Engineering Materials Vol II (microstructures processing design) 2nd ed. - M. Ashby_ D. Jones (1999) Episode 5 pps

The light alloys 111 Thermal stability Aluminium and magnesium melt at just over 900 K. Room temperature is 0.3 T m , and 100°C is 0.4 T m . Substantial diffusion can take place in these alloys if they are used for long periods at temperatures approaching 80–100°C. Several processes can occur to reduce the yield strength: loss of solutes from supersaturated solid solution, over- ageing of precipitates and recrystallisation of cold-worked microstructures. This lack of thermal stability has some interesting consequences. During supersonic flight frictional heating can warm the skin of an aircraft to 150°C. Because of this, Rolls-Royce had to develop a special age-hardened aluminium alloy (RR58) which would not over-age during the lifetime of the Concorde supersonic airliner. When aluminium cables are fastened to copper busbars in power circuits contact resistance heating at the junction leads to interdiffusion of Cu and Al. Massive, brittle plates of CuAl 2 form, which can lead to joint failures; and when light alloys are welded, the properties of the heat-affected zone are usually well below those of the parent metal. Background reading M. F. Ashby and D. R. H. Jones, Engineering Materials I, 2nd edition, Butterworth-Heinemann, 1996, Chapters 7 (Case study 2), 10, 12 (Case study 2), 27. Further reading I. J. Polmear, Light Alloys, 3rd edition, Arnold, 1995. R. W. K. Honeycombe, The Plastic Deformation of Metals, Arnold, 1968. D. A. Porter and K. E. Easterling, Phase Transformations in Metals and Alloys, 2nd edition, Chapman and Hall, 1992. Problems 10.1 An alloy of A1–4 weight% Cu was heated to 550°C for a few minutes and was then quenched into water. Samples of the quenched alloy were aged at 150°C for Table 10.5 Yield strengths of work-hardened aluminium alloys Alloy number s y (MPa) Annealed “Half hard”“Hard” 1100 35 115 145 3005 65 140 185 5456 140 300 370 112 Engineering Materials 2 various times before being quenched again. Hardness measurements taken from the re-quenched samples gave the following data: Ageing time (h) 0 10 100 200 1000 Hardness (MPa) 650 950 1200 1150 1000 Account briefly for this behaviour. Peak hardness is obtained after 100 h at 150°C. Estimate how long it would take to get peak hardness at (a) 130°C, (b) 170°C. [Hint: use Fig. 10.10.] Answers: (a) 10 3 h; (b) 10 h. 10.2 A batch of 7000 series aluminium alloy rivets for an aircraft wing was inadvert- ently over-aged. What steps can be taken to reclaim this batch of rivets? 10.3 Two pieces of work-hardened 5000 series aluminium alloy plate were butt welded together by arc welding. After the weld had cooled to room temperature, a series of hardness measurements was made on the surface of the fabrication. Sketch the variation in hardness as the position of the hardness indenter passes across the weld from one plate to the other. Account for the form of the hardness profile, and indicate its practical consequences. 10.4 One of the major uses of aluminium is for making beverage cans. The body is cold-drawn from a single slug of 3000 series non-heat treatable alloy because this has the large ductility required for the drawing operation. However, the top of the can must have a much lower ductility in order to allow the ring-pull to work (the top must tear easily). Which alloy would you select for the top from Table 10.5? Explain the reasoning behind your choice. Why are non-heat treatable alloys used for can manufacture? Steels: I – carbon steels 113 Chapter 11 Steels: I – carbon steels Introduction Iron is one of the oldest known metals. Methods of extracting* and working it have been practised for thousands of years, although the large-scale production of carbon steels is a development of the ninetenth century. From these carbon steels (which still account for 90% of all steel production) a range of alloy steels has evolved: the low alloy steels (containing up to 6% of chromium, nickel, etc.); the stainless steels (con- taining, typically, 18% chromium and 8% nickel) and the tool steels (heavily alloyed with chromium, molybdenum, tungsten, vanadium and cobalt). We already know quite a bit about the transformations that take place in steels and the microstructures that they produce. In this chapter we draw these features together and go on to show how they are instrumental in determining the mechanical properties of steels. We restrict ourselves to carbon steels; alloy steels are covered in Chapter 12. Carbon is the cheapest and most effective alloying element for hardening iron. We have already seen in Chapter 1 (Table 