Compilation of metallurgy job interview question and answers for engineering students.
Metallurgy Job Interview Question # Q.1. What are the Properties of Non-Crystalline Solids?
Ans. i. Non-crystalline solids are isotropic, i.e., exhibit same physical and chemical properties in every direction. This is because on an average, the extent of disorder is the same in every direction. But a crystal shows anisotropy, i.e., directionality in properties like electrical and thermal conductivity, thermal expansions, etc.
ii. All non-crystalline solids are not in their metastable states, and tend to, if possible, form crystals. Amorphous metals usually crystallise when warmed up, even to as little as 20°K.
iii. As crystalline solids are more closely packed, they have higher densities and higher strength of secondary bonds than glassy materials.
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iv. A pure crystalline metal has a definite crystallisation temperature and it maintains, on heating, its rigidity right up to its melting point. Non- crystalline solids do not have such definite melting points, but soften gradually as the temperature is raised, and also harden gradually as the temperature is lowered.
They gradually become more viscous over a range of temperature. This is because all the sub-units of a non-crystalline solid do not have identical surroundings, meaning thereby they do not have identical bond energies. Thus, different bonds break at different temperatures leading to gradual softening as the temperature is raised.
In a polycrystalline metal, the grains have proper regularity in arrangement of atoms (crystalline nature), but the atoms at the grain boundaries are not properly arranged, i.e., grain boundaries have non-crystalline structure. Thus, when metals are used in service at high temperatures (> 0.5 Tm.p.), the grain boundaries behave like a very viscous liquid, separating the neighbouring grains, and thus, allow them to slide against each other, causing the creep failure of metals.
Curve II hypothetically indicates, had the crystallisation taken place at the solidification temperature, a sharp decrease in volume would have occurred due to the change of state from liquid to solid. In the absence of crystallisation, as indicated by curve I, thermal contraction continues to lower temperatures.
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There is a temperature called glass-transition temperature, Tg, or glass point (though the glass point, is also a function of rate of cooling), below which the atoms, or molecules cannot be rearranged to give more efficient packing. Below this temperature, the solid is called glass, because it is brittle and non- deformable.
Above this temperature, it is called super cooled liquid (as it is highly viscous and semi-rigid), but above the so called solidification temperature, it is truly liquid. If the glass is kept for varying time above Tg and below Tm.p., varying amount of crystallisation occurs, thereby varying properties can be induced.
Curve I is for furnace cooling. As the rate of cooling is increased, the freezing starts at a lower temperature under a super cooling, and occurs in a shorter time (horizontal part becomes smaller because the rate of nucleation increases), till there is no horizontal part (curve IV), i.e., no crystallisation occurs at this rate of cooling. Thus, in ideal conditions, the metal can be obtained as metallic glass (non-crystalline metal).
Liquid metals are least suitable substances for forming glasses:
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(i) They have low viscosity.
(ii) They have simple crystal structures, having mostly one atom at each lattice point. The process of crystallisation is easier, and entanglement does not occur.
(iii) Extremely high rates of cooling (>106K°/s) are required to avoid crystallisation, but such rates are difficult to attain.
(iv) As Tg temperatures of the metals are low, the amorphous metals produced are very unstable, and usually crystallise when warmed up even to as low a temperature as 20°K.
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However, certain alloys (normally transition metal based) can be made as alloy glasses and few of them are even stable at room temperatures (60% Ni, 40% Nb; 80% Fe, 20% B; 40% Fe, 40% Ni, 14% P, 6% B). Some of these metallic glasses are strongly ferromagnetic being extremely ‘soft’ magnetically with very high permeability.
As these are also very strong mechanically, materials such as Fe(40)-Ni(40)- P(19)-B(6) are used in the form of fabric (woven from ribbon) as a magnetic shielding material. Miniature transformers and magnetic cores are also made out of these. Some of metallic glasses, such as Fe(80)-B(20) is stronger than best carbon fibre and almost as stiff.
The commonly used methods for producing metallic glass are vapour quenching, ‘splat cooling’, or melt spinning. Copper wheel rotates at a peripheral speed up to 50 m/s when molten-melt-jet strikes under pressure. The ‘pool’ freezes quickly forming a continuous ribbon.
Metallurgy Job Interview Question # Q.2. What are the Types of Intermediate Phases?
Ans. Most alloy systems do not show complete solid solubility. When the amount of the solute element (say solute B in solvent A) is more than the limit of solid solubility, a second phase also appears apart from the primary (terminal) solid solution of solute B in solvent A. This second phase in some cases may be the solid solution of element A in B (the second terminal solid solution).
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Generally, apart-from these terminal solid solutions, the second phase which forms is an intermediate phase. It is a phase formed at intermediate composition between the two primary (or terminal) solid solutions. The crystal structure of intermediate phase is different from both the parent elements.
