Volume 20 - Materials Selection and Design Part 6 doc

the stress situation and design A balanced approach is necessary; this often requires the collaboration of two or more individuals with the required analytical strengths The trick is knowing when to call in another person with the required engineering background Naturally, the primary focus of the failure analysis is the determination of the cause of failure Indeed, this may be the sole charge given to the analyst However, in the larger scheme of things, someone must view the reported failure mechanism, material properties, manufacturing procedure, service conditions, and so forth, and decide what must be done if other similar components could fail in the same way If the failure was a unique case, the material may be quite adequate However, if the failure indicates the potential for similar failures in other components, either in service or to be built, or if the percentage of failures is higher than acceptable (depending on the degree of risk or potential consequences), or if the component life span is less than desired or required, changes must be made In any given situation there may be a number of potential ways in which a component can be improved; each must be evaluated as to its potential to cure the problem, its cost, and its potential to produce other problems (which can happen) One option may be the selection of a different material References cited in this section 7 G.F Vander Voort, Conducting the Failure Examination, Met Eng Q., Vol 15 (No 2), May 1975, p 31-36 8 Failure Analysis and Prevention, Vol 11, ASM Handbook (formerly Metals Handbook, 9th ed.), American Society for Metals, 1986 9 V.J Colangelo and F.A Heiser, Analysis of Metallurgical Failures, John Wiley & Sons, 1974 10 R.D Barer and B.F Peters, Why Metals Fail, Gordon and Breach Science Publishers, 1970 11 D.J Wulpi, Understanding How Components Fail, American Society for Metals, 1985 12 C.R Brooks and A Choudhury, Metallurgical Failure Analysis, McGraw-Hill, 1993 Use of Failure Analysis in Materials Selection George F Vander Voort, Buehler Ltd Methods for Analyzing Failures to Improve Materials Selection To evaluate the value of a new, better material, it is necessary to define the required properties of the component Manufacturing processes can alter the expected properties, as can the environment Here, the failure study may reveal factors not initially considered, or incorrectly estimated A host of properties must be considered, as well as their relevance to the part One must remember that mechanical property tests pertain to idealized situations that have been standardized to reveal comparative information about the material These data may not be fully relevant to the conditions that exist in the actual component The uniaxial tension test specimen, toughness tests, such as the Charpy V-notch impact specimen, and hardness tests provide useful comparative data under the test conditions, but the data ranking of materials may not be the same if used in the actual component Nevertheless, testing by standard procedures is very useful as fullscale testing of components is very difficult and expensive Besides these common tests, there are a myriad of other tests and characteristics that can be evaluated for materials under fixed conditions; for example, ductility, fatigue strength, fatigue crack growth rate, notch sensitivity, formability (drawability, stretchability, bendability, etc.), wear resistance (abrasion resistance, adhesion resistance, galling resistance, etc.), machinability, weldability, plus other tests relating to corrosion behavior, thermal expansion/contraction, electrical resistivity, magnetic permeability, and so forth The failure analysis may show that one or more of these attributes are deficient, and the analyst should be asked to consider these problems, not merely determine the cause of failure Once the failure mechanism has been determined, it is important that contributing factors from the design, materials (and quality level), manufacturing and assembly processes, and service conditions be determined In general, failure analysts tend to do a fairly good job evaluating design and service condition contributions However, they may overlook the influence of the materials and manufacturing processes unless their assignment includes a specific request to make recommendations for dealing with similar components that may experience the same problems (to prevent failures) or recommendations on avoiding such problems with components to be manufactured When recommendations are made, they should be carefully reviewed to ensure that new problems are not created by the recommended changes (as was not done in the precipitator wire problem discussed in Example 2 in this article) Implementing Changes Once all of the relevant information is assembled, the analyst is faced with the problem of informing those who can implement the required changes This can be quite frustrating Some analysts are hired to simply perform the analysis, and their input is not sought beyond the report generation phase This is often true for independent laboratories and consultants Their job is done when the report is written, or perhaps after they testify in court They may not be requested to comment on corrective actions, and the person who hired them may not be concerned about this either If a manufacturer or supplier is performing the analysis, they will be more concerned with the work that follows the analysis than with the analysis itself because of their view of the larger picture Years ago, there was greater resistance to interdepartmental criticism in organizations, even when totally constructive, and resistance to changes required to fix obvious problems with components Today, most of these barriers have disappeared as companies try to be competitive and to improve products "Getting the message out" to the required individuals to implement changes, be they in materials or whatever, used to be one of the most difficult tasks of the analyst, but this has changed dramatically over the past decade because of many factors (of which product liability cannot be ignored) Use of Failure Analysis in Materials Selection George F Vander Voort, Buehler Ltd Historical Evolution of Improved Materials While failures stimulate manufacturers to upgrade their product through design changes, manufacturing process changes, or materials substitution they also stimulate research to improve existing alloys and to develop better new alloys One classic example of this process, the historical evolution of rail steels, is described in the article "Effects of Composition, Processing, and Structure on Properties of Irons and Steels" in this Volume Despite some notable historical catastrophes such as the 15 Jan 1919 failure of a 90 ft diam riveted molasses tank in Boston killing 19 people (Ref 13, 14, 15, 16, 17, 18) and the 14 March 1938 failure of the all-welded Vierendeel truss bridge near Hasselt, Belgium (Ref 19, 20, 21) it was not until the failures of welded T-2 tankers and Liberty ships during World War II were analyzed that brittle fracture was demonstrated to occur in ordinary mild steels This research (Ref 22, 23, 24, 25, 26) stimulated not only an understanding of brittle fracture and the measurement of toughness, but also extensive alloy development programs to develop stronger, tougher, weldable carbon- and low-alloy steels in all product forms These alloys are being continually "tweaked" to further improve properties through advances in steelmaking technology (control of residual elements, gases, inclusion content, segregation, grain size, and microstructure) It is interesting to speculate that, had the RMS Titanic sunk on the night of 14-15 April 1912 in shallow waters so that studies could have discovered the extreme brittleness of the steel used, this failure might have initiated brittle fracture studies three decades earlier References cited in this section 13 Disastrous Explosion of a Tank of Molasses, Sci Am., Vol 120 (No 5), 1 Feb 1919, p 99 14 Bursting of Molasses Tank in Boston Charged to Bad Design, Eng News-Rec., Vol 82 (No 7), 13 Feb 1919, p 353 15 B.S Brown, Details of the Failure of a 90-Foot Molasses Tank, Eng News-Rec., Vol 82 (No 20), 15 May 1919, p 974-976 16 Boston Molasses-Tank Trial: The Case for the Defense, Eng