The knowledge for a design engineer about the properties of different materials has great significance. If the engineer designs any machine element, it should have sufficient strength to withstand stresses produced at the time of operation.
Classification of engineering materials
Engineering materials are classified in two main categories which are following
- Metals and their alloys
First category is further classified into two sub categories which are following
- Ferrous metals
- Non-ferrous metals
On the otherhand, non-ferrous metals are those metals in which iron is not their main element of composition, like aluminium, brass, copper, tin, zinc etc.
Ferrous metals are defined to be those metals which have iron as their main element, the best example of ferrous metals are cast iron, steel and wrought iron. Principal raw material for the manufacturing of ferrous metals is pig iron and we obtain pig iron from smelting iron ore with limestone and coke in blast furnace.
Some principal ores of iron are shown in table
Cast iron is manufacture when we re-melt pig iron with coke and limestone in furnace known as cupola. If we talk about the main components of cast iron, these are carbon and iron in which 1.7 to 4.5 percent is carbon. Along with that it also has some amount of silicon, sulphur, phosphorous and manganese. There are two forms of carbon in cast iron which are free graphite and the other one is combined carbon respectively.
Since cast iron is a brittle material but it has some properties which make its engineering use valuable are its good casting, high compressive strength, wear resistance and excellent machinability. The ultimate tensile, compressive and shear strength of cast iron is 100 to 200 MPa, 400 to 1000 MPa and 120 MPa respectively.
Types of cast iron
Types of cast iron are following
- Grey cast iron
- White cast iron
- Chilled cast iron
- Mottled cast iron
- Malleable cast iron
- Nodular cast iron
Grey cast iron
This type of cast iron is an ordinary cast iron and its grey color is due to the presence of carbon as free graphite. The composition of grey cast iron is as following
Carbon = 3 to 3.5%; Silicon = 1 to 2.75%; Manganese = 0.40 to 1%; Phosphorous = 0.15 to 1%; Sulphur = 0.02 to 0.15% and remaining percentage is of iron.
Grey cast iron has low tensile strength but has very high compressive strength. Since it is not a ductile material therefore it can be easily machined. Due to the presence of carbon as free graphite, it acts as a lubricant. This property makes its use in those areas where we have sliding action.
We commonly use grey cast iron in pipes and pipe fitting, cylinder blocks, housings, agricultural implements.
A table which has different grades of grey cast iron is shown in which FG designation is given to grey cast iron according to Indian standard specifications.
White cast iron
The composition of white cast iron is as following
Carbon = 1.75 to 2.3%; Silicon = 0.85 to 1.2%; Manganese = less than 0.4%; Phosphorus = less than 0.2%; Sulphur = less than 0.12% and remaining composition is of iron.
Since the reason behind white of color of this type of cast iron is due to the presence of carbon as cementite and is the hardest component of cast iron.
This type has high tensile strength but low compressive strength. Since this type is hard so it cannot be machined by simple cutting tools. For this reason, we do cutting operation on this material by using shaping process.
We can produce cast iron by metal chills. Chills are used where we need to use a material having high wear resistive property.
Chilled cast iron
In terms of composition chilled cast iron is similar to white cast iron but is different in the method of manufacturing. Chilled cast iron is prepared by abrupt cooling of molten iron and the process of abrupt cooling is called chilling. During casting of all materials, we chilled their outer surface with cool sand. In most cases, chilling process is carried out in less than 1 mm depth of the material. But sometimes, we do chilling process according to design condition and sometimes it takes place accidentally.
During chilling process, when outer surface of molten iron comes in contact of chill, it extracts heat through conduction process abruptly. We do chilling process only to make our material surface harder so that our material can withstand wear and friction.
Mottled cast iron
This type of cast iron is actually an intermediate stage of grey cast iron and white cast iron having intermediate properties. We use this type where we need to chill wear surfaces.
Malleable cast iron
Malleable cast iron is actually a cast iron-carbon alloy in which carbon is present as cementite. This type is ductile in nature which means that it can bent into different shape without any breaking. This material has more tensile strength as compare to white cast iron.
In common life, we can use this material where we cannot use forge steel due to cost. There are two methods for the manufacturing of malleable cast iron which are following
- Whiteheart process
- Blackheart process
In whiteheart process, we pack white iron casting into the boxes of steel or iron which are surrounded by haematite ore. After that we slowly heats upto 900 to 950°C for several days. In this time duration, some amount of carbon contents oxidize and leave white cast iron and remaining disperse into the structure.
After heating for several days, we cool our material for several days and obtain a tough material which will not break during heat treatment.
