Fatigue Damage Evaluation Of Coarse-grained Austenitic Stainless Steel Based On EBSD And Ultrasound Methods

Z3CN20-09 M austenitic stainless steel has been widely used in nuclear power pipelinesystem for its excellent mechanical properties.The equiaxed crystal grain size is about 1-5mm,while columnar crystals can up to 10 mm,which is a typical coarse material.Its service conditions are harsh,so it will produce mechanical damage inevitably,of which fatigue is one of the main forms of injury.Therefore,it is very important to study the basic characteristics of early fatigue damage and evaluate it for the safe operation of nuclear power pipeline.Electron Back-scatter Diffraction(EBSD)can combine micro-organization with crystallography,which can analyse material information from the grain scale.While ultrasonic pulse echo test method is more flexible,in which the immersion method can achieve the effective control of acoustic parameters,the acoustic information can reflect the evolution of microstructure within the material.Both of them have many advantages in evaluating the fatigue damage of materials,which has attracted the attention of researchers.In this paper,we mainly use EBSD and ultrasonic pulse echo technology to study the fatigue damage behavior of Z3CN20-09 M austenitic stainless steel.The main contents are as follows:(1)The microstructure of Z3CN20-09 M austenitic stainless steel was found to be composed of austenite matrix and about 15% ferrite.EBSD tests found that the relationship between the austenite and ferrite orientation is mainly K-S,and some of them satisfy the N-W relationship.This is because the austenite phase is formed on the specific surface of the high temperature ferrite,and the two phases satisfy the K-S relationship during the shear process,and the little orientation satisfies the N-W relationship due to centrifugal casting.(2)Pull-pull fatigue was done on Z3CN20-09 M material,found that the material appears obvious surface relief after damage,the fluctuation curve of rough surface in different stages can be reflected by Root Mean Square Deviation,RMSD and deformation rate ?? to meet the expression2RMSD(%)(28)1.75(10)0.17?(35)?(10)0.08?(35)?,the occurrence of PSB is affected by the crystal orientation,the ferrite morphology and the orientation relation of the two phases;with the accumulation of damage,the quality of the pattern is decreasing and the local misorientation is gradually increasing.The difference between the mean local misorientation? and the deformation rate ?? is similar to the linear expression,o?(28)1.77(10)0.022?(35)?.In the process of damage,the pole figure shows that the crystal orientations of different crystal orientations are different,and the grains in the vicinity of 001> rotate along the <111>-<001>orientation,and the grains near the <111> orientation rotate to <111> direction,while the grains near <101> show different rotation mechanisms,which shows that different grain orientation has different damage mechanism.(3)Based on the flooding focused ultrasonic pulse-echo technique,the different crystal orientations of different crystal orientations were characterized by longitudinal wave velocity and sound attenuation coefficient.It is found that the velocity of the longitudinal wave is the largest in the <101> direction and the minimum in the <100> direction on the ultrasonic propagation path,and the attenuation coefficient shows the opposite trend.On this basis,the relationship between the longitudinal velocity,the acoustic attenuation coefficient and the crystal orientation is plotted.(4)After the fatigue damage of the specimen,when the deformation rate is less than3.57%,the attenuation coefficient of the three directions increases with the increase of the damage,and when the deformation rate is 3.57% it reached the peak,while the <101>,<111>and <001> were 4.74 d B/mm,5.79 d B/mmand 3.43 d B/mm,respectively.When the deformation rate reaches 4.26%,the attenuation coefficient began to decrease from the peak and reached to a lower level.The coefficient showed the trend of increase first and then decrease with the increase of ?ML,while attenuation coefficient on <111> orientation changed the fastest,<101> the second,<001> the smallest.This is related to the evolution of dislocation structures in different crystal orientations.