News-Rec., Vol 85 (No 15), 7 Oct 1920, p 691-692 17 Experts Deny Bomb Caused Collapse of Boston Molasses Tank, Eng News-Rec., Vol 87 (No 9), 1 Sept 1921, p 372-373 18 Bursting of Boston Molasses Tank Found Due to Overstress, Eng News-Rec., Vol 94 (No 5), 29 Jan 1925, p 188-189 19 Welded Bridge Failure in Belgium, Eng News-Rec., Vol 120 (No 18), 5 May 1938, p 654-655 20 O Bondy, Brittle Steel a Feature of Belgian Bridge Failure, Eng News-Rec., Vol 121 (No 7), 18 August 1938, p 204-206 21 A.M Portevin, Collapse of the Hasselt Bridge, Met Prog., Vol 35 (No 5), May 1939, p 491-492 22 C.F Tipper, The Brittle Fracture Story, Cambridge University Press, 1962 23 M.L Williams and G.A Ellinger, Investigation of Structural Failures of Welded Ships, Weld J., Res Suppl., Vol 32, Oct 1953, p 498s-537s 24 H.G Acker, Review of Welded Ship Failures, Welding Res Council Bull Ser., No 19, Nov 1954 25 J Hodgson and G.M Boyd, Brittle Fracture in Welded Ships, Inst Naval Arch Q Trans., Vol 100 (No 3), July 1958, p 141-180 26 M.L Williams, Correlation of Metallurgical Properties and Service Performance of Steel Plates from Fractured Ships, Weld J., Res Suppl., Vol 37, Oct 1958, p 445s-454s Use of Failure Analysis in Materials Selection George F Vander Voort, Buehler Ltd Failure Analysis Examples Three examples are provided to illustrate the use of failure analysis in materials selection and materials development/refinement Example 1: Use of Failure Analysis Results in the Improvement of Line Pipe Steels A superb example of continual product refinement, stimulated by product failures, concerns gas transmission line pipe steels Brittle fracture of some line pipes occurred, and these fast, full-running fractures were spectacular demonstrations of a poor combination of materials, environment, manufacturing and installation problems, and loads (Ref 27, 28) Following previous research on brittle fracture, the initial efforts were to decrease the Charpy ductile-to-brittle transition temperature (DBTT) Other toughness tests, such as full-section drop-weight tests, were used in an effort to make the toughness tests more relevant Indeed, fracture initiation tests were even performed on actual full lengths of pressurized line pipes (see Ref 29, 30, for example) To illustrate, Fig 1 shows a full-size section of an X60 grade line pipe that was pressurized internally to 40% of its yield strength and tested at 56 °F (13 °C), 8 °F above its 50% shear-area drop weight transition temperature (DWTT) A 30-grain charge was detonated beneath an 18 in (45.7 cm) notch cut into the pipe The crack, moving at 279 ft/s (85 m/s), stopped after a short distance The fracture was ductile, and the line pipe was tough enough at this temperature to arrest the crack However, when another pipe was tested in similar fashion at -15 °F (-26 °C), 40 °F below its 50% shear-area DWTT, the crack moved at 2215 ft/s (675 m/s), the fracture was fully brittle, and the line pipe was not ductile enough to stop the crack Figure 2 shows that the full length of the line pipe opened up in this test Fig 1 Ductile fracture of a full section X60 grade line pipe tested at 56 °F (13 °C), which is 8 °F above its 50% shear-area DWTT Fig 2 Brittle fracture of a full-section X60 grade line pipe tested at -15 °F (-26 °C), well below its 50% sheararea DWTT Despite substantial reductions in the DBTT of line pipe steels, line pipe failures still occurred; this time, however, they occurred under a ductile fracture mode (Ref 31) It was subsequently learned that the absorbed energy on the upper shelf of the Charpy energy-temperature curve was critical for arresting a moving crack Hence, researchers concentrated on improving the upper-shelf energy Meanwhile, the strength of these tougher alloys (hot rolled, non-heat treated) was also raised while designers increased the line pipe diameter In the 1960s, steelmaking technology was found to be inadequate as pipe sizes increased, stresses increased, and service temperatures decreased Failure analysis revealed that brittle fracture was the culprit However, application of the standard approach of reducing the ductile-to-brittle transition temperature of the line pipe steel merely changed the failure mode Both fracture initiation and fracture propagation had to be controlled Further studies showed that the upper shelf energy had to be improved in order to stop a crack, once initiated, from propagating This work led to still further enhancements permitting development of weldable higher-strength, larger-diameter pipe but with satisfactory fracture control Overall, the changes in steel composition were relatively minor, at least on a total weight percent basis However, these changes, chiefly in residual gas content, sulfur content, and grain refining additives, required steelmaking technology enhancements Improved steel processing procedures, chiefly hot-working temperature and deformation control, were also required to optimize microstructure and properties Example 2: Failure Analysis Leading to Improved Materials Selection for Precipitator Wires in a Basic Oxygen Furnace This example of how a component failure stimulated materials selection concerns the failure of wires used in a wet precipitator for cleaning the gases coming off a basic oxygen furnace (BOF) The system consisted of six precipitators, three separate dual units, each composed of four zones Each zone contained rows of wires suspended between parallel collector plates The original 0.109 in (2.77 mm) diam AISI 1008 (for zones 2 to 4 in each precipitator) carbon steel wire was cold drawn to a tensile strength of 100 ksi (690 MPa) One end was looped around an insulator spool at the top and fastened with a ferrule made from AISI 430 stainless steel The top end of the wire is attached to the insulator on the framework while the bottom end of the wire was attached to a bottle-shaped weight A potential was placed between the rows of wires and the interspersed parallel collector plates to remove the particulate matter from the gas Wires began failing about one year after start-up of the BOF shop The frequency of failures varied in the different zones Figure 3 shows an example of an unwrapped 1008 wire and two views of a failed wire The maintenance metallurgist determined that the 1008 wires failed because of corrosion fatigue It was decided to replace all of the wires in the two zones with the highest rates of failure with stainless steel wire Type 304 austenitic stainless steel wire was chosen, and it was ordered cold drawn (150 ksi, or 1034 MPa, tensile strength), mainly because the 1008 wire was cold drawn Fig 3 Precipitator wires from a basic oxygen furnace (a) Original AISI 1008 carbon steel wire, wrapped around an insulator spool and fastened with a ferrule made from type 430 ferritic stainless steel One ferrule has been removed (b) Close-up view showing the fractured wire face inside the ferrule Seven days after the more expensive type 304 wires were installed, the first failed Thus, switching from inexpensive 1008 carbon steel wire to a much more expensive type 304 austenitic stainless steel wire changed the time to first failure from one year to one week! As a result, further failure analysis was requested For the new type 304 wires, ferrules were not used at the ends Instead, 18 in (45.7 cm) long cold-drawn 1010 carbonsteel tubes were used The type 304 wire was inserted into one end of the tube, and the tube was crimped in two places to fix the wire The other end of the tube was bent so that it could be looped over the insulator on the frame, as shown in Fig 4 The wires broke at the apex of the tube or just slightly below it, as shown in Fig 5 Fig 4 Replacement precipitator wires (a) View of a type 304 replacement precipitator wire and the AISI 1010 tube bent at one end to place over the insulators The arrows point to the two crimps used to fix the wire in the tube (b) Close-up view of one of the crimps Fig 5 Fractured replacement precipitator wires (a) View of fractured type 304 precipitator wires (b) Close-up view of one of the wires Note the deformation at the inside diameter of the tube due to the motion of the wire The new type 304 stainless steel wires failed by transgranular SCC The plant metallurgist was either unaware of the potential for SCC or did not realize that the hot gas from the BOF was cooled with