Application Of High Carbon Low Silicon Gray Cast Iron With Nb In The Brake Disc

Brake disc and brake drum are the security parts of cars and the most important parts of the automobile braking system. The real working environment of brake is relatively bad so that it would be invalid because of wear and cracking after repeated mechanical friction and temperature changes. With the development of the times and improvement of living standards, cars are required to be more convenient, comfortable and lightweight, so the brake material requires high strength, good wear resistance, thermal conductivity, thermal fatigue resistance and shock absorption.Gray cast iron is a kind of traditional material for the production of brake disc, its advantages are reflected in low prices, easy production, high yield and a variety of ways to improve performance. This topic uses alloying strengthening method. We first get enough graphite by regulating the addition of carbon and silicon to improve the heat conductivity, heat dissipation and thermal fatigue resistance of grey Cast Iron. After that, we adding different content of Nb elements when melting and then research the influence of Nb element on the microstructure and properties of gray cast iron.Research shows that high carbon equivalent gray cast iron that carbon equivalent exceeds 4.0% owns plenty of graphite and has high heat conductivity and heat dissipation. For the gray cast iron with the same carbon equivalent, if appropriately reduce the Si/C ratio, the length of the graphite sheets will be reduced but its’quantity will be increased,the ratio of ferrite will decrease and the ratio of pearlite will increase in the matrix, the distance of the pearlite laminae will be reduced, the tensile strength, hardness and wear resistance of the material will be improved. When reducing the Si/C ratio,the heat conductivity of gray cast iron will be improved in the certain degree because of the refinement of graphite sheets, so the ability to resist thermal crack initiation and growth will be enhanced. Thermal fatigue resistance of gray cast iron with high carbon and low silicon is superior than other gray cast iron with the same carbon equivalent but different components and superior than common gray cast iron with low carbon equivalent.The niobium element can be used to refine the graphite sheets of high carbon equivalent gray cast iron. Along with the increase of Nb in the range of 0～0.3%, graphite sheets gradually transformed from three-level to four-level,but still keep the shape of sheet and distribute evenly. Nb could obviously refine the pearlite lamellar spacing, enhance the strength of the matrix,in addition, improve the hardness and wear resistance of the material in various degree.The thermal conductivity and thermal fatigue resistance of gray cast iron could be improved only after adding a small amount of niobium, meanwhile, the initiation and growth of thermal crack would be delayed. The improvement of thermal fatigue resistance of gray cast iron is most visible when niobium content increase from 0 to 0.14 percent. After Niobium content is increased to more than 0.20%, the graphite sheets would be too thin and the thermal conductivity of materials would be reduced. The sharp corners of graphite sheets can be easier to cause stress concentration and promote the initiation of thermal fatigue crack, the thermal fatigue resistance of gray cast iron will be reduced. So niobium need not to be added too much.After adding a small amount of Nb in the gray iron brake disc with high carbon and low silicon content, the graphite sheets are obviously refined, both the tensile strength and hardness of gray cast iron are increased, the deficiency of the mechanical properties caused by the high carbon equivalent is made up. Considering the economic factors, the adding amount of Nb had better be controlled at about 0.10%.Simulate the filling and solidification process of gray cast iron with Magmasoft, it can get a good quality work plan when facing down the disk and using a bottom-gating system, and the shrinkage porosity caused by hot section could be eliminated by adding chills. This system can provide a certain reference to the actual production.