Metals Cheatsheet

Cheatsheet Content

Introduction to Metals Four Types of Engineering Materials: Metals, Ceramics, Polymers, Composites Importance of Metals: Properties satisfy wide design requirements. Manufacturing processes are well-developed. Engineers have deep understanding. Why Metals Are Important: High stiffness and strength, can be alloyed for rigidity, strength, hardness. Toughness: capacity to absorb energy. Good electrical and thermal conductivity. Competitive cost (e.g., steel). Starting Forms of Metals Cast metal: Starting form is a casting. Wrought metal: Metal has been worked or can be worked after casting. Powdered metal: Very small powders converted into parts using powder metallurgy. Classification of Metals Ferrous: Based on iron. Steels Cast irons Nonferrous: All other metals. Aluminum, magnesium, copper, nickel, titanium, zinc, lead, tin, molybdenum, tungsten, gold, silver, platinum. Superalloys Alloys and Phase Diagrams Metals and Alloys: Some metals used as pure elements (e.g., gold, silver, copper). Most engineering applications require enhanced properties obtained by alloying. Alloying increases strength, hardness, and other properties. Alloy Definition: A mixture or compound of two or more elements, at least one of which is metallic. Main Categories of Alloys: Solid solutions Intermediate phases Solid Solutions An alloy where one element is dissolved in another to form a single-phase structure. Phase: Any homogeneous mass of material with the same crystal lattice structure. Solvent (base element) is metallic; dissolved element can be metallic or nonmetal. Two Forms of Solid Solutions: Substitutional: Atoms of solvent element replaced by dissolved element in unit cell. Interstitial: Atoms of dissolving element fit into vacant spaces between base metal atoms. In both forms, alloy structure is generally stronger and harder than component elements. Intermediate Phases Limits to solubility of one element in another. When dissolving element amount exceeds solid solubility limit, a second phase forms. Term intermediate phase describes it due to chemical composition being intermediate between two pure elements. Crystalline structure is different from pure metals. Types of Intermediate Phases: Metallic compounds: Consist of a metal and nonmetal (e.g., Fe$_3$C). Intermetallic compounds: Two metals forming a compound (e.g., Mg$_2$Pb). Some two-phase alloys can be heat-treated for much higher strength than solid solutions. Phase Diagrams Graphical means of representing phases of a metal alloy system as a function of composition and temperature. Binary Phase Diagram: For an alloy system of two elements at atmospheric pressure. Composition plotted on horizontal axis, temperature on vertical axis. Any point indicates overall composition and phases present at given temperature under equilibrium. Copper-Nickel (Cu-Ni) Alloy System Solid solution alloy throughout entire range of compositions below the solidus. No intermediate solid phases. Mixture of phases (solid + liquid) in the region bounded by the solidus and liquidus. Chemical Compositions of Phases: Overall composition given by position on horizontal axis. Liquid and solid phase compositions differ. Found by drawing a horizontal line at temperature of interest, intersecting liquidus and solidus. Inverse Lever Rule Used to determine amounts of each phase present at a given temperature. Step 1: Measure distances from aggregate composition to liquidus (CL) and solidus (CS) intersection points on a horizontal line. Step 2: Liquid phase proportion = $\frac{CS}{(CS + CL)}$ Solid phase proportion = $\frac{CL}{(CS + CL)}$ Applications: Applicable to solid region and liquidus-solidus region where two phases are present. Not applicable when only one phase is present. Tin-Lead (Sn-Pb) Alloy System Widely used in soldering for electrical connections. Includes two solid phases: alpha ($\alpha$) and beta ($\beta$). $\alpha$-phase: Solid solution of tin in lead (left side of diagram). $\beta$-phase: Solid solution of lead in tin around $200^\circ C$ ($375^\circ F$) (right side of diagram). Mixture of $\alpha + \beta$ phases between these solid solutions. Melting: Pure tin melts at $232^\circ C$ ($449^\circ F$). Pure lead melts at $327^\circ C$ ($621^\circ F$). Tin-lead alloys melt at lower temperatures. Two liquidus lines meet at $61.9\%$ Sn, which is the eutectic