river water before entering the precipitators Because the river water goes through the plant piping system it was treated three times daily with chlorine to prevent algae growth in condensers, pipes, and so forth The air temperature within the precipitator varied from about 425 to 200 °F (218 to 93 °C) Thus, the new attachment method produced an ideal stress concentrator, the wire had a high level of residual stresses, the gas contained chlorine ions and the temperature was above 200 °F ideal conditions for chloride ion SCC of type 304 Furthermore, the framework holding the wires was vibrated periodically at 60 Hz to dislodge particulate matter from the wires The bottle weight produced a tensile load of slightly less than 2 ksi (14 MPa) During operation, both ends of the wires were constrained so that the force of the wind through the precipitator concentrated stresses at the tube opening Calculation of the natural frequency of the wire showed that its third harmonic was almost exactly 60 Hz Consequently, it was felt that this could be another source of stresses To prevent future problems, either from corrosion fatigue or SCC, wires were made from annealed type 430 ferritic stainless steel, which is immune from chloride ion SCC Attachment was made using type 430 ferrules The wire diameter and bottle weight were changed so that the natural frequency was about 90 Hz Over the next few years, most of the precipitator zones (2 to 4, zone 1 used barb wire) were restrung with type 430 wires, and they performed for more than twenty years (except for failures due to electrical short circuits) until the BOF shop was closed down recently This is a good example of the problems that can occur in practical failure analyses work The original carbon steel wire was deemed to be inadequate for the application The metallurgist decided to replace these wires with a more corrosionresistant alloy, but did not realize that his choice, under the operating conditions, was unwise The more expensive type 304 stainless steel wire failed catastrophically after only a week of service time Use of a ferritic stainless steel solved the problem, but other solutions were also possible Example 3: Failure of Chipper Knives This example describes how a steel research engineer used failure analysis results to identify the need for a better alloy In this case, a new alloy had to be developed because no suitable composition existed This example concerns the development of an alloy steel for knives used to chip logs, either hardwoods or softwoods Traditionally, chipping of logs for making paper, cardboard, or particle board took place in a paper mill using an alloy with a nominal composition of Fe0.48C-0.30Mn-0.90Si-8.50Cr-1.35Mo-1.20W-0.30V This alloy performed well in pulp mill applications where the knives are typically 20 to 30 in (51 to 76 cm) long, from about 5 to 7 in (13 to 18 cm) wide, and usually 0.5 to 0.625 in (13 to 16 mm) thick One of the long edges is beveled and sharpened However, with the development of total tree harvesters, where the chipper is taken out in the field to chip the logs (see Fig 6), knife failures were frequently encountered due to the less rigid support of the knives and because fewer knives are used (Fig 7) than in a pulp mill Typical examples of failed knives are shown in Fig 8; Fig 9 shows close-ups of edge damage Fig 6 A 75 in Morbark total tree harvester Fig 7 Chipper knife being installed in a 75 in Morbark total tree harvester Fig 8 Examples of a few of the different types of failed chipper knives examined Arrows point to fractures (a) Knife from a 75 in Morbark total tree harvester (b) Knife from a 66 in CM&E sawmill chipper (c) Knife from a Carthage Norman chipper (d) Knife from a 96 in Bush chipper Fig 9 Close-up of the fracture on the Carthage Norman chipper knife shown in Fig 8(c) In this case, a tougher alloy steel was needed that would still exhibit all of the other necessary characteristics of a knife steel, for example, edge retention, resistance to softening under frictional heating, wear resistance, ease of heat treatment, dimensional stability in heat treatment, grindability, low alloy cost, and so forth The failure analysis revealed longitudinal Charpy V-notch (room temperature) impact strengths of 3 to 5 ft · lbf (4 to 6.8 J), hardnesses from 56 to 59 HRC, and retained austenite contents varying from a trace to 8% in failed knives The alloy design program was established to produce a lower-cost composition (based on the cost to add various alloying elements) with significantly higher toughness, which is easily heat treated and yields a hardness of at least 58 HRC with as high a tempering temperature as possible Furthermore, tempering must produce high dimensional stability The program developed the nominal composition of Fe-0.50C-0.30Mn-0.40Si-5.00Cr-2.00Mo (Ref 32, 33), which achieved the goals described above Optimal austenitizing temperature was 1850 °F (1010 °C), and either air or oil quenching could be used Many manufacturers oil quench using a press to maintain flatness The optimal tempering temperature was 980 °F (527 °C), followed by a second temper at 940 °F (504 °C), yielding hardness of 59 HRC and absorbed energy of 6.5 to 7.5 ft · lbf (8.8 to 10.2 J) on longitudinal, room-temperature Charpy V-notch specimens With this tempering practice, all residual retained austenite was eliminated Knives were made from the above composition, and blind trials were conducted using a 75 in Morbark Chip Harvester (Fig 6 and 7) Knives were also tested from steel of the old composition using two different sources Knives (B) of the old composition were made by the same company that made the knives from the new composition, while old knives A were made by a competitive knife manufacturer using the former composition Table 3 presents the results from field trials conducted by a knife user It is quite obvious that the new composition outperformed the standard composition (and A and B performed similarly) in all areas Furthermore, in this and in further usage none of the knives made from the new composition broke in service Table 3 Results of chipper knife field trials See Example 3 in text Knife material New Old A Average results per run Run time, No boxes min of chips 75.0 1.67 58.75 1.19 Resharpening time, min 29.0 34.4 Stock removal in mm 0.036 0.91 0.057 1.45 Production per in of knife Run time, No boxes No of runs min of chips 27.78 2083.3 46.4 17.54 1030.7 20.9 This example is typical of many such studies conducted by researchers working for steel and specialty alloy manufacturers A problem was observed through contacts with manufacturers of knife blades Knives from a variety of pulp mill, portable tree harvesters, and saw mills were studied to characterize the steel and the reason for their failures These data then served as the basis for an alloy development program where all of the relevant parameters were evaluated to develop an optimal composition This was followed by trials in which a knife manufacturer made knives from trial compositions and then evaluated their performance in the field This information was used to refine the final composition, which was evaluated in blind trials by a disinterested party The key to the development was a careful study of a number of failed knives with different problems, but chiefly gross fracture, used in different types of operations References cited in this section 27 G.D Fearnehough, Fracture Propagation Control in Gas Pipelines: A Survey of Relevant Studies, Int J Pressure Vessels Piping, Vol 2, 1974, p 257-281 28 J.E Hood, Fracture of Steel Pipelines, Int J Pressure Vessels Piping, Vol 2, 1974, p 165-178 29 J.B Cornish and J.E Scott, Fracture Study of Gas Transmission Line Pipe, Mechanical Working & Steel Processing Conf., Vol VII, American Institute of Mining, Metallurgical, and Petroleum Engineers, 1969, p 222-239 30 J.F Kiefner, W.A Maxey, R.J Eiber, and A.R Duffy, Failure Stress Levels of Flaws in Pressurized Cylinders, Progress in Flaw Growth and Fracture Toughness Testing, STP 536, ASTM, 1973, p 461-481 31 J.E Hood and R.M Jamieson, Ductile Fracture in Large-Diameter Pipe, J Iron Steel Inst., Vol 211, May 1973, p 369-373 32 G.F Vander Voort, "Steel Composition for Chipper Knife," U.S Patent 4,353,743, 12 Oct 1982 33 G.F Vander Voort, "Method of Heat Treating a Steel Composition