| With the majority of domestic oil fields entering high water-cut development stage, due to the influence of high water-cut, high sand content, hyper salinity, high-temperature steam heavy oil extract, polymer injection oil extract and the strong corrosive medium, the corrosion and wear of oil well tube get more and more serious.In the paper, compound nitriding layers were produced successfully by laser quenching/nitrided compound processing technique.The phase composition, microstructure, hardness, wear resistance, corrosion resistance, brittleness of nitriding layers were systematically analysised, the promotion effect of laser quenching on nitrding and the influence of laser quenching on the microstructure and properties were researched.Both the white layers of gaseous nitriding and laser quenching/nitrided are composed ofζ-Fe2N,ε-Fe3N and Cr2N. The volume fractions ofζ-Fe2N,ε-Fe3N and Cr2N are separately 79.95%,14.74%,5.31% in the gaseous nitriding layers and 25.03%,69.45%,5.52% in the laser quenching/nitrided layers, of which the content of Cr2N stays stable, the N content inζ-Fe2N is higher and the white layers are looser. The structure ofε-Fe3N is body-centered hexagonal andε-Fe3N is less brittle.The structure ofζ-Fe2N is orthorhombic lattice structure and is more brittle.The content ofε-Fe3N increased in the laser quenching/nitrded layers and the content ofζ-Fe2N decreased. Brittle fracture Critical stresses of nitriding layers increased from 3N of gas nitriding to 8N of laser quenching/nitrided layers.The grain ofεphase and Cr2N in gas nitriding layers are more coarse, while in the compound nitriding layers, the grain are finer and the quantities of Cr2N grain are more.Tiny Cr2N grains play a pinning effect and the micro hardness of compound nitriding layers increased by 100HV0.2,compared with gas nitrding layers.The micro hardness of laser quenching/nitrded layers dramatically improved.Compared with gas nitriding layers,γ'phase andα-Fe in the laser quenching/nitrded layers are finer.The ultra lattice structure existed inγ'-Fe4N in the diffusion layer, in which the Fe and N atoms arranged orderly. There have been certain orientation relationship between y'-Fe4N and a-Fe.The coherency and compatibility of y'-Fe4N and a-Fe is very excellent.Tiny Fe3C grains dispersively distribute on the structure of the strip lower bainite.The grains of surface layer processed by Laser quenching/nitrded were refined, which lead to the increase of the quantities of the grain boundary and the dislocation. These defects were at a higher energy state.Part of the energy can provide additional driving force to form the nitride compounds, making the standard free energy of generatingεphase andγ'phase reduced, and form the nitride compounds at the warming stage before reaching the nitriding temperature (530℃).Simultaneously, the diffusion path of N atoms along the grain boundaries increased and diffusion speed also increased, which increased the depth of laser quenching and nitriding layers.The quantities of y'phase andεphase,which have excellent ductibility, in the laser quenching/nitrided layers increased. Tiny wearable nitride alloy particles hinder the development of gridding crack. The hindering and pinning effect play a forceful friction block effect during friction process, which make friction process more stable, decrease wear losses and friction coefficient.In corrosive wear experiment, corrosion is inclined to start on grain boundary and phase boundary, which weaken the binding strength between Cr2N and base and make some Cr2N particles dropping from base during wear process.Therefore, corrosion speeds up wear process.Wear morphology of corrosive wear are Cutting furrows and corrosion pits.The mechanism of corrosive wear is corrosion abrasion wear. The white layers'microstructures of laser quenching/nitrided layers are denser, less brittle and less dropping.So the corrosion wear resistance of laser quenching/nitrided layers is better than that of gas nitriding layers.
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