Here is a compilation of exam questions with answers on metallurgy for engineering students.
Q.1. What do you mean by Space Lattice?
Ans. Crystal structures are regular three dimensional patterns of atoms in space. These regular arrangements of atoms arise due to geometrical conditions which are imposed by directional bonding (if present in the bond) and close packing. In a crystalline solid, the regular arrangement of atoms continues to infinity in three dimensions (Two hundredth of a centimeter of a crystal of iron has 1018 atoms, which approximates to infinity).
Before describing the actual three dimensional patterns made by atoms in crystal structures, it is useful to consider briefly what patterns are actually possible for identical points dispersed in space. Such patterns of points are called space lattices. Every crystal structure is based on one of the possible space lattices.
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Space lattice is the distribution of points in three dimensions in such a way that every point has identical surroundings, i.e., it is an infinite array of points in three dimensions in which every point has surroundings identical to every other point in the array as X has in Fig. 1.18 (a), which illustrates a portion of general space lattice (for clarity, the points have been joined by lines to describe the space lattice in the form of geometric figure formed by these lines).
A unit cell, Fig. 1.18 (b) is the sub-division of the space lattice that still retains the overall characteristics of the space lattice. Repetition of the unit cell in three dimensions generates the space lattice.
As a space lattice has regularity in the distribution of points, the geometry of the space lattice is defined completely by three lattice constants (vector lengths) a, b, c and the three interaxial angles α, β and γ, the meaning of them become clear from Fig. 1.18 (b). If all the combinations of equality and inequality in the interaxial angles and in lengths are counted, then Bravais was the first to show that only fourteen possible arrangements of points arc feasible, i.e., there are only fourteen possible different and distinguishable ways of arranging points in three dimensions, i.e., there are 14 space lattices.
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These are called Bravais lattices. The simple hexagonal space lattice can be represented by either of the unit cells. Even the smaller unit cell having one point per unit cell can generate the entire space lattice by packing in three dimensions.
The fourteen Bravais space lattices can be classified into possible seven crystal systems. The basis of seven crystal systems is the relative lengths of axes and the interaxial angles. For example, the cubic crystal system means (a cube) all the three axes (a, b and c) are equal and are at right angles, i.e., α= β = ϒ = 90°.
Thus, three out of the fourteen space lattices (namely, simple cubic, body centred cubic and face-centred cubic) can be clubbed together under the heading of cubic crystal system. Thus, such clubbing leads to only seven crystal systems. The number varies from one to four.
Q.2. What do you Understand by Crystalline and Non-Crystalline Solids?
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Ans. Solids may be classified broadly as crystalline or non-crystalline (also called amorphous, vitreous, or glassy). In a crystalline solid, there is regular, periodic three-dimensional pattern of atoms in space over a long range, i.e. atoms display both short range and long-range order. Long range periodicity is the characteristic of crystalline state.
The structures of non-crystalline solids are not composed of repetitive, three- dimensional patterns of atoms, at least over a long range. These may exhibit some local order such as in a sub-unit, but because the sub-units are packed randomly, they lack the long range order.
Let us take example of glass (silica) as illustrated in Fig. 1.14. The basic repeating unit, Fig. 1.14 (a) is a silicate tetrahedron. The structure of crystalline form such as quartz consists of tetrahedrons arranged in a periodically repeating pattern in three dimensions as illustrated in Fig. 1.14 (c). In non-crystalline form, such as silica glass, the sub-unit tetrahedra has perfect order, but tetrahedrons are randomly bonded to other tetrahedra, Fig. 1.14 (d), i.e., long range order is missing.
A non-crystalline solid is like a super-cooled liquid (glass) differing from it in its physical properties. Commonly, the value of viscosity is used to distinguish a non-crystalline solid from a liquid, i.e., it is called a liquid if its viscosity is less than 1015 poises.
The most important common feature of all non-crystalline solids is that their structures are such that the sub-unit arrangements get entangled so easily and so completely in the liquid state that it is almost impossible to untangle to form (like noodles entangle in the Chinese food) regular arrangements of a crystal.
