Physical Metallurgy: Drill Bit - Research Paper Example

Physical Metallurgy: Drill Bit 1. State and explain the main properties required in each case by a material used in the manufacture of: a. Drill bit A drill bit is subject to high stress during its effective service life. It is supposed to remove material and transport it from the substance being drilled. Other than the stresses, the drill bit is also exposed to corrosive environmental settings where it may corrode rapidly if an unsuitable material is used. Given the requirements for a drill bit, it can be seen that a drill bit needs to have good hardness. This ensures that the drill bit is able to drill through intended materials without deforming itself. The material of the drill bit therefore must have the properties of hardenability as well as tempering. Moreover, the material of the drill bit should possess enough strength that it does not buckle or deform under the stress offered in well drilling. Moreover, given the corrosive work environment, it is essential that the drill bit material is not subject to corrosion easily. As the drill bit works, there are great chances for wear and tear as the drill bit handles all kinds of grit and debris. Thus, the drill bit must also be well prepared against wear and tear. Typical materials include carbide tips as well as diamond base tips for drills. (Welll Drilling School, 2011) (Drill bits used in oil and gas drilling, 2001) b. Casing pipe Once drilling has been done, a series of casing pipes are introduced into the bore hole. This ensures that the bore hole does not collapse and that drilling mud can be pumped to greater depths for more drilling to occur. These pipes need to be flexible (over long lengths). Moreover, the material used to build them should not be brittle as it would cause fractures and other defects to appear during service. The material for casing pipes needs to have great formability. Moreover, it needs to be ductile and malleable too so that it can be easily formed into the shape of pipes. Casing pipes need to have great strength too as the pressures of the transmitted fluid are often high. Generally iron is employed to create these pipes. Carbon content is kept sizable enough to avoid brittleness. (Pipe Industries, 2011) (Typical installation of casing pipes during drilling, 2002) c. Drill string A drill string is a column or string of drill pipes and associated assemblies that move drilling fluid as well as torque to the drill bit. The working pressures for drill strings are high and so a material with good strength is required to ensure that the drill string does not burst when exposed to high pressures. Drill string material needs to have good formability including ductility and malleability so that drill string can be created with ease. Moreover, the drill string material needs to be flexible enough over long lengths so that the drill string does not fracture or break up when it has to bend over long lengths. (Ocean Drilling, 2011) (Creating a drill string connection, 2011) d. Travelling block hook A travelling block hook is utilised to move loads from one place to another in an oil rig. The travelling block hook must be capable of supporting large loads without moving beyond its elastic limit. Localised plastic deformations are acceptable. This implies that the material used to construct a travelling block hook must possess a high tensile strength. Moreover, the material used must offer great formability so that it can be easily formed into various shapes and sizes. Typically low alloy steels are used to create travelling block hooks. (Jereh, 2011) (Typical travelling block hook, 2011) 2. Explain in detail the mechanism of crystal formation and growth as a metal cools from liquid state to solid state. Typically metals change their allotropic state as they are cooling down. The physical state changes from liquid to solid too. Several allotropic forms may exist and transitions occur between various states as temperatures decrease. A typical case of metal cooling down and changing allotropic states is offered by iron. As iron is cooling down, it changes various crystal structures. Iron has four different allotropic forms better known as α, γ, δ, and ε iron. As iron begins to cool down from its molten state, the crystallisation process begins at 1538o C. This causes iron to turn into its δ allotrope. This particular allotrope has a BCC (body centred cube) crystal structure. When iron cools more down to some 1394o C, then the crystal structure of iron changes to a FCC (face centred cube) crystal structure. This allotrope of iron is better known as γ iron or austentite. As the iron transitions further to a temperature of 912o C the crystal structure reverts back to BCC and this allotrope is better known as α iron or ferrite. As the iron cools down further to 770o C (better known as the Curie point. Tc), the iron begins to display magnetic behaviour. As the iron moves through the Curie point, there is no change in the crystalline structure of the iron but there is a certain domain structure change. However, all metals may not display such changes in domain structure. (Bramfitt & Benscoter, 2002) (Iron phase diagram showing crystal structures, 2010) 3. Draw Fe-Fe3C phase diagram and discuss why is the Fe-Fe3C phase diagram a metastable phase diagram instead of a true equilibrium phase diagram. (Fe-C Phase Diagram, 1988) A true equilibrium phase diagram represents a situation where the components have all come to equilibrium. Any changes in time or any small interactions are unlikely to cause any new changes to occur. Such situations are easily represented by a true equilibrium phase diagram. On the other hand, the situation where components have come to an equilibrium but are susceptible to switch to a new state of equilibrium if small changes in interaction occur, the need for a metastable phase diagram occurs. The Fe-C system is also a metastable system as