| High chromium cast irons are applied to metallurgy, mine and building materialsprocessing due to the high hardness, high wear resistance and corrosion-resistance ofacids and bases. In practical application, it is well known that high chromium cast ironswere easily subjected to intergranular corrosion in corrosion medium, especially instrong acid medium. With the aggravation of corrosion, the support roles of matrix toeutectic carbides were weakened. In the slurry washout conditions, carbides wouldcompletely fall off or fracture from matrix, which has serious consequences forfunctions of good wear resistance. White cast irons with different Cr contents werechosen as research object. The microstructure morphology and distribution of wearresistant phase were obtained by the change of technological conditions. The influencelaw about technological conditions on carbides amount, morphology and distributionwas discussed. The characteristics during corrosion were explained by the law.The study results showed that the solidification microstructure of eutectic whitecast irons containing10%Cr，12%Cr，15%Cr，18%Cr，23%Cr，28%Cr hadsignificant changes with the change of cooling condition, especially the morphologiesand distributions of carbides. The corrosion tests results were influenced accordingly. Inthe samples of experimental conditions, the carbide size of solidification microstructureby sand mold casting (slow cooling rate) generally was bigger than that of metal moldcasting (relatively quick cooling rate), in addition, the carbides were lath shaped by sandmold casting. The net M3C carbides with big spacing were observed in white cast ironscontaining10%Cr, which directly influenced wear resistance and anti-corrosion. Withthe gradually increase of cooling rate, the precipitation amount of primary austenite in solidification microstructure increased, the eutectic microstructure amount decreasedaccordingly. The similar microstructure was obtained with different cooling rate andadjacent chromium contents. That was to say the microstructure of metal mold samplewas similar to that of adjacent low chromium by water cooling, carbides sizes anddistribution were quite similar, which indicated that the chromium content and coolingconditions to a certain extent impact on organizations. Meanwhile, the differences ofcorrosion resistance caused by different amount of chromium can also be reducedwithin a certain range by controling the cooling rate.As for the10%Cr，12%Cr and15%Cr white cast irons, M7C3carbides of sandmold casting (slow cooling rate) were cross-distribution with lath and blocky shaped,the sizes of carbides and eutectic colonies were relatively big. Carbides of metal moldcasting were blocky shaped and the amounts of eutectic colonies with chrysanthemumshaped increased. The carbide size was reduced and had a tendency of granulation, themicrostructure was fine and carbides distribution was uniform, the corrosion resistancewas better. As for the18%Cr，23%Cr and28%Cr white cast irons, the difference ofmicrostructure was the same as stated above, but the corrosion resistance difference wasare different due to the high Cr contents. Water cooling not only made more C and Crsolid solution in austenite matrix, but also had better carbides, morphology anddistribution, which decreased the electrode potential between austenite matrix andcarbides, the corrosion resistant property was improved. It indicated that owing to thechange of matrix microstructure, the influence degree of carbides parameter oncorrosion resistant property was weakened when the Cr content reached a certaindegree.It was believed that the effect of different cooling rate on the corrosion resistance of these chromium alloys was critically attributed to the role of Cr distribution in matrixduring solidification. Cr content had not only a relationship with the structure and thickness of surface oxidizing film, but also been largely considered for determining the corrosion potential difference between martix and carbides. Therefore, the corrosion rate ofmaterial could been retarded by the increase of Cr content and the change of technological conditions, thus enabling a good corrosion resistance of high chromium cast iron incorrosion environment. |
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