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'STEEL' is used for various alloys of iron. These vary from each other depending on the content and % of material added to the iron base solution. On must keep an important factor in mind, all steels contain a % level of carbon + Manganese + Silicone + Sulpher. By increasing carbon to base solution the material will increase strength and hardness thus improves drastically the harden-ability. On the other hand carbon reduces the capability of weld-ability and increases brittleness. Several types of steels exist in our markets, the most common being; plain carbon, stainless, alloyed and tool steel.
Carbon steel is one of the mostly used steels in several industries. As explained previously, its properties are dependent mostly on the % of carbon added but the other elements have minor influence too. One important factor regarding Plain Carbon Steel, is that this is a type of steel containing up to 1.5% of carbon. One must highlight that most of the carbon additions in carbon steel are less than that of 1%. Some applications of these steels are structural beams, car frames, etc. (Capudean, 2003) (Iron Making & Steel MAking, 2008) (Jr., 2007)
According to the 'Steel Classification Society', Carbon steels are sub divided into four main categories;-
Low Carbon Steels /Mild Steels; these steels have less than 0.25% of carbon, and these are the mostly used grade of carbon steels. Normally this type being unresponsive to heat treatments, intended to form a martensitic structure, strengthening is achieved by cold working the part due to low carbon content. These are very easy to form steels with achieving popularity mostly for non high strength applications. Some properties are; Very tough, cheap regarding costs (least expensive to produce), they are easily machined (more ductile than high carbon steels) and welding properties are also very good. Typically these have 275MPa - yield strength, 450 - 550 MPa - tensile strength, and 25%EL - Ductility. For such a reason, these are commonly used in applications as bridges due its ability to deform under load and return to its original. (B) (Capudean, 2003) (Groover, 2010)
Medium Carbon Steels; contain 0.25% up to 0.55% carbon. When it comes to properties, increasing carbon also means a direct influential increase in hardness + tensile strength. On the other hand this also effects badly the ductility which makes it more difficult to machine. Heat treatment to very thin sections can improve such machinability issues. These may be austinized by quenching and tempering afterwards to improve such properties. These are relatively stronger than the previous category, whilst weaker than the high carbon steels. One common application of medium carbon steel is the production of crankshafts where high strength and stiffness is required to withstand high loads and resistance to fatigue when it comes to continuous torsion and bending. Typical values are, 450-580MPa - yield strength, 600-750 MPa - tensile strength, and 20-30%EL - Ductility. (Corus, 2010) (Jr., 2007) (Iron Making & Steel MAking, 2008)
High Carbon Steels; contain 0.6% up to 1.4% of carbon. In a heat treated condition it is able to withstand high shear and wear therefore, these are used in applications where stiffness and hardness play an important part such as in rope wire, also good in sustaining sharp cutting edges. Unfortunately, these are the least ductile and very hard to weld, in fact preheating and post heating (controlling of cooling temperature) is critical in order to obtain good welding segments and achieve good mechanical properties. Since at their maximum hardenable conditions, these are the most brittle making this a big drawback for this steel. (B) (Jr., 2007) (Groover, 2010)
Very High Carbon Steel / Tool Steels; are a 'sub group' of high carbon steels and contain minimum of 0.8% carbon. These have weak welding properties and ductility, but are very good when it comes to strength, hardness and wear resistance that's why it is very commonly used in centre punches. An important factor for these kinds of steels is that most of them are used in a hardened + tempered condition. (Jr., 2007) (Groover, 2010)
P7: Question 2
Select an industry and provide a brief over view of one metal forming technique used.
The precision metal forming process I will be discussing is the Powder Formation (PM). This gives an alternative cost effective reach towards stamping, forging, machined parts and casting. This is characterized one of the most implemented precision metal forming technique in the modern industry and it is commonly found in the automotive industry. Most vehicle parts nowadays are produced by such a process. PM combines powder compaction with optimal physical properties with controlled high temperatures atmosphere sintering. (Turkish PM)
Such a process offers ideal benefits that you can not achieve with utilizing other metal working processes. Mentioning just a few, it is very material efficient since the finished part uses 100% of the material, whilst in other forming techniques one will have wastes. The process is very good in creating intricate shapes with minimal cost addition. Some examples could be splines, gears, off centric parts, etc). Sometimes this also reduces assemblies since most of the assembly parts can be produced in a single formation by PM. The PM punches and rods produce parts with closer tolerances. A variety of standard alloys are specifically suitable for PM processing, however blending can be easily done to meet specific property requirement. Automatic cycles make the PM process very labor efficient. The only back draw is to set up the parameters for the tooling, but as this is done, the process can produce parts at a very efficient rate. High temperature Powder Metal (HTPM), is able to achieve 88-96% of the material theoretical density. Since every material properties depends on density (elongation, yield strength, etc), HTMP outperform other processes. (GT.b, 2012) (J, 2011) (Marciniak, 1988)
The high temperature metal powder process is divided into 4 main sectors;
Mixing - here high purity metals powders and additives are studied then funneled down to select the optimum choice for the application in order to meet the desired properties to the application. For example in the automotive industry, this process is used to form panels. Here aluminum powder with anti oxidization additives and silicone for elastic properties are added in order to form the ideal mixture for the door panel. All the powders are mixed with special lubricants which helps processing the powder flow. As mentioned before, mixing the powders in a solid state provides new opportunities to come up with new properties unique only to HTPM.