The high viscosity of a silicate is due to the network of strong chemical bond which links the molecules together. It is difficult for a molecule to break away from its neighbours and thus, the diffusion and thereby the crystallisation is slowed down. It is easy to undercool a liquid to such a degree that while it has not crystallised, it becomes a solid-like in its mechanical behaviour (when viscosity becomes higher than 1015 poises). This high viscosity prevents the material from crystallising and keeps it in this metastable state. The energy of the non-crystalline state is higher than the crystalline state.
Crystallisation of non-crystalline solid is prevented due to:
1. Complex Molecular Configuration to Get Entangled:
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Metals are always almost crystalline as a few atoms, often only one is present at a lattice point, i.e., crystal structure is not complex.
2. Slow Rate of Process of Crystallization:
In silicate glass, the process of crystallisation is very slow such that a cooling rate of a fraction of Kelvin per hour prevents it. But, it is much faster in metals. A cooling rate faster than 106 K/sec is required to prevent crystallisation and to form metallic glass.
Q.3. What is Multiphase Alloys?
Ans. Multi-phase materials are mixtures of two, or more phases. These phases may be produced through mechanical mixing, or by some phase transformation. Whether the alloy is a single phase material, or has two phases, can be easily distinguished. For example, Fig. 2.22 (a) illustrates a photomicrograph of two phase brass (α + β), also called Muntz metal (60/40, Cu/Zn), etched with ferric chloride solution.
The following two aspects must be clearly understood:
1. Etching Characteristics:
The alpha phase (Fig. 2.22 a) etches white, whereas beta-phase etches black. It appears different than Fig. 2.21 (b) for a pure metal and Fig. 2.17 for solid solutions, which are single phase materials.
2. Shapes of the Grains:
Fig. 2.22 (a) illustrates clearly that the shape of the grains of alpha is distinctly different from the shape of grains of beta-phase. In a two-phase alloy, the relative amounts of phases, the interfacial energy between phases, and the method of obtaining the phase mixture control the shapes and sizes of the two phases.
A single phase alloy has all the grains which have almost similar shapes and sizes under normal conditions as illustrated in Fig. 2.21 (b) for a pure metal and in Fig. 2.17 for solid solutions. The schematic picture of two phase alloy-Muntz metal-at the atomic level (Fig. 2.22 b) can be compared with similar picture for pure metal and a solid solution in Fig. 2.18.
The phase shapes vary from equiaxed to flake-like to fibroid. In the first, all the three dimensions are approximately equivalent, whereas in the second, one dimension is markedly smaller than the other two, while in third, one dimension is greatly elongated.
Applications:
The magnitude of dihedral angle has important practical significance.
Some of them are:
1. When the second phase is brittle such as when bismuth is present as impurity in small amounts (even less than 0.05%). This thin film of brittle bismuth (dihedral angle ≈ 0°) makes copper (a highly ductile metal) to fracture along the grain boundaries, and is no more ductile. An equal amount of lead if present makes copper ductile again as ϒ12 increases to increase dihedral angle.
2. Hot Shortness in Steel:
Sulphur in steel (in absence of Mn) combines with Fe to form FeS eutectic which melts at 988°C. When steel is heated for hot working, this compound (eutectic actually) melts. As the dihedral angle is around zero degree, it wets the grain boundaries, i.e., a thin film of liquid is present at all the grain boundaries as very, small amount of liquid is needed to cover them.
As the stress is applied for hot-working, the steel cracks and breaks. This is called ‘hot shortness of steel’. Hot shortness is overcome by the common practical method of increasing the value of ϒ12, by adding a suitable element such as Mn, which is soluble in the injurious liquid.
As the dihedral angle is increased, the compound (MnS) forms discrete lenticular or rounded particles. Mn has more affinity for sulphur and a small amount of 0.5% Mn is enough to take care of normal sulphur present in steels. MnS has a melting point of 1610°, and is quite ductile at the hot working range of steel to get deformed.
