In addition, it is important to consider both the duty and the shape of the component in order to select a material with the required processing characteristics, whether it be metallic or non-metallic. Property requirements are equally important in the choice of engineering materials. Any material chosen must have the most suitable properties.
As in the other requirements, a compromise has to be made. Property requirements include mechanical properties such as strength and hardness, and physical properties such as melting point and density. Load Load 1. Load Load Instead of the word strength, the word stress is usually used.
If a material shatters, it is brittle eg glass. Rubbers and most plastic materials do not shatter, therefore they are tough. It is the state of a material which has been loaded beyond its elastic limit, so as to course the material to deform permanently. Under such conditions, the material takes a permanent set. Ductility materials can be used in such processions as wire drawing, tube drawing, and cold pressing low carbon steel sheers into motor car body panels. Materials that are malleable can be used in such processes as forging, rolling, rivet heading.
It is an indication of the wear resistance of the material. Aluminium is used in constructing airplanes because of its low density. Only puree substances have a single melting temperature. Alloys of metals have a melting range of temperature. Examples: Tungsten has melting temperature of oC Tin has a temperature of C Tungsten is used for making the filament of light bulbs due to its high melting temperature.
Tin is used for making soft solders due to its low melting temperature. Bad conductors, also known as insulators, allow very few electrons to pass through them, whereas good conductors offer very little resistance to the flow of electrons. In between conductors and insulators lie a group of materials known as semi- conductors. These can be good or bad conductors depending upon their temperature. The conductivity of semi-conductors increases very rapidly for relatively small temperature increases.
This enable them to be used as temperature sensors in electronic thermometers. Permittivity is exploited in devices known as capacitors. Permittivity is dimensionless, since it is a ratio.
Good magnetic conductors have a low reluctance, and examples are the ferromagnetic materials. All other materials are non-magnetic and offer a high reluctance to the magnetic flux field. Permeability This is the ease with which material can be magnetised. It is a constant for any given material, and has no units. Hysteresis This is the lagging effect of the magnetic field in relation to the current in a material subjected to alternating fields, such as when AC current flows in the transformer.
Hysteresis causes energy loss which ends up as heat in the core of the transformer. Classification Magnetic materials can be classified as either hard or soft. Hard magnetic materials are those which retain their magnetism after the initial magnetising force has been removed. These materials are hard and cannot be machined. Examples of such materials are alnico, alcomax and columax. An example of a good conductor of heat is copper, from which the bits of soldering irons are made.
Expansivity This is the expansion of a material due to an increase in temperature. The linear expansivity of a material is the measure of the amount by which a unit length of the material expands when its temperature is raised by 10C.
Different substances require different amounts of heat energy to produce the same rise in temperature. For example water requires more heat energy to raise its temperature by any given amounts than any equal mass of any other liquid. This is why water is such a good coolant. Solder melts easily, and has the property of high fusibility. On the other hand, fire bricks, used for furnace linings only melt at very high temperatures, and so have the properties of low fusibility.
Heat treatment This is the controlled heating and cooling of metals to change the properties, to improve their performance or to facilitate processing. An example of heat treatment is the hardening of a piece of high carbon steel rod. It is heated to a dull red heat and plunged into cooled water to cool it rapidly quenching , it will become hard and brittle.
If it is again heated to dull read heat but allowed to cool very slowly. It will become softer and less brittle more tough. In this condition it is said to be annealed.
In this condition it will be too soft and the grain will be too course for it to machine to a good surface finish, but it will be in its best condition for flow forming.
During flow forming waxing the grains will be distributed and this will result in most metals becoming more hardened if flow-formed at room temperature.
To remove any locked in stresses resulting from the operations and to prepare the material for machining the material has to be normalized. This consists of again healing the metal to dull red but cooling less slowly than for annealing. The hot metal is removed from the furnace and allowed to cool in still air away from draughts. This results in a finer grain than annealing and improves the metal machining properties and strength. Processing Metals are said to be worked when they are squeezed or stretched or beaten into shape.
Metals which have being shaped in this manner one said to be in the wrought state. Therefore, metals which are worked into shape must possess the property of plasticity. A metal is either cold-worked or hot-worked depending upon the temperature at which it is flow formed to shape.
When metals are examined under the microscope, it can be seen that they consist of very small grains. When most metals are bent or worked at room temperature cold worked , these grains become distorted and the metal becomes hard and brittle.
That is why metals which have been cold- worked become work-handed. Care must be taken to avoid excessive cold working, because this can cause the metal to crack. Composition The properties of a material depends largely upon the composition of the material. An example is that of plain carbon steels which are alloys of iron and carbon. If the carbon content is between 0. This is relatively soft and ductile with moderate strength and it cannot be hardened by heating and quenching. However, if the carbon content is increased to between 0.
This is much less ductile, but is stronger and can be hardened by heating and cooling rapidly quenching to make cutting tools. Environmental Reactions The properties of materials can also be affected by reactions with the environment in which they are used.
Examples are: i Dezinfication of brass on alloy of copper and zinc. This obviously weakens the material which falls under normal working conditions.
The rust will eat into the steel, reduces its thickness and therefore it strength iii Degradation of polymers: many plastics degrade and become weak and brittle when exposed to the ultraviolet content of sunlight. Special life stuffs have to be incorporated into the plastic to filter out these harmful rays. State the main property required in each case by a material used in the manufacture of: a coins b copper wire c steel girders d the front wheel axle of a car e a cutting tool 2.
Explain briefly the essential difference between the following terms as applied to the physical properties of metals: a Resistance and reluctance b Conductor and insulator c Permittivity and permeability d Specific heat capacity and thermal expansivity 3. Sort the following materials into groups of ferrous metals, non ferrous metals, and non metals: iron, carbon, sulphur, low carbon steel, low carbon steel, bronze, polystyrene, brass, stainless steel, zinc, PVC, phosphorus, silicon, duralumin,titanium, grey cast iron.
List the essential criteria for selecting a material for a specific application. Select suitable material for each of the following applications and discuss the reason for your selection in each case in terms of cost and availability, suitability for the manufacturing processes required and suitability for the applications listed below.
Explain the essential differences between tensile strength, shear strength and impact strength 7. With reference to the physical properties of metals, choose a suitable material for: a A wire wound resistor that has to maintain a stable resistance at all normal working temperatures.
List the factors that can affect the mechanical properties of a material. In each case, give an example of how the factor chosen affects their property. You will find this unit interesting in that it analyses the make-up of materials and shows how atoms bond. An atom is electrically neutral because it has an equal number of negatively charged electrons and positively charged protons.
The diagram below shows an atom of hydrogen gas this is the simplest atom 2. Protons: These are positively charged particles of mass greater than the electrons. Neutrons: These particles have the same mass as protons but carry no electrical charge. Electrons: These particles are negatively charged and orbit the nucleus like planets around the sun. The chemical properties of an atom are determined by the number of electrons it has. Electrons also determine the electrical and magnetic properties of a material.
Loss of an electron makes an atom electro-positive. Since there well be positively charged protons without balancing electron. Such an ion is called a positive ion. Gaining an electron makes the atom electro- Negative. Rahghavan 5. Engineering Materials and Their Applications — R. Follow us on Facebook and Support us with your Like. Average rating 4. Vote count: No votes so far! Be the first to rate this post. Mandi Govindgarh municipal limit and khanna area. View PDF At a pre-determined level, dictated by the coil metallurgy, the furnace must be de-coked to restore its performance and carbon is burned to carbon dioxide.
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