Compilation of useful notes on metallurgy.
Note # 1. Introduction to Metal Forming:
Metal forming is the backbone of modern manufacturing industry besides being a major industry in itself. Throughout the world hundreds of million tons of metals go through metal forming processes every year. As much as 15-20% of GDP of industrialized nations comes from metal forming industry. Besides, it fulfills a social cause by providing job opportunities to millions of workers.
Metal forming industry, in general, is a bulk producer of semi-finished and finished goods and this is one reason that it is viable to undertake large scale research and development projects because even a small saving per ton adds up to huge sums.
In metal forming processes, the product shapes are produced by plastic deformation. Hence it is important to know the plastic flow properties of metals and alloys for optimizing the processes. Also the resulting component properties depend upon the intensity and the conditions of plastic deformation during forming.
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Many forming processes produce raw materials for other processes which in turn produce finished or semi-finished products. For example, steel plants produce sheet metal which is used by automobile industry to manufacture components of automobiles and their bodies.
In fact sheet metal is used by a number of manufacturers for producing a large variety of household and industrial products. Similarly billets produced by steel plants are used by re-rolling mills for rolling into products like angles, channels, bars etc.
Bars may be further used for manufacturing forgings, wires, bright bars and machined products. Similarly the manufacturers of rivets, screws, bolts and nuts buy wire from wire manufacturers and process them further. Therefore, the producers of semi-finished materials such as sheet metal, bar stock and wires, etc. have to consider that they produce such properties in their products which are required by downstream industry engaged in further processing of these products.
For example, deep draw ability of sheet metal increases with increase in anisotropy ratio, therefore, rolling parameters such as finishing temperature, cold reduction etc., are adjusted to produce higher anisotropy ratio in the sheet metal which is to be used for deep drawing.
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The properties of metals and alloys are highly influenced by their microstructure which may be modified or altered by alloying elements, by heating or heat treatment or by plastic reformation. For example, metals and alloys may be hardened by plastic deformation. It would, therefore, be helpful if we look at metals at the micro level.
Note # 2. Metal Forming Processes:
Metal forming processes are the oldest manufacturing processes known to mankind. In fact, metal forming started when man first learnt to change the shape of metal pieces by hammering. In those early days little was known about metals or alloys but the inquisitive human mind developed many processes by hit and trial. Even a village smith came to know how to forge a piece of iron to shape of an axe and how to harden and temper it.
Wire drawing process, for instance, was in practice as early as 1400 B.C. Then it was mainly used for making fine gold and silver threads for use in royal robes. Thus many different metal forming processes that we see today were developed on the shop floor. These have been grouped into a few basic ones like forging, rolling, drawing, extrusion, sheet metal forming etc.
Also, combinations of these processes have been developed in order to manufacture economically a greater variety of product shapes with the desired surface quality, dimensional accuracy and mechanical properties.
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In early days metal forming was regarded as primary process and components shaped by forming were finished by metal cutting and grinding. However, with the development of hard materials for tools and dies, components can now be made to near exact size, thus minimizing machining.
Metal forming has now become the back bone of modern manufacturing industry. A large variety of products are made by forming processes. The primary products which are either used as such or are used as raw material for other processes are bars, angle sections, beams, channels, sheets, plates, rails, rail wheels, drawn products such as wires and tubes, forged products such as shafts, crank shafts, gear blanks and gears, auto components etc. (Fig. 2.1) shows some of the products made by forming processes.
Most of the developments in metal forming processes have been the result of innovations on the shop floor. Theoretical analysis of various forming processes lagged behind because of complexities of theory of plastic flow of metals. Serious attempts in analyzing metal forming problems started only during World War II. Even today, designs of some metal forming dies and tools are based on empirical data.
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Nevertheless, the theoretical analyses, though not exact, have been extremely helpful in understanding the metal flow conditions and in optimizing the machines, tools and process parameters so that products with better surface quality and micro- structure are obtained and there is saving of energy.
A component may be manufactured by casting, machining, welding, forming or by other processes.
As compared to the various processes, metal forming has following advantages:
(i) During forming the cast structure gets homogenized. The grain structure gets refined. This results in better mechanical properties.
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(ii) Shrinkage cavities and some blow holes get welded. Forming by compressive forces reduces voids in the material.
(iii) The component shapes are produced by plastic deformation, therefore, there is no loss of material in the form of chips. However, in case of some components material is lost by the way of trimming as in die forgings and in sheet metal blanking.
(iv) In hot forming the material has low strength and higher ductility, therefore, large components may be formed.
(v) During forming the grains and the impurities get compressed in the direction of compressive forces, and are elongated in the directions perpendicular to it. Thus if we see the deformed structure under a microscope it looks as if fibers are developed. (Fig. 2.35) shows some examples. The material gets directional properties and can take high loads across the fibers.
(vi) In cold forming, the strain hardening improves the mechanical properties of the formed components.
(vii) The directional properties which develop during cold rolling of sheet metal is usefully employed to get deeper draws in sheet metal forming.
(viii) Also in cold forming high surface finish and very close dimensional tolerances may be obtained.
Note # 3. Work Hardening for Strengthening Metals:
During plastic deformation most of the metals and alloys become stronger due to work hardening and develop directional properties.
The work hardening effect may be taken as consisting of following two factors:
i. Isotropic Work Hardening:
In this case the yield strength increases equally in all directions. It is illustrated in Fig. 4.7. The extent or magnitude of work hardening is generally related to plastic work done or the total strain suffered by the material. The relationship between flow stress and the strain suffered by some materials. For use of this data in complex conditions of strains we may take the effective strains and effective stresses.
ii. Kinematic Work Hardening:
In this case the yield strength may not increase in magnitude but the whole of the yield diagram shifts in the direction of strain vector (Fig. 4.8). It explains the Bauschinger effect. The magnitude of shift may be related to the magnitude of strain suffered. Very few attempts have been made to determine this relationship. The data on the relationship of shift of yield diagram with the strain suffered by the material is still very scanty.