3. For the production of useful cermets (ceramic grains bonded by metal films), wetting of all the grains by a liquid metal is essential-this is called liquid-phase sintering. Cemented tungsten carbide- tip tools are made by it for use as high speed cutting tools. Powders of tungsten carbide and cobalt are mixed and pressed together. The powder compact is heated to melt the cobalt which spreads (as dihedral angle is ≈ 0) and binds the carbide grains as illustrated in Fig. 2.29 (b).
4. Soldering:
A good solder must wet the surfaces of the metals to be soldered. Thus, the dihedral angle of the drop of liquid solder on the metal surface must be around zero degree. Thus, ϒ12 should be small to happen this. A good flux helps to achieve this condition.
Problem:
There are three alloys with the following weight composition:
In what weight proportion should the above three alloys be mixed to produce a new alloy of the following weight composition.
56% element-A, 29% element-B, 15% element-C. Assume that the total weight is conserved.
Solution:
Let X wt.% of alloy-1, Y % of alloy-2 and (100-X-Y)% of alloy-3 be used to make the alloy.
Now weight of each element from each alloy is then added to get weight % in the new alloy:
Q.4. What are the Properties of Crystalline Materials?
Ans. Actually, the properties of the crystalline materials can be categorised into two classes depending on their sensitivity to the presence of defects in them:
1. Structure-Insensitive Properties:
These properties of the crystalline solids are hardly affected by the presence of defects in crystals. For example, density, specific heat etc.
2. Structure-Sensitive Properties:
These properties of the crystalline solids are profoundly affected by the relatively minor changes in the crystal structure caused by the presence of crystal defects. These are the properties of greatest engineering importance such as, hardness, mechanical strength, ductility, etc.
Line defects, called dislocations, are the most striking defects, responsible for the useful property of ductility in metals (even in ceramics and crystalline polymers), but are also responsible for drastically decreasing the mechanical strength of the metals. Defects like point defects such as impurity atoms in a perfect lattice (just a few parts per million of aluminium in silicon) are responsible for the functioning of semi-conductor devices.
The defects (or imperfections) are the occasional disruptions in the periodicity within the crystals. The defects which disrupt the crystal structure the most are the defects in the crystal structure.
Since crystal structure is a geometric concept, it is natural to classify the defects based on their basic geometry as:
i.. Point defects, i.e., zero-dimensional defects.
ii. Line defects, i.e., one-dimensional defects.
iii. Surface defects, i.e., two-dimensional defects.
iv. Volume defects, i.e., three-dimensional defects.
Q.5. What is Constitutional Supercooling?
Ans. Some alloys show supercooling of a different kind called constitutional supercooling, i.e., due to their constitution, when a solid freezes with a composition different from that of the liquid from which it forms (A different aspect of this has been discussed in zone-refining). Such alloys have separate curves for solidus and liquidus such as in an isomorphous system with phase diagram illustrated in Fig. 5.16.
Let us take a liquid alloy of composition ‘l’, Fig. 5.16 in a tube, Fig. 5.17. Let us assume that heat is being lost on left end. The heat flow is linear from right to left. Let us safely assume that there is no stirring of liquid with no liquid convection currents.
As the liquid at left end of tube attains a temperature T1, a solid of composition given by point, k forms from the layer of liquid of immediate vicinity of the interface. As this solid has larger proportion of A atoms than does the liquid layer from which it forms, the liquid gets enriched in B atoms. As the temperature falls to T2, solid formed has composition of point m, and liquid gets further enriched in B atoms to have a composition of point n.
This change in the composition of the liquid just ahead of the interface takes place with continuous increase of atoms of B in it, but there is a concurrent change in the composition of the solid, till steady-state condition is reached. This happens when the concentration of the excess of B atoms in the liquid adjacent to the interface reaches a maximum, i.e., when liquid attains a composition of point p.
This is the steady state condition as at this instant the liquid is able to freeze a solid of composition l, that is, as of the original alloy. At this state, the solid which forms from liquid enriched in B atoms, is the same as the liquid drawn into this layer and the temperature now of the interface is T3. Fig. 5.18 (a) illustrates variation of composition in the solid and the liquid when steady-state condition has been just attained.