composition . Eutectic Alloy: Composition where solidus and liquidus are at the same temperature. Eutectic temperature: Melting point of eutectic composition, always the lowest melting point for an alloy system. Derived from Greek eutektos , meaning "easily melted". Ferrous Metals Based on iron, one of the oldest metals. Engineering importance due to iron-carbon alloys: Steel Cast iron Constitute $\approx 85\%$ of metal tonnage in the US. Iron-Carbon Phase Diagram Shows phases for iron-carbon system up to about $6\%$ carbon. Phases of Iron: $\alpha$ (alpha) called ferrite (BCC) at room temperature. At $912^\circ C$ ($1674^\circ F$), ferrite transforms to $\gamma$ (gamma) called austenite (FCC). At $1394^\circ C$ ($2541^\circ F$), $\gamma$ transforms to $\delta$ (delta) (BCC). Pure iron melts at $1539^\circ C$ ($2802^\circ F$). Iron as a Commercial Product: Electrolytic iron: Purest ($\approx 99.99\%$), for research. Ingot iron: $\approx 0.1\%$ impurities (e.g., $0.01\%$ carbon), for high ductility/corrosion resistance. Wrought iron: $\approx 3\%$ slag, very little carbon, easily shaped in hot forming. Solubility Limits of Carbon in Iron Ferrite can dissolve $\approx 0.022\%$ carbon at $723^\circ C$ ($1333^\circ F$). Austenite can dissolve up to $\approx 2.1\%$ carbon at $1130^\circ C$ ($2066^\circ F$). Difference in solubility provides opportunities for strengthening by heat treatment. Steel and Cast Iron Defined Steel: Iron-carbon alloy with $0.02\%$ to $2.1\%$ carbon. Cast iron: Iron-carbon alloy with $2.1\%$ to $\approx 4\%$ or $5\%$ carbon. Both can contain other alloying elements. Cementite in Iron-Carbon System At room temperature, iron-carbon alloys form a two-phase system at carbon levels slightly above zero. Second phase is Fe$_3$C, known as cementite . Cementite is an intermediate phase: metallic compound of iron and carbon, hard and brittle. Eutectic and Eutectoid Compositions Eutectic composition (Fe-C system): $4.3\%$ C. Phase changes from solid ($\gamma + \text{Fe}_3\text{C}$) to liquid at $1130^\circ C$ ($2066^\circ F$). Eutectoid composition (Fe-C system): $0.77\%$ C. Phase changes from $\alpha$ to $\gamma$ above $723^\circ C$ ($1333^\circ F$). Below $0.77\%$ C: hypoeutectoid steels . From $0.77\%$ to $2.1\%$ C: hypereutectoid steels . Iron and Steel Production Iron making: Iron reduced from its ores. Steel making: Iron refined to obtain desired purity and composition (alloying). Iron Ores Required in Iron-making Principal ore: Hematite (Fe$_2$O$_3$). Other ores: Magnetite (Fe$_3$O$_4$), Siderite (FeCO$_3$), Limonite (Fe$_2$O$_3$-xH$_2$O). Iron ores contain $50\%$ to $70\%$ iron (hematite $\approx 70\%$ iron). Scrap iron and steel are also widely used. Other Raw Materials in Iron-making Coke (C): Supplies heat, produces CO to reduce iron ore. Limestone (CaCO$_3$): Used as a flux to remove impurities as slag. Hot gases (CO, H$_2$, CO$_2$, H$_2$O, N$_2$, O$_2$, fuels): Used to burn coke. Iron-making in a Blast Furnace Refractory-lined chamber, $9-11$ m diameter, $40$ m height. Charge of ore, coke, and limestone dropped into top. Hot gases forced into lower part for combustion and iron reduction. Chemical Reactions: Hematite reduction: Fe$_2$O$_3$ + CO $\rightarrow$ 2FeO + CO$_2$ CO production: CO$_2$ + C (coke) $\rightarrow$ 2CO Final FeO reduction: FeO + CO $\rightarrow$ Fe + CO$_2$ Proportions of Raw Materials (per ton of iron): $2.0$ tons of iron ore $1.0$ ton of coke $0.5$ ton of limestone $3.5$ tons of gases Byproducts are largely recycled. Iron from the Blast Furnace Pig iron: Iron tapped from blast furnace, contains $>4\%$ C, plus impurities (Si, Mn, P, S). Further refinement needed for cast iron and steel. Cupola furnace: Converts pig iron into gray cast iron. For steel, compositions must be closely controlled, impurities reduced. Steel-making Processes developed since mid-1800s to refine pig iron into steel. Two most important processes today: Basic oxygen furnace (BOF) Electric furnace Both produce carbon and alloy steels. Basic Oxygen Furnace (BOF) Accounts for $\approx 70\%$ of US steel production. Adaptation of Bessemer converter (air blown to burn impurities). BOF uses pure oxygen. Typical vessel: $\approx 5$ m diameter, processes $150-200$ tons/heat. Cycle time: $\approx 45$ min. Electric Arc Furnace Accounts for $\approx 30\%$ of US steel production. Scrap iron and steel are primary raw