Engineering Materials Vol II (microstructures processing design) 2nd ed. - M. Ashby_ D. Jones (1999) Episode 6 pps

Case studies in steels 141 Postscript Although there is a British Standard for hammers, there is no legislation in the UK which compels retailers to supply only standard hammers. It is, in fact, quite difficult to get standard hammers “over the counter”. But reputable makers spot check their hammers, because they will not knowingly sell improperly heat-treated hammers. Further reading K. E. Easterling, Introduction to the Physical Metallurgy of Welding, Butterworth, 1983. D. T. Llewellyn, Steels – Metallurgy and Applications, 2nd edition, Butterworth-Heinemann, 1994. Problems 13.1 The heat exchanger in a reformer plant consisted of a bank of tubes made from a low-alloy ferritic steel containing 0.2 weight% carbon. The tubes contained hydrocarbon gas at high pressure and were heated from the outside by furnace gases. The tubes had an internal diameter of 128 mm and a wall thickness of 7 mm. Owing to a temperature overshoot, one of the tubes fractured and the resulting gas leak set the plant on fire. When the heat exchanger was stripped down it was found that the tube wall had bulged over a distance of about 600 mm. In the most expanded region of the bulge, the tube had split longitudinally over a distance of about 300 mm. At the edge of the fracture the wall had thinned down to about 3 mm. Metallurgical sections were cut from the tube at two positions: (i) immediately next to the fracture surface half-way along the length of the split, (ii) 100 mm away from the end of the split in the part of the tube which, although slightly expanded, was otherwise intact. Fig. 13.10. Austenitising a striking face. 142 Engineering Materials 2 The microstructure at position (ii) consisted of grains of ferrite and colonies of pearlite. It was noticed that the pearlite had started to “spheroidise” (see Problem 5.2). The microstructure at position (i) consisted of grains of ferrite and grains of lower bainite in roughly equal proportions. Estimate the temperatures to which the tube been heated at positions (i) and (ii). Explain the reasoning behind your answers. Answers: (i) 800°C; (ii) 700°C. 13.2 In 1962 a span of Kings Bridge (Melbourne, Australia) collapsed by brittle frac- ture. The fracture started from a crack in the heat-affected zone (HAZ) of a transverse fillet weld, which had been used to attach a reinforcing plate to the underside of a main structural I-beam (see the diagram on the next page). The concentrations of the alloying elements in the steel (in weight%) were: C, 0.26; Mn, 1.80; Cr, 0.25. The welding was done by hand, without any special precau- tions. The welding electrodes had become damp before use. Account for the HAZ cracking. After the collapse, the other transverse welds in the bridge were milled-out and rewelded. What procedures would you specify to avoid a repeat of the HAZ cracking? 13.3 Steels for railroad rails typically contain 0.80 weight% carbon, 0.3 weight% silicon and 1.0 weight% manganese. The steel is processed to give a fine-grained pearlitic structure with a hardness of approximately 2.8 GPa. However, after a period in service, it is commonly found that a thin, hard layer (the “white layer”) forms in patches on the top (running) surface of the rail. The microhardness of this white layer is typically around 8 GPa. Given that frictional heating between the wheels of rail vehicles and the running surface of the rail can raise the temperature at the interface to 800°C, explain why the white layer forms and account for its high hardness. Transverse weld HAZ Web σ σ 76 3 Web Tension flange Crack Tension flange Cover plate Production, forming and joining of metals 143 Chapter 14 Production, forming and joining of metals Introduction Figure 14.1 shows the main routes that are used for processing raw metals into finished articles. Conventional forming methods start by melting the basic metal and then cast- ing the liquid into a mould. The casting may be a large prism-shaped ingot, or a continuously cast “strand”, in which case it is worked to standard sections (e.g. sheet, tube) or forged to shaped components. Shaped components are also made from stand- ard sections by machining or sheet metalworking. Components are then assembled into finished articles by joining operations (e.g. welding) which are usually carried out in conjunction with finishing operations (e.g. grinding or painting). Alternatively, the casting can be made to the final shape of the component, although some light machin- ing will usually have to be done on it. Increasing use is now being made of alternative processing routes. In powder metal- lurgy the liquid metal is atomised into small droplets which solidify to a fine powder. The powder is then hot pressed to shape (as we shall see in Chapter 19, hot-pressing is Fig. 14.1. Processing routes for metals. 