Ferrous Alloys - Material Science for Engineers - Lecture Slides, Slides of Material Engineering

Download Ferrous Alloys - Material Science for Engineers - Lecture Slides and more Slides Material Engineering in PDF only on Docsity! The Science and Engineering of Materials, 4th ed Chapter 12 – Ferrous Alloys Docsity.com 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. Docsity.com 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: www.steel.org. Used with permission of the American Iron and Steel Institute.) Docsity.com • 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 Docsity.com © 2 0 0 3 B ro o k s/ C o le , a d iv is io n o f T h o m so n L ea rn in g , In c. T h o m so n L ea rn in g ™ i s a tr ad em ar k u se d h er ei n u n d er l ic en se . Figure 12.2 (a) The eutectoid portion of the Fe-Fe3C phase diagram. (b) An expanded version of the Fe-C diagram, adapted from several sources. Docsity.com 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 steel’s 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 steel’s carbon content. Example 12.1 Design of a Method to Determine AISI Number Docsity.com Example 12.1 SOLUTION The first way is to heat the steel to a temperature just below the A1 temperature and hold for a long time. The steel overtempers and large Fe3C 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% Fe3C using this method, the carbon content is: %086.1or 16100 )0218.067.6( )0218.0( CFe % 3          x x A better approach, however, is to heat the steel above the Acm 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 Fe3C, then: %065.1or 95100 77.067.6 -6.67 Pearlite %         x x Docsity.com  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 Docsity.com Figure 12.6 The microstructure of spheroidite, with Fe3C particles dispersed in a ferrite matrix ( 850). (From ASM Handbook, Vol. 7, (1972), ASM International, Materials Park, OH 44073.) Docsity.com Recommend temperatures for the process annealing, annealing, normalizing, and spheroidizing of 1020, 1077, and 10120 steels. Example 12.2 Determination of Heat Treating Temperatures Docsity.com © 2 0 0 3 B ro o k s/ C o le , a d iv is io n o f T h o m so n L ea rn in g , In c. T h o m so n L ea rn in g ™ i s a tr ad em ar k u se d h er ei n u n d er l ic en se . Figure 12.2 (a) The eutectoid portion of the Fe-Fe3C phase diagram. (b) An expanded version of the Fe-C diagram, adapted from several sources. Docsity.com ©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. Docsity.com © 2 0 0 3 B ro o k s/ C o le , a d iv is io n o f T h o m so n L ea rn in g , In c. T h o m so n L ea rn in g ™ i s a tr ad em ar k u se d h er ei n u n d er l ic en se . Figure 12.8 The TTT diagrams for (a) a 1050 and (b) a 10110 steel. Docsity.com 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 © 2 0 0 3 B ro o k s/ C o le , a d iv is io n o f T h o m so n L ea rn in g , In c. T h o m so n L ea rn in g ™ i s a tr ad em ar k u se d h er ei n u n d er l ic en se . Figure 12.8 The TTT diagrams for (a) a 1050 and (b) a 10110 steel. Docsity.com © 2 0 0 3 B ro o k s/ C o le , a d iv is io n o f T h o m so n L ea rn in g , In c. T h o m so n L ea rn in g ™ i s a tr ad em ar k u se d h er ei n u n d er l ic en se . Figure 12.9 Producing complicated structures by interrupting the isothermal heat treatment of a 1050 steel. Docsity.com Figure 12.10 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 44073.) Docsity.com  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 MS and holding until the temperature is equalized throughout the steel before further cooling to produce martensite. Section 12.4 Quench and Temper Heat Treatments Docsity.com Figure 12.12 Retained austenite (white) trapped between martensite needles (black) ( 1000). (From ASM Handbook, Vol. 8, (1973), ASM International, Materials Park, OH 44073.) Docsity.com © 2 0 0 3 B ro o k s/ C o le , a d iv is io n o f T h o m so n L ea rn in g , In c. T h o m so n L ea rn in g ™ i s a tr ad em ar k u se d h er ei n u n d er l ic en se . Figure 12.13 Increasing carbon reduces the Ms and Mf temperatures in plain-carbon steels. Docsity.com ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 12.14 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. Docsity.com ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 12.16 The CCT diagram (solid lines) for a 1080 steel compared with the TTT diagram (dashed lines). Docsity.com ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 12.17 The CCT diagram for a low-alloy, 0.2% C Steel. Docsity.com  Hardenability - Alloy steels have high hardenability.  