The drastic softening in the heat affected zone (HAZ) of friction stir welded 0.2C–2Si–3Mn martensitic steel occurred due to the martensite tempering in the subcritical temperature range just below A1 temperature in HAZ. It was revealed that the softening in the subcritical HAZ was successfully suppressed by Mo addition. The optimum condition was found to be 3 wt-% Mo addition with the solution temperature of 1120°C, where densely distributed nanometer-sized Mo2C were confirmed by transmission electron microscopy and atom probe tomography in the subcritical HAZ. These fine Mo2C precipitates gave rise to an increase in the hardness of approximately 110 HV compared with a no-Mo-added sample, which was in good agreement with the predicted precipitation strengthening value based on the Ashby–Orowan model.
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