Hypoeutectic / eutectic solidification structure of cast chromium alloys

According to the different carbon content, the solidification structure of chromium alloys can be divided into hypoeutectic, eutectic and hypereutectic. The solidification of the alloy begins with the formation of primary phase, which may be austenite in hypoeutectic alloy or primary carbide in hypereutectic alloy, and then the two-phase eutectic mixture precipitates simultaneously. The solidification structure is usually hard, and different types of primary and eutectic carbides are embedded in the soft iron matrix (i.e. austenite, martensite, ferrite pearlite or bainite). The matrix structure is usually the product of austenite transformation and can be changed after heat treatment. Generally speaking, carbide in solidified structure is the main hard wear-resistant phase, while metal matrix is to provide enough toughness. The wear resistance depends on the amount, size and distribution of carbides, as well as the hardness, fracture toughness and the matching between them. Due to the high thermodynamic stability of carbides, the thermal hardness and corrosion resistance of materials mainly depend on the metal matrix.

(1) Solidification structure of Hypoeutectic chromium alloy

The alloy with hypoeutectic solidification structure is one of the most commonly used materials in chromium alloys, and its solidification structure is mainly composed of primary austenite and eutectic structure. The commonly used hypoeutectic white cast iron (C content between 2.0 ~ 4.3 wt.%) precipitates primary austenite when the liquid metal is cooled to the liquidus, and the temperature continues to decrease. The amount of austenite precipitated between the liquidus increases and the liquid phase decreases. With the decrease of temperature, the content of C in austenite increases gradually. Because the average content of C in this kind of cast iron is more than 2.0 wt.%, and the austenite can not hold so much C, the extra C exists in the remaining liquid phase, resulting in the increasing content of C in the decreasing remaining liquid. Until the eutectic temperature is 1130 ℃, the C content of all austenite precipitated directly from the liquid phase is 2.0 wt.%, and the carbon content of the remaining liquid phase is just 4.3 wt.%, so the liquid phase transforms into ledeburite. Therefore, the structure of solidified cast iron is composed of austenite and ledeburite.

(2) Solidification structure of eutectic chromium alloy

The results show that the eutectic clusters of eutectic high chromium white cast iron are formed randomly, because under the condition of equilibrium solidification, there is no prior austenite dendrite as the base of eutectic clusters. Therefore, the structure and morphology of eutectic clusters are similar to those of hypoeutectic high chromium white cast iron.

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