Effect on the Phase Stability - When alloying elements are added to steel, the binary Fe-Fe3C 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 Docsity.com ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 12.20 When alloying elements introduce a bay region into the TTT diagram, the steel can be ausformed. Docsity.com ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 12.21 The effect of alloying elements on the phases formed during the tempering of steels. The air-hardenable steel shows a secondary hardening peak. Docsity.com  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 Docsity.com TABLE 12-3 @ The relationship between cooling rate and Jominy distance Jominy Distance (in.) Cooling Rate (°C/s) - 315 % 110 3% 50 £ 36 3 28 & 22 % 17 = 15 z 10 # 8 % 5 2 3 8 28 8 2.5 # 2.2 Docsity.com 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 © 2 0 0 3 B ro o k s/ C o le , a d iv is io n o f T h o m so n L ea rn in g , In c. T h o m so n L ea rn in g ™ i s a tr ad em ar k u se d h er ei n u n d er l ic en se . Figure 12.23 The hardenability curves for several steels. Docsity.com TABLE 12-1 & Compositions of selected AISI-SAE steels AISI-SAE Number %C % Mn % Si % Ni % Cr Others 1020 0.18-0.23 0.30-0.60 1040 0.37-0.44 0.60-0.90 1060 0.55-0.65 0.60-0.90 1080 0.75-0.88 0.60-0.90 1095 0.90-1.03 0.30-0.50 1140 0.37-0.44 0.70-1.00 0.08-0.13% S$ 4140 0.38-0.43 0.75-1.00 0.15-0.30 0.80-1.10 0.15-0.25% Moa 4340 0.38-0.43 0.60-0.80 0.15-0.30 1.65-2.00 0.70-0.90 0.20-0.300% Mo 4620 0.17-0.22 0.45-0.65 0.15-0.30 1.65-2.00 0.20-0.30% Mo 52100 0.98-1.10 0.25-0.45 0.15-0.30 1,30-1.60 8620 0.18-0.23 0.70-0.90 0.15-0.30 0.40-0.70 0.40-0.60 0.15-0.25% Y 9260 0.56-0.64 0.75-1.00 1.80-2.20 Docsity.com 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 Docsity.com ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 12.24 The Grossman chart used to determine the hardenability at the center of a steel bar for different quenchants. Docsity.com © 2 0 0 3 B ro o k s/ C o le , a d iv is io n o f T h o m so n L ea rn in g , In c. T h o m so n L ea rn in g ™ i s a tr ad em ar k u se d h er ei n u n d er l ic en se . Figure 12.23 The hardenability curves for several steels. Docsity.com Figure 12.25 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.) Docsity.com  Selectively Heating the Surface - Rapidly heat the surface of a medium-carbon steel above the A3 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 Docsity.com ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 12.26 (a) Surface hardening by localized heating. (b) Only the surface heats above the A1 temperature and is quenched to martensite. Docsity.com 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 A3 temperature (about 770oC). 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 A3 temperature (about 860oC), 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. Docsity.com Section 12.9 Weldability of Steel © 2 0 0 3 B ro o k s/ C o le , a d iv is io n o f T h o m so n L ea rn in g , In c. T h o m so n L ea rn in g ™ i s a tr ad em ar k u se d h er ei n u n d er l ic en se . Figure 12.29 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. Docsity.com 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 5oC/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 Docsity.com TABLE 12-4 m™ Typical compositions and properties of siainless steels Tensile Yield Strength Strength % Steel %C %Cr %Ni Others (psi) (psi) Elongation Condition Austenitic: 201 0.15 17 5 6.5% Mn 95,000 45,000 40 Annealed 304. 0.08 19 10 75,000 30,000 30 Annealed 185,000 = 140,000 9 Cold-worked 304L = (0.08 19 10 75,000 30,000 30 Annealed 316 0.08 17 12 2.5% Mo 75,000 30,000 30 Annealed 321 0.08 18 10 04% Ti 85,000 35,000 55 Annealed 347 0.08 18 1 0.8% Nb 30,000 35,000 50 Annealed Ferritic: 430 0.12 17 65,000 30,000 22 Annealed 442 0.12 20 75,000 40,000 20 Annealed Martensitic: 416 0.15 13 0.6% Mo 180,000 = 140,000 18 Quenched and tempered 431 0.20 16 2 200,000 150,000 16 Quenched and tempered 440c 1.10 17 0.7% Mo 285,000 =275,000 2 Quenched and tempered Precipitation hardening: 17-4 0.07 17 4 0.4% Nb 190,000 ~=—-170,000 10 Age-hardened 17-7 0.09 17 7 1.0% Al 240,000 230,000 6 Age-hardened Docsity.com Figure 12.31 (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 44073.) Docsity.com 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 treatments—such as annealing, cold working, or quench and tempering—the 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 Docsity.com ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 12.33 The iron-carbon phase diagram showing the relationship between the stable iron-graphite equilibria (solid lines) and the metastable iron-cementite reactions (dashed lines). Docsity.com ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 12.34 The transformation diagram for austenite in a cast iron. Docsity.com ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 12.35 (a) Sketch and (b) photomicrograph of the flake graphite in gray cast iron (x 100). Docsity.com Figure 12.38 (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 44073.) (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). Docsity.com TABLE 12-5 @ Typical properties of cast irons Tensile Yield Strength Strength (psi) (psi) %E Notes Gray irons: Class 20 12,000—40,000 _— _— Class 40 28,000-54,000 _— _ Class 60 44,00-66,000 _— _ Malleable irons: 32510 50,000 32,500 10 Ferritic 35018 53,000 35,000 18 Ferritic 50005 70,000 50,000 5 Pearlitic 70003 85,000 70,000 3 Pearlitic 90001 105,000 90,000 1 Pearlitic Ductile irons: 60-40-18 60,000 40,000 18 Annealed 65-45-12 65,000 45,000 12 As-cast ferritic 80-55-06 80,000 55,000 6 As-cast pearlitic 100-70-03 10,000 70,000 3 Normalized 120-90-02 120,000 90,000 2 Quenched and tempered Compacted graphite irons: Low strength 40,000 28,000 5 90% Ferritic High strength 65,000 55,000 1 80% Pearlitic Docsity.com ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 12.17 (Repeated for Problem 12.20) The CCT diagram for a low-alloy, 0.2% C steel. Docsity.com