Electron beam simulation of disruptions into stainless steel

Abstract

The effects of repetitive pulsed heating and melt layer formation on type 304 stainless steel are reported. A line-source electron beam with pulse times of 0.5 or 1.5 ms and power densities up to 100 kW/cm 2 has been used for 1–40 pulse cycles at 500°C substrate temperatures. Numerical calculations of the temperature profiles and their time evolution during melt layer formation are also carried out. Significant metallurgical changes are observed, both in the melt layers and in the adjacent heat-affected zone. Melt depths (10–20 μm) are consistent with calculations and the resolidified regions exhibit columnar or columnar-dendritic microstructure. Appreciable lateral motion within the molten layer results in very rough surfaces and localized cracking occurs. Extensive slip deformation is seen in the adjacent heat-affected zone, and low-cycle fatigue crack initiation is observed after only 20 pulses. Chemical changes in the stainless steel melt layer result from the preferential vaporization of Mn from the melt layer and correlate with the time in the liquid phase. We suggest that the preferential loss of high vapor pressure species such as Mn in the early phases of plasma device operation might provide a unique signature of the impurity introduction mechanism. Disruption melted compositions are distinctly different from the more-nearly stochiometric ratios of Mn/Cr/Ni expected for sputter erosion.