Taking both these effects in an analysis is rather difficult, therefore, most of the researchers prefer to work with isotropic work hardening. The formulation of the shift in case of kinematic work hardening is not in the scope of present text.
Note # 4. Products Produced by Longitudinal Rolling of Metals:
Of all the different rolling processes, major developments have taken place only in longitudinal rolling, simply because the maximum variety of long products as well as maximum tonnage of metals is rolled by this process. Also of all the metals and alloys the maximum tonnage that is rolled comprises low and medium carbon steels.
It is quite natural that most of the development in rolling technology and equipment have been evolved with respect to these two alloys. Some products produced by longitudinal rolling are illustrated in Fig. 8.1.
The raw material for rolling various shapes is the ingot which is cast out of molten metal. In case of low carbon steels the ingot is quite large. It is first rolled into blooms (Fig. 8.1). The blooms are rolled into smaller sizes called billets. Large structural sections such as rails, beams, girders, channels, angle sections are also rolled out of blooms. Bloom and billets are also rolled into cylindrical shape for further rolling into seamless tubes.
The above practice still goes on in older plants. But now a days the trend is to install continuous casting units to cast smaller sections directly from liquid metal and thus eliminate bloom rolling. In one such machine shown in the Fig. 8.2 the molten metal is poured into tundish from where it is regulated into a groove formed by an endless steel band and a grooved copper wheel.
The steel band moves with the wheel till it is detached from the copper wheel by a band guide rolls. By this time the cast billet is partially solidified particularly on its surfaces. The cast billet is detached from copper wheel with the help of a knife, it is straightened out and further cooled to rolling temperature.
In some plants billets of still small cross sections may also be continuously cast thus eliminating even the billet rolling mill. Installation of continuous casting results in substantial saving in capital cost of the plant. That is why the continuous casting plants are becoming popular.
Steel alloys such as stainless steel have a very narrow temperature range for rolling and hence it requires repeated reheating for rolling into final product. An alternative technology based on powder metallurgy has been developed, in which the molten metal is first turned into metal powder which is rolled into sheets and strips. The resulting green strip is sintered and further rolled into finer gauges. The metal particles get welded and the final strip is as sound as that rolled conventionally.
Note # 5. Thread Rolling:
Thread rolling is widely used in industry engaged in the manufacture of bolts, nuts and screws. Almost all types of threads such as Vee threads, acme, worm, buttress, metric, single start and multiple pipe threads and other special forms may be rolled.
The machines are designed for plunge type action as well as through feed rolling so that headed components as well as studs may be rolled. The available machines can roll threads on diameters of a few mm to as big as 600 mm. Precision threads such as in lead screws, ball screws, gear box worms and high helix threads for mechanical actuators are possible to roll.
Three types of thread rolling processes are in use:
(i) Thread rolling by flat dies.
(ii) Thread rolling by cylindrical dies.
(iii) Planetary thread rolling.
One of the dies may be kept stationary while the other rolls the job on it. The process is generally carried out in cold state. However, for large diameters and coarse threads the blank may be heated to reduce the load on dies and the machine.
Along with rotation, the rolls may be given ‘in feed’ motion to roll the threads. Side stays are provided so that the job does not slip out. This problem is not there in a three-roll thread rolling machine. The job is automatically located centrally with respect to the three rolls.
The planetary thread rolling machine shown in Fig. 12.31 consists of an inner cylindrical roll on the outer surface of which the thread forms are machined. On the outer side of this roll, segmented circular dies are fixed. The jobs are introduced at the inlets provided between the dies and after rolling they get out of similarly placed outlets. If there are three segmented dies, three bolts can be threaded simultaneously.
In other machines instead of segmented dies single circular cylinder having thread forms on its inner surface is fixed eccentrically with respect to the inner threaded roll. The jobs are introduced between the two cylinders at the wider side of the gap between the two.
The thread are rolled as the job passes through the narrow side of the eccentric gap between the two cylinders, and are taken out on the wider side. Planetary thread rolling machines can roll 2000 pieces or more per minute.
Modern thread rolling machines can also roll shapes on the bar besides threads. Some shapes rolled on thread rolling machines are shown in Fig. 12.32.
Note # 6. Lubrication in Extrusion Process of Metals:
Both in cold as well hot extrusion the appropriate lubricant properties are of utmost importance. The tool design has to be such that lubrication persists through out the process. It should not get squeezed out in the initial stages of deformation or get locked in the tool cavities due to improper metal flow conditions.
The general properties for extrusion lubricant are as under:
1. Provide a lubricant film on work piece and tool interface surfaces.
2. The lubricant should not get squeezed out before the extrusion starts or during the process.
3. Should prevent metal to metal contact between tool and work piece during the process.
4. Should provide effective insulation between work piece and tools. Thus in case of hot extrusion it should inhibit the flow of heat from work pieces to tools.
5. It should be stable at the working temperatures.
6. Non-toxic in nature. Should not be a health hazard during handling or application of lubricant.
7. Low cost and should be easily available.
8. Should be easily remove-able from extruded components.
9. Should not react and damage tool or work piece materials.
For cold extrusion of carbon steels and alloy steels phosphate coating as lubricant carrier is universally used. Without the coating it is not possible to carry out cold extrusion of steel.
The general steps for coating are as below:
1. Clean the surfaces to be coated of oil and grease by an alkaline solution.
2. Water rinse.
3. Pickle in acid to remove oxides.
4. Rinse in cold and hot water. Dip in a solution of phosphoric acid and Zn-phosphate.
5. Rinse in water.
6. Apply lubricant.
Oxalate Coating:
High alloy steels including stainless steels do not respond to phosphate coating. For these steels oxalate coatings are used as lubricant carrier. The lubricants generally used are metallic stearates (soaps), molybdenum disulfide or their mixtures.
