Study On The Obdurability Of Intercritical Hardening Plasma Nitrocarburizing Composite Process Of Oil Drill Pipe

The complexity of oilfield drilling fluid media imposes higher requirements on the corrosion and wear resistance of drill pipe surface.However,there are disadvantages such as easy peeling of inner coating and poor mechanical properties in the anti-corrosion technology of oil drill pipe inner coating.Nitrogen technology has the problem of poor obdurability.In order to solve the problem of poor obdurability,in this paper,a mixed gas of nitrogen,hydrogen and carbon dioxide is used as a carbonitriding medium.Sub-temperature quenching ion-nitrogen-carbonitriding composite technology is used to study the microstructure,thickness of the carbonitriding layer of the sample with different process parameters,Corrosion wear resistance,strength and impact toughness.On the basis of ensuring the corrosion resistance of oil drill pipe,the best process parameters of oil drill pipe toughness is obtained.In this paper,orthogonal experiments were used to study the effects of different process parameters on the microstructure and properties of the carbonitriding layer by changing the gas ratio,the carbonitriding temperature,the time of carbonitriding and the gas pressure of plasma nitrocarburizing.Experimental results show that the optimal plasma nitrocarburizing process parameters are H₂:N₂:CO₂=2:2:1,temperature at 580 degrees centigrade,carbonitriding time at 4 h,the pressure of500Pa in the furnace.The surface of the sample consists of a white bright layer and a nitrided layer,and the inner layer consists of tempered sorbite.The carbonitriding layer has good corrosion resistance and wear resistanc e.Compared with the conventional plasma nitriding,the strength and elongation of the optimal carbonitriding samples slightly increased,and the impact toughness increased by25%.Using the optimal carbonitriding parameters as tempering parameters,the effects ofdifferent process parameters on the microstructure and mechanical properties of the carbonitriding layer were investigated by changing the temperature,holding time,and cooling medium of the sub-temperature quenching.Experimental results show that the carbonitriding layer cosists of white bright layer and nitrogen-enriched layer.Furthermore,the inner layer is basically ferritin Body and tempered sorbite.,and the crystal grains become apparently fine and uniform.With the increase of intercritical hardening temperature,the yield and tensile strength of the sample increase s,and the impact toughness decreases.With the increase of holding time,the yield and tensile strength of the sample increase sfirst,then decreases,and the impact toughness gradually increases.Compared with domestic water,the yield and tensile strength of the 10%NaCl sample increases,but the impact toughness is less obvious.The impact toughness of the 10#engine oil sample increases significantly,but the yield and tensile strength decline obviously.The optimum parameters of the intercritical hardening process were finally obtained,namely,the intercritical hardening temperature was 790°C,the holding time was 5 min,and the cooling medium was domestic water.At the composite process parameters,the yield strength of sample reaches 98.8%of the base material,and the impact toughness reached 90.2%of the base metal,which basically meet use requirements of the actual production.