Treatment and Processing Methods of Metals - Assignment Example

Table of Contents Task 1 2 Treatment methods 2 Heat treatment 2 Annealing 2 Case hardening 3 Quenching 4 Processing methods 6 Liquid metal processing methods 6 Mechanical metal processing methods 8 Rolling 8 Forging 9 Task 2 10 Macro and microscopic examination of Fabricated Bracket 10 Task 3 12 Tensile tests on polymers 12 Conclusion 16 References 17 Task 1 Treatment methods Heat treatment Heat treatment is a group of metal working processes used to alter the physical, sometimes chemical, properties of a material to achieve a product with the desired properties. It involves the heating or chilling the material, usually to extreme temperature, to cause hardening or softening depending on the desired property. Some of the techniques used in the heat treatment process are tempering, quenching, annealing, case hardening and normalizing. Heat treatment aims at manipulating the grains in the material by controlling the rate of diffusion and cooling within the microstructure, resulting in alterations in the properties of the metal. Heat treatment is usually applied to metallic alloys, causing significant alterations in properties such as strength, toughness, elasticity and hardness. Annealing This is a heat treatment process that alters the physical and chemical properties of a material to make it more malleable and easy to work with. The material is heated to above its recrystallization temperature, maintained at a predetermined temperature, and then cooled. This results in a reduction of the hardness and an increase in the ductility of the metal, allowing it to prepared for further work, such as shaping, stamping and forming. The procedure of heating the metal and holding it at a specific temperature has the effect of causing atoms within the solid material to diffuse, resulting in the material progressing towards its equilibrium state. Raising the temperature of the metal past its recrystallization point provides the atoms with the energy to break bonds and ‘move about’. The redistribution of atoms increases the ductility of the metal by eradicating dislocations, allowing it to deform more easily. This redistribution of atoms is a thermodynamically spontaneous process referred to as ‘stress relief’, which is slow in room temperatures, with the high temperatures needed in the annealing processing serving to accelerate the process. The annealing process takes place in 3 stages, namely [Ver75];- 1. Recovery – This is the removal of dislocations and their associated internal stresses, causing the material to soften. This occurs in the lower temperatures of the process, but the grain size and shape are not altered. 2. Recrystallization – New strain-free grains nucleate and grow to replace the grains deformed by internal stresses 3. Grain growth – The microstructure of the metal starts to coarsen, and may result in significant loss in the material’s original strength Figure 1: Effect of annealing on microstructure[Tew15] Case hardening Also known as surface hardening, case hardening refers to the process of hardening the surface of the metal while allowing the inside of the material to remain soft, creating a layer of harder metal on the surface called a ‘case’. Case hardening is usually done to the material after it has attained its final shape, but it can also be done to increase the hardening element content of assembly component. Case hardening provides the material with some added properties, such as shock absorbing without cracking due to the soft core, and increased surface wear resistance. However, case hardening is not advisable for components that will be subject to certain kinds of stresses, such as impact stresses. Case hardening more or less revolves around the amount and position of carbon atoms within the metal object. Carbon is sold and immobile at case-hardening temperatures and can only be transported to the surface of the metal as gaseous carbon monoxide , when the metal is heated. The process requires significant carbon atoms to be present in the material. Metals that have low carbon concentration, such as iron and steel, the object is packed with a substance high in carbon, such as charcoal or ground bone [Hig83]. The choice of material used to pack the metal depends on the final desired appearance, with organic substances causing some discolouration, which is used for decorative finishes and providing a degree of corrosion resistance. The packing is then heated to a high temperature, but below that of the metal’s melting point, and held there for a length of time. Longer periods will result in more carbon atoms diffusing into the metal’s surface, and this time is determined by the eventual use of the metal. Figure 2: Cross-section of multi-layered microstructures showing case hardening[Kab14] The cold chisel (Item D) is a cutting tool and case hardening would be used in its manufacture. Quenching This refers to a heat treating process where the workpiece is cooled rapidly to obtain certain desired material properties. Quenching is done to prevent the occurrence of certain low temperature processes that would compromise the physical properties of the final object. Quenching works by reducing the window of time that these undesirable reactions are kinetically accessible and thermodynamically favourable. The process causes significant change in the microstructure of the metal, such as reducing the crystal grain size, increasing the material’s hardness. Quenching can aslo change the composition of the grains, such as introducing martensite in steel by cooling the steel rapidly through its eutectoid point. The steps followed in the quenching process are as follows;- 1. Heating – The material is heated to between and , paying careful attention that the workpiece is heated evenly throughout. Uneven heating or overheating could result in the development of undesired material properties. 2. Soaking – The heated material is soaked in air, a liquid bath, or a vacuum, depending on the desired properties and material composition. The soaking should be done evenly such that the temperature within the sample remains uniformly distributed. 