Materials: Nodular Iron Casting Crankshaft - Term Paper Example

Materials (engineering Nodular Iron Casting Crankshaft A crankshaft, also commonly referred to as the “crank”, translates the linear motion of pistons into rotational motion in an engine. All of the force created by the process of the explosion of the air and fuel in the combustion chamber creates a force through the rod and piston, which ends up on the journal of the crankshaft. As such, the crankshaft may undergo extreme conditions of pressure, heat, torsional forces, and vibrational forces that necessitate careful planning when designing these parts and choosing the materials with which they will be constructed. The crankshaft has since the steam engine, which often used only a single piston and commonly appears in four-stroke and six-cylinder engines used in contemporary automotive design. Manufacturing of crankshafts has changed dramatically over the last century, including a bigger number of main bearings, more counterbalances, and more sophisticated heat treating processes (Huppertz 2008). The following is an account of the heat treating process for the nodular iron crankshaft. There are three ways of making crankshafts: forging, casting, and machining from a solid billet. The basic iron crankshaft is cast, and has the lowest strength, with a tensile strength of about 80,000 PSI. Nodular Iron is actually a slightly improved version of cast iron, with a composition by weight of C 3.4, P 0.1, Mn 0.4, Ni 1.0, Mg 0.06 and tensile strength around 95,000 (Walbot 2008). It has a yield strength of 63 KSI, elongation of 18% in two inches, and ahardness index of 170 according to the Brinell scale (Lyons et al. 2006). Nodular iron parts, such as crankshafts, are heat treated after casting so that the graphite particles form distinctive spherical particles rather than flakes, which inhibit cracks and give nodular iron its name (Watson 2010). The heat treatment process for nodular iron crankshafts is sophisticated, as they are complex parts that sometimes require additional stress relief. For such large parts, nodular iron castings should be mold-cooled to 600°F/315°C before shakeout to ease structural stress. Additional stress relief should be performed only when required at a temperature of 1150-1250°F/620-675°C. The Annelaing process will aid in the decomposition and spheroidization of carbides in the metal, creating structural strength by preventing crack formation. This process should be conducted at 1750-1900°F/960-1035°C for for at least one hour and up to five, depending on the size of the part and degree of spheroidization necessary. After annealing, the part should be cooled in the air or furnace, for maximum elongation and minimum hardness (Watson 2010). In many engines parts will reach extremely high temperatures, and so need additional treatment. When nodular iron parts are to be operated at temperatures of 900°F/480°C and higher, the casting will need to additionally be stabilized against warpage and growth that may occur by keeping the part at 1600°F/870°C for an additional two hours, followed cooling in a furnace 1000°F/540°C, and final air cooling to reach room tempurature. Dimensional stability can be assured by holding at 1600°F/870°C for two hours plus an hour for each additional inch of section size added to the part, furnace cooling to 1000°F/540°C) holding for one hour for each inch of section size, and then slowly cooling to room temperature expose to air. After rough machining of the part is complete, the part must be reheated to 850-900°F/450-460°C) and held for an additional one hour for each inch of section size to relieve machining stresses. At this point the part will need to be furnace cooled to below 500°F/260°C (Watson 2010). Forged steel crankshafts are the most common choice for high performance engines. With a tensile strength of about 110,000 PSI they can easily handle horsepower from a few hundred to over 1,500 horsepower. Even though there strength is not much higher than some other metals, they have an elongation of 22 percent—much higher than the mere 6 percent seen in cast steel and slightly higher than the 18 percent seen in nodular iron (Walbot 2008). The forged part will also have smaller counterweights because the material is more dense (VWvortex 2010). Though forged steel offers slightly superior properties to cast iron, the cast iron crankshaft has some benefits for street-driving, or other operations where high performance is not required. Where the rob journal meets the web is a weak point where cracks often occur in the crankshaft, and in cast iron varieties flow can be made around these joints to give a stronger reinforced joint at a significantly lower cost that the alternative forged steel (Walbot 2008). A durable camshaft, a shaft to which a cam is fastened or forms an integral