for Chipper Knife," U.S Patent 4,353,756, 12 Oct 1982 Use of Failure Analysis in Materials Selection George F Vander Voort, Buehler Ltd Conclusions When failures occur in existing products or prototypes, study of the failures provides valuable information to guide the materials selection process, at least in those cases where the material used appears to be part of the problem Many failures, of course, are not caused by the use of inadequate materials, but a significant percentage of failures do result from use of materials that may not be optimal for the application Published failure analyses generally concentrate on failure modes and mechanisms and do not always consider if the best material is being used However, by examining the reasons for the failure and the role of the chosen materials, the appropriateness of the chosen materials can be determined The analyst should always consider what steps must be taken to prevent such failures in other similar components The materials development engineers need to be sensitive to the deficiencies of materials under certain operating/environmental conditions Only by recognizing needs can new and better materials be developed The materials selection engineer needs to be sensitive to deficiencies in product performance in order to make materials substitutions in a timely manner Use of Failure Analysis in Materials Selection George F Vander Voort, Buehler Ltd References 1 R.K Penny, Failure Types, Consequences and Possible Remedies, Int J Pressure Vessels Piping, Vol 61, 1995, p 199-211 2 F.R Hutchings, The Laboratory Examination of Service Failures, British Engine Tech Report, New Series, Vol III, British Engine Boiler and Electrical Insurance Company, Manchester, England, 1964, p 174-219 3 R.W Wilson, Diagnosis of Engineering Failures, Br Corros J., No 3, 1974, p 134-146 4 G.J Davies, Performance in Service, Essential Metallurgy for Engineers, E.J Bradbury, Ed., Van Nostrand Reinhold, London, 1985, p 126-155 5 T.J Dolan, Preclude Failure: A Philosophy for Materials Selection and Simulated Service Testing, Exp Mech., Jan 1970, p 1-14 6 M.F Ashby, Materials Selection in Mechanical Design, Pergamon Press, 1992 7 G.F Vander Voort, Conducting the Failure Examination, Met Eng Q., Vol 15 (No 2), May 1975, p 31-36 8 Failure Analysis and Prevention, Vol 11, ASM Handbook (formerly Metals Handbook, 9th ed.), American Society for Metals, 1986 9 V.J Colangelo and F.A Heiser, Analysis of Metallurgical Failures, John Wiley & Sons, 1974 10 R.D Barer and B.F Peters, Why Metals Fail, Gordon and Breach Science Publishers, 1970 11 D.J Wulpi, Understanding How Components Fail, American Society for Metals, 1985 12 C.R Brooks and A Choudhury, Metallurgical Failure Analysis, McGraw-Hill, 1993 13 Disastrous Explosion of a Tank of Molasses, Sci Am., Vol 120 (No 5), 1 Feb 1919, p 99 14 Bursting of Molasses Tank in Boston Charged to Bad Design, Eng News-Rec., Vol 82 (No 7), 13 Feb 1919, p 353 15 B.S Brown, Details of the Failure of a 90-Foot Molasses Tank, Eng News-Rec., Vol 82 (No 20), 15 May Malleable cast iron encompasses yet another form of graphite called temper carbon The microstructure of a typical malleable cast iron is shown in Fig 47 This form of graphite is produced by the heat treatment of white cast iron, which does not contain graphite, but does contain a high percentage of cementite When a white cast iron is heated for an extended period of time (about 60 h) at a temperature of 960 °C (1760 °F), the cementite decomposes into austenite and graphite By slow cooling from 960 °C (1760 °F), the austenite transforms into ferrite or pearlite, depending on the cooling rate and the diffusion rate of carbon The ductility and toughness of malleable iron falls between that of ductile cast iron and gray cast iron Because white iron can only be produced in cast sections up to about 100 mm (4 in.) thick, malleable iron is thus limited in section size The yield strength of typical malleable cast irons ranges from 207 to 621 MPa (30 to 70 ksi), and the tensile strength ranges from 276 to 724 MPa (40 to 105 ksi) Total elongation ranges from 1 to 18% Applications for malleable iron include pipe fittings, valves, crankshafts, transmission gears, and connecting rods Fig 47 Microstructure of a typical malleable cast iron showing graphite in the form of temper carbon 4% picral etch 250× Courtesy of A.O Benscoter, Lehigh University Cementite A major microstructural constituent in white cast iron is cementite The microstructure of a typical white cast iron is shown in Fig 48 The cementite forms by a eutectic reaction during solidification: Liquid Cementite + Austenite (Eq 15) The eutectic constituent in white cast iron is called ledeburite and has a two-phase morphology shown as the smaller particles in the white matrix in Fig 49 The eutectic is shown in the Fe-C binary diagram in Fig 7(b) The austenite in the eutectic (as well as the austenite in the primary phase) transforms to pearlite, ferrite-pearlite, or martensite, depending on cooling rate and composition (e.g., in Fig 47 the austenite transformed to pearlite) Because of the high percentages of cementite, white cast irons are used in applications requiring excellent wear and abrasion resistance These irons contain high levels of silicon, chromium, nickel, and molybdenum and are termed alloy cast irons Such applications include steel mill rolls, grinding mills, and jaw crushers for the mining industry Hardness is the primary mechanical property of white cast iron and ranges from 321 to 400 HB for pearlitic white iron and 400 to 800 HB for alloy (martensitic) white irons Fig 48 Microstructure of a typical white cast iron 4% picral etch 100× Courtesy of A.O Benscoter, Lehigh University Fig 49 Microstructure of the eutectic constituent ledeburite in a typical white cast iron 4% picral etch 500× Courtesy of A.O Benscoter, Lehigh University References cited in this section 2 3 4 5 6 G Krauss, Principles of the Heat Treatment of Steel, American Society for Metals, 1980 R.W.K Honeycombe, Steels Microstructure and Properties, American Society for Metals, 1982 W.C Leslie, The Physical Metallurgy of Steels, McGraw-Hill, 1981 F.B Pickering, Physical Metallurgy and the Design of Steels, Applied Science, 1978 G Krauss, Microstructures, Processing, and Properties of Steels, Properties and Selection: Irons, Steels, and High-Performance Alloys, Vol 1, ASM Handbook, 1990, p 126 7 E.C Bain and H.W Paxton, Alloying Elements in Steel, 2nd ed., American Society for Metals, 1961, p 62 8 Microalloying 75, Conference Proceedings (Washington, D.C., Oct (1975), Union Carbide Corporation, 1977, p 5 9 T.B Massalski, J.L Murray, L.H Bennett, and H Baker, Ed., Binary Alloy Phase Diagrams, Vol 1, American Society for Metals, 1986, p 822 10 W Heller, R Schweitzer, and L Weber, Can Metall Q., Vol 21 (No 1), 1982, p 3 11 J.M Hyzak and I.M Bernstein, Metall Trans A, Vol 7A, 1976, p 1217 12 G.F Vander Voort and A Roósz, Metallography, Vol 17 (No 1), 1984, p 1 13 H Ichinose et al., paper I.3, Proc First Int Heavy Hauls Railway Conf., Association of American Railroads, 1978, p 1 14 Atlas of Time-Temperature Diagrams for Irons and Steels, G.F Vander Voort, Ed., ASM International, 1991, p 570 15 B.L Bramfitt, Proc 32nd Mechanical Working and Steel Processing Conference, Vol 28, ISS-AIME, 1990, p 485 16 F.B Pickering, Towards Improved Toughness and Ductility, Climax Molybdenum Co., 1971, p 9 17 G.J Roe and B.L Bramfitt, Notch Toughness of Steels, Properties and Selection: Irons, Steels, and HighPerformance Alloys, Vol 1, ASM Handbook, ASM International, 1990, p 739 18 E.C Bain, The Sorby Centennial Symposium on the History of Metallurgy, TMS-AIME, 1963, p 121 19 B.L Bramfitt and J.G Speer, Metall Trans A, Vol 21A, 1990, p 817 20 Atlas of Time-Temperature Diagrams for Irons and Steels, G.F Vander Voort, Ed., ASM International, 1991, p 249 21 W Steven and A.G Haynes, J Iron Steel Inst., Vol 183, 1956, p 349 22 R.W.K Honeycombe and F.B Pickering, Metall Trans A, Vol 3A, 1972, p 1099 24 K.W Andrews, J Iron Steel Inst., Vol 203, 1965, p 271 25 G.R Speich and H Warlimont, J Iron Steel Inst., Vol 206, 1968, p 385 26 G Krauss, J Iron Steel Inst Jpn., Int., Vol 35 (No 4), 1995, p 349 27 R.A Grange, C.R Hribal, and C.F Porter, Metall Trans A, Vol 8A, 1977, p 1775 Effects of Composition, Processing, and Structure on