Effect Of Heat Treatment On Microstructure And Properties Of Austenitic Ductile Iron And Development Of Turbocharger Shell

Turbocharger can improve the engine power, reduce automobile exhaust emissions, play a huge role in the energy-saving emission reduction. But the turbocharger housing, continue to experience the cooling and heating up the alternative transformation process in the car parking and start process, the working conditions are very bad, so the performance of the material has a high demand. High nickel austenitic ductile cast iron has become a widely used material for the production of turbocharger shell due to its excellent high temperature performance.In this paper, the heat treatment process of high nickel austenitic ductile iron was studied by orthogonal test, study the heat treatment of high Ni Austenitic Ductile iron structure, mechanical properties, thermal fatigue, high temperature antioxidant and anti corrosion performance by means of metallographic observation, SEM and spectrum analysis, X-ray diffraction analysis and other means, and the development of the production process of high Ni Austenitic Ductile Iron turbine shell of supercharger.The research conclusions are as follows:The heat treatment process was formulated by orthogonal test as follows:Heat insulation for 3 hours under 1040?, the cooling mode selection of air cooling. The degree of influence of each factor is the cooling mode, the heating temperature and the holding time.The microstructure of high Ni Austenitic Ductile Cast are Austenite+spherical graphite+carbide. The number of carbides in the as cast microstructure are large, and the morphology is irregular shape, and the size is not uniform. After heat treatment, the carbide morphology is isolated island, and the shape rules are evenly distributed, and the quantity is obviously reduced. High nickel austenite ductile iron heat treatment, the tensile strength by 488MPa decreased 442MPa hardness by 158.8HBW decreased 144.2HBW elongation by 20.6% rise to 25.2%.After heat treatment the carbides are distributed in the intergranular Island, can strengthen the grain boundary and effectively prevent high temperature mobile dislocation, preventing the crack generation and propagation, improve the thermal fatigue property of high Ni Austenitic Ductile iron. Under 850?,900?,950? for the upper cycling temperature of 200 times thermal fatigue cycling, cast alloys crack length respectively are 4.51mm,6.02mm,7.11mm; heat treatment state of the specimen crack length respectively are 4.05mm,5.56mm,6.43mm.Heat treatment of high Ni Austenitic Ductile iron oxidation resistance at high temperature had no effect, after 900? high temperature oxidation 150h and casting state and heat processing state of the specimen surface is formed consisting of Fe2O3, Cr2O3, NiCr2O4, FeCr2O4 and compact oxide film, can effectively prevent the internal organization to oxygen, has good oxidation resistance, weight gain without significant difference between state samples per unit area oxidation treatment cast state specimens and heat and oxidation level reached antioxidant levels.After heat treatment, the corrosion potential of high Ni Austenitic Ductile iron is increased from -0.389V to -0.382V, and the corrosion current decreased from 1.561×10-6A/cm2 to 8.238×10-7A/cm2, and the corrosion resistance is significantly improved.Turbocharger housing production process is:the iron mould tectorial sand molding process design live piece of cold iron on isolated hot spot to chill; with large flow and high temperature rapid casting process, the first tank pouring temperature is less than 1560?, the end of the box pouring temperature is more than 1470?, the tundish cover spheroidizing process and package bottom and flow compound inoculation process; heat treatment process for 3H under 1040? and air cooling.The development of turbocharger shell microstructure of austenite and carbides, the spheroidization class for 1-2 grade, the spheroidization rate more than 90%, the graphite size class for 6-7 grade, porosity less than 1%; tensile strength meanwhile and elongation is 20.5%, and the hardness is 146HBW, the fabric and performance indicators are meet the turbocharger housing requirements.