Formation and evolution of craters in carbon steels during low-energy high-current pulsed electron-beam treatment

The authors investigated in detail the formation and evolution of microcraters induced by low-energy high-current pulsed electron-beam treatment on several quenched and tempered carbon steels. They have shown that the crater formation mechanism is the same for the three selected steels regardless of the carbon content and original microstructure state. Melting starts at the subsurface layer during treatment, resulting in the nucleation of small droplets preferentially at grain or phase boundaries. Under further heating, the boiling droplets erupt through the surface. The liquid around these craters shrinks to supply the lost part and, during the cooling process, leads to the formation of the funnel-like crater morphology. Microirregularities help retain locally the heat flux and, consequently, serve as nucleation sites for crater formations. By increasing the number of pulses, microirregularities were gradually removed and melted layer depth increased. As a result, crater formation became less effective. On the other hand, some of the already formed craters were deepened, while others were eliminated during the following pulses. The above processes together cause the crater density to first increase and then decrease, whereas the surface roughness first increases and then remains at the same level with increasing number of pulses.