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

Carbidic autempered ductile iron (CADI) is a new type of engineering structure materials originated from autempered ductile iron (ADI) in recent years. CADI is obtained by controlling the amount of cardide in ductile cast iron and austempering heat treatment. With the excellent combined properties of hardness, toughness and wear resistance, CADI has a better application prospect.In this paper microstructure morphology observation, scanning electron microscopy, spectrum analysis, X-ray diffraction analysis, rock hardness test, impact toughness test and abrasive wear test were conducted to study the effects of boron and austempering temperature on microstructure and properties of CADI.The results of microstructure morphology observation indicate that microstructure of as-cast specimen is composed of nodular graphite, pearlite, a certain amount of carbide and little ferrite. With the increase of boron content within the addition range (0.020%-0.062%), nodule size increased, nodule count reduced and roundness level of nodule declined gradually. The increasing boron content not only promotes the formation of pearlite and carbide, it also leads to the decline of interlamellar spacing in pearlite.After austempering heat treatment with austenitizing temperature900℃for1.5h and austempering temperature250℃for1.5h, matrix structure consists of acicular ferrite, retained austenite, a certain amount of carbide and little martensite. Boron can make contribution to the formation of acicular ferrite and its grain size is refined as the boron addition, while the retained austenite count is inversely proportional to acicular ferrite. The consequences of SEM energy spectrum analysis and X-ray diffraction analysis show that Mo and Cu distribute uniformly in the matrix, however Cr, Mn and B tend to dissolve out to form (Cr, Fe)7C3,(Fe, Mn)3C, Fe2B and Fe23(CB)6respectively. The rock hardness ascend from55.8HRC to57.1HRC, while wear rate presents the similar trend from0.198mg/m to0.181mg/m, with the increasing boron content, compared to the decline of impact toughness from9.6J/cm2to8.3J/cm2. The effect of austempering temperature on the microstructure and properties were studied by operating the different austempering temperature with220℃,250℃,280℃and310℃. Matrix of CADI is composed of martensite, a certain amount of carbide, small amount of retained austenite at220℃. With austempering temperature ascent, martensite is replaced by acicular ferrite gradually and retained austenite count increase, however the acicular ferrite grain grows constantly. Rock hardness drops from59HRC to53HRC, to the contrary, wear rate go up gradually0.206mg/m, similarly, impact toughness ascends from6.4J/cm2to10.2J/cm2.In this process with the increasing austempering tempeature, the main features of impact fracture transform cleavage fracture into quasi-cleavage fracture, however wear surface characteristics present from micro-cutting to the mixture of micro-cutting and mirco-spalling.In this paper, after researching the effects of boron and austempering temperature on microstructure and properties of CADI, specimen represents excellent combination of microstructure, rock hardness, impact toughness and abrasive wear resistance, which contains0.047%boron content with austenitizing temperature900℃for1.5h and austempering temperature250℃for1.5h. Under this condition, the matrix consists of acicular ferrite, a certain amount of retained austenite and carbide, meanwhile rock hardness, impact toughness and wear rate reach57.8HRC,7.7J/cm2and0.183mg/m respectively.