Subsurface Crack Initiation And Propagation Mechanism In GCr15 Steel

Along with the light requirement of machine, a lot of high strength steels are used in machine components and structures. In the past the fatigue researchers considered that only the surface crack can initiate in the steel materials and the materials show the fatigue limit duing to crack closure before 10⁶ cycles with the reduction of stress amplitude. Although lots of machine components and structure are performed over 10⁷, even 10¹⁰ loading cycles, it is considered that the loading stress at 10⁷ cycles can satisfy the requirement of design when doing the experiment and research. However, in recent teen years it is found that the high strength steel can also fracture from the defect as internal inclusion in the very high cycle regime above 10⁷ loading cycles at the stress amplitude below the convention fatigue limit. The purpose of this paper is revealing the subsurface crack initiation and propagation property in GCr15 steel.The object of this research is high-carbon-chromium bearing steel, GCr15. The simulation analysis of subsurface crack initiation and propagation property in GCr15 steel was carried out by using the fracture surface obtained from the fatigue tests performed by four-axis cantilever-type rotary bending fatigue machine and ultrasonic fatigue machine. Based on different kinds of methods subsurface crack initiation mechanism was discussed, meanwhile, the size of internal inclusion's influence on fatigue life was quantitatively analyzed. The particular contents are as follows:From detailed observation of the fracture surface by scanning electron microscopy, it was discovered that the materials fractured from surface at high-stress amplitude and low cycles, whereas fatigue failure occured at small internal defects at low stress amplitude in a high-cycle region.The simulation analysis of subsurface crack initiation and propagation property was carried out by the FRASTA method. The result shows that on the fracture surface with GBF area, multiple dispersive micro-cracks formed around the inclusion and coalesced each other to form the GBF area. After that, the crack grew continually. On the other hand, on the fracture surface without GBF area the fatigue crack was initiated around the inclusion and then propagated continually. The fracture surface obtained from the two kind of fatigue test system had the same results.Detail observation and detection of GBF area indicated that the roughness of this area was large, the area of convex particles in it corresponded to those of carbide particles in the microstructure of the tested materials and there was rich carbon distribution in the area. According to these results it can be concluded that the GBF area was formed by the micro-cracks growth owing to the carbide particles around the inclusion dispersively desquamated from the matrix.The subsurface crack growing speed was calculated based on the S-N data obtained from the rotary bending fatigue test. The size of internal inclusion's influence on fatigue life was quantitatively analyzed according to this speed.The detailed observation on fracture surface which was obtained from the fatigue test performed by ultrasonic fatigue machine shows that on the internal inclusion induced fracture surface there is burned area around the inclusion owing to the deficient cooling. The temperature induced by the ultrasonic frequency makes influence on the fatigue property of the materials.