The composition of solid increases from its point k to that of original liquid, I. At this interface, there is a sudden increase of B in the composition of liquid to point p as one move from solid to liquid. The composition of the liquid then decreases exponentially from that of point p to the composition of the original liquid as given by point I. The shape of the distance x of this liquid versus composition curve depends on the rate of freezing and the rate of diffusion of atoms.
Fig. 5.18 (b) illustrates the contour of composition with distance curve at a later stage of solidification. It is clear that the exponential region of the liquid ahead of the interface is being carried alongwith the interface.
When a liquid metal is poured in a mould, the freezing starts from the mould-wall as heat is removed through the mould wall. Thus, the temperature is lowest at the mould wall and rises towards the centre of the mould. This increase of temperature of the liquid can be safely assumed to be linear with distance as illustrated in Fig. 5.19.
But the freezing point of the liquid as a function of the distance from the interface (in which the composition changes exponentially) is a curved graph as illustrated by second curve in Fig. 5.19 (a). In this liquid layer of ‘x’ thickness, the freezing point is higher than the actual temperature of the alloy, i.e., this layer of liquid is effectively supercooled.
The amount of supercooling increases as one moves from the interface towards inside the liquid. This is called constitutional supercooling as it is due to the constitution of the alloy, i.e., due to concentration gradient in the liquid in-front of the interface. In some cases, the graph may appear as in Fig. 5.19 (b).
Q.6. How do you Define the Term Electrode Potential?
Ans. When a rod of a metal, say zinc, is immersed in an aqueous solution containing its own ions (if zinc sulphate is dissolved in water, it has Zn2+ ions, SO2-4 ions, and ZnSO4 in the solution). Some of the Zn atoms detach themselves from the metal surface and go into solution as ions leaving their valence electrons on the metal surface. Some of the zinc-ions of the solution deposit themselves on the metal surface.
The rate of dissolution depends on the binding forces holding the atom in the surface, and the rate of deposition depends on the concentration of ions in the solution. In the beginning, these two rates are different, but soon attain equilibrium with no net flow of metal-ions.
In case of zinc, as more zinc atoms have dissolved (than deposited), a net negative charge builds up at the metal (due to valence electrons left behind), and a net positive charge in the adjacent solution (due to more Zn2+-ions present). This electrical double-layer decreases further dissolution of zinc atoms resulting in attainment of equilibrium.
This electric potential of a metal, when the equilibrium is reached, depends also on the concentration of metal-ions in the solution (and the temperature). If the metal-ion concentration in the solution is kept fixed, normally one mole of ions per litre (having unit activity) at a constant temperature (25°C), the electric potential of the metal now becomes the property of the metal, as it gives the tendency of a metal to go into solution, i.e., to corrode, or to get plated.
It is difficult to measure this potential of the metal in absolute terms, and as only a potential difference can be measured in an experiment, the potential of the electrode is determined against a standard hydrogen electrode (it is a platinum electrode coated with black platinum, dipped in hydrogen gas at one atmosphere in contact with HCl solution of 1.2 mole/litre) at 25°C, whose potential is arbitrarily taken to be 0.00 Volts.
This is similar to measuring height of a place with reference to sea-level. The value of the potential, so obtained, is called the standard electrode potential of that metal, such as for zinc, it is-0.763 volts. The elements, when arranged in order of their standard electrode potentials give rise to the electrochemical series, or emf series as shown in Table 14.2.
According to Table 14.2, the reactive metals are at the bottom of the list with large negative potentials, i.e., these metals dissolve readily even in concentrated solutions of their ions. Lithium is the most active and base metal. Noble metals are at the top of the list with large positive potentials, i.e., these metals are not dissolved easily but are deposited from the dilute solutions. Gold at the top of the list is the most noble metal. This difference in behaviour of metals shows that the valency electrons are strongly bound to the positive cores in the noble metals because of the short distance of interaction, i.e., datomic = dionic
Ans. When an equilibrium is attained on immersing a metal, M in its electrolyte (some particular half-cell reaction), the rate of oxidation, roxid (i.e., M → Mn+ + ne̅) and the rate of reduction rred (Mn+ + ne̅ → M) are equal,
roxid = rred
and thus, there is no net current-flow. However, this equilibrium is really in a dynamic state on the atomic level, i.e., two equal and opposite electric currents How, called the exchange current density, i0. This exchange current density is defined as the rate of oxidation, or reduction at the equilibrium, expressed in terms of current density (current per unit surface area of corroding material), and is given as
roxid = rred = i0 / nF
The value for i0 is determined experimentally, and varies from system to system as it depends on electrode composition, the electrolyte, the surface roughness and surface impurities, but is almost constant for a particular half-cell.
