Influência do cromo no envelhecimento após deformação em fios de aço perlítico trefilados

Abstract

Strain aging tests were performed in two stelmorized pearlitic steels, after drawing to an area reduction of 86%. One of the steels contains 0,25% of chromium in its chemical composition. Two stages of strain aging were observed for both steels in the treatment temperature interval considered. The mechanism responsible for the strain aging in the first stage, which occurred between 100 and 120ºC, was the locking of dislocations by nitrogen and carbon atoms in solid solution in the ferrite. The values of the activation energy found for this stage were close to the activation energy for the diffusion of these elements in ferrite. Thevalues of the time exponent n were close to 1/3, in agreement with the model of dislocation locking in the cell walls by atoms following a perpendicular flow to the cell walls. The second stage of aging was associated with dislocation locking in the ferrite/cementite interfaces, and the activation energy values were close to the activation energy corresponding to cementite decomposition. The values of n approaching 1/2 are compatible with the kinetic law for a planar precipitate dissolution.The stelmorized pearlitic steels showed changes in the mechanical properties with the aging time similar to those generally observed in patented pearlitic steels. With respect to the kinetic parameters, the values obtained permitted the association with the same aging mechanisms attributed by other authors to the patented pearlitic steels.The effect of chromium was the reduction of the pearlite interlamellar spacing, increasing the intensity of the strain aging effects in the mechanical properties and in the area reduction by necking.No significant differences were observed in the activation energy for the second stage ofaging, meaning that at least for the investigated amount, 0.25%, chromium does not affect the stability of cementite.