Compacting - Powdered metal is fed from a hopper into a feed shoe. The feed shoe delivers it into a die cavity. Here multiple pulses compress it into 50% of its original capacity. Second intensity compacting forces realign and deform the particles whilst implementing local mechanical bondings forming a part known as 'a green part'. These are ejected carefully by automated machines and conveyed to the next step. Compacting presses vary from 5 up to 900 tones and are capable of multiple motions.
Sintering - this involves placing the green part in temperatures above 1300°C (but below melting points). By such a process a lot of benefits are achieved, metallurgic bonding is enhanced, ductility and toughness are also improved. The initial art of the process cleans lubricants and surface of the metallic particles to ensure perfect mechanical properties regarding the finished component. At the second part of this process (leaving part in high temperatures), this allows energy to transform mechanical bonds to metallurgical bonds.
Finishing - sizing / coining are post operations that can be used for more complicated and minor tolerance dimensions, (example - engine parts like cylinder valves, connecting rods, etc). Additional finishing assets could be milling, grinding etc. From here it can progress to a joining process. After joined this can also be heat treated and plated. Heat treatment goes up to 550°C to make a layer of Fe3O4 which acts as corrosion resistant, and increase in hardness. (N.A.) (GT.b, 2012) (J, 2011) (Marciniak, 1988)
Note- A presentation will be held in class explaining the process in more detail.
P8: Question 3
Distinguish between fusion & non fusion, also select one process and discuss. Finally provide one application and also discuss.
Welding is a process to produce high strength permanent joints between two (or more) parts that need to be connected with each other by means of a weldment (metallurgic joining section). There are various ways to obtain such a connection and with today's technology we are achieving better property outcomes when it comes to post welding properties. Mainly the process involves heating a part, melting the part / base metal (depending on which process is adopted) and joining of the material occurs as temp start to cool down. Thus the process required relies mainly on the application of heat and pressure. One common welding factor is that the metallurgic structure of any metal will be changed post to welding. This could lead to a post welding heat treatment process. (T, 2010) (Key to metals, 2007)
Welding is divided into two main groups, depending on the physical state of the base metal during the process that of the base metal being in liquid state or in solid state. These processes are better known as fusion and non fusion welding techniques. In order to select which welding process needs to be selected one should first study both the welding processes and gain information regarding pre welds and post welds. (YÄ±lmaz, 2007)
Fusion / Liquid State Welding; this is the most common type of welding found in the industry due that the weld produced is very strong. In this kind of joining process, the base metal is melted in order for the weld to occur. Apart from the heat source a filler material (consumable electrode) may be fed also into the welding pool. This process may also use a protective layer between our surrounding atmosphere and the molten metal (gas shield or flux). This protection results in a slag on top of the welded seam which later on solidifies and can be removed. There is a variety of fusion welding processes that can be used having a different property outcome. (Marinov)
Three major categories of fusion welding are found which are sub divided into different processes;
Arc Welding: in this kind of process heating and melting of the material is done by an electric arc. Some examples of arc welding could be; Shield metal arc welding, ,Gas tungsten arc welding, Plasma arc welding, Gas metal arc welding, Flux cored arc welding, Submerged arc welding, Electroslag welding.
Gas Welding; for this kind of welding the gas produces a flame that melts the base material. An example of such process is; Oxyacetylene welding
High energy beam welding; here a high energy beam is shot between the two parts held together under pressure. As the temperature starts decreasing after the beam stops, the parts start to bond together. Some examples of this process are; Electro beam welding & Laser beam welding.
