Engineering Materials - Metals PDF

CHEME - Chemistry for Engineers Engineering Materials – Metals METALS Metallic elements are those that are found in the periodic table with valence electrons ranging from one to three that take part in chemical bonding. Metallic solids, also simply called “...

CHEME - Chemistry for Engineers Engineering Materials – Metals METALS Metallic elements are those that are found in the periodic table with valence electrons ranging from one to three that take part in chemical bonding. Metallic solids, also simply called “metals,” are composed of metallic elements that are held together by metallic bonds. PROPERTIES OF METALS: 1. Malleable: the ability to be hammered or pressed permanently out of shape without breaking or cracking 2. Ductile: ability to be drawn into wire. 3. Good electrical and thermal conductivity: ability to allow the transport of an electric charge and heat, respectively. 4. High melting point 5. Dense – has a high mass for the size or volume that the substance takes up. 6. Has a high characteristic of luster – the shiny appearance of the metals. Alloy is a material that is composed of two or more metals, or sometimes a metal and nonmetal, which have been intimately mixed by fusion, electrolytic deposition and the like; and where the metallic bond is the one that predominates. ALLOY COMPOSITION 1. Base metal: element present in major proportions. 2. Alloying elements: other elements present in minor proportions CLASSIFICATIONS OF ALLOYS: 1. Cast: brittle alloys that cannot undergo appreciable deformation during forming or shaping 2. Wrought: alloys that are amenable to mechanical deformation  CHEME - Chemistry for Engineers Engineering Materials – Metals CATEGORIES OF ALLOY 1. Substitution alloy – when atoms of the solute in a solid solution occupy positions normally occupied by a solvent atom. They are formed when the two metallic components have similar atomic radii and chemical-bonding characteristics. When two metals differ in radii by more than about 15%, solubility is generally more limited. 2. Interstitial alloy – when the solute atoms occupy interstitial positions in the “holes” between solvent atoms. For an interstitial alloy to form, the solute atoms must have a much smaller bonding atomic radius than the solvent atoms. Typically, the interstitial element is a nonmetal that makes covalent bonds to the neighboring metal atoms. The presence of the extra bonds provided by the interstitial component causes the metal lattice to become harder, stronger, and less ductile. 3. Heterogeneous alloy – when the components in the alloy are not dispersed uniformly. In general, the properties of heterogeneous alloys depend on both the composition and the way the solid is formed from the molten mixture. 4. Intermetallic compounds – are compounds rather than mixtures; hence, they have definite properties, and their composition cannot be varied. Unlike the atoms in substitutional and interstitial alloys, the different types of atoms in an intermetallic compound are ordered rather than randomly distributed. The ordering of atoms in an intermetallic compound generally leads to better structural stability and higher melting points than what is observed in the constituent metals. On the negative side, intermetallic compounds are often more brittle than substitution alloys. CLASSIFICATIONS OF METALS AND ALLOYS 1. FERROUS ALLOYS are alloys whose primary constituent is iron. Alloys of iron and carbon have up to 2% by weight carbon. It is widely used because of three primary factors. I. Iron-containing compounds exist abundantly on the Earth’s crust. II. May be produced using economical extraction, refining, alloying, and fabrication techniques. III. Extremely versatile. A. STEELS i. Low alloy/plain carbon steels are steels that contain only residual concentrations of impurities other than carbon and little manganese. a. Low-carbon steels contain less than 0.25 % carbon and are produced in the greatest quantities because of usage as automobile parts and structural shapes such as beams, sheets, pipelines, buildings, etc. *** High Strength Low Alloy (HSLA) Steels contain other alloying elements such as copper, vanadium, nickel, and molybdenum in combined concentrations of up to 10% by weight, carbon content between 0.05 – 0.25 % by weight carbon. These steels possess higher strengths than plain low-carbon steels and are not made to meet a specific chemical composition but rather to specific mechanical properties. These are used in structural strength such as columns in high-rising buildings, bridges, and pressure vessels. b. Medium carbon steels contain 0.25 to 0.60 % by weight carbon and are heat treatable with tempered martensite as the most common form. These are used in tracks, gears, and high strength structural components. c. High carbon steels contain 0.60 to 1.40 % by weight carbon and are wear resistant and capable of holding sharp edge. These are used in cutting tools, dies, razors, blades, springs and high strength wires. ii. High alloy/alloyed steels have alloying elements which are intentionally added in specific concentrations to improve specific properties. a. Tool steels are mainly used for tools and are used for general machine parts where strength, wear resistance and dimensional stability are required. b. Stainless steels contain at least 11 % chromium and highly resistant to corrosion in a variety of environments. Corrosion resistance is attributed to the formation of a thin, adherent, stable chromium oxide or nickel oxide film that effectively protects the steels  CHEME - Chemistry for Engineers Engineering Materials – Metals against many corrosive media (passivation effect). These are utilized in gas turbines, high temperature boilers, heat treating furnaces, aircrafts, etc. 1. Martensitic stainless steels are primary chromium steels with 11.5 – 18 % Cr and respond to heat treatment such that martensite id formed as the primary constituent. 