Study On Tensile-compressive Fatigue Properties Of High-strength Gray Cast Iron

Gray cast iron is widely used in industrial production and people's daily life because of its excellent performance and low price.It is one of the most commonly used metal materials.Gasoline and diesel engine cylinders and cylinder heads are generally made of gray cast iron.With the promotion of lightweight vehicles,high-strength gray cast iron began to enter people's vision.The promulgation of the National Engine Six Emission Standard for Automobile Engines takes the tensile-compressive fatigue performance of high-strength gray cast iron as the engine material as an inspection standard.Therefore,studying the tensile-compressive fatigue performance of gray cast iron to provide a scientific basis for engine material selection has become a very important Meaningful subject.In this paper,three types of gray cast iron are used as research objects to determine the tensile properties of three types of gray cast iron at room temperature,and the effects of loading frequency and surface roughness on the tensile-compressive fatigue properties of gray cast iron are studied.The tensile-compressive fatigue limit of gray cast iron is used to draw the fatigue life curve.ZEISS metallographic microscope was used to detect the metallographic structure of three grades of gray cast iron.Field emission scanning electron microscope was used to observe the tensile fracture and fatigue fracture morphology of the three types of gray cast iron.The results show that the flake graphite in the three types of gray cast iron is A-type graphite,and the matrix is a pearlite matrix.The addition of alloying elements Cr and Mn refines the flake graphite in the gray cast iron and improves the gray cast iron matrix.Compared with ordinary HT250,the low alloy HT250 with added alloy elements has a tensile strength increased by 17.3% at room temperature and a tensile-compressive fatigue strength increased by 13.1%.The surface roughness and loading frequency of the specimen have no significant effect on the tensile-compressive fatigue properties of gray cast iron.The fatigue fracture of gray cast iron is mainly composed of fatigue crack initiation zone,crack propagation zone and final instantaneous fracture zone.There are a large number of quasi-cleavage planes and cracks extending along the grain boundary in the tensile-compression fatigue fracture of gray cast iron,indicating that high-strength gray compression fatigue fracture mechanism belongs to the compound fracture mechanism of quasi-cleavage and intergranular fracture.The initial investigation of the tensile-compressive fatigue failure mechanism of gray cast iron is that the fatigue cracks of gray cast iron mainly originate from the casting holes and flaky graphite tips located on or near the surface of thematerial.The crack nucleation starts to expand to the surroundings when the nucleus initiation occurs.Then a sudden break occurred.The presence of the alloy compound hindered the further expansion of fatigue cracks,changed the direction of crack progression,produced a large number of secondary cracks,and extended the fatigue life of high-strength gray cast iron materials.