This work presents the microstructural changes of a carbon steel during tempering at low temperatures (first stage of tempering), and its effects in the second and third stages. These changes are determined by internal friction. Internal friction is measured in a forced vibration mode at 5 Hz, in a temperature range from 300 to 600K. Four internal friction peaks appear: S1 at 350K, P1 at 380K, P2 at 440K, P3 at 480K. S1 only appears in the sample tempered at 380K for 20 hours, and this can be related to the reduction of the martensite tetragonality. The height of P1 is lowest when the intensity of tempering is increased. This height can be related to the quantity of epsilon carbide than precipitate. The heights of P2 and P3 are not changed significantly, but the difference of temperature between both is lowest when the tempering intensity is increased. This suggests than the dissolution of epsilon carbide and formation of cementite takes places in a lowest range of temperature.
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