Surface Engineering Technology - Term Paper Example

Surface Engineering Technology Used In Industry for Surface Modification ID Contents 1 Thermo-Chemical Processes5 1.1.1 Nitriding 5 1.1.2 Carburizing 7 1.1.3 Carbonitriding 8 1.1.4 Ferritic Nitrocarburizing/ Tuffriding 10 1.2 Localized Thermal Treatment 11 1.2.1 Flame hardening 11 1.2.2 Induction hardening 12 2 Wear Resistance Hip Prosthesis 13 2.1 Physical Vapor Deposition (PVD) 13 2.2 Cobalt Chromium Alloys (CoCr) 14 2.3 Aluminium Oxide Ceramic Bearings 14 2.4 Discussion 14 3 Colored and Electrically Conducting Thin Films onto Steel Sheets 15 3.1 Batch/ general hot dip galvanizing 15 3.2 Continuous hot dip galvanizing 15 3.3 Spraying 16 3.4 Zinc plating/ electro-galvanizing 16 3.5 Sherardizing 16 3.6 Metal/alloy-rich paints 16 3.7 Mechanical platting 17 4 List of References 18 Table of Figures Figure 1: Schematic diagram of a nitriding process. 6 Figure 2: Schematic diagram of a laser carburizing setup 7 Figure 3: Diagram showing carbonitriding using vacuum method 9 Figure 4: Iron-nitrogen phase diagram 10 Figure 5: Diagram showing the principle of flame hardening 11 Figure 6: Surface hardening of steel with induction hardening 12 Surface Hardening of Steel Engineers in the building and construction industry have used steel over to develop structures over the years. This has been made feasible because steel is strong, flexible, and durable, thus, making it a suitable material for construction. However, there are instances where some modifications have to be made to the metal to make it suitable for some types of construction. The resolve to have steel that is strong, tough, wear, and shock resistant leads engineers to manipulate the surface of the metal, leaving its inner core intact (Davis, 2003). This process is called surface hardening of steel, and it can be done in many ways. Surface hardening techniques can be grouped into two main categories; local thermal treatment and thermochemical processes. The choice of treatment to be used depends on engineering requirement s as well as commercial competition. In this section, this article presents an overview of the procedures used to harden the surface of steel. Each describes the procedures involved in the two categories. 1.1 Thermo-Chemical Processes Engineers modify the local chemical composition of steel at the surfaces by induction of nitrogen, carbon or both. Sometimes they may also use boron. The techniques used in this category depend on the method of heat treatment applied and can further be broken down into four subcategories: i. Pack processes such as metalizing and pack carburizing ii. Salt-bath processes such as carbonitriding (cyanide hardening) iii. Gaseous processes such as gas nitriding and gas carburizing iv. Vacuum based processes including ion nitriding, carburizing, and plasma nitriding The most salient features of these processes are the processing temperature, mechanical properties and depth of the case, as well as the service behavior of the case’s core composite. Temperature is particularly significant because it affects the level of distortion directly. 1.1.1 Nitriding This can only be done on materials that have already been hardened and tempered. It produces better results when used with a range of alloys of steel that form stable nitrides such as vanadium, aluminium, tungsten, chromium, and molybdenum. The resultant nitrides are dispersed evenly throughout the surface of steel. Figure 1: Schematic diagram of a nitriding process. The process occurs at around 540 degrees Celsius where distortion does not take place. The nitride layer on the surface of the metal makes it expand, thus creating large, compressive stresses that in turn increase surface hardness and improve fatigue strength. In addition, the process also results in steel that has a reduced friction coefficient. Nitriding is, therefore, the best method to use when developing surfaces that are intended to minimize friction such as crank shafts and ball bearings. Advantages of Nitriding 1. It is easy to control the nitrogen and oxygen content in the nitriding chamber by monitoring the gas flow rates. 2. Low distortion due to low temperatures 3. No need for machining operations 4. Nitriding produces wear and corrosion resistant steel in high volumes. Disadvantages of Nitriding 1. The salts used in the process are highly toxic, hence posing a health risk. 2. It requires long cycle times to process (between ten to 130 hours per cycle depending on the requirements) 3. The procedures and processes involved are expensive to implement. 4. The cases formed are brittle. 