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Engineering Materials 2
An Introduction to Microstructures and Processing
By Michael F. Ashby David R. H. Jones
Butterworth-Heinemann
Copyright © 2013 Michael F. Ashby and David R. H. Jones
All right reserved. ISBN: 978-0-08-096669-4
Chapter One
Metals
1.1 INTRODUCTION
This first group of chapters looks at metals. There are so many different metals—literally hundreds of them—that it is impossible to remember them all. It isn't necessary—nearly all have evolved from a few "generic" metals and are tuned-up modifications of the basic recipes. If you know about the generic metals, you know most of what you need.
This chapter introduces the generic metals. But rather than bore you with a catalogue, we introduce them through real engineering examples. They allow us not only to find examples of the uses of the main generic metals but also to introduce the all-important business of how the characteristics of each metal determine how it is used in practice.
1.2 METALS FOR A MODEL STEAM ENGINE
Model making is big business. The testing of scale models provides a cheap way of getting critical design information for things from Olympic yacht hulls to tidal barrages. Architects sell their newest creations with the help of miniature versions correct to the nearest garden shrub. And many people find an outlet for their energies in making models—perhaps putting together a miniature aircraft from a kit of plastic parts or, at the other extreme, building a fully working model of a steam engine from the basic raw materials in their own "garden-shed" machine shop.
Figure 1.1 shows a model of a nineteenth-century steam engine built in a home workshop from plans published in a well-known modelers' magazine. Everything works just as it did in the original—the boiler even burns the same type of coal to raise steam—and the model is capable of towing several people. But what interests us here is the large range of metals that were used in its construction, and the way in which their selection was dictated by the requirements of design. We begin by looking at metals based on iron (ferrous metals). Table 1.1 lists the generic iron-based metals.
How are these metals used in the engine? The design loads in components like the wheels and frames are sufficiently low that mild steel, with a yield strength σy of around 220 MN m-2, is more than strong enough. It is also easy to cut, bend, or machine to shape. And last, but not least, it is cheap.
The stresses in the machinery—like the gear-wheel teeth or the drive shafts—are higher, and these parts are made from either medium-carbon, high-carbon, or low-alloy steels to give extra strength. However, there are a few components where even the strength of high-carbon steels as delivered "off the shelf" ([sigmay [equivalent] 400 MN m-2) is not enough. We can see a good example in the mechanical lubricator, shown in Figure 1.2, which is essentially a high-pressure oil metering pump. This is driven by a ratchet and pawl. These have sharp teeth which would quickly wear if they were made of a soft alloy. But how do we raise the hardness above that of ordinary high-carbon steel? Well, medium- and high-carbon steels can be hardened to give a yield strength of up to 1000 MN m-2 by heating them to bright red heat and then quenching them in cold water. Although the quench makes the hardened steel brittle, we can make it tough again (though still hard) by tempering it—a process that involves heating the steel again but to a much lower temperature. And so the ratchet and pawls are made from a quenched and tempered high-carbon steel.
Stainless steel is used in several places. Figure 1.3 shows the fire grate—the metal bars which carry the burning coals inside the firebox. When the engine is working hard, the coal is red hot; then, both oxidation and creep are problems. Mild steel bars can burn out in a season, but stainless steel bars last indefinitely.
Finally, what about cast iron? Although this is rather brittle, it is fine for low-stressed components like the cylinder block. In fact, because cast iron has a lot of carbon, it has several advantages over mild steel. Complicated components like the cylinder block are best produced by casting. Now cast iron melts much more easily than steel (adding carbon reduces the melting point in just the way that adding antifreeze works with water) and this makes the pouring of the castings much easier. During casting, the carbon can be made to separate out as tiny particles of graphite, distributed throughout the iron, which make an ideal boundary lubricant. Cylinders and pistons made from cast iron wear very well; look inside the cylinders of a car engine when the head is taken off, and you will be amazed by the polished, almost glazed look of the bores—and this after perhaps 108 piston strokes.
These, then, are the basic classes of ferrous alloys. Their compositions and uses are summarized in Table 1.1, and you will learn more about them in Chapters 12 and 13, but let us now look at the other generic alloy groups.
An important group of alloys are those based on copper (Table 1.2).
The most notable part of the engine made from copper is the boiler and its firetubes (see Figure 1.1). In full size, this would have been made from mild steel, and the use of copper in the model is a nice example of how the choice of material can depend on the scale of the structure. The boiler plates of the full-size engine are about 10 mm thick, of which perhaps only 6 mm is needed to stand the load from the pressurized steam safely—the other 4 mm is an allowance for corrosion. Although a model steel boiler would stand the pressure with plates only 1 mm thick, it would still need the same corrosion allowance of 4 mm, totaling 5 mm altogether. This would mean a very heavy boiler, and a lot of water space would be taken up by thick plates and firetubes. Because copper hardly corrodes in clean water, this is the obvious material to use. Although weaker than steel, 2.5 mm thick copper plates are strong enough to resist the working pressure, and there is no need for a separate corrosion allowance. Of course, copper is expensive—it would be prohibitive in full size—but this is balanced by its ductility (it is very easy to bend and flange to shape) and by its high thermal conductivity (which means that the boiler steams very freely).