materials. Capacities: $25-100$ tons/heat. Melting: $\approx 2$ hrs, tap-to-tap time: $4$ hrs. Associated with alloy, tool, and stainless steels. Better quality steel but higher cost per ton compared to BOF. Casting Processes in Steel-making Steels solidified as cast ingots or by continuous casting. Casting of Ingots: Discrete production process. Steel ingots: discrete castings, $ Molds made of high carbon iron, tapered for removal. Mold placed on a stool; lifted after solidification, leaving casting. Continuous Casting: Widely applied in aluminum and copper production. Most noteworthy application: steel-making. Dramatic productivity increases over ingot casting. Reduces solidification time by an order of magnitude (vs. $10-12$ hrs for ingot). Steel Alloy of iron with $0.02\%-2.11\%$ carbon by weight. Often includes other alloying elements: nickel, manganese, chromium, molybdenum. Four Categories of Steel Alloys: Plain carbon steels Low alloy steels Stainless steels Tool steels Plain Carbon Steels Carbon is principal alloying element ($\approx 0.5\%$ manganese is normal). Strength increases with carbon content, but ductility reduces. High carbon steels can be heat treated to form martensite (very hard and strong). AISI-SAE Designation Scheme: $4$-digit number 10XX. 10 indicates plain carbon steel. XX indicates carbon $\%$ in hundredths of percentage points (e.g., 1020 steel has $0.20\%$ C). Types: Low carbon steels ($ Auto sheetmetal, plate steel, railroad rails. Medium carbon steels ($0.20\%-0.50\%$ C): Machinery components, engine parts (crankshafts, connecting rods). High carbon steels ($>0.50\%$ C): Springs, cutting tools, wear-resistant parts. Low Alloy Steels Iron-carbon alloys with additional alloying elements totaling $ Superior mechanical properties to plain carbon steels. Higher strength, hardness, hot hardness, wear resistance, toughness. Heat treatment often required for improved properties. AISI-SAE Designation Scheme: $4$-digit number YYXX. YY indicates alloying elements. XX indicates carbon $\%$ in hundredths (e.g., 13XX - Manganese steel, 20XX - Nickel steel). Stainless Steel (SS) Highly alloyed steels designed for corrosion resistance. Principal alloying element: Chromium ($>15\%$). Cr forms a thin, impervious oxide film for corrosion protection. Nickel (Ni) is an alloying ingredient in certain SS to increase corrosion protection. Carbon strengthens and hardens SS, but high C reduces corrosion protection (Cr carbide forms). Noted for strength and ductility, but difficult to work and more expensive. Types of Stainless Steel (by predominant phase): Austenitic stainless: $18\%$ Cr, $8\%$ Ni. Ferritic stainless: $15\%-20\%$ Cr, low C, no Ni. Martensitic stainless: Up to $18\%$ Cr, no Ni, higher C than ferritic. Designation Scheme (AISI): $3$-digit numbering scheme. First digit indicates general type, last two digits specific grade. Type 302 - Austenitic SS ($18\%$ Cr, $8\%$ Ni, $2\%$ Mn, $0.15\%$ C). Type 430 - Ferritic SS ($17\%$ Cr, $0\%$ Ni, $1\%$ Mn, $0.12\%$ C). Type 440 - Martensitic SS ($17\%$ Cr, $0\%$ Ni, $1\%$ Mn, $0.65\%$ C). Additional Stainless Steels: Precipitation hardening stainless: $17\%$ Cr, $7\%$ Ni, small Al, Cu, Ti, Mo. Duplex stainless: Mixture of austenite and ferrite. Tool Steels Highly alloyed steels for industrial cutting tools, dies, molds. Must possess high strength, hardness, hot hardness, wear resistance, toughness under impact. Tool steels are heat treated. AISI Classification of Tool Steels: T, M: High-speed tool steels (machining). H: Hot-working tool steels (hot-working dies). D: Cold-work tool steels (cold working dies). W: Water-hardening tool steels (high carbon). S: Shock-resistant tool steels (high toughness). P: Mold steels (molds for plastics/rubber). Cast Irons Iron alloys with $2.1\%-4\%$ carbon and $1\%-3\%$ silicon. Highly suitable as casting metals. Tonnage of cast iron castings is several times that of all other cast metal parts combined (excluding steel ingots). Overall tonnage second only to steel among metals. Types of Cast Irons: Gray cast iron (most important). Ductile iron, white cast iron, malleable iron, alloy cast irons. Ductile and malleable irons result from special processing treatments. Nonferrous Metals Metal elements and alloys not based on iron. Most important: Al, Cu, Mg, Ni, Ti, Zn, and their alloys. Generally not as strong as steels, but offer