3. Cooling – The material is submerged in a quenching liquid, with different liquids imparting different properties to the final object, as the liquid used determines the rate of cooling of the metal. Water is the most commonly used quenching liquid, giving the workpiece maximum hardness, but with the possibility of tiny cracks and distortion. Figure 3: Microstructure of the specimen after quenching at different distances[Fer14] Cooling can also be achieved using inert gases, as is the case with nitriding. The introduction of some elements to the steel, such as tungsten, give the illusion that the steel has been cooled more rapidly that it has, meaning that the metal can be cooled slowly in air and achieve the desired properties. Quenching can result in the material having excessive hardness, making it brittle and other heat treatment methods may be necessary to increase its toughness, such as tempering. Processing methods Liquid metal processing methods Liquid metal processing techniques manipulate the metal while it is in its molten form, more commonly referred to as casting. Casting refers to the process of pouring liquid metal into a mould and allowing it to cool and solidify, taking the shape of the mould. Casting is employed when producing components with complex shapes, which would be irrational, impossible or too expensive to utilize other methods of metalworking [Deg03]. Casting is referred to as a solidification process since the phenomenon of solidifying dictates the properties of the final casting. The process of solidification takes place in two steps, namely;- 1. Nucleation – In this stage, solid particles form within the molten metal, which have a lower internal energy than the surrounding liquid, creating an energy interface between the two states. At this interface, energy is required to form the surface of the solid materials cooling the material below its freezing temperature. Nucleation occurs on a pre-existing solid surface since less energy is required for partial interface surface as opposed to a complete spherical surface interface. This has the effect of producing fine-grained microstructures in the metal, giving it more desirable mechanical properties. 2. Crystal growth – After the internal solid surface cools below the freezing point at the energy interface, the metal heats back up to allow for the crystal growth stage to proceed, in a process called ‘recalescence’. All nucleations formed represent a crystal which grows by extracting the heat of fusion from the molten metal until no more liquid is left. The rate, type and direction of crystal growth can be controlled to produce a final product that possesses the desired properties The most preferred mode of solidification is called ‘directional solidification’, which allows the liquid state to compensate for shrinkage of the solid part, reducing the likelihood of occurrence of internal stresses within the finished product [Kal06]. Figure 4: A typical cast metal structure Investment casting is a manufacturing process where a wax pattern is coated with a refractory ceramic material and cast to provide the mould where the molten metal will be poured. This is done for components that have a complex and precise geometric shape, such as turbine blades (Item C). Mechanical metal processing methods Mechanical metal processing methods are defined as the intentional deformation of metals plastically under the action of an externally applied force. [Lib17]. Mechanical metal working is broadly categorized as ‘hot working’ and ‘cold working’. Hot working refers to working the metal while it is above its recrystallization temperature, while cold working is done with the metal object below the recrystallization temperature [Mec13]. Rolling In this process, the metal workpiece is passed through one or more pairs of rolls to reduce the thickness, and produce a product with uniform thickness. In hot rolling, recrystallization occurs simultaneously with deformation, and thus keeps pace with the actual working process. This has the advantage that the metal will not work harden and can be quickly and continuously reduced to the desired dimension using minimum energy. The recrystallization process rids the workpiece of internal dislocations, making it more malleable at high temperature. This also has the effect of creating a uniformly fine grain in the recrystallized material, making the final product stronger, tougher and more ductile than the original workpiece. However, hot rolled workpieces have an overall poor surface condition due to oxidation and scaling that occurs at high working temperatures. The accuracy of the dimension of the final workpiece is also compromised since the form tools need to be of simpler design to work under high temperatures. Hot rolled workpieces usually undergo further metalworking processes to clean the surface and improve its quality. Figure 5: Microstructures of cold rolled steel[Sri13] Cold rolling, on the other hand, is done below recrystallization temperature and increases the strength of the workpiece via strain hardening. Workpieces that have been cold rolled have good surface finish and are more tolerant to stresses. Cold rolled sections have better mechanical properties than sections that have been machined from solid pieces. Forging Metal forging is a metal forming process that involves applying compressive forces to a work piece through various dies and tools to deform it, in a bid to create a desired change in the geometric shape of the material. Forging is an integral part in the industrial metal manufacturing industry, particularly in the manufacture of iron and steel. The aim of metal forging is to produce forgings of desired geometry and specific material properties, for use as individual tools but more often as components put together as part of an assembly [Lib17]. In hot forging, the workpiece is heated and then pressure is applied to form it into the desired shape and size. This causes the alignment of grains to be continuous throughout the formed component, giving it improved strength characteristics over equivalent cat or machined pieces [Deg03]. Hot forging is almost always done to iron and steel pieces to prevent work hardening, which may increase the difficulty of performing other metalworking operations. The forging process can be widely categorized as the following;- 1. Drawn out forging – tensile forces are applied to the workpiece causing the length to increase while the cross-section decreases 2. Upset – compressive forces are applied to the workpiece causing an increase in the cross-section while the length decreases 3. Squeezed – compressive forces are applied to the workpiece while in closed compression dies, causing multidirectional flow depending on the type of mould used. Figure 6: Cast ingot cross-section Forging is the most likely process used in the manufacture of the connecting rod (Item B). Aluminium forging could also be used to produce the aluminium piston (Item A) Task 2 Macro and microscopic examination of Fabricated Bracket The forming process used to manufacture the fabricated bracket is cold rolling. Cold rolling is done below recrystallization temperature and increases the strength of the workpiece via strain hardening. Workpieces that have been cold rolled have good surface finish and are more tolerant to stresses due to the limitation on sizes that can be cold rolled. Cold rolled sections have better mechanical properties than sections that have been machined from solid pieces. Cold rolled steel is created by working the metal at temperatures close to room temperature, creating a product that is more dimensionally precise than a hot rolled product [Mid15]. Welding is a fabrication process that is used to join metals by melting them to form a pool of molten material, often with filler materials added, to form a joint that is usually stronger than the base metals. Welding can be classified as a type of covalent bonding where constituent atoms are of the same type and don’t combine to form a chemical bond. The atoms in the microstructure lose electrons to create an array of positive ions, which share the electrons. This means that the electron are free to move about the lattice, giving metals their relatively high electrical and thermal conductivity, as well as their ductility [Lan99]. Welding has been shown to affect the mechanical properties of the base metals, with increase in welding current and speed of welding increasing the strength of the workpiece, but also reducing the toughness, yield stress and tensile strength. This was attributed to the fact that increased current meant increased heat input, providing room for deformations that reduce the material’s mechanical properties [Tal14]. Task 3 Tensile tests on polymers Tensile tests measure the force required to break a sample specimen, in this case polymer materials, and the extent to which the specimen stretches before the breaking point. Table 1: Polymer tensile tests results No. Material UTS Elongation 1 High Density Polyether (HDPE) 89.333 87.1 2 Low Density Polyether (LDPE) 56.5 100 3 Polystyrene 495.333 40.4 4 Toughened Polystyrene 1692 6.25 5 Nylon 2044.667 273.5 6 Nylon Glass Filled 4248 10.7 The plots below show the stress- strain diagrams for the 6 different polymer materials. Figure 7: Stress-Strain curve for High density polyether High density polyether is a polymer composed of organic units composed of urethane links. The stress-strain curve shows that the polymer will withstand significant loading in the early stages coupled with very little deformation of the material. However, when the loading reaches a certain point (close to 600N), the polymer reaches its yield point and freely deforms without the addition of further load. Figure 8: Stress-Strain curve for low density polyether Low density polyether is a polymer composed of organic units composed of urethane links that have some chain extenders and cross linkers within the material’s microstructure. The plot shows that the polymer quickly deforms when a load is placed. However, this mechanical deformation induces reorientation in the microstructure towards the direction of the stress, giving the polymer high tensile strength and elongation resistance. This continues until the mechanical reorientation is overcome at the second yield point [Gum92]. Figure 9: Stress-Strain curve for Nylon 66 Nylon 66 is a polyamide material. The stress-strain plot in Figure 3 shows that it has significant tensile strength, with minimal elongation even when subjected to heavy loads (approximately 1700N). However, once the material is past its yield point, it deforms freely without any additional load. The introduction of glass fibre to the Nylon 66 results in increased tensile strength, with the stress-strain plot in Figure 4 showing significantly little deformation with sizeable increasing loads Figure 10: Stress-Strain curve for Nylon 66 (glass filled) Figure 11: Stress-Strain curve for Polystyrene Polystyrene is a synthetic aromatic polymer that is usually a clear, hard and brittle solid. The stress-strain plot shows that the material will withstand some loading with little deformation, until its yield stress where it freely deforms without any additional loading [Ame12]. Toughened polystyrene is manufactured by extruding and forming normal polystyrene, creating a product with high toughness and increased tensile strength. The stress-strain plot in Figure 6shows that the material is able to withstand huge loads with very little associated deformation before reaching its yield point Figure 12: Stress-Strain curve for toughened Polystyrene Conclusion This reports finds that the piston is made by high pressure die casting. The con rod is made by closed die drop forging. The stainless steel turbine was made by investment casting, while the chisel was made by hot forging, followed by quenching and tempering. The fabrication of the bracket by cold rolling resulted in a stronger material that is resistant to stresses and has a good surface finish. The welding carried out between the individual components resulted in a metallic bond stronger than the parent materials. References Ver75: , (Verhoeven, 1975), Tew15: , (Tewary, et al., 2015), Hig83: , (Higgins, 1983), Kab14: , (Kabir & Islam, 2014), Fer14: , (Ferro & Bonollo, 2014), Deg03: , (Degarmo, et al., 2003), Kal06: , (Kalpakjian & Schmid, 2006), Lib17: , (LibraryOfManufacturing, n.d.), Mec13: , (Lohani, 2013), Sri13: , (Srikanth, et al., 2013), Mid15: , (MidCitySteel, 2015), Lan99: , (Lancaster, 1999), Tal14: , (Talabi, et al., 2014), Gum92: , (Gum, et al., 1992), Ame12: , (AmericanChemistryCouncil, 2012), Read More