In this article we will discuss about the mechanical properties and heat treatment of metals.
Mechanical Properties of Metals:
The properties of metals are defined as the characteristics of the metal which represent the quality of the metal and Heat treatment is the process of changing the mechanical properties of metals by controlling the heating and cooling rates.
Properties of metals actually predicts the behavior of the metal in manufacturing operation and also during its use. In other words, mechanical properties of metals are defined as the behavior of metal under the application of external forces (load). On the other hand, it represents the ability to resist failures under the action of external forces.
Heat treatment of metals and its alloys is the combination of processes applied during the manufacturing of machine components to change their mechanical properties such that they gain the desirable working property. The processes involved may be heating the metal up to some desirable temperature and then cooling it. This is for the purpose of achieving some desirable mechanical properties to the metal.
The mechanical properties of metals are as follows:
Elasticity is the property of the metal to retain or regain its original shape after the removal of load. In fact, there is no such metal which is perfectly elastic over the entire range of forces. By the application of load, the material is elastically deformed which is called strain and thereby stress is developed in the metal to offer resistance.
Strain = Change in dimensions/Original dimensions
Stress = Load applied/Area of cross section
The steel is said to be elastic up to a wide range of stress.
Plasticity is a property of metal which represents permanent deformation (without fracture) due to the action of external forces. Plasticity depends on its nature and environmental condition, i.e., the form of metal in hot or cold condition. Lead has good plasticity even at room temperature. Cast iron does not possess good plastic properties even when it is red hot.
Ductility is a property of metal which is defined as the ability to draw into wires or to elongate without fracture. It depends on the grain size of the metal crystal. The percentage of the elongation of metal is the measurement of its ductility.
Elongation = Increase in length × 100/Original length
The metal with more than 15% elongation is considered as ductile. Gold has the maximum ductility among all other metals.
Brittleness is the property of the metal which represents the fracture without any appreciable deformation. Cast iron and glass are the brittle materials.
Hardness is the property of the metal which represents the ability to resist abrasion, indentation, and scratching by hard material. Mohs scale of hardness is based on 10 standard materials as shown in Table 7.1. The materials are shown in decreasing order of their hardness on Mohs scale.
The hardness test is conducted to know the metal’s resistance against indentation (penetration).
The following is the list of tests to know the hardness of different materials:
(a) Brinell hardness test
(b) Rockwell hardness test
(c) Vickers hardness test
(a) Brinell Hardness Test:
This test is conducted by pressing a spherical steel ball known as indenter on a test piece by load as shown in Fig. 7.2. Load is applied on the surface for 15 sec. The indentation made depends on the applied load and the shape of indenter and the time for which load is applied.
In case of Brinell hardness test, we have:
Where P is the applied load; D is the diameter of the ball (mm); and d is the diameter of the indentation (mm).
(b) Rockwell Hardness Test:
This test is conducted when quick reading is required. The measurement of hardness is faster as compared to the other methods. This test is useful when the hardness of material is beyond the Brinell hardness number. Rockwell hardness test is based on the principle that the depth of penetration of the indenter is proportional to the hardness of material. In Rockwell hardness test, a sphero-conical diamond cone of 120° angle and a spherical apex of radius 0.2 mm are used to make the indentation and the depth of indentation. Hence,
R = 100 – 500h
Where h is used to calculate the hardness number and R is the Rockwell hardness number.
(c) Vickers Hardness Test:
In Vickers hardness test, a square base pyramidal diamond indenter having 136° between the opposite faces is used.
The Vickers hardness number (VHN) can be calculated by the expression:
VHN = 1.854L/D2
Where L is the applied load in kilograms which is about 30-120 kg available in testing machine for harder material and D is the measured average diagonal of the indentation in millimeters.
Toughness is the property of the metal which represents the maximum energy absorbed at the time of fracture under impact loading. In other words, toughness is the amount of work done on the material at the time of fracture. The toughness of a metal decreases with increase in temperature and is measured by the means of impact test.
There are two important types of impact loading machine to measure toughness:
(a) Charpy testing machines
(b) Izod testing machines
The Charpy test and Izod test are conducted in a machine as shown in Fig. 7.3.
In these tests, a test bar is prepared from the test metal to be tested as per the standards shown in Figs. 7.4(a) and 7.4(b). The test bar is held in the vice and a hammer is allowed to swing from a known height in such a way that it hits the notched bar and breaks it.
Since the material has absorbed some amounts of energy during its fracture, the hammer mass loses part of energy and therefore, it will not be able to reach the same height after striking in opposite direction. The loss in height multiplied by the weight is equal to the energy absorbed by the specimen.
