Introduction to Welding - Essay Example

Introduction to Welding Conventional wisdom defines welding to be a process of joining two or more pieces of fusible material, most often metals. Itfalls in the category of Fabrication processes. The basic welding process is carried out in 3 steps. First, heat is applied by some method onto the metal pieces, such that majority of the heat is concentrated on the areas to be bonded. This causes the metal to melt and on subsequent application of pressure, the liquid regions undergo a process called coalescence. When cooled, this coalesced liquid metal mixture undergoes solidification and the weld is complete, thus giving us one continuous piece of metal. The heat energy required for the process may be obtained from a number of sources. Some options that can be used are gas flames, electric arcs and ultrasound. Most often carried out in an industrial environment in open air, there may arise certain situations where the welding is required to be carried out marine or even in space. The sources of energy will have to be selected accordingly, since certain sources may not work well in certain environments. For instance, an open oxyacetylene flame in a vacuum or even underwater, is obviously impossible. The quality of a weld, its strength and durability are largely dependant on the base metals used in the welding process. Some of the major base metals which can be joined by using the process of welding are Steels The suitability of alloys such as steel to welding depend on the contents, which may be a diverse collection. Steel, or more accurately, plain carbon steel is chosen as a reference material for this. To judge alloys made up of many distinct materials, we make use of a factor called the equivalent carbon content. This is used to compare the relative weldabilities of different alloys by comparing their properties to plain carbon steel. Considerable effects are seen on the weldability of a metal alloy which contains elements like carbon, chromium and vanadium, while copper and nickel have only negligible effects. As the equivalent carbon content rises, the weldability of the alloy decreases (Lincoln Electric, 1994). But this can't be helped, because plain carbon and low-alloy steels have unacceptably low strength levels, especially from an industrial perspective. High strength, low-alloy steels which contain a very small percentage of carbon and include additive elements like manganese, phosphorus, sulphur and small amounts of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, silicon, or zirconium(Schoolscience.co.uk, 2007) were developed especially for welding applications during the 1970s. The high chromium content of stainless steel makes it less preferable for welding. Those varieties which may have been deemed weldable are susceptible to distortion due to their high coefficient of thermal expansion, and hence are prone to cracking and reduced corrosion resistance. Aluminum The chemical composition of aluminum alloys, as with any alloy, decides the weldability. Hot cracking of the alloy on welding is prevented by preheating. This reduces the temperature gradient across the welding area. However, this can reduce the mechanical properties of the base material. Another alternative is to alter the design of the joint, with a more compatible filler alloy to decrease hot cracking. Aluminum alloys should also be cleaned prior to welding, with the goal of removing all oxides, oils, and loose particles from the surface to be welded. This is especially important because of an aluminum weld's susceptibility to porosity due to hydrogen and dross due to oxygen(Lincoln Electric, 1994) Residual stresses Stresses caused in a rigid structure as a result of internal strains are referred to by the term Residual Stresses. These strains are usually of a permanent nature and may have its origins at any stage in the component life cycle. Welding is one of the most significant causes of residual stresses and may cause large tensile stresses whose maximum value is approximately equal to the yield strength of the materials being joined, balanced by lower compressive residual stresses elsewhere in the component(TWI, 2006). Residual stresses have a major effect on fracture in the brittle and transitional regimes, and hence the stress intensity, or energy release rate, due to residual stresses must be calculated and included in the fracture assessment. X-ray diffraction, neutron diffraction and magnetic and ultrasonic methods are utilized to measure the residual stress in a structure in a non-destructive manner. On the other hand, hole drilling, ring core method, deep hole methods and sectioning methods including block removal, splitting, slicing, layering and the contour method are all destructive methods for the same purpose. The optimum measurement technique is chosen by taking into account a number of factors such as volumetric resolution, material, geometry and access(TWI, 2006). Rather than being directly responsible for damage to metallic structures, tensile residual stresses may add to the natural course of damage by fatigue, creep or environmental degradation and thus reduce the performance or cause failure. They contribute to failure by brittle fracture, or cause other forms of damage such as shape change or