Study On The Relationship Between Microstructure And Corrosion Resistance Of Cold-drawn Pearlitic Steels

In this work,cold-drawn pearlitic steels with diverse strains were used to study the relationship between microstructure and corrosion behavior.The electrochemical test,microstructure characterization and in-situ tension test were used as the primary measurements to study the relationship between cold-drawn microstructure and corrosion resistance.It was assumed that the integrity of surface was destroyed by corrosion effect,and the stress concentration facilitated the crack initiation.Thereby,the crack propagation of cold-drawn pearlitic microstructure was further investigated.It was found by electrochemical measurements and corrosion morphology that the corrosion resistance of longitudinal section decreased with increasing the strain to a critical value,whereas it increased with strain up to a final value.But the corrosion resistance of cross section continually decreased with increasing the strain of cold drawing.By characterizing the distribution of pits in the evolutionary microstructure,the grain boundary,pearlite colony boundary and phase boundary were found sensitive to pitting.Thereby,the densities of interface were markedly enhanced by cold drawing,giving a rise to the corrosion susceptibility of pearlite microstructure.The change in the misorientation angle distribution was induced by ferrite <110> texture,giving a rise to the corrosion resistance of ?=1.6.The lamellar structure suggested that the volume fraction of cementite in per lamellar structure was slightly increased under a strain of 0.8 and evidently decreased for a plastic strain of 1.6.The change in volume fraction of cementite in per lamellar structure influenced the corrosion resistance of microstructure.The result of APT suggested that a part of cementite dissolved at the strain of 1.6.Moreover,excess carbon atoms after cold drawing tended to dissolve into the microstructure defects.The unstable defects in the ferrite were stabilized by the dissolved carbon atoms.Thus,corrosion susceptibility induced by defects was decreased.Result of in-situ tension revealed three patterns of crack propagation in the pearlitic microstructure: shear cracking,cracking along the pearlite colony interface,and cracking with respect to pearlite phase boundary.In terms of ? = 0.8,the path of crack propagation gradually deflected towards the tensile axis obviously.At a higher strain of 1.6,the degree of deflection decreased.The local grain orientation distribution of ? = 0.8 suggested that the content of <110> texture component in the central part remarkably developed.However,at a higher strain of ? = 1.6,<110> texture components in the middle and center parts were remarkably developed.The variation of grain orientation distribution with respect to tensile axis induced the change of angle between slip plane trace and tensile axis,causing the deflection of crack propagation path.