Research On Heat Treatment Processes And The Microstructure And Properties Of CADI

Carbidic austempered ductile iron(CADI)is derived from austempered ductileiron(ADI).It contains a certain amount of elements which would produce somecarbide when the molten iron freezed.The mechanical properties of CADI are asexcellent as that of ductile iron but the wear resistance property is better than thatof ADI.CADI is an attractive engineering material which has better applicationprospect.In this paper，microstructure observation，determination of mechanical propertiesand impact abrasion test, the results and conclusions were as follows:1.Using suitable composition design and appropriate technology,we can obtainthe samples whose as-cast microstructure was made up pearlitic,spheroidalgraphite and carbides.The graphite spheroidization grade reachede level1-2and theball size reached level5-6;with the chromium content’s increasing,the content ofcarbide in the as-cast structure significantly increased.The carbide’s morphologyimproved after joining SG modifier.2.A significant impact was showed in improving carbide morphology anddistribution by the high-temperature pretreatment process.With the hightemperature pretreatment temperature and the holding time increased,the amount ofcarbide in the sample was gradually decreased,and a slight decline on the hardness.3.After the high temperature pretreatment process on CADI, the samples ofmacro-hardness decreased in some extent,but the impact toughness has a moresignificant improvement. Compared with non-hightemperature pretreatmentsamples,the hardness of the A、B、C three groups samples after high temperaturepretreatment respectively decreased by3.3%,4.7%and5.5%;the impact toughnesswere increased respectively by51.1%,78.4%, and62.2%.4.SEM analysis on the sample showed that: the samples withouthigh-temperature pretreatment, the fracture morphology was some typical cleavagesteps,indicating that the fracture mechanism is brittle fracture; afterhigh-temperature pretreatment,the fracture morphology was the regional of thecleavage step became less.Around the graphite the dimple began to increase,indicating that the fracture mechanism were both brittle fracture and ductilefracture. 5.After different heat treatment process on the samples,the wear mechanism isalso different:without the high-temperature pretreatment of the samples,on the wearsurface existed short and deep furrows,the main wear mechanism was micro-cuttingmechanism;after high-temperature pretreatment(980℃/3min),on the wear surfaceof samples the number of the cutting furrows significantly reduced,and the furrowis shallower,the brittle fracture of the pits formed a peeling piece of brittlefracture,which indicated the main wear mechanism was micro-fracturemechanism;after15min,990℃high temperature pretreatment,the specimen wornsurface formed intensive pressure pit and lip flange,we can see the traces thatmaterial detach from the surface under repeated plastic deformation,whichillustrated the wear mechanism was repeatedly plastic deformation mechanisms.In this paper，the wear resistance of CADI was the integrated contribution ofcarbide morphology， distribution， quantity and the matrix structure.It wasemphasized that amount of carbides present in the matrix will help improve thewear resistance of CADI；when the size of carbide in the base is excessivelysmall,it was not conducive to improve wear resistance.