Selection and use of engineering materials free download

Topic Page No. Selection of material to the economy working and the life of machinery. The subject of engineering material has been designed to overcome the above aspects. Forexample Alloys for air craft, Semi-conductor chips for pc, Photovoltaic for energy storage, Semi-conductor, Semi- conductor scanners. Metals are usually found in the form of ores which are raw or crude form impurities includes oxides, sulphides, nitrites, sulphates and traces elements like inert gasses.

Ferrous materials are usually refers to the materials that have a high content of iron in them. Ferrous compounds are usually garnished in color.

Occurrence of Iron: Iron is never available in pure form it is available in the form of different ores the most common ore is hematite various form of iron and steel are obtained by purifying and adjusting the composition of pig iron by suitable methods.

Iron is extracted from hematite or the separation of iron by reduction with carbon is very reactive. This process takes place in a blast furnace at c0. To remove the impurities from the ores some treatments are necessary. Carbon is added by melting it the product obtained is called pig iron.

Cast Iron: When we melt the pig iron in the presence of coke and calcium carbonate the product obtained is called cast iron. It has gray white color its gray color is due to the presence of graphite and white due to the presence of carbon carbides. It can be transferred into different molds of desired shapes and size.

Wrought Iron: It is the purest form of iron. It contains In wrought iron corrosion will be large. It is the most common type of iron used in engineering.

Steel: Steels are the large family of metals which consists mostly of iron and other elements usually carbon ranging 0. Steel goes harder and tougher with a n increase in carbon content up to 1. When a force is subjected to an engineering material it may result in translation, rotation and deformation of that material.

Aspects of translation and rotation of materials are deled by engineering dynamics. We restrict our cells here to the subject of materials under deformation forces. For example when using a rope to lift a load. For example a column that supports an overhead beam is in compression. Shear force can separate the materials by sliding part of it in one direction and rest of part is in opposite direction.

Shear stress in this case is the function of applied torque. Shear strain is related to the angle of twist. In short torsion is twisting any object due to an applied torque. Permanent deformation is irreversible and stays even after the removal of applied forces such a deformation is called as plastic deformation while the temporary deformation is reversible and disappears after the removal of applied forces such a deformation is called as elastic deformation.

So elastic deformation is recoverable. Both kinds of deformation can be a function of time or independent of time. Inelastic Deformation: Time dependent recoverable deformation under load is called as inelastic deformation. Creep: Time dependent progressive permanent deformation under constant load is called creep. Stress: When a material is subjected to an external force it will either totally comply with that force and be pushed away or it will set up the internal forces to oppose that forces.

Solid materials are generally act wither like a spring when stressed or compressed the internal forces come into play as it is easily seen when spring is released. A material is subjected to an external force that tends to stretch it is called as tension. Whereas forces which squeeze the material are put in compression. Since strain is the ratio of two lengths so it has no units and it frequently expressed as percentage.

If a spring is gradually stretched the force needed to increase but the material spring that to its original shape when the force is increased. This linear relationship between stress and strain can be shown in the form of a graph as this graph shows that as we increase or decrease the stress the strain also increase or decrease with same proportion respectively.

So both are directly proportional to each other. The point at which the straight line behavior ceases is called limit of proportionality. Beyond this the material will not spring back to its original shape and said to exhibit some plastic behavior. This stress at which the material starts to exhibits permanent deformation is called elastic limit or yield point. If stress is increased beyond yield point the sample will eventually break.

The slope of stress over strain graph varies with stress so we gradually take the slope of initial straight line portion. It is the one of the important property of metals. When metals are heated they expand and become larger while cooling the metals causes them to contract or shrink in size. It is very important for metals that are used in process industry to consider temperature changes and how they affect the metals.

Metal density is very important factor in different structures i. We measure the amount of penetration and then compare it with the standard scale for ferrous metals which are usually harder than the non ferrous metals a diamond strip is used which is indicated by a Rockwell number represented by C.