144 Engineering Materials 2 the method used for shaping high-technology ceramics). Melt spinning (Chapter 9) gives high cooling rates and is used to make amorphous alloys. Finally, there are a number of specialised processes in which components are formed directly from metallic com- pounds (e.g. electro forming or chemical vapour deposition). It is not our intention here to give a comprehensive survey of the forming processes listed in Fig. 14.1. This would itself take up a whole book, and details can be found in the many books on production technology. Instead, we look at the underlying prin- ciples, and relate them to the characteristics of the materials that we are dealing with. Casting We have already looked at casting structures in Chapter 9. Ingots tend to have the structure shown in Fig. 14.2. When the molten metal is poured into the mould, chill crystals nucleate on the cold walls of the mould and grow inwards. But the chill crystals are soon overtaken by the much larger columnar grains. Finally, nuclei are swept into the remaining liquid and these grow to produce equiaxed grains at the centre of the ingot. As the crystals grow they reject dissolved impurities into the re- maining liquid, causing segregation. This can lead to bands of solid impurities (e.g. iron sulphide in steel) or to gas bubbles (e.g. from dissolved nitrogen). And because most metals contract when they solidify, there will be a substantial contraction cavity at the top of the ingot as well (Fig. 14.2). These casting defects are not disastrous in an ingot. The top, containing the cavity, can be cut off. And the gas pores will be squashed flat and welded solid when the white-hot ingot is sent through the rolling mill. But there are still a number of dis- advantages in starting with ingots. Heavy segregation may persist through the rolling operations and can weaken the final product*. And a great deal of work is required to roll the ingot down to the required section. * Welded joints are usually in a state of high residual stress, and this can tear a steel plate apart if it happens to contain layers of segregated impurity. Fig. 14.2. Typical ingot structure. Production, forming and joining of metals 145 Fig. 14.3. Continuous casting. Many of these problems can be solved by using continuous casting (Fig. 14.3). Con- traction cavities do not form because the mould is continuously topped up with liquid metal. Segregation is reduced because the columnar grains grow over smaller dis- tances. And, because the product has a small cross-section, little work is needed to roll it to a finished section. Shaped castings must be poured with much more care than ingots. Whereas the structure of an ingot will be greatly altered by subsequent working operations, the structure of a shaped casting will directly determine the strength of the finished article. Gas pores should be avoided, so the liquid metal must be degassed to remove dissolved gases (either by adding reactive chemicals or – for high-technology applications – casting in a vacuum). Feeders must be added (Fig. 14.4) to make up the contraction. And inoculants should be added to refine the grain size (Chapter 9). This is where powder metallurgy is useful. When atomised droplets solidify, contraction is immaterial. Segregation is limited to the size of the powder particles (2 to 150 µ m); and the small powder size will give a small grain size in the hot-pressed product. Shaped castings are usually poured into moulds of sand or metal (Fig. 14.4). The first operation in sand casting is to make a pattern (from wood, metal or plastic) shaped like the required article. Sand is rammed around the pattern and the mould is then split to remove the pattern. Passages are cut through the sand for ingates and risers. The mould is then re-assembled and poured. When the casting has gone solid it is removed by destroying the mould. Metal moulds are machined from the solid. They 146 Engineering Materials 2 Fig. 14.4. Sand casting. When the casting has solidified it is removed by destroying the sand mould. The casting is then “fettled” by cutting