Some of these intermediate phases have a fixed composition and are called intermetallic compounds. They have metallic bonding at least to a large extent. Only a few of the intermetallic compounds obey the normal rules of valency.
In many cases, the intermediate phase has a wide range of solubility and it is not appropriate to call it a compound. For example, the beta-brass having a large solubility be better called as secondary (intermediate) solid solution, has BCC crystal structure, different than the parent metals Cu (FCC) and Zn (HCP). At about 50 at% Cu and 50 at% Zn, this alloy is disordered BCC solid solution, but becomes ordered below 470°C temperature, when zinc atoms occupy body corner sites and copper-atoms occupy body centre sites. The structure then is called as superlattice. The superlattice as well as intermetallic compounds in general have poor ductility.
Intermediate phases can be classified in the following categories depending on their structures:
The carbides, nitrides, hydrides and borides of transition metals (Fe, Mn, Cr, Mo, etc.) are important group of phases in this category. When the solubility of the interstitial solid solution forming element exceeds the solubility limit, an interstitial compound is formed. The small sized solute atoms are present in the interstices in between metal atoms.
These interstitial phases are true alloys with metallic properties. These usually form at compositions equivalent to formulae- M4X (Fe4N), M2X (Mo2C, W2C), MX (VC, WC, TiC, NbC, TiN etc.), MX2. The crystal structure depends on the ratio of radius of interstitial atom to solvent metal atom, (RX/RM).
When this ratio is less than 0.59, the crystal structure is FCC, HCP or BCC, but with larger ratio, the crystal structure becomes complex. In plain carbon steels, the ratio Rc/RFe = 0.63. The intermediate phase is the interstitial compound Fe3C, cementite, which has metallic properties but a complex crystal structure (orthorhombic with 12 iron atoms and 4 carbon atoms per unit cell).
Transition metal carbides are of immense industrial importance. Some of these carbides are among the hardest and most refractory of all known substances. The melting points of some of them are, TaC, 3800°C; ZrC, 3500°C; NbC, 3500°C, TiC; 3150°C; VC, 2800°C; WC, 2750°C. The high-speed tool tips owe their characteristics to carbides like WC and TaC. These carbides are also the basis of high-strengths at high temperatures as finely dispersed second phase particles in Heat-resisting steels and other alloys.
2. Electron Compounds (Hume Rothery Compounds):
In many alloy systems, phases of similar crystal structures are formed at the same ratios of valence electrons to atoms. These critical ratios are 3/2, 21/13 and 7/4.
A. BCC (beta brass) structure- Electron atom ratio = 3/2
CuZn, AuZn, CuBe, FeAl.
Complex-cubic structure; ratio 3/2
Ag3Al, Au3Al, Cu5Si
HCP structure; ratio = 3/2
AgCd, Ag7Sb.
B. Complex cubic (γ -brass); ratio = 21/13
Cu5Zn8, Ag5Zn8
C. HCP (ɛ-brass) structure; ratio = 7/4
CuZn3, AgZn3, Cu3Sn, CuBe3
For example, copper (FCC) being monovalent has as pure copper, the electron atom ratio of one. When zinc (HCP), a divalent metal is added in copper, then at 50 atomic percent zinc, beta phase (BCC) appears; the ratio is 3/2 (As the composition is 50 atomic percent, there is one atom to one atom ratio for Cu and Zn.
For two atoms-one of copper and other of zinc-the number of valence electron due to them is 1 + 2 = 3. So, the electron atom ratio is 3/2 = 1.5). A complex cubic γ -phase appears at a ratio of 1.62, and a close packed hexagonal ԑ -phase appears at 1.75.
Electron compounds are essentially metallic in nature as they have metallic bonding and exist over a range of compositions.
These are intermetallic phases which in certain ranges of concentration develop unusually large number of vacant lattice sites. For example, in Ni-Al system, when aluminium content is about 54%, it has unusually high vacancies of 8%. These phases are of great scientific importance because the large vacancy concentration helps in the process of diffusion.
These phases are based on chemical formula AB2. For example- MgCu2 (cubic), MgZn2 (hexagonal), MgNi2 (hexagonal) and -like AgBe2, CaMg2, TiFe2. These phases exist because there is an atomic size difference of about 22.5%, and thus can pack in crystal structures of higher coordination number than the common maximum, 12. The high coordination is possible due to free electron bond.
Sigma phases are found in certain heat resisting alloys and high alloy steels (including stainless steels). For example, FeCr, FeW, Mn3V, CoCr. These have complex structures and are brittle.
Metallurgy Job Interview Question # Q.3. What are the Properties of Eutectic-Type Alloys?