Properties of Irons and Steels Bruce L Bramfitt, Homer Research Laboratories, Bethlehem Steel Corporation Evolution of Microstructural Change in Steel Products It is interesting to study how microstructural and compositional changes have evolved in certain steel products In some products, these changes have taken place over a 150-year span, and in other products over just a few decades Examples of long-time evolution are rail steel and ductile/malleable cast iron, and an example of a short-term evolution is automotive sheet steel Evolution of Rail Steels Up until the 1860s, most rails in the United States were either cast iron, wrought iron, or plain carbon steel imported from England The steel rails were much preferred because the cast iron rails were brittle and the wrought iron rails were soft and had poor wear resistance In 1865, the first steel rails were produced in the United States and since then, steel has become the dominant rail material Early steel rails had carbon contents much below the current eutectoid carbon level This is because higher carbon content rails had a tendency to fracture and cause serious derailments Not until the 1930s did railroad engineers realize that the problem was not the higher carbon content, but hydrogen Laboratory and commercial studies proved that the problem of premature failure was caused by hydrogen flakes, or what the rail engineers called shatter cracks Practices were put in place in the late 1930s to remove hydrogen by slow cooling of the rails during the manufacturing process in order to allow the hydrogen to diffuse out of the rails With this problem solved, the carbon content of rail steel quickly rose to eutectoid levels (0.76 to 0.82%) With this increase in carbon, the wear resistance increased and the railroads could haul heavier axle loads Unfortunately, as axle load steadily increased from the 1940s to the 1980s, even the fully pearlitic rails began to wear more rapidly During this time, it was discovered that if one produced a finer pearlite interlamellar spacing, the hardness of the rail increased as did the wear resistance This led to manufacturing processes in which accelerated cooling was employed to attain a finer pearlite interlamellar spacing These processes included oil quenching (in the 1940s), forced-air cooling, water-spray cooling, and aqueous-polymer quenching (in the 1980s) Large improvements in wear resistance were obtained by these new processes, and it was soon discovered that rail life was no longer determined by the rail wearing out Indeed, rail life in the 1990s is now determined by fatigue life and fracture toughness The fatigue life could be improved by cleaner steels (fewer inclusions), but fracture toughness could not be improved sufficiently in a fully pearlitic steel, even by finer interlamellar spacings That is, the pearlitic steel has essentially been pushed to its limit as serving as a rail steel Thus, a revolutionary change must take place in rail steel technology One such possible change is to completely switch from a fully pearlitic microstructure to a fully bainitic microstructure Rail producers around the world are currently investigating high-strength bainitic rail steels A German rail manufacturer has an experimental bainitic steel rail in track This example is used to illustrate how the demands in the field produce evolutionary microstructural, property, and processing changes to keep pace However, as these demands increase, there comes a point where a revolutionary change must take place because the useful properties attributed to the original microstructure have been fully exhausted Evolution of Cast Iron Another historic evolutionary change in microstructure took place in the manufacture of cast iron Both gray and white cast iron, being among the earliest ferrous materials, were brittle and could not be fabricated into shapes other than those made by the casting process Although quite useful, gray and white cast iron were relegated to simple shapes such as cooking pots, cannon balls, window sashes, stove panels, and radiators that did not demand a material with ductility or malleability The reason for the lack of ductility is the flake-shaped graphite in gray cast iron and the cementite networks in white cast irons The only way to attain ductility and malleability and therefore expand the usefulness of these irons was to alter the microstructure The first commercial success was that of R.A.F de Réaumer, a French metallurgist, in 1720 Réaumer used the process of decarburization of white cast iron by packing the castings in iron ore and heating the material to "bright redness" for several days The lengthy heat treatment allowed the cementite to decompose into iron and carbon The iron ore provided oxygen for decarburization The final casting was completely free of cementite and the microstructure was fully ferritic or a mixture of ferrite and pearlite Because the process depended on decarburization (and thus carbon diffusion), only thin castings could be treated in this way In larger castings, cementite remained in the thicker sections, thus limiting their usefulness About 110 years later, S Boyden of the United States invented what is known today as malleable iron The advantage of malleable iron was that the process did not depend on days and weeks of decarburization, but depended on the decomposition of the cementite in white cast iron into iron and "free-carbon." This free-carbon formed as particles of carbon called "temper carbon," (see Fig 47) The Boyden process involved a heat treatment of only 30 h and large castings could be heat treated This process of microstructure alteration greatly expanded the usefulness of cast iron for more than a century and a half Another microstructural alteration process was invented in the 1960s Here liquid cast iron is inoculated with a magnesium alloy, resulting in spheroids, rather than flakes, of graphite forming during solidification The cast iron is called ductile iron and has a microstructure as shown in Fig 46 The matrix in ductile iron can consist of ferrite, pearlite, ferrite-pearlite, martensite, or bainite, depending on alloy composition and heat treatment Currently, austempered ductile irons, with a bainitic matrix, provide high strength and toughness heretofore not achieved in a cast iron product Ductile iron castings now replace many components that were historically produced from plain-carbon and low-alloy steels This evolutionary path of microstructural alteration in cast iron is an important example of the role microstructure plays in the development of structure-sensitive mechanical properties Evolution of Steel Sheet An example of rapid-paced evolutionary change is in sheet steel for the automotive industry For years, automobiles were produced using inexpensive, low-carbon sheet steel However, during the oil crisis in 1974, the drastic increase in the price of gasoline and the Western world's almost total dependence on imported oil, mandated improved fuel economy for automobiles This translated into production of lighter weight automobiles; this, in turn, created a revolution in sheet steel metallurgy Since the oil crisis, ordinary low-strength, low-carbon sheet steel has been replaced by a number of higher-strength sheet steels requiring new process technology These new steels include the high-strength, precipitation-strengthened steels, the dual-phase and tri-phase steels, and the bake-hardenable steels Also, new coating techniques have been developed to protect these new steels from corrosion This rapid change in automobile sheet steel is one example of the importance of basic understanding of metallurgy and the application of physical metallurgy to engineering product design Effects of Composition, Processing, and Structure on Properties of Irons and Steels Bruce L Bramfitt, Homer Research Laboratories, Bethlehem Steel Corporation References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Engineering Properties of Steel, P.D Harvey, Ed., American Society for Metals, 1982 G Krauss, Principles of the Heat Treatment of Steel, American Society for Metals, 1980 R.W.K Honeycombe, Steels Microstructure and Properties, American Society for Metals, 1982 W.C Leslie, The Physical Metallurgy of Steels, McGraw-Hill, 1981 F.B Pickering, Physical Metallurgy and the Design of Steels, Applied