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A flash tempering process has been demonstrated for low-carbon low-alloy lath martensite, which includes heating the sample to 750 °C (Ac1: 752 °C), dwelling at 750 °C for 2 s, and quenching directly to room temperature. The high temperature accelerated the spread of the supersaturated carbon atoms in the lath martensite, activated the dislocation migrations and thus their combining and annihilation, and promoted the mass recrystallisation. The reduced dwelling time inhibited the coarsening of the martensitic laths and suppressed the growth of the recrystallized structures. The combination of higher temperature and the shorter dwelling period, so-called flash tempering, gives rise to the same microstructure and mechanical properties as those obtained by the conventional tempering process (i.e., dwelling at 600 °C for 1 h). The high efficient flash tempering, possessing significantly reduced energy and time consumptions, may have important consequence in industrialization of tempering process.