The general steps are as below:
1. Modification of scale by dipping in a molten salt bath.
2. Water rinse.
3. Acid pickling – hydrochloric or sulfuric acid.
4. Rinsing in water.
5. Pickling in a solution of nitric acid and hydrochloric acid.
6. Water rinsing twice.
7. Application of coating by dipping in oxalate bath.
8. Rinsing in water.
9. Neutralizing rinse.
10. Application of lubricant.
Lubrication in Warm Extrusion of Steel:
The warm extrusion of steel is generally carried out in the temperature range of 300°C to 500°C. The work piece may be coated with a dry coating prior to heating the billet. Graphite is the main ingredient in these coatings.
At temperatures higher than 550°C the oxidation of graphite limits its usefulness. However, commercial preparations containing graphite are available which may be used up to 800°C.
Lubrication for Hot Extrusion:
For extrusion of non-ferrous alloys such as of aluminium which are generally extruded between 350° C to 500°C, oil and graphite are the main lubricants. Copper-zinc alloys which are hot extruded between 700°C – 850°C oil and graphite are the general lubricants.
Hot extrusion of steel alloys are carried out at temperatures more than 1100°C. In such cases glass is found be the best lubricant. The method of application consists of (i) rolling the hot billet over glass powder or (ii) spraying hot billet with glass powder or (iii) wrapping the billet with glass fiber.
Besides this, a disc of glass is also placed on the dies before introducing the billet in the cylinder. As the extrusion proceeds the glass disc provides a continuous supply of lubricant as it melts under hot billet and prevents metal to metal contact between die and the billet.
Note # 7. Central Burst in Direct Extrusion of Metals:
Central burst also known as chevrons occur in direct cold extrusion of steel and in wire drawing. It is observed to occur in the final light reduction step in multistage extrusion sequence. The defect is the periodic appearance of arrow shaped internal cracks.
On the surface, the extruded component appears sound, the crack may be detected by ultrasonic crack detection method or by destructive testing. The load carrying capacity of the component is obviously seriously affected. Thus the extruded shafts and other such components which are used at critical applications in automobiles and aircrafts must be tested for central burst.
The defect occurs with certain combination of die angle, reduction, frictional stress and work hardening. Zimerman et al. have carried out an experimental investigation on 1000 pieces of 4 different heats of 1024 steel. It is concluded that the bursting criterion developed by Zimerman and Avitzur may be used successfully for designing the extrusion sequence.
The criterion is shown graphically in Fig. 10.23 which shows the safe and unsafe zones of parameters such as friction, strain hardening index, semi die angle that can cause or prevent the occurrence of central burst.
Note # 8. Tractrix Dies and Die Profile:
Ordinarily cup shaped components cannot be made without adequate blank holder pressure which is required to prevent the sheet from buckling and wrinkle formation. However, with tractrix dies sheets may be drawn without any blank holding pressure. The process is cheap because there is no blank holding.
The process may be combined with ironing of wall. The combined drawing and ironing may be carried out in the same stroke. Shape of a tractrix die is illustrated in Fig. 11.43(a). It was first suggested by May, Shawki suggested a simpler profile illustrated in Fig. 11.43(b) which is easier to manufacture though not as effective as tractrix curve.
Manish carried out experimental work on brass, M.S. and aluminum sheets using tractrix die. With brass and M.S sheets of 1 mm thickness a draw-ability of as much as 1.35 could be obtained. He also carried out experiments with aluminum sheets of 1, 2 and 3 mm thickness. The results are given in Table 11.5.
For successful draws the ratio of blank diameter to sheet thickness varies from 25 to 40.
Note # 9. Forming Limit Diagram for a Sheet Metal:
Apart from cup drawing, sheet metal is formed in a large variety of shapes which involve complex state of strain paths and total strains. The cup drawing tests are of little help for predicting whether the particular forming operation for forming non-symmetric component would be a success or not. There is no theoretical method to predict the same.
The forming limit diagram (FLD) which gives an indication whether the material can sustain certain ratio of strains without failing is of great help. Keeler pioneered the application of forming limit diagrams (FLD) for accessing the formability of sheet metal. Many researcher.,s have tried to determine the FLD diagrams for different sheet metals commonly used in forming operations either experimentally or theoretically.
The experimental method consists of printing a grid pattern of circles of appropriate diameter (generally 2 mm to 4 mm) on the surface of sheet. The sheet is deformed as required but in stages. After each stage the grid pattern is examined. The advantage of printing circular grids is that during the deformation the circles will get deformed into ellipses with their major and minor axes directed along the principal directions of strain.
The measurement of the axes and knowing the original circle diameter we can determine principal strains and their directions. As the forming progresses, at some region neck formation may occur. The ratio of strains is determined at the region.
This is a point on FLD diagram or curve which separates the safe and unsafe regions. The states of different strain ratios may be obtained by drawing strips of different widths with the help of hemispherical punch and determining the strain ratio when necking occurs.
It is better to take circular blanks and cut off material on two diametrically opposite sides to prepare the specimen. Since the test results are dependant on test conditions it is necessary to lay down standard test conditions.
The factors which affect the FLD diagram are as below –
(i) Material properties—strain hardening and strain rate exponent.
(ii) Thickness of sheet—FLD for thicker sheet is placed higher than for a thinner sheet with little or no change in the shape of diagram.
(iii) Anisotropy in the sheet.
(iv) The forming limit curve of softened sheet of the same alloy and same thickness is positioned higher to that of hard sheet.
(v) Type of coating on the sheet.
(vi) Type of pre-straining prior to testing. The FLD may be modified by altering strain path. It can be positioned higher by selecting proper strain path.
(vii) The orientation of test specimen with respect to rolling direction.
The material property such as strain hardening exponent n help to distribute the strain in the sheet, so the sheet can bear higher strains. The strain-rate sensitivity index (m) in fact helps to distribute the strain after the neck has started. Naturally material can bear higher strain before fracture.
For the same material the FLD curves for thicker sheets are placed slightly higher than for thinner sheets. Mahmudi has investigated the FLD of aluminum in biaxial stretching and compared the limit forming curves obtained by him with the ones obtained by other investigators for brass (70/30), cold rolled aluminum, and 2936 T6 aluminum and aluminum killed steel. Figure 11.32 show a comparison of FLDs for brass, steel and aluminum determined by different researchers.