1.1) that carbon is added to iron in quantities ranging from 0.04 to 4 wt% to make low, medium and high carbon steels, and cast iron. The mechanical properties are strongly dependent on both the carbon content and on the type of heat treatment. Steels and cast iron can therefore be used in a very wide range of applications (see Table 1.1). Microstructures produced by slow cooling (“normalising”) Carbon steels as received “off the shelf” have been worked at high temperature (usu- ally by rolling) and have then been cooled slowly to room temperature (“normalised”). The room-temperature microstructure should then be close to equilibrium and can be inferred from the Fe–C phase diagram (Fig. 11.1) which we have already come across in the Phase Diagrams course (p. 342). Table 11.1 lists the phases in the Fe–Fe 3 C system and Table 11.2 gives details of the composite eutectoid and eutectic structures that occur during slow cooling. * People have sometimes been able to avoid the tedious business of extracting iron from its natural ore. When Commander Peary was exploring Greenland in 1894 he was taken by an Eskimo to a place near Cape York to see a huge, half-buried meteorite. This had provided metal for Eskimo tools and weapons for over a hundred years. Meteorites usually contain iron plus about 10% nickel: a direct delivery of low-alloy iron from the heavens. 114 Engineering Materials 2 Fig. 11.1. The left-hand part of the iron–carbon phase diagram. There are five phases in the Fe–Fe 3 C system: L , d, g, a and Fe 3 C (see Table 11.1). Atomic packing d.r.p. b.c.c. f.c.c. b.c.c. Complex Table 11.1 Phases in the Fe–Fe 3 C system Phase Liquid d g(also called “austenite”) a(also called “ferrite”) Fe 3 C (also called “iron carbide” or “cementite”) Description and comments Liquid solution of C in Fe. Random interstitial solid solution of C in b.c.c. Fe. Maximum solubility of 0.08 wt% C occurs at 1492°C. Pure d Fe is the stable polymorph between 1391°C and 1536°C (see Fig. 2.1). Random interstitial solid solution of C in f.c.c. Fe. Maximum solubility of 1.7 wt% C occurs at 1130°C. Pure g Fe is the stable polymorph between 914°C and 1391°C (see Fig. 2.1). Random interstitial solid solution of C in b.c.c. Fe. Maximum solubility of 0.035 wt% C occurs at 723°C. Pure a Fe is the stable polymorph below 914°C (see Fig. 2.1). A hard and brittle chemical compound of Fe and C containing 25 atomic % (6.7 wt%) C. Steels: I – carbon steels 115 Table 11.2 Composite structures produced during the slow cooling of Fe–C alloys Name of structure Description and comments Pearlite The composite eutectoid structure of alternating plates of a and Fe 3 C produced when g containing 0.80 wt% C is cooled below 723°C (see Fig. 6.7 and Phase Diagrams p. 344). Pearlite nucleates at g grain boundaries. It occurs in low, medium and high carbon steels. It is sometimes, quite wrongly, called a phase. It is not a phase but is a mixture of the two separate phases a and Fe 3 C in the proportions of 88.5% by weight of a to 11.5% by weight of Fe 3 C. Because grains are single crystals it is wrong to say that Pearlite forms in grains: we say instead that it forms in nodules . Ledeburite The composite eutectic structure of alternating plates of g and Fe 3 C produced when liquid containing 4.3 wt% C is cooled below 1130°C. Again, not a phase! Ledeburite only occurs during the solidification of cast irons, and even then the g in ledeburite will transform to a + Fe 3 C at 723°C. Fig. 11.2. Microstructures during the slow cooling of pure iron from the hot working temperature. Figures 11.2–11.6 show how the room temperature microstructure of carbon steels depends on the carbon content. The limiting case of pure iron (Fig. 11.2) is straight- forward: when γ iron cools below 914°C α grains nucleate at γ grain boundaries and the microstructure transforms to α . If we cool a steel of eutectoid composition (0.80 wt% C) below 723°C pearlite nodules nucleate at grain boundaries (Fig. 11.3) and the micro- structure transforms to pearlite. If the steel contains less than 0.80% C (a hypoeutectoid steel) then the γ starts to transform as soon as the alloy enters the α + γ field (Fig. 11.4). “Primary” α nucleates at γ grain boundaries and grows as the steel is cooled from A 3 116 Engineering Materials 2 Fig. 11.3. Microstructures during the slow cooling of a eutectoid steel from the hot working temperature. As a point of detail, when pearlite is cooled to room temperature, the concentration of carbon in the a decreases slightly, following the a/a + Fe 3 C boundary. The excess carbon reacts with iron at the a–Fe 3 C interfaces to form more Fe 3 C. This “plates out” on the surfaces