Published on January 4, 2008
Slide1: Chapters 11 (Part II) Metal Alloys Slide2: Part I: Thermal Processing of Metal Alloys Heat Treatment Precipitation Hardening Part II: Metal Alloys and Fabrication of Metals Outline of Chapter 11 Slide3: Ferrous Alloys: Alloys containing more than 50wt.%Fe Classification of Steels Designation of Steels Nonferrous Alloys: Alloys containing less than 50wt.%Fe Aluminum Copper Magnesium Titanium Refractory metals Superalloys Noble metals Metal Alloys Slide4: Classification of Ferrous Alloys Slide5: Steels (0.008 ~ 2.14wt% C) In most steels the microstructure consists of both a and Fe3C phases. Carbon concentrations in commercial steels rarely exceed 1.0 wt%. Cast irons (2.14 ~ 6.70wt% C) Commercial cast irons normally contain less than 4.5wt% C Classification of Ferrous Alloys Based on carbon content Pure iron (< 0.008wt% C) From the phase diagram, it is composed almost exclusively of the ferrite phase at room temperature. Slide6: The carbon content is normally less than 1.0 wt%. Plain carbon steels: containing only residual concentrations of impurities other than carbon and a little manganese About 90% of all steel made is carbon steel. Alloy steels: more alloying elements are intentionally added in specific concentrations. Stainless steels Ferrous Alloys — Steels Slide7: Low-carbon steels Less than 0.25 wt%C Medium-carbon steels 0.25 ~ 0.60 wt%C High-carbon steels 0.60 ~ 1.4 wt%C Classification of Steels According to Their Carbon Contents Slide8: A four-digit number: the first two digits indicate the alloy content; the last two, the carbon concentration For plain carbon steels, the first two digits are 1 and 0; alloy steels are designated by other initial two-digit combinations (e.g., 13, 41, 43) The third and fourth digits represent the weight percent carbon multiplied by 100 For example, a 1040 steel is a plain carbon steel containing 0.40 wt% C. The Designation of Steels Slide9: A four-digit number: the first two digits indicate the alloy content; the last two, the carbon concentration 41 40 Identifies major alloying element(s) Percentage of carbon The Designation of Steels Slide10: AISI: American Iron and Steel Institute SAE: Society of Automotive Engineers UNS: Uniform Numbering System Table 11.2a AISI/SAE and UNS Designation Systems Steel Alloys: Steel Alloys Slide12: Less than 0.25 wt%C Unresponsive to heat treatments intended to form martensite; strengthening is accomplished by cold work Microstructures: ferrite and pearlite Relatively soft and weak, but having outstanding ductility and toughness Typically, sy = 275 MPa, sUT = 415~550 MPa, and ductility = 25%EL Machinable, weldable, and, of all steels, are the least expensive to produce Applications: automobile body components, structural shapes, and sheets used in pipelines, buildings, bridges, etc. Low-Carbon Steels Slide13: TTT Diagram of Some Hypoeutectoid Alloys Slide14: Table 11.1a Compositions of Five Plain Low-Carbon Steels Slide15: Table 11.1b Mechanical Characteristics of Hot-Rolled Material and Typical Applications for Various Plain Low-Carbon Steels Slide16: 0.25 ~ 0.60 wt%C May be heat treated by austenitizing, quenching, and then tempering to improve their mechanical properties Stronger than low-carbon steels and weaker than high-carbon steels a Classified as high-carbon steels Typical Tensile Properties for Oil-Quenched and Tempered Plain Carbon Medium-Carbon Steels Slide17: 0.60 ~ 1.4 wt%C Used in a hardened and tempered condition Hardest, strongest, and yet least ductile; especially wear resistant and capable of holding a sharp cutting edge Containing Cr, V, W, and Mo; these alloying elements combine with carbon to form very hard and wear-resistant carbide compounds (e.g., Cr23C6, V4C3, and WC) Applications: cutting tools and dies for forming and shaping materials, knives, razors, hacksaw blades, springs, and high-strength wire High-Carbon Steels Slide18: Table 11.3 Designations, Compositions, and Applications for Six Tool Steels Slide19: Alloy steel is more expensive than carbon steel; it should be used only when a special property is needed. Comparison of the Advantages Offered by Carbon Steels and Alloy Steels Slide20: Table 11.2a AISI/SAE and UNS Designation Systems Slide21: What makes stainless steels “stainless”? Slide22: Stainless steels are selected for their excellent resistance to corrosion. Stainless steels are divided into three classes: martensitic, ferritic, or austenitic The predominant alloying element is chromium; a