Here is a list of commonly used engineering materials.
1. Tungsten Steel:
1. It contains 4.5 to 6% tungsten, 0.5 to 0.7% carbon and the remainder is iron.
2. Saturation flux density = 1.2 Wb/m2.
3. Coercivity = 8000 AT/m,
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4. It has high magnetic reluctance.
5. It has excellent water and shock resistance; it is quite good in toughness too.
6. It is so hard that it necessitates a special care during heat treatment, to avoid distortion and cracks.
7. When suitably hardened, it has a high value of B-H product.
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It is used in making permanent magnets for dynamos, motors etc.
2. Cobalt Steel:
1. It contains 34% cobalt, 5% chromium, 3.5 to 6% tungsten and remainder iron.
2. Saturation flux density = 2.4 Wb/m2.
3. Coercivity = 104 AT/m.
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4. It is adequately hard and strong.
5. It is one of the most satisfactory and efficient magnetic materials.
6. It is expensive due to greater content of cobalt.
7. It can be hot forged and machined after annealing.
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It is used in motors, fans and heavy duty instruments of commercial importance.
3. Chromium Steel:
1. It contains 2-6% chromium, 0.6-1% carbon, 0.3-0.5% manganese, remainder being iron.
2. Saturation flux density = 1.5 Wb/m2.
3. Coercivity = 7500 AT/m.
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4. Its magnetic properties are of the same tune as that of tungsten steel.
5. It has a reasonable degree of hardness and toughness.
6. It requires a good amount of care during ‘hardening’ as due to high carbon content it is liable to break.
7. It possesses high magnetic properties.
8. Magnets are manufactured either by casting or by sintering.
It finds its use in portable and light instruments.
4. Alnico:
1. It contains 18% nickel, 10% aluminium, 5% copper, 15% cobalt and remainder iron.
2. It is more expensive than Alni.
3. Its magnetic properties are even better than Alni. In fact it is known as one of the best magnetic materials.
4. Its saturation flux density is 1.2 Wb/m2.
5. Its coercivity is 105 AT/m.
6. It is available in many grades, each possessing varying properties.
7. The permanent magnets formed from alloy are smaller in size and lighter in weight as compared to those made from cobalt or tungsten steel.
8. Alnico alloys are very hard and brittle, therefore, they cannot be machined and have to be cast to shape and finished by grinding.
9. It has a more rectangular hysteresis loop and high remanence.
5. Cunife:
1. It contains 50% copper, 30% nickel and 20% iron.
2. This alloy is malleable and ductile.
3. It can be punched, machined and cold rolled.
4. It is suitable for producing small-size magnets.
6. Hypernic:
1. It contains 50% nickel and 50% iron.
2. Saturation flux density = 1.2 Wb/m2.
3. Hysteresis loss is low.
4. Magnetic property is high.
5. Its permeability is 70,000.
6. It is used in loading coils (in the form of cores) placed at intervals in long distance telephone lines to make the electrical impulses distinct.
7. Commonly Used Material for Semiconductors:
Germanium:
Germanium, the material in which transistor action was first observed, is one of the most interesting and most important semiconductor.
The following points relate to Germanium:
1. It is a grey metallic looking material.
2. It is brittle and glass like in its mechanical properties.
3. It crystallises in the diamond cubic lattice.
4. It has an intrinsic resistivity of 47 Ω cm but may be doped with antimony or arsenic to give N-type resistivity of 0.01 or less Ω cm and with boron, gallium or aluminim to give P-type resistivity of 0.001 Ω cm or less.
5. The valence band of germanium is of interest, since it degenerate, so that there are two types of positive-hole conducting current namely, “light” and “heavy” holes , the former having 0.0044, the latter having 0.28, the mass of free electron. The third degenerate “spin- orbit split” band is of minor significance.
6. Germanium is usually prepared in high purity single-crystal form for electronics use by pulling from the melt, either vertically or horizontally. To obtain intrinsic material, impurities of active elements must be in the range of 1 part/billion or less.
