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,...

Carbon Steels and the Iron-Carbon Phase Diagram...

Carbon Steels and the Iron-Carbon Phase Diagram Steels are alloys having elements of iron (Fe) and carbon (C). C gets dissolved in Fe during the production of steels. Pure Fe melts at a temperature of 1540 deg C, and at this temperature, C readily dissolves into the liquid iron, generating a liquid solution. When this liquid solution solidifies, it generates a solid solution, in which the C atoms are dissolved into the solid iron. The individual C atoms lie in the holes between the Fe atoms of the crystalline grains of austenite (at high temperatures) or ferrite (at low temperatures). Austenite has a face centred cubic (fcc) structure while the ferrite has a body centred cubic (bcc) structure (Fig 1). If the amount of C dissolved in the liquid iron is kept below 2.1 %, the product is steel, but if it is above this value, then the product is cast iron. Although liquid iron can dissolve C at levels well above 2.1 % C, solid iron cannot. This leads to a different solid structure for cast irons (iron with total C greater 2.1 %). In addition to C, all the types of steels contain the element manganese (Mn) and low levels of the impurity atoms of phosphorus (P) and sulphur (S). Hence, steels can be considered as alloys of three or more elements. These elements are Fe, C, other element/elements additions, and impurities. It is normal to classify steel compositions into two categories namely (i) plain C steels, and (ii) alloy steels. In plain C steels, other elements consist only of Mn, P, and S, whereas in alloy steels, one or more additional alloying elements are added. Solid solutions are similar to the liquid solution; that is, after the solid substance is dissolved,...

Malleable Cast Iron

Malleable Cast Iron  Malleable cast iron is essentially white cast iron which has been modified by heat treatment. It is formed when white cast iron is heated to around 920 deg C and then left to cool very slowly. Graphite separates out much more slowly in this case, so that surface tension has time to form it into spheroidal particles rather than flakes. Due to their lower aspect ratio, spheroids are relatively short and far from one another, and have a lower cross section vis-a-vis a propagating crack. They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems faced by the gray cast iron. In general, the properties of malleable cast iron are more like mild steel. There is a limit to how large a part can be cast in malleable cast iron, since it is made from white cast iron. The white cast iron is converted to malleable cast iron by a two stage heat treatment process to a condition having most of its carbon content in the form of irregularly shaped nodules of graphite, called temper carbon. The structure of malleable cast iron consists of ferrite, pearlite and tempered carbon as compared to the fracture inducing lamellar structure of gray cast iron. Malleable cast irons are a class of cast irons with mechanical strength properties that are intermediate to those of gray or ductile cast irons. The microstructure provides it properties that make malleable cast irons ideal for applications where toughness and machinability are required, and for components that are required to have some ductility or be malleable so that they can be bent or flexed into position without cracking. Malleable cast iron besides less sensitive to cracking has a range of features, such as higher values of...

Microstructures of Iron and Steels...

Microstructures of Iron and Steels The microstructures of iron and steels is complicated and diverse which is influenced by composition, homogeneity, heat treatment, processing and section size. Microstructure of castings looks different than those of the wrought products even if the composition is same and even if the same heat treatment is given. Pure iron is polymorphic. Two allotropic phases exist for pure iron in solid state depending on the temperature. One is bcc (body centered cubic) and the other is fcc (face centered cubic). The bcc crystalline form (?-iron) is stable until a temperature of 912 deg C when it is transformed to fcc (?-iron). The ?-iron remains stable until 1394 deg C, and then it reverts to bcc structure (?-iron). ?-iron is stable until the melting point of 1538 deg C. High purity iron is very weak. The ability of iron to accommodate heavy interstitials, namely carbon and nitrogen, is mostly responsible for the strength and the hardening effects. Ferrite ?- iron refers only to the bcc form of pure iron that is stable below 912 deg C. Ferrite is a solid solution of one or more elements in bcc iron. The carbon solubility of ferrite depends on the temperature: the maximum being 0.02 % at 723 deg C. Ferrite may precipitate from austenite in acicular form with certain cooling conditions. Ferrite is a very soft, ductile phase, although it loses its toughness below some critical temperature. ?-ferrite is magnetic below 768 deg C. Austenite ? – iron refers to fcc form of pure iron that is stable between 912 deg C and 1394 deg C. Austenite is a solid solution of one or more elements in fcc iron. Austenite is stable above 723 deg C depending upon C content. It can...

Wear Resistant Structural Steels...

Wear Resistant Structural Steels Wear is described as ‘the phenomenon of metal surfaces that are moving relative to each other getting worn out due to the surfaces scratching each other or due to metallic adhesion’. Wear resistance can be said to be the property in which such a phenomenon is difficult to occur. The properties of wear resistant steels enable them to resist wear, due to rubbing, impact or compressive loads from external agents such as cement, sand, stones etc., and are intended for use in equipment construction and for replacement of wearing parts. Numerous structures, such as dump bodies, materials handling equipment and crushing machines, for instance, are exposed to continuous, abrasive and impact wear, which is costly. As a solution, special structural steels have been developed that are highly resistant to wear and abrasion. Factors affecting wear resistance of steels There are four main factors which have considerable effect on the wear resistance of steels. These are (i) heat treatment, (ii) alloying additions, (iii) influence of carbon content, and (iv) effects of carbides, both primary and secondary. A big factor affecting wear resistance is ‘hardness’. In general, the wear resistance increases as the material becomes harder. There is a direct relationship between hardness and wear resistance. The resistance of a steel surface against wear is primarily a function of the ‘effective hardness’ resulting from the destructive action of the abrasive particles and depend on the strain hardening rate of the steel under the applied conditions. Factors affecting plastic deformation, such as grain size, recrystallization temperature, hardness, strain rate etc. also affect the wear of steels. Unlike single crystals which have free boundaries, the grains of a polycrystalline steel are influenced by their neighours during deformation, their constraining action on deformation is least...