Science, 1978 G Krauss, Microstructures, Processing, and Properties of Steels, Properties and Selection: Irons, Steels, and High-Performance Alloys, Vol 1, ASM Handbook, 1990, p 126 E.C Bain and H.W Paxton, Alloying Elements in Steel, 2nd ed., American Society for Metals, 1961, p 62 Microalloying 75, Conference Proceedings (Washington, D.C., Oct (1975), Union Carbide Corporation, 1977, p 5 T.B Massalski, J.L Murray, L.H Bennett, and H Baker, Ed., Binary Alloy Phase Diagrams, Vol 1, American Society for Metals, 1986, p 822 W Heller, R Schweitzer, and L Weber, Can Metall Q., Vol 21 (No 1), 1982, p 3 J.M Hyzak and I.M Bernstein, Metall Trans A, Vol 7A, 1976, p 1217 G.F Vander Voort and A Roósz, Metallography, Vol 17 (No 1), 1984, p 1 H Ichinose et al., paper I.3, Proc First Int Heavy Hauls Railway Conf., Association of American Railroads, 1978, p 1 Atlas of Time-Temperature Diagrams for Irons and Steels, G.F Vander Voort, Ed., ASM International, 1991, p 570 B.L Bramfitt, Proc 32nd Mechanical Working and Steel Processing Conference, Vol 28, ISS-AIME, 1990, p 485 F.B Pickering, Towards Improved Toughness and Ductility, Climax Molybdenum Co., 1971, p 9 G.J Roe and B.L Bramfitt, Notch Toughness of Steels, Properties and Selection: Irons, Steels, and HighPerformance Alloys, Vol 1, ASM Handbook, ASM International, 1990, p 739 E.C Bain, The Sorby Centennial Symposium on the History of Metallurgy, TMS-AIME, 1963, p 121 B.L Bramfitt and J.G Speer, Metall Trans A, Vol 21A, 1990, p 817 Atlas of Time-Temperature Diagrams for Irons and Steels, G.F Vander Voort, Ed., ASM International, 1991, p 249 W Steven and A.G Haynes, J Iron Steel Inst., Vol 183, 1956, p 349 R.W.K Honeycombe and F.B Pickering, Metall Trans A, Vol 3A, 1972, p 1099 Atlas of Time-Temperature Diagrams for Irons and Steels, G.F Vander Voort, Ed., ASM International, 1991, p 544 K.W Andrews, J Iron Steel Inst., Vol 203, 1965, p 271 G.R Speich and H Warlimont, J Iron Steel Inst., Vol 206, 1968, p 385 G Krauss, J Iron Steel Inst Jpn., Int., Vol 35 (No 4), 1995, p 349 R.A Grange, C.R Hribal, and C.F Porter, Metall Trans A, Vol 8A, 1977, p 1775 Effects of Composition, Processing, and Structure on Properties of Nonferrous Alloys Ronald N Caron, Olin Corporation; James T Staley, Alcoa Technical Center Introduction A BROAD RANGE OF PROPERTIES are available with commercially available nonferrous alloys; this articles provides an overview of these alloys and describes the specific microstructure/property relationships that are used to make specific properties available to designers of structural applications Control of the microstructure during processing is as important for fabricability as it is for providing the desired engineering properties in the final product Manufacturing alloy components with the desired structural properties requires applying the proper choice of processing method to the alloy composition capable of delivering those desired properties Alloy and process development have sought to improve upon the properties available with metals by adjusting alloy additions along with processing parameters to meet the requirements of new technology This process is ongoing today with the base alloys available for centuries (iron, copper, tin) as well as with the newer base alloys currently recruited to meet modern needs with unique property requirements (titanium, tungsten, niobium) When currently available alloys fall short of meeting the needs of the newest structural designs, the solution will be found as it has in the past by cooperative alloy/process development efforts with the supplier of the particular base-alloy system that has the properties closest to the particular design need No metal or alloy is entirely unique; similarities in process and property characteristics do exist among the wide variety of commercially available nonferrous alloys However, each metal and alloy offers unique combinations of useful physical, chemical, and structural properties that are made available by its particular composition and the proper choice of processing method This article focuses on the monolithic form of nonferrous alloys However, it should be recognized that these alloys are also used in structural combinations, either as simple or complex composites, or with coatings in order to provide even more unique, economical combinations of useful properties The importance of availability and cost are generally ignored in the following discussion except where these attributes are significant Detailed information and additional references to the literature on processing and property characteristics of nonferrous alloys are provided in Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Volume 2 of ASM Handbook (Ref 1) Basic data on nonferrous alloy phase relationships are found in Alloy Phase Diagrams, Volume 3 of ASM Handbook (Ref 2) References 1 Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Vol 2, ASM Handbook (formerly 10th ed., Metals Handbook), ASM International, 1990 2 H Baker, Ed., Alloy Phase Diagrams, Vol 3, ASM Handbook, ASM International, 1992 Effects of Composition, Processing, and Structure on Properties of Nonferrous Alloys Ronald N Caron, Olin Corporation; James T Staley, Alcoa Technical Center Aluminum and Aluminum Alloys Aluminum alloys are second in use only to steel as structural metals More than 500 alloys are registered with the Aluminum Association Typical tensile strengths of aluminum alloy products range from 45 MPa (6.5 ksi) for 1199-O sheet to almost 700 MPa (100 ksi) for 7055-T77511 extruded products The low density combined with high strength have made aluminum alloys the standard material for applications such as aircraft, where specific strength (strength-toweight ratio) is a major design consideration Because of their corrosion resistance, moderate strength, and good ductility, they are also used for a wide variety of applications including beverage cans; building and construction; marine, rail, truck, and auto transportation; and tool and jig plate Wrought aluminum products are produced using all available metalworking techniques, and castings are made using all standard solidification processes Their properties depend on a complex interaction of chemical composition and microstructural features developed during solidification, thermal treatments, and (for wrought products) deformation processing Although pure aluminum is very resistant to corrosion because of the presence of a film of aluminum oxide, corrosion resistance generally decreases with increasing alloy content, so tempers have been developed to improve the corrosion resistance of the highly alloyed materials Comprehensive information about the properties, processing, and applications of aluminum alloys is provided in the ASM Specialty Handbook: Aluminum and Aluminum Alloys (Ref 3) Other useful reference sources are listed as Ref 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, and 17 Aluminum Alloy Phase Diagrams Although few products are sold and used in their equilibrium condition, equilibrium phase diagrams are an essential tool in understanding effects of composition and both solidification and solid-state thermal processing on microstructure For aluminum alloys, phase diagrams are used to determine solidification and melting temperatures, the solidification path, and the equilibrium phases that form and their dissolution temperatures In addition to determining appropriate temperatures for casting and thermal treatments, phase diagrams are used to determine the maximum levels for ancillary element additions of certain elements to prevent the crystallization of coarse primary particles The most important liquidto-solid transformations for aluminum alloys are the eutectic and the peritectic Examples of phase diagrams illustrating eutectic and peritectic reactions are discussed in the following paragraphs, and phase diagrams for aluminum alloys can be found in Ref 2, 3, 10, 11, and 17 The eutectic reaction is illustrated by the aluminum-copper system (Fig 1) When the liquidus temperature of aluminumrich alloys is reached during solidification, the liquid begins to solidify into a solid solution of copper in aluminum ( aluminum) As temperature approaches the solidus, the -aluminum becomes more enriched with copper When the temperature falls below the solidus temperature in alloys containing