Ans. As a binary eutectic-type alloy system has mechanical mixtures of two different types of crystals in varying proportions, then ideally, the physical and mechanical properties should be an average of the properties of two phases weighted according to the proportion of each.
But actual alloys depart from this ideal behaviour, because the hardness, strength and ductility depend not only on the individual characteristics of the phases, but also on their size, number, shape and distribution. Let us take one situation, which is quite common, but not always true. If the hardness of the two phases is similar, then the hardness increases of alloys when approached from either side of the phase diagram.
The individual crystals in a eutectic mixture are, on the average, much finer than are primary crystals of either phase and fine grains make the alloy stronger and harder as motion of dislocations are more frequently blocked by their boundaries.
There occurs increase of average hardness and strength of alloys, the maximum value being at eutectic composition. Thus, yield strength, impact resistance, electrical resistivity, etc. vary like this, while reverse happens, i.e., decrease occurs with its minimum at eutectic composition of elongation and reduction in area (ductility decreases).
This increased brittleness, at exact eutectic composition, is largely responsible for eutectic alloys not finding many uses as industrial alloys. The properties of such alloys are also dependent on the properties of the phase which is continuous in the alloy, which normally is the phase present in larger amount in the eutectic mixture, i.e., dependent on the properties based on the metal or its solid solution having lower melting point. If the phase has ductility, alloys shall show some ductility, but if brittle, then alloys are relatively brittle.
Phase rule can be applied to this diagram. At the eutectic temperature, P = 3, and thus,
F = 3 – 2 + 1 = 0
The degree of freedom in zero. Eutectic temperature is invariant temperature. Here, at the eutectic temperature, 183°C, the compositions of α (is 19% Sn), of β (is 97.5% Sn) and of liquid (61.9% Sn) are fixed, and none of them can be changed arbitrarily. If temperature is raised above 183°C, either one, or both of the α and β phases would disappear If the temperature is lowered, then liquid disappears as it transforms to α + β. In two-phase region (L + α),
F = 2 – 2 + 1 = 1
It is a univariant region, i.e., has one degree of freedom. If the temperature T is arbitrarily fixed (one degree of freedom has been used), then, the compositions of the two phases are automatically fixed, i.e., at a particular temperature, only one specific composition of α and of liquid are in equilibrium.
Now suppose, we use one degree of freedom to fix arbitrarily the composition of α, then this α can exist at only one temperature and with one specific composition of liquid under equilibrium. See Fig. 3.26. Similarly in (α + β) phase field, F = 1, i.e., it is univariant. At temperature T3, (we have used one degree of freedom), compositions of α as Cα’ and of β as Cβ’ are automatically fixed.
Problem:
Cadmium and bismuth form a eutectic at 40% Cd, 60% Bi. Eutectic mixture has parallel laminations. Calculate the relative width of them if relative densities are 8.65 and 9.8 respectively. (Assume the contact areas of them to be equal.
Solution:
Metallurgy Job Interview Question # Q.4. What do you mean by Unary Diagrams?
Ans. The equilibrium diagrams are often classified on the basis of the number of components in the system. The diagrams of single component systems are called unary diagrams, for two component systems are binary diagrams, for three-component systems are ternary diagrams and so on.
Unary diagrams are for single component systems, and thus, there is no composition variable. The only two variables are pressure and temperature. The co-ordinates of the unary equilibrium diagram are pressure and temperature with temperature along the ordinate (y-axis) and the pressure along the abcissa (x- axis). A typical unary diagram is illustrated in Fig. 3.1, which is for pure iron. The solid phases are normally designated by Greek alphabets, α, β, ϒ etc.
There are a few single phase regions having gas, liquid or different crystal forms of pure iron. Phase rule can be applied to any one of them- C = 1, P = 1, and thus, F = 2, i.e. there are two degrees of freedom. Hence, temperature as well as pressure can be varied independently within the limits outlined by the boundaries of the region. In other words, single phase equilibrium is characterised by an area.
When two phases are in equilibrium (C = 1, P = 2), then F = 1. That means, either pressure, or temperature can be varied independently. Both them cannot be varied simultaneously. Two phase equilibrium is possible only along phase boundaries. Suppose ‘X’ is a point on the phase boundary having two phases ϒ and liquid in equilibrium.
If the pressure is increased by an amount, say P1, one degree of freedom has now been used, but now the system has only one phase ϒ. To maintain the two phases in equilibrium, the temperature has to be increased by such an amount, here by T1 to return to a point X1 on the phase boundary, i.e., this increase in temperature by T1 is not arbitrary, but already fixed for an increase of pressure by P1.
Thus, temperature is not a degree of freedom. We could as well have first exercised the increase of temperature by T1 as one degree of freedom, the required increase of pressure was automatically fixed to come to point X1, the point of two phase equilibrium.