Effect of Coating the Steel Sheet on Drawability:
Gronostajski et al. have investigated the effect of strain path on pre-coated sheet steel. Following four coatings have been tested and compared with bare sheet-
(i) Polyethylene- Zn-coated steel.
(ii) Acryl-Zn – coated
(iii) PVC-Zn-coated
(iv) Only Zn coated.
The coatings being of soft metal, help reduce the co-efficient of friction between the work piece and tools and thus enhances the formability. The average value of coefficient of friction (µ) observed for bare steel was 0.19 while for Zn-coated sheet was 0.16. The PVC coating gives the lowest value of µ, i.e. 0.09.
For polyethylene-Zn-coated and acryl-Zn-coated sheet the values observed are 0.11 and 0.13 respectively. The effect of coating on FLD is shown in Fig. 11.33. Hillier has also observed that coating affects the drawability because it reduces the coefficient of friction on the die surface. The LDR could be increased from 2.11 to 2.33 by Teflon (PTFE) coating.
Pre-straining alters the shape of FLD along with shifting of the diagram latterly. The FLD diagrams for uni-axially deformed materials are shifted towards the negative minor strain and for the equi-biaxially pre-strain specimens the FLD diagrams are shifted toward positive minor strain.
Use of aluminum for automobile bodies is gaining popularity. Being lighter than steel, it reduces weight and hence the fuel consumption. Therefore, there is renewed interest in study of formability of aluminum. It is generally accepted that formability of aluminum alloys is inferior to that of deep drawing quality low carbon steels.
Kohra has done experimental study on development of forming limit diagram (FLD) for non-hardening aluminum alloys. He also studied the effect of pre-straining on FLD. His work concludes that forming limit curves may be shifted up by suitable pre-strain. The softer variety of same alloy gives FLD higher than for hard variety.
Note # 10. Modes of Redrawing Sheet Metals:
Redrawing of already drawn cup is often necessary for the following objectives:
(i) To reduce the diameter or to increase the length of cup.
(ii) To reduce the wall thickness of cup.
The maximum drawing ratio in the first draw is in general limited to 2, i.e. blank diameter/cup diameter ≈ 2. The ratio may be increased by having material with higher draw-ability or by drawing in a pressure chamber, i.e. hydro-mechanical deep drawing or aqua-drawing. These processes are described below.
In all these processes we cannot get cups with ratio of cup height to diameter ratio of more than 0.75-1.2. The cup height may be calculated by equating the volumes of blank sheet with that of cup and if the thickness of cup wall is nearly same as that of blank, we may get the cup height by equating the surface areas of blank with that of cup. Thus if we take zero corner radius of cup we may write,
However, for exact calculation, the corner radii after each draw has to be considered as well.
The general values of λ1, λ2, λ3 as recommended by various researchers are given in Table 11.3. Because of work hardening the drawing ratio decreases after every draw. In case of multiple draws the profile radii of punch and die should be higher in the first draw and it should successively decrease in subsequent draws. The influence of these radii on draw-ability is discussed below.
The following three types of redrawing modes are used, these are illustrated in Fig. 11.18:
(a) Redrawing with Retaining Ring:
The process is used for small diameters. The corner radius is progressively decreased after each redraw.
(b) Redrawing thorough a Conical Die:
This is more common method. The conical die surface is generally at 45 degrees with the vertical axis. In this process the material flow is in better condition because there is less shear both at entry and exit from die. Therefore, lower forces are required.
(c) Reverse Redrawing:
In drawing the different layers of material across the thickness of sheet suffer different strains. In reverse redrawing those very layers suffer strains of opposite type. Because of Bauschinger effect the material yields at a lower stress and hence it is easier to deform the material.
Note # 11. Drawing of Tubes from Annular Sheet Discs:
In many applications tubes of short length are required. If we wish to make such a component from sheet metal, we will have to draw the sheet into cup shaped component and then shear off the bottom. However, by a technique developed by Juneja the annular flat discs of sheet metal can be drawn directly into tubular components.
The tools required are:
(i) Die
(ii) Blank holding plate
(iii) A steel ring with tapered internal surface
(iv) Punch
The assembly of the equipment is mounted on a machine in which the ram moves upwards. The steel ring is loosely kept in the recess in the die. Blank with proper size hole is placed between die and blank holding plate. The blank holding pressure is applied. The punch is moved up. First the punch enlarges the hole and makes a flange which goes into the ring.
The top potion of punch is also tapered. With further motion of punch the flange gets clamped automatically between tapered surfaces of ring and punch. As the punch moves up, the ring, the flange and punch move together thus pulling the remaining sheet into a tube. It is similar to the part of deep drawing in which sheet between blank holding plate and die is pulled into cup wall.
Note # 12. Tests Performed on Sheet Metal:
It would be worthwhile to know about the tests that are generally carried out on sheet metal to test the suitability for deep drawing or forming.
i. Swift Cup Draw Test:
In this test flat bottom cups are drawn out of the sheet metal to be tested for drawability, however, each time the blank diameter is increased by certain value. The maximum diameter that may be drawn successfully is determined.
The test aims to determine the limiting drawing ratio for the sheet. It is therefore necessary to standardize the procedure, the punch size, die clearance, tool surface roughness and lubrication. This test is commonly done in industry.
ii. Erichsen Cupping Test:
Circular disc of the material to be tested is pressed between two plane surfaces with a set force. A spherical punch is pressed against the sheet at its centre. This deforms the sheet into a dome shape at the centre. The pressing is carried out till a crack appears on the dome. The height of dome is taken as the index for drawability of sheet. It is simpler than Swift cup test. However, the correlation between the drawability determined by Swift cup draw test and Erichsen test has not been established.
iii. Sachs Wedge Draw Test:
This test is a simulation of flange drawing in the deep drawing operation. Wedge shaped sheet metal strip is drawn through conical die. This represents a piece of flange between two radii. Let w be the outlet width of die and wm be the maximum width of strip that can be drawn. The ratio wm/w is an indicator of drawability.
iv. Fukui Cup Drawing Test:
In this test the circular sheet blank is drawn through a conical die with a circular punch but without a blank holder. The drawing is continued till a crack appears. The ratio of the initial blank diameter to the minimum cup diameter at which the crack does not occur represents the drawability of sheet metal.
