Effect Of Molybdenum On Microstructure And Properties Of Carbidic Austempered Ductile Iron

Carbidic austempered ductile iron (CADI) was researched and developed by theUnited States in the1990s, basing on a new type of ADI by adding carbide formingelements, and its microstructure is needle-like ferrite, retained austenite and carbide.Thanks to the particular characteristics of the matrix, which was widely used inmachinery, automobile manufacturing and other industries, but in mine metallurgyand other industries that the components worked under the condition of wet millingconditions require high wear resistance and certain application of toughness andcorrosion resistance, its performance still cannot satisfies the use requirement.The purpose of this article is to research the influences of molybdenum and theaustempered temperature on microscopic structure and performances of CADI. Theoptical microscope (OM), scanning electron microscope (SEM), and X-raydiffractometer (XRD) and other equipment was usied to analyzed themicrostructure,properties,wear resistance of CADI.The results of the present research showed that the microscopic structure ofas-cast CADI containing pearlite and minute quantity of ferrite with graphite nodule,carbide distributed in, the graphite spheroidizing is2~3grade level, graphite sizereaches6~7magnitude; The increasing Mo content promotes the quantity of pearliteand carbide, meanwhile declines the ferrite, the content and size of graphite nodulewas decreased. After austempered, the microstructure of CADI consists of needle-likeferrite, retained austenite, with graphite nodule, carbide distributed in, The increasingMo content promotes the size of needle-like ferrite, he quantity of retained austenite.When the component contains Mo, it is main enrichment in carbide, which tends todissolve out to form (Fe,Mo)3C, but the very small amount of solid solution in thematrix, moreover, there is the very small amount of Mo form (Cr2.5Fe4.3Mo0.1)C3with Cr, when the samples containing0.735wt.%Mo, the carbide edge appearedwhite area with high Mo.With the increasing of Mo content, the rock hardness ascend from50.38HRC without Mo content to53.88HRC with0.735wt.%Mo content; while the impacttoughness showed a fluctuation and the samples containing0.416wt.%Mo presentedthe best toughness of16.9J/cm2; wear rate performed the declined tendency andpresented the best wear resistance with0.210mg/m with0.735wt.%Mo content; Thecorrosion potential Ecorr presented a slightly grows with the increasing of Mo content,but there is a slightly decrease with0.735wt.%Mo content and its corrosionresistance deviation.The samples containing0.416wt.%Mo after austempering heat treatment withdifferent austempering temperature, with austempering temperature increase,needle-like ferrite grain grows gradually and retained austenite count increase, therock hardness dropped from55.54HRC with240℃to48.58HRC with300℃; whilethe impact toughness presents the opposite trend from8.1J/cm2to18.6J/cm2; wearrate performed the ascended tendency and presented the worst wear resistance with0.2410mg/m with300℃; The corrosion potential Ecorr presented a slightly growswith the increasing of austempering temperature, while there is a slightly breakdownwith300℃and its corrosion resistance deviation.In conclusion, appropriate addition of Mo could effectively refine themicroscopic structure and performances of CADI. After austenitizing temperature900℃f or1.5h and austempering temperature270℃for1.5hheat treatment, specimenwhich contains0.416wt.%Mo content represents excellent combination ofmicrostructure. Under this condition, its rock hardness and impact toughness reach52.74HRC and16.9J/cm2respectively, the wear rate reaches0.221mg/m, meanwhilethe corrosion potential Ecorr and corrosion current Icorr reach-0.5786V,8.406×10-8A cm-2.