it has multiple states of equilibrium with multiple minimas and maximas. This means that although the Fe-C situation can come to equilibrium but there are chances for it to switch to a new state of equilibrium as soon as small changes are introduced. In the Fe-C metastable phase diagram above it can be seen that multiple phases exist for the Fe-C system. Moreover, the transition between the α, γ, δ and ε allotropes of iron is transitive not distinctive. Similarly, small variations in carbon content especially along the phase boundaries of the Fe-C system give rise to different kinds of iron and steel varieties. In order to enlist the entire Fe-C system, a metastable phase diagram is utilised. (Pollack, 1988) 4. Explain how plain carbon steel differs from wrought iron and cast iron in respect of their compositions and properties. Plain carbon steels are steels where the major interstitial alloying constituent is carbon but they may include other alloying elements too. Carbon steels are often classified into four different categories which are low carbon steel, mild steel, high carbon steel and ultra high carbon steel. Low carbon steels contain from 0.05% to 0.15% of carbon content. On the other hand, mild steel contains between 0.16% and 0.29% of carbon content. This makes mild steel neither brittle nor ductile. Overall mild steel has low tensile strength but it is cheap as well as being malleable. Treatment such as carburizing can be used to enhance surface hardness. Mild steel is used for structural steel as well as applications where a high tensile strength is not required. High carbon steels contain between 0.3% to 1.7% carbon content and can undergo heat treatment successfully. They are used for forging, automotive parts, large industrial parts, springs, high strength wires etc. Ultra high carbon steel contains carbon content of up to 2%. These steels can be tempered to produce great hardness. These steels are used for specialised uses such as knives, axles, punches etc. (Key to Metals, 2010) Cast iron contains carbon content of between 2.1% and 4% while it also has silicon as a component. Cast iron is brittle for most applications except in the case of malleable cast irons. Moreover, it displays a low melting point as well as good fluidity, castability and machinability. Oxidation does not affect cast iron much either. Therefore, it has found extensive use in applications such as pipes, machine components, automotive components, casings for large machines etc. (Laird et al., 2000) Wrought iron contains very low carbon content and is tough, malleable, easily welded and malleable. Before the production of mild steel on a commercial scale, wrought iron was used to manufacture objects that are made out of mild steel today. Most wrought iron produced today is used for making furniture and other such accessories. (McArthur & Spalding, 2004) (Cast iron automotive part, 2011) 5. Suggest a suitable heat treatment process for a hook that was made of steel rod having microstructure containing a mixture of ferrite, bainite and martensite after quenching. After an object is quenched, the formation of martensite causes the object to become brittle. Internal stress levels in the object are high too as a result of quenching. The formation of BCC (body centred cube) and BCT (body centred tetragonal) structures makes the object very hard and brittle but such brittleness is often impracticable. Tempering is the optimal process to apply to quenched objects with the presence of martensite as well as bainite. This ensures that the object in question relives all its internal stress that would otherwise cause it to develop cracks and subsequently fragment. Tempering allows for the conversion of martensite and bainite into ferrite and cementite. This not only helps to relieve internal stress but also helps to decrease brittleness to acceptable levels. Ductility also increases but the total amount can be well controlled. Tempering allows the shift from the distorted BCT structure to the BCC structure that ferrite inhabits. (McArthur & Spalding, 2004) (Comparison between martensitic structure (left) and ferritic structure (right), 2011) 6. What is hardenability? How does it differ from hardness? Describe them. Hardenability is the measure of a material’s capability to become hardened after appropriate heat treatment. On the other hand, hardness refers to the resistance of a material to indentation or scratching. Both hardness and hardenability are important properties as weldability depends on them inversely. Hardenability is a function of the carbon content, presence of other alloying elements and of the grain size of the austentite present. Another important factor for hardenability is the geometry of the involved component. A larger surface area promotes greater cooling and this greater hardenability. (Kalpakjian, 2001) Hardenability directly represents the depth to which a material can be hardened using heat treatment. The hardenability of a material is tested by using the Jominy test where a round metal bar is transformed to an austentite structure through heat treatment. One end is then quenched using water at room temperature. Hardness along the bar is then measured. The greater that the hardness is found along the bar, the greater is the material’s hardenability. On the other hand, hardness is a measure of the resistance offered by a material when it is subjected to a force or stress. Hardness of a material depends on a large number of factors such as the ductility, elasticity, plasticity, strain strength, toughness, visco elasticity and viscosity of the material. Hardness is measured by indenting a material with standard apparatus and checking for the depth of the indentation formed. Multiple scales are available for hardness measurement such as Brinell, Rockwell etc. (Typical metal hardness testing equipment, 2011) 7. Explain the need of destructive testing in testing Well Engineering components. Destructive testing is employed to determine microstructure and mechanical properties of materials to be used for construction of well engineering components. Some of the widely employed 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 (Avner, 1997) 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 caused 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 the figure below. 