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1 The Science and Engineering of Materials, 4 th ed Donald R. Askeland – Pradeep P. Phulé Chapter 12 – Ferrous Alloys.

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Presentation on theme: "1 The Science and Engineering of Materials, 4 th ed Donald R. Askeland – Pradeep P. Phulé Chapter 12 – Ferrous Alloys."— Presentation transcript:

1 1 The Science and Engineering of Materials, 4 th ed Donald R. Askeland – Pradeep P. Phulé Chapter 12 – Ferrous Alloys

2 2 Objectives of Chapter 12 Discuss how to use the eutectoid reaction to control the structure and properties of steels through heat treatment and alloying. Examine two special classes of ferrous alloys: stainless steels and cast irons.

3 3 Chapter Outline 12.1 Designations and Classification of Steels 12.2 Simple Heat Treatments 12.3 Isothermal Heat Treatments 12.4 Quench and Temper Heat Treatments 12.5 Effect of Alloying Elements 12.6 Application of Hardenability

4 Specialty Steels 12.8 Surface Treatments 12.9 Weldability of Steel Stainless Steels Cast Irons Chapter Outline (Continued)

5 5 Figure 12.1 (a) In a blast furnace, iron ore is reduced using coke (carbon) and air to produce liquid pig iron. The high-carbon content in the pig iron is reduce by introducing oxygen into the basic oxygen furnace to produce liquid steel. An electric arc furnace can be used to produce liquid steel by melting scrap. (b) Schematic of a blast furnace operation. (Source: Used with permission of the American Iron and Steel Institute.)

6 6 Designations - The AISI (American Iron and Steel Institute) and SAE (Society of Automotive Engineers) provide designation systems for steels that use a four- or five-digit number. Classifications - Steels can be classified based on their composition or the way they have been processed. Section 12.1 Designations and Classification of Steels

7 7 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure 12.2 (a) The eutectoid portion of the Fe-Fe 3 C phase diagram. (b) An expanded version of the Fe-C diagram, adapted from several sources.

8 8 Figure 12.3 Electron micrographs of (a) pearlite, (b) bainite, and (c) tempered martensite, illustrating the differences in cementite size and shape among these three microconstituents ( 7500). (From The Making, Shaping, and Treating of Steel, 10th Ed. Courtesy of the Association of Iron and Steel Engineers.)

9 9

10 10 An unalloyed steel tool used for machining aluminum automobile wheels has been found to work well, but the purchase records have been lost and you do not know the steels composition. The microstructure of the steel is tempered martensite, and assume that you cannot estimate the composition of the steel from the structure. Design a treatment that may help determine the steels carbon content. Example 12.1 Design of a Method to Determine AISI Number

11 11 Example 12.1 SOLUTION The first way is to heat the steel to a temperature just below the A 1 temperature and hold for a long time. The steel overtempers and large Fe 3 C spheres form in a ferrite matrix. We then estimate the amount of ferrite and cementite and calculate the carbon content using the lever law. If we measure 16% Fe 3 C using this method, the carbon content is: A better approach, however, is to heat the steel above the A cm to produce all austenite. If the steel then cools slowly, it transforms to pearlite and a primary microconstituent. If, when we do this, we estimate that the structure contains 95% pearlite and 5% primary Fe 3 C, then:

12 12 Process Annealing Eliminating Cold Work: A low- temperature heat treatment used to eliminate all or part of the effect of cold working in steels. Annealing and Normalizing Dispersion Strengthening: Annealing - A heat treatment used to produce a soft, coarse pearlite in steel by austenitizing, then furnace cooling. Normalizing - A simple heat treatment obtained by austenitizing and air cooling to produce a fine pearlitic structure. Spheroidizing Improving Machinability: Spheroidite - A microconstituent containing coarse spheroidal cementite particles in a matrix of ferrite, permitting excellent machining characteristics in high-carbon steels. Section 12.2 Simple Heat Treatments

13 13 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure 12.4 Schematic summary of the simple heat treatments for (a) hypoeutectoid steels and (b) hypereutectoid steels.

14 14 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure 12.5 The effect of carbon and heat treatment on the properties of plain-carbon steels.

15 15 Figure 12.6 The microstructure of spheroidite, with Fe 3 C particles dispersed in a ferrite matrix ( 850). (From ASM Handbook, Vol. 7, (1972), ASM International, Materials Park, OH )

16 16 Recommend temperatures for the process annealing, annealing, normalizing, and spheroidizing of 1020, 1077, and steels. Example 12.2 Determination of Heat Treating Temperatures

17 17 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure 12.2 (a) The eutectoid portion of the Fe-Fe 3 C phase diagram. (b) An expanded version of the Fe-C diagram, adapted from several sources.