Note # 13. Constituents of Iron and Steel:
The different microscopic constituents of iron and steel which commonly occur are:
1. Ferrite
2. Cementite
3. Pearlite
4. Martensite
5. Austenite
6. Troostite
7. Sorbite.
The other constituents comprise the three allotropic forms of nearly pure iron, graphite and slag.
1. Ferrite:
Iron which contains little or no carbon is called ferrite. It is very soft and ductile and is known as alpha iron by the metallurgists. Ferrite is present to some extent in a great range of steels, particularly those low in carbon content, and it is also present, in soft cast iron. Ferrite does not harden when cooled rapidly. It forms smaller crystals when cooled from a bright red heat at a rapid rate.
2. Cementite:
This is a definite carbide of iron (Fe3C) which is extremely hard, being harder than ordinary hardened steel or glass. Cementite increases generally with the proportion of carbon present, and the hardness and also the brittleness of cast iron is believed to be due to this substance.
It contains 6.6 percent carbon and occurs either in the form of a network or in globular or massive form, depending on the analysis of the steel and the heat treatment to which it is subjected. It is magnetic below 25°C. Its presence in iron or steel decreases the tensile strength but increases the hardness and cutting qualities.
3. Pearlite:
Pearlite is the name given to a mixture of about 87.5 percent ferrite and 12.5 percent cementite. It consists of alternate layers of ferrite and cementite in steel. Under high magnification the ferrite and cementite can be seen to be arranged in alternate laminations or plates.
When seen in the microscope the surface has appearance like mother of pearl, hence the name pearlite. The thickness of alternate plates and the distance between them is governed by the rate of cooling, slow cooling produces a coarser structure than rapid cooling. Pearlite is eutectoid of steel.
It has been found that the proportion of pearlite increases from nothing in the case of pure carbonless iron upto 100%, or saturation, for steel containing 0.90% of carbon thus a 0.3 percent carbon steel will consist of about 33 percent pearlite and rest ferrite. It is the characteristic of soft steels that they contain ferrite and pearlite, and the hardness increases with the proportion of pearlite. Hard steels are mixtures of pearlite and cementite.
4. Martensite:
It is hard brittle mass of fibrous or needle like structure and is the chief constituent of hardened steel. The vickers pyramid numeral is anything upto 900 for an original carbon content of 0.9 percent. It has been found that martensite is produced by the rapid quenching of high carbon steel from a slightly higher temperature than the maximum temperature of critical interval. It is not as tough as austenite. It differs from austenite in being magnetic.
5. Austenite:
It is a solid solution of iron-carbon which is stable only within a particular range of composition and temperature, and is non-magnetic. On cooling below 700°C it is completely transformed into ferrite which is magnetic and cementite to form the eutectoid pearlite, together with free ferrite or free cementite, depending on whether the carbon content is less or greater than 0.87 percent respectively.
It is formed when carbon steel with more than 1.1 percent carbon is quenched rapidity from about 1000°C. The amount of austenite increases with the proportion of carbon, 0 upto 1.1 percent carbon, upto 70 percent for 1.6 to 1.8 percent carbon. Austenitic steels cannot be hardened by usual heat treatment methods and are non-magnetic.
6. Troostite:
It is a structure in steel (consisting of very finely divided iron carbide in what is known at “alpha-iron”) produced either by tempering a martensitic steel at between 250 and 450°C or by quenching steel at a speed insufficient to suppress the thermal change point fully. The structure produced by the latter method should be more accurately termed very fine pearlite.
7. Sorbite:
It is a structure which consists of evenly distributed carbide of iron particles in a mass of ferrite, formed when a fully hardened steel is tempered at between 550 and 650°C. A sorbitic structure is characterised by strength and a high degree of toughness.
Note # 14. Aluminium:
Bauxite is first purified and then dissolved in fused cryolite (double fluoride of aluminium and sodium). The aluminium is then separated from this solution by electrolysis at about 910°C.
Physical and Mechanical Properties of Aluminium:
(i) Pure aluminium has silvery colour and lustre, while the commercial grades show a characteristic bluish tinge.
(ii) The high purity aluminium has a much greater resistance to corrosion than the ordinary steel.
(iii) It is ductile and malleable.
(iv) Its specific gravity is 2.7.
(v) In proportion to its weight it is quite strong.
(vi) Melting point = 658°C, boiling point = 2057°C.
(vii) Its electrical resistivity is 2.669 micro ohms/cm3 at 20°C.
(viii) Its tensile strength varies from 95 to 157 MN/m2.
(ix) It is a good conductor of heat and electricity.
(x) It forms useful alloys with iron, copper, zinc and other metals.
(xi) It is unaffected by ordinary atmospheric influences but is corroded in sea water. It is soluble in solutions of caustic alkalies and in hydrochloric acid. When there is an excess of silicon present in the metal it does not withstand atmospheric actions.
(xii) Aluminium is electron positive to most other metals e.g., iron, chromium, zinc, copper, nickel, tin, lead etc. Care is necessary therefore, to prevent it from coming into metallic contact with other metals under conditions where moisture is present in order to avoid electrolytic action i.e., corrosion.
Uses of Aluminium:
1. Because of its softness and difficulty of making sound castings, little pure aluminium is used in the cast form. The largest quantity is employed after it has been mechanically worked in some manner, as by rolling, wire drawing, drop forging or extruding. After being rolled into sheets, it may be stamped into a variety of shapes.
