Study On Plasma Nitriding Of 40Cr Steel With Hollow Cathode Assisted

Low alloy steel with high strength,high toughness,low cost,widely used in chemical industry,shipbuilding,aerospace and other fields,but its low surface hardness,poor wear and corrosion resistance characteristics.Plasma nitriding is one of the most widely used material surface modification technologies at present.After nitriding,a nitride layer is formed on surface of alloy steel,resulting in a significant increaseand wear resistance.The traditional plasma nitriding pressure is at 300~1000 Pa.Duo to the influence of the discharge characteristics of the gas and the electric field effect,which often leads to the edge effect and uniformity of the layer and so on.The nitriding atmosphere is usually a single ammonia gas or the mixture of nitrogen and hydrogen,but the wear and corrosion resistance of its nitrided sample is difficult to meet some specific environmental requirements.In this study,the alloy steel were plasma nitriding in the home-made ion-nitriding furnace.A double cylinder with different diameters is used as a hollow cathode.The hollow cathode discharge makes the gas ionized to produce active nitrogen ions.At the same time,the hollow cathode electrode can also be used as an efficient heat source to heat the sample.The sample materials used in this study are 40Cr steel.The influence of low pressure,the ratio of ammonia and nitrogen,nitriding temperture and time on nitriding process is studied.By using the optical microscope,scanning electron microscope,microhardness test,X-ray diffraction analysis and electrochemical workstation,charactering nitriding layer thickness,microhardness,phase composition,and evaluating the corrosion and wear resistance.The results show that the characters of plasma nitriding sample at low pressure is almost the same as that of plasma nitriding under traditional pressure,so as to achieve energy saving and emission reduction.The ratio of NH3/N2 has an important influence on the thickness,hardness,phase composition,wear resistance and corrosion resistance of plasma nitriding samples.Under NH3/N2 =1:4,the nitrided sample has the thickest compound layer thickness(12 ?m),and the maximum surface hardness is about 980 HV0.1.Compared with the nitriding samples with ammonia as nitriding gas,the maximum surface hardness is increased by 0.3 times,the thickness of the compound layer is increased by 2.2 times,and the wear and corrosion resistance is improved significantly.Meanwhile,increasing the nitriding temperature or prolonging the nitriding time is beneficial to nitriding process.