18 18 Example 12.2 SOLUTION From Figure 12.2, we find the critical A1, A 3, or A cm, temperatures for each steel. We can then specify the heat treatment based on these temperatures.

19 19 Austempering - The isothermal heat treatment by which austenite transforms to bainite. Isothermal annealing - Heat treatment of a steel by austenitizing, cooling rapidly to a temperature between the A 1 and the nose of the TTT curve, and holding until the austenite transforms to pearlite. Section 12.3 Isothermal Heat Treatments

20 20 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure 12.7 The austempering and isothermal anneal heat treatments in a 1080 steel.

21 21 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure 12.8 The TTT diagrams for (a) a 1050 and (b) a steel.

22 22 A heat treatment is needed to produce a uniform microstructure and hardness of HRC 23 in a 1050 steel axle. Example 12.3 Design of a Heat Treatment for an Axle ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure 12.8 The TTT diagrams for (a) a 1050 and (b) a steel.

23 23 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure 12.2 (a) The eutectoid portion of the Fe-Fe 3 C phase diagram. (b) An expanded version of the Fe-C diagram, adapted from several sources.

24 24 Example 12.3 SOLUTION 1.Austenitize the steel at (30 to 55) = 805 o C to 825 o C, holding for 1 h and obtaining 100% γ. 2.Quench the steel to 600 o C and hold for a minimum of 10 s. Primary ferrite begins to precipitate from the unstable austenite after about 1.0 s. After 1.5 s, pearlite begins to grow, and the austenite is completely transformed to ferrite and pearlite after about 10 s. After this treatment, the microconstituents present are: 3. Cool in air-to-room temperature, preserving the equilibrium amounts of primary ferrite and pearlite. The microstructure and hardness are uniform because of the isothermal anneal.

25 25 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure 12.9 Producing complicated structures by interrupting the isothermal heat treatment of a 1050 steel.

26 26 Figure Dark feathers of bainite surrounded by light martensite, obtained by interrupting the isothermal transformation process ( 1500). (ASM Handbook, Vol. 9 Metallography and Microstructure (1985), ASM International, Materials Park, OH )

27 27 Retained austenite - Austenite that is unable to transform into martensite during quenching because of the volume expansion associated with the reaction. Tempered martensite - The microconstituent of ferrite and cementite formed when martensite is tempered. Quench cracks - Cracks that form at the surface of a steel during quenching due to tensile residual stresses that are produced because of the volume change that accompanies the austenite-to-martensite transformation. Marquenching - Quenching austenite to a temperature just above the M S and holding until the temperature is equalized throughout the steel before further cooling to produce martensite. Section 12.4 Quench and Temper Heat Treatments

28 28 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The effect of tempering temperature on the mechanical properties of a 1050 steel.

29 29 Example 12.4 Design of a Quench and Temper Treatment A rotating shaft that delivers power from an electric motor is made from a 1050 steel. Its yield strength should be at least 145,000 psi, yet it should also have at least 15% elongation in order to provide toughness. Design a heat treatment to produce this part. Example 12.4 SOLUTION 1.Austenitize above the A 3 temperature of 770 o C for 1 h. An appropriate temperature may be = 825 o C. 2.Quench rapidly to room temperature. Since the M f is about 250 o C, martensite will form. 3.Temper by heating the steel to 440 o C. Normally, 1 h will be sufficient if the steel is not too thick. 4.Cool to room temperature.

30 30 Figure Retained austenite (white) trapped between martensite needles (black) ( 1000). (From ASM Handbook, Vol. 8, (1973), ASM International, Materials Park, OH )

31 31 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure Increasing carbon reduces the M s and M f temperatures in plain-carbon steels.

32 32 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure Formation of quench cracks caused by residual stresses produced during quenching. The figure illustrates the development of stresses as the austenite transforms to martensite during cooling.

33 33 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The marquenching heat treatment designed to reduce residual stresses ands quench cracking.

34 34

35 35 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The CCT diagram (solid lines) for a 1080 steel compared with the TTT diagram (dashed lines).

36 36 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The CCT diagram for a low-alloy, 0.2% C Steel.

37 37 Hardenability - Alloy steels have high hardenability. Effect on the Phase Stability - When alloying elements are added to steel, the binary Fe-Fe 3 C stability is affected and the phase diagram is altered. Shape of the TTT Diagram - Ausforming is a thermomechanical heat treatment in which austenite is plastically deformed below the A1 temperature, then permitted to transform to bainite or martensite. Tempering - Alloying elements reduce the rate of tempering compared with that of a plain-carbon steel. Section 12.5 Effect of Alloying Elements

38 38 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure (a) TTT and (b) CCT curves for a 4340 steel.

39 39 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The effect of 6% manganese on the stability ranges of the phases in the eutectoid portion of the Fe-Fe 3 C phase diagram.