On the otherhand in blackheart process, we use casting having less carbon and sulphur contents. We pack this casting in neutral substance like sand. The purpose of placing it in neutral substance is to reduce the contents of sulphur so that we can accelerate our process. This casting is heated upto 850 to 900°C for 3 to 4 days. The material we obtain from this process will be more malleable.
Nodular graphite cast iron
This type of cast iron is also known as spheroidal graphite cast iron, high strength cast iron or ductile cast iron. This type is prepared when we add 0.1 to 0.8% magnesium into molten grey cast iron. The reason behind is addition is to change the mixing behavior of graphite into small nodules.
After that when we obtain this type, it has high fluidity, tensile strength, wear resistance, castability, pressure tightness, weldability and machinability. This type of cast iron is defined by Indian standards as SG 400/15 in which 400 is the tensile strength of the cast iron and 15 is the percent elongation.
Alloy cast iron
The above types of cast iron have very small percentage of other elements but if we add more quantity of other elements in it, we will a big positive change in mechanical properties. So these elements are called alloy cast iron which has better features than simple cast iron types. Due to this behavior, we extensive applications of alloy cast iron.
Effect of impurities of cast iron
Impurities are added in cast iron in very small quantity but create large influence on mechanical properties of the cast iron. The impurities are following
Silicon is present in cast iron upto 4%. This element increases the formation of free graphite. In the result of this formation, cast iron becomes soft and can be easily machined. By adding this element, we can make sound castings free from blow holes.
This element is present in cast iron less than 0.1%, increases the hardness and brittleness property of cast iron.
This element is present in cast iron less than 0.75%, increase hardness and give cast iron white color. This element helps cast iron to control the harmful effects of sulphur.
This element rarely exceeds to 1%, increases fusibility and fluidity of cast iron. We use phosphorus cast iron where we need light engineering casting applications and for casting of intricate design.
This form of iron is the purest form of iron in which iron is present from 99.5% to 99.9%. The composition of wrought iron is as following
Carbon = 0.020%; Silicon = 0.120%; Sulphur = 0.018%; Phosphorus = 0.020%; Slag = 0.070% and remaining percent is for iron.
We prepare wrought iron molten pig iron when we remelt in puddling reverberatory type furnace. From this furnace, we obtain wrought iron in the form of balls and each balls having 45 to 65 kg weight. After that we use these balls to squeeze out slag so that it can get any commercial shape.
If we talk about the properties of wrought iron, it is tough, malleable and ductile but cannot bear sudden or shocks which impure cast iron types can do. Its ultimate tensile and compressive strength is low as compare to cast iron and the values are 250 MPa to 500 MPa and 300 MPa respectively.
We use it as chains, railway couplings, water and steam pipes and chains respectively because its tensile strength is high.
Non-ferrous metals are those types of metals in which iron is not present as their main element in the composition. There are several reasons behind using non-ferrous metals like these materials are easy to fabricate, good resistant to corrosion, good electrical and thermal conductivity and its weight factor. There are many non-ferrous metals in which aluminium is one of them
Aluminium is very light metal and is white in color. It is prepared from its oxide alumina. Tensile strength of aluminium ranges from 90 MPa to 150 MPa. Its specific gravity is 2.7 and melting point 658°C.
Naturally aluminium is very light and soft but to make to useful for some particular applications like die casting, forging etc, we make its alloy so that it becomes hard and rigid. Aluminium has good electrical conduction and this property makes it useful in overhead cables. Similarly its light weight property makes it useful in the manufacturing of aircrafts and automobiles.
The alloys of aluminium are following
The composition of this alloy is 3.5 to 4.5% copper, 0.4 to 0.7% copper, 0.4 to 0.7% magnesium and rest of the percent composition is of aluminium
This alloy is called wrought alloy and have upto 400 MPa maximum tensile strength after age hardening and heat treatment.
We widely use this alloy in the manufacturing of aircraft and automobile components, stamping, sheets, tubes, rivets and forging. This alloy can do hot working at temperature 500°C only if it is prepared from forging and annealing process. Similarly this alloy can also do cold working.
The composition of this alloy is 3.5 to 4.5% copper, 1.2 to 1.7% manganese, 1.8 to 2.3% nickel,0.6% silicon, 0.6% iron, 0.6% iron and remaining percent composition is of aluminium. This type of aluminium alloy is also called as copper-aluminium alloy.
Mostly we use this alloy for casting purposes but we can also use for forging purposes. This alloy has more strength as compare to duralumin at high temperature. Due to this reason, we prefer to manufacture cylinder heads and pistons of aircraft engines from this alloy.
The composition of this alloy is 2 to 10% magnesium, 1.75% copper and remaining percent composition is of aluminium. This alloy has better mechanical properties and less weight so we mainly use this type of alloy for the manufacturing of aircraft and automobile components.