Effect Of Vanadium On Microstructure And Mechanical Properties Of Carbidic Austempered Ductile Iron

Carbidic austempered ductile iron (CADI) is derived from austempered ductile iron (ADI). It contains a certain amount of elements which would produce some carbide when the molten iron freezed. The mechanical properties of CADI are as excellent as that of ADI but the wear resistance property is better than that of ADI. CADI is an attractive engineering material which has better application prospect.In this paper, microstructure observation, scanning electron microscopy, spectrum analysis, X-ray diffraction analysis, rockwell hardness test, tensile test, impact toughness test, impact fracture observation, wear test and wear morphology analysis were done to research the effect of vanadium and heat treatment process on the microstructure, mechanical properties and wear resistance performance of CADI.The effect of vanadium on the microstructure of as-cast samples has been researched. The result showed that the microstructure of as-cast samples was pearlite, a very small amount of ferrite and some certain amount of carbide. With increasing of the content of vanadium, the amount of carbides increased gradually. The form of carbide were (Cr, Fe)7C3,VC and V4C3. As the content of vanadium was increasing, the rockwell hardness of the samples increased, and the impact toughness reduced gradually.The research of effect of vanadium on microstructure and mechanical properties of carbidic austempered ductile iron was shown in the below. With increasing of content of vanadium, the amount of bainite increased and the grain became finer, the amount of retained austenite decreased. The rockwell hardness of all these specimens was 51 to 54HRC and increased with the vanadium increased. The impact toughness reached to 28.26 J/cm2 when the sample contained 0.45% vanadium, and the result was higher than other samples. With increasing of the content of vanadium, the tensile strength of these specimens increased to 1070MPa but it decreased when the amount surpassed 0.45%. The wear rate of the specimens decreased with the vanadium content increasing.The result of austenitic temperature test showed that the microstructure morphology of the samples was blocky at first then changed to acicular as the increase of the austenitic temperature, and the amount of the retained austenitic increased. With the increase of austenitic temperature, the hardness of the specimens showed a case of increased at first then reduced when the temperature surpassed 880℃, while the impact toughness increased with austenited temperature increasing.The austempering temperature test showed that in the matrix there was more martensite than bainite when the austempering temperature was 220℃. When the temperature was 250℃,280℃,320℃, the amount of bainite increased with the austempering temperature rose. With increasing of the austempering temperature, the hardness decreased from HRC58.7 to HRC47.7, while the impact toughness increased from 9.77J/cm2 to 65.74J/cm2. The tensile strength of the samples in the beginning increased to 1390MPa but decreased when the temperature surpass 280℃. After austempering at 220℃, the specimen had the lowest wear rate, the wear surface which had no adhesive surface and metallic transfer was smooth and flat. While at 280℃, its wear mechanism was micro-cutting and micro-spalling. But after austempering at 320℃, its wear mechanism was mainly furrow action and a little micro-cutting and micro-spalling.The result of austempering time test had been shown as below. The specimen which was kept in the coolant for 0.5h showed the highest rockwell hardness, it proved that there was a lot of matensite in the matrix. The specimen kept for 1.5h had the best impact toughness.Through the research on heat treatment process, microstructure and properties of CADI, the optimized chromium content was 0.45%. While after austeniting at 900℃for 1.5h and then austempering at 250℃for 1.5h, there were 10% carbide content and 15% retained austenite content. Bainite morphology was needle in the microstructure of samples, which had good comprehensive properties. The hardness was HRC 52.9, the tensile strength was 1070MPa, the impact toughness was 28.26 J/cm2 and the wear rate was 0.61 mg/m. This specimen of the above chemical composition and heat treatment had the best comprehensive performance.The carbidic austempeing ductile iron developed in this paper had high strength, high toughness, high hardness and high wear-resistant. Its wear-resistant could be comparable to wear-resistant alloy cast iron, high manganese cast steel and high chromium cast iron. Its high impact toughness, less prone to break during use, greatly extend the life of the work piece, and because less alloy elements was added, the cost will be low, economic returns will be well. This material can be used in mining, machinery, buildings, railways and construction machinery, transportation and other fields. So it has broad application prospects.