7. Junctions of P-N type have been made in germanium, alloyed junctions involve greater amounts of impurity, are produced at lower temperatures, and are made directly from metals, which form a eutectic with germanium. Such alloyed junctions tend to be abrupt in transition and usually involve recrystallization phenomena.
8. The ordinarily active impurities such as gallium, indium, arsenic, and antimony tend to diffuse rather slowly in germanium. The donors diffuse roughly an order of magnitude faster than the acceptors.
9. Germanium is notable, not only for the number of impurity elements with which it may be used to create deep impurity energy levels, but also for sensitivity it has to other elements (notably copper, lithium, nickel and gold) which can move about in the crystal at high speeds at low temperature.
Silicon has become increasingly important as a transistor material, to such an extent that its properties are now being studied more than those of germanium. Its higher band gap offers a greater temperature range than that of germanium, but silicon is handicapped by the lower carrier mobilities, 1300 and 450 electrons and holes respectively compared with values of 3900 and 1800 for germanium. This drawback limits applications of silicon in high frequency transistors.
The following points relate to silicon:
1. Silicon is somewhat more difficult than germanium to produce and purify because of its higher melting point (1420°C).
2. The properties of silicon are notably sensitive to the presence of oxygen, which is usually present to a level of 1017 to 1018 atoms/cm3, unless special care is taken to exclude it.
3. Oxygen in silicon tends to introduce instabilities when the material is subjected to heat treatments at high temperatures. The mechanism is not well understood, although it is likely that inferential oxygen acts as a donor. It may also be combined with other impurity atoms to produce neutral complexes.
4. Silicon is one of the most sensitive elements to nuclear radiations, stemming from its low atomic weight and high resistivity of the material as generally used.
5. The bulk properties of silicon under radiation are particularly sensitive to the presence of oxygen, since the defects produced seem to combine readily with oxygen to produce active centres.
6. Silicon behaves much like germanium in its sensitivity to a wide variety of impurities.
7. Silicon, like germanium, is also characterised by the number of impurities which besides having deep levels, move rather rapidly through the lattice at moderate to high temperatures. Among these are copper, iron, manganese, nickel and cobalt.
8. Compound Semiconductors:
Even before germanium and silicon became important, compound semiconductors were the objects of much interest. The foundations of semiconductor science, as a matter of fact, were laid on the basis of studies with copper oxide, zinc sulphide, silicon carbide, and zinc oxide, among other.
Some of the compound semiconductors are discussed below:
Gallium Arsenide:
The characteristics of gallium arsenide are very closely related to those of germanium. This is understable, since gallium is the third column neighbour and arsenic the fifth-column neighbour of germanium in the periodic table.
The following properties of gallium arsenide make it inferior to silicon and germanium:
(i) The high melting point (1300°C) combined with the high vapour pressure of arsenic at 1200 to 1300°C, makes the production of gallium arsenide of electronic grade an extremely difficult one.
(ii) Horizontal furnaces using zone refining techniques have proved the best, but the resulting material still is not competitive with Silicon and Germanium in purity or structural perfection.
(iii) Resistivities of 1 to 10 W cm are generally obtained and mobilities of 5000 to 6000 are common. However life time values are low.
Gallium arsenide has proved to be useful for a number of very important devices, including some varieties of switching and parametric diodes, tunnel diodes, semiconductors lasers, and hot electron “Gunneffect” diodes. Material-production problems remain one of the chief difficulties in the field, since good examples of material are obtained by selection rather than by planning.
It is of interest because it has the highest room-temperature electron mobility of any known material. Because of the low melting point, 525°C, Indium antimonide (In Sb) is much easier to prepare in single crystal form than gallium arsenide (Ga As).
Cadmium sulphide (Cd) melts only under high pressure. It has been used commercially as a photoconductor for many years, as well as constituent of cathode-ray phosphorus.
