the grinding surface in the form of sulfide membrane, sulfide layer promoted the formation of oxide film(Fe3O4, Fe2O3 and Cr2O3) on the surface of friction pairs. A simplified model that sulfurizing FeS film playing as a solid lubricant during friction is presented, and its anti-friction mechanism is discussed. It is supposed to be a result of the comprehensive action of many aspects, such as characteristics of sulfide layer, low hardness, easy deformed and porous; decomposition, regeneration and diffusion of FeS during friction; the interaction of oxide film; the compound modified layer is an ideal friction surface and the two layers possess a strong chemical bonding, etc.. |
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