Surface nanostructure and improved microhardness of 40CrNiMo7 steel induced by high current pulsed electron beam treatment

In this paper, surface modification of 40CrNiMo7 steel was investigated with high current pulsed electron beam (HCPEB) treatment. The scanning electron microscope (SEM), electron back-scattered diffraction (EBSD), electron probe micro analysis (EPMA), transmission electron microscopy (TEM) and X-ray diffraction (XRD) results show that a composite microstructure of mainly refined austenite and a little martensite was produced in the surface modified layer of depth ∼7 µm. The average size of small cells on modified surface was decreased to ∼120 nm after 25 HCPEB pulses. XRD analysis indicates a preferred orientation of austenite (2 2 0) crystal plane, and TEM results show the broken and dissolved cementite in the surface modified layer. After HCPEB treatment, all the samples exhibited a remarkable improvement in surface microhardness measurement, up to ∼1000 HK for 15 HCPEB pulses, as tripled of the initial 40CrNiMo7 steel.