The single electrode potentials of the two metals, and the potential of the electrochemical cells formed between them are measured under standard conditions when virtually no current flows. Corrosion is a dynamic process, i.e., it occurs only when the current flows in the corrosion-cell formed.
Thus if a standard electrochemical cell is short-circuited, so that a finite current flows through it, the potentials of the electrodes change from their standard values (as given in Table 14.2), because the system is now in non-equilibrium state. The potential of the anode increases in the cathodic direction, while that of cathode decreases in the anodic direction (Fig. 14.24). This change of each electrode-potential from its equilibrium value (when the net current flows) is called polarisation.
The magnitude of this displacement (i.e., polarisation) is measured as overvoltage, which is normally represented by the symbol, η (eta). The overvoltage is expressed in terms of plus or minus volts (or millivolts) relative to the equilibrium potential.
Q.8. How to Repair Semiconductor Materials?
Ans. Germanium and silicon are the two most commonly used semiconductors; their details of preparation are discussed below:
Germanium:
a. Germanium is a hard dense element (discovered in 1886) obtained as a by-product of zinc refining or from the flue dust of certain coals.
b. It has grey metallic lustre.
c. Commercially, it is available in the oxide form (GeO2) which becomes reduced germanium powder when heated at about 650°C in an atmosphere of hydrogen. When it is further heated above the melting point of 937°C, germanium is obtained in the form of bars of low impurity. Further purification is achieved by the process of zone-refining in inert gas atmosphere.
d. Seed crystals are obtained by taking a portion of an ingot of germanium obtained from zone-refining process. The desired single crystals are grown by dipping a small seed crystal into a bath of molten germanium and withdrawing it uniformly as it grows. The bath is maintained at an accurately-controlled temperature under an inert gas atmosphere.
Fig. 7.27 shows a cross-sectional view of a typical piece of equipment used for the purpose of crystal pulling.
Steps:
(i) Germanium melt is kept in a graphite crucible lined by quartz liner and is maintained just above the melting temperature by electric heater.
(ii) To start the growth, tiny seed crystal is positioned in a metal chuck and then immersed in the liquid germanium.
(iii) The seed is rotated (to promote stirring in the melt) and is very slowly withdrawn as the liquid freezes on to the colder seed. The atoms in the solidified melt arrange themselves in such a way that they have the same crystals structure as the seed crystal.
(iv) The growing crystal is raised at such a rate as to keep the interface between solid and the liquid at the surface of the melt.
(v) Carefully controlled amounts of donor or acceptor type impurities can be added via quartz, tube for producing N- type or P-type semiconductors respectively.
Generally the whole apparatus is placed in a sealed vessel containing as inert atmosphere of hydrogen-argon which can be introduced via the side tube.
(vi) Final crystals are usually many centimetres in length and about 2 to 3 cm in diameter. These are sliced by diamond saws into small wafers only in fraction of a mm in thickness.
Silicon:
a. Hard element having bluish-grey metallic lustre; melting point of 1420°C.
b. Obtained by reduction of tetrachloride with zinc followed by use of hydrogen and other reducing agents.
c. Required purification is obtained by zone-refining process.
d. Single crystals are grown by a method similar to that used for germanium.
Production of P- and N-Type Crystals:
If a junction is produced by simply placing P- and N-type germanium (or silicon) in contact, it will not have any useful electrical properties.
There is a need to grow the junction in a single crystal.
