Effect of phosphide eutectic on the properties of a cast iron alloy
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The most technologically advanced material for valve seats of internal-combustion engines are the phosphorus-alloyed cast irons strengthened by a reinforcing phase consisting of the phosphide eutectic and cementite, the ratio between which depends on the Cr to P ratio, the technology of the modification, and the cooling rate of the castings.
The optimum structure of the cast iron for valve seats, which produces a combination of good machinability and high wear resistance is the troostomartensitic or troostosorbitic matrix with a network of the FÉ5-type phosphide eutectic (with cementite lamellae), and clusters of graphite combined with fine- and medium-size flakes with less than 5% of the structurally free graphite. The optimum combination of wear resistance, machinability, and strength of the valve seats is attained at the ratio Cr/P=3–4 and modification by 0.5–0.8% ferrosilicon. The wear of the phosphorous-containing cast iron with a troostomartensitic or troostosorbitic matrix is determined by the amount of the cementite in the ledeburite and, when its content is small or it is absent, by the type of the phosphide eutectic. A phosphorus-containing cast iron of the type of 360S3KhGMD has been suggested which has gone into production. It has the composition: 3.5–3.8% C, 2.5–3.0% Si, 0.7–1.0% Mn, 0.9–1.2% Cr, 0.9–1.2% Mo, 0.3–0.6% P, 0.7–1.2% Cu, and up to 0.1% S.
KeywordsIron Graphite Wear Resistance Cementite Cast Iron
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