Abstract
Because the size of the eutectic colony, and the shape and distribution of the eutectic carbide closely influence the properties of high chromium cast iron, it is necessary to express the eutectic structure quantitatively. Hypo·eutectic, eutectic and hyper-eutectic high chromium cast irons containing about 15 to 40%Cr were cast into 30φ×70mm exothermic molds on a watercooled copper chill plate, and solidified unidirectionally. The morphology of the eutectic structure could be expressed quantitatively by both the colony size and the carbide spacing, even when it showed some defferences in appearance according to the chemical compositions. The eutectic colony size (EW) was given by the following equation as a function of the eutectic growth rate (RE). EE=A·RE-0.72 (A>0, EW; μ. RE; mm/min) A, a constant depending on the chemical composition of the specimen, was smallest at 30%Cr, therefore the value EE became smallest. The value of hypo-eutectic irons was much smaller than those of eutectic and hypereutectic irons when it was compared at around the same chromium level. The coarseness ofeutectic structure was determined by both the carbide spacing at the colony center (F??) and at thecolony boundary (F??). Each value, F?? and F?? increased in proportion to RE-1/2. The value F?? was little affected by the chemical composition of the specimen. On the other hand, the value F?? was affected greatly by the Cr and C content, and it was smallest at 30%Cr at the same eutectic ratio. Consequently the cutectic carbides existing in the eutectic colonies of 30%Cr irons were found to be most finely dispersed.
