Study On The Technology And Process Of Plasma Nitriding In Inner Wall Of Oil Drill Pipe

The oil drill pipe is prone to corrosion,wear and other failure accidents in the course of service.The protective effect of the inner coating is poor,and the potential of the plasma nitriding method is great.However,the conventional plasma nitriding technology can not realize the nitriding treatment of the large length diameter ratio pipe wall,and the conventional plasma nitriding process easily has some problems such as thin and brittle nitrided layer,large hardness gradient of nitrided layer and so on.Aiming at the above problems,this paper proposed to realize the nitriding treatment of the large length diameter ratio pipe wall by independent anode plasma nitriding technology,and studied the thermal cycle and argon composite plasma nitriding process,and researched the effects of process parameters on microstructures and properties of nitrided layer after nitriding.In this paper,an independent anode and its clamping and fixing device,which was placed in the nitriding furnace,was designed.The material of the independent anode was chosen as 304 stainless steel,and the shape of the solid steel rod with equal diameter was round.Dimensions were designed with two lengths of independent anodes,named L600 half furnace short rod of 10mm×600 mm,and L1700 through rod of 10 mm×1700 mm.According to the test results of independent anode nitriding test,under this experimental conditions,the independent anode of L1700 through rod was more suitable.Compared with conventional ion nitriding,after thermal cycle and argon composite ion nitriding process,the microhardness gradient of nitrided layer decreased,the surface hardness decreased to a certain extent,the thickness of the nitrided layer increased,whose maximum thickness was up to 0.462 mm,and the extension of nitrided sample increased,whose maximum was 8.6%.Cracks appeared on the nitrided layer when the samples are turned to 45°,so the surface brittleness was larger.However,the process of thermal cycle and argon composite plasma nitriding had no cracks on the surface of nitrided layer,and the brittleness of nitrided layer reduced.The orthogonal optimization of the process parameters of thermal cycle and argon composite ion nitriding was carried out.The factors affecting the comprehensive properties of nitrided samples from main to subordinate were the high temperature,the high and low temperature holding time,the low temperature temperature and the gas ratio.Under this experimental conditions,the optimum combination of parameters was high temperature at 540℃,low temperature at460℃,high and low temperature holding time at 1 h（i.e.high and low temperature cycle was4 times）,and N₂:H₂:Ar=1:3:1.The research results of this task can provide theoretical support for the nitriding of drill pipe,and save a lot of material and cost for the oilfield,which has high scientific and economic value.