It can be prepared in the resistivity range from 10 to about 1012, depending on the defects present and the impurities. It is extremely difficult, if not impossible, to produce P-type Cds.
Silicon carbide is extremely refractory, subliming in the region of 2800°C. Because of this and because of large band gap it has long been hoped that silicon carbide would be useful for very high-temperature rectifiers and transistors.
The ferrous metals are iron base metals which include all varieties of irons and steels. These metals go a long way in bringing prosperity to a country.
Ordinarily the terms, iron, cast iron and steel in reference to a metal in which the element iron (Fe) is the major element do not refer to a specific metal or alloy, but are loosely used to indicate af general type of iron alloy. The term iron should be used only when reference is made to the element iron (Fe).
In speaking of the commercial forms of iron such terms as Pig iron, grey cast iron, wrought iron etc. may be used. Each of these terms represents some commercial form of element iron, and each form may occur in many variations of chemical composition which influence the functions within each class. Due to tremendous production tonnage of these metals, and to their many forms and varied uses, a detailed study is not a simple and easy task.
Steel (master metal) is obtainable in great quantities, both in wrought and cast form. Its plasticity, whether at room temperature or at elevated temperatures, allows it to be worked either hot or cold. Its combination of strength with plasticity makes it the most important metal for use in large structures.
By varying the carbon content and by suitable heat treatments, we can alter the properties from a very soft workable steel of the type used, in pressed metal parts, wire and similar materials to a hard, strong steel suitable for use in tools and machinery where great strength and hardness are required.
Wrought iron is the oldest form of iron made by man. It was originally produced by slow reduction of the metal from the ore in the forge fire. This reduction process resulted in a very impure iron which required further refining by mechanical working that is by hammering or shaping to the form in which it is used.
Wrought iron is a metal containing high purity iron and iron silicate in physical association. It is very low in carbon and the iron silicate or slag is distributed throughout the base metal in fibers which gives it a woody or fibrous appearance when fractured.
Cast iron is fundamentally an alloy whose chief elements are iron, silicon and carbon. Cast irons are available with a wide range of properties. Pig iron, grey cast iron, white cast iron, chilled cast iron and malleable iron are all referred to as cast iron, chiefly because these forms of irons are not plastic enough, even when hot to be forgeable; therefore they are always produced commercially by a process of melting and casting into shape, the commercial form of each of these metals is in castings.
9. Iron Ores:
The principal ores which yield ferrous metals, the percentage of iron content and the countries where available are given in the Table 2.3.
10. Pig Iron:
Pig iron is the basic material from which, wrought iron and steel are manufactured. It is extracted from the abovementioned ores in a tall, continuous working furnace called ‘Blast furnace’. The product obtained from the blast furnace is crude and impure iron.
Composition of Pig Iron:
In addition to iron, pig iron contains varying quantities of other elements amongst which carbon, silicon, manganese, sulphur and phosphorus are the most important. These may amount to as much as 10% of the weight and 25% of the volume of pig iron.
Effects of Impurities on Iron:
The impurities (such as silicon, phosphorus, sulphur and manganese) affect the iron in the following ways:
A. Increases the fluidity of metal.
B. Induces softness in the iron.
C. Reduces the melting point but enhances the percentage content of uncombined carbon.
D. Produces castings which are free from blow holes.
2. Phosphorus:
A. Increases fluidity of metal.
B. Gives rise to cold shortness (brittleness at ordinary temperature).
3. Sulphur:
A. Encourages the formation of blow holes and makes the casting unsound.
B. Exercises an injurious effect on the metal, therefore its percentage should be kept below 0.1%.
C. Causes red shortness (brittleness at high temperature).
4. Manganese:
A. Increases hardness and brittleness.
B. Checks the bad effect of sulphur by forming MnS which is not injurious in small quantity.
Classification of Pig Iron:
Pig iron is classified by chemical composition into the following three grades:
1. Basic Pig Iron:
A. Basic pig iron must be low in sulphur (0.04%) since sulphur is an active impurity in steel and is not eliminated in the refining furnaces.