1.1.2 Carburizing This process involves the dispersion of carbon atoms into the surface of the metal. This, therefore, means that it is only suitable in low carbon steels (where carbon can be added up to desirable levels). This is done by putting the carbon gradient in the carburizing chamber at a level that is higher than the metal’s (Vautard, Ozcan, and Meyer, 2012). Figure 2: Schematic diagram of a laser carburizing setup Typically, steel is packed into a chamber containing graphite and is heated at 880 degrees Celsius over a period. The time taken depends on the depth of hardness needed; the harder steel required the longer the period of exposure (Gregory, et al. 2011). Next, the steel is quenched (cooled rapidly) to create a local martensite layer on the surface. Finally, the metal is tempered to obtain the required surface hardness. Tempering involves heating metal to a lower temperature than was used during the hardening process. It reduces hardening to a desired level. The amount of hardness removed is directly proportional to the amount of temperature used during tempering; hence, engineers should be careful when determining the tempering temperature if they are keen on obtaining a metal that meets their requirements. In some cases, the high temperatures used in the two processes of carburizing result in distortions to the metal; hence, it may be necessary to grind the metal to the desired size and shape after completing the treatment. This technique is used in situations where certain parts of the metal, for example, the gear wheel, need to be hardened to reduce wear. The other sections of the metal can be left tough and fatigue resistant. Advantages of Carburizing 1. It guarantees higher resistance to corrosion and wear because of the depth of cases; it is possible to achieve depths greater than 0.3 inches. 2. It results in steel with greater strength than other surface hardening techniques. Disadvantages of Carburizing 1. The high temperatures involved lead to distortion, thus resulting in losses and increased expenses. 2. It takes long to complete a single carburizing cycle. 1.1.3 Carbonitriding In this process, carbon and nitrogen are diffused together into the surface of pure carbon, or low alloy steel, at around 850 degrees Celsius in a salt bath. Great care has to be taken when heating to reduce distortion. This can be done by providing support to the metal. Figure 3: Diagram showing carbonitriding using vacuum method Whereas carbonitriding increases surface hardening, it cannot go beyond 0.5 mm into the surface of the metal. The steel used in this situation is weak; therefore, there is no need of developing deep, hardened layers. Engineers have to be careful to ensure that fatigue resistance is not damaged and that the hard layer does not crush. The addition of layers of metallic nitrides to the carbon layers results in better properties of the metal. The low temperatures used also ensure that there is reduced distortion. Carbonitriding is used in situations whereby just a little hardness is required of steel. This is mostly done to improve the performance of economical materials used in the mass assembly of components. Advantages of Carbonitriding 1. It offers better case hardenability in plain carbon and lower alloy steels. 2. It lowers the critical cooling rate of steel; hence helping reduce distortion. Disadvantages of Carbonitriding 1. It offers limited case depth 2. The proportion not being carbonitrided has to be covered with a copper plate, making the process expensive and tedious. 1.1.4 Ferritic Nitrocarburizing/ Tuffriding Figure 4: Iron-nitrogen phase diagram This procedure can be used with almost all mild and low alloy steels. It introduces nitrogen and carbon with traces of sulphur and oxygen onto the surface of steel. A thin layer of iron carnbonitride (Fe2(CN)) measuring between 0.01 and 0.02 mm in thickness is developed at 570 degrees Celsius. The diffusion of nitrogen in the surface, during quenching, results in a supersaturated nitrogen solution that increases fatigue life. This method’s advantages are related to galling and friction; hence it is mainly used in tools. Advantages of Ferritic Nitrocarburizing 1. It induces little case distortion 2. Improved fatigue properties 3. Increases corrosion resistance Disadvantages of Ferritic Nitrocarburizing 1. Its cases have reduced impact strength, making it unsuitable for such parts as gear teeth. 1.2 Localized Thermal Treatment This category of techniques employs the use of selective surface heating to modify steel. This leaves local hardening to occur at the surface while leaving the properties of the core intact. 1.2.1 Flame hardening Figure 5: Diagram showing the principle of flame hardening Surface hardening in steel can be done by rapid local heating ad quenching. An Oxy-acetylene or oxy-propane flame is moved by a carriage over the metal. At the same time, the carrier sprays water to quench the metal. The result is that the process terminates before the metal’s temperature rises to the hardening temperature (Davis 2003: 58). The process is mainly used for hardening sliding surfaces of machine tools spins, gears, and spindles since they can be spun between centers. Advantages of Flame Hardening 1. Hardening can easily be localized to areas where it is required. 2. It results in little distortion and deformation; only a tiny section of the surface is heated. Disadvantages of Flame Hardening 1. High surface temperatures during quenching may cause cracking. 