40 40 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure When alloying elements introduce a bay region into the TTT diagram, the steel can be ausformed.

41 41 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The effect of alloying elements on the phases formed during the tempering of steels. The air-hardenable steel shows a secondary hardening peak.

42 42 Jominy test - The test used to evaluate hardenability. An austenitized steel bar is quenched at one end only, thus producing a range of cooling rates along the bar. Hardenability curves - Graphs showing the effect of the cooling rate on the hardness of as-quenched steel. Jominy distance - The distance from the quenched end of a Jominy bar. The Jominy distance is related to the cooling rate. Section 12.6 Application of Hardenability

43 43 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The set-up for the Jominy test used for determining the hardenability of a steel.

44 44 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The hardenability curves for several steels.

45 45

46 46 A gear made from 9310 steel, which has an as-quenched hardness at a critical location of HRC 40, wears at an excessive rate. Tests have shown that an as-quenched hardness of at least HRC 50 is required at that critical location. Design a steel that would be appropriate. Example 12.5 Design of a Wear-Resistant Gear ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The hardenability curves for several steels.

47 47

48 48 Example 12.5 SOLUTION From Figure 12.23, a hardness of HRC 40 in a 9310 steel corresponds to a Jominy distance of 10/16 in. (10 o C/s). If we assume the same Jominy distance, the other steels shown in Figure have the following hardnesses at the critical location: 1050 HRC HRC HRC HRC HRC 60 In Table 12-1, we find that the 86xx steels contain less alloying elements than the 43xx steels; thus the 8640 steel is probably less expensive than the 4340 steel and might be our best choice. We must also consider other factors such as durability.

49 49 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The Grossman chart used to determine the hardenability at the center of a steel bar for different quenchants.

50 50 Design a quenching process to produce a minimum hardness of HRC 40 at the center of a 1.5-in. diameter 4320 steel bar. Example 12.6 Design of a Quenching Process

51 51 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The Grossman chart used to determine the hardenability at the center of a steel bar for different quenchants.

52 52 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The hardenability curves for several steels.

53 53 Example 12.6 SOLUTION Several quenching media are listed in Table We can find an approximate H coefficient for each of the quenching media, then use Figure to estimate the Jominy distance in a 1.5- in. diameter bar for each media. Finally, we can use the hardenability curve (Figure 12.23) to find the hardness in the 4320 steel. The results are listed below. The last three methods, based on brine or agitated water, are satisfactory. Using an unagitated brine quenchant might be least expensive, since no extra equipment is needed to agitate the quenching bath. However, H 2 O is less corrosive than the brine quenchant.

54 54 Tool steels - A group of high-carbon steels that provide combinations of high hardness, toughness, or resistance to elevated temperatures. Secondary hardening peak - Unusually high hardness in a steel tempered at a high temperature caused by the precipitation of alloy carbides. Dual-phase steels - Special steels treated to produce martensite dispersed in a ferrite matrix. Maraging steels - A special class of alloy steels that obtain high strengths by a combination of the martensitic and age-hardening reactions. Section 12.7 Specialty Steels

55 55 Figure Microstructure of a dual-phase steel, showing islands of light martensite in a ferrite matrix ( 2500). (From G. Speich, Physical Metallurgy of Dual-Phase Steels, Fundamentals of Dual- Phase Steels, The Metallurgical Society of AIME, 1981.)

56 56 Selectively Heating the Surface - Rapidly heat the surface of a medium-carbon steel above the A 3 temperature and then quench the steel. Case depth - The depth below the surface of a steel at which hardening occurs by surface hardening and carburizing processes. Carburizing - A group of surface-hardening techniques by which carbon diffuses into steel. Cyaniding - Hardening the surface of steel with carbon and nitrogen obtained from a bath of liquid cyanide solution. Carbonitriding - Hardening the surface of steel with carbon and nitrogen obtained from a special gas atmosphere. Section 12.8 Surface Treatments

57 57 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure (a) Surface hardening by localized heating. (b) Only the surface heats above the A 1 temperature and is quenched to martensite.

58 58 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure Carburizing of a low-carbon steel to produce a high-carbon, wear-resistant surface.

59 59 Design the materials and heat treatments for an automobile axle and drive gear (Figure 12.28). Example 12.7 Design of Surface-Hardening Treatments for a Drive Train ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure Sketch of axle and gear assembly (for example 12.7).

60 60 Example 12.7 SOLUTION The axle might be made from a forged 1050 steel containing a matrix of ferrite and pearlite. The axle could be surface- hardened, perhaps by moving the axle through an induction coil to selectively heat the surface of the steel above the A 3 temperature (about 770 o C). After the coil passes any particular location of the axle, the cold interior quenches the surface to martensite. Tempering then softens the martensite to improve ductility. Carburize a 1010 steel for the gear. By performing a gas carburizing process above the A 3 temperature (about 860 o C), we introduce about 1.0% C in a very thin case at the surface of the gear teeth. This high-carbon case, which transforms to martensite during quenching, is tempered to control the hardness. This high-carbon case, which transforms to martensite during quenching, is tempered to control the hardness.