The commonly used methods are given below:
First Method:
a. A pure germanium crystal is pulled up very slowly from an N-type melt.
b. Then a P-type impurity is added to the melt in such a quantity as is sufficient to make holes the majority carriers and crystal growing is continued.
In this way a junction between N and P is formed which can be located by etching the crystal in hydrofluoric acid.
c. Diamond cutters are employed to slice the crystals to produce in large number of pieces.
Second Method:
a. An N-type germanium crystal is sliced into thin wafers and an indium (a trivalent material) is placed on each slice as shown in Fig. 7.28.
b. Many such wafers are heated at the same time in a hydrogen atmosphere.
c. The indium melts (melting point = 156°C) at a lower temperature than germanium (melting point = 142°C) and thus dissolves some germanium from the slice to form P-type material. A strict temperature control is essential during the process.
d. On cooling, the contaminated germanium solidifies first and then the rest of the indium pallet, thereby forming the P-N junction.
e. A wire is soldered to the indium and a nickel tag to the germanium as shown in Fig. 7.29.
f. The P-N diode so formed is etched to remove surface contamination, washed in de-ionized water, dried and mounted in water-repellent grease in a light-tight metal container.
Q.9. Explain the Effect on Magnetic Properties of Magnetic Materials.
Ans. Effect of Temperature, Heat Treatment, Direction of the Grain on the Magnetic Properties of Magnetic Materials:
If heated to a high enough temperature, any ferromagnetic material will lose its magnetism. The temperature at which this occurs is known as Curie point. It is 770°C for iron, 358°C for nickel, and 1120°C for cobalt, which has the highest known value. For some metals the Curie point lies near a temperature of absolute zero. The relative magnetisation under a given field decreases as temperature approaches the Curie point.
In metals, near Curie point, often abnormal thermal expansion characteristics occur due to change in atomic forces associated with ferromagnetism. The medium nickel alloys (30 to 70% Ni) decrease in expansion co-efficient and then increase when cooled through their Curie point. This appears to be the result of superposing “magnetic” expansion on the thermal contraction, thus suppressing normal change.
The magnetic properties of iron and iron-silicon alloys are slightly affected by the small temperature increases. The permeability is usually improved, and the hysteresis and eddy current losses are decreased.
The core losses continue to decrease as the temperature approaches the Curie point, and for medium and high inductions, the permeability reaches unity at this point.
The type of heat treatment given to magnetic materials amply affects their magnetic properties. Annealing is usually accomplished in an inert or reducing atmosphere and tends to remove the carbon and to relieve internal elastic strains, thus reducing the area of hysteresis loop and altering the saturation curve.
Strains not affected by heat treatment are those caused by impurities and magnetisation. Since impurities disrupt the orderly arrangement of the atoms and set up lattice strains, freedom from dissolved impurities is very important. There are also strains set up by magnetisation, the change in length being called “magnetostriction”. These strains are positive with magnetisation for some materials, negative for others. In iron, magnetostriction is different for different crystal directions.
3. Effect of Direction of the Grain:
Electrical strip steels have been developed that have considerably different properties, depending on the direction of the flux relative to the direction of rolling. Strip steel, which is rolled in a continuous mill, must not be confused with sheet steel, which, during finishing, is alternately rolled along the sheet and then across it.
The magnetic curves for some transformers are wound from coiled strip so that the flux direction is aligned with the grains.
Effect of Impurities and Alloying Elements on Electromagnet Materials:
The magnetic qualities that are usually specified or evaluated for use of ordinary industrial electromagnet materials are:
(i) Saturation curve and intrinsic saturation.
(ii) Hysteresis loss.
(iii) Eddy current loss.
(iv) Maximum permeability.
The order of importance to the users largely depends on the application.
In addition, there are many other qualities that must be considered, some of which are given below:
(i) Cost.
(ii) Mechanical properties (strength, ductility etc.).
(iii) Manufacturing properties (drawing, brazing, welding etc.).
(iv) Electrical resistivity, which is one of the factors determining eddy-current loss.
(v) Thermal conductivity.
(vi) Ageing characteristics.
