The fatigue fracture process of metal mainly includes the process of fatigue crack nucleation and initiation, fatigue crack propagation and final instantaneous fracture. The morphological characteristics of fatigue fracture are left by fatigue crack initiation and propagation. Therefore, fatigue fracture retains all the characteristics of the whole fatigue failure process and records a lot of fracture information, Generally, there are obvious morphological characteristics in each stage of fatigue fracture. Observation and analysis of fatigue fracture morphology is an important method to study fatigue fracture mechanism.
1.The graphite morphology of three grades of gray cast iron samples is mainly A-type graphite, and the graphite grade of low alloy HT250 and HT300 is higher than that of ordinary HT250; The matrix structure of three grades of gray cast iron samples is pearlite matrix, and the pearlite content is slightly different: ordinary HT250 is 90.6%, low alloy HT250 and HT300 are 95.1% and 97.6% respectively; Ordinary HT250 eutectic has coarse size, scattered distribution and less eutectic per unit area, while low alloy HT250 and HT300 eutectic have small size, uniform distribution and obvious increase in the number of eutectic per unit area.
2.The tensile strength of three grades of gray cast iron samples at room temperature: 239mpa for ordinary HT250, 280.4mpa for low alloy HT250 and 308.7mpa for HT300. The tensile strength of low alloy HT250 increased by 17.3% compared with ordinary HT250 and 10.1% compared with low alloy HT250
3.Tensile compressive fatigue strength limit of three grades of gray cast iron samples at room temperature: 75.9mpa for ordinary HT250, 85.9mpa for low alloy HT250 and 101.1mpa for HT300. The fatigue strength of low alloy HT250 increased by 13.1% compared with ordinary HT250, and that of HT300 increased by 17.7% compared with low alloy HT250.
4.The tension compression fatigue loading frequency is 20hz-140hz, and the sample surface roughness Ra = 0.3-2.5 μ M will not affect the tensile compressive fatigue strength of gray cast iron. The tensile compressive fatigue strength of gray cast iron is mainly determined by the properties of the material itself.
5.The tension compression fatigue fracture of gray cast iron is mainly composed of fatigue crack initiation zone, fatigue crack propagation zone and final instantaneous fracture zone. The fatigue crack nucleation of gray cast iron mainly originates from the casting holes and the tip of flake graphite on the surface or near the surface of the material; There are fatigue bands and radial river patterns in the fatigue crack growth zone; There are fatigue cracks along grain boundaries in the final transient fracture zone. The results show that the tension compression fatigue fracture of gray cast iron belongs to the composite fracture mechanism of quasi cleavage and intergranular fracture.
6.The tension compression fatigue failure mechanism of gray cast iron is that under the tension compression cyclic load, the fatigue crack is generated at the casting hole or flake graphite tip on the surface or near the surface, and expands in different directions along the grain boundary. The existence of alloy compound phase hinders the further expansion of fatigue crack, changes the forward direction of crack, and produces many secondary cracks, The fatigue life of gray cast iron is prolonged.