An Essential Tool in Inspecting Engineering Components - Essay Example

?Applications of Materials Science and Engineering in Well Engineering Q1. Metallic materials have emerged as the materials of choice for modern industrial era. Like most of the engineering components, most of well engineering components like drill pipe, crown block, drill string etc are also made of metallic materials. Some of the reasons for using metallic materials are the following. (a) Ease of Manufacturing: Metallic materials are easiest to manufacture into different forms by different shaping processes like casting, hot and cold rolling, hot and cold forging, extrusion, drawing, pilgering etc. exhibit very good formability. One can convert metallic materials into different forms like plates, bars, tubes etc. to name a few. This is possible because metallic materials have very good formability by virtue of non-directional metallic bond. Not just that, metallic materials are easy to assemble to form different assemblies and structures by different joining processes from mechanical fastening to welding processes. (b) Excellent Combination of Mechanical Properties Different set of mechanical properties are required from a component to match the demands posed by the service conditions and environment. Some applications demand very high strength and hardness, while small to moderate ductility is good enough, such as tools for drilling of hard rocks. On the other hand some applications cannot allow relaxation on ductility and toughness; while strength can be sacrificed to some extent. Metallic materials offer excellent combination of these mechanical properties. Not just that, mechanical properties of metallic materials can be tailored as well by means of alloying, mechanical working, surface treatments and suitable heat treatment. Therefore, metallic materials have an edge over other class of materials when it comes to offer desired combination of mechanical properties to suit an application. (c) Very Good Wear and Corrosion Resistance Components used in well engineering face very hostile service conditions in terms of wear and corrosion. Therefore, there is need of a material which can offer matching wear and corrosion resistance. Metallic materials offer very good wear and corrosion resistance. Therefore, it is but natural that most of the well engineering components are made of metallic materials. Q2. Relevant properties required by the materials used for manufacturing the “Centralizer” and “Travelling Block Hook” are briefly discussed below. (a) Centralisers: These are used to ensure centering of the drilling string in the well bore. Three types of centralizers are there - drill pipe centralizer with changeable sleeve, bit centralizer with changeable sleeve and casing centralizer [1]. Casing ring centralisers are used to provide clearance gap or stand off between the wall of well and the casing. The centralisers rub off against the wall of the well. This rubbing action leads to wear and tear, heat generation and many times sparking. The material, therefore, should have low coefficient of friction, so that less heat is generated and chances of spark generation are minimized. The material should provide high wear resistance. It should possess high thermal conductivity and resistance against spark. Zinc and aluminum alloys posses these properties and are therefore, used for manufacturing of centralisers. Nowadays, spray metal technology is also being explored for fabrication of the casing centralisers [2]. (b) Travelling Block Hook: Travelling block and hook are used for lifting in drilling rig [3]. The material for manufacturing of this component should be high on strength, wear resistance, fatigue resistance and toughness. This is because catastrophic failure of this component can jeopardize safety of the personnel besides other tangible losses. Low alloy steels are used for manufacturing this component. The material is hot forged into the shape and then hardened to improve strength of the component. However, in hardened condition its ductility and toughness drops drastically and therefore, it is then tempered to introduce some toughness into this component. Q3. Atoms can be seen as hard spheres, which pack themselves into different crystallographic forms. It is known from our experience that when hard spheres pack together some interstitial space remains empty. In other words, hard spheres cannot pack themselves to occupy 100% of the available volume. Thus, atomic packing factor (APF) is the volume fraction occupied by atoms in a crystal [4]. A crystal is made by putting the unit cell together in repetitive manner; therefore, atomic packing factor of a crystal is the same as that of the corresponding unit cell. APF of