Enhancement of metal properties by irradiation with intense, high-energy electron beams

There has been considerable interest in low energy (30–120 keV) electron beam surface treatment for both hardening and corrosion protection of steels and other alloys^.[1] Techniques involve either melting the surface or raising the temperature to near melt and relying on self-quenching to rapidly cool the material below the critical transition temperature. With iron, for example, both carbon and carbides are dissolved in both the austenite (above 910 °C) and liquid phases. The rapid cooling to below 710 °C prevents growth of large ferric carbide (Fe3C) or cementite crystals and thus produces a much harder surface. At sufficiently high cooling rates martensite or even amorphous compositions can be produced.^[2] With previously used low-energy electron beams, only the top 10 to 100 μm of the material is directly heated by the electrons, although the heat affected zone may extend much further into the material. Recent accelerators have been developed in the high-current beam propagation program which have both high current density (>10 kA/cm²), higher energy (up to 50 MeV), and high average beam power (150 kW). The advantage of using a high-energy machine is that the electron range is much greater. This leads to modifying bulk properties of the material and not just changing surface properties. For example, the range of 10 MeV electrons is nearly 1 cm in iron. The question that remains is whether it is possible to self-quench sufficiently fast when the material is heated to greater depths. We propose theory and experiments to study these effects. Theory will involve use of available PIC codes, such as ISIS, for electron deposition, and a radiation transport code, such as CYLTRAN, for radiation transport and deposition. These will provide input to thermal transport codes for determination of heat flow rates. Experiments will consist of irradiating samples with high power, high-energy bea- s and measuring the resulting materials changes.