Toughness of metal should be able to absorb energy up to fracture. It enables materials with stand shocks and to be deformed without rupturing when a rod is bend its outer surface is stretches and the inside radius of the rod is compresses the more a material is bends the more outer surface is stretches an inner radius is contracts a tough material is one that gives relatively small changes in length when subjected to tension and compression in the other words the small value of stress over strain.

Tough materials are desirables to vehicles, machines and large structures. Elasticity is the ability of the materials to return in to its original shape after the load is removed theoretically the elastic limit of a material is the limit to which material is loaded and still recovers its original shape after the load is removed.

Ductile metals are vitals in creating wires or tubes because of its easy of forming. While cast iron and cast aluminum very hard steel and glass is the one of the best example of the brittle materials. Generally a brittle metal are very high in the compression strength and in tensile strength.

Brittle metals are not suitable for the heavy loads as they could break easily and can cause the damage. Here metals are liquefied and then joined together when it becomes harden it becomes one piece. Steel liquefy at oF while aluminum alloy at oF. It occurs as the result of the long term exposure to a high level of stress that are below the yield point of the material. Creep is more swear in materials that are subjected to heat for the long periods and near the melting points.

Creep deformation is the time dependent deformation. The temperature ranges in which the creep deformation may occur is different in various metals. Some important non ferrous metals are aluminum, copper, lead, tin and zinc. Aluminum: Aluminum found its maximum use in every field of engineering due to its particular properties softness, lightweight it has become very useful metal in all over the world. Modified metallurgical processes have improved strength and durability of different metals to such an extent that it has made maximum use of aluminum in engineering processes.

Copper: Copper is one of the most widely used metal but due to its high price we use it with some limitations in engineering work. Tin: Tin is very common metal in the family of non ferrous metals. It is mostly use as a protection layer for the protection of different metals. Flexible - Read on multiple operating systems and devices. Easily read eBooks on smart phones, computers, or any eBook readers, including Kindle.

We cannot process tax exempt orders online. If you wish to place a tax exempt order please contact us. Add to cart. As an example, few woUld now choose carbon steel table knives or garden tools because of the care required in cleaning, drying and greasing, and yet for their purpose they would be cheaper and more readily maintained to a sharper cutting edge than the normal stainless steel alternatives.

In other extreme cases we have situations where the use of technically superior materials would be discouraged for what has come to be regarded as a short-term replacement item by the public, even if the long-term economics were favourable.

An interesting example here is the exhaust system of cars, where the long-term economy of stainless steel cannot be disputed, but where in many countries the increase in initial capital cost or early replacement cost is not generally found acceptable, since the first ownership is usually short.

HMSO, 2. Materials Selection. Chapman and Hall, Placing a product on the market inevitably involves risk, and in a capitalist economy calculations prior to marketing must aim at the certainty of profit within a foreseeable period of time. The allowable margin of error associated with these calculations, and thus the vigour with which they are carried out, depends upon the state of the market and the activities of competing manufacturers. Increase in costs from superior materials or components has to be offset by substantial improvement in performance, as previously indicated, if it is not to appear finally as an increased increment of cost for the project as a whole.

A change of material also brings inhouse costs such as those associated with changes of instruction and stocking, particularly in the latter where the variety of materials being used is increased by the change.

Whilst in any given set of circumstances the competition between materials or components may be finally decided on costs where otherwise similar performance is obtainable, the precise level of performance and cost must depend on the type of application involved. In the interaction between performance and cost it is possible to see a continuous spectrum stretching from, at one end, applications which demand the maximum achievement of performance i.

Typical examples of fully performanceoriented products would be advanced armaments e. In these cases the over-riding need for complete reliability in service means that, once the decision to manufacture has been made, considerations of cost will frequently be subordinate.

However, expenditure which does not improve the level of performance and reliability will only lead to reducefl sales or increased resistance to project funding even where the level of cost is not the most important consideration. Such funding may well be politically controlled and external sales may not be involved, although for many advanced armaments there is still a competitive market. A less clear-cut example is a train for a commuter network.