2. Ferritic stainless steel are straight-chromium stainless steels with approximately 14 – 27 % Cr and have low in carbon content. These steels have ferrite, α as the primary constituent at room temperature and does not respond to heat treatment and are hardenable by cold working. 3. Austenitic stainless steels are chromium-nickel (3xx) and chromium-nickel- manganese (2xx) types in which nickel promotes austenite texture. These contains total nickel and chromium content of at least 23 % and has austenite, 𝛾 as the primary constituent at room temperature and does not respond to heat treatment and are hardenable by cold working. B. CAST IRONS are class of ferrous alloys with carbon contents above 2% as graphite and not as cementite and classified according to metallographic structure. These have low melting points, which makes them easy to cast, and are generally brittle. Graphite formation is promoted by the presence of silicon in amounts greater than 1 % and slow cooling rate. i. Gray cast iron is the most widely used form of cast iron and has carbon and silicon contents between 2.5 and 4.0 % (hypoeutectic) and 1.0 to 3.0 %, respectively. Graphite exists in the form of “flakes”, making the fractured surface have a gray color. Gray cast iron is weak and brittle in tension because graphite flakes act as stress concentrators, are effective in damping vibrational energy, and are wear-resistant and least expensive among the metals. ii. Ductile or nodular cast iron is also known as spheroidal graphite iron or spherulitic iron. The addition of magnesium, sometimes cerium, promotes the formation of the graphite in the form of nodules or spheres. This is stronger and more ductile, and common applications are valve, pump bodies, gears, etc. iii. White cast iron contains low amounts of silicon and produced from high cooling rates, and carbon exists as cementite instead of graphite (fractured surface is white). All white cast irons are hypoeutectic alloys. Typical microstructure consists of dendrites of transformed austenite(pearlite) in a white interdendritic network of cementite. White cast irons are harder and more brittle, difficult to machine, are used in limited applications that necessitate a very hard and wear-resistant surface such as rollers in rolling mills and serves as the starting material for malleable iron. iv. Malleable cast iron is produced by heating white iron at temperatures between 800 to 900 °C for a prolonged period of time causing decomposition of cementite, forming graphite (temper carbon) which exists in the form of cluster or rosettes. Malleable cast irons are relatively high strength, has appreciable ductility, and are used for connecting rods, transmission gears, differential cases for cars, pipe fittings, valve parts for railroad, marine, and other heavy-duty services. 2. NON-FERROUS ALLOYS are alloys whose primary constituent is a metal other than iron and are utilized primarily because of their unique properties in specific applications. A. Copper and its alloys are widely used in its pure and alloyed form, are soft and ductile, corrosion resistant, medium tensile strengths, good electrical and thermal conductivities, machinability, ease of fabrication, non-magnetic, not responsive to heat treatment, and possible strengthening mechanism included alloying and cold working. 1. Brass is an alloy of Cu and Zn, with some amounts of lead, tin, or aluminum. It is harder and stronger than pure copper and are used as custom jewelry, cartridge casings, automotive radiators, musical instruments, electronic packaging, and coins.  CHEME - Chemistry for Engineers Engineering Materials – Metals 2. Bronze is an alloy of Cu and Sn. It is stronger than brass and has a high degree of corrosion resistance. It is used where corrosion resistance and good tensile properties are required, such as bearings. 3. Cupronickels are copper-nickel alloys that contain up to 30 % Ni, can exist as single-phase alloys, and have high resistances to the corrosive and erosive action of rapidly moving seawater. 4. Nickel Silver is essentially a ternary alloy of copper, nickel, and zinc. It has a pleasing silver- blue color and exhibits good corrosion resistance to food, chemicals, water, and the atmosphere. B. Aluminum and its alloys are relatively low density (2.7 g/ml as compared to 7.9 g/ml for steel), have high electrical and thermal conductivities, are non-toxic, and are used extensively for food containers and packaging, have good corrosion resistance (forms passivating layer), has high ductility that can be produced in sheets and other wrought form, and is used where light weight is a necessity such as in transportation. C. Nickel and its alloys are highly resistant to corrosion and oxidation in many environments, are often plated to other metals as a protective measure, and are white in color, and have good workability and mechanical properties. D. Magnesium and its alloys have a density of 1.74 g/ml, compete with aluminum in light-weight applications, are disadvantageous in terms of cost, difficulty to cast, and are not easily deformable at room temperature. E. Titanium and its alloys are relatively new and possess an extraordinary combination of properties: extremely strong, highly ductile, and easily forged and machined. Pure titanium has a relatively low density of 4.5 g/ml and a high melting point of 1668 °F. F. Refractory metals are used for high-temperature applications, possess high melting points, interatomic bonding is extremely strong, resulting in high strengths and hardness, and are used as structural parts in space vehicles, light filaments, x-ray tubes, and welding electrodes. Refractory metals include niobium, molybdenum, tungsten, and tantalum.