part, can be produced from low carbon alloy steel if the proper heat treatment procedure is followed because low carbon steel has toughness and ductility as carbon content drops. Low carbon steel has acarbon content up to thirty percent. This form of the metal is ductile and soft with the ability to be sheared, punched, rolled, and worked either hot or cold. Machining this metal is easy, with little hardening observed other than surface hardening (“Properties, Identification, and Heat Treatment…” 2010). For parts like the camshaft, carbon content of no more than 0.1-0.12% will ensure that hardening does not occur during use, keeping the core hardness below RC 30. For high volume production, chilled iron casting for camshafts is a good choice, producing hardened camshafts. This method, however, requires changes to the alloy before casting in order to ensure that it is suitable to retain the desired hardness level (Prabhudev 1998). Camshafts begin as a solid steel bar of the material chosen. In contemporary facilities, computer aided programs control mills and lathes that carve the parts and grind the cores. The initial core is carved into an unground lobe (UGL) with large lobes that are yet to assume the final configuration. After heat treatment, the lobes are ground into their final configuration and hardened. Low carbon steel parts can be heat treated by a process called carbonitriding after they take their final shapes, which is most suitable when the parts are composed of particularly low carbon steels and steel alloys. This process allows both carbon and nitrogen to diffuse into the surface of the part. The parts themselves are placed in an environment of hydrocarbons, often propane or methane, and mixed with ammonia in a process that resembles the carburizing and nitriding processes combined. This is done at a temperature of 1400-1598°F/760-870°C. The part is then quenched in an atmosphere of natural gas that is oxygen free, resulting inevitably in some inherent distortion. The distortion is generally much smaller than oil or water quenching, but can still be enough to damage some high-precision parts. The hardness achieved by this process is similar to the process of carborizing at 60-65 RC. It is not as high as the process of nitriding at 70 RC. Tempering is necessary for these parts to reduce brittleness (“Diffusion Treatment Hardening” 2010). The carburizing temperature has a large effect on the grain size of the core of the finished part and will be determined by the intended use of the resultant part. After carburizing the part usually undergoes quenching and tempering before hardening. A quench refers to a rapid cooling of the part, usually in a liquid. Phase transformations are prevented by reducing the time in which these processes can occur as thermodynamically or kinetically favorable reactions. Quenching reduces crystallininty and polymerization but produces strong stresses that must be relieved by tempering . Extremely rapid quenching can prevent formation of all crystal structure, creating what is known as an amorphous metal or “metal glass” ("Intro to Quenching" 2010). Tempering is the heat treatment technique that transforms relatively brittle martensite or bainite into a combination of cementite and ferrite, making it ductile and much tougher after the process is complete. The process is accomplished by controlled reheating to a temperature below that of the metal’s critical temperate. Trapped carbon atoms caused by oil or water quenching are allowed to diffuse out of the body-centered-tetragonal structure of the metal, resulting in an almost completely pure ferrite structure. The tempering process is one of balancing ductility and strength, with a delicate balance that is controlled by many variations in time and temperature that affect the exact mechanical properties of the metal part (Todd et al. 1994). Case hardening or surface hardening refers the process by which the steel part is submerged in a liquid and allowed to cool, forming martensite, a hard solution of iron and carbon, on its surface. This makes the top layer wear and fatigue resistant. This normally occurs after the final shape is given to the part, but sometimes is done sooner to increase the strength of interior aspects, such as in steel beams (Wolf 2010). Carbon and alloy steels are able to be case hardened, but typically low carbon content steels are used. Case hardening hardens only the exterior of the steel surface in low carbon alloy steels, but in medium and high carbon alloy steels, case hardening may penetrate the core and cause brittleness. There are some cases where a harder core may be required, and in these cases case hardening of medium to high carbon alloy steels may prove mechanically beneficial. The two most common processes are gas and electric heat hardening, with the requirement