of the existing Fe 3 C plates which become very slightly thicker. The composition of Fe 3 C is independent of temperature, of course. Fig. 11.4. Microstructures during the slow cooling of a hypoeutectoid steel from the hot working temperature. A 3 is the standard labelling for the temperature at which a first appears, and A 1 is standard for the eutectoid temperature. Hypo eutectoid means that the carbon content is below that of a eutectoid steel (in the same sense that hypodermic means “under the skin”!). Steels: I – carbon steels 117 Fig. 11.5. Microstructures during the slow cooling of a hypereutectoid steel. A cm is the standard labelling for the temperature at which Fe 3 C first appears. Hyper eutectoid means that the carbon content is above that of a eutectoid steel (in the sense that a hyperactive child has an above-normal activity!). Fig. 11.6. Room temperature microstructures in slowly cooled steels of different carbon contents. (a) The proportions by weight of the different phases . (b) The proportions by weight of the different structures . 118 Engineering Materials 2 to A 1 . At A 1 the remaining γ (which is now of eutectoid composition) transforms to pearlite as usual. The room temperature microstructure is then made up of primary α + pearlite. If the steel contains more than 0.80% C (a hypereutectoid steel) then we get a room-temperature microstructure of primary Fe 3 C plus pearlite instead (Fig. 11.5). These structural differences are summarised in Fig. 11.6. Mechanical properties of normalised carbon steels Figure 11.7 shows how the mechanical properties of normalised carbon steels change with carbon content. Both the yield strength and tensile strength increase linearly with carbon content. This is what we would expect: the Fe 3 C acts as a strengthening phase, and the proportion of Fe 3 C in the steel is linear in carbon concentration (Fig. 11.6a). The ductility, on the other hand, falls rapidly as the carbon content goes up (Fig. 11.7) because the α –Fe 3 C interfaces in pearlite are good at nucleating cracks. Fig. 11.7. Mechanical properties of normalised carbon steels. Quenched and tempered carbon steels We saw in Chapter 8 that, if we cool eutectoid γ to 500°C at about 200°C s −1 , we will miss the nose of the C-curve. If we continue to cool below 280°C the unstable γ will begin to transform to martensite. At 220°C half the γ will have transformed to martensite. And at –50°C the steel will have become completely martensitic. Hypoeutectoid and hypereutectoid steels can be quenched to give martensite in exactly the same way (although, as Fig. 11.8 shows, their C-curves are slightly different). Figure 11.9 shows that the hardness of martensite increases rapidly with carbon content. This, again, is what we would expect. We saw in Chapter 8 that martensite is a supersaturated solid solution of C in Fe. Pure iron at room temperature would be b.c.c., but the supersaturated carbon distorts the lattice, making it tetragonal Steels: I – carbon steels 119 Fig. 11.8. TTT diagrams for (a) eutectoid, (b) hypoeutectoid and (c) hypereutectoid steels. (b) and (c) show (dashed lines) the C-curves for the formation of primary a and Fe 3 C respectively. Note that, as the carbon content increases, both M S and M F decrease . Fig. 11.9. The hardness of martensite increases with carbon content because of the increasing distortion of the lattice. 120 Engineering Materials 2 Fig. 11.10. Changes during the tempering of martensite. There is a large driving force trying to make the martensite transform to the equilibrium phases of a + Fe 3 C. Increasing the temperature gives the atoms more thermal energy, allowing the transformation to take place. (Fig. 11.9). The distortion increases linearly with the amount of dissolved carbon (Fig. 11.9); and because the distortion is what gives martensite its hardness then this, too, must increase with carbon content. Although 0.8% carbon martensite is very hard, it is also very brittle. You can quench a 3 mm rod of tool steel into cold water and then snap it like a carrot. But if you temper martensite (reheat it to 300–600°C) you can regain the lost toughness with only a moderate sacrifice in hardness. Tempering gives the carbon atoms enough thermal energy that they can diffuse out of supersaturated solution and react with iron to form small closely spaced precipitates of Fe 3 C (Fig. 11.10). The lattice relaxes back to the undistorted b.c.c. structure of equilibrium α , and the ductility goes up as a result. The Fe 3 C particles precipitation-harden the steel and keep the hardness up. If the steel is [...]