Although the level of performance required is not as high as in the previous two examples, it is still at a substantial level, or should be, to provide a reliable service on crowded networks.

Yet the builder of trains is faced with the fact that there is hardly a railway system throughout the world that is not running at a loss.

Nevertheless, wherever the money is to come from, once the decision to build is taken performance must be provided to the required degree and this fixes the level of cost. Examples of cost-oriented products are a mass-market motor car and a washing machine. The mass-production industries must market their products at a price the public will pay so that once an acceptable performance has been achieved, i. The essential point here is that the manufacturer does not have to provide the maximum level of performance of which he is technologically capable.

He has merely to ensure that his 'value-for-money' parameter is no worse, and preferably better, than that of his competitors; he therefore seeks to provide the level of 17 Cost basis for selection performance which is economically right, i.

This must, of course, be acceptable to the consumer. As well as varying from product to product, the acceptable level of optimum performance may vary from time to time as the general climate of public opinion changes. But how do you measure a 'value-for-money' parameter?

The current trend is to move away from the volume manufacturing of uniform products towards products meeting the needs of the individual. The 'mass market' is becoming a mass of 'niche markets'. Whereas in the 70s and 80s the price of a product may have been of paramount importance to the customer when it came to the decision to purchase, now, it seems, the consumer is becoming more educated in terms of the real value of quality, good design and, in particular, the benefits of the sensible and responsible use of our finite resources.

We are, slowly, moving away from being a throw-away society. The properties of a given design and material may be regarded according to the extent to which they are cost-effective; that is to say, the extent to which they may be dispensed with in the interests of reducing costs. The designer will be prepared to incur costs for the provision of a certain property in proportion to the penalties that will result when it is absent. Thus, the civil engineering contractor will not regard toughness as a cost-effective property when designing a bridge, since if his bridge breaks then his professional reputation is destroyed with it.

One of the contributions that the materials engineer can make as a member of a design project team is his ability to distinguish between material-sensitive and design-sensitive properties. A tough material is one that is resistant to the initiation and propagation of cracks, whereas a tough design is one that is free from notches and stress-raisers. It may be quite expensive to obtain an especially tough material for a critical application but relatively cheap to free a design from stress-raisers.

It is technical incompetence to solve a problem more expensively than is necessary. Cost-effective decisions should only be made in the light of full knowledge relating to: 1 the special requirements of anticipated service; 2 the properties of all available materials and their relationship to those requirements. An important aspect of the service requirement may be formal regulations laid down by an appropriate Safety Board.

Inevitably, cost-effective decisions act to inhibit technological advance. Every commercial product is required to give a satisfactory return on capital expenditure in the shortest possible time, so that the cost of any improvement in technology must be more than recouped from corresponding savings resulting from improved performance.

More highly alloyed ferritic steels are being developed for this purpose e. The more advanced plant will also require austenitic steel superheaters with improved creep strength and corrosion resistance. However, if the increased efficiency and any improved environmental performance over the lifetime of the power plant were unlikely to make up for the increased material costs, then such improvement in materials may not be worthwhile.

It is anticipated that nickel-based and austenitic materials will have to be developed for many components. Whether or not a manufacturer operates in a competitive market, but particularly if he does, reduction in the cost of products to the consumer should be the aim, and in this it is as important to reduce the costs of ownership as it is to reduce the purchase price.

Unfortunately, most attention is usually directed towards reduction of purchase price since this is the simplest and most direct way of increasing sales of cost-oriented products. Although reducing the costs of ownership is equally valuable to the consumer, there is often less emphasis in this direction since it will usually increase the basic purchase price.

The justification is, of course, long-term in that when spread over a reasonable life the decrease in running costs more than compensates for the increase in purchase price. Thus in the automobile field, the wider use of galvanized steel for motor car bodies would help eliminate the rust problem and greatly extend the life of the whole car, which at present in the bulk sales market tends to be limited by the body rather than the mechanical components.