concentration of at least 11 wt% Cr is required It permits a thin, protective surface layer of chromium oxide to form when the steel is exposed to oxygen. The chromium is what makes stainless steel stainless! Stainless Steels Slide23: Aluminum and aluminum alloys are the most widely used nonferrous metals. Aluminum alloys: strengthened by cold working and alloying (Cu, Mg, Si, Mn, and Zn) Nonheat-treatable: single phase, solid solution strengthening Heat treatable: precipitation hardening (MgZn2) Properties Low density (2.7 g/cm3), as compared to 7.9 g/cm3 for steel High electrical and thermal conductivity Resistant to corrosion in some common environments Easily formed and thin Al foil sheet may be rolled Al has an FCC crystal structure; its ductility is retained even at very low temperatures Limitation: low melting temperature (660°C) Aluminum and Its Alloys Aluminum Alloy Desginations: Aluminum Alloy Desginations Aluminum’s use in vehicles is rapidly increasing due to the need for fuel efficient, environmentally friendly vehicles: Aluminum’s use in vehicles is rapidly increasing due to the need for fuel efficient, environmentally friendly vehicles Al alloys can provide a weight savings of up to 55% compared to an equivalent steel structure It can match or exceed crashworthiness standards of similarly sized steel structures The Ford Motor Company now has aluminum-intensive test vehicles on the road, providing 46% weight savings in the structure, with no loss in crash protection. Aluminum plate is used in the manufacture of aircraft and for fuel tanks in spacecraft: Aluminum plate is used in the manufacture of aircraft and for fuel tanks in spacecraft Aircraft manufacturers use high-strength alloys (principally alloy 7075) to strengthen aluminum aircraft structures. Alloy 7075 has zinc and copper added for ultimate strength, but because of the copper it is very difficult to weld. 7075 has the best machinability and results in the finest finish. Lightweight aluminum is a good material for conductor cables: Lightweight aluminum is a good material for conductor cables Electrical transmission lines are the largest users of aluminum rod/bar/wire products. In fact, this is the one market in which aluminum has virtually no competition from other metals. Aluminum is simply the most economical way to deliver electrical power. Copper and Its Alloys: Unalloyed copper: So soft and ductile that it is difficult to machine Unlimited capacity to be cold worked Highly resistant to corrosion in diverse environments Copper alloys: strengthened by cold working and/or solid-solution alloying. Bronze and brass are two common copper alloys. Applications: costume jewelry, cartridge casings, automotive radiators, musical instruments, electronic packaging, and coins Copper and Its Alloys Bronze and Brass: Bronze is an alloy of copper and tin. The first metal purposely alloyed by the smith May contain up to 25% tin Brass is an alloy of copper and zinc. Contain 5-30% zinc The zinc increases the strength of the copper. Ductility and formability are also increased. Bronze and Brass Slide30: Brass — An Alloy of Copper and Zinc Fig. 9.17 The copper-zinc phase diagram. Titanium and Its Alloys: Relatively new engineering materials that possess an extraordinary combination of properties Low density (4.5 g/cm3) High melting temperature (1668°C), high elastic modulus (107 GPa) Extremely strong: 1400 MPa tensile strength at room temperature, highly ductile and easily forged and machined Limitations Chemical reactivity with other materials and oxidation problem at elevated temperatures Cost Applications: airplane structures, space vehicles, and in the petroleum and chemical industries Titanium and Its Alloys An Example of Titanium Alloy (Table 11.9): Typical Applications: High-strength prosthetic implants, chemical-processing equipment, airframe structural components An Example of Titanium Alloy (Table 11.9) Ni-Base Superalloys: Superlative combinations of properties Nickel-based alloys Other alloying elements: Nb, Mo, W, Ta, Cr, and Ti IN792: Ni-12Cr-10Co-2Mo-4W-3.5Al-4Ti-4Ta- 0.01B-0.09Zr-0.1C-0.5Hf Applications: aircraft turbine components Turbine blades and discs, high creep and oxidation resistance at elevated temperatures (1000°C) Density is an important consideration because centrifugal stresses are diminished in rotating parts when the density is reduced Ni-Base Superalloys Material Strength with Increased Temperature: Titanium Alloy Steel Aluminum Alloy Nickel Alloy Material