(a) Charpy Test:
Charpy test is conducted on a specimen which is 55 mm × 10 mm × 10 mm in size and has a 2-mm deep notch made at the center with an angle of 45° on one side face. The specimen is placed horizontally like simply supported beam as shown in Fig. 7.5. The load is suddenly applied by striking hammer at the center opposite to the notch. The test piece is broken by the swinging strike hammer. The striking hammer rises with some height noted by the angle.
(b) Izod Test:
Izod test is conducted on a specimen which is 75 mm × 10 mm × 10 mm in size and has a 2-mm deep notch made at center making an angle of 45° as shown in Fig. 7.6. The specimen is placed vertically as cantilever between two jaws and the striking hammer strikes on the same face as that of notch as shown in Fig. 7.6.
The strength of metals can be measured by a tensile and compression tests machine or universal testing machines.
The tensile test of a metal is performed in the laboratory to determine the following items:
i. Proportional and elastic limit
ii. Yield point
iii. Percentage elongation and reduction of area
iv. Ultimate tensile strength
The test is carried out with the help of universal testing machine/tensile testing machine. The machine is shown in Fig. 7.7. It has a hydraulic pressure in which oil is supplied by an oil pump. The test specimens are shown in Figs. 7.8(a)-7.8(c).
The ISI code of practice has recommended the size of specimen to be tested as:
L = 5. 65√A = 5d
Where A is the cross-sectional area (mm2) and d is the diameter of specimen (mm), i.e., mean gauge length of 50 mm and the diameter of rod should be 10 mm.
The load is gradually increased and the extension is recorded. Now, a graph is plotted between the stress and the corresponding strain. Such plot is named as stress-strain graph which may be different for different materials.
Stress-Strain Curve for Mild Steel:
The graph shown in Fig. 7.9(a) reveals the following points:
(a) Portion OA is a long straight line which indicates that stress is proportional to strain and Hook’s law holds good in between OA (elastic limit) which is known as limit of proportionality or proportional limit.
(b) Portion AB is very small which indicates that stress is not proportional to strain. In this range the metal behaves like perfectly elastic. The point B is known as elastic point.
(c) Graph BC is another small portion which indicates that strain increases more quickly than stress. Point C is called upper yield point and corresponding to this stress is known as upper yield stress.
(d) Portion CD is also very small in which point D is called lower yield point and corresponding to this stress is called lower yield stress. Thus, there are two yield points such as upper yield point and lower yield point. The curve indicates that near the elastic limit, there is a sudden yield and fall-offload. The material continues to deform at a lower load.
(e) Graph DE is an upward curve, which explains that the specimen regains some strength and a high value of stresses is required for higher strains. The curve rises up to the point E. The stress at E is known as ultimate tensile stress. The work done on specimen is transferred to heat and the specimen becomes very hot.
(f) The portion of graph EF is a downward curve which reveals that a neck is formed which decreases the cross-sectional area of the specimen. It requires minimum load to continue elongation till fracture is attained at F.
Stress-Strain Curve for Ductile Material:
The stress-strain curve for ductile material is shown in Fig. 7.9(b) .The graph CD in Fig. 7.9(a) is a small portion of curve which indicates that the stress increases without any appreciable increase in strain. The variation of stress and strain curves for ductile and brittle materials is shown in Figs. 7.10(a) and 7.10(b).
The compression test is just opposite to the tensile test. The test is conducted for testing brittle materials such as CI (cast iron), stone, and concrete. The specimen used for this test is made in the form of cubical or cylindrical shape as shown in Fig. 7.8(c).
It is defined as the property of the metal which represents resistance of the metal against deformation.
It is defined as the property of the metal which represents energy stored and capability to resist shock or impact. The property is important in selecting metal for manufacturing springs.
It is defined as the property of the metal which represents the ability to deform continuously under a steady load. Creep property is always considered in designing of IC engines, boilers, turbines, etc. The creep occurs at high temperature objects such as rockets and nuclear reactors.
It is defined as the property of the metal which represents the ability to withstand varying stresses. It is of great importance in the design of components in reciprocating engines and components subjected to vibrations.
Factors Affecting Mechanical Properties:
The following is a list of factors affecting the property of a material:
Metals are formed by crystals or grains. If the orientation of grains is of smaller size, it is known as fine-grained metal. If the grains are large, it is known as coarse-grained metal. Fine-grained metals have more tensile and fatigue strength. On the other hand, coarse-grained metals have greater harden ability and better creep resistance. They have less toughness and have a greater tendency to cause distortion.
Mechanical properties are greatly influenced by temperature. It has been observed that a decrease in temperature increases the tensile and yield strength but the toughness and ductility decreases.
It is the process in which metals are heated and cooled in their solid state. It is desirable to obtain some properties of the metal. In fact, heat treatment increases tensile strength, hardness, ductility, and shock resistance.
Sometimes a metal is exposed to atmospheric environment in the presence of moist air for a considerable time. The exposure forms the metal oxide films over the surface.
