Research On Microstructure And Properties Of QT400-18 Ductile Iron

To be used as an important engineering material, ductile cast iron, the best modern form of cast iron, this has excellent casting properties, good shock absorption and superior mechanical processing properties, low price. It is now used extensively in various fields such as mining machinery, automobile, railway rail, ship, metallurgy, and agricultural machinery. However, the microstructure of as-cast ductile iron characterizes "bull's-eye-shaped" which consists of ferrite, pearlite and nodular graphite. The inferior comprehensive mechanical property of as cast ductile iron constrains its application, so we should adopt proper heat treatment process to change the microstructure and then enhance the comprehensive mechanical properties and broaden its application.In this paper, QT400-18 ductile cast iron is used for the study. First, EDS spectrum analyzer, CS600LCS carbon and sulfur analyzer, and the whole spectrum of direct reading OPTIMA5300DV plasma emission spectrometer are used to determine the composition. Then optical microscope is used to observe microstructure. Based on the results above, specific annealing and isothermal quenching heat treatment are designed, then CMT5305 electronic universal testing machine and pendulum impact testing machine are used for mechanical properties tests, color 3D laser microscope is used to observe microstructure, at last optimum annealing and austempering heat treatments are got. It is concluded:The two-step annealing treatment can completely eliminate the phosphorus eutectic and hard-brittle phase such as cementite structure, and improve the mechanical properties of as-cast low-manganese cast iron QT400-18. The optimum heat treatment process was done by first austenitizing the ductile iron by heating to 920℃in furnace and holding the temperature at that level for 2h, then quenching and holding the temperature at 720℃for 3.5h, at last cooling in furnace to 600℃and then air cooling. This treatment improves the mechanical properties drastically. At room temperature, tensile strength increases from 371MPa of as-cast state to 428MPa, at the same time elongation rate increases from 8.1% to19.8%, low-temperature impact properties increase so obviously that impact value of samples with V notch is about 11.3 J at the temperature of -40℃.The process of isothermal quenching can promote the formation of austempered ductile iron (ADI), which enhances the tensile strength dramatically, which is similar with that of QT800-2 and QT900-2, elongation rate reaches two times that of QT800-2 and QT900-2.The isothermal quenching treatment was done by first austenitizing the ductile iron by heating to 920℃in furnace and holding the temperature at that level for 1h. Then the samples were quenched in a salt bath (NaNO246%+KNO354%) at different temperatures. Compare to as-cast iron, the tensile strength increases dramatically from 428MPa to 917MPa, elongation rate decreases from 19.8% to 4.61%.Studying the above two treatment methods, optimum heat treatments that meet the need of low temperature toughness and high strenghth are found, which is significant for actual production to some extent.