Similar remarks apply to the use of stainless steel for silencers. In both of these cases the necessary technology is available, but there is often little incentive for the manufacturer to use the more expensive materials because by the time failure has occurred he is no longer involved, and the case that the initial consumer would be willing to pay more for a longer life product is not always clear-cut.

Total cost to the consumer! I Cost of ownership Purchase price I, Variable c o s t s cost of production a Cost of basic materials b Cost of manufacture, i. The use of galvanized steel for structural purposes in cars, by such manufacturers as Rolls-Royce to give greatly longer body life, has been established for a long time and this approach is now being followed also by some of the better bulk manufacturers such as Audi and BMW. This may well reflect a growth in a more performance-oriented purchasing sector, but with safety and reliability of increasing importance.

However, costs have to be calculated on the basis of properties for a particular design criterion and not merely on costs based on weight or volume, and for reasons explained in Chapter 18, these materials are gaining acceptance among automobile design engineers.

The variable costs i. The primary cost can be markedly affected by supplies, marketing methods, international politics including tariffs , metal stocks strikes, dumping, etc. Fabricating industries for the most part are limited in outlook to their own countries and do not possess effective priceregulating organizations or mechanisms, which, in any case, may be banned by the State anti-trust laws.

Frequently the fabrication costs are low in relation to the value of the material particularly for non-ferrous metals and plastics and the scope for manipulation and influence is small. The main cost worries in components made from the more expensive metals are caused by the variations in base metal price, and by abrupt changes in trade activity. Compound stability In metals, the more stable the compound in which the element is found, the greater will be the amount of energy and thus cost in the process of reducing that compound for the recovery of the metal value.

Interestingly the history of metal usage relates to the stability of its compounds, i. Gold, silver and copper occur in the elemental state, and copper and lead are relatively easily reduced from accessible minerals.

Relative abundance Relative abundance, and the degree of complexity in mineralogical association, are obviously important factors since the less concentrated a material source is, the more effort must be devoted to its extraction.

Thus iron, where the reduction from oxides is only marginally more energy-consuming than copper and which has also the richest and most easily recovered ores, is the cheapest metal. A typical copper ore contains In at least the medium term there is ample capacity to meet present and prospective demand for nearly all minerals, even with due allowance for typical disruptive influences on supply. Whilst the overall world production of metals has continued to grow, almost certainly the fall in Analysis of cost demand for metals in industrialized countries outside of commercial reccessions is the result of challenge from competing materials.

Whilst cost savings and productivity improvements can to some extent offset weak prices, a satisfactory situation for the mineral industries can only come about when supply and demand are restored to an approximate balance, and endemic excess capacity is eliminated. In a recent survey 4 the productivity as measured by the value added per head was considered high in the metals sector of UK industry, but the forecast for growth for these industries in the UK over the period was considered low, unlike the manufacture and processing of plastics, where over the same period the forecast for growth is high.

For plastics, the raw materials cost is dependent on the prepolymers, derived largely from oil. This is only part of the picture, however, as there are many stages between the oil platform and a product for sale in the shopping precinct, including: oil recovery, oil refining, base chemical production, polymer manufacture, compounding, processing, assembly and, finally, sale.

The price at each stage is affected by both subsequent and preceding stages in a complex manner. Supply and demand The elementary theory of economics considers that the price of a commodity is fixed by a unique equilibrium between supply and demand.

This price is given by the point at which the demand curve intersects the supply curve curves D and S in Figure 3. Prices vary as a result of horizontal shifts in one or other of the d e m a n d and supply curves.

When demand rises, prices tend to rise because a buoyant market lessens the keenness of competition between different suppliers and enables them to maintain wider profit margins. Although the consumer is then paying more for the product this is not necessarily disadvantageous overall if it leads to improved capital investment and efficiency, which thereby adds to the future stability of the company concerned, with a maintained contribution to national wealth and employment.

When there is surplus productive capacity, prices should fall as competing producers pare their profit margins to avoid shutting down large-scale plant.