B. Carbon content varies from 3.5 to 4.4%. Phosphorus is normally held to less than 1% and magnese to a range of 1 to 2%.
C. It is used for steel making and is low in silicon (1.5% max.) to prevent attack of the refractory linings of refining furnaces and to control slag formation.
2. Foundry Pig Iron:
A. It includes all the types that are used for the production of iron castings.
B. Foundry iron contains- C = 3 to 4.5%, Si = 0.5 to 3.5%, Mn = 0.4 to 1.25%, S = upto 0.05%. P = 0.035 to 0.9% and Fe = remainder.
3. Ferro-Alloys:
A. These are alloys of pig iron, each rich in one specific element.
B. These are used as additives, in iron and steel industries, to control or alter the properties of iron and steel.
Examples:
(i) Ferromanganese—pig iron that contains from 74 to 82% manganese.
(ii) Ferrosilicon—pig iron with 5 to 17% of silicon content.
11. Cast Iron:
The product of the blast furnace i.e. pig iron is unsuitable for castings as it contains impurities in high percentage. To render it suitable for desired purpose it is refined in the furnace known as cupola. The refined product is termed as cast iron.
Cupola:
It is very similar to a blast-furnace in principle i.e., it is a vertical shaft furnace, into which the raw materials and fuel are charged at the top. Air for combustion of fuel is introduced through one or more rows of tuyeres a short distance above the bottom.
Since the cupola is only concerned with the melting of the metal and not with the reduction of ores as in the blast furnace, it is considerably smaller than a blast furnace of the same output. Its diameter varies from 1 to 2 metres with a height of 4 to 5 times diameter.
In a cupola, the first operation is to lit the fire at the bottom. When the fire is burning strongly, coke is added gradually till the level above the tuyeres is about 0.6 metres. This coke serves as a bed for the alternate charges of metal and coke which follow.
When the shaft of the cupola is filled level with the charging door the blast is put on and the combustion of the coke near the tuyeres increases rapidly until a very intense heat is attained. The gases of combustion move upwards and pass on a portion of the heat to the metal and coke waiting to descend.
In 5 to 10 minutes the first charge of metal starts melting and trickles down through the coke and finally collects at the bottom of the cupola. When an adequate quantity (say 1 or 2 tonnes) has accumulated the plug of clay called ’bout’ is removed from the tap hole and metal allowed to run into the ladle.
The temperature of tapping metal is 1200- 1400°C. After melting a number of charges as per requirements the bed coke is removed through a drop-bottom door and quenched with water so as to be available for use the next day.
Although it is usual practice to operate a cupola with cold blast (since no reduction of ores is required) a few cupolas have been equipped for hot blast. Whereas a blast furnace operates continuously, a cupola works intermittently.
12. Alloy Steels:
When certain special properties are desired some elements such as nickel, chromium, manganese, vanadium, tungsten etc. are added to the carbon steels. The steels thus obtained are called alloy steels.
The first investigation on the effect of alloying elements in steel were made from 1875 to 1890. But the use of alloyed steel found little application until 1901, when reduced cost of alloys made their use practicable.
Purposes of Alloying:
The alloying elements are added to accomplish one or more of the following:
1. To impart a fine grain size to steel.
2. To improve casehardening properties.
3. To improve elasticity.
4. To improve corrosion and fatigue resistance.
5. To improve hardness, toughness and tensile strength.
6. To improve machinability.
7. To strengthen the fertitie.
8. To improve high or low temperature stability.
9. To improve cutting ability.
10. To improve wear resistance.
11. To improve ductility.
The Effects of Alloying Elements:
Nickel:
(i) Increases toughness.
(ii) Improves response to heat treatment especially in large sections.
(iii) In large amounts provides special electrical and magnetic properties.
(iv) Improves forming properties of stainless steel.
Chromium:
(i) Provides stainless property in steel.