crazing. Residual stresses in welded structures may be minimized by appropriate selection of materials, welding process and parameters, structural geometry and fabrication sequence. Residual Stress Management Residual stresses, as detailed above, may be disastrous, in combination with other factors to structures involving welded metals. The minimization of this residual stress is critical fror prolonging the life of such structures. Cold working techniques, such as shot peening, laser shock peening, ultrasonic peening, planishing, hammering, burnishing, low plasticity burnishing, rolling, coining and split sleeve expanding, can generate compressive residual stresses which counter act on the innate residual stress forces. Also, hot working techniques, such as heat treatment, controlled cooling and localized heating may also be employed to minimize or reduce the magnitude of residual stress in components(Proto, 2006) Residual stresses may be reduced by various special welding techniques including low stress non-distortion welding (LSND), last pass heat sink welding (LPHSW) or inter-run peening(TWI, 2006) Overview of Welding Processes Contrary to what the term suggests, welding in all its forms essentially follows basically the same procedure of heating, application of pressure and subsequent cooling. But welding methods are classified on the basis of the modality of the energy supply for the generation of adequate heat to facilitate the weld. In this regard, the basic classifications of welding processes are: Shielded Metal Arc Welding This is one of the most common types of arc welding. It is also referred to by terms such as manual metal arc welding or stick welding. It is based on the phenomenon where electric current can be used to strike an arc between two metallic electrodes. The base material acts as one of the electrodes and the welding electrode acts as a consumable electrode rod. This is made of steel and covered with a flux that protects the weld area. A large amount of CO2 gas is produced in the process, and this protects the metals from oxidation and other contamination. The, making A separate filler is not required, as the electrode core itself acts as filler material. The process is generally limited to welding ferrous materials and may consume considerable time, since the electrodes must be frequently replaced and slag must be chipped away after welding. Figure 1 Gas Metal Arc Welding Also known as metal inert gas welding, this process offers very high speeds, on account of the fact that it uses a continuous wire feed as an electrode and an inert or semi-inert gas mixture to protect the weld from contamination. The arc size is smaller compared to the shielded metal arc welding process. However, the equipment used is more complex and expensive than that required for shielded metal arc welding, and requires an extensive installation and setup procedure. This makes it less portable, and due to the use of a separate shielding gas, it is not particularly suitable for outdoor work. However, owing to the higher average rate at which welds can be completed, GMAW is well suited to production welding. The process can be applied to a wide variety of metals, both ferrous and non-ferrous (Lincoln Electric, 1994) Figure 2 Flux-cored arc welding This method also uses similar equipment as in shielded metal arc welding, the difference being that a powdery material is enclosed in the steel electrode. This cored wire is more expensive than the standard solid wire and can generate fumes and/or slag, but it permits even higher welding speed and greater metal penetration (Weman, 2003). Figure 3 Gas tungsten arc welding This is a manual welding process that uses a durable tungsten electrode, an inert gas mixture and a separate filler material. A stable arc and high quality welds are salient features of this technique, which is used extensively in welding of metals which are thin in cross section; most often applied to stainless steel and light metals. On a parallel, plasma arc welding, also uses a tungsten electrode but uses plasma gas to make the arc. The plasma welding arc is more concentrated than the gas tungsten arc welding arc, and because of its stable current, the method can be used on a wider range of material thicknesses and is much faster. Gas welding Oxyacetylene welding, which makes use of a mixture of oxygen and acetylene as fuel for a high temperature flame is one of the oldest, most popular and most versatile welding processes. It is widely used for welding pipes and tubes, as well as repair work. The equipment is relatively inexpensive and simple,and is capable of producing a temperature of about 3100C. Concentration of the flame is very inefficient in this process. Other gas welding methods, such as oxygen hydrogen welding, and pressure gas welding are quite similar, generally differing only in the type of gases used, and hence the temperature of the flame obtained. Figure 4 Resistance welding Resistance welding in its simplest form makes use of Ohmic resistance heating. This involves the generation of heat by passing current through the resistance caused by the contact between the two metal surfaces to be joined. A high current of the order of 105 A is passed through the metal. Efficiency is higher but equipment cost can be