The durability and strength of a farm implement depends upon the quality of the material used for construction. The success and failure of an implement is directly connected with the proper choice of the material.
The common materials used are:
1. Ferrous Material:
I. Cast iron
II. Steel
I. Cast Iron:
Cast iron (C.I.) is the iron containing 2.2 to 4.3% carbon.
The main advantages are:
(i) Its cheapness,
(ii) Low melting point temperature (1150-1200°C),
(iii) Its fluidity in molten condition,
(iv) It is easily machined and,
(v) Free graphite seems to act as a lubricant.
The fluidity of cast iron enables easy casting of intricate shapes. Its strength is much greater in compression than tension. It is brittle material.
Cast iron is of several types:
(a) Gray C.I.
(b) White C.I.
(c) Malleable C.I. and
(d) Ductile C.I.
(a) Gray Cast Iron:
It contains carbon in form of iron carbide. It is suited to the parts subjected to abrasion. It is cheap and common material for casting. It is strong in compression but relatively weak in tension. Gray C.I. lacks in ductility and in the ability to withstand impact or shock load. It is good in wear resistance.
(b) White Cast Iron:
When cast iron with low silicon content is cooled rapidly, all the carbon remains in the combined form and white cast iron is formed. It is somewhat white in colour due to absence of graphite flakes. It is hard, brittle but difficult in machining. It may be used for bearings of disc harrow gangs or wheel bearings for implements. It may be used for mould boards also.
(c) Malleable Cast Iron:
Malleable cast iron is obtained when white cast iron is properly annealed by heating to a temperature of about 1600°F for several days and then cooling slowly. It changes all its graphite in form of fine nodules. It is tough, easily machined and much stronger than gray cast iron.
(d) Ductile Cast Iron:
Ductile cast iron is formed when magnesium and ferrosilicon are added to the molten cast iron, such that it changes the graphite flakes to a spheroidal form. It has high ductility and can take up greater impact resistance than gray cast iron. It may be used for gears, sprockets and plough shares.
II. Steel:
Steel is malleable alloy of iron and carbon or of iron, carbon and some other elements. If the alloying material is only carbon, it is known as Carbon steel, otherwise Alloy steel.
Steel is divided into following types:
(i) Low Carbon Steel,
(ii) Medium Carbon Steel, and
(iii) High Carbon Steel.
(iv) Tool Steel
(v) Soft Centre Steel
(i) Low Carbon Steel:
Low carbon steel contains less than 0.25% carbon. It is also called Mild steel. It is used for nuts, bolts and other simple components of agricultural implements.
(ii) Medium Carbon Steel:
It contains 0.25% to 0.6% carbon. It is used for shafts, gears, axles etc.
(iii) High Carbon Steel:
High carbon steel contains 0.6 to 1.5% carbon. It is used for plough shares, blades, shovels, disc, coulters and cutting tools etc.
(iv) Tool Steel:
It is a special type of steel used for making cutting tools. It contains more than 0.7% carbon.
(v) Soft Centre Steel:
It consists of three layers of steel. The two outer layers are of hard steel and central one of the soft steel. All the three layers are combined together to form one unit. Mould board is usually made of this type of steel.
Several components used in farm equipment are made of steel, having different carbon contents.
Heat Treatment of Steel:
Heat treatment is the combination of operations of heating and cooling of a metal in solid state for obtaining certain desirable properties of the metal.
