Cast irons and their Classification...

Cast irons and their Classification  The term ‘cast iron’ represents a large family of ferrous alloys. Cast irons are multi-component ferrous alloys, which solidify with a eutectic. The major elements of cast irons are iron, carbon (2 % or more), silicon (1 % to 3 %), minor elements (less than 0.1 %), and often alloying elements (less than 0.1%). Cast iron has higher carbon and silicon contents than steel. The structure of cast iron displays a richer carbon phase than that of steel because of its higher carbon content. Cast iron can solidify according to the thermodynamically metastable Fe-Fe3C (iron carbide) system or the stable iron-graphite system depending principally on composition, cooling rate, and melt treatment. Cast iron in its basic form is a brittle material which has a very little impact strength. It has a little or practically no toughness when compared to low carbon steels.  It has a fraction of the tensile strength of low carbon steels.  When a cast iron piece fails it does not deform in a noticeable way and appears to snap apart or break in a manner consistent with a snap.  There is no early warning of a failure. The graphite phase which is pure carbon acts as a natural defect in the material.  The iron is so saturated with carbon that graphite forms (free carbon) and causes the cast iron to be weaker.  Much smaller amounts of carbon is combined with iron (Fe) in the form of iron carbide (Fe3C, cementite) which is hard and brittle. During the solidification process, when the metastable route is followed, the rich carbon phase in the eutectic is the iron carbide and when the stable solidification route is followed, the rich carbon phase is graphite. Referring only to the binary Fe-Fe3C or...

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...

Ductile Cast Iron

Ductile Cast Iron  Ductile cast iron also known as nodular cast iron, spheroidal graphite iron or SG iron, and spherulitic cast iron. The ductile iron process was developed by The International Nickel Company in 1948. As the name ductile iron suggests this grade of cast iron has a degree of ductility. The main characteristic of this material is the structure of the graphite. Ductile iron is a family of cast graphitic irons which possess high strength, ductility and resistance to shock. Annealed cast ductile iron can be bent, twisted or deformed without fracturing. Its strength, toughness and ductility duplicate many grades of steel and far exceed those of standard gray irons. Yet it possesses the advantages of design flexibility and low cost casting procedures similar to gray iron. The difference between ductile iron and gray iron is in the graphite formation. Ordinary gray iron is characterized by a random flake graphite pattern in the metal. In ductile iron the addition of a few hundredths of 1 % of magnesium or cerium causes the graphite to form in small spheroids rather than flakes. These create fewer discontinuities in the structure of the metal and produce a stronger, more ductile iron. This nodular graphite structure inhibits the creation of linear cracks hence the ability to withstand distortion. Fig 1 shows typical micro structure of ductile iron. Fig 1 Typical micro structure of ductile iron  With ductile iron, the safety and reliability of process equipment is improved. The improved mechanical properties increase its resistance to breakage from physical load, or mechanical and thermal shock far above that of gray iron. The corrosion resistance of ductile iron is equal or superior to gray cast iron and to cast steel in many corrosives. Its wear resistance is comparable to...

White Cast Iron

White Cast Iron The term cast iron refers to those iron carbon silicon alloys which contain 1.8 % – 4 carbon (C) and usually 0.5 % – 3 % 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 % – 2 % 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...

Gray Iron

Gray Iron Gray iron (also called grey iron) is a type of cast iron that has a graphitic microstructure. It is named after the grey color of the fracture it forms, which is due to the presence of graphite. It is the most common cast iron and the most widely used cast material. Gray iron is one of the oldest cast ferrous products. In spite of competition from newer materials and their energetic promotion, gray iron is still used for those applications where its properties have proved it to be the most suitable material available. MacKenziein his 1944 Howe memorial lecture referred to gray cast iron as ‘steel plus graphite’. Although this simple definition still applies, the properties of gray iron are affected by the amount of graphite present as well as the shape, size, and distribution of the graphite flakes. Composition and effect of composition on properties  Gray iron is commercially produced over a wide range of compositions. The range of compositions which one may find in gray iron castings is given below. Carbon (C) – 2.75 % to 4.00 % Silicon (Si) – 0.75 % to 3.00 % Manganese (Mn) – 0.25 % to 1.50 % Sulfur (S) – 0.02 % to 0.20 % Phosphorus (P) – 0.02 % to 0.75 % One or more of the alloying elements namely molybdenum, copper, nickel, vanadium, titanium, tin, antimony, and chromium may be present in varying amounts. Nitrogen is generally present in the range of 20 to 92 ppm. Si is important for the gray iron since it is a graphite stabilizing element in cast iron, which means it helps the iron to produce graphite instead of iron carbides. Another factor affecting graphitization is the solidification rate. The slower is the rate, the greater is the tendency for graphite...