TRANSFORMATION HARDENING OF STEEL USING HIGH-ENERGY ELECTRON-BEAMS

Megavolt/megawatt high-energy electron beams (HEEBs) represent a new g eneration of charged particle beams that have unique capabilities for advanced processing of materials. These beams have energies in the 1 t o 10 MeV range, which enables them to deposit energy volumetrically wi thin the material being processed, under rapid and controlled conditio ns. One material processing application for HEEBs is the localized tra nsformation hardening of steel. The high accelerating voltage of the e lectron beam allows its energy to be deposited subsurface, so that the steel can be heat treated at depth without melting, while the beam's high average power allows transformation hardening to be performed at rapid surface coverage rates. In this investigation, a 6 MeV electron beam was used to process plain carbon steel and O-1 tool steel using s tationary and traveling beams, at normal and glancing angles of incide nce. These experiments investigated the beam fluence thresholds for tr ansformation hardening, melting, and vaporization of steel. Localized transformation hardening of steel was performed at a surface fluence o f approximately 20 J/mm2, which hardened the steel to a depth of nearl y 2 mm without melting, thus producing a microstructure that was free of solidification-related defects such as cracking and porosity. Achie ving deep localized heat treating of steel to its peak hardness withou t surface melting and at high surface coverage rates is not possible b y conventional directed energy beam processing techniques.