Study On Regulating Technology Of Structure And Properties Of Cast Austempered Ductile Iron

Austempered ductile iron?English for short ADI?,is one of the major discoveries of ferrous metallurgy in the 20th century.Because of its high strength,good plastic toughness and in some respects it can even be compared with the performance of steel.But the cost is much lower than steel and it has good economic benefits,so more and more widely used in the automotive,ship heavy industry,rail transportation and other fields.At present,the access of getting ADI is mainly through the isothermal quenching heat treatment.But the austempering heat treatment makes the production cycle longer and sometimes also need to use large-scale heat treatment equipment,which will greatly increase production costs.And the use of NaNO₂ and KNO₃ quenching molten salt also causes pollution to the environment.These disadvantages have limited the development of isothermally quenched ductile iron to some extent.Therefore,it is proposed that preparation of austempered ductile iron in the as-cast state which has an important engineering value.The topic mainly studied the effect of different alloying elements and cooling technology on the microstructure and properties of cast austempered,optimizing the process for preparing as-cast austempered ductile iron.The alloying elements mainly include Mo,Ni,Cu,and Si,and their comprehensive action is expressed by the alloy equivalent.The alloy equivalent is1/10(?_Ni+3.5?_Mo)2+0.35_Cu-0.27?_Si.The modulus m of the casting?m=V/F,V-volume of the casting,surface area of the F-casting,dimension cm?represents its cooling capacity.In this paper,based on the approximate relationship between the alloy equivalent-modulus-matrix structure,the amount of alloying elements added and the cooling process are changed to change the shape and position of the alloy equivalent-modulus-matrix relationship diagram,thereby affecting the ductile iron structure in the as-cast state.The composition,the homogeneity of the Abbott structure,the morphology and distribution of the bainite structure,the proportion of austenite and bainite,and the process for optimizing the preparation of cast austempered ductile iron.Under the condition of air-cooling and 400°C for 1 hour?modulus m<1.84,the same applies hereinafter?,the effects of four alloy equivalents?0.150,0.400,0.704,and 1.054?on the microstructure and properties of cast austempered ductile iron were studied.The test shows that when the alloy equivalent weight is 0.704 and 1.054,austenite bainite structure can be obtained without pearlite and martensite.The maximum tensile strength can reach825MPa,elongation 3.1%,and hardness 310HBW.Based on this alloying element,the effects of cooling process?cooling mode,holding temperature and holding time?on the microstructure and properties of as-cast austempered ductile iron were studied.Among the two cooling methods,the structure obtained by air cooling and fog cold cooling is more uniform,and the residual austenite is less.The best tensile strength is 1106 MPa,the elongation is preferably 7.5%,and the hardness is 362 HBW.In addition,the effects of different holding temperatures?425°C and 375°C?and different holding times?1h and 2h?on the microstructure and properties of as-cast austempered ductile iron were also studied.The results showed that the microstructure of austenite and bainite was the best at 375°C for 2h,and the best comprehensive performance was obtained.The tensile strength was 974MPa,the elongation was 6.7%,the impact power was 96J and the hardness was 274HBW.Based on the good experimental results obtained in the laboratory,the above process was applied to the production of crankshaft under the conditions of the factory to explore the application of the process in crankshaft production initially.The study of different parts of the crankshaft showed that all parts of the crankshaft can be completely austenite and bainite structure,and tensile strength can reach more than 850 MPa,elongation is about 6%.