Experimental Research On The Effect Of Banded Structure On Uniaxial Tensile And Fatigue Crack Growth Behavior Of Pipeline Steel

Banded structure,as a common defect structure in pipeline steel,often causes anisotropy in the mechanical properties of steel,which tends to lead to a decrease in strength,ductility and toughness of steel.Most of its research is limited to the common banded structure of ferrite/pearlite,and the research on the relationship between banded structure and mechanical behavior of steel is not enough.Digital image correlation(DIC)method has great advantages in the research on the deformation mechanism of the non-uniform materials such as banded structure because of its advantages of non-contact,full field and high precision.Based on the DIC method,the effects of banded structure on the uniaxial tensile and fatigue crack growth behaviors of pipeline steel were studied.In addition to the ferrite band,the class II structure banding is mainly a mixed phase of granular bainite,degraded pearlite and martensite/austenite(M/A)island structure.This article has mainly completed the following works:Firstly,metallographic tensile specimens containing banded structure were prepared by heat treatment process,and the characteristics of the banded structure were analyzed and then the multi-scale deformation measurement systems for banded structure based on DIC method were constructed.The natural texture after corrosion of the specimen surface was used as the speckle mark for DIC.And the zero deformation experiment proved that the systems had high precision and stability.Secondly,the influence of banded structure on the tensile properties of pipeline steel was studied based on the DIC test system.The results show that the ferrite band in transverse direction(TD)specimen undergoes major deformation under tensile load.The rolling direction(RD)specimen has better coordination deformation ability between the ferrite band and the class II structure banding.That is the main reason for the strength of RD better than TD.In addition,the continuity of banded structure has an important influence on the tensile properties of pipeline steel.Banded structure with poor continuity is prone to strain concentration in the discontinuous zone of the class II structure banding,and the RD specimen containing banded structure with poor continuity is even inferior to the TD specimen containing banded structure with better continuity.Finally,the effect of banded structure on the fatigue crack growth behavior of pipeline steel was studied.The fatigue crack growth behaviors of specimens on the longitudinal section along longitudinal direction(LD)and TD specimens of pipeline steel were compared and analyzed.In addition,the cumulative strain field during fatigue crack growth was measured ex situ by DIC combined with laser confocal microscopy.The results show that the banded structure in the LD specimen will cause wide range of crack deflections,especially in the early stage of fatigue crack growth,which tends to cause a lower fatigue crack growth rate relative to the TD specimen.At the later stage,due to the increase of the plastic zone of crack tip,more ferrite in the crack growth path of the TD specimen is also beneficial for crack tip blunting,thereby slowing down the increase of crack growth rate.Overall,the LD specimens have a slightly higher fatigue crack growth resistance than the TD specimens under the set load condition.