Machine Design 1 Terms G PDF

***Age hardening*** (precipitation hardening) occurs in some metals, notably certain stainless steel, aluminum, and copper alloys, at ambient temperature after solution heat treatment, the process being one of a constituent precipitating from solid solution. Where used, the consequences include incr...

***Age hardening*** (precipitation hardening) occurs in some metals, notably certain stainless steel, aluminum, and copper alloys, at ambient temperature after solution heat treatment, the process being one of a constituent precipitating from solid solution. Where used, the consequences include increased strength and hardness, decreased ductility. Aging at moderately elevated temperature expedites the process and is called *artificial aging.* ***Alloy*** is a substance with metallic properties, composed of two or more elements of which at least one is metal. ***Alloying elements*** in steel are usually considered to be the metallic elements added for the purpose of modifying the properties. ***Anisotropy*** is the characteristic of exhibiting different properties when tested in different directions (as tensile strength "with the grain" or "across the grain"). ***Brittleness*** is a tendency to fracture without appreciable deformation. See *ductility.* ***Charpy test*** is one in which a specimen, supported at both ends as a simple beam, is broken by the impact of a falling pendulum. The energy absorbed in breaking the specimen is a measure of the impact strength of the metal. See *Izod test.* ***Cold shortness*** is the brittleness of metals at ordinary or low temperatures. ***Cold working*** is the process of deforming a metal plastically at a temperature below the recrystallization temperature and at a rate to produce strain hardening. Cold-drawn steel is frequently used because it increases strength and machinability and improves surface finish. It reduces ductility. Commercial amounts of cold working of steel are of the order of 10-20%. ***Dumping capacity*** is the ability of a material to absorb or damp vibrations, which is a process of absorbing kinetic energy of vibration owing to hysteresis. The absorbed energy is eventually dissipated to the surroundings as heat. At a particular stress level, cast iron is a much better damping material than steel. ***Decarburization*** is a loss of carbon from the surface of steel, occurring during hot rolling, forging, and heat treating, when the surrounding medium reacts with the carbon (as oxygen and carbon combining) ***Ductility*** is that property that permits permanent deformation before fracture in tension. There is no absolute measure of ductility, but the *percentage elongation* and the *percentage reduction of area* are used as indices; the higher these indices, the more ductile the material is said to be. Ductility is the opposite of brittleness, but there is no sharp division line. For purposes of definition, it is frequently assumed that *Ductile* material -- Elongation greater than 5% in 2-in. gage, *Brittle* material -- Elongation less than 5% in 2-in. gage. Ductility is frequently a valuable property because, by virtue of it, a member may take an occasional exceptionally high load without breaking. ***Elasticity*** is the ability of a material to be deformed and to return to the original shape. Stress is proportional to strain only during an elastic deformation (see *proportional limit,* p. 8). ***Embrittlement*** involves the loss of ductility because of a physical or chemical change of material. ***Free carbon*** is that part of the carbon content of steel or iron that is in the form of graphite or temper carbon. ***Hard drawn*** is a temper produced in a wire, rod, or tube by cold drawing. See *temper* and 2.16, 2.17. ***Homogeneous materials*** (have homogeneity) have the same structure at all points. (Steel consists of randomly oriented iron crystals of different sizes, with other matter in between and is thus not homogeneous. ***Isotropic*** materials have the same properties in all directions. (Wood has a grain; rolled steel is not isotropic.) ***Izod test*** is a test in which a specimen, supported at one end as a cantilever beam, is broken by the impact of a falling pendulum. The energy absorbed in breaking the specimen is a measure of the impact strength. Impact values in the tables should be considered more qualitative than quantitative because the actual variation of samples from the same universe is quite wide. ***Killed steel*** is steel that has been deoxidized with a strong deoxidizing agent, such as silicon or aluminum, in order to eliminate a reaction between the carbon and oxygen during the solidification. Ingots of killed steel are sounder, containing fewer gas holes, and more homogeneous than non-killed or *rimmed steel;* these are desirable characteristics for forgings and heavy rolled sections. ***Machinability*** is a somewhat indefinite property that refers to the relative ease with which a material can be cut. In the case of steels, cold-drawn AISI B1112 being cut with a high-speed tool-steel tool and with a proper cutting oil is usually taken as 100%. Free-cutting brass is a reference for copper alloys. Such data as in Table AT 7 are roughly relative at best, since the actual conditions of operation vary widely. There are significant production variables, such as the sharpness and shape of the cutting tool, exact nature of the material, the cutting lubricant, and the use of carbide tools. ***Malleability*** is a material's susceptibility to extreme deformation in rolling or hammering. The more malleable the metal, the thinner the sheet into which it can be formed (usually cold). Gold and aluminum are quite malleable. ***Mechanical properties*** are those that have to do with stress and strain: ultimate strength and percentage elongation, for example. See *Physical properties.* ***Percentage elongation*** is the extension in the vicinity of the fracture of a tensile specimen, expressed as a percentage of the original gage length, as 20% in 2 in. ***Percentage reduction area*** is the smallest area at the point of rupture of a tensile specimen divided by the original area. ***Physical properties*** exclude mechanical properties, and are other physical properties such as density, conductivity, coefficient of thermal expansion, see *mechanical properties. Chemical* properties include corrosion resistance. ***Plasticity*** is the ability of a metal to be deformed considerably without rupture. In a plastic deformation, the material does not return to its original shape. See *elasticity.