off the ingate and the feeder head. must come apart in enough places to allow the casting to be removed. They are costly, but can be used repeatedly; and they are ideal for pressure die casting (Fig. 14.5), which gives high production rates and improved accuracy. Especially intricate cast- ings cannot be made by these methods: it is impossible to remove a complex pattern from a sand mould, and impossible to remove a complex casting from a metal one! This difficulty can be overcome by using investment casting (Fig. 14.6). A wax pattern is coated with a ceramic slurry. The slurry is dried to give it strength, and is then fired (as Chapter 19 explains, this is just how we make ceramic cups and plates). Fig. 14.5. Pressure die casting. Production, forming and joining of metals 147 Fig. 14.6. Investment casting. During firing the wax burns out of the ceramic mould to leave a perfectly shaped mould cavity. Working processes The working of metals and alloys to shape relies on their great plasticity: they can be deformed by large percentages, especially in compression, without breaking. But the forming pressures needed to do this can be large – as high as 3 σ y or even more, depend- ing on the geometry of the process. We can see where these large pressures come from by modelling a typical forging operation (Fig. 14.7). In order to calculate the forming pressure at a given position x we apply a force f to a movable section of the forging die. If we break the forging up into four separate pieces we can arrange for it to deform when the movable die sec- tions are pushed in. The sliding of one piece over another requires a shear stress k (the shear yield stress). Now the work needed to push the die sections in must equal the work needed to shear the pieces of the forging over one another. The work done on each die section is f × u, giving a total work input of 2fu. Each sliding interface has area 2 (d/2)L. The sliding force at each interface is thus 2 (d/2)L × k. Each piece slides a distance ( 2 )u relative to its neighbour. The work absorbed at each interface is thus 2 (d/2)Lk( 2 )u; and there are four interfaces. The work balance thus gives 242224fu d Lk u dLku (/) ( ) ,== (14.1) or f = 2dLk. (14.2) 148 Engineering Materials 2 The forming pressure, p f , is then given by p f dL k f y ===2 σ (14.3) which is just what we would expect. We get a quite different answer if we include the friction between the die and the forging. The extreme case is one of sticking friction: the coefficient of friction is so high that a shear stress k is needed to cause sliding between die and forging. The total area between the dies and piece c is given by 2 22 2 W x d Lw xdL       −+             =−− ( ). (14.4) Piece c slides a distance 2u relative to the die surfaces, absorbing work of amount (w − 2x − d)Lk2u. (14.5) Production, forming and joining of metals 149 Fig. 14.7. A typical forging operation. (a) Overall view. (b) to (d) Modelling the plastic flow. We assume that flow only takes place in the plane of the drawing. The third dimension, measured perpendicular to the drawing, is L . Pieces a and b have a total contact area with the dies of 2dL. They slide a distance u over the dies, absorbing work of amount 2dLku. (14.6) The overall work balance is now 2fu = 4dLku + 2(w − 2x − d)Lku + 2dLku (14.7) 150 Engineering Materials 2 Fig. 14.8. How the forming pressure varies with position in the forging. or fLkd w x .=+−       2 2 (14.8) The forming pressure is then p f dL wx d f y (/) .== + −       σ 1 2 (14.9) This equation is plotted in Fig. 14.8: p f increases linearly from a value of σ y at the edge of the die to a maximum of p w d ymax =+       σ 1 2 (14.10) at the centre. It is a salutory exercise to put some numbers into eqn. (14.10): if w/d = 10, then p max = 6 σ y . Pressures of this magnitude are likely to deform the metal-forming tools themselves – clearly an undesirable state of affairs. The problem can usually be solved by heating the workpiece to ≈ 0.7 T m before forming, which greatly lowers σ y . Or it may be possible to change the geometry of the process to reduce w/d. Rolling is a good example of this. From Fig. 14.9 we can write (r − b) 2 + w 2 = r 2 . (14.11) Provided b Ӷ 2r this can be expanded to give wrb .= 2 (14.12) Thus w d rb d r d b d / / ==             22 12 12 . (14.13) [...]