(vii) Corrosion resistance.
(viii) Initial permeability.
The most common impurities in iron are carbon, oxygen, nitrogen, manganese, sulphur and phosphorus. These impurities cause in general, a decrease in permeability and an increase in hysteresis loss.
a. A slight amount of carbon reduces the permeability tremendously.
b. Lowers the saturation point.
c. Increases the area of the hysteresis loop which causes an increase in coercive force and a decrease in residual induction.
d. The presence of carbon increases resistivity.
The form in which carbon appears, that is cementite or graphite, modifies its influence on magnetic properties.
a. It improves the permeability at low inductions.
b. Decreases hysteresis loss and eddy current loss.
c. Its presence increases the electrical resistivity.
d. It reduces ageing to a negligible figure.
e. It is useful to the magnetic property, but decreases the ductility appreciably.
Silicon is the most important non-magnetic element with which iron is alloyed for the sake of magnetic properties.
In small amount, it does not affect the magnetic property. If the content reaches to 13%, the steel becomes practically non-magnetic.
Sulphur has the greatest detrimental effect, next to carbon. Even very small amounts are harmful.
Phosphorus and oxygen lower permeability and increase the hysteresis loss, and attempts are made to minimise their percentages.
Copper is often added to steel to increase its corrosion resistance. In amounts upto 0.5% it has little effect on magnetic properties.
Q.10. Explain Time-Temperature-Transformation Relations in Heat Treatment.
Ans. When a phase has been brought into an unstable region, e.g., austenite (0.8 w/oC) cooled below 723°C, the equilibrium diagram no longer applies nor does it tell us anything about the structure of transformation products or the mechanism of their formation.
It is well known that very slow cooling produces pearlite and very rapid cooling produces martensite. This leaves a middle ground for isothermal transformation, continuous cooling transformation, or some combination of both.
Isothermal transformation of austenite at subcritical temperatures (less than 723°C for 0.8 w/oC) is known as- “Time-Temperature-Transformation (TTT) diagrams”, whereas continuous cooling is shown on “Continuous-Cooling-Transformation (CCT) diagrams”.
One diagram is required for each alloy composition in contrast to an equilibrium diagram which covers an entire alloy system.
TTT Diagrams:
Depending on the temperature of transformation, austenite may transform to pearlite, bainite, or martensite. The kinetics of the above phase transformation is indicated on TTT diagrams.
Importance:
I. These diagrams indicate the phases existing in steel at various temperatures and times.
II. They are very much useful in heat-treatment of steel.
III. With the help of these diagrams, one can choose a proper cooling cycle to obtain the desired transformation product (microstructure) so as to obtain the required properties in the component.
IV. Whereas iron-carbon (Fe-C), equilibrium is of limited use in the study of steel cooled under non-equilibrium conditions, TTT diagram is the principal source of information on the actual process of austenite decomposition under non-equilibrium conditions.
Determination of TTT Diagram:
TTT diagrams are graphical summaries of isothermal transformation data obtained as follows:
I. A large number of relatively small specimens, at equilibrium above critical temperature, are quenched in a liquid bath maintained at constant subcritical temperature.
II. Specimens are withdrawn at regular intervals and quenched in iced brine to prevent further transformation.
III. Microstructures are examined to determine the beginning (usually 0.5% or 1%) and the end (99.5% or 99% respectively) of transformation at given temperature.
IV. A series of these tests at several temperatures allow construction of diagrams such as Fig. 5.12 (for eutectoid carbon steel). Because of their shape, these curves are sometimes called S curves or C curves although more common practice is to call them TTT diagrams.
V. Transformation of eutectoid steel just below critical temperature is to pearlite at a very slow rate. As transformation temperature is decreased, the rate increases rapidly to a maximum at about 540°C. Transformation of austenite to pearlite is a nucleation and growth process, dependent on diffusion and therefore, on temperature. Nucleation rate increases, and growth rate decreases with decreasing temperature.