Strength with Increased Temperature Ni-Based Superalloys Used for Turbine Blades: Modern aeroengine design constantly seeks to increase the engine operating temperature to improve overall efficiency. Materials for turbine blades are required to perform at higher and higher temperatures. Use of advanced nickel-based alloys, together with innovative cooling design Ni-Based Superalloys Used for Turbine Blades Improvement in Creep Resistance of Turbine Blades through Casting Technologies: Polycrystalline turbine blade Improvement in Creep Resistance of Turbine Blades through Casting Technologies Columnar grain structure produced by a directional solidification technique Creep resistance is further enhanced with single-crystal blades. Thermal Barrier Coatings (TBCs): Thermal Barrier Coatings (TBCs) Demands for higher efficiency and lower emission require higher operating temperatures in aeroengines The typical melting points of the superalloys used for the turbine components range from 1230-1315°C The temp. in a combustion gas environment is > 1370°C The key to meeting the higher temperature requirements lies in providing an insulating ceramic thermal barrier coating (TBC) to lower the surface temperature of superalloy underneath Refractory Metals: Melting temperatures range between 2468°C for niobium (Nb) and 3410°C for tungsten (W) Interatomic bonding is extremely strong. Large elastic moduli and high strength and hardness at ambient and elevated temperatures Applications: Ta and Mo are alloyed with stainless steel to improve its corrosion resistance. Molybdenum alloys: extrusion dies and structural parts in space vehicles Tungsten alloys: filaments, X-ray tubes, welding electrodes Refractory Metals Design Example 11.1: It is necessary to select a steel alloy for a gearbox output shaft. The design calls for a 1-in diameter cylindrical shaft having a surface hardness of at least 38 HRC and a minimum ductility of 12%EL. Specify an alloy and treatment that meet these criteria. Design Example 11.1 Design Example 11.1: To select a steel alloy for a gearbox output shaft, a 1-in diameter cylindrical shaft. Surface hardness 38 HRC Ductility >12%EL Specify an alloy and treatment that meet these criteria Cost is always an important design consideration. This would eliminate relatively expensive steels, such as stainless steels. Examine plain-carbon and low-alloy steels, and what treatments are available to alter their mechanical properties. Two approaches Cold work Heat treatment martensite Design Example 11.1 Relationships between Hardness and Tensile Strength for Steel, Brass, and Cast Iron: Relationships between Hardness and Tensile Strength for Steel, Brass, and Cast Iron Adapted from Fig. 6.19, Callister 6e. Fig. 7.17 For 1040 steel, brass, and copper, (b) the increase in tensile strength, and (c) the decrease in ductility (%EL) with percent cold work: Fig. 7.17 For 1040 steel, brass, and copper, (b) the increase in tensile strength, and (c) the decrease in ductility (%EL) with percent cold work The Correlation between Cold Work and Tensile Strength and Ductility for Various Alloys Design Example 11.1: Design Example 11.1 1-in diameter cylindrical shaft. Having a surface hardness of at least 38 HRC and a minimum ductility of 12%EL Cold work From Fig. 6.19, a hardness of 38 HRC corresponds to a tensile strength of 1200 MPa. From Fig. 7.17(b), at 50% cold work, a tensile strength is only ~900 MPa and the ductility is ~10%EL. Both of these properties fall short of those specified in the design. Design Example 11.1: Cold working other plain-carbon or low-alloy steels would probably not achieve the required minimum values. To perform a series of heat treatments in which the steel is austenitized, quenched (to form martensite), and finally tempered. Examine the mechanical properties of various plain-carbon and low-alloy steels that have been treated in this manner. The surface hardness of the quenched material will depend on both alloy content and shaft diameter. Design Example 11.1 Design Example 11.1: Design Example 11.1 The only alloy-heat treatment combinations that meet the stipulated criteria are 4150/oil-540°C temper, 4340/oil-540°C temper, and 6150/oil-540°C temper. The costs of these three materials are probably comparable. The 6150 alloy has the highest ductility (by a narrow margin), which would give it a slight edge in the selection process. Steel Alloys: Steel Alloys Steel Alloys: Steel Alloys
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