... “atomised” liquid metal to give semi-finished seamless tubes But modifications of conventional processes can give useful economies too In examining a production line it is always worth questioning whether a change in processing method could be introduced with economic benefits Background reading M F Ashby and D R H Jones, Engineering Materials I, 2nd edition, Butterworth-Heinemann, 19 96 Further reading S Kalpakjian,... construction materials and the most durable The pyramids are 5000 years old; the Parthenon 2200 Stone used in a load-bearing capacity behaves 164 Engineering Materials 2 Table 15.5 Generic natural ceramics Ceramic Composition Limestone (marble) Sandstone Granite Ice Typical uses 5 6Building foundations, construction 7 Arctic engineering Largely CaCO3 Largely SiO2 Aluminium silicates H2O Table 15 .6 Ceramic... 30–50 50 50 7 7 12 12 Cost of mining and transport 2.7 2 .6 0.92 20 23 1.7 – – – 63 60 –80 9.1 30–80 65 –150 6 Weibull exponent m 5 4 6 Assume 10 4 in design 7 Ceramics and glasses 165 like any other ceramic; and the criteria used in design with stone are the same One natural ceramic, however, is unique Ice forms on the Earth’s surface in enormous volumes: the Antarctic ice cap, for instance, is up to 3... 1000 (1400) 2.48 2.23 260 –1000 ( 360 –1400) 2.3–2.5 4 × 10 8 (6 × 10 8 ) Expensive at present Potentially 350–1000 (490–1400) Young’s modulus (GPa) Compressive strength (MPa) Modulus of rupture (MPa) 74 65 1000 1200 50 55 70 350 45 3.52 3.9 3.2 3.2 5 .6 3.2 1050 380 410 310 200 300 5000 3000 2000 1200 2000 2000 – 300–400 200–500 300–850 200–500 500–830 10 10 – 10–21 15 52 (73) 26 ( 36) 2.4–2.5 2.4 20–30... be taken as 0 .6 0.7 0 .60 0 .68 1.4 1.5 ∞ 0.95 0.97 1.00  −125 kJ mol −1  D = 9 × 10 6 m 2 s −1 exp   RT   where R is the gas constant and T is the absolute temperature Calculate the time required for carburisation, if the depth of the case is taken to be the value of x for which C = 0.5 wt% carbon Answer: 8.8 minutes Ceramics and glasses B Ceramics and glasses 159 160 Engineering Materials 2 Ceramics... by forming chains, sheets, or three-dimensional networks Often these are non-crystalline; all commercial glasses, for instance, are three-dimensional amorphous networks based on silica, SiO2 We will first examine the simple structures given by ionic and covalent bonding, and then return to describe the microstructures of ceramics  168 Engineering Materials 2 Fig 16. 1 Ionic ceramics (a) The rocksalt,... 2nd edition, Butterworth-Heinemann, 19 96 Further reading S Kalpakjian, Manufacturing Processes for Engineering Materials, Addison-Wesley, 1984 J A Schey, Introduction to Manufacturing Processes, McGraw-Hill Kogakusha, 1977 J M Alexander and R C Brewer, Manufacturing Properties of Materials, Van Nostrand, 1 968 G J Davies, Solidification and Casting, Applied Science Publishers, 1973 C R Calladine, Plasticity... and so forth (c) The new high-performance ceramics, now finding application for cutting tools, dies, engine parts and wear-resistant parts  162 Engineering Materials 2 (d) Cement and concrete: a complex ceramic with many phases, and one of three essential bulk materials of civil engineering (e) Rocks and minerals, including ice As with metals, the number of different ceramics is vast But there is no... four, neighbours) so its density is low The very hard structural ceramics silicon carbide, SiC, and silicon nitride, Si3N4 (used for load-bearing components such as high-temperature bearings and engine 170 Engineering Materials 2 Fig 16. 3 Covalent ceramics (a) The diamond-cubic structure; each atom bonds to four neighbours (b) Silicon carbide: the diamond cubic structure with half the atoms replaced by... needed: the diamond stylus of a pick-up, or the diamond anvils of an ultra-high pressure press Its structure, shown in Fig 16. 3(a), shows the 4 coordinated arrangement of the atoms within the cubic unit cell: each atom is at the centre of a tetrahedron with its four bonds directed to the four corners of the tetrahedron It is not a close-packed structure (atoms in close-packed structures have 12, not four, . sand for ingates and risers. The mould is then re-assembled and poured. When the casting has gone solid it is removed by destroying the mould. Metal moulds are machined from the solid. They 1 46. following parameters: sheet width 300 mm, starting thickness 1 mm, reduced thickness 0.8 mm, yield strength 100 MPa, maximum forming pressure 200 MPa, roll radius 16. 2 mm, roll length 300 mm. Calculate. is 0.4 mm. Calcu- late the liquid head needed to force the molten silver into the mould cavity. It can be assumed that molten silver does not wet the mould walls. [Hint: the pressure needed to