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. A typical stress – strain curve 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 an square impression on the surface; size of which is indicative of hardness of the material. Length of diagonals is measured and averages and from a table a 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. (Basic principle behind hardness tests, 2001) 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. 8. Why is non destructive testing (NDT) an essential element of a programme of quality control? Explain. Give examples of where NDT would be used. 5. Non destructive testing (NDT) is widely used to inspect various engineered and manufactured components. Explanation for parts is as below. 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 property 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. (Hellier, 2003) (Underwater NDT being carried out, 2011) Myriad methods cold be used for NDT in this situation including methods such as MPT (magnetic particle testing), ECT (Eddy current testing), ultrasonic testing etc. However, I would prefer to use ultrasonic testing above other methods because it can detect such defects satisfactorily. Moreover, UT can detect parallel and perpendicular defects. Using UT apparatus is easiest compared to the other two methods because UT apparatus is far more compact and easier to use. Another advantage of UT apparatus is that little surface preparation is required. Moreover, UT is preferably used for objects such as pipes, shafts etc. 9. Suggest a suitable NDT method for identifying invisible defects in mechanical instruments. Invisible defects exist in mechanical devices below the surface. Surface testing NDT methods are not effective enough in dealing with these defects. The testing could be carried out with either UT, radiography, magnetic detection etc. for mechanical components of different nature. However, mechanical instruments used have smooth surfaces. The smooth surfaces are highly conducive to conducting UT (ultra sonic testing) because the produced signal is highly accurate. Therefore, UT is an optimal method to test for invisible defects in mechanical instruments. (UT inspection of a vessel in progress, 2011) Bibliography Avner, S. H., 1997. Introduction to Physical Metallurgy. New York: McGraw Hill International. Bhadeshia, H. 2011. Comparison between martensitic structure (left) and ferritic structure (right). [image online] Available at: [Accessed 20 July 2011] Beardmore, Roy 2010. Iron phase diagram showing crystal structures. [image online] Available at: [Accessed 20 July 2011] Bramfitt, B.L. & Benscoter, A.O., 2002. Metallographer's guide: practice and procedures for irons and steels. ASM International. China A Ogpe 2011. Typical travelling block hook. [image online] Available at: [Accessed 20 July 2011] FMA 2002. Typical installation of casing pipes during drilling. [image online] Available at: [Accessed 20 July 2011] Frank, Stefan, 2001. Basic principle behind hardness tests. [image online] Available at: [Accessed 18 July 2011] Hellier, Charles, 2003. Handbook of Nondestructive Evaluation. New York: McGraw-Hill. Jereh, 2011. Travelling Block and Hook. [Online] Available at: HYPERLINK "http://www.jereh-oilfield.com/uploadfiles/Travelling-Block-and-Hook.pdf" http://www.jereh-oilfield.com/uploadfiles/Travelling-Block-and-Hook.pdf [Accessed 20 July 2011]. Kalpakjian, S., 2001. Manufacturing Engineering and Technology. Pearson Education. Key to Metals, 2010. Classification of Carbon and Low-Alloy Steels. [Online] Available at: HYPERLINK "http://www.keytometals.com/Articles/Art62.htm" http://www.keytometals.com/Articles/Art62.htm [Accessed 20 July 2011]. Laird, G., Gundlach, R. & Röhrig, K., 2000. Abrasion-Resistant Cast Iron Handbook. ASM Internationalz. McArthur, H. & Spalding, D., 2004. Engineering materials science: properties, uses, degradation and remediation. Horwood Publishing. Metallurgy for Dummies 2010. Cast iron automotive part. [image online] Available at: [Accessed 20 July 2011] NB United, 2011. UT inspection of a vessel in progress. [image online] Available at: < http://www.nbunited.com/Quality.htm> [Accessed 19 July 2011] NDT Resource Centre, 2011. A typical stress-strain curve. [image online] Available at: [Accessed 18 July 2011] Ocean Drilling 2011. Creating a drill string connection. [image online] Available at: [Accessed 20 July 2011] Ocean Drilling, 2011. Drill String. [Online] Available at: HYPERLINK "http://www-odp.tamu.edu/publications/tnotes/tn31/pdf/drill_s.pdf" http://www-odp.tamu.edu/publications/tnotes/tn31/pdf/drill_s.pdf [Accessed 20 July 2011]. OSHA, 2001. Drill bits used in oil and gas drilling. [image online] Available at: [Accessed 20 July 2011] Pipe Industries, 2011. Rolled and Welded Carbon Steel Pipe. [Online] Available at: HYPERLINK "http://www.pipeindustries.com/products/rolled-and-welded-carbon-steel-pipe.html" http://www.pipeindustries.com/products/rolled-and-welded-carbon-steel-pipe.html [Accessed 20 July 2011]. Pollack, 1988. Material Sciences and Metallurgy. 4th ed. New York: Prentice Hall. Seaward Marine Services, 2011. Underwater NDT being carried out. [image online] Available at: [Accessed 19 July 2011] Welll Drilling School, 2011. Drilling Methods. [Online] Available at: HYPERLINK "http://welldrillingschool.com/courses/pdf/DrillingMethods.pdf" http://welldrillingschool.com/courses/pdf/DrillingMethods.pdf [Accessed 20 July 2011]. Yamayo 2011. Typical metal hardness testing equipment. [image online] Available at: [Accessed 20 July 2011] Read More