61 61 Section 12.9 Weldability of Steel ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The development of the heat-affected zone in a weld: (a) the structure at the maximum temperature, (b) the structure after cooling in a steel of low hardenability, and (c) the structure after cooling in a steel of high hardenability.

62 62 Compare the structures in the heat-affected zones of welds in 1080 and 4340 steels if the cooling rate in the heat-affected zone is 5 o C/s. Example 12.8 SOLUTION The cooling rate in the weld produces the following structures: 1080: 100% pearlite 4340: Bainite and martensite The high hardenability of the alloy steel reduces the weldability, permitting martensite to form and embrittle the weld. Example 12.8 Structures of Heat-Affected Zones

63 63 Stainless steels - A group of ferrous alloys that contain at least 11% Cr, providing extraordinary corrosion resistance. Categories of stainless steels: Ferritic Stainless Steels Martensitic Stainless Steels Austenitic Stainless Steels Precipitation-Hardening (PH) Stainless Steels Duplex Stainless Steels Section Stainless Steels

64 64 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure (a) The effect of 17% chromium on the iron-carbon phase diagram. At low- carbon contents, ferrite is stable at all temperatures. (b) A section of the iron- chromium-nickel-carbon phase diagram at a constant 18% Cr-8% Ni. At low-carbon contents, austenite is stable at room temperatures.

65 65

66 66 Figure (a) Martensitic stainless steel containing large primary carbides and small carbides formed during tempering ( 350). (b) Austenitic stainless steel ( 500). (From ASM Handbook, Vols. 7 and 8, (1972, 1973), ASM International, Materials Park, OH )

67 67 In order to efficiently recycle stainless steel scrap, we wish to separate the high-nickel stainless steel from the low-nickel stainless steel. Design a method for doing this. Example 12.9 SOLUTION Performing a chemical analysis on each piece of scrap is tedious and expensive. Sorting based on hardness might be less expensive; however, because of the different types of treatmentssuch as annealing, cold working, or quench and temperingthe hardness may not be related to the steel composition. The high-nickel stainless steels are ordinarily austenitic, whereas the low-nickel alloys are ferritic or martensitic. An ordinary magnet will be attracted to the low-nickel ferritic and martensitic steels, but will not be attracted to the high-nickel austenitic steel. We might specify this simple and inexpensive magnetic test for our separation process. Example 12.9 Design of a Test to Separate Stainless Steels

68 68 Cast iron - Ferrous alloys containing sufficient carbon so that the eutectic reaction occurs during solidification. Eutectic and Eutectoid reaction in Cast Irons Types of cast irons: Gray cast iron White cast iron Malleable cast iron Ductile or nodular, cast iron Compacted graphite cast iron Section Cast Irons

69 69 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure Schematic drawings of the five types of cast iron: (a) gray iron, (b) white iron, (c) malleable iron, (d) ductile iron, and (e) compacted graphite iron.

70 70 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The iron-carbon phase diagram showing the relationship between the stable iron-graphite equilibria (solid lines) and the metastable iron-cementite reactions (dashed lines).

71 71 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The transformation diagram for austenite in a cast iron.

72 72 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure (a) Sketch and (b) photomicrograph of the flake graphite in gray cast iron (x 100).

73 73 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure The effect of the cooling rate or casting size on the tensile properties of two gray cast irons.

74 74 Figure The heat treatments for ferritic and pearlitic malleable irons.

75 75 Figure (a) White cast iron prior to heat treatment ( 100). (b) Ferritic malleable iron with graphite nodules and small MnS inclusions in a ferrite matrix ( 200). (c) Pearlitic malleable iron drawn to produce a tempered martensite matrix ( 500). (Images (b) and (c) are from Metals Handbook, Vols. 7 and 8, (1972, 1973), ASM International, Materials Park, OH ) (d) Annealed ductile iron with a ferrite matrix ( 250). (e) As-cast ductile iron with a matrix of ferrite (white) and pearlite ( 250). (f) Normalized ductile iron with a pearlite matrix ( 250).

76 76

77 77 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure (Repeated for Problem 12.20) The CCT diagram for a low-alloy, 0.2% C steel.

78 78 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure (Repeated for Problem 12.54) The hardenability curves for several steels.

79 79 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Figure 12.30b (Repeated for Problem 12.48) (b) A section of the iron-chromium- nickel-carbon phase diagram at a constant 18% Cr-8% Ni. At low-carbon contents, austenite is stable at room temperature.


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