Three phase equilibrium is present at points where three phase boundaries meet, say at point Y (Fig. 3.1.) Applying phase rule here (C = 1, P – 3), results in F = 0, that is, three phase equilibrium exists only for a fixed value of pressure and temperature i.e., at only one particular combination of pressure and temperature. If the temperature or the pressure is changed from the fixed value, one or two of the phases disappear.
Metallurgy Job Interview Question # Q.5. What are the Three Dimensional Defects in Crystal?
Ans. The common three dimensional defects are:
1. Precipitates
2. Voids
3. Pores
1. Precipitates:
These are defects of three dimensional natures, which break the continuity of regular crystalline structure of the matrix.
Precipitates are classified in three structural categories depending on the nature of the interface with the matrix which areas follows:
(a) Coherent Precipitates:
These precipitates form by the replacement of solvent atoms by an equal number of solute atoms in localised regions. The interfaces of precipitate are coherent with the matrix, but depending on the difference in the size of the solute and solvent atoms, there are long range elastic strain-fields (coherency strains).
When a new precipitate tries to form, it is normally a coherent precipitate. The surface energy of the coherent boundary is low as there is no true interface boundary, but elastic strain-energy is high.
(A) Semi-Coherent Precipitates:
When the interface between the precipitate and the matrix consists of regular network of dislocations as illustrated in Fig. 4.105 b). This happens as the size of the precipitate becomes larger than the coherent precipitates. As the interface is unable to bear the large elastic-strains, dislocations get introduced. The elastic strain-energy and the interfacial-energy for the semi-coherent precipitate are both relatively small.
(c) Incoherent Precipitates:
These precipitates are formed when a given volume of the matrix is replaced by an equal volume of the precipitate. In fact, this gives no elastic strains, but by having a normal boundary with the matrix with no coherency at all, it has high surface energy (interface energy). This happens when the size of the lattice constant of the precipitate increases much more than the semi-coherent state, leading to no coherency at all at the interface. (Fig. 4.105 c)
2. Voids:
Voids can be regarded as an agglomeration of vacancies. These are generally formed when the materials are subjected to drastic treatments such as irradiation with high energy particles. Voids formed in aluminium metal are normally octahedral in shape and are bounded by the {111} planes.
Voids formed in irradiated metals offer sites for the nucleation of gas bubbles. If large numbers of molecules of gases get collected in voids, and the pressure developed becomes high, the material fails. This is a great problem of considerable technological importance for materials to be used in nuclear reactors.
3. Pores:
It is common in ceramic materials as well as in powder-metallurgy products, which are processed by powder Processes such as compacting, setting, and sintering to have pores in them. The presence of pores leads to decrease of strength, thermal conductivity, and magnetic properties.
Ceramic transparent material may appear opaque due to pores (even in ice). Pores have been used to an advantage in powder metallurgy products such as in self-lubricating bearings to have a reservoir of ever ready lubricating oils.
Metallurgy Job Interview Question # Q.6. What are the Types of Segregation?
Ans. A liquid alloy may be homogeneous in composition, but the resulting solid invariably varies in composition from one point to another. This variation in composition is called ‘segregation’. Solids show several types of segregation, and more than one type may be present at a time in a piece of the alloy.
1. Normal Segregation:
Normal segregation is the direct consequence of the rejection of solute (alloying elements or impurities) at an advancing interface because it is more soluble in the liquid state than in the solid. The liquid becomes increasingly richer in the solute as the freezing progresses.
Thus, there takes place increase in concentration of the solute in regions which solidify in the end, such as the centre of the ingot. Such long-range variations in the composition are called normal segregation. Normal segregation occurs more often when the direction of growth is inwards, as in the columnar zone. This is also called macro-segregation.
2. Gravity Segregation:
Gravitational effects, i.e., due to the difference in density of various constituents, can cause difference in composition in the upper and lower parts of a casting. This is also macro-segregation.
It takes place by two mechanisms during the process of freezing:
(a) Vertical motion of the enriched layer (at the head of the interface) as a result of a difference of density due to the change in composition that is caused by rejection of solute. (b) Flinting or sinking of the equiaxed crystals depending on its density with respect to surrounding liquid.
In Pb-Sb system, eutectic reaction occurs at 252°C at 11.1% Sb, and 88.9% Pb. When a hypereutectic alloy containing, say, 15% Sb solidifies, proeutectic almost pure antimony crystals freeze before eutectic temperature, 252°C is attained.
Before the eutectic reaction occurs, antimony crystals being lighter than the liquid from which they form rise towards the surface. Such a cast alloy has antimony crystals with some eutectic product in the upper portion of the casting but the lower portion of it is composed almost entirely of the eutectic mixture.
