Investigations On Micro-structure And Magnetic Domain During Fatigue Process Of Pressure Vessel Steel

This study applies microstructure and domain changes to Metal MagneticMemory Testing (MMMT) method through the tension-tension fatigue experiment ontypical pressure vessels steel16MnR and4130. It provides the theory foundation forthe21st century new technique of Non-Destructive Testing (NDT).The changing regularity of microstructure, crystal orientation and misorientationwas analyzed in fatigue process to establish a relationship between fatigue stress andmicrostructure via Scanning Electron Microscope (SEM) with Electron BackscatterDiffraction (EBSD) attachment. By using “Bitter method”, the transformations ofmagnetic domains for pressure vessels steel16MnR and4130in fatigue process werestudied, and the relationship between fatigue stress concentration and magneticdomains was analyzed. The relationship of fatigue cycles and microstructure, crystalorientation and magnetic domains were also studied.Magnetic suspending liquid which can help observe the morphology of magneticdomains was obtained by using co-precipitation method through adjusting pH value,reaction temperature, magnetic stirring speed, concentration time, the sodium oleatecoated et al parameters.Quasi “in situ” SEM experiments on steel16MnR and4130were carried outunder different fatigue cycles to analyse the effect of fatigue on morphology of thesamples. It shows that fatigue can’t cause destructive plastic deformation in thematerial but make grains rotate. The further study of EBSD shows that, in the fatigueprocess, the rotation angle of grains in16MnR steel was large at the beginning, andbecame smaller then and increased sharply in the end. While it showed "large-small-large-small" changing orderliness in4130. The above results were caused by fatiguehardening which exists in annealed material. In other word, the smaller rotation anglein the middle stages of fatigue was caused by fatigue hardening which blocked therotation of grains, and the4130grains showed "large-small-large-small" changingregularity, making the grains rotation angle demonstrate big-small interchange.Quasi “in situ” EBSD experiments on steels16MnR and4130were carried outunder different fatigue cycles. The Kernel Average Misorientation (KAM) was used toqualitative responses to the elastic deformation degree of materials during fatigueprocess. The results showed that a nonlinear relationship exists between the value ofKAM and fatigue cycles. The reason is that the elastic deformation degree of materialis not proportional to fatigue cycles. And the fatigue hardening slows down thematerial deformation rate, while it increases when the hardening reach saturationpoint. The plastic deformation was non-uniform. The characteristic of deformationshowed the grain boundary and the grains with some orientations crystal are plasticdeformation sensitive areas. The magnetic domain structures of two materials (steel16MnR and4130) wereobtained by quasi “in situ Bitter method”, and the magnetic domain structures haddifferent forms, such as90degree domains,180degree domains, branch domains and“tetragonum” domains. The deflexion of domain wall caused the magnetic domainstructures transmute into each other, the transformations achieved by90degree orquasi90degree domain-walls rotation and domains recombination, by which thecorrespondence relationship between fatigue cycles and the transformations ofmagnetic domains was build. The alterations of magnetization direction caused byfatigue strain induced the transformations of magnetic domains. Therefore, it hadfurther developed the core mechanismof magnetic memory test “centralized area ofstress and deformation will occur with magnetostrictive magnetic domain structureoriented and not reversible reorientation".Quasi “in situ” research results of fatigue-EBSD and fatigue-magnetic domainon16MnR and4130showed that the grains rotated, while the magnetic domainsoriented in the fatigue process. The transformations of magnetic domains and grainsrotation were associated with fatigue strain. The inner strain energy increases with theincreasing of fatigue cycles, to minimize system energy, the grains rotate to coordinatethe increasing inner strain energy. The magnetic domains rotated from the easymagnetization direction to stress loading direction due to the grains rotation andshowed a45°angle with it in the end.