Basically all the processes mention previously for fusion has the same principals. A heat / energy source is concentrated on a very small area and a weld pool is produced. These heat sources for fusion are arc, high energy beams and gas. Obviously the power density rises from gas flame to an electric arc to a high energy beam. For the categories mentioned below we can study a graph showing the heat input to the work piece vs power density of heat source. It is pretty obvious that as the power of the heat source increases, the heat input to the welded part decreases. Therefore the less the power density of the source the larger the affected area. Excessive time for heating can cause damage to the work piece, by varying the properties including distortion and weakening. (Schwartz, 1979)
Therefore one of the advantages on power density increaser is deeper penetrations, optimal welding qualities, and less exposure time for the welded part. This process is also versatile, and adaptable for confined spaces. On the other hand some disadvantages are, it is not as productive as continuous wire applications, current limits are critical to determine the joint potential. (Liverpool, 2000)
Non Fusion / Solid State Welds; for this type of welding heat is applied only for the base metal to soften in its solid state in order to ease plastic deformation or speed up the solid state diffusion. As the name states, in non fusion welding the materials are joined together without the need of melting the base metal. Here the materials to be joined are brought together at equilibrium spacing for plastic deformation through high temperatures (below melting point) and pressures. Some of the main non fusion welding techniques are;
Cold welding: In this process the weld is achieved by the pressure and slight elevation in temperature due to cold working. Some typical examples are; Press welding & Roll Welding.
Friction welding: For this process the weld is achieved due to the heat produced due to applying pressure between the parts i.e creating friction. Some examples are; Radial Friction welding & Orbital Friction Welding.
Diffusion Welding: here both parts are held together under constant pressure, elevating the temperature slightly and left constant for a particular time frame (depending on the material). Some examples are; Conventional Diffusion welding & Deformation Diffusion welding
Solid State Deposition Welding: Chemical reaction bounding.
The first advantage of such a process is that low heat is required, therefore minimal disruptors in crystalline structure are noticed since no melting temperatures are required. This could also be applied for materials within the same class or even within different classes due that no intermixing is involved. The main disadvantage is that for such a process large, fixed machineries are required and this is a result for large capital funds to buy the equipment required. Another disadvantage is that this process is not very viable to be applied for small scale applications since its complicity. (Jr, 2004) (Lancaster, 1999)
Describing one particular welding method, the method I will be discussing is the 'Hyperbaric Welding Process' commonly known as under water welding. This process falls under the category of Fusion welding. This is a very good example of welding in an unusual, critical environment. For such a process the welder must be an A grade welder plus a commercial diver in order to withstand the harsh and dangerous sea conditions nature has to offer. (Mukund, 1974)
In order to assign immediately the process with an application mentioning advantages, disadvantages, equipment, limitations and precautions, the underwater welding is a process used in the offshore industry. The offshore industry covers the repairs of vessels (any sort), drilling platforms (jack up rigs, semi submersible rigs, self propelled rigs), bridges, etc. These structures face severe weather conditions and are constantly flexing in multi conditional environments, therefore the risk of damage and failure is far greater. Basically this kind of welding is ideal and it is very flexible since the work could be done under the waterline, therefore the floating plant could be repaired in position without the need of docking the platform to obtain dry conditions. This results in less expanses and loss of valuable drilling time. Limitations of such a process for our drilling platform application are that the system is inevitably bulky and expensive to run.
For such an application usually low to intermediate carbon steels are used. Steels are designed as CT and a digit after to show the carbon content. The most common steel found in this industry is CT3. Low and intermediate carbon steels are ideal since these have high plasticity and reasonable weld ability. The weld ability of depends on their carbon + manganese content in conjunction with their impurity levels. At low carbon levels that of less than 0.15%, the steels are not hard and the weld ability is excellent. For the medium carbon type, 0.30 - 0.60% carbon is found and these can be welded easily by means of arc welding. Unlike the surface normal arc welding, where such intermediate carbon have higher hardness and martinsite formation, in dry conditions pre heating and post heating are required. For deep sea welds instead of pre heating and post heating, the weld is not performed at one go but with intermediate pauses. (Martikainen) (Sacrificial Metal, 2012)
Basically there are two classifications to perform underwater welding. The selection of the proper method is selected after an underwater structural / hull survey and the parameters of the underwater environment. The welding classifications are 'Dry Welding' & 'Wet welding'.
Wet Welding; here the welding is performed to the direct exposure of the wet conditions. Here the 'Manual Metal Arc' process is used. The principals of such a process are basically the same as that of Arc Welding on dry land but in this case a special water proof electrode is required in order to protect the electrode's core. This is the most efficient and cost effective welding method found amongst all under water welding. For this method the power supply is located on the surface of the water (dry conditions) and the cables and hoses are fed to the diver via cable protectors (large flexible piping).
