Technical Paper Title: A Study paper on “CAST IRONS”
Authors: K.ANIL KUMAR, 2nd Year B.Tech, Mechanical Engineering
College: CMR College of Engineering and Technology
The term cast iron refers to those iron-carbon-silicon alloys which contain 2.5% – 4% carbon and usually 1-3% silicon. Cast iron is an important engineering material with a number of advantages, mainly good castability and machinability and moderate mechanical properties. This Application Note describes the production and the materialographic preparation of this group of materials.
If %c is greater than 2% then the alloys are called cast irons. Cast irons are cheaper than steels and find many applications some of which are quite unique to cast irons. It contain a greater amount of carbon than that necessary to saturate austenite at the eutectic temperature .cast irons contain between 2-6.67%c. High carbon content tends to make the cast iron very brittle, most commercially manufactured types are in the range of 2.5-4%c.
The ductility of cast iron is very low and it cannot be rolled, drawn or worked at room temperature Most of the cast irons are not malleable at any temperature .However, they melt readily and can be cast into complicated shapes which are usually machined to final dimensions. Casting is the only suitable process applied to alloys ,they are known as cast irons .The common cast irons are brittle and have lower strength properties than most steels, they are cheap can be cast more readily than steel.
By proper alloying , good foundry control and appropriate heat treatment the properties of any type of cast iron may be varied over a wide range.
TYPES OF CAST IRONS
The best method of classifying cast iron is according to metallographic structure .there are four variables to be considered which lead to the different types of cast iron.
a) carbon content
b) The alloy and impurity content
c) The cooling rate during and after freezing
d) The heat treatment after casting
The carbon may be combined as iron carbide in cementite ,or it may exist as free carbon in graphite. The different types of cast irons are as follows
1. White cast irons
2. Gray cast irons
3. Malleable cast irons
4. Chilled cast irons
5. Nodular cast irons
6. Alloy cast irons
WHITE CAST IRONS
White cast irons in which all the carbon is in the combined form as cementite . All white cast irons are hypo eutectic alloys and the cooling of a 2.50%c alloy. The typical microstructure consisting of dendrites of transformed austenite (pearlite) in a white inter dendritic network of cementite . Higher magnification of the same sample reveals that the dark areas as pearlite .As white cast iron contains a relatively large amount of cementite .The cast iron is hard and wear resistant but extremely brittle and difficult to machine .
They are limited in engineering applications because of this brittleness and lack of machinability . They are mainly used for liners for cement mixtures , ballmills, drawing dies and extrusion nozzles.
MALLEABLE CAST IRONS
It consists of graphite in the form of more or less spherical particles or nodules in a ferritic or pearlitic matrix. This is obtained by heat treating a white cast iron structures which is called malleabilising treatment .It involves a two stage heating of the white cast iron and casting or wshite iron piece is heated to near eutectoid temperature and is soaked for a long time. Through an annealing heat treatment ,the brittle white cast irons structure is transformed into the malleable form. Three basic types of malleable iron are recognized within the casting industry
a) Black Heart malleable cast irons
b) white Heart malleable cast irons
c) pearlitic malleable cast irons
Malleable iron also exhibits better fracture ,toughness properties in low temperature environments than the other nodular irons ,due to its lower silicon content . The ductile to brittle transformation temperature is lower than many other ductile iron alloys.
Used for small castings ,requiring good tensile strength and ability to flex without breaking. Examples are hand tools, pipe fittings, washers, brackets, machine parts etc.
CHILLED CAST IRONS
It is difficult to cool thick castings fast enough to solidify the melt as white cast iron all the way through. However ,rapid cooling can be used to solidfy a shell of white cast iron, after which the remainder cools more to form a core of gray cast iron. The resulting casting called chilled cast iron has the benefits of a hard surface(white cast iron) and some what tougher interior consisting of graphitized cast iron.
NODULAR CAST IRONS
Nodular cast iron also known as ductile iron ,spheroidal graphite iron, and spherulitic iron is cast iron in which graphite is present as tiny balls or spheroids.The total carbon content of nodular iron is same as in gray cast iron. Nodular irons with a matrix having a maximum of 10% pearlite are known as Ferritic irons.
ALLOY CAST IRONS
An alloy cast iron is one which contains a specially added element or elements in sufficient amount to produce a measurable modification in the physical or mechanical properties .Alloying elements are added to cast iron for special properties such as resistance to corrosion, heat or wear and to improve mechanical properties.
GRAY CAST IRONS
Gray cast irons in which most or all of the carbon is uncombined in the form of graphite flakes. This group is one of the most widely used alloys of iron. In the manufacture of gray cast irons the tendency to separate into graphite and austenite or ferrite is favoured by controlling alloy composition and cooling rates. Most Gray cast irons are hypoeutectic alloys containing between 2.5 to 4% carbon. High carbon content increases the amount of graphite.Gray iron also has very good damping capacity and hence it is mostly used as the base for machine tool mountings. Rapid cooling partly or completely suppresss graphitization and leads to formation of cementite which is called white iron. Graphite-ferrite mixture is the softest and weakest gray iron,the strength and hardness increase with the increase in combined carbon,reaching a maximum with the pearlitic Gray iron.
So many cast irons are etched by using a etchant to reveal their structure .Due to presence of high carbon it is very brittle.It can be minimised by having low carbon. It is manufactured by only one process as it is a casting. The ductility is also low.White cast iron is hard and wear resistant but extremely brittle and difficult to machine.By adding alloying elements to cast irons resistance,corrosion,heat or wear can be improved. Gray cast irons are mostly present in graphite flakes having ferrite.
1.INTRODUCTION TO PHYSICAL METALLURGY BY SIDNEY H AVENER (423-458 PAGES).
2. Struers Company Brouchers.
3. CASTING TECHNOLOGY AND CAST ALLOYS BY A. K. CHAKRABARTI