CAST IRON

CAST IRON:

          Cast irons contain more than 2% carbon, which is the theoretical limit for steels. However, in actual practice, carbon content of most cast irons is between 3 to 4 %. One characteristic of cast irons (except white cast iron) is that much of the carbon content is present in free form as graphite. It is this fact, which determines, largely, the properties of cast iron. Cast iron is generally produced in coke-fired cupola furnaces by melting a mixture of pig iron, scrap cast iron and a small percentage (usually not exceeding 5%) of small sized steel scrap. Melting point of cast iron is much larger that that of steel. Most of the castings produced in a cast iron foundry are of grey cast iron. These are cheap and widely used. There are many varieties of cast iron. These are listed below :

(i) Grey cast iron, 

(ii) White cast iron,

(iii) Malleable cast iron, 

(iv) Nodular cast iron, and

(v) Alloy cast iron.

(1) Grey cast iron:

          Grey cast iron is charecterised by presence of a large portions of its carbon in the form of graphite flakes. Although grey iron is often defined as steel containing graphite, its properties are far different from those of steel. As already mentioned, Grey cast iron is very widely used in the form of castings. In fact, it is so widely used that the term cast iron has come to mean grey cast iron. If a finger is rubbed on a freshly fractured surface of grey cast iron, the finger will get coated with grey color due to the graphite present in the cast iron. grey cast iron has good compressive strength, but is weak in tension. It is relatively soft but brittle. It is very easy to machine and the resulting surface finish is good. It is self lubricating due to the presence of graphite and has good vibration damping characteristics. Compares to steel, it resists corrosion. 

Due to these properties, it is used extensively for making machine beds, slides, gear-housing, steam engine cylinders, manhole covers, drain pipes etc., 

(2) White cast iron:

White cast iron has 2 to 2.5% carbon and most of it is in the form of cementite. If molten cast iron is cooled very quickly and its chemical composition lacks graphite-promoting elements like Si and Ni, then carbon remains in combined form as . However, white cast iron does not have much use as such. It is very hard and shows white colored fracture. Highly resistance to wear. Due to its poor fluidity it does not fill the mould freely.

Only crushing rolls are made of white cast iron. But it is used as raw material for production of malleable cast iron. Used for parts subjected to excessive wear.

(3) Malleable cast iron:

         Malleable cast iron is manufactured by a complex and prolonged heat treatment of white cast-iron castings. Grey cast iron is brittle and has no or very little elongation. Malleable cast iron castings loose some of grey iron's brittleness and become useful even for those applications where some ductility and toughness is required. (Note: 'molten iron' is a name given to cast iron whose structure shows part grey and white cast iron characteristics).

Malleable cast iron is of two types: 1. Black hearth and  2. White hearth. Malleable cast iron can be obtained by keeping temperature and time comparatively of high values.

It is used in differential and steering gear housing, brake pedals, tractor springs, hangers and washing machine parts.

(4) Nodular cast iron:

          This cast iron is also known under the name of spheroidal graphite cast iron. If a little bit of magnesium (0.5%) is added to molten cast iron, the graphite, which is normally present in grey iron in the form of graphite flakes, changes its shape to small balls/ spheres and remain distributed throughout the mass of cast iron. This change in the shape of graphite particles has a very big effect on the properties of resulting castings and their mechanical properties improve considerably. The strength increases, yield point improves and brittleness is reduced. Such castings can even replace some steel-components.

(5) Alloy cast iron:

          The properties of cast iron can be improved by addition of certain alloying elements like nickel, chromium, molybdenum and vanadium, etc. Among the alloying elements, nickel is predominating alloying constituent whose addition to the extent of 0.5 to 1.5% avoid the tendency of chilling and hard spots. Alloying cast irons have higher strength, heat-resistance and grater wear-resistance etc. Such enhanced properties increase the application and uses of cast irons. I.C. Engine cylinders, cylinder liners, piston rings etc, are made of alloy cast irons.

Tool steels

(ii)Tool steels:

          The requirements in a tool steel are that it should be capable of becoming very hard and further, that it should be able to retain its hardness at high temperatures commonly developed during cutting of steel and other materials. This property is called "red hardness". Further tool steel should not be brittle and should have good strength. High speed steel (HSS) is the name given to a most common tool steel. Its name implies that it can cut steel at high cutting speeds. At high cutting speed, the temperature rise is higher but high speed steel tools can retain their hardness up to 600-625. The property of red hardness comes from addition of tungsten. A typical composition of H.S.S is tungsten 18%, chromium 4%, vanadium 1%, carbon 0.75 - 1%, rest iron. Tungsten is a costly metal. It has been found that molybdenum can also impart "red hardness" to steel and actually half per cent of molybdenum can replace one percent of tungsten. Molybdenum is far cheaper than tungsten. H.S.S with tungsten are known as T-series and H.S.S with molybdenum are known as M- series steels. A very useful H.S.S has a composition of tungsten 6%, molybdenum 6%, chromium 4% and vanadium 2%, besides iron and carbon. Another version of H.S.S is called super high speed steel. It is meant for heavy duty tools and has about 10-12% cobalt, 20-22% tungsten, 4% chromium, 2% vanadium, 0.8% carbon, rest iron. These days, tools are made of tungsten carbide and other materials, besides H.S.S.

Special Alloy Steels

(iii) Special Alloy Steels:

(a) Manganese steels:

          All steels contain small amounts of manganese to mitigate the bad effects of sulphur. The true manganese alloy steels contain much larger amounts of Mn. More manganese reduces strength and ductility. Manganese steels show high percentage of elongation. Specific gravity = 7.92. Melting point = 1343°C. They have work hardening properties. Manganese steels can be forged but special care is necessary to avoid degrading the steel. After forging the steel should be heat-treated by raising it to temperature of 1010 and quenching in water. They are used for railway points and crossings, and with usage, they become more wear-resistant.

(b) Nickel steels:

         Nickel can be added in steels up to 50%. Nickel makes the steel highly resistant to corrosion, non-mgnetic, and having very low coefficient thermal expansion. Invar (Ni=36%) and super invar (Ni=31%) are the popular materials for least co-efficient of expansion and are used for measuring instruments, surveyor tapes and clock pendulums. Such steels are used for turbine blades, internal combustion engine valves etc.,

(c) Chromium steels:

         Chromium makes steel corrosion resistant, and increases its UTS. And IZOD strength. Very often alloy steels are used with both chromium and nickel being added. Ni-Cr steel wires are often used in furnaces, toasters and heaters.

(d) Silicon steels:

        A steel containing 0.05% carbon, about 0.3% Mn and 3.4% of silicon possesses extremely low magnetic hysteresis and is used widely for making laminations of electrical machines. Silico-manganese steels are also used frequently for making springs.