corrosion resistance and/or high strength-to-weight ratios. Properties make them ideal for applications where steel is unsuitable. Aluminum and Magnesium Light metals, often specified for this feature. Abundant on Earth (Al on land, Mg in sea), but difficult to extract. Aluminum Production: Principal ore: Bauxite (hydrated Al$_2$O$_3$). Extraction: Washing/crushing ore, Bayer process (bauxite to pure Al$_2$O$_3$), Electrolysis (Al$_2$O$_3$ to Al and O$_2$). Properties of Aluminum: High electrical and thermal conductivity. Excellent corrosion resistance (hard, thin oxide film). Very ductile, good formability. Pure Al is low strength, but can be alloyed/heat treated to compete with steels (especially for weight). Designation Scheme for Aluminum: $4$-digit code for composition. Distinguishes wrought from cast: cast aluminums have a decimal point after the third digit (e.g., 1XX.X). Temper Designation: Attached to $4$-digit code by a hyphen. F: As fabricated (no special treatment). H: Strain hardened (wrought aluminums). O: Annealed (relieves strain hardening, improves ductility). T: Thermal treatment (produces stable tempers other than F, H, O). Magnesium and Its Alloys: Lightest structural metals. Available in wrought and cast forms. Relatively easy to machine. Small metal particles oxidize rapidly, fire hazard during processing. Magnesium Production: Sea water contains $\approx 0.13\%$ MgCl$_2$ (source). Mixed with milk of lime (Ca(OH)$_2$) to precipitate Mg(OH)$_2$. Slurry filtered, mixed with HCl to form concentrated MgCl$_2$. Electrolysis decomposes MgCl$_2$ into Mg and Cl$_2$. Mg cast into ingots; Cl$_2$ recycled. Properties of Magnesium: Pure Mg is soft, lacks strength for most engineering applications. Can be alloyed/heat treated to achieve strengths comparable to aluminum alloys. High strength-to-weight ratio is advantageous in aircraft/missile components. Designation Scheme for Magnesium: $3-5$ character alphanumeric code. First two characters: letters identifying principal alloying elements. Next two digits: amounts of alloying ingredients to nearest percent. Last symbol: letter for composition variation or chronological order. Magnesium alloys also require temper specification (similar to aluminum). Example: AZ63A - aluminum $6\%$, zinc $3\%$, magnesium $93\%$. Copper One of the oldest metals. Low electrical resistivity (pure copper used as electrical conductor). Excellent thermal conductor. Noble metal (like gold, silver), so corrosion resistant. Copper Production: In ancient times, available as free element. Today, extracted from ores like chalcopyrite (CuFeS$_2$). Ore crushed, concentrated by flotation, then smelted. Resulting copper is $98\%-99\%$ pure. Electrolysis used for higher purity. Copper Alloys: Pure copper has low strength/hardness; alloyed to improve. Bronze: Cu-Sn alloy ($\approx 90\%$ Cu, $10\%$ Sn), ancient and modern use. Brass: Cu-Zn alloy ($\approx 65\%$ Cu, $35\%$ Zn). Beryllium-copper: Highest strength alloy ($\approx 2\%$ Be), heat-treatable for high strengths, used for springs. Designation Scheme (UNS): $5$-digit number preceded by 'C'. Includes wrought and cast copper/alloys. C10100 - $99.99\%$ pure copper. C17000 - $98\%$ Cu, $1.7\%$ Be (beryllium-copper). C24000 - $80\%$ Cu, $20\%$ Zn (brass). C52100 - $92\%$ Cu, $8\%$ Sn (bronze). Nickel and Its Alloys Similar to iron in some respects (magnetic, modulus of elasticity E). Differences with iron: Much more corrosion resistant (alloying element in SS, plating metal). High temperature properties superior. Nickel Production: Extracted from pentlandite ((Ni,Fe)$_9$S$_8$). Ore crushed, ground with water. Flotation separates sulfides. Nickel sulfide heated to burn sulfur, then smelted to remove iron/silicon. Further refinement yields high-concentration NiS. Electrolysis recovers high-purity nickel. Nickel Alloys: Commercially important for corrosion resistance and high temperature performance. Many superalloys are nickel-based. Applications: SS alloying, plating, high temp/corrosion resistance. Titanium and Its Alloys Abundant in nature ($\approx 1\%$ of earth's crust). Density between aluminum and iron. Importance grown due to aerospace applications (light weight, good strength-to-weight ratio). Titanium Production: Principal ores: Rutile ($98\%-99\%$ TiO$_2$), Ilmenite (FeO and TiO$_2$). TiO$_2$ converted to TiCl$_4$ by reacting with