An Experimental Study Of Surface Modification Of40CrNiMo7Steel By High Current Pulsed Electron Beam

High current pulsed electron beam (HCPEB) is a new kind of charged particle beam technology, and has developed rapidly in recent years. HCPEB can generate transient heating (109K/s), and cooling (107～108K/s) of the surface layer (10-5～10-6m) within a very short time and to which the energy has been delivered. The element distribution, the stress state and microstructure change greatly within the heat affected zone, and unequilibrium microstructures, such as metastable phase, ultra-fine grain and high-density defects could be formed in the surface layer, which lead to, significant surface modification effect unattainable for the conventional methods.In this study, surface modification and surface alloying of40CrNiMo7steel was carried out with HOPE-I type high current pulsed electron beam (HCPEB) equipment. The microstructure and phase state in modified surface layer were detected by optical microscopy and X-ray diffraction methods. The elements distribution on the cross-section of HCPEB treated samples was tested by EPMA technique. The surface microhardness and wear resistance were measured accordingly.Under the HCPEB irradiation, the crater morphology was formed on the modified surface, and the crater density decreased with the increasing number of HCPEB pulses. The different physical properties of ferrite and pearlite phase in surface (hot melt and thermal conductivity, etc.) give rise to the formation of crater eruption. XRD results showed that the mixture of martensite and γ-Fe phase was generated in the surface modified layer. The surface melted layer (7-8μm) with the finer grain size and compacted microstructure was showed in cross-sectional optical morphology. The surface microhardness of HCPEB modified sample was improved from313HK of initial state to above800HK, and the wear resistance increased by36%approximately for the HCPEB irradiated40CrNiMo7steel with accelerating voltage23.4kV and6pulses.Using HCPEB alloying of40CrNiMo7steel with carbon pre-deposition, the crater morphology was still formed on modified surface, and the average thickness of modified layer was about～7μm, where existed a～2μm difference between initial ferrite and pearlite phases. XRD results showed that the mixture of ferrite, martensite and γ-Fe phase has been generated after HCPEB alloying treatment. The effect of carbon alloying was not realized from the initial sample surface. However, the content of γ-Fe phase on the sample after pretreatment with10pulses of HCPEB increased enormously. According to EPMA results, the average carbon content in the remelted surface layer was higher than that of ferrite phase about78%. and the surface alloying was realized. The surface microhardness of40CrNiMo7steel after HCPEB alloying treatment was improved remarkably to355HV as compared to its initial state of190HV. It was also found that the modified surface of40CrNiMo7steel after HCPEB alloying treatment was composed of different zones that were correlated with the initial ferrite and pearlite phases. The hardening of ferrite zone was attributed to the grain refinement resulting from the fast quenching of melt, and also the increment of carbon composition diffused from the melted pearlite zone. While for the initial pearlite zone, except for the grain refinement, the main important factor is the martensite transformation during fast cooling cycles. It is recommended that an reasonable pretreatment should be conducted before the HCPEB alloying treatment to eliminate the influence of crater eruptions.