that the exterior of the part have a high carbon content either from being made of a high carbon alloy content metal or from carburizing or similar process (“Case Hardening” 2010). In electric, or inductive, hardening electric current flow heats the part from the outside inward using the principle of electrically induced magnetism. This system is more costly, but has the ability to provide controlled heating on very small elements. In gas heating, the part is surrounded by a carbonaceous gas and heated. In flame hardening the surface of the metal is heated using an oxyacetylene flame to quickly reach the metal’s critical temperature. This allows a thin surface layer to become hardened while the core of the material retains its original properties. In the flame method, temperatures are normally determined by visual inspection, and care must be taken not to overheat the metal. In all three methods, the metal is immediately quenched afterwards, normally in cold water spray and then cold base metal. Automated systems for hardening generally produce more consistent and reliable results (Elgun 1999). Commercial automotive plastic components are welded by a variety of techniques, including ultrasonic welding and laser welding, as well as hat sealing and traditional electron beam welding that must be carried out under a vacuum (Weber 2007). Automotive headlight and taillight assemblies are generally constructed using laser-spot welding and are made of polycarbonate plastic parts. The two parts are suspended under pressure while the laser beam moves along the joining line, passing through the first and creating enough heat in the second to soften the plastic along the weld line, creating a permanent weld. Laser welding works well in industrial settings for most thermoplastics, acrylics, nylon, polypropylene, and polycarbonate plastics, among others. Laser welding is likely chosen because it allows for precision welds with no nozzles or parts to clog, no materials to mare the surface of the part, no consumables, a high throughput, a low-power density, and capability to produce complex geometrical patterned welds (“Laser Spot Welding…” 2002). Thermoplastics may also be joined by solvent welding, wherein the polymers are freed by a chemical agent and allowed to move on the surface of each component enough to chemically entangle with those on the surface of another component. This is the basis for the functioning of PVC glue that can join PVC pipes. In heat-based welding processes, heat is applied to polymers to reach transition temperatures, above which polymer entanglement may proceed when the parts are brought in contact. The resulting welds from both solvent welding and heat based welding are permanent, as the chemical structure of the polymers are entangled upon process completion (Wise 2001). Solvent welding has been widely used in a variety of industrial applications such as manufacturing of plumbing and piping materials and even in the assembly of plastic toys. Many solvents used have disposal costs which are rising as legislation is going in place to reduce the use of organic solvents in industry. Because many organic solvents may pose both health risks and environmental concerns upon disposal, solvent welding is being reducing in favor of heat based welding whenever appropriate in contemporary industrial settings (Wise 2001). Works Cited Huppertz, Herald. “ Crankshaft (history).” CarTecc Website. May 2008. 18 August 2010 Walbolt, Jim. “Crankshaft Selection Dictated by Design”. Engine Builder Website. April 2008. 18 August 2010. Watson, Laura. “Ductile Iron Data for Design Engineers.” Ductile Website. 2010. 18 August 2010. “Crank Shaft Differences and Manufacture.” VWvortex: Volkswagen enthusiast website. 2006. 18 August 2010. Lyons, William C. and Plisga, Gary J. (eds.) Standard Handbook of Petroleum & Natural Gas Engineering, Elsevier, 2006. Properties, Identification, and Heat Treatment of Metals. Army Manual TC 9-524. 2010. 18 August 2010. Prabhudev, K H. Handbook of heat treatment of steels. Tata McGraw-Hill. 1998. “Diffusion Treatment Hardening”. Efuna. 2010. 18 August 2010. Wolf, Andrew. Comp Cams Shows How a Camshaft is Made. 2010. 18 August 2010. "Intro to Quenching". University of Notre Dame Radiation Laboratory. 2010. 18 August 2010. Todd, R, Allen, D, and Alting, L. Manufacturing Processes Reference Guide. 1994. Industrial Press Inc. “Case Hardening”. Integrated Publishing. 2010. 18 August 2010. Elgun, Serdar. "Case Hardening Methods". 1999. 18 August 2010. Weber, Austin. "Welding Still Ensures High-Strength Joints". Assembly Magazine. 2007. 18 August 2010. "Laser Spot Welding of Plastics". Application Tech Note. Coherent. 2002. 18 August 2010. Wise, Rogers. "Solvent Welding of Thermoplastics". TWI Technology Engineering. 2001. 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