Study On The Process And Properties Of Nitriding Layer Of Plasma Nitriding On The Tubing Wall

In order to solve the problems about the efficiency of nitriding treatment and the brittleness of nitrided layer,use self-developed large length diameter ratio ion nitriding device to explore the influnces of technologies based on the conventional ion nitriding,such as purification treatment process,lost nitrogen treatment process and composite technology nitriding,on overall properties of tubing.Use orthogonal experiment method to optimize parameters and explore the the mechanism of action of each process.The orthogonal experiment analysis shows the optimum parameters of conventional ion nitriding are nitriding temperature 550℃,holding time 8h and pressure 180 Pa.Owning to the optimum parameters,nitrided layer thickness is about 0.46 mm,the maximum hardness of cross-sectiona is about 617 HV_0.2,corrosion resistance is about 4 times the matrix and the elongation is about 12%.Nitrided layer contains theεphase,γ’phase andαphase.The purification treatment process make sample obtain carbon-poor surface and improve ion nitriding by way of sputtering and thermal action of the active hydrogen ions.The optimum parameters are purification time 20 min,purification temperature 540℃and hydrogen partial pressure 180Pa.Owning to the optimum parameters,nitrided layer thickness is about 0.57 mm,the maximum hardness of cross-sectiona is about 670 HV_0.2,the elongation is about 13%,the corrosion resistance is increased by 1.4 times than the conventional ion nitriding sample,the nitrogen content of nitrided layer increases about 25%.Nitrided layer contains moreγ’phase.The lost nitrogen treatment process makes the white layer thiner by mechanical and thermal effects and promote the further diffusion of the active nitrogen atoms,then reduce the brittleness of the nitriding layer.The optimum parameters are hydrogen partial pressure 160Pa,lost nitrogen voltage 850V and lost nitrogen time 20min.Compared with conventional ion nitriding,the highest sectional hardness of lost nitrogen diffusion layer decreases by 150HV_0.2.The the elongation is about 16%and is 1.25 times the conventional ion nitriding elongation.After the lost nitrogen treatment,nitrided layer surface morphology roughness increases.Nitrided layer mainly containsγ’phase andαphase.The optimum parameters of composite technology nitriding are purification time 25min,lost nitrogen time 30min and purification temperature 560℃.The nitrided layer thickness is about 0.52mm.The maximum cross-sectional hardness is about 550 HV_0.2.The elongation is about 14%.The γ’ phase content of nitrided layer is higher and the nitrided layer has a higher quality.