Brass is stronger than copper, is much easier to machine, and is fairly corrosion proof (although it can "dezincify" in water after a long time).
A good example of its use in the engine is for steam valves and other boiler fittings (Figure 1.4). These are intricate and must be easy to machine; dezincification is a long-term possibility, so occasional inspection is needed. Alternatively, corrosion can be avoided altogether by using the more expensive bronzes, although some are hard to machine.
Nickel and its alloys form another important class of nonferrous metals (Table 1.3). The superb creep resistance of the nickel-based superalloys is a key factor in designing the modern gas turbine aeroengine. But nickel alloys even appear in a model steam engine. The flat plates in the firebox must be stayed together to resist the internal steam pressure (see Figure 1.3). Some model builders make these stays from pieces of monel rod because it is stronger than copper, takes threads much better and is very corrosion resistant.
1.3 METALS FOR DRINKS CANS
Few people would think that the humble drinks can (Figure 1.5) was anything special. But to a materials engineer, it is high technology. Look at the requirements. As far as possible we want to avoid seams. The can must not leak, should use as little metal as possible, and be recyclable. We have to choose a metal that is ductile to the point that it can be drawn into a single-piece can body from one small slug of metal. It must not corrode in beer or coke and, of course, it must be nontoxic. And it must be light and must cost almost nothing.
Aluminum-based metals are the obvious choice (Table 1.4)—they are light and corrosion resistant. But it took several years to develop the process for forming the can and the alloy to go with it. The end product is a big advance from the days when drinks only came in glass bottles and has created a new market for aluminum (now threatened by polymers). Because aluminum is lighter than most other metals, it is also the obvious choice for transportation—aircraft, high-speed trains. Most of the alloys listed in Table 1.4 are designed with these uses in mind. We will discuss the origin of their strength and their uses, in more detail, in Chapter 11.
1.4 METALS FOR HIP JOINTS
As a last example we turn to the world of medicine. Osteoarthritis is an illness that affects many people as they get older. The disease affects the joints between different bones in the body and makes it hard—and painful—to move them. The problem is caused by small lumps of bone which grow on the rubbing surfaces of the joints and which prevent them sliding properly. The problem can only be cured by removing the bad joints and putting artificial joints in their place. The first recorded hip joint replacement was done as far back as 1897—when it must have been a pretty hazardous business—but the operation is now a routine piece of orthopedic surgery. In fact, half a million hip joints are replaced worldwide every year.
Figure 1.6 shows the implant for a replacement hip joint. In the operation, the head of the femur is cut off and the soft marrow is taken out to make a hole down the center of the bone. Into the hole is glued a long metal shank which carries the artificial head. This fits into a high-density polythene socket which in turn is glued into the old bone socket. The requirements of the implant are stringent. It has to take large loads without bending. Every time the joint is used ([equivalent] 106 times a year), the load on it fluctuates, giving us a high-cycle fatigue problem as well. Body fluids are as corrosive as seawater, so we must design against corrosion, stress corrosion, and corrosion fatigue. The metal must be biocompatible. And, ideally, it should be light as well.
One material which meets these tough requirements is based on titanium (although stainless steel and cobalt–chromium alloy are also used). The α–β alloy shown in Table 1.5 is as strong as a hardened and tempered high-carbon steel and is more corrosion resistant in body fluids than stainless steel but is only half the weight. A disadvantage is that its modulus is only half that of steels, so that it tends to be "whippy" under load. But this can be overcome by using slightly stiffer sections. The same alloy is used in aircraft, both in the airframes and in the compressors of the gas turbine engines.
1.5 DATA FOR METALS
When you select a metal for any design application you need data for the properties. Table 1.6 gives you approximate property data for the main generic metals, useful for the first phase of a design project. When you have narrowed down your choice, you should turn to more exhaustive published sources of data. Finally, before making final design decisions you should get detailed material specifications from the supplier who will provide the materials you intend to use. And if the component is a critical one (meaning that its failure could precipitate a catastrophe), you should arrange to test it yourself.
(Continues...)
Excerpted from Engineering Materials 2 by Michael F. Ashby David R. H. Jones Copyright © 2013 by Michael F. Ashby and David R. H. Jones . Excerpted by permission of Butterworth-Heinemann. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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