Levantamento da cinética de revenimento de um aço-carbono ligado ao V, B, Cr, Ni e Mo

Abstract

The effects of temperature and time on the kinetics of tempering has been evaluated by hardness, tensile and Charpy V-notch tests of specimens quenched and tempered in the range 580 to 680°C. Johnson, Mehl and Avramis equation was applied to study the kinetics of tempering from the variation of strength and Brinell hardness. The values found for the time exponent at 655 and 680°C can be expressed by a t1/6 kinetic law. It was found that the mechanical properties did not change with tempering time, but were strong influenced by the tempering temperature. A greater resistance to tempering was observed for the tensile strength and hardness as compared to the yield strength, probably associated with the softening of the martensitic matrix during tempering and the coarsening of fine precipitates, which tends to lower the tension required for the dislocation motion, which reduces the density of dislocations. A slight temper embrittlement occurred in the tempering temperature of 580°C, identified by the increase in the ductile-brittle transition temperature and a decrease in the absorbed energy. The activation energy for tempering found in terms of the hardness results was 476.2 kJ/mol and it is suggested that the mechanism which controls the kinetics of tempering is the diffusion in ferrite and along dislocations of the carbide forming elements, such as Mo, V and Cr. The best combination of strength and toughness was obtained in the heat treatments with Hollomon-Jaffe parameter between 17.300 and 18.284.