Glow Discharge In Long And Thin Stainless Steel Tube And Nitriding Of Inner Wall

Because of the good mechanical and physical properties,such as good corrosion resistance,good plasticity,highly polished,and good surface appearance,austenitic stainless steel is widely used in industry.However,low hardness and abrasion resistance repressed the lifespan of stainless steel,and make great damage to enterprises.It's difficult to improve the properties of austenitic stainless steel by usual heat treatment crafts,and the optimal craft for austenitic stainless steel to improve its surface hardness and abrasion resistance is surface modification.Compared to plane components,the working environment for tube is much more severe,and the tube can get damaged more easily and it's much more difficult to deal with the inner surface.In this thesis,the stainless steel tube is chosen as the object of study.In order to improve properties of inner wall of stainless steel tube,plasma nitriding is utilized.To achieve uniform nitriding,the equipment of grow discharge was optimized.The filament was chosen as anode,and went through the tube to nitriding uniformly.The characteristics of glow discharge,plasma emission spectroscopy,and nitriding technology have been investigated to understand the nitriding better.The 304 austenitic stainless steel tube size:inner diameter of 10mm,external diameter of 14mm,and the length of 200mm.Pulse bias power supply was adopted,and the characteristics of glow discharge,including pulse current waveform,average current,peak current and current distribution have been investigated.By changing voltage,duty cycle and pressure,the high current value and peak current may be achieved.In the same condition,max discharge current appears in Ar gas,while minimum discharge current is obtained in N₂ gas,and it may be related to ionization rate of different gases.As duty cycle increasing,average current and peak current also increase,and current distribution gets more evenly,however,current distribution is not so good in pure N₂ gas.As gas pressure increasing,average current and peak current increase,and current uniformity also gets better as pressure increasing,over 90%uniformity in discharge current is achieved when the working pressure is 80Pa.In Ar gas,emission spectrum intensity of Ar^*and Ar⁺increase as the Ar flow rate,duty cycle and voltage increasing.The highest intensity of Ar^*is found in Ar^*?750.4nm?,while the highest intensity of Ar⁺is achieved in Ar⁺?656.3nm?.In N₂ gas,emission spectrum intensity of N₂^*and N₂⁺also increases as flow rate of Ar,duty cycle and voltage increasing.N₂⁺?391.5nm?is influenced by processing parameters heavily.Besides,there is highest intensity in N₂⁺?391.5nm?.In mixed gas,Ar and N₂ interact with each other,and new particles will be created.In atom emission spectrum,the highest intensity us found in N₂^*?750.4nm?,while the highest intensity is detected in N₂⁺?391.5nm?in ion emission spectrum.Nitriding layer has proved to be there by SEM,XRD,energy spectrum analysis and indentation test.Nitriding layer can improve the hardness of inner surface.The highest nitriding rate of inner wall is 5?m/h with processing parameters are voltage of 600V,duty cycle of 20%and gas composition of n?Ar?:n?H₂?:n?N₂?=100:300:300.Before nitriding,Ar ion cleaning is carry out to excite the surface.Ar can help increase the possibility of collision to promote ionization.The thickness of nitriding layer in Ar/N₂/H₂ mixture gas is over twice as that in N₂/H₂.Increasing the concentration of N₂ can increase nitriding rate.Duty cycle slightly affects the thickness of nitriding layer,but changes processing temperature and nitrogen concentration.