This simple market mechanism does not, of course, always operate, and there are considerable incentives to maintain prices at artificially high levels by arrangement. When, for any particular product, there is only one major producer in the field then it is easy for price control mechanisms to be distorted away from the public interest and most countries have anti-monopoly laws to prevent this.

On a national basis this may work well, but internationally it is more difficult. There is little to be done about the fact that if a single country is the sole large-scale producer of a certain commodity for which there is a large and continuing demand throughout the rest of the world then that country has the ability to maintain the price of the commodity at a level which is quite inappropriate to its true value.

Even when two countries are involved, they can arrange to control its marketing to the benefit of them both. When a number of major producers join together to control prices, this is known as a cartel.

The prices of some metals still appear to be controlled in this way. Level of consumption is important because when production is low, unit costs are high. Reducing unit costs requires high-volume production methods which are only obtainable with large-scale plant and equipment. But however much it is desired to reduce prices, the rightward limit of any supply curve is set by the productive capacity of available plant.

Such a project requires the investment of substantial risk capital and calls for considerable confidence in the level and consistency of future demand. This can be done. For example, when a new material becomes available it is usually produced at first in small quantities and the price is correspondingly high. There is then a production barrier which must be sumounted before the price can be significantly reduced 21 Cost basis for selection D S Sl Quantity bought or sold per unit time Figure 3.

However, once this barrier has been surmounted the price should fall sharply and remain steady so long as there is no further major change in the equilibrium between supply and demand. We have only to look at the history of aluminium and titanium to see materials move from being rare and expensive exotics to being relatively moderately priced items of everyday industrial use under the influence of demand in a few decades. There is currently much interest in the metallocene catalysts being used to improve the properties of polyolefins e.

There must be a clear commercial benefit, both for producer and end-user. Cost fluctuations When a material is in general short supply its price may sometimes fluctuate violently as a result of non-technical factors. In the pro22 ducer price for nickel was s per tonne. There was then a strike at Falconbridge which brought production to a halt.

Immediately the price of nickel on the open market rose to s per tonne. The consequence of such an increase was to cause traditional applications of austenitic stainless steels to be examined to see if there was any possibility of using low nickel ferritic stainless steels instead. The combined basin and draining sections incorporated into kitchen sink units had normally been made wholly of austenitic stainless steel. The ferritic variety of stainless steel is capable of functioning in the draining section of the unit but had not been widely used because of its being less amenable to the forming method and slightly inferior performance as regards corrosion resistance.

Modern steel-making methods enabling the control of interstitial solutes at lower levels improved the formability of the material and widened its application. The incentive to use substitutes has been even stronger in the case of copper and its alloys, where the price situation for copper has, for many years, been extremely fluid and unstable. In the mids the price of copper on the London Metal Exchange fell from s to s as a result of overproduction against more general depressed economic growth.

Since then the 'normal' slope of the approximate price curve, reflecting inflation, has been frequently swamped by massive oscillations due to political factors, industrial strikes, local wars and world recessions see Figure 3.

Similar effects may be found with other commodities, prices tending to collapse during recessions and rise if it happens that production difficulties coincide with increased demand at the end of a recession Figure 3. In the case of tin, trading was suspended over a substantial period, as a result of large stocks Analysis of cost Figure 3.

Data from London Metal Exchange. Figure 3. Continued overleaf 23 Cost basis for selection Figure 3. The analogous advice to a manufacturer would be to stock up when prices are low and de-stock when they are high.

Whether or not he does this depends upon his perception of the time scale over which the price fluctuations occur, because money in the bank earns interest whereas metal in the warehouse earns nothing. This is mainly a matter of confidence and, in fact, companies seem to de-stock during a recession, probably because cash flow becomes a problem when sales are low.

Material substitution is sometimes possible. For example, aluminium is an obvious substitute for copper in many electrical and heat conduction applications but there are problems. The inherent low strength of aluminium can be overcome by the use of steel-cored cables but difficulties associated with the joining of aluminium by soldering have been particularly significant in maintaining the use of copper in many cases.