bcc and fcc unit cells are calculated below. This is because, most of the metals are either bcc or fcc or hcp in structure and packing factor of hcp is the same as that of an fcc crystal. APF can be expressed as the following formula: Where, Natom is number of atoms per unit cell Vatom is volume of individual atom Vunit cell is volume of unit cell APF of BCC Unit Cell: BCC unit cell is made of atoms at each of eight corners of the cube and one atom in the centre of the cube. Let us assume that atomic radius is ‘r’ and cube edge is ‘a’. The body centre atom is completely enclosed in the unit cell and the 8 corner atoms are shared by eight unit cells. Therefore, number of atoms per unit cell is 1 + 8/8 = 2. For closest possible packing of identical hard sphere in BCC lattice; the atoms along the body centre will have to be in contact. In other words length of body diagonal of a BCC unit cell will be equal to twice the diameter or four times the radius of the atoms. i.e. ………. (1) Hence, or 68% APF of FCC Unit Cell: BCC unit cell is made of atoms at each of eight corners of the cube and one atom on each of the six faces of the cube. Let us assume that atomic radius is ‘r’ and cube edge is ‘a’. The face centre atom is shared between two adjacent unit cells and the 8 corner atoms are shared by eight unit cells. Therefore, number of atoms per unit cell is 6/2 + 8/8 = 4. For closest possible packing of identical hard sphere in FCC lattice; the atoms along the face centre will have to be in contact. In other words length of face diagonal of a FCC unit cell will be equal to twice the diameter or four times the radius of the atoms. i.e. ………. (1) Now, or 74% Q4. When a pure metal is cooled from a temperature above its melting point i.e. from liquid state, it is expected to get solidified. But there is a barrier to the solidification of a liquid. For solidification process to begin within the liquid; nuclei should form and formation of nuclei is thermodynamically not feasible. Nucleation is a statistical phenomena and its probability increases with increasing undercooling of the liquid below the solidification temperature. Once nuclei form it grows an thus solidification process propagates. When the liquid is poured into a mould; it experiences very high undercooling or chill effect at the mould wall. This leads to formation of very large number of small chilled grains on the mould wall. Some of these grains grow towards the mould centre leading to formation of columnar grains. After some solidification has been over the temperature gradient in the mould flattens and equiaxed grains form in the central zone of the mould. As a result of solidification, the liquid contracts and this leads to formation of open and / or closed pipes in the central zone, which is a solidification effect. Q5. (a) Fe-C phase diagram is shown in Fig. 2 [5]. Fig. 2: Fe-C Phase Diagram [5] This phase diagram is termed as mother of steel metallurgy. Different phase field of Fe-C system up to 6.67 wt% C is shown in this diagram. From this diagram one can find out critical temperatures associated with different phase transformations. Fe-C alloys with up to 2.14 wt% C are termed as steel. Up to 2.14 wt% C, the alloy can be heated to single phase austenite (?) and therefore, it can be hot worked into different shapes. Fe-C alloys with more than 2.14 wt% C have lower melting point an high fluidity an therefore suitable for casting an hence known as cast iron. Using this diagram one can predict equilibrium phase composition for steel or cast iron of known composition [6]. Q5. (b) Eutectoid is solid state reaction in which one solid phase transforms into two soli phases. Composition of the eutectoid is 0.76 wt% C and the temperature is 727 oC. The material is high carbon steel. It can be heated to a temperature above 727 oC and becomes single phase austenite. In this phase field, the steel has high ductility and hence it can be hot worked forged, rolled, extruded etc.). The eutectoid reaction is the following. The ? + Fe3C produced upon cooling of ? consist of alternate lamellae of ? + Fe3C and is known as Pearlite structure [6]. The eutectic reaction involves liquid phase On the other hand, the eutectoid region occurs at the composition is ~ 4.3 wt% C and the temperature is 1130 oC. This temperature corresponds to the minimum melting point in Fe – C system. At this point, the material is cast iron. It can be heated to a temperature above 1147 oC and becomes single phase liquid. In this phase field, the steel has high fluidity and hence it can be cast into different shapes. When cooled below the eutectic temperature, the material undergoes solidification reaction and simultaneously solidifies into two solid phases ? and Fe3C. The eutectic