Microstructure And Properties Of Surface Layer Of M50Steel Treated By High Current Pulsed Electron Beam Irradiation

In order to improve wear and corrosion resistance of M50steel, high currentpulsed electron beam (HCPEB) was used to irradiate the M50steel. Microstructureof the melted layer was characterized using TEM, SEM and XRD techniques. Theformation mechanism, tribological and corrosion resistance of the melted layer wereinvestigated. Influences of cryogenic and tempering on the microstructure of themelted layer were studied.Results of the simulation indicate that the irradiation energy density has biginfluence on the conditions of M50steel. When the time duration fix to2.5μs,surface of the M50steel would not be melted when the irradiation energy density islower than3J/cm2. When the irradiation energy density is between3J/cm2and6J/cm2, the M50surface would be melted. As the energy density is higher than9J/cm2, vaporation of the M50surface could not be neglected. Changing rate of thetemperature could reach as high as~109K/s and the temperature gradient is about~108K/m. As the energy density is fixed, highst temperature of the surface layerdecreases when the time duration increases. Time duration has little influence on thethickness of the melted layer.After the electron beam irradiation, crater-like defects formed on the surface layerof M50steel. Dimension of the defects is about t0μm. Density of the defectsdecreased as the irradiation number or irradiation energy density increases. But thedefects could not be eliminated by many irradiation numbers. Thickness of themelted layer is about6μm. When the irradiation number is less, undissolvedcarbides and penetrating craters could be seen in the melted layer. But as theirradiation number increases, carbides dissolved and depths of the craters decrease.Results of XRD indicate that martensite transformation is depressed and muchretained austenite formed in the melted layer. Index of (200) appeared. Proportion ofaustenite could increase to100%as the irradiation number increases.During solidification process, nucleation embryo formed between the meltingpool and the substrate. When the irradiation number is less, the melted layer iscomposed of twin martensite and cellular-like retained austenite. At the martensitetransformation zone, nucleation embryo formed earlier than other zones. Metallicelements satured in the solid phase due to the superfast growth rate and martensitetransformation occurs. Cellular retained austenite formed corresponds to the localregions of the carbides because of the high concentration of the metallic elements.During the solidification process, the dopant will be pulled off the growing crystal into the melt and move into the near layers. This leads to the formation of thecellular structure. As the irradiation number increases, a more homogeneous meltedlayer formed. The melted layer consists of absolute cellular retained austenite.Surface hardness of the M50steel decreased due to the formation of the muchretained autenite. When the melted layer is composed all of austenite, surfacehardness decreased from12.5GPa to about7.5GPa. And also, a softening zone withthickness about40μm appeared on the surface layer of the M50steel.Type and level of the residual stress are determined by the phase composition ofthe melted layer. When the melted layer composed all of austenite, tensile stressformed. The value is about1.1GPa. When the melted layer composed of martensiteand austenite, the tensile stress changed into compressive stress. As the proportionof martensite increases, level of the compressive stress increased first and thendecreased.Under dry sliding contact conditions, all of the samples have the same frictioncoefficient. The value is about1.0. Formation of the oxidation film make the frictioncoefficient did not change dramatically. Appearance of the austenite makes theincrease of the wear coefficient. When the melted layer contains martensite andaustenite, the surface has highest wear coefficient,5times higher than the untreatedsample.The retained austenite could not transforme to martensite at the cryogentictreatment because of the austenite stablizaition in the melted layer. Hardness of themelted layer changed a little. Tempered treatment could induce decomposition of theretained austenite. Tempering behavior of the melted layer depends on the phasecomposition. For the melted layer contains100%austenite, when the temperingtemperature is below500℃, the retained austenite could not decompose. As thetemperature increases to525℃, austenite begins to decompose. The decompositionplace locates to the boundary between melted layer and the substate. As theincreasing of the temperature, the decomposition zone enlarged to the surface.Second hardness peak appears at600℃. The value can reach up to13.5GPa. As thetemperature is higher than600℃, hardness of the melted layer decreases again. Forthe melted layer contains austenite and martensite, uniform decomposition occurswhen the temperature is higher than500℃. As the temperature is below675℃,hardness of the melted layer is about10.7GPa.