2. It is not suitable for irregular shaped parts 1.2.2 Induction hardening This process resembles flame hardening, except that, in this case, it is held in a stationary position, and heated simultaneously by electromagnetic-induction (Parsapour, Khorasani, and Fathi, 2011). Figure 6: Surface hardening of steel with induction hardening Advantages of Induction Hardening 1. The process can easily be automated using quenching nozzles, heating coils, and tempering coils. 2. Shorter heating times make the process fast and reduces oxidation and decarburization (Pecheur, et al. 2012). Disadvantages of Induction Hardening 1. Results in poor surface properties compared to other methods. 2. It is uneconomical for small scale hardening as the initial cost of acquiring and setting up the equipment is high. 2 Wear Resistance Hip Prosthesis Biomedical prosthetic devices are used to replace failed human organs. These devices need to be biologically compatible with the human body. The biocompatibility is necessary to ensure that the prosthesis works well with the human body. Hip replacement is the most common remedy for patients with advanced arthritis. It is the final solution available for any patient who would wish to continue walking on his/ her own. Initially, the hip prostheses were made out of two materials, metal and a soft material that aligned with the body. However, the prostheses wore out quickly because of friction. Manufacturers opted for the use of hard on hard hip replacements to prolong the life of the prosthesis. Whereas this has works, there are still questions on the impacts of metallic ions that are generated due to their gradual wear and tear; they have been blamed for premature loosening, inflammation, and allergic reactions. Nevertheless, several technologies have been employed to try and minimize the wear of hip prostheses. 2.1 Physical Vapor Deposition (PVD) One method of reducing wear at the articulating surfaces of prostheses is the use of Titanium-Niobium-Nitride (TiNbN) through Physical Vapor Deposition (PVD). In this process, the prosthesis is coated with nitrogen in a high-vacuum chamber (10-2 to 10-4 mbar). The prosthesis is bombarded with energetic, positively charged ions of titanium, and Niobium. Nitrogen gas is then introduced into the chamber during metal deposition (Marcos, et al. 2012). This creates a coating on the surface, resulting in a strong union between the nitrogen coating and substrate and the structural, physical, and tribological properties of the film. 2.2 Cobalt Chromium Alloys (CoCr) Originally meant for the aerospace industry, CoCr have excellent mechanical properties that make them suitable for use in hip prostheses; they have better corrosion and mechanical properties than steel. The alloys may take the form of CoCrMo or CoNiCrMo and could also include elements like iron and tungsten. Their low wear levels and ability to support high weights make them suitable alloys for bearing surfaces for hip prostheses (the hip joints should be able to sustain the weight of the upper torso of the human body). 2.3 Aluminium Oxide Ceramic Bearings Alumina ceramic bearings are being used to reduce osteolysis and wear amongst young, active patients. This is because the aluminium oxide has high lubrication, is highly resistant to wear, and does not release ions (Gakovic, et al. 2012). 2.4 Discussion The use of hip prosthesis has evolved from the years when it was done using steel, which was at that time the best material to use. However, the need for more efficient prostheses necessitated the development of alternative compounds; these are compounds that are cost-effective, inert, durable, easy to implant, and produce minimal wear debris. It is crucial to reduce the amount of debris and ions since studies have established that they cause side effects. An accumulation of debris would also increase wear since they would increase friction. These techniques have the following advantages: They allow the joint to stay intact for long since they possess high abrasion resistance. Their low friction coefficient makes them generate less heat compared to steel; hence increasing their longevity (heat accelerates the degeneration of hip prostheses). They are biocompatible with the human body, hence, reducing the chances of rejection, or infection of the body. 3 Colored and Electrically Conducting Thin Films onto Steel Sheets Colored and electrically conducting thin films can be added onto metal sheets for various reasons. Some of these include increasing the metal’s conductivity and giving steel a special appearance. The level to which these can be implemented depends on the function for which the metal is to be used (Bayer, et al. 2012). Zinc coatings are normally used, in iron and steel, to provide protection from corrosion and wear. They are used mostly to protect finished products like nuts, bolts, structural steelwork for bridges and buildings, strips, and sheet. Apart from offering protection from corrosion, zinc also gives the finished product a new, attractive color (Schindhelm, et al. 2011). Zinc coats and other metallic, thin films, can be added to steel using the following techniques: 3.1 Batch/ general hot dip galvanizing The steel components are cleaned using chemicals and then immersed in zinc, or any other metal to be used to make the thin film. The steel is immersed into molten zinc at about 450 degrees Celsius. Metallurgical reactions between zinc and steel create a series of steel-zinc layers. A layer of zinc remains on the surface of the metal as it is removed from the molten zinc. The bond between base steel and zinc results in a surface that is not only resistant to corrosion, but also less susceptible to wear (Luciano, et al. 2012). In addition, Zinc’s perfect electrical and thermal conductivity enhances the electrical conductivity of steel sheets. 