Effects Of Ni Content And Heat Treatment On Impact Toughness Of QT400-18L At Low Temperature

Ductile iron has been significant engineering materials for its excellent mechanical properties. Crankcases of engine used in high-speed railway are made of QT400-18L. On account of safety, a higher property at low temperature is necessary. Especially, its impact toughnessαkv at -40℃is required to exceed 12.0 J/cm2. But a common nodular graphite cast iron can not meet this requirement. For this reason, appropriate measures should be taken, such as alloying and heat-treatment to control components and microstructure of ductile iron so that the castings with required performance can be manufactured in practice. In this work, the ductile iron with 3.5-3.9 wt%C, 1.4-2.0 wt%Si, 0.050-0.20 wt%Mn, 0.020-0.040wt%P, 0.040-0.060 wt%Mg, and 0.030-0.040wt%S were designed and 0-1.8wt% Ni added into it for alloying and heat treated by 950℃×2h +750℃×3h as two-step graphitization annealing.The alloy is molten and poured into the molds from sodium silicate sands cured by CO₂ and standard Y-block ingots were obtained. The test specimens were taken from ingots and then heat treated for graphitization. Absolute ethyl alcohol and dry-ice were mixed in a sealed container in a right proportion so that three low temperature conditions were simulated. The impact specimens were put into a container and kept enough time so that they can reach the expected temperatures. Once reaching the temperature, the impact toughness of the specimens was measured rapidly. In addition, the tensile strength, extensibility, and hardness of the samples at different temperatures were tested. An optical microscope was used to observe microstructure of specimens. The phase compositions of the specimens were analyzed by X-ray. The fracture appearances of impact samples were observed by SEM.The results show that all the graphite in samples are normally nodularizing. The samples with 0.4wt% Ni as heat-treated have an excellent mechanical property. The tensile strength, elongation, and hardness at room temperature are ca 401.28MPa, 24.2%, and HRB80.6, respectively. Its impact toughnessαkv at -20℃, -40℃, and -60℃is 20.2, 17.9, and 14.5 J/cm², separately.The tensile strength, elongation and hardness of the samples contained 0.7wt% Ni as heat-treated are ca 411MPa, 20.8%, and HRB92.6, individually. The impact toughnessαkv at -20℃, -40℃, and -60℃is 28.2, 20.7, and 13.7 J/cm², individually. It indicates that the amount of Ni in excess of 0.40wt% has not an obvious effect on the improvement of impact toughness at -40℃.It was also found that the impact toughness of ductile iron without nickel at -40℃is ca 10.5J/cm² and its microstructure is composed of ferrite, a little pearlite and nodular graphite. As the amount of Ni increasing from 0.10 to 0.70wt%, the impact toughness of the samples increase, particularly, from 10.9 to 20.7J/cm² at -40℃and the microstructure contains ferrite and spheroidal graphite; Besides, little residual austenite exists in it. However, the impact toughness of the samples reduces as the content of Ni increasing from 1.0 to 1.8wt%, from 14.6 to 5.83J/cm² at -40℃, and the microstructure of ductile iron are composed of ferrite, nodular graphite, pearlite and a traces of carbonization, without residual austenite. It illustrates that the amount of Ni has to be controlled within a correct range so that a ductile iron with required properties can be obtained.