(ii) Used widely in tool steels and in electric plates.
Vanadium:
(i) Improves response to heat treatment.
(ii) Provides control of structure.
(iii) Used in high speed tool steels.
Tungsten:
(i) Retention of hardness and toughness at high temperatures.
(ii) Used in tools, dies, valves, magnets etc.
Silicon:
(i) High electrical resistance and magnetic permeability.
(ii) Used in electrical machinery.
Copper:
(i) In small amounts improves atmospheric corrosion resistance.
(ii) Acts as a strengthening agent.
Carbon:
(i) Affects melting point.
(ii) Affects tensile strength, hardness and machinability.
Silicon:
(i) Improves oxidation resistance.
(ii) Strengthens low alloy steels.
(iii) Acts as a deoxidiser.
Titanium:
(i) Prevents formation of austenite in high chromium steels.
(ii) Reduces martensitic hardness and hardenability in medium chromium steels.
(iii) Prevents localized depletion of chromium in stainless steel during long heating.
Molybdenum:
(i) Enhances corrosion resistance in stainless steels.
(ii) Makes steel usually tough at various hardness levels.
(iii) Promotes hardenability of steel.
(iv) Forms abrasion resisting particles.
(v) Raises tensile and creep strength at high temperatures.
(vi) Makes steel fine grained.
(vii) Counteracts tendency towards temper brittleness.
Manganese:
(i) Counteracts brittleness from sulphur.
(ii) Increases strength and hardness markedly.
(iii) Lowers both ductility and malleability if it is present in high percentage with high carbon content in steel.
Boron:
(i) Increases hardenability or depth to which steel will harden when quenched.
Aluminium:
(i) Acts as a deoxidiser.
(ii) If present in an amount of about 1%, it helps promoting nitriding.
Cobalt:
(i) Refines the graphite and pearlite.
(ii) Improves heat resistance.
(ii) Contributes to red-hardness by hardening ferrite.
(iii) It is a mild stabilizer of carbides.
(iv) Improves mechanical properties such as tensile strength, fatigue strength and hardness.
13. Phenol Formaldehyde (PF) — “Bakelite”:
a. PF is made by condensation polymerisation of phenol and formaldehyde in an acid or alkaline medium.
b. Asbestos, glass fibers, pigments and other additives can be added to improve its properties.
Properties:
(i) The colour range is limited to black or brown (however, it discolours on ageing by sunlight).
(ii) It is strong, rigid and dimensionally stable.
(iii) It is resistant to heat, most chemicals and solvents.
Uses:
(i) Lavatory seats
(ii) Thermal insulation as cellular/foamed
(iii) Paints and adhesives
(iv) Vacuum cleaner parts
(v) Electrical parts
(vi) Impregnants for paper and formica
(vii) Handles, knobs for domestic appliances.
14. Amine Formaldehyde (Urea and Melamine Formaldehyde):
a. These plastics are obtained by condensation of urea or melamine with formaldehyde.
b. These are highly cross-linked polymers.
c. They can be compounded with fillers, pigments and other additives to form moulding materials of different colours.
Properties:
Their general properties are similar to formaldehyde.
a. Melamine has more resistance to chemicals, heat and moisture. However, they are attacked by strong acids.
b. It is scratch free and more expensive.
c. It has better electrical properties.
d. It is slightly affected by the sunlight.
Other properties are given in Table 9.2.
Uses:
(i) Glues for plywood
(ii) Cellulose and foamed products
(iii) Paints
(iv) Surface coating
(v) Adhesive
(vi) Plugs, switches, buttons etc.
.
15. Polysters (Unsaturated):
a. These plastics are manufactured by condensation polymerisation of dicarboxylic acid (mallic acid) and dihydric alcohol (e.g., glycol) followed by curing with cross-linking agent (styrene).
b. A wide variety of products can be made by varying monomers and curing agents.
Properties:
(i) They have good resistance to heat and most chemicals except strong acids and alkalies.