reaction is the following. The microstructure of ? + Fe3C produced upon cooling of L (Fe-C) consists is known as Ledeburite [6]. Q6. Mild steel is Fe-C alloy with less than 0.3 wt% C. It is hypoeutectoid steel. When it is cooled from high temperature ? phase field, the resulting final phase field and microstructure depends on the rate of cooling [7]. When cooled slowly the microstructure consists of pro-eutectoid ferrite (?) and pearlite. This equilibrium transformation requires diffusion of carbon for formation of ? and Fe3C. However, if the steel is quenched (water or brine quenching) from the high temperature?? phase field carbon does not get the time to diffuse out of ? phase and this causes formation of a distorted lattice – body centered tetragonal (bct). This phase is known as martensite. This is very hard and brittle phase. The critical cooling rate required for formation of martensite depends on alloying element concentration. One can get the value of critical cooling rate from the CCT diagram. The CCT diagram for hypoeutectoid steel is presented in Fig. 3 [8]. For martensite formation the cooling rate should be such that it surpasses the nose of the CCT diagram. However, for mil steel the time to surpass the nose of the CCT diagram is less than even 1 s. This means very high cooling rate is needed for complete transformation of mild steel into martensite. This is not possible even by water quenching except in a thin surface zone. Therefore, hypoeutectoid steels do not produce 100% hardenability even by water quenching. Fig. 3: CCT Diagram of Hypoeutectoid Steel [8] Q7. Offshore drilling platforms and flexible pipes in oil and gas fields are made of mild steels. The main concern for these structures is safety. To ensure safety the material of construction should have sufficient ductility and fracture toughness so that there is no catastrophic failure as that would lead to huge losses in terms of asset and life. Ductility and fracture toughness helps in warding off catastrophic failures by giving sufficient time and warning signals in prior to fracture and therefore, one can take remedial action. Annealing of mild steel or any other steel helps in improving ductility and fracture toughness of the material by either forming a fresh set of stress free grains or by relieving pre-existing residual stress in the material [8]. Besides, removal of residual stress also helps in minimizing the danger of stress corrosion cracking a major problem in marine atmosphere. These are the considerations why offshore drilling platforms and flexible pipes used in oil and gas fields are treated by annealing. Q8. For hardening of engine cam shafts, revolvers and self-drilling screws surface hardening methods are used. This is because hardness is required only on the case while a non-hardened core provides the necessary toughness to the component. The surface hardening methods are – Carburizing, Nitriding, Carbonitriding, Flame hardening, Induction hardening, Laser Transformation Hardening etc [10]. These processes are briefly described below. Case Carburizing The job to be hardened is put in carbon rich medium and heated to allow diffusion of carbon into the surface. This leads to enrichment of carbon in the surface region. The component is then hardened to give very high hardness to the surface by martensitic transformation. Case Nitriding The job is heated in nitrogen rich media. Nitrogen diffuses into the surface and the surface gets hardened by formation of nitries and carbonitrides of iron. Flame, Induction and Laser Surface Hardening In these processes a heat source like flame, induction coil or laser beam is applied onto the surface of the job to rapidly heat the surface into ? phase field. The power density and the time of application of the power source determine the case depth. Once the heat source is removed the hot surface gets self quenched by the underlying cold bulk leading to martensitic transformation on the surface. This is how surface hardening is of Cam Shaft, revolvers and self-drilling screws is carried out. Q9. Destructive testing is employed to determine microstructure and mechanical properties of materials to be used for construction of well engineering components. Some of the destructive tests are – Metallography, Tensile test, Hardness test, Fracture Toughness test etc. These are briefly described below. (a) Metallography: For metallography a sample is taken and polished to mirror finish. Afterwards it is etched with suitable etchant like 2% Nital (2% v / vnitric acid in distilled water) for carbon steel. This reveals the microstructure i.e. grain size and distribution of different phases under optical microscope at high magnification (100X, 200X, 400X). From this test one can