VI. Transformation temperature influences both reaction rate and resulting microstructure. Pearlite which forms just below 723°C has relatively thick lamellae which are essentially parallel. The pearlite structure becomes progressively finer until transformation temperature is about 500-600°C, i.e., at the “nose” in Fig. 5.12. Below this temperature, austenite transforms to bainite.
VII. The change in spacing between alternate lamellae in pearlite has a pronounced effect on mechanical properties, this is indicated by the hardness numbers given at the right side of Fig. 5.12.
The Bainite Transformation:
I. Bainite is the name given to the structures that form on isothermal transformation at temperatures below the nose of the TTT curves.
II. Bainite is an isothermal transformation product and cannot be produced by continuous cooling.
III. Bainite is an intimate mixture of ferrite and cementite, as in pearlite. Pearlite has alternate layers of ferrite and cementite. In bainite, however, cementite apparently exists as tiny spheroids uniformly distributed throughout a ferrite matrix.
In upper bainite (formed at temperatures just below the nose of TTT curve) there is evidence of some patterns in the cementite arrangement since the microstructure has a feathery appearance. In lower bainite (formed at temperatures approaching Ms) the cementite becomes too fine for resolution and an acicular (needle like) pattern is found.
IV. Bainite is normally harder, stronger and tougher than fine pearlite of the same chemical composition (due to differences in size, shape, and distribution of cementite).
V. Bainite exhibits considerable variation in properties depending on the temperature at which it is formed.
Martensite in the TTT Diagram:
I. Though most of TTT diagram is based on isothermal transformation of austenite, it is common to show the transformation to martensite which occurs only on continuous cooling (Fig. 5.12 shows Ms and Mf temperatures).
II. The critical cooling rate for martensite transformation is one which allows us to just get through the “gate”, i.e., past the “nose”, without any transformation of austenite. At temperatures below Ms, part of the austenite will transform to martensite during cooling, making impossible a strictly isothermal transformation of all austenite below Ms.
As temperature is decreased, a larger fraction of austenite is transformed to martensite. If a specimen is held isothermally at temperature between Ms and Mf the remaining fraction of austenite will eventually transform to bainite.
Q.11. How does a Blast Furnace Work?
Ans. It is so named as a very high temperature is developed inside it by means of forcing a blast of heated air. Its height is about 30 metres and interior diameter 8 metres. To permit proper flow of materials it is made narrow at the top and bottom. The outside of the furnace is constructed of steel plates and these are lined on the inside with fire bricks.
There is a wall or hearth at the bottom of the furnace in which molten iron collects prior to its tapping. The widest part of the furnace is at the top of the ‘bosh’ and its diameter depends upon the output. The raw materials after weighing in the desired proportions are raised to the top of the blast furnace through mechanical charging equipment.
A double bell arrangement is provided at the top to check the escape of gas from the top of the furnace. The materials are charged first on the upper bell which is then lowered allowing them to fall on the lower bell. After closing upper bell the lower bell is lowered to enable the materials to fall on to the stockline.
Ore, coke and limestone are charged in relation at a rate adequate to maintain the stockline in the correct height. At the top of the hearth (above molten metal and slag) are situated the ‘tuyeres’ through which blast of heated air is introduced. There may be 8-18 tuyeres round the furnace but usually there are 10.
Working:
After charging the furnace with the raw materials, the furnace is lit and a blast of hot air is passed through the blast pipe and tuyeres. At the bottom of the furnace (hearth) a high temperature to the tune of 1350 to 1600°C is obtained.
At this high temperature carbon present acts as a reducing agent and reacts on the ore as follows:
The tapped molten metal is run into channels and branches formed in the sandybed. The solidified iron after cooling is cut into pieces of small lengths. These are called sows and pigs because when inverted their tops have a curved, hog-back profile.
The gases leaving the blast furnace contain about 30% carbon monoxide (CO) which is combustible. The gases after travelling through dust catchers are burnt in a chamber, the heat of which is used in heating fire bricks in a stove.
The heat stored by the firebricks (in a stove) is utilised for heating the blast of air (to a temperature of about 560°C) to be supplied to the furnace. Usually 3 to 5 units of stoves are employed with each furnace for heating of air.