Penning Discharge Plasma Anode For High Current Pulsed Electron Beam Source And Surface Modification Research Of Turbines Steel

High current pulsed electron beam(HCPEB)has recently been developed as an effective technique of material surface modification.In the process of modification,high-energy irradiation beam instantaneously penetrates the material surface(up to sereval micrometers),causing rapid heating and cooling(temperature rate of change can reach 109 K/s)and resulting in solidification with thermal stress effect.Unbalanced structure can be observed on the modified surface.In this paper,a new Penning discharge structure was designed to improve the plasma anode which is used in high current pulsed electron beams source(HOPE-I).The modified device was adopted to conduct surface modification on 40CrNiMo7 steel(normalized,quenched and tempered),martensite stainless steel 2Cr13 and precipitation hardening martensitic stainless steel FV520B.By using scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),back-scattered diffraction(EBSD),and electron probe micro analysis(EPMA),this paper intents to study the evolution mechanism of microstructure structure in the modified surface based on the testing results and aims to explore effective ways to improve the surface mechanical properties of mentioned steels.Since Penning discharge can produce large-caliber,high density plasma and long-distance transmission channel,we designed the new electron-beam source.Results show that two main stages of Penning discharge:high-voltage with low-current stage and high current discharge stage.With the optimized plasma discharge condition of working pressure 7.0×10-2 Pa,anode voltage 5 kV,magnetic field intensity 2000 Gauss and ballast resistor 200?,a stable plasma transmission channel with large-caliber(80 mm),high plasma density(1012-1013cm-3)and long distance transmission(170 mm)can be obtained.In this research,HCPEB equipment(HOPE-I)was adopted to perform surface modification on two types of 40CrNiMo7 steels(normalized,quenched and tempered).A composite nanometer structure(containing a large amount of austenite and a small amount of martensite)was formed on the modificated surface layer,and the martensite transformation and the dissolution and fracture of cementite can be observed.After initial irradiation,the high cooling rate caused the formation of nanocrystalline on the surface.After continuous irradiation treadments,the cooling rate of the modification surface gradually reduced,and the carbon kept dissolutimg and ended with surface composition homogenization.Both competitive factors result in the evolution rule of nanometer dimensions of surface structure.The average size of austenite phase on the modified surface decreased from micron-sized to?120 nm and?150 nm after HCPEB treadments.Due to thermal conductivity differences of pearlite and ferrite on normalized steel modified surface,a melting depth difference was formed on the cross section with initial pulse treatment.The depth difference decreased as the pulses number increased.After HCPEB treatment,the samples exhibited a remarkable improvement in surface microhardness,up to?1000 HK,which is as tripled of the initial 40CrNiMo7 steel.As for quenched and tempered steel,the surface microhardness is doubled to 553 HV,and the wear rate decreased to one third of the initial state correspondingly.The corrosion rate of modified surface decreased from 0.12 mm/a to 0.02 mm/a,showing remarkable improvement of corrosion resistance.In addition,new austenite phases were formed on the surface of 2Crl3 steel.While the surface phase structure of modified FV520B steel has not changed,and XRD analysis indicates a new preferred crystal orientation<200>in the remelting layer.The extreme low carbon content 0.06(wt.%)resulted in the fact that austenite converted to martensite during the rapid cooling process,which caused the austenite phase stabilization cannot be fully realized.The main factors of the improvements of wear and corrosion resistance property are the flat,dense structured and preferred crystal orientation on the modification layer of the treated material surface.