Again, copper has maintained its position as the principal material for the and a breakdown of producers' agreements. The average price of commodity plastics in the UK for the period is shown in Figure 3. This illustrates how plastics are similarly affected by fluctuating prices. Increasingly, end users are involved in direct discussion with polymer producers, agreeing on grades and forecasts of demand.

This may allow a longer term view of prices, but the scene may be just too complicated for this to be effective. The classical market response to plunging prices is for the producers to lower their production rate or otherwise restrict supplies.

However, this does not always happen; sometimes because the economy of a whole country is dependent on the revenue from a single commodity, or perhaps because severe cutbacks in state-owned companies would be expected to produce unacceptable political and social consequences.

There are several options open to the manufacturer who must buy a material which is subject to severe price instabilities. Three possibilities are: 1 advanced stock control, 2 material substitution and 3 diversification of operations. Copper has also maintained its position for low-temperature heat exchanger applications as in water heaters, again because of the ease of assembly by soldering as compared with aluminium. Diversification of some proportion of a company's operations into some other less sensitive area is another way of lessening the problem.

Within the conditions of a volatile market, large users of a metal may prefer to negotiate a future supply price with the producers and risk a change in market forces. But for many purchasers there will always be a need to buy directly or indirectly through the commodity exchange, where the dealing reflects the supply and demand position and fixes prices.

Commodity exchanges - the London Metal Exchange LME As pointed out by Gibson-Jarvie, 5 under the impetus of the industrial revolution, Britain moved from a net exporter to net importer of metals on a large scale.

The result was that prices began to fluctuate with shipments of ore or metal arriving at very irregular intervals and the value of the cargoes varied greatly as supplies temporarily exceeded or lagged behind demand.

It is a characteristic of the different industries that producers would like to see a steady, smooth demand or a predictably smooth increase or decrease , whereas stockists and consumers are operating at a different rhythm. Fairly soon, fast packets and eventually the telegraph, made it possible for a merchant in London to know of the departure of a particular ship some time before she could be expected to dock in this country.

By making use of this intelligence a merchant could to some extent iron out the wider of these fluctuations in price by dealing in a cargo while it was still at sea, or selling it forward. The result was a smoother price characteristic although there w e r e still 26 major difficulties in that metal was arriving in all sorts of shapes and sizes and at different purities.

This could make the non-physical buying and selling of cargoes difficult, and it was clearly necessary to insist on standard forms and purities assays. Dealings therefore became standardized on Straits Tin and Chile Bar copper; lots were at fixed tonnages and the forward trading period was settled at 3 months, this being the average time for a voyage from Chile or the Malay Straits.

From forward dealing it was an obvious step to 'hedging'. Hedging is used as an insurance against adverse price movements. For every physical transaction when there is an interval of time between the commencement and completion long enough for prices to move appreciably, a hedging contract will be entered into such that a possible loss on the one will be offset by a profit on the other.

As an example, a cable manufacturer may contract to supply cables using tons of copper wirebars. As he starts the order and draws copper from his stock, he will buy forward on the LME tons, this price being used for his quote for cable supply.

When the cable contract is completed, he replenishes his physical stocks by buying tons at the then current cash price on the LME. Finally, he sells his forward-bought copper also at the LME cash price for that day, and so closes his hedge. Note that the cable manufacturer has not only protected himself from an adverse movement in the copper price, but he was also able to establish a firm price with his own customer, as to its copper content for the order, the moment it was accepted.

Such forward dealing also attracts speculators. It is always said that the presence of such professional risk-takers serves to make the market more flexible. There is, of course, considerable risk, since delivery is explicit in all contracts, and a dealer must be prepared to deliver against a forward sale, either by delivering warrants of purchase on the market on the forward date at the market price or physically delivering from his warehouse.