2. It is employed, often alloyed with small amounts of other metals, in the manufacture of furniture, rail-road and trolley cars, automobile bodies and pistons, electric cables and bus bars, rivets, kitchen utensils and collapsible tubes for pastes.
3. In a finely divided flake form, aluminium is employed as a pigment in paint. Aluminium paint is used as a priming coat for wood, as a protective coat for metals.
4. Aluminium is used in deoxidizing molten iron and steel, especially in the top of the ingot when steel is poured in the ingot mould. In a similar manner it is used to prepare the metals from their oxides by heating a mixture of powdered aluminium and the oxide of the metal to be reduced. The mixture is known as Thermit.
5. Aluminium foil is used as silver paper for packing chocolates etc.
Annealing of Aluminium:
Aluminium sheet which has been hardened by cold working, such as hammering or rolling can be annealed by heating it to about 350°C and afterwards cooling in air or water. The period of heating need only be for a few minutes. As a rough guide to correct temperature of heating for annealing, the surface may be rubbed, during the heating process, with a dry matchstick from time to time, the heating being stopped when wood begins to char.
The annealing of aluminium wire for electrical purposes require more care, since the aim is to obtain a high electrical conductivity by obtaining the correct structure. For high conductivity a long exposure at a lower temperature, namely 250°C to 300°C appears to give the best results. In passing, it may be remarked that the aluminium alloys require higher annealing temperature, from about 350°C to 400°C.
Note # 15. Fibers:
The fibers is a filament or thread like piece of any material. This term sometimes also refer to a raw material that can be drawn into threads.
Fibers may be of the following types:
1. Mineral fibers.
2. Animal fibers.
3. Vegetable fibers.
Mineral fibers include asbestos, glass fiber, slag wool and metal wool. Asbestos due to its fibrous structure combined with inflammability is peculiarly suited for thermal and electrical insulation. Glass fibers and slag wool are employed for thermal insulation. The metal wool is used for fibers and cleaning applications.
a. Animal fibers are of two types:
(i) Animal hair obtained from sheep, goats, pigs etc.;
(ii) Silk produced by the mulberry silkworm or from the larvae of other moths.
b. The most important hair fiber is sheep wool. Its length varies from 2.5 to 20 cms and its diameter from 0.0045 to 0.01 cm. Pig bristles are used in paint brushes. Wool waste for packing glands etc. is made from carpet yarn with fibers not less than 8 cm long.
c. Silk from the mulberry silkworm possesses high strength while varieties obtained from other moths are of lower strength comparatively.
Vegetable fibers consist mainly of cellulose. They may be seed hairs, such as cotton, or the inner bark of plants, such as flax, hemp, jute. The colours of cotton, flax and jute and hemp fibers are white, grey brown and brown respectively. The woody types are stiff and brittle whereas those with a high proportion of cellulose are flexible and elastic.
Cotton is used for making batting, guncotton, cloth and jute is obtained from the stalks of the jute plant by retting (a process which involves steeping in water). The short fibers are used in paper making whereas longer ones are used in the manufacture of coarse woven fabrics such as burlap, carpet bindings, as a substitute for hemp in turn and small ropes, as a filler in cable, and as adulterant for other fibers.
Hemp is the name given to the fibers obtained by retting the hemp plant. It is very strong and of great durability. Some grades of hemp are of dark brown colour and contain a mixture of cellulose and ligno cellulose. They are more hygroscopic than cotton, less affected by moisture and are disintegrated by bleaching. They are used mainly for turn and rope.
The following points in respect of fiber polymers are worth noting:
(i) The fiber polymers are capable of being drawn into long filaments having at least 100: 1 length-to-diameter ratio. Most commercial fiber polymers are utilized in the textile industry, being woven or knit into cloth or fabric. The aramid fibers are employed in composite materials.
(ii) The molecular weight of fiber materials should be relatively high.
(iii) Fiber polymers most exhibit chemical stability to a rather extensive variety of environments, including acids, bases, bleaches, drycleaning solvents, rid sunlight. In addition they must be relatively nonflammable and amenable to drying.
Production/Manufacture of Synthetic Fibers:
There are, in general, following three methods of producing synthetic fibers:
(i) Melt extrusion;
(ii) Dry spinning;
(iii) Wet spinning.
a. In this method of producing synthetic fibers, a polymer is forced through a series of orifices, through a spinneret and solidifies into thin filaments by stream of cool air.
b. Polyamide (Nylon), Dacron and PVC are produced by melt extrusion.
a. Dry spinning is accomplished by dissolving the polymer in a suitable solvent which is then extruded through a spinneret. As warm air is circulated in the system, the solvent evaporates and the filament solidifies.
b. This method is used to produce the commercially known copolymers Orion (cellulose acetate) and Acrilon (polyacrylonitrile).
a. This method is used to produce rayon fibers.
b. An alkaline viscous solution is forced through a spinneret into an acid bath where coagulation of fibers takes place resulting in solid filaments.
c. After the production of fibers the next treatment is improving the chain molecules by stretching the filament. To increase crease resistance, water-proofing and fire proofing qualities, a number of chemical treatments are required. Like polymers metals and glass fibers are also produced and familiar example is of fiber glass which has many industrial applications (e.g., safety helmets, racing cars, bullet proof bodies of automobiles and durable furniture items) because of its high strength.
Note # 16. Copper:
Manufacture of Copper:
Copper is extracted from its ores by several different methods that chosen depending upon the character of the ore and local conditions.
Refining of the metal is usually considered to begin when the copper is in the blister stage, the surfaces of the cast material being irregular and blistered due to the generation of gases during cooling. This copper is 99% pure and is further refined in the furnace by oxidation process which removes sulphur and other impurities.
The excess of oxygen is removed from the metal by operation known as poling. Green wooden poles or tree trunks are thrust under the surface of the molten metal, which is covered with charcoal, coke or similar material rich in carbon.
Although this process may seem crude it is still almost invariably used, owing to its cheapness and efficiency. Poling is discontinued when sample casting indicates that the oxygen content has been reduced to 0.08 – 0.025 percent. The copper is then known as tough pitch copper.