(ii) They are affected by sunlight unless stabilized.
Other properties are given in Table 9.2.
(i) Safety helmets
(ii) Automobile body components
(iv) Paints
(v) Binder for glass fibers
(vi) Fiber glass boats
(vii) Jointing and repair work.
16. Epoxy Resins (Epoxies):
a. These are obtained by condensation polymerisation of epichlorhydrin and polyhydroxy compound (e.g., bisphenol).
b. As adhesive these materials have shown extremely high bond strengths without the need for pressure curing.
c. They are transparent, light amber colour and have very little shrinkage.
d. As coating materials, they have shown superior toughness, elasticity and chemical resistance.
e. To increase hardness and strength glass fibers or carbon fibers are compounded with epoxies. (Other properties are given in Table 9.2).
f. While handling epoxies, following precautions are necessary (because styrene can cause irritation and peroxide catalyst can cause skin irritation).
I. No use of inflammable solvents for cleaning;
II. No smoking, no naked flame;
III. Proper ventilation of room;
IV. Use of protective equipments like goggles, gloves etc.
Uses:
(i) They are used as an insulating material in cable-end boxes, cable joint boxes, instrument transformers etc.
(ii) Epoxies are used as/for adhesive (Araldite) and jointing and repair work.
(iii) They have also found considerable use as casting materials.
17. Glass Ceramics:
I. These are special glass compositions that are thermally treated prior to forming operations to divertify or precipitate a crystalline phase from the material; this phase gives that material special properties such as zero thermal expansion for applications involving high thermal-shock application.
II. The compositions (typical of glasses) in which nucleation and crystallization have been commercially produced are- MgO-Al2O3-SiO2; LiO2-Al2O3-SiO2; LiO-MgO-SiO2.
Characteristics:
(i) Very low coefficient of thermal expansion.
(ii) Relatively high mechanical strengths.
(iii) High thermal conductivities.
(iv) Can be easily fabricated (conventional glass-forming techniques may be employed conveniently in the mass production of nearly pore-free ware).
Uses:
(i) Owing to their excellent resistance to thermal shock and their high conductivity, glass ceramics are used as ovenware and tableware.
(ii) As insulators.
(iii) As substrates for printed circuit boards.
18. Dielectric Ceramics:
I. The use of ceramic materials is made both as electrical insulators and as functional parts of an electrical circuit. Since the electrical insulators can breakdown under high electrical voltages, the insulators are designed with lengthened surface paths to decrease the possibility of surface shorting. Since internal pores and cracks provide opportunity for additional surface failure, the insulators are glazed to make them non-absorbent.
II. Non-linear dielectric ceramics are suitable in the miniaturization of electronic parts which have led to the development of increasingly sophisticated electrical circuitry.
III. These ceramics are also used in capacitors.
IV. Some typical non-linear dielectric ceramics are- Lead zirconate-titanate, lead niobates, barium titanate, etc.
19. Electronic Ceramics:
Ferrites ferroelectric ceramics etc., are the ceramic materials with unusual properties that are of specific use in electronic circuits.
I. Ferrites are mixed-metal-oxide ceramics (almost completely crystalline). They assimilate high electric resistivity and strong magnetic properties. Soft ferrites can be used for specific uses such as memory cores for computers and cores for radio and television loop antennas. Barium and lead ferrites are widely used in permanent-magnet motors in automobiles, portable electrical tools and small appliances.
II. Ferroelectric ceramics can convert electrical signal into mechanical energy (such as sound); and can also change sound, pressure or motion into electrical signals. Thus they function as transducers.
Examples:
Barium titanate (most common), tantalates, zirconates, niobates, etc.
20. Cermets:
I. Cermets are ceramic-metal composites.
II. Cermets contain alumina (Al2O3) and chromium in varying proportions.
III. These are used in brake shoe linings, oxidation-resistant parts and inject engines.
IV. The most common cermet is cemented carbide and such like composites are extensively used as cutting tools for hardened steels.