Where the supplies are plentiful the forward price tends to be at a premium over cash, the Analysis of cost difference being known as a contango. Should supplies be scarce, or should there be a heavy demand for nearby metal, the cash price may rise above that for 3 months forward and the market is said to have gone into backwardation.

The extent of a contango is, in practice, limited to the cost of financing and carrying metal for the 3-month period. An interesting aspect is that a consumer can take advantage of a contango as an opportunity to build up stocks, at the same time selling forward. The difference in selling forward, being the extent of the ruling contango, will cover his costs of finance and storage. Effects on cost of composition and metallurgical complexity- effect of purity A metallic alloy is made up of a basis metal of a certain purity to which is added the required range of alloying elements, either as pure metals or as 'hardeners' concentrated mixtures of the element and the basis metal produced independently which enables more ready solution and distribution in the melt under normal foundry conditions.

The degree of purity required in the basis metal will vary with the type of alloy being produced. In the aluminium alloy field material intended to be used for general purpose, moderately stressed castings is able to tolerate higher quantities of impurities than, say, a high strength casting alloy for use in aircraft.

The higher the purity of the basis metal the more expensive the alloy will be as shown by the approximate costs given in Table 3. TABLE 3. In the LM10 aluminium-magnesium casting alloy the silicon impurity reacts with the magnesium in the alloy to form the intermetallic constituent Mg2Si, which has a serious embrittling effect if present in excessive amounts, and 0.

The basis aluminium used for manufacturing the alloy must therefore be of at least The wider specification of LM4 with regard to certain elements will not only permit the use of lower-grade aluminium for virgin ingot, at reduced cost, but will also more easily enable the composition to be achieved by the melting of scrap to produce secondary ingot, again with reduced costs.

Costs of alloying If an alloying element costs more than the basis metal to which it is being added then it is selfevident that the alloy must cost more than the metal, and vice-versa. Many other factors, such as the scale of the alloy usage and the practical difficulties in alloying to a tight specification in complex systems, can have a marked effect on costs. If, for example, an alloy contains small quantities of a readily oxidizable element, expensive melting procedures to avoid losses on melting may be required.

Consider the relative costs of the aluminium alloys and Table 3. Typical aluminium alloy costs s A! Clearly, other factors are operating; in this case one of the most important being metallurgical complexity.

This is almost equivalent to saying that a given order has to be made three times before deliverable quality is attained, and it is therefore not surprising that the alloy is expensive.

Filling and blending of plastics Fillers have been used in plastics ever since wood flour, asbestos, mica or cotton fabric were added to Bakelite's phenolic resin to enhance toughness.

These fillers frequently had another advantage, that of lowering the price of the material. Of course, not all fillers will result in cheaper plastic components. Glass- and carbonfibre filled plastics are used in demanding applications where their improved strength and stiffness is required.

Not only will the fibre-filled materials be up to ten times more expensive than the virgin polymer, but processing will also be more demanding. A number of plastic blends have also been successful commercially.

The resulting materials display properties not attainable with the unblended starting polymers, and occasionally the blend properties can be better than those of the base resins. Blends have frequently opened up new markets, filling gaps in the properties of engineering thermoplastics.

Examples of polymer blends include: ABS and polycarbonate connectors and housings , nylon and ABS sports goods, gears, housings and nylon and polyphenylene oxide automotive mouldings.

Typical properties are listed in Table 3. The larger the size of an order for material the smaller will be the unit cost. Even in the case of common, well-established materials the surcharge to be paid on small quantities can be alarming. It is to be emphasized that the additional charges are not necessarily levied to offset the cost of special manufacture, since it is usually the case that completion of a small order still has to await the passage through the factory of normal quantity.

The higher charges result from the fact that the irreducible administrative procedures and delivery charges represent a higher proportion of the total cost of the order.

Clearly the highest costs will be paid when buying from a small local stockist. Value-added costs The usual industrial procedure is for a manufacturer to buy in material in a form which is suitable for his purpose, process it and then sell it in its new form.

