Cementite dissolution in heavily cold drawn pearlitic steel wires

Abstract

The influence of cold wire drawing on the structure of a pearlitic steel containing 0.7 wt% of carbon and subjected to a strain of approximately 3.5 has been studied. The as-deformed microstructure is very fine, with an interlamellar spacing close to 20 nm, and contains high internal elastic strains. The main result concerns the cementite phase which undergoes a dissolution, at least partial, during wire drawing. The proposed interpretation of the cementite dissolution is based on the destabilization of this phase due to the increase of its free energy arising from thinning of cementite lamellae and creation of slip steps during wire drawing. Post-deformation annealings lead to restoration of the phase equilibrium by precipitation of the cementite, and also to recovery of the defects, mainly interfaces, stored in the microstructure during deformation.

Résumé

Nous avonse´tudie´l'influence de l'ope´ration de tre´filagea`froid sur la structure d'un acier perlitiquea`0.7% pds de carbone ayant subi une de´formation d'environ 3.5. La microstructurea`l'e´tat brut de de´formation est tre`s fine, la distance interlamellaire valant environ 20 nm, et contient des de´formationse´lastiques internese´leve´es. Le re´sultat majeur concerne la ce´mentite qui subit au cours du tre´filage une dissolution au moins partielle. La dissolution de la ce´mentite est interpre´te´e par la de´stabilisation de cette phase du fait de l'augmentation de sone´nergie libre provenant de l'amincissement des lamelles ainsi que de la cre´ation de marches de glissement au cours du tre´filage. Les recuits post-de´formation entraiˆnent le retoura`l'e´quilibre des phases par pre´cipitation de la ce´mentite ainsi que la restauration des de´fauts, principalement les interfaces, stocke´s dans la microstructure au cours de la de´formation.