3. Micro-Segregation:
It is possible to have variations in composition within a grain (crystal) of an alloy. It is a localised type of segregation in which the composition of the crystal varies gradually from is centre to periphery, then the crystals are said to be ‘cored’.
The equiaxed central zone nucleate with the composition of the initial concentration of the liquid, but as liquid changes in composition due to the rejection of solute, the crystals thus formed have cored structure. Metallographic section reveals it easily.
The interdendritic segregation refers to cored dendrites. The initial shoot out of dendritic arm is relatively pure metal. As the surrounding liquid gets enriched in solute, which on solidification in spaces between the arms become regions having high solute content.
4. Inverse Segregation:
It has been observed in some cases that the solute content is maximum at, or near the surface of the ingot. This phenomenon may be due to the outward movement of solute- enriched interdendritic liquid. Under certain conditions, channels between the dendrite arms offer a path by which the liquid from the centre of the casting can make its path back towards the surface.
The two methods by which it can happen are:
(a) A space may be created between the mould and the metal surface, because of differential contraction of the mould and the metal in it. This creates a suction that pulls the inside-solute- enriched liquid towards the surface.
(b) If large amount of gas is evolved near the end of the freezing, it pushes the inner-solute enriched liquid through the interdendritic channels to the surface. ‘Tin sweat’ is due to evolution of large amount of the hydrogen gas which pushes the tin-rich liquid from the centre to the surface and solidifies there as white alloy.
Metallurgy Job Interview Question # Q.7. How to Determine the Growth of a Solid Nucleus?
Ans. Growth is the increase in size of the solid nucleus after it has been nucleated. It usually occurs by the thermally activated jump of atoms from the liquid to the solid phase. Consider a moving solid-liquid interface as shown in Fig. 5.10 (a).
Its movement can be considered to be the resultant of two atomic processes occurring at the interface:
Solid atom → Liquid atom (melting reaction)
Liquid atom → Solid atom (freezing reaction)
If ∆Gm and ∆Gf are activation energies for an atom jumping from solid to liquid phase, and from liquid to solid phase respectively as illustrated in Fig. 5.10 (b), the former is larger by the latent heat of freezing as compared to latter, then-
where, Kf and Km are constants. Depending on whether the rate of melting is larger or smaller than the rate of freezing, the interface moves inside the solid or liquid. At the equilibrium temperature, the flux of atoms jumping into liquid must be exactly matched by atom flux onto solid. Fig. 5.11 illustrates the variation of rate of melting or freezing with the temperature, and that, it is not possible to freeze the metal if liquid-solid interface temperature is exactly at the freezing temperature.
For the interface to move, it must be at some temperature below freezing temperature, i.e., a solidifying interface must always be undercooled by some finite amount. Also that when supercooled metal freezes, the rate of solidification is very fast.
One of the main factors effecting the constants Km and Kf is the accommodation factor. It is the probability that an atom in a given phase can find a position to be accommodated on the other side of the interface. As the structure of all liquid metals is similar, this accommodation factor for atom to diffuse from solid to liquid is constant being independent of the chemical nature of atoms of the liquid.
However, the accommodation factor for the diffusion of atom from the liquid to solid depends on the nature of the solid, and the indices of the crystallographic plane of the solid nucleus facing the liquid. Fig. 5.12 illustrates two different planes of FCC crystal structure, one having high density of atoms, {111}, and the other having low density of atoms, {100}.
It is clear that an atom jumping from liquid phase can be better accommodated (bonding is better) on less dense plane than on more densely packed plane. The result is that low-density planes of fast growth soon tend to grow themselves out of existence. The close- packed planes of low growth rate remain behind.
The growth of the solid demands the interface to be undercooled, and the growth rate is thus a function of the amount of supercooling of the liquid ahead of the interface. The growth rate is also dependent on the nature of temperature gradient in-front of the interface.
There can be two types of such temperature gradients as illustrated is Fig. 5.13:
(i) Temperature in the liquid rises ahead of the interface, i.e., the positive temperature gradient.
(ii) Temperature falls ahead of the interface, i.e., the negative temperature gradient, or more commonly called temperature inversion.
Metallurgy Job Interview Question # Q.8. How to Add Carbon to the Surface Metals?
Ans. 1. Pack Hardening:
This method is the oldest. The articles to be carburised are packed in metal boxes or pots surrounded by a suitable compound which is rich in carbon. The boxes are sealed with clay to exclude air, and are placed in an oven, or furnace, where they are heated to a temperature of between 900 and 920°C, depending on the composition of steel.
The carbon from the carburizing compound soaks into the surface of the hot steel to depth which depends on the time that the box is left in the furnace, so that the low-carbon steel is converted into high-carbon steel in the form of thin case. The internal section of the steel, and any parts, which have been protected by tinning, however, remain unaffected, the result being a piece of steel with a dual-structure. The steel is allowed to cool slowly in the box.