Deoxidized copper is needed for intricate castings, welding and certain other processes. Special reducing agents such as phosphorus, silicon, lithium, magnesium, beryllium or calcium are added to the molten metal to eliminate the oxygen just sufficient of the deoxidizer being used to prevent any residue in the metal.
Arsenic, in amounts upto approximately 5 percent is added to improve the strength and toughness of the metal, and most copper products, other than electrical gear, are manufactured from arsenical copper.
Physical and Mechanical Properties of Copper:
(i) Copper is a reddish-brown metal.
(ii) Although pure copper is one of the best conductors of heat and electricity, its electrical conductivity is highly sensitive to the presence of impurities.
(iii) If copper is heated to red heat and cooled slowly it becomes brittle; but if cooled readily it is soft, malleable and ductile. The brittleness is due to the coarsely crystalline structure that develops during slow cooling.
(iv) Copper can be welded at red heat.
(v) Like aluminium, pure copper does not cast well. When molten it absorbs gases, such as carbon monoxide, hydrogen and sulphur dioxide which separate out on cooling and cause blow holes.
(vi) Melting point = 1084°C, boiling point = 2595°C.
(vii) Specific gravity = 8.9, Electrical resistivity = 1.682 microhms per cm.
(viii) It is highly resistant to corrosion by liquids.
(ix) Its tensile strength varies from 300 to 470 MN/m2.
(x) It forms important alloys like bronze and gun metal.
(xi) It is strongly attacked by nitric acid but only very slowly by dilute hydrochloric and sulphuric acids in the absence of air; ammonical solutions also attack copper.
For copper that is to be worked, lead is sometimes added in order that metal may be worked more easily, but if more than about 0.5 percent is employed, it causes the copper to be brittle. In copper alloys, for casting, as much as 10 to 20 per cent may be added to cheapen the product but this lead largely separates out in globular masses on cooling. At the melting point of lead, copper is practically insoluble in liquid lead and less than 0.05 percent of the lead enters into the solid solution in the copper.
Uses:
1. It is largely used in making electric cables and wires and electric machinery and appliances.
2. Used in electroplating, electrotyping and I or soldering iron bits.
3. Used as a damp proof material and for making alloys.
4. It is used for sheeting, roofing, spouts, boilers, condensers and other purposes where corrosion resistance with fair strength and flexibility is essential.
Annealing Temperature of Copper:
The annealing temperature of copper varies between 200°C and 600°C, according to the impurities present and condition of the metal, i.e., upon the amount of cold work it has been subjected to. Work-hardened copper of high purity may be partly softened at temperature as low as 120°C, but the minimum softening temperature is usually at least 200°C. For most commercial copper a temperature of 500°C is employed but for heavy sections the furnace temperature may be raised to 600°C.
After raising to the annealing temperature it may be cooled in any convenient manner, namely, in air or by water quenching; the latter method facilitates the removal of dirt and scale.
Note # 17. Wrought Iron:
Manufacture of Wrought Iron:
The manufacture of wrought iron from pig iron (laborious and expensive process) involves the following operations:
1. Refining
2. Puddling
3. Shingling
4. Rolling.
1 Refining:
This operation consists in passing through the molten pig iron a strong current of air and keeping it well stirred in order that liquid mass may come in complete contact with air and get oxidised. The oxygen present in the air eliminates a portion of carbon and some other impurities. The liquid mass or iron is then cast into moulds and made brittle by sudden cooling.
2. Puddling:
Puddling consists in melting the refined pig iron (broken into lumps) in a reverberatory furnace. The term ‘reverberatory’ is applicable to any furnace in which the charge does not come in direct contact with fire but receives heat from the roof by reflection.
The molten metal is mixed with oxidising substances such as haemetite, oxide of iron etc., and subject to enormous amount of heat and strong current of air. Major portion of carbon content remaining in the iron gets converted into carbonic acid gas. The silicon gels oxidised and is removed in the form of slag. As the iron is purified, it thickens up and removed from the furnace in the form of balls. These plastic balls are known as ‘puddle balls’.
3. Shingling:
In this operation, the puddle balls are hammered to remove any particles of cinder associated with them and the iron particles welded together to form a “bloom”.
4. Rolling:
The blooms obtained during the previous operation are passed through grooved rollers as a result of which they get converted into puddle bars. These bars are wrought iron of lowest quality. Then quality is improved by subsequent processes of piling, reheating and rerolling.
Aston Process (For Manufacturing Wrought Iron):
This process was invented by James Aston (U.S.A.) in 1925. In this process the molten steel, obtained from Bessemer converter which contains large amount of dissolved gas and at a temperature of 1500°C is poured into liquid slag maintained at 1200°C.
As the steel strikes the slag it gets frozen and the dissolved gases are given off. Thus a spongy mass composed of spherical particles is produced at a temperature of about 1375°C. With this process just within a few minutes as much as 3 or 4 tonnes of metal is produced. This method produces wrought iron at a much cheaper cost compared to puddling process.
Properties of Wrought Iron:
Wrought iron entails the following properties / characteristics:
1. It possesses a high resistance towards corrosion.
2. It possesses high ductility and can be easily forged and melted.
3. Its ultimate strength can be increased considerably by cold working followed by a period of ageing.
4. It is never cast. All shaping is accomplished by hammering, pressing, forging, etc.
5. It possesses the property of recovering rapidly from overstrain, which enables it to accommodate sudden and excessive shocks without permanent injury. It has high resistance towards fatigue.
6. Owing to the nature of the slag distribution, tensile strength and ductility are greater in the longitudinal direction or rolling direction than in the direction transverse to rolling.
