Study On Microstructure And Properties Of High Carbon Equivalent And High Strength Passivation Flake Gray Iron

Gray cast iron is widely used in automobile braking system because of its good thermal conductivity.However,with the increasing requirements for the performance of braking system components,the traditional low and medium carbon equivalent gray cast iron can no longer meet the requirements.Therefore,it is urgent to develop high carbon equivalent gray cast iron brake parts,the increase of carbon equivalent will reduce the strength of cast iron,so it is necessary to improve the strength of high carbon equivalent gray iron by some technical means.In this paper,the micro structure and properties of gray cast iron were improved by adding passivation elements(sulfur,nitrogen and rare earth)to the molten iron.The orthogonal experiment and single factor experiment are combined to explore the effects of alloying elements on the microstructure and mechanical properties of high carbon equivalent gray cast iron.The research indicates:The microstructure of passivated high carbon equivalent gray cast iron consists of flake graphite,pearlite matrix and a small amount of inclusions.When sulfur,nitrogen and rare earth are added together and each element is Ws=0.10%,WN=80ppm,WRE=0.10%,the graphite passivation rate in the microstructure reaches to 18%,the tensile strength of the alloy is 248MPa,the hardness is 212HBW,the amount of austenite dendrite precipitation is more,the morphology is very developed,and the size of eutectic colony is small.The range analysis and variance analysis of various performance indexs showed that,the nitrogen is a significant factor affecting the comprehensive properties of gray cast iron.Under the condition of other technological conditions are roughly constant,when the nitrogen content increases from 99ppm to 135ppm,the thickness of flake graphite increases slightly,the length becomes shorter,the bending degree increases,the ends become round and blunt,the stress concentration decreases,the number of dendrites increases,and a small amount of E-type graphite exists.The passivation rate increased from 22%to 27%.The number of pearlite increases and the lamellar spacing decreases.The strength of the alloy decreased from 404MPa to 360.3MPa,and the hardness increases from 245HBW to 260HBW,and the elastic modulus range from 115GPa and 121GPa.The fracture morphology was typical cleavage fracture,namely brittle fracture.During the thermal fatigue test,black oxide layers appear on the surface of three materials.With the increase of thermal cycles,the oxidation phenomenon becomes more and more serious,and the crack propagation rate increases with the serious of oxidation degree;the crack initiation occurs at the tip of graphite or inclusion;the effect of graphite morphology on the crack is greater than that of matrix structure;in phase under the same cycle conditions,the crack of vermicular graphite cast iron was the shortest,HT250 is the second,and the crack of passivated gray iron was the longest.