The steel is then removed from the box and reheated to a temperature just above its critical point, or approximately 915-925°C for fine grain steel, followed by quenching in water, brine or oil. This hardens the skin and at the same time refines the core. Smaller articles and thin sections are heated to a lower temperature in order to avoid distortion. The steel is usually given a second heat treatment at about 760-780°C, in order to improve the ductility and impact resistance of the core and case.
Small parts and single jobs are often carburized by heating them in forge, and covering them with a carburizing powder when the metal has reached a bright red heat. The carburizing compound melts and flows over the surface of the metal, which is then returned to the forge and maintained at a bright red heat for sufficient time to allow the carbon to penetrate the surface, quenching then follows as usual.
Many commercial ‘carburizing’ compounds are available in suitable mixed form. Among the ingredients, combined in different percentages, are powdered charred leather, wood charcoal and horn. Wood charcoal is very largely used, although its value varies with the type of wood. Hickory gives the best results, and a normal rate of penetration gradually decreases and ceases after eight hours. Wood charcoal gives the slowest rate of penetration of any of the carburizing materials.
2. Liquid Carburizing:
Where a fairly thin case is required a more economical process is to carburize the parts in a liquid bath. This consists of a container filled with a molten salt, such as sodium cyanide, which is heated by electrical immersion elements or by a gas burner.
Salt bath carburizing reduces distortion of the parts to the minimum, while equal heating is assured. The parts leave the bath with a clean, bright finish, the scaling experienced during pack hardening being avoided.
Heat treatment following liquid bath carburizing is much the same as that used after pack hardening, although for cheaper classes of work the parts may be quenched immediately after removal from the salt bath. It is more usual to reheat the parts to about 760°C and quench them again. Even in this case a considerable saving in time results from the fact that quenching can follow carburizing, instead of waiting for a red-hot box to cool before the parts can be removed for heat treatment.
3. Gas Carburizing:
It is another method of introducing extra carbon into the surface of the steel, in this case by heating the metal in a furnace into which a gas which is rich in carbon such as methane, propane, butane, is introduced. It is necessary to maintain a continuous flow of carburizing gas into the furnace, and to extract the spent gas.
The first cost of gas carburizing equipment is high and the process is economical only for large output. The horizontal, rotary type of gas carburizing furnace has a retort of muffle which revolves slowly so that the parts are rotated in the stream of gas; this is suitable for smaller parts such as ball and roller bearings, chain links, pins, axles and so on. Larger parts are usually carburized in a vertical rotary furnace, in which gas is given a swirling rotary motion so that it circulates around the parts.
Metallurgy Job Interview Question # Q.9. What are the Applications of Semiconducting Materials in Electrical Engineering?
Ans. Applications of Semiconductor Materials:
1. Copper oxide and selenium were the first materials to be used to serve as rectifiers.
2. Germanium and silicon rectifiers came into commercial use somewhat later after copper- oxide and selenium. Germanium rectifiers found application earlier than silicon ones. One of the reasons for this is that is easier and simpler to produce germanium monocrystals, although the process involves considerable technological difficulties.
The Germanium and Silicon semiconductors find wide use in both high frequency and commercial frequency circuits particularly as non- controlled rectifiers (diodes) and controlled rectifiers (for example, silicon controlled rectifiers).
3. Non-Linear Resistors:
These are also called varistors. These are the semiconductors whose resistance is marked by dependence on the applied voltage, due to which the current rises non-linearly with rise in voltage.
These are made mainly from silicon carbide obtained by electrically heating a mixture of quartz sand with carbon to a temperature of about 2,000°C. This is commonly known as synthetic (electrical) carborundum.
4. Temperature-Sensitive Resistors:
These are also called Thermistors. They possess a negative temperature resistivity of high absolute value.
They are made from oxide of certain metals such as copper, manganese, cobalt, iron and zinc. Thermistors are produced in the form of discs, short rods, beads etc. by ceramic techniques.
5. Photoconductive and Photovoltaic Cells:
These are prepared from the materials which possess high sensitivity to light. The materials are sulphides, selenides and tellurides. Sometimes germanium and silicon are also used for the purpose.
These days considerable use is made of silicon photovoltaic cells, more commonly called solar cells which serve to convert solar radiant energy into electric energy for spacecraft power supply, etc.
Photovoltaic Cells find wide applications in the following:
a. Automatic control systems.
b. Television circuits
c. Sound motion picture recording and reproducing equipment.
Besides applications mentioned above, the semiconductor materials specifically find uses in:
(i) Heating appliances
(ii) Refrigerators
(iii) Power absorbers in radio systems
(iv) Elements in measuring instruments
(v) Remote control system
(vi) Surge arrestors, and
(vii) Memory elements of computers.