* ***Poisson's ratio*** is the ratio of the lateral strain (contraction) to the longitudinal strain (extension) when the element is loaded with a longitudinal tensile force. ***Precipitation heat treatment*** brings about the precipitation of a constituent from a supersaturated solid solution by holding the body at an elevated temperature, also called *artificial aging*. In some alloys, precipitation may also occur at ambient temperatures, a process called *aging.* ***Proof stress*** is that stress which causes a specified permanent deformation of a material, usually 0.01% or less. See *yield strength,* 1.8. ***Red shortness*** is a brittleness in steel when it is red hot. ***Relaxation,*** associated with creep, is the decreasing stress at a constant strain; important for metals in high-temperature service. ***Rimmed steel*** is incompletely deoxidized steel. Ingots of these steel have a surface layer quite free of slag inclusions and gas pockets, which results in the optimum surface on rolled sheets. ***Solution heat treatment*** is the process of holding an alloy at a suitably high temperature long enough to permit one or more constituents to pass into solid solution and then cooling fast enough to hold the constituents as a supersaturated solution. (Precipitation may occur with time.) ***Stiffness*** is the ability to resist deformation. It is measured by the modulus of elasticity in the elastic range; the higher the modulus, the stiffer is the material. ***Strain hardening*** is increasing the hardness and strength by plastic deformation at temperatures lower than the recrystallization range. See *temper.* ***Temper*** is a condition produces in a non-ferrous metal by mechanical or thermal treatment: for example, annealed temper (soft), hard temper, spring temper. ***Toughness*** is the capacity of material to withstand a shock load without breaking. The impact strength (see Charpy and Izod test), though not an absolute measure, evaluates toughness. Formerly, the energy required to pull a standard tensile specimen in two was taken as the toughness, but this quantity is not representative because of the effect of the cold working of the specimen during the slow-speed test. ***Transverse strength*** refers to the results of a transverse bend test, the specimen being mounted as a simple beam; also called *rupture modulus.* It is frequently applied to brittle materials, especially cast iron ***Work hardening*** is the same as strain hardening. ***Wrought steel*** is steel that has been hammered, rolled, or drawn in the process of manufacture; it may be plain carbon or alloy steel. **HEAT-TREATMENT TERMS.** ***Aging** (and **age hardening**)* is a change in a metal by which its structure recovers from an unstable or metastable condition that has been produced by quenching or cold working. The change in structure, which proceeds as a function of time and temperature, consists in precipitation often submicroscopic. The result is a change of mechanical and physical properties, a process that may be accelerated by using a temperature slightly higher than room temperature. ***Annealing,*** a comprehensive term, is a heating and slow cooling of a solid metal, usually done to soften it. Other purposes of annealing include those of altering the mechanical and physical properties, producing a particular microstructure, removing internal stresses (stress relieving), and removing gases. See *normalizing* below. ***Critical range*** has the same meaning as *transformation range* (below). ***Drawing*** is often used to mean *tempering,* but this usage conflicts with the meaning of drawing of a material through a die (2.9) and is to be avoided. ***Graphitizing,*** and *annealing* process, causes the combined carbon to transform wholly or in part into graphitic or free carbon; it is applied to cast iron, sometimes to high-carbon steel. ***Hardening*** is the heating of certain steels above the transformation range and then quenching, for the purpose of increasing the hardness. In the general case, hardening is any process of increasing the hardness of a metal. ***Malleablizing*** is an annealing process whereby combined carbon in white cast iron is transformed wholly or in part to temper carbon. Temper carbon is free (graphitic) carbon in the form of rounded nodules, characteristic forms in graphitizing and malleablizing. Concerning malleable iron. ***Normalizing*** is the heating of an iron-base alloy to some 100F above the transformation range with subsequent colling to below that range in still air at room temperature. The purpose is to produce a uniform structure. ***Spheroidizing*** is any heating and cooling of steel that produces a rounded or globular form of carbide. Typically, it is a prolonged heating at a temperature slightly below the transformation range, usually followed by slow cooling; or, for small objects of high-carbon steel, it may be prolonged heating alternately within and slightly below the transformation range. ***Stress relieving*** (thermal) is the heating of metal body to a suitable temperature (generally just below the transformation range for steel, say 1100-1200F) and holding it at that temperature for suitable time (1 to 3 hours for steel) for the purpose of reducing internal residual stresses. The internal stresses may be present because the body has been cast, quenched, normalized, machined, cold worked, or welded. ***Tempering*** is a reheating of hardened or normalized steel to a temperature below the transformation range, followed by any desired rate of cooling. Quenched steel is tempered in order to reduce internal stresses, to restore a certain amount of ductility, and to improve toughness If, for example, a particular steel with a particular yield strength is desired, do not specify the tempering temperature. This temperature can be varied slightly to produce closely the desired mechanical property. ***Transformation age*** for ferrous metals is the temperature interval during which austinite is formed during heating; it is also the temperature interval during which austenite disappears during cooling. Thus, there are two ranges; these may overlap but never coincide. The range on heating is higher than that cooling.