Failure Analysis On Cracked Rotation Shaft Of Chemical And Mechanical Device

Cracking occurred on rotation shaft of chemical and mechanical device after accumulative service 5 years.Crack morphology,macro micro fractographic features were observed visually and SEM to determine failure modes.The microstructure and present phases in various cracked regions were observed and analyzed by OPM,SEM,TEM and XRD.According to the fractographic and metallographic analysis,the failure causes of rotation shaft was determined.In addition,based on Ansys Workbench finite element software,the static structure analysis of the shaft was carried out,and the structure stress of the shaft was simulated to determine the mechanical factors responsible for cracking failure of shaft.The main conclusions of the paper are shown as follow:1)Two crack sources S1 and S2 were presented on the cracked shaft and the crack sources were located at the root of the O-ring groove.No metallurgical or forging defects were found in the crack source regions.Four axial cracks,two transverse cracks and two deflection cracks were presented on the surface of shaft and a circumferential crack was presented on the subsurface.Axial cracking occured on both the inner and outer rings of the bearing.The failure mode of the rotation shaft and the bearing are attributed to brittle fracture.2)The failed shaft was subjected to a short-term age hardening treatment after solid solution treatment.An abnormal black layer was produced on the outer zone of shaft(about?30mm),which is an over-aged zone.The micro structure of black layer was composed of tempered martensite and longitudinally strip ferrite.The micro structure in subsurface layer of the hardening peak-value zone(30-60mm)and the matrix(?60mm)was composed of tempered martensite and longitudinally strip ferrite,and tempering degree of the martensite was lower than that in the surface layer.The grains size of surface layer and the matrix are coarse(1-2 grade).3)The hardness values on the outer layer(about ?30mm)in the crack source region,the subsurface layer and matrix are,respectively,36.0-37.5HRC,38.5-41.5HRC and 36.0-38.5HRC,which are corresponding to the specification(?32HRC).The difference in micro structure and hardness should be due to the short heating time and the large temperature difference between the inner and outer layers of the shaft in aging process.4)The material of the first bearing outer ring is different from the material of the inner ring,both of them were not in agreement with E52100 bearing steel.The average hardness values of matrix for the inner and outer rings are 58.7HRC and 60.2HRC,respectively,which meet the specification of general bearing steel.The bearing inner ring and bearing outer ring have a relatively uniform structure,whose microstructure were composed of fine tempered martensite.There are no metallurgical and forging defects in the crack source region.5)The maximum stress value of the finite element model reaches 428 MPa under the bending torsional load,and the structure stress has a certain influence on the crack initiation.Under the action of the bending torsional load and the structure stress,the maximum stress value just appears in the crack source region.However,the stress is not great enough to cause rapid brittle fracture of the rotation shaft,and there should be a load as the main force to cause cracking failure.