There are several methods of heat treatment such as:
a. Hardening
b. Annealing
c. Tempering and
d. Case hardening
a. Hardening:
The metal is heated to a high temperature about 2000°F and cooled suddenly by quenching in water or oil. This process increases the hardness, tensile strength and brittleness of the metal. It increases the strength and wear resistance of the metal but reduces its ductility.
b. Annealing:
The metal is heated to temperature of 1550° to 1750°F and allowed to remain at maximum temperature for one to several hours according to the size of the metal. It is then allowed to cool slowly. It refines the coarseness of the grains, removes strains, caused due to uneven cooling and increases the tensile strength. The ability to resist the shock load is also improved.
c. Tempering:
Tempering is done to reduce brittleness and to increase the toughness and ductility of the metal. The process consists of heating the hardened metal to a temperature lower than the hardening temperature and then cooling by quenching the metal in a liquid. In this process, tensile strength of the metal is reduced.
d. Case Hardening:
It is a process of introducing additional carbon into the outer shell of the steel pieces. This process increases the wear resistance, corrosion resistance and resistance to scaling in the surface layer of the metal.
Case hardening can be done by several processes such as:
(i) Carburising,
(ii) Nitriding,
(iii) Carbonitriding,
(iv) Cyaniding,
(v) Induction hardening, and
(vi) Flame hardening
(i) Carburising:
It is a process, in which steel is packed in charcoal and heated to about 1600°F for a longer period of time to give desired depth of surface carbon in the metal. It is then removed, quenched and tempered in usual manner.
(ii) Nitriding:
It is a process of case hardening by placing the steel in air tight box and heating to a temperature of about 1000°F as ammonia gas is injected into the chamber.
(iii) Carbonitriding:
It is a process of hardening steel by adding ammonia gas or other similar gases having high % of carbon.
(iv) Cyaniding:
It is a process where the steel is dipped into a molten bath of potassium cyanide for a short time.
(v) Induction Hardening:
Induction hardening is done by introducing high frequency alternating current for a short duration of time. After heating is over, the water is flooded on the surface of the metal.
(vi) Flame Hardening:
It is a process in which an oxyacetylene flame is used to heat the metal surface quickly to a temperature above the critical temperature, after which it is quenched in water which is cold enough to make it hardened.
2. Non-Ferrous Material:
i. Copper
ii. Aluminium
iii. Brass
iv. Bronze
v. Wood
vi. Plastic and
vii. Rubber
i. Copper:
Copper is soft and non-corrosive metal having high electrical conductivity. It is used for milk coolers, refrigerator coils, generators, motors, fuel tubes etc. Copper is soft enough to be rolled or hammered into thin sheets or drawn into fine wires.
ii. Aluminium:
It is very light and corrosion resistant metal. It is ductile and malleable. It is used for connecting rods, pistons, cylinder head, milk cans, cooling vats and aluminium casting. It is a white metal with bluish tinge. It has a specific gravity of 2.7 and a melting point of 678°C. It can be dissolved by alkali and hydrochloric acid.
iii. Brass:
It is an alloy of copper and zinc. It contains 60 to 75% copper and 30 to 40% zinc. It is used for fluid valves, piping, wires, welding rods, radiators and pipes.
iv. Bronze:
It is an alloy mainly of copper and tin. The most important metal under this class is gun metal which has an average composition of 88% copper, 10% tin, and 2% zinc. It is used for bearings, bushings, piston, springs and valves.
v. Wood:
Wood is very common material used for farm implements. Wood is composed of about 60% cellulose, 28% lignin and other minor elements. Wood is used for beams, bullock carts, cart wheel, tool handles, sugarcane crusher beam, country plough, hoe handle, pitman of mower, cattle yokes, levelers, clod crusher, crowbar and wooden spades.
The common woods available for use are:
(a) Babul,
(b) Jambul,
(c) Khair,
(d) Mango,
(e) Shisham,
(f) Teak,
(g) Tamarind, and
(h) Sagada
vi. Plastics:
Plastic is an organic material, capable of being moulded by heat and pressure. It is used for bearing, washers, bushings, handles, steering, castings, tubings and many other items.
vii. Rubber:
Rubber may be natural or artificial. Natural rubber is obtained from the latex (milky fluid of plant) of special type of trees. It is also made by synthetic process. Rubber is used for tires, tubes, belts, insulation of wires, rubber bushing etc.