Study On Low Cycle Fatigue Crack Propagation Of TP347H

For recent years,modern industry has highly developed,meanwhile industrial equipment has gradually begun to ultra-large or miniaturization,lightweight development of two routes,which for the structure of equipment and the strength of materials put forward a huge challenge.Due to its excellent intergranular corrosion resistance and high allowable stress and creep fracture strength,TP347 H steel has been widely used in the oil refining industry for the manufacture of high-temperature mechanical equipment components.Therefore,it is of great significance to study the fatigue cycle characteristics of TP347 H and research the initiation and propagation laws of fatigue cracks for improving the fatigue life of TP347 H,extending the service life of mechanical equipment and reducing the occurrence of fatigue failure accidents.Firstly,TP347H standard fatigue specimens were tested at room temperature.It was studied how the strain amplitudes affect fatigue life,to set a series of gradient strain amplitudes of 0.16%,0.18%,0.20%,0.22% and 0.24%.At the same time,the strain and stress data of each group during fatigue test were tracked and recorded to explore the fatigue cycle characteristics of the material.It is found that the fatigue life of the specimen shows an overall downward trend with the increase of the load strain amplitude,and as the maximum strain increases,the fatigue life of the specimen drops more slowly until it remains constant.By analyzing the stress-strain data of fatigue tests for each group,it can be concluded that:with the decrease of loading strain amplitude,the hysteresis loop of the half-life cycle becomes narrower,the area decreases,and the fatigue life increases accordingly.During the fatigue experiment,the cyclic stress of the specimen decreases rapidly,and then remains stable for a long time.TP347 H material displays good cyclic softening properties.The fracture of the standard fatigue specimen was polished and polished,and the austenite structure was etched using 10% oxalic acid solution.Low-cycle fatigue tests were performed under 0.2% strain amplitude on the processed specimens.The initiation and propagation of fatigue cracks were tracked by interrupt test method under metallography microscope,and the effects of microstructure on the growth process of cracks were observed.The experimental data show that fatigue cracks generally occur in discontinuous areas of microstructure,for example,pores,grain boundaries,etc.The initial fatigue crack growth direction is 45 degrees to the load axis,and then the crack starts to grow in the direction perpendicular to the load axis.In the process of fatigue crack propagation,it is found that two different types of cracks fit together.At the same time,there will be interference between adjacent cracks,so that only part of the cracks gradually expand into the main cracks.The fatigue crack always propagates along the favorable slip plane in the grain.When the fatigue crack expands to the grain boundary in a grain,cracks will enter adjacent grains after rotating at a certain angle at the grain boundary.The deflection Angle is determined by the Angle between the favorable slip planes in the adjacent grain.Finally,ANSYS Workbench finite element software was used to carry out finite element numerical simulation of TP347 H material,and static analysis and fatigue analysis of the specimen under multi-dimensional stress state were carried out.The results show that the changes of stress and strain at the fracture are most complex and concentrated.At the same time,the fatigue cycle life of the notch is the lowest and the fatigue failure is the most likely to occur,which is in accordance with the fatigue experimental results.