Advantages and Disadvantages of Semiconductor Devices:
Advantages:
Semiconductor devices have following main advantages over vacuum tubes:
1. Semiconductor devices are very compact and portable. Physical size of these devices (transistor and junction diode etc.) is very small as compared to that of electron tubes. Therefore circuits using semiconductor devices are very compact and are portable.
2. These devices are operated with relatively low voltages, therefore the power consumption is less and battery may be used for biasing purposes.
3. In semiconductor devices there is no warm up time. No heating is required to produce current carrier. Therefore these are set into operation as soon as the circuit is switched on.
4. These devices have almost unlimited life unless they are subjected to wrong voltage.
5. Transistors can withstand mechanical shocks as they are solid crystals.
6. Semiconductor devices are low priced as compared to corresponding vacuum tubes.
7. In transistors there is no vacuum deterioration trouble as no vacuum has to be created. The vacuum tubes however suffer from such defects.
Disadvantages:
The semiconductor devices have the following disadvantages as compared to electron tubes:
1. These devices are temperature sensitive. Their characteristics change considerably above the permissible ambient temperature (above 50°C). This is due to the breaking of more covalent bonds which increases the percentage of minority carriers.
2. These devices give excessive reverse current and consequently noise level is high.
3. The power output is relatively low.
4. There is less uniformity in characteristics among the individual of the same type.
Metallurgy Job Interview Question # Q.10. What are the Types of Brushes Used in Electrical Machines?
Ans. Carbon and Graphite Brushes:
Commutator type machines utilize carbon brushes which are prepared in the varieties of grades for varying types of requirements. The brushes lend a leading hand in the reversal of the flow of current in the armature coils, i.e., in commutation.
Some of important types of brushes are discussed as follows:
The preparation of carbon brushes consists of:
(i) Reduction of amorphous carbon (available in various forms such as gas carbon, lamp black or coal) to very fine powders;
(ii) Mixing with an appropriate binding material such as pitch;
(iii) Carbonization of whole mass by baking the blocks at high temperature. The brushes obtained by this process are less expensive and have better mechanical and electrical properties.
i. These are particularly used in small machines where rubbing speed, commentation or current densities are of small magnitudes.
ii. These brushes can be used in medium size machines if a small amount of graphite is added to the original carbon before firing, because graphite addition improves adequately the high speed running and current carrying capacity of the brushes.
Manufacture of these brushes is carried out the same way as discussed under carbon brushes. Graphite possesses a high degree of lubricating property and thermal conductivity.
Uses:
i. They are employed in large heavy duty D.C. generators and motors.
ii. They are also used in turbo-alternators.
3. Electrographitic Brushes:
In the manufacture of these brushes the materials are subjected to a very high temperature (under controlled atmospheric conditions) to convert the carbon and binder into almost pure artificial graphite.
These brushes have the following characteristics:
1. Low co-efficient of friction.
2. Adequate toughness and mechanical strength.
3. High current carrying capacity.
4. Capacity of operation even under most arduous conditions is satisfactory.
5. High speed running capacity is quite good.
Uses:
It is used in largest D.C. machines having the highest peripheral speeds.
Such type of brushes are generally formed by mixing finely powdered copper or bronze with graphite and suitable binder. The metal contents vary from 50 to 90%.
Uses:
The brushes are suitable for low and medium speed slip rings and low voltage D.C. generators where commutation is not difficult and low contact resistance is required.
The main technical requirements for different kinds of brushes are given in Table 7.6.
The non-metallic brushes are used in commutator machines and metallic brushes in non-commutator machines. This is due to the following reasons.
A carbon brush (non-metallic) has a high contact resistance, which is an essential requirement for better commutation in case of commutator type machines undergoing heavy sparking. In addition these machines have small rubbing speed and carry small current which a carbon brush can easily stand. Other types such as natural graphite and electrographitic brushes which have low friction loss, high mechanical strength and current carrying capacity can also be used for high speed applications.
Metallic brushes can carry large currents as they have low resistance. Their temperature co-efficient of resistance is very low and can withstand high vibrations. It is due to these characteristics that these brushes are used in heavy current machines mostly in automobiles starting motors where running speed is high and starting current is heavy. There being no commutation difficulty, the metallic brushes are used in slip ring motors as they have capacity to withstand a heavy current.
Metallurgy Job Interview Question # Q.11. Give the Classification of Electrical Contact Materials?
Ans. Depending on the power level of the circuits, the contact materials are classified in the following way:
A. Lightly and moderately loaded….. When the current does not exceed 1 ampere and the voltage causing arcing is to the tune of 10 to 20 V.
B. Heavily loaded……. When the power levels are higher. The primary requirement of contact materials is probably stability of contact resistance.
