Alloy Cast Irons

Alloy Cast Irons Alloy cast irons are the casting alloys which are based on the iron (Fe) – carbon (C) – silicon (Si) system. They contain one or more alloying elements intentionally added to improve one or more properties. The addition to the ladle of small amounts of substances such as ferrosilicon (Fe-Si), cerium (Ce), or magnesium (Mg)) that are used to control the size, shape, and/or distribution of graphite particles is termed as inoculation. The quantities of material used for inoculation neither change the basic composition of the solidified cast iron nor alter the properties of individual constituents. Alloying elements, including Si when it exceeds about 3 %, are usually added to increase the strength, hardness, hardenability, or corrosion resistance of the basic iron and are often added in quantities sufficient to affect the occurrence, properties, or distribution of constituents in the microstructure. In gray and ductile cast irons, small amounts of alloying elements such as chromium (Cr), molybdenum (Mo), or nickel (Ni) are added primarily to achieve high strength or to ensure the attainment of a specified minimum strength in heavy sections. Otherwise, alloying elements are used almost exclusively to enhance resistance to abrasive wear or chemical corrosion or to extend service life at elevated temperatures. Classification of alloy cast irons Alloy cast irons can be classified as (i) white cast irons, (ii) corrosion resistant cast irons, and (iii) heat resistant cast irons (Fig 1). Fig 1 Classification of alloy cast irons White cast irons White cast irons are so named because of their characteristically white fracture surfaces. They do not have any graphite in their microstructures. Instead, the C is present in the form of carbides, mainly of the types Fe3C and Cr7C3. Frequently, complex carbides such as (Fe,Cr)3C and (Cr,Fe)7C3,...

White Cast Iron

White Cast Iron The term cast iron refers to those iron carbon silicon alloys which contain 1.8 % Р4 carbon (C) and usually 0.5 % Р3 % silicon (Si). Cast iron is an important engineering material with a number of advantages, mainly good castability and machinability and moderate mechanical properties. White cast iron contains 1.8 % -3.6 % C, 0.5 % -1.9 % Si and 1 % Р2 % manganese (Mn). White cast irons are so called because when broken, the fracture surface is white. This is unlike the grey fracture surface normally associated with other cast irons which contain graphite. White cast iron is a cast iron without any alloy addition and with low C and Si content such that the structure is hard brittle iron carbide (Fe?C, also called cementite) with no free graphite. A fast cooling rate prevents the precipitation of C as graphite. Instead the C, which is in solution in the melt, forms iron carbide. The structure of white cast iron consists of pearlite and ledeburite, a eutectic mixture of pearlite (converted from austenite) and cementite. Cementite is hard and brittle and dominates the microstructure of white cast iron. Thus, white cast iron is hard and brittle and has a white crystalline fracture because it is essentially free of graphite. Typical micro structure of white cast iron is shown in Fig 1. Fig 1 Typical micro structure of white cast iron White cast iron does not have the easy castability of other cast irons because its solidification temperature is generally higher, and it solidifies with C in its combined form as iron carbide. White cast iron has a high compressive strength and excellent wear resistance, and it retains its hardness for limited periods even up to a red...