The power supply feeds direct current (DC) 300 - 400 amps through cables with negative polarity. If positive polarity is used in DC electrolysis will occour and causes rapid deterioration of the metallic components in the electrode hand held holder. DC currents are used instead of AC (Alternating) due to electrical safety and the difficulty of maintaining the arc in wet conditions. Motor generator welding machines are commonly used. For the wet type welding it is important that the circuit includes a knife switch (Positive type). This kind of switch is operated on the surface but finally commanded by the diver for optimum safety. This switch should be capable of breaking the full welding current and stops the weld ax the switch is non-operated. As mentioned previously, even the electrode itself should have a special coating of waterproofing. Basically all connections should be insulated in order that no water comes in contact with any metal parts. If this insulation is damaged, current will leak and will not be available in the arc. (Levi, 2010)
This method is low cost compared to dry welding, it is also an onsite job i.e. no need of complex equipment an the diver can reach positions of the offshore structure that could not be reached by other types of welding. Some disadvantages are that there is an immediate quench of the welded joint due to surrounding waters (increases tensile strength - decreases ductility & increasure in porosity and hardness).large amounts of hydrogen is present leading to dislocations. H2 dissolves in heat affective zone and the welded joint could crack. The main disadvantage is the safety issue regarding visibility and currents
Hyperbaric Dry Welding; here the weld is done in a sealed chamber. This chamber is usually filled up with helium containing 0.5 bar of oxygen at positive pressure slightly above ambient. These welds are slightly higher in quality that of wet welding and meet X-ray coding requirements. For such a process the 'Manual Metal Arc' is used or the 'Gas Tungsten Arc' welding is used (depending on the welder skills). The welding is done in dry conditions but at hydrostatic pressures of the depth of the surrounding sea. For the dry weld process the welding power generator could be AC and DC.
Some advantages of dry welding are that the diver is safer since the weld is performed in an enclosed chamber, therefore no direct distractions from currents, and visibility is much better than wet welding since the chamber is illuminated. Good quality welds are achieved comparable to surface welding and H2 levels are much lower than water welds, non distructave testing could be performed in the chamber. Some disadvantages are that the process requires much more capital and support at the surface as the method is much more complex, the cost increases with depth, and the process is not as flexible as the open water weld as the joining sections should fit the enclosed chamber. (Akers, 2012)
To describe the process of Manual Arc Welding, the work that is going to be welded should be directly connected to the positive side of the electric circuit via cables. Sometimes the earth cable incorporates more than 1 connection in order to obtain the best circuit continuity for the ground. A flux (plus water proofing) electrode is connected to the hand held holder via another cable which are connected to a power welding generation source. When the tip of the electrode touches the work piece, the current arcs from the gap and causes a spark which melts the base metal and the electrode forming a weld puddle.
Comparing both diagrams we can notice some differences. For the wet process, DC current, a knife switch and double insulation cables are required. The polarity is -ve, whilst the circuit is basically the same.
As the arc is moved away from the work piece, the pool solidifies and the joints are fused together. This welding pool is controlled by varying the current which flows through the arc being produced and by changing the electrode diameters. Typical temperatures are those of 5000°C. by such a process a result of metallurgical bonding is achieved through the work piece. At these elevated temperatures, metals are 'active' and if it comes in contact with air it rusts instantly and the mechanical properties of the joint fail. For such a reason the arc welding provides means of shielding to the weld pool (gas, slag). This is known as metallic arc shield and is accomplished by the flux covering the electrode core. The slag produced as the pool cools and solidifies has its function to minimize contact of weld to air until temp decreases.
The arc creates a small cavity formed inside the flux. This is for the flux to burn slower than the metal barrel i.e. aiding in the protection and control of the metal deposit that leave the electrode. For the welding process this is a critical in order to maintain a constant arc and weld joint. Even in poor under water visibility, the diver needs to keep pressure on the electrode to maintain a constant good feed rate. In under water welding in is very important to mention that the arc behaves differently than it does in air. In water gas bubbles are a very important factor as these create an unstable arc for good welding conditions. In conjunction with this, it is much more difficult to control the weld puddle. The diver must set currents by trial and error depending on the waters conditions (temp, salinity, etc). Apart from this there is no difference between surface and under water welding. (Kaets, 2000)
When compared to other welding processes Manual arc welding is commonly used because the equipment is relatively easy to use and inexpensive (compared to other types), it is flexible, portable, and ideal for the diver to use in confined spaces, no gas shield is required and finally it is suitable for almost all kind of metals and alloys. Some disadvantages of arc welding are that the electrode needs to be replaced frequently (more exposed to shock), the slag should be removed after the weld is performed, and the process is slightly slower since the rate of deposition is slower than continuously fed electrode process (Gas Tungsten Arc). (Kaets, 2000)
One last note regarding under water welding; there is a process not commonly used which features a specially built holding torch which sprays a constant cone made of high pressure water, with protective gas under pressure to isolate the welded are from the water during the weld is performed. Also a recent development was done involving under water laser beam welding. This method is still being improved but will soon be applicable for the offshore industry. (Clutter, 2012)