less than the maximum solubility, 5.7% Cu, solidification is complete At temperatures below the solvus, Al2Cu particles precipitate, depleting the -aluminum of copper When cooled to room temperature under near-equilibrium conditions, the -aluminum contains little copper, so strength is low To increase strength, the material must be solution heat treated, quenched, and aged to develop metastable precipitates as described later in this section In alloys containing more than 5.7% Cu, some liquid remains when the eutectic temperature is reached This liquid solidifies at this temperature by a eutectic reaction to -aluminum and Al2Cu intermetallic particles On cooling below the eutectic temperature, the -aluminum rejects copper as Al2Cu precipitates It is important to realize that the eutectic reaction can occur in alloys containing less than the maximum solid solubility under commercial casting conditions, even though the equilibrium phase diagram does not predict that Consequently, Al2Cu particles form during solidification of most aluminum alloy ingots and shaped castings Therefore, they are "preheated" or homogenized to dissolve the intermetallic particles Fig 1 Aluminum-copper phase diagram illustrating the eutectic reaction The peritectic reaction in aluminum alloys is typified by the aluminum-chromium system (Fig 2) During equilibrium solidification of alloys containing more than the peritectic composition, 0.41% Cr, but less than the maximum solid solubility of 0.77%, an intermetallic compound, Al7Cr, forms when the liquidus temperature is reached When the temperature falls to the peritectic temperature, 661 °C, the remaining liquid along with the Al7Cr transforms to aluminum Under commercial solidification conditions, however, the primary particles of Al7Cr would not have the opportunity to transform to -aluminum, so they would remain Consequently, maximum chromium limits are established so that all of the chromium remains in supersaturated solid solution in the ingot It precipitates as chromiumbearing dispersoids during ingot preheat Fig 2 Aluminum-chromium phase diagram illustrating the peritectic reaction Aluminum Alloy Impurity and Alloying Elements All commercial aluminum alloys contain iron and silicon as well as two or more elements intentionally added to enhance properties The phases formed and the function of the alloying elements are described Iron Virtually all aluminum alloys contain some iron that is an impurity remaining after refining bauxite and smelting The phase diagram predicts that during solidification of an aluminum-iron alloy containing a few tenths of a percent of iron, most of the iron remains in the liquid phase until a eutectic of solid solution plus Al3Fe intermetallic constituent particles having a monoclinic crystal structure freezes Depending on solidification rate and on the presence of other elements such as manganese, constituent particles of the metastable orthorhombic Al6Fe phase can form instead of the equilibrium Al3Fe The maximum solid solubility of iron in aluminum is 0.05%, but the solubility is much lower in most structural alloys Silicon This element is also a ubiquitous impurity in commercial aluminum alloys Two ternary phases, cubic Al12Fe3Si and monoclinic -Al9Fe2Si2, form by a eutectic reaction At low silicon contents, almost all of the iron is present as Al3Fe With increasing silicon contents, first the - then the -Al-Fe-Si phases appear Phases in commercial products may not be those predicted by the equilibrium phase diagrams because of the long times at high temperatures required to approach equilibrium In large amounts, silicon improves castability and fluidity Consequently, it is used in 4xxx brazing sheet and in 3xx.x and 4xx.x casting alloys Silicon ranges from about 5 to 20% in casting alloys Hypereutectic alloys (those containing >12.6% Si, the eutectic composition) are used for engine blocks because the primary silicon particles are wear resistant Some 3xx.x casting alloys contain small additions of magnesium to render them capable of being age hardened Silicon is deliberately added to some alloys containing magnesium to provide precipitation hardening The Al-Mg-Si system is the basis for the 6xxx alloys At low magnesium contents, elemental silicon may be present as second-phase particles As magnesium increases, both silicon particles and equilibrium hexagonal Mg2Si constituents may be present At higher magnesium contents, only Mg2Si is present Ternary alloys are strengthened by precipitation of metastable precursors to Mg2Si With the addition of copper, a complex quaternary Al4CuMg5Si4Q phase can form A precursor to this quaternary phase strengthens Al-Cu-Mg-Si alloys Manganese The aluminum-manganese system is the basis for the oldest aluminum alloys Such alloys, known as 3xxx, are the most widely used wrought alloys because of their excellent formability and resistance to corrosion Commercial aluminum-manganese alloys contain both iron and silicon During solidification of commercial size ingots, some of the manganese forms Al6(Mn,Fe) and cubic Al12(Fe,Mn) Si by eutectic reactions The remaining manganese remains in solution and is precipitated during the ingot preheat as Al12(Mn,Fe)Si and Al6(Mn,Fe) dispersoids These dispersoids strengthen the material and control recrystallized grain size In alloys containing copper, manganese precipitates as Al20Cu2Mn3 dispersoid particles Effects on strength are minor, but the dispersoids aid in grain size control after solution heat treatment Magnesium The aluminum-magnesium system is the basis for the wrought 5xxx and cast 5xx.x non-heat-treatable aluminum alloys, which provide excellent combinations of strength and corrosion resistance by solid-solution strengthening and work hardening Although in principle this phase diagram exhibits a positively sloping solvus, a necessary condition for a precipitation-hardening system, difficulty in nucleating the face-centered cubic (fcc) Al3Mg2 precipitates has precluded commercialization of heat-treatable aluminum-magnesium alloys, unless they contain enough silicon, copper, or zinc to form Mg2Si, Al-Cu-Mg, or Al-Zn-Mg precipitates Copper The aluminum-copper system is the basis for the wrought 2xxx and cast 2xx.x alloys, and many other heattreatable alloys contain copper In commercial aluminum-copper alloys, some of the copper chemically combines with aluminum and iron to form either tetragonal Al7Cu2Fe or orthorhombic (Al,Cu,Fe) constituent particles during solidification These constituents cannot be dissolved during subsequent thermal treatments, but one can transform to the other during thermal treatments of ingots or castings During heat treatment of aluminum-copper alloys containing little magnesium, Al2Cu precipitates as the strengthening phase Adding magnesium to aluminum-rich aluminum-copper alloys results in the formation of the Al2CuMg phase by eutectic decomposition Metastable precursors to face-centered orthorhombic Al2CuMg precipitates are used to strengthen several structural alloys used in the aerospace industry because they confer a desirable combination of strength, fracture toughness, and resistance to the growth of fatigue cracks Zinc This element confers little solid-solution strengthening or work hardening to aluminum, but Al-Zn-Mg precipitates provide the basis for the 7xxx wrought alloys and the 7xx.x cast alloys Two phases can form by eutectic decomposition in commercial Al-Zn-Mg alloys: hexagonal MgZn2 and body-centered cubic (bcc) Al2Mg3Zn3 Depending on the zinc/magnesium ratio, copper-free alloys are strengthened by metastable precursors to either MgZn2 or Al2Mg3Zn3 In AlZn-Mg-Cu alloys, copper and aluminum substitute for zinc in MgZn2 to form Mg(Zn,Cu,Al)2 Al2CuMg particles can also form in these alloys by eutectic decomposition and solid-state precipitation Chromium In commercial alloys, the solubility can be reduced to such an extent that Al7Cr primary particles can form by a peritectic reaction at chromium contents lower than that indicated by the binary aluminum-chromium phase diagram Because coarse primary particles are harmful to ductility, fatigue, and fracture toughness, the upper limits of chromium depend on the amount and nature of the other alloying and impurity elements In 5xxx alloys, fcc cubic Al18Mg3Cr2 dispersoids precipitate during ingot preheating