The manufacturer is not selling material but rather the value that he has added to the material in ;its passage through his factory. Whatever the precise nature of the processes that are operated in the factory, the quantities that are added to the material, and which will determine its final price on exit, must include the variable costs of skilled and unskilled labour, energy, technical development and supervision, royalty payments, etc.

The more a material is altered the greater the value-added component of its final cost should be. The extent of fabrication costs is frequently not appreciated. For example, in a low-cost material such as mild steel, the working cost to produce annealed thin sheet, or complex girder section, may approach that of crude steel supplied from the steelworks to the rolling mill. The more complex the section in rolling, with higher roll maintenance and general operating costs, the higher the price of mechanical reduction.

Hot stampings and drop forgings, generally involving higher labour costs and die replacements, are more expensive per unit weight than rolled products, particularly for non-repetitive parts. As with all fabrication techniques which involve the expense of shaped dies, the longer the run up to the full life of the die, the lower will be the component of die cost in the product see Chapter An inspection of typical product prices will sometimes indicate a higher cost of castings as compared with wrought products per unit weight, dependent on the complexity of shape and quantity.

In the case of steel this is partly a function of the normal foundry costs of mould preparation, sand reclamation, etc. It may be that the properties or the shape required in the product favour a particular fabricating route, but more often the required level of performance may be achievable by more than one method of fabrication, with direct competition in cost. At one time it was taken as axiomatic that wrought products were always more reliable and gave greater toughness than castings, but there has been such an improvement in the quality of high-grade castings that this is not now necessarily the case.

In some instances the use of a particular fabrication route is built into the product specification. As an example, the British Standard for domestic gas appliances requires that gas handling components in, for example, water heaters, are produced as brass hot-stampings, although aluminium alloy castings would be satisfactory other than for the possibility of lack of pressure tightness if there were undetected macro- or microshrinkage.

The dimensional tolerance required is also an important factor in the choice of both material and fabrication route, since it controls part of the cost accruing during manufacture. The level of tolerance required must be matched up to those that may be readily obtained with the fabrication techniques best suited to the material, otherwise costs will escalate.

In machining, the higher the degree of accuracy required, the more expensive the operation, since finishing cuts become protracted. Frequently, the material chosen will dictate the quality of finish obtainable and the speed with which it can be achieved. Such precise control and high accuracy implies rigid inspection and quality control. This is an expensive procedure requiring extra staff, space, equipment and held-production i. It may be that there are specific dangers in the use of materials which have the same fabrication function and appearance, but which have greatly different properties in the service conditions of a component.

As an example, in the manufacture of radio transmission valves external soldering of the fins to an anode was achieved by a silversolder containing cadmium. The use of such a solder at hotter points internally, within the glass envelope, would have led to cadmium vapour formation, possible melting, and breakdown of valve operation.

In order to be sure that such a mistake could not be made the physical form of the solders taken into the factory was very different- the silver-cadmium in slug form, and those without cadmium as wire. This distinction greatly eased identification of stock until final use. Stock control aspects Holding stocks of materials represents tied-up money, and clearly the narrower the range of materials required within a factory the easier stock-holding becomes.

It enables larger orders of, say, steel bar stock, to be negotiated at lower unit cost and simplifies storage and identification. At the same time, of course, it is seldom attractive to use an expensive low-alloy steel for applications equally well served by a carbon steel; the point is that each main area of application should be studied in order that ideally one material should be employed for any one type of usage, minimizing the range of specifications employed overall.

This also helps 30 References 1. Iliffe, CEC, Cost 4. It would be satisfactory to be able to say that such decisions are always based on a quantitative analysis of the form and extent of all the various demands anticipated in service for a particular design, and it might seem that, provided the designer has a clear idea of the properties required in his materials and the modes of failure to be avoided in service, this should be a simple process.

Unfortunately, simple situations arise only rarely in engineering practice. It has already been made clear that any application requires its own special combination of properties, and usually the demands are conflicting. We may, therefore, be seeking a combination of properties which it is impossible to achieve fully in any one material and a compromise has to be reached.