Electronbeam (EB) welding was used in T250 maraging steel, microstructures of both base material and heat affected zone (HAZ) were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and microhardness was tested. The results showed that during EB welding, the HAZ of T250 maraging steel exhibited a continuous gradient structure. The microstructure of the entire HAZ, from fusion line, could be divided into four zones: fusion zone, overheated zone, transition zone, and hardened zone. The microhardness showed a distinct regularity in each area. The softest region was the fusion zone, whereas the hardest was the hardened zone. In the overheated zone, the hardness increased as the grain size decreased. Furthermore, in the transition zone, the hardness level dropped noticeably. The peak temperature during the thermal cycle had a great influence on the formation of reverted austenite and dissolution of the precipitated particles, which contributed a lot to the microstructure and hardness of this material.
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