3.2 Continuous hot dip galvanizing Non-oxidized, clean steel sheets are passed rapidly in a bath of molten zinc alloy and wiped to ensure the thickness of the coat is uniform. The sheet is the cooled, forming a continuous, thin zinc coating. A number of thermal and chemical processes may then be applied to the sheet depending on its intended final use. For example, paint and plastic coatings may be added to provide a colored finish and additional protection respectively. This method is mostly applied in cladding and roofing for industrial, commercial, and domestic buildings, as well automotive bodies and domestic appliances. 3.3 Spraying In this case, arc pistols or special flames are used to project atomized particles of metal or alloy onto a grit-blasted surface of the steel sheet. This method is used mostly in situations where the steel is too large to be immersed into a chamber containing the molten metal. 3.4 Zinc plating/ electro-galvanizing This technique is used in situations where the sheet requires a finer finish than conventional galvanizing can offer. The coating is deposited electrically from a solution of the metal’s salt. If the coating is zinc based, then a salt of zinc will be used. 3.5 Sherardizing Sherardizing is a process whereby the steel sheet is heated in a mixture of sand and zinc dust. This is done in a drum that is rotating slowly, and at temperatures just below zinc’s melting point, until a zinc-steel alloy coating is formed over the sheet. This results in an even coating that is grey. The technique is recommended for tiny sheets only since it is difficult to heat large sheets evenly. 3.6 Metal/alloy-rich paints Paints with high levels of the desired metal/alloy are used on the surface of the steel sheet. The paint can be applied by spraying, brushing, or immersion. Once dry, the thin film formed exhibits the properties of the parent metal/alloy. If zinc-based paint is used, then the resultant film will, to some extent, act as a metallic zinc coat. It will be electrically conductive, hence enhancing the conductivity of the sheet. This method can be used to make repairs to damaged zinc coatings, as well as to protect car bodies, ship hulls, and factory steel work. 3.7 Mechanical platting Steel sheets are cleaned and immersed, into a copper sulphate solution, to create an extremely thin copper coating. The sheet is then tumbled in a mixture of ballotini and zinc dust, in an aqueous medium, to impact zinc into it. In addition to the electrical and thermal conductivity enhanced by copper, this forms a thin zinc coating that increases the sheet’s resistance to wear and corrosion. 4 List of References Ana M. C. et al., 2012. Surface modifications by gas plasma control osteogenic differentiation of MC3T3-E1 cells. Acta Biomaterialia, 8 (2012) pp.2969–2977. Bayer, E. et al, 2012. The influence of compressive stress applied by hard coatings on the power loss of grain oriented electrical steel sheet. Journal of Magnetism and Magnetic Materials, 323 (2011) pp.1985–1991. Davis, J. R., 2003. Surface Hardening of Steels: Understanding the Basics. Ohio: ASM International. Gakovic, B. et al., 2012. Selective single pulse fem to second laser removal of alumina (Al2O3) from a bilayered Al2O3/TiAlN/steel coating. Surface & Coatings Technology, 206 (2012) pp.5080–5084. Gregory, S. et al., 2011. In situ surface hardening during turning via pyrolytic carburization. Precision Engineering, 36(2012) pp.668-672. Luciano, R. et al., 2012. Atomic force microscopy applied to the quantification of nano- precipitates in thermo-mechanically treated microalloyed steels. M a t e r i a l s C h a r a c t e r i z a t i o n, 6 9 ( 2 0 1 2 ) pp.9 – 1 5. Marcos, C. G. et al., 2012. High-throughput analysis for preparation, processing and analysis of TiO2 coatings on steel by chemical solution deposition. Journal of Alloys and Compounds, 540(2012) 170-178. Parsapour, A., Khorasani, S. N., & Fathi, M. H., 2011. Effect of surface treatment and metallic coating on corrosion. J. Mater. Sci. Technol, 28(2) pp.125–131. Pecheur, A. et al (2012) Polycrystal modelling of fatigue: pre-hardening and surface roughness effects on damage initiation for 304L stainless steel. International Journal of Fatigue, 45 (2012) pp.48–60. Schindhelm, J. et al. (2011) Combination of zinc alloy coating with thin plasma polymer films for novel corrosion protective systems on coated steel. Surface & Coatings Technology, 205 (2011) pp.S137–S140. Vautard, F., Ozcan, S. & Meyer, H. (2012) Properties of thermo-chemically surface treated carbon fibers and of their epoxy and vinyl ester composites. Composites: Part A 43, (2012) pp.1120–1133. Read More