... (CCR in °C s −1 ) = 4.3 − 3.27 C − Steel Weight percentages C A B C D E F G Mn Cr Mo Ni 0.30 0.40 0.36 0.40 0.41 0.40 0.40 0.80 0.60 0.70 0.60 0. 85 0. 65 0.60 0 .50 1.20 1 .50 1.20 0 .50 0. 75 0. 65 0.20 0.30 0. 25 0. 15 0. 25 0. 25 0 .55 0 .55 1 .50 1 .50 1 .50 0 .55 0. 85 2 .55 [Hint: there is a log–log relationship between bar diameter and cooling rate.] Answer: Steels B, C, D, G Case studies in steels 133 Chapter... is a creep-resistant steel which can be used at about 450 °C It is a standard material for pipes and pressure vessels in chemical plants and oil refineries The specification is: C թ 0. 15% , Mn 0. 25 to 0.66%, Si թ 0 .50 %, Cr 1.88 to 2.62%, Mo 0. 85 to 1. 15% ; σ TS 51 5 to 690 MPa; σy ը 310 MPa The carbon equivalent for the maximum composition figures given is CE = 0. 15 + 0.66 2.62 + 1. 15 + = 1.01 6 5 (13.2) Case... isothermal transformation diagram for a coarse-grained, plain-carbon steel of eutectoid composition Samples of the steel are austenitised at 850 °C and then subjected to the quenching treatments shown on the diagram Describe the microstructure produced by each heat treatment 124 Engineering Materials 2 700 Temperature (˚C) 600 50 0 400 1% 300 50 % 99% MS · e 200 100 M50 · –100 MF 1 · ·· · c b 0 a f d 10 102... bulk of the tube the cooling rate was less, which is why bainite formed instead The hoop stress in the tube under the working pressure of 50 bar (5 MPa) is 5 MPa × 50 mm /5 mm = 50 MPa Creep data indicate that, at 900°C and 50 MPa, the steel should fail after only 15 minutes or so In all probability, then, the failure occurred by creep rupture during a short temperature excursion to at least 870°C How... 13.4 Part of the iron–carbon phase diagram 1 35 136 Engineering Materials 2 Fig 13 .5 Temperature distribution across the water-tube wall deposit scale inside the tubes (Fig 13 .5) This scale will help to insulate the metal from the boiler water and the tube will tend to overheat Secondly, water circulation in a natural convection boiler can be rather hit-and-miss; and the flow in some tubes can be very... Properties of Engineering Materials, Van Nostrand Reinhold, 1978 R W K Honeycombe and H K D H Bhadeshia, Steels: Microstructure and Properties, 2nd edition, Arnold, 19 95 Smithells’ Metals Reference Book, 7th edition, Butterworth-Heinemann, 1992 (for data on uses and compositions of steels, and iron-based phase diagrams) A H Cottrell, An Introduction to Metallurgy, 2nd edition, Arnold, 19 75 D T Llewellyn,... of Engineering Materials, Van Nostrand Reinhold, 1978 R W K Honeycombe and H K D H Bhadeshia, Steels: Microstructure and Properties, 2nd edition, Arnold, 19 95 R Fifield, “Bedlam comes alive again”, in New Scientist, 29 March 1973, pp 722–7 25 Article on the archaeology of the historic industrial complex at Ironbridge, U.K D T Llewellyn, Steels – Metallurgy and Applications, 2nd edition, Butterworth-Heinemann,... iron carbide at room temperature are 7.87 and 8. 15 Mg m−3 respectively Calculate the percentage by volume of a and Fe3C in pearlite Answers: α, 88.9%; Fe3C, 11.1% Steels: II – alloy steels 1 25 Chapter 12 Steels: II – alloy steels Introduction A small, but important, sector of the steel market is that of the alloy steels: the lowalloy steels, the high-alloy “stainless” steels and the tool steels Alloying... 2nd edition, Arnold, 19 75 D T Llewellyn, Steels – Metallurgy and Applications, 2nd edition, Butterworth-Heinemann, 1994 132 Engineering Materials 2 Problems 12.1 Explain the following (a) The critical cooling rate (CCR) is approximately 700°C s–1 for a fine-grained 0.6% carbon steel, but is only around 30°C s–1 for a coarse-grained 0.6% carbon steel (b) A stainless steel containing 18% Cr has a bcc structure... specifications: C թ 0. 25% , Mn թ 1.60%, Si թ 0 .50 %; σ TS 430 to 51 0 MPa; σ y ը 240 MPa The maximum carbon content of 0. 25% is well below the 0 .5 to 0.6% that may give HAZ problems But, in common with all “real” carbon steels, 4360 contains manganese This is added to react with harmful impurities like sulphur Any unreacted manganese dissolves in ferrite where it contributes solid-solution strengthening . at 150 °C for Table 10 .5 Yield strengths of work-hardened aluminium alloys Alloy number s y (MPa) Annealed “Half hard”“Hard” 1100 35 1 15 1 45 30 05 65 140 1 85 5 456 140 300 370 112 Engineering Materials. Si, 0.4% Mn, 4% Cr, 5% Mo, 6% W, 2% vanadium (V) and 5% Co. The steel is used in the quenched and tempered state (the Mo, Mn and Cr give good hardenability) and owes its strength to two main factors:. Engineering Materials, Van Nostrand Reinhold, 1978. R. W. K. Honeycombe and H. K. D. H. Bhadeshia, Steels: Microstructure and Properties, 2nd edition, Arnold, 19 95. R. Fifield, “Bedlam comes alive