Study On Microstructure And Magnetic Domain During Tensile And Fatigue Process For Pipeline Steel

Microstructure and magnetic domain structure during uniaxial tensile deformationand fatigue process in X80pipeline steel which is the high grade pipeline steel and iswidely used in the project now were investigated quasi “in situ” by electronbackscattered diffraction(EBSD) and Magnetic force microscopy. And the relationshipamong the stress, microstructure and magnetic domain structure under differenttensile strain and fatigue cycles was discussed.The microstructures of the samples with different strain were observed, and thetransformation of the crystal orientation, distribution of grain boundary andmisorientation angle were analysed. The results indicated that the orientation of thegrains changed during the tensile deformation, and many small subgrains formed insome grains when the strain exceeded1%. According to the transformations oforientation of the individual graisn, the grains with the same or similar originalorientation have the same rotating tendency. And the orientation rotation of the grainwas affected by the orientation and deformation behavior of grains around it, and therotation was not successional and was salutatory for some grains. The magnitude ofrotation was not linearly increasing with the macro strain. Some grains performed noobvious rotation or reversing rotation.The distribution of grain boundary and misorientation were studied. It was foundthat the amount of grain boundary changed during tensile, especially when the strainreached1.5%or2%, there were much high angle boundary. Combined with theorientation maps and misorientation angle maps, it was inferred that the angle of thegrain boundary of the subgrains was within27°～33°. From the comparison ofmicrostructure change for different morphologies ferrite, it was found that thevariation of microstructure was different.The microstructures of the samples which have been loaded under different fatiguecycles were analyzed. The results indicated that when the maximum fatigue loadingclose to the yield stress with the increase in the number of fatigue loading, theorientation of the grains in the analysis area did not show much change. Accordingto the transformation of orientation of the individual grain, there was almost nochange in orientation of grains during the fatigue process no matter what position isthe original orientation of the grains. The fraction of the high and low angleboundary and the misorientation angle of the sample were analysed after differentfatigue cycles. There was no obvious change in the fraction of the high and low boundary and misorientation.MFM observations shows that, the magnetization direction of the domain wasaffected by the tensile stress, and the tensile stress enhanced the magneticdirectionality of the material. In the initial stage, the magnetization directiondistributed along the direction which was perpendicular to or close to perpendicular tothe surface of the sample, resulting in the distinct dark and bright parts of the image.With the increasing of the stress, the magnetization direction turned continuously.When the stress reached to a certain value, almost all of the magnetization direction ofthe domain exhibited uniaxial tendency.MFM observations of the fatigue process shows that, the magnetization direction ofthe magnetic domain changed under the fatigue loading, but the influence of thefatigue loading on the magnetization direction was complex. And the changing of thesystem energy during the fatigue cycle and the elastic stress which is different fromthe plastic stress may make the complexity happen.