Heat Treatment of Metals:
The process of heat treatment may carry the following sequence of operation:
(a) Heating of metal to a specified temperature.
(b) Keeping the metal at that temperature for a specified period.
(c) Cooling of metal by quenching. Quenching may be in air, water, caustic soda, brine, or oil.
Objectives of Heat Treatment:
Following are the important objectives of heat treatment:
(a) Internal stresses, developed due to hot or cold working of metals, may get relieved.
(b) To get the metal softer.
(c) Fine grain structure can be obtained.
(d) The metal surface may become hard.
(e) Substantial gain in tensile strength.
(f) Ductility may be increased.
(g) Gain in shock resistance.
(h) Wear and tear resistance may be improved.
(i) Corrosion resistance may be improved.
Types of Heat Treatment:
Following are the processes of heat treatment:
(e) Case hardening
Annealing is a widely and universally used heat treatment process. The process of annealing involves heating of steel to the austenitic temperature (723°C) and then the metal is cooled slowly in the furnace. The rate of cooling depends on the composition of steel.
The advantages of this process are as follows:
(i) Internal stresses are relieved which might have been produced during solidification, forging, rolling, and machining.
(ii) It improves ductility.
(iii) Hardness is reduced.
(iv) Machinability is increased.
(v) Refinement in grain sizes.
It is the process in which steel is heated to the austenitic temperature (723°C) and then the metal is cooled in still air, i.e., somewhat faster rate of cooling. The process provides higher tensile strength and hardness more than in case of annealing. A uniform fine grain structure is obtained. It also improves ductility and the internal stresses are reduced.
The process of hardening is applied to almost all machine components of steel and its alloys. In this process, the metal is heated up to the austenitic temperature (723°C) and the metal is kept for considerable time and the cooling is done at a faster rate by quenching it in water or oil bath. The process is done to increase hardness, tensile strength, and high wear resistance.
Hardening of metal provides a fine grain structure, maximum hardness, minimum ductility, and severe internal stress. The metal parts become unsuitable for the use. To avoid cracking and distortion, tempering is done after hardening. The process relieves the stresses. It decreases brittleness and restores ductility and improves toughness. In this process, the metal parts are reheated to a temperature below the lower critical temperature and keep it for a considerable time and then cooling is done at a slower rate.
(e) Case Hardening:
Following are the five methods of case hardening:
(iv) Flame hardening
(v) Induction hardening
(i) Carburizing: vector
Carburizing is a process of changing the chemical composition of metal components. It is the process of introducing carbon on low-carbon steel surface to make the surface hard. The process is used to make the surface hard up to a certain depth. A low-carbon steel part (0.2% carbon) is placed in carbon monoxide vicinity. The process of carburizing is done at temperature 920°C. Thus the outer surface becomes very hard even though the inside portion is not so hard.
Nitriding is also a process of changing the chemical composition of metal components. This is a process to make the surface of steel or alloy steel hard in the presence of nitrogen. The process is well employed for steel alloyed with chromium and molybdenum. This process is well employed in a furnace where temperature is maintained in between 450°C and 550°C. The process is used for making internal combustion engine parts with high wear resistance at high temperature such as valves, gudgeon pin, and cylinder liner.
Cyaniding is a process of changing the chemical composition of metal components. It is a surface-hardening process in which both carbon and nitrogen are absorbed by the metal surface to get the surface hardened. In this case, steel pieces are dipped in a carbon- and nitrogen-producing liquid salt (sodium cyanide or potassium cyanide) bath available at 760-870°C. The process is very fast and less time-consuming. It is mainly applied to low-carbon steel for making sleeve, speed year box, oil pump gears, cam, etc.
(iv) Flame Hardening:
Sometimes a particular surface area of steel pieces is desired to be made hard to take up wearing and shock load. The process of making such localized area hard by means of flame is known as flame hardening. In this process, selected area is heated from 750°C to 950°C by means of an oxy-acetylene flame and then a spray of water quenching is done over the surface.
The depth of surface to be heated depends upon the rate of flame movements. Excess heating of the steel pieces should be avoided otherwise crack may be developed after quenching. Tempering is done at 200°C for flame-hardened surface.
(v) Induction Hardening:
Induction hardening is the process of making surface of working steel pieces hard. The working steel pieces which act as secondary circuit is surrounded by an inductor block which acts as primary coil of a transformer. A high-frequency current is passed through the block. The heating is done by means of induced eddy current.
The inductor block will have a cooled water tube with a number of small holes to spray on the heated surface. As soon as the surface temperature becomes 750°C-760°C for 0.5% carbon steel and 790°C-800°C for alloy steel, it is automatically quenched by spray. The method is rapid because heating takes a few seconds only. The method is used for making crankshaft, piston rod, camshaft, pump shaft, etc.