In 7xxx alloys, the composition of the dispersoids is closer to Al12Mg2Cr Chromium dispersoids contribute to strength in non-heat-treatable alloys and control grain size and degree of recrystallization in heat-treatable alloy products Zirconium This element also forms a peritectic with aluminum The phase diagram predicts that the equilibrium Al3Zr phase is tetragonal, but fine dispersoids of metastable cubic Al 3Zr form during ingot preheating treatments Most 7xxx and some 6xxx and 5xxx alloys developed since the 1960s contain small amounts of zirconium, usually less than 0.15%, to form Al3Zr dispersoids for recrystallization control Lithium This element reduces the density and increases the modulus of aluminum alloys In binary alloys it forms metastable Al3Li precipitates and combines with aluminum and copper in Al-Cu-Li alloys to form a large number of Al- Cu-Li phases Because of its high cost relative to other alloying elements, lithium alloys have been found to be cost effective thus far only in space and military applications Aluminum Alloy Designation System The alloy designation system classifies alloys by their composition In the United States, aluminum alloys are designated by two distinct four-digit systems, one for wrought aluminum (Table 1) and one for cast products (Table 2) The system for designating wrought alloys has been adopted worldwide No worldwide system for designating aluminum casting alloys exists; European manufacturers often use tradenames as identification Table 1 Designation system for wrought aluminum alloys Aluminum, at least 99.00% 1xxx Aluminum alloys grouped by major alloying elements: Copper 2xxx Manganese 3xxx Silicon 4xxx Magnesium 5xxx Magnesium and silicon 6xxx Zinc 7xxx Other element 8xxx The last two digits indicate the aluminum purity or identify the aluminum alloy The second digit indicates modifications of the original alloy or impurity limits Table 2 Designation system for cast aluminum alloys Aluminum, at least 99.00% 1xx.x Aluminum alloys grouped by major alloying elements: Copper 2xx.x Silicon with added copper or manganese 3xx.x Silicon 4xx.x Magnesium 5xx.x Zinc 7xx.x Tin 8xx.x Other element 9xx.x The second two digits indicate the aluminum purity or identify the aluminum alloy The last digit indicates the product form, castings (xxx.0) or foundry ingot (xxx.1 and xxx.2) A modification of the original alloy or impurity limits is designated by a serial letter before the numerical designation The series 6xx.x is unused Nominal compositions for several wrought structural alloys are presented in Tables 3 and 4 Nominal compositions of several casting alloys are presented in Table 5 Table 3 Nominal chemical compositions of selected 3xxx and 5xxx aluminum alloys Alloy Alloying element content, wt % Cu Mn Mg Cr 3003 0.12 1.2 3004 1.2 1.0 3005 1.2 0.4 3105 0.6 0.5 5005 0.8 5050 1.4 5052 2.5 0.25 5252 2.5 5154 3.5 0.25 5454 0.8 2.7 0.12 5056 0.12 5.0 0.12 5456 0.8 5.1 0.12 5182 0.35 4.5 5083 0.7 4.4 0.15 5086 0.45 4.0 0.15 All contain iron and silicon as impurities Table 4 Nominal chemical compositions of selected 2xxx, 6xxx, and 7xxx aluminum alloys Alloy Alloying element content, wt % Fe Si Cu Mn Mg Cr Zn Zr 2008 0.40(a) 0.65 0.9 0.3(a) 0.4 2219(b) 0.30(a) 0.20(a) 6.3 0.3 0.18 2519(b) 0.39(a)(c) 0.30(a)(c) 5.8 0.3 0.25 0.18 2014 0.7(a) 0.8 4.4 0.8 0.5 2024 0.50(a) 0.50(a) 4.4 0.6 1.5 2124 0.30(a) 0.20(a) 4.4 0.6 1.5 2224 0.15(a) 0.12(a) 4.4 0.6 1.5 2324 0.12(a) 0.10(a) 4.4 0.6 1.5 2524 0.12(a) 0.06(a) 4.25 0.6 1.4 2036 0.50(a) 0.50(a) 2.6 0.25 0.45 6009 0.50(a) 0.8 0.4 0.5 0.6 6061 0.7(a) 0.6 0.3 1.0 0.2 6063 0.50(a) 0.4 0.7 6111 0.4(a) 0.9 0.7 0.3 0.8 7005 0.40(a) 0.35(a) 0.45 1.4 0.13 4.5 0.14 7049 0.35(a) 0.25(a) 1.6 2.4 0.16 7.7 7050 0.15(a) 0.12(a) 2.3 2.2 6.2 0.12 7150 0.15(a) 0.10(a) 2.2 2.4 5.4 0.12 7055 0.15(a) 0.10(a) 2.3 2.1 8.0 0.12 7075 0.50(a) 0.40(a) 1.6 2.5 0.25 5.6 7475 0.12(a) 0.10(a) 1.6 2.2 0.20 5.7 (a) Maximum allowable amount (b) 2219 and 2519 also contain 0.10% V and 0.06% Ti (c) 0.40% max Fe plus Si Table 5 Nominal chemical compositions of selected aluminum casting alloys Alloy Alloying element content, wt % Fe Si Cu Mn Mg 354.0 0.2(a) 9.0 1.8 0.5 A356.0 0.2(a) 7.0 0.1(a) 0.1(a) 0.3 365.0 0.15(a) 10.5 0.65 0.3 (a) Maximum allowable amount Aluminum Alloy Temper Designation System The temper designation system (Table 6) classifies tempers by certain thermal and mechanical processes that control strength and other characteristics of both wrought and cast aluminum alloy products In general, wrought alloys in the 2xxx, 6xxx, and 7xxx series and cast alloys in the 2xx.x, 3xx.x, and 7xx.x series are referred to as heat-treatable alloys because they can be precipitation hardened and are used in T tempers Typical tensile properties of heat-treatable wrought and cast alloy products are presented in Tables 7 and 8, respectively The remaining are referred to as non-heat-treatable alloys and are strengthened by solid-solution strengthening, dispersion strengthening and, for wrought alloys, work hardening They are used in either O or H tempers Typical tensile properties of wrought non-heat-treatable alloy sheet products are presented in Table 9 Table 6 Temper designation system for aluminum alloys Designation Processing Comments F As fabricated Products of shaping processes in which no special control over thermal conditions or work hardening is employed O Annealed Wrought products that are annealed to obtain the lowest strength temper and to cast products that are annealed to improve ductility and dimensional stability H Work hardened (wrought products only) Products that have their strength increased by work hardening, with or without supplementary treatments to produce some reduction in strength The H is always followed by two or more digits H1 Work hardened only The number following this designation indicates the degree of work hardening H2 Work hardened annealed H3 Work hardened and stabilized Products that are work hardened and whose properties are stabilized either by a low-temperature thermal treatment or as a result of heat introduced during fabrication The number following this designation indicates the degree of work hardening remaining after the stabilization procedure W Solution heat treated An unstable temper applicable only to alloys that spontaneously age at room temperature after solution heat treatment This designation is specific only when and partially Products that are work hardened more than the desired final amount and then reduced in strength by partial annealing The number following this designation indicates the degree of work hardening after the product has been partially annealed the period of natural aging is indicated For example, W h T Thermally treated to produce stable properties other than F, O, or H Products that are thermally treated, with or without supplementary work hardening, to produce stable tempers The T is always followed by one or more digits T3 Solution treated, quenched, cold worked, and naturally aged to a substantially stable condition Products that are cold worked after solution treatment and the effect of cold work is recognized in mechanical property limits T4 Solution treated, quenched, and naturally aged to a substantially stable condition Products that are either not cold worked after solution heat treatment or are cold worked and the effects of cold work might not be recognized in mechanical property limits T5 Quenched from an elevatedtemperature shaping process and then artificially aged T6 Solution heat treated, quenched, and artificially aged Products that are either not cold worked after solution heat treatment or are cold worked and the effects of cold work might not be recognized in mechanical property limits ... Molasses Tank, Eng News-Rec., Vol 82 (No 20) , 15 May 1919, p 97 4-9 76 16 Boston Molasses-Tank Trial: The Case for the Defense, Eng News-Rec., Vol 85 (No 15), Oct 1 920, p 69 1 -6 92 17 Experts Deny Bomb... Belgium, Eng News-Rec., Vol 120 (No 18), May 1938, p 65 4 -6 55 20 O Bondy, Brittle Steel a Feature of Belgian Bridge Failure, Eng News-Rec., Vol 121 (No 7), 18 August 1938, p 204 -2 06 21 A.M Portevin,... Molasses Tank, Eng News-Rec., Vol 82 (No 20) , 15 May 1919, p 97 4-9 76 16 Boston Molasses-Tank Trial: The Case for the Defense, Eng News-Rec., Vol 85 (No 15), Oct 1 920, p 69 1 -6 92 17 Experts Deny Bomb