Effects Of Technological Conditions On Microstructure And Corrosion-Resistance Behavior Of High Chromium Cast Irons

The formation characteristics of matrix were studied by varying technological conditions aimed at different chemical compositions of high chromium white cast iron; the effects of technological conditions variation on austenite precipitation, its stability and corrosion-resistance behavior were analyzed profoundly; and the constituent characteristics of carbides in high chromium cast iron were further discussed. The results showed that:As for as a 15%Cr, 2.6%C hypoeutectic white cast iron, solidified microstructure changed remarkably with increasing cooling rate: under our experimental conditions, when the pouring size of specimen was (?)70×100mm in crucible, the microstructure consisted of martensite and eutectic structure, which was typical hypoeutectic microstructure; as the pouring size was (?)30×100mm in sand mold, primary austenite disappeared and almost 100% pseudoeutectic structure was obtained. Both the primary and eutectic austenite in above-mentioned specimens were unstable due to low carbon,chromium content, thereby transforming into martensite. However, with the increase in cooling rate (that was pouring the specimen of (?)20×50mm in metallic mold and that of (?)10×25mm with hydro-cooling), the amount of primary austenite increased whereas that of eutectic structure decreased, and the amount of carbon,chromium atoms soluted in primary austenite increased. The more carbon,chromium atoms soluted in primary austenite were, the more stable austenite was, however, the tendency of secondary carbides precipitation also enhanced. The increase of carbon,chromium atoms leaded to primary austenite more stable when no secondary carbides precipitating, however, once secondary carbides precipitate, it resulted in the instability of primary austenite.The solidified structure of high chromium cast iron changed with the cooling conditions variation, and its corrosion-resistance varied correspondingly. No matter Cr15 or Cr30 cast iron, both of them had better corrosion-resistance at rapid cooling velocity. The austenitic amounts and its stability of Cr15 cast iron increased at rapid cooling velocity, and compared with Cr15 cast iron with martensite matrix at slow cooling velocity, the corrosion-resistance of Cr15 white iron under rapid cooling conditions improved greatly; for Cr30 cast iron, although both of the matrix structure obtained at slow and rapid cooling velocity were austenite, the amount of austenite obtained at rapid cooling velocity increased, it soluted more carbon,chromium atoms and had better uniformity, which decreased the electrode potential difference between austenite matrix and carbides, and thus, the corrosion-resistance of Cr30 cast iron under rapid cooling conditions was significantly improved.For a 15%Cr, 3.2%C white cast iron, the type of both primary and eutectic carbides was (Fe,Cr)₉C₃, however, the proportion between Fe and Cr atoms was varying. For primary carbides, because of its higher forming temperature and full diffusion of carbon and chromium atoms, its chemical formula was Fe₂Cr₅C₃; but for eutectic carbides, the chemical formula of its center was also Fe₂Cr₅C₃ ,while that of its periphery was Fe_2.4Cr_4.6C₃ due to the decrease of temperature and slowing up the diffusion of carbon,chromium atoms, especially chromium atoms.