(1) The graphite morphology of the three grades of gray cast iron samples is mainly type A, with a small amount of type D and e graphite. In terms of graphite grade, ordinary HT250 graphite is mixed with grade I and grade II, and low alloy HT250 and HT300 graphite are both grade II. The matrix of three grades of gray cast iron is pearlite matrix, the lowest pearlite content of ordinary HT250 is 90.6%, and the highest pearlite content of HT300 is 97.6%. In terms of eutectic clusters, the grains of ordinary HT250 eutectic clusters are coarse and scattered. The grains of low alloy HT250 and HT300 eutectic clusters are fine and uniform. The number of eutectic clusters per unit area of low alloy HT250 and HT300 are higher than that of ordinary HT250.
(2) The tensile test results of three grades of gray cast iron show that the tensile strength of ordinary HT250 is 239mpa, the tensile strength of low alloy HT250 is 280.4mpa, which is 17.3% higher than that of ordinary HT250, and the tensile strength of HT300 is 308.7mpa, which is 10.1% higher than that of low alloy HT250. The elongation of the three grades of gray cast iron is about 0.5%, indicating that the three grades of gray cast iron are brittle materials. By observing the tensile fracture morphology of three grades of gray cast iron, it is found that the tensile fracture of low alloy HT250 and HT300 added with trace alloying elements has local fragmentation, which prevents the continuous propagation of cracks, produces secondary cracks, and enhances the tensile strength of low alloy HT250 and HT300.
(3) The fatigue test results of different loading frequencies and different surface roughness show that the loading frequency between 20Hz and 140hz will not affect the fatigue strength of low alloy HT250; Surface roughness Ra = 0.3-2.5 μ M, it will not affect the fatigue strength of low alloy HT250. By observing the fatigue fracture morphology of low alloy HT250, it is found that the fatigue fracture with different loading frequencies and different surface roughness has river patterns and fatigue bands, which has typical fatigue fracture characteristics and belongs to normal fatigue fracture. In conclusion, the fatigue strength of the gray cast iron sample used in this test is mainly determined by the material itself.
(4) The lifting farad compression fatigue test results of three grades of gray cast iron show that the tensile compression fatigue strength of three grades of gray cast iron is 75.9mpa, 85.9mpa and 101.1mpa respectively. The fatigue strength of low alloy HT250 added with trace alloying elements is 13.1% higher than that of ordinary HT250, and that of HT300 is 17.7% higher than that of low alloy HT250. The overall trend of fatigue life curves of three grades of gray cast iron samples is the same. The cycle times in the high stress section decrease rapidly with the increase of loading load. After the loading load reaches the horizontal section, the cycle times begin to increase steadily until it reaches 10000000 cycles, and the sample is not broken.