Applications of Wrought Iron:
The fields of application of wrought iron are as under:
1. Building Construction:
A. Soil, waste, vent and downspout piping.
B. Underground service lines and electrical conduit.
2. Public Works:
A. Bridge railings.
B. Blast plates.
C. Drainage lines
D. Sewer outfall lines.
3. Industrial:
A. Unfired heat exchangers.
B. Condenser tubes.
C. Skimmer bar.
D. Acid and alkali process lines etc.
4. Railroad and Marine:
A. Tanker heating coils.
B. Diesel exhaust and air brake piping.
C. Blast and brite protection plates.
5. Miscellaneous:
A. Coal handling plant.
B. Cooling tower and spray pond piping.
C. Gas collection hoods.
Note # 18. Magnesium:
Manufacture of Magnesium:
Metallic magnesium is prepared commercially chiefly by two methods:
I. Chloride Process and
II. Oxide Processes.
I. In the ‘chloride process’ a melted mixture of the chlorides of sodium, potassium, and magnesium is electrolysed. The presence of alkali chlorides is necessary to prevent the decomposition of magnesium chloride during heating. The electrolysis is carried out in an air tight iron vessel, the walls of which act as the cathode, with a centrally located graphite anode, which is protected by a porcelain hood from which the chlorine escapes.
As the magnesium is liberated it rises to the surface and is ladled out. The metal is refined by remelting in iron pots and fluxing it with sodium magnesium chloride to remove oxide inclusions. If especially pure magnesium is desired, it is prepared by distillation.
II. ‘Oxide process’ consists in dissolving the oxide in bath containing the fused fluorides of magnesium, barium and sodium and separating the metal by electrolysis.-The process, however, is no longer used.
Physical and Mechanical Properties of Magnesium:
(i) It is the lightest metal used as engineering material.
(ii) The tensile strength of cast metal is the same as that of ordinary cast aluminium, but when reduction of weight is important magnesium is preferable because of its low density of 1.74 which is a little less than sevenths of that of aluminium.
(iii) The tensile strength of rolled and annealed magnesium is about the same as that of a good quality grey cast iron.
(iv) It is harder than aluminium.
(v) Melting point = 650°C, boiling point = 1107°C.
(vi) Its electrical resistivity is 4.46 microhms.
(vii) Its Brinell hardness is 33 (500 kgf load, 10 mm ball).
(viii) In casting magnesium and its alloys special care is necessary because of its tendency to oxidise readily.
(ix) Because of its lightness, it tends to trap gases during casting and form gas flames.
(x) It is readily machined and under the buffing wheel it takes a high polish.
(xi) It hardens very rapidly with cold working, but it is suitably plastic and workable at 350°C.
(xii) It is readily attacked by weak acids and also by saline solution, it is not affected, however, by most alkaline solutions.
(xiii) It ignites easily at above its ignition temperature.
(xiv) It can be welded, but because it combines with oxygen more readily than any other common metal, a special technique is required.
(xv) It may be soldered but the joints must afterwards be protected to prevent them from corroding.
Uses of Magnesium:
1. It is used in the form of sheets, wires, rods, tubes etc.
2. The ribbon is chiefly used for degasification of radio tubes.
3. In the powdered form, mixed with an oxidising agent such as barium nitrate or potassium chlorate, magnesium is employed in the manufacture of flash light powder and in military pyrotechnics for production of rocket, signals and flares.
Note # 19. X-Rays:
The Origin of X-Rays:
An atom of a substance consists of a heavy central positive part called the nucleus surrounded by suitable number of negatively charged particles called electrons which revolve in more or less circular orbit. The electron on the innermost orbits is attracted by the nucleus with the greatest force and to detach it from the atom maximum energy is required.
On the other hand, electrons lying in the outer orbits experience a comparatively smaller force by the nucleus and a smaller amount of energy is required to detach them from these orbits. The innermost orbit is called K orbit and the outer are called L, M etc., respectively.
When the cathode ray beam falls on the anti-cathode (See Fig. 35) it penetrates deep into the atom of the anti-cathode and displaces one of the electrons in the innermost orbit say the if-orbit if the beam possesses a high energy. Immediately an electron from one of the higher orbits jumps to occupy the vacant space and difference in energy is emitted in the form of X-rays.
When the incident cathode ray beam carries a lesser amount of energy, an electron is displaced from the L-orbit and M- orbit and the vacant space is filled by the electron jumping into this space from M or other higher orbits. The difference of energy emitted in this case will be smaller than in the previous case.
Properties of X-Rays:
1. X-rays are highly penetrating.
2. They affect a photographic plate and are even more effective than light.
3. They are not deflected by electric and magnetic fields.
4. They ionise a gas.
5. They are propagated in straight lines with the velocity of light.
6. They have a destructive effect on living tissues.
7. They produce photoelectric effect.
8. They cause fluorescence in many substances.
9. They are invisible to eyes.
10. They are reflected like ordinary rays of light, under suitable conditions.
Bragg’s Law:
When a beam of monochromatic X-rays falls on a crystal it is scattered by the individual atoms which are arranged in sets of parallel layers. Bragg’s reflection takes place when the following two conditions are satisfied.
1. X-rays are reflected by the regularly arranged parallel layers of atom in a crystal.
2. The atoms on which X-rays fall become centre of disturbance and develop spherical wavelet whose envelope provides size to the reflected wave front by Huygen’s construction.
Bragg’s law can be explained as follows:
Refer to Fig. 36. It represents three layers of a crystal. An X-ray wavefront enters the upper left, making an angle θ with the crystal surface. It may be noted that the rays reflected from the second layer travel a greater distance than those from the first layer. In order that reflections from the second and successive layers shall reinforce, it is necessary that these additional distances shall be some integral multiple of the X-ray wavelength h or n λ.
If we construct the lines LP and LQ perpendicular to the directions of the incident and reflected rays respectively, we see that each of these lines makes an angle θ with line LM whose length is the separation d of the layers. The additional length of the path of the ray reflected from the layer is (PM + MQ), each of which is equal to d sin θ.
Thus 2d sin θ = n λ, where n is the order of spectrum and the value of n may be 1, 2, 3 etc., for first order, second order and third order maxima respectively.
This equation is called Bragg’s law for diffraction at crystal surfaces and is basis for the determination of crystal structures.