determined whether the material is in proper heat treated condition – annealed / normalized / hardened etc. or not [11] (b) Tensile test: This is performed at room temperature or even at elevated temperature. In this test a sample (cylindrical or rectangular) is pulled at certain speed between cross-heads. This leads to strain in the material and the load required to cause this strain is supplied by the testing machine. This test provides load – deformation plot; which is then normalized by cross-sectional area and length respectively to give stress-strain plot of this material as shown in Fig. 4 [12]. From this test one can determine material properties like yield strength, ultimate tensile strength, % elongation, strain hardening coefficient etc. which are very important design parameters. Fig. 4: A typical stress – strain curve [12] (c) Hardness test: Hardness test quantifies resistance of material against plastic deformation. One important hardness test is Vickers’ Hardness Test. In this test a square base pyramidal diamond indentor is pressed against the test piece at certain load for certain time and the load and the indentor is then removed. This leaves rhombus impression on the surface. The applied load divided by area of the indentation mark gives hardness of the material. However, in actual practice area is not calculated every time; instead, length of the diagonals is measured and averaged and from a table the hardness value is read corresponding to the applied load and the diagonal size. This test method is very popular in industry and is widely used for case depth determination and other applications. (d) Fracture Toughness test: Fracture toughness of a material is a measure of the resistance offered by the material against crack growth. Therefore, this value is very critical in assessing vulnerability of materials for catastrophic failures and hence is of particular interest to the well engineering industry. Q10. (a) Non-destructive testing or (NDT) is an essential tool in inspecting engineering components. This is because, destructive tests are sample based and therefore, cannot be used to inspect all finished components. At the same time one cannot take the risk of in-service failure and consequent losses of life and wealth and hence it is essential that all the components going for critical applications be checked thoroughly for detecting any undesired defect arising out of material or process imperfections. Therefore, there is no option but to go for NDT of engineering components. NDT does help not just in preventing accident and consequent losses; but also helps in salvaging a defective component and assessing residual life of a component in service and hence contributes to the economy of the entire process. Q10. (b) There are two methods which can be used to detect shallow embedded defects – (i) Magnetic Particle Testing (MPT) and (ii) Eddy Current Testing (ECT). While MPT is suitable only for magnetic materials; ECT can be employed to all the conducting materials. While other process are capable of detecting either surface defects (Visual Inspection, Die Reentrant Test (DPT) etc.) or only deep bulk defects (Ultrasonic Testing (UT), Radiography etc); these two methods – MPT and ECT are able to detect shallow embedded defects. References: [1] http://00e13f9.netsolhost.com/centralizer.htm [2] Gammage J., “Advances in Casing Centralization Using Spray Metal Technology”, Offshore Technology Conference held in Houston, Texas, USA, 2–5 May 2011 [3] http://www.jereh-oilfield.com/uploadfiles/Travelling-Block-and-Hook.pdf [4] Smallman R. E. and Bishop R. J., Chapter 3 in “Modern Physical Metallurgy and Materials Science”, 6th ed., Butterworth-Heinemann, Oxford, 1999, p. 44 [5] Avner S. H. “Metal Structure and Crystallization”, In “Introduction to Physical Metallurgy”, 2nd Eds., McGRAW –HILL International Editions p 80 – 85 [6] http://web.utk.edu/~prack/MSE%20300/FeC.pdf [7] Avner S. H. “The Iron – Iron Carbide Equilibrium Diagram”, In “Introduction to Physical Metallurgy”, 2nd Eds., McGRAW –HILL International Editions p 224 – 248 [8] Avner S. H. “The Heat Treatment of Steel”, In “Introduction to Physical Metallurgy”, 2nd Eds., McGRAW –HILL International Editions p 270-72 [9] http://app.eng.ubu.ac.th/~edocs/f20061122Suriya101.pdf [10] Avner S. H. “The Heat Treatment of Steel”, In “Introduction to Physical Metallurgy”, 2nd Eds., McGRAW –HILL International Editions p 317 – 335 [11] Avner S. H. “Metal Structure and Crystallization”, In “Introduction to Physical Metallurgy”, 2nd Eds., McGRAW –HILL International Editions p 101-103 [12] Smallman R. E. and Bishop R. J., Chapter 7 in “Modern Physical Metallurgy and Materials Science”, 6th ed., Butterworth-Heinemann, Oxford, 1999, p. 198 Read More