chlorine gas. TiCl$_4$ reduced to metallic titanium by magnesium ( Kroll process ). Resulting metal cast into ingots. Properties of Titanium: Relatively low coefficient of thermal expansion. Stiffer and stronger than Al. Retains good strength at elevated temperatures. Pure Ti is reactive, presents processing problems in molten state. At room temperature, forms thin adherent oxide coating (TiO$_2$) for excellent corrosion resistance. Applications of Titanium: Pure Ti: corrosion resistant components (marine, prosthetic implants). Titanium alloys: high strength components up to $550^\circ C$ ($1000^\circ F$) (aircraft, missile components). Alloying elements: aluminum, manganese, tin, vanadium. Zinc and Its Alloys Low melting point, attractive for die casting. Provides corrosion protection when coated onto steel or iron. Galvanized steel: Steel coated with zinc. Widely used as alloy with copper (brass). Production of Zinc: Principal ore: Zinc blende or sphalerite (ZnS). ZnS concentrated by crushing, grinding with water to create slurry. Slurry agitated, mineral particles float to top and skimmed off. Concentrated ZnS roasted to form zinc oxide (ZnO). Zn liberated from ZnO by electrolysis or thermochemical reactions. Lead and Tin Often considered together due to low melting points and use in soldering alloys. Lead: Dense, low melting point, low strength, low hardness, high ductility, good corrosion resistance. Applications: Solder, bearings, ammunition, type metals, X-ray shielding, storage batteries, vibration damping. Tin: Lower melting point than lead, low strength, low hardness, good ductility. Applications: Solder, bronze, "tin cans" for food storage. Refractory Metals Metals enduring high temperatures, maintaining high strength and hardness. Most important: Molybdenum, Tungsten. Other: Columbium, Tantalum. Molybdenum: Properties: high melting point, stiff, strong, good high temperature strength. Used as pure metal (99.9+% Mo) and alloyed. Applications: heat shields, heating elements, resistance welding electrodes, high temperature dies, rocket/jet engine parts. Alloying ingredient in steels (e.g., high speed steel) and superalloys. Tungsten: Properties: highest melting point among metals, densest, stiffest (highest modulus of elasticity), hardest pure metal. Applications: filament wire in incandescent bulbs, rocket/jet engine parts, arc welding electrodes. Used as element in tool steels, heat resistant alloys, tungsten carbide. Precious Metals Gold, platinum, silver. Called noble metals because chemically inert. Limited supplies. Used throughout history for coinage and to underwrite paper currency. Widely used in jewelry and similar applications. Properties: high density, good ductility, high electrical conductivity, corrosion resistance, moderate melting temperatures. Superalloys High-performance alloys for demanding strength and surface degradation resistance at high service temperatures. Many contain substantial amounts of three or more metals. Commercially important (expensive) due to unique properties. Why Superalloys are Important: Good room temperature strength. Excellent high temperature performance (tensile strength, hot hardness, creep/corrosion resistance at elevated temps). Operating temperatures often around $1100^\circ C$ ($2000^\circ F$). Applications: gas turbines, jet/rocket engines, steam turbines, nuclear power plants (where efficiency increases with higher temps). Three Groups of Superalloys: Iron-based alloys: Iron can be $ Nickel-based alloys: Better high temp strength; alloyed with Cr, Co, Fe, Mo, Ti. Cobalt-based alloys: $\approx 40\%$ Co, $\approx 20\%$ Cr; alloyed with Ni, Mo, W. In virtually all superalloys, strengthening is by precipitation hardening. Manufacturing Processes for Metals Metals shaped by: casting, powder metallurgy, deformation, material removal. Joined by: welding, brazing, soldering, mechanical fastening. Heat treating enhances properties. Finishing processes (e.g., electroplating, painting) improve appearance and/or provide corrosion protection. How to Enhance Mechanical Properties Alloying: Increases strength of metals. Cold working: Strain hardening during deformation increases strength (reduces ductility). Strengthening occurs as byproduct of forming. Heat treatment: Heating and cooling cycles change mechanical properties. Alters microstructure, which determines properties.