Contacts operate more satisfactory in a vacuum than in gaseous medium because the vacuum eliminates any possible arcs between them.
A. Metals for Lightly Loaded Contacts:
The following metals are oftenly used for making lightly loaded contacts:
1. Platinum:
(i) It does not oxidise in air.
(ii) It has no tendency to arc but it may form bridges and needles at low current.
(iii) It is often alloyed with iridium for making light contacts of extra-high reliability.
(iv) It is one of the most stable of all metals under the combined action of corrosion and electrical erosion.
(v) It has high meling point and does not corrode and surfaces remain clean and low in resistance under most adverse atmospheric and electrical conditions.
2. Palladium:
(i) It has properties similar to that of platinum.
(ii) Being much cheaper, it is frequently used to replace platinum.
(iii) It is less resistant to cathodic erosion and oxidation in air.
3. Silver:
(i) It has the highest thermal conductivity of any metal.
(ii) It has low contact surface resistance, since its oxide decomposes at approximately 150°C.
(iii) As silver oxides readily dissociate at a relatively low temperature, the stability of contact resistance is slightly affected by them. Nevertheless it is not recommended to use silver for contacts in light and precise devices with low contact pressures. Otherwise, silver is widely used as a contact materials, either alone or alloyed with copper.
(iv) Since silver readily combines with sulphur therefore, measures should be taken not to allow it to come into contact with sulphur-bearing substances, e.g., rubber.
4. Gold:
(i) It is similar to platinum in corrosion resistance but has a much lower melting point.
(ii) Gold and its alloys are ductile and easily formed into a variety of shapes.
(iii) Because of its softness, it is usually alloyed.
(iv) Gold alloys of silver and other metals are used to impart hardness and improve resistance to mechanical wear and electrical corrosion.
(i) It is hard, dense, slow wearing metal.
(ii) It is good thermal and electrical conductor.
(iii) It has a high melting point of 3380°C.
(iv) It is manufactured in several grades having various grain sizes.
(v) It shows no tendency to arc.
(vi) The drawback of the tungsten is that the atmospheric oxygen oxidises it. Owing to this, a certain minimum pressure must be provided between contacts made of it.
(vii) Tungsten contacts operating in an atmosphere of high relative humidity and temperature typical of moist tropical regions should not be located near phenol bearings materials such as backlisted paper or laminates bounded with phenolic resins because the gaseous product they evolve strongly corrode the tungsten.
(viii) Forged bars of tungsten have a longitudinal fibrous structure and contacts from them will have greater wear resistance than those from sheet tungsten.
(i) It has contact surfaces about midway between tungsten and fine silver, it often replaces either metal where greater wear resistance than that of silver or lower contact surface resistance than that of tungsten is desired.
(ii) It erodes faster than tungsten and corrodes in atmospheric condition.
It is an excellent contact material in light and precise devices.
Such materials are produced by powder metallurgy techniques. The blanks are pressed formed from a mixture of metal and non-metal powders and sintered at a high temperature. Contacts as compared to metal contacts entail the advantages of greater resistance to fusion, welding and wear.
B. Metals for Heavily Loaded Contacts:
The materials used for heavily loaded contacts are discussed below:
I. (i) Silver.
(ii) Silver-palladium.
(iii) Silver-copper.
(iv) Silver-cadmium.
Operating Conditions:
Voltage upto 500 V, arcing during opening, high contact pressure.
Field of Application:
“Circuit breakers of medium current rating.”
II. (i) Silver
(ii) Copper, cadmium.
Operating Conditions:
Main contacts.
Field of Application:
“Air circuit breakers.”
III. (i) Silver
(ii) Copper
Operating Conditions:
Main contact.
Field of Application:
“Oil circuit breakers”.
IV. (i) Silver
(ii) Silver-copper
(iii) Copper
(iv) Copper-cadmium
(v) Copper-silver.
Operating Conditions:
Medium and heavy alternating currents and voltage upto 500 V main contacts.
Field of Application:
Industrial-control contactors and magnetic starters.
V. (i) Copper cadmium
Operating Conditions:
Arcing contacts
Field of Application:
Industrial control contactors and magnetic starters.
VI. (i) Silver
Operating Conditions:
Low voltage direct current.
Field of Application:
D.C. contactors.
In addition to the above materials the following are also employed in the making of heavily loaded contacts:
(i) Silver-nickel
(ii) Silver-graphite
(iii) Silver-cadmium oxide
(iv) Silver-tungsten carbide
(v) Silver-tungsten
(vi) Silver-tungsten carbide
(vii) Silver lead oxide
(viii) Silver cadmium-nickel
(ix) Copper-graphite, and
(x) Copper-tungsten.
Note:
Circuit opening contacts are often made of kind able materials in which working layers consist of the main contact material and the support layer of copper, nickel steel etc.