high. Figure 5 Energy beam welding Energy beam welding is a collective term for laser beam welding and electron beam welding. As the names suggest, laser beam welding uses a highly focused laser beam, while electron beam welding uses an electron beam. Both have a very high energy density. This facilitates a deep reaching weld, while keeping the weld area fairly small and giving very high speeds. The primary disadvantages are their very high equipment costs (though these are decreasing) and a susceptibility to thermal cracking. Developments in this area include laser-hybrid welding, which uses principles from both laser beam welding and arc welding for even better weld properties (Weman, 2003) Precautions for safe welding practice 1. The main steps to be taken for achieving a satisfactory welding performance, in addition to safety measures, may be enumerated as below: 2. Wooden floors and such flammable furnishing must be avoided 3. Flammable material, such as cotton, oil, gasoline, etc., must be removed from the vicinity of welding. 4. Proper clothing or goggles. 5. Remove any assembled parts from the component being welded that may become warped or otherwise damaged by the welding process. 6. Hot rejected electrode stubs, steel scrap, or tools on the floor or around the welding equipment must be promptly removed. 7. Mark all hot metal after welding operations are completed. Soapstone is commonly used for this purpose. (TC,2005) Marine Welding and Shipbuilding Marine welding, on a broad perspective, refers to the application of welding techniques on structures meant for operation in a marine environment. This includes steel bridges, ships, buoys, ports, etc. The actual welding procedure may be carried out either on land or in the water itself, but the finished product shall be put to work on a water based environment, and the welding has to be optimized keeping this in mind. The primary application of marine welding is in ship building. Modern ships, given the renewed requirements in strength and durability, are largely composed of welded steel. Brittle fracture was a common disability in older ships until specialized steels such as ABS Steels with good properties were developed. Most modern shipyards tend to pre-install equipment, pipes, electrical cables, and other components within units called blocks, which are welded together in place for assembly. Marine welding is also known as Hyperbaric welding, owing to the elevated pressures at which this kind of welding is often carried out. Hyperbaric welding can take place in the 'wet' of the surrounding water or inside a specially constructed pressure chamber and hence a 'dry' environment. Therefore, marine welding techniques are typically classified as wet marine welding and dry marine welding. However, the term "Hyperbaric Welding" is predominantly used in dry marine welding. Wet marine welding, chiefly comprises a variation of shielded metal arc welding. It makes use of a waterproof electrode. Alternatively, flux-cored arc welding and friction welding are also employed. The welding equipment gets its power through heavy cables and hoses which are laid in the water at required depths. Hydrogen-caused cracking is prevalent in this kind of marine welding. In dry marine welding, a pressure chamber is set up, filled with a gas mixture sealed around the joint being bonded. The method of choice here is gas tungsten arc welding, and the results are considerably superior to those obtained from wet marine welding. Another similar form of welding is called Coffer dam welding, which is carried out in the dry, in air, where a rigid steel structure to house the welders is sealed against the side of the structure to be welded, and is open to the atmosphere(TWI, 2006). Marine welding is often used in the repair of ships, offshore oil drilling platforms, and in underwater pipeline maintenance. Preferred material is steel. The primary disadvantage of marine welding is that defects are difficult to detect especially if the defects are beneath the surface of the weld, because a proper inspection is next to impossible in comparison to a regular open air welding procedure. Risks involved in Marine Welding The risks involved in marine welding can be broadly divided into two - One, the risk to the personnel involved, and two, the risk to the structure. The primary risk to the personnel is that of electrocution. The highly saline sea water acts as a near perfect electrolyte. Further, hydrogen and oxygen produced by the arc in marine welding and cutting may cause the build-up of gas pockets which are explosive. Exposure of the welder to extreme pressures may cause an influx of nitrogen into the blood stream. Precautions include the provision of an emergency air or gas supply, stand-by divers, and decompression chambers to avoid decompression sickness following saturation diving or too rapid return to the surface from a deep dive (TWI, 2006). References 1. Lincoln Electric, 1994. The Procedure Handbook of Arc Welding. Cleveland (Lincoln Electric, 1994) 2. Schoolscience.co.uk, 2007, "High strength low alloy steels", Retrieved on 2007-08-14 (Schoolscience.co.uk, 2007) 3. The Welding Institute, 2006, TWI World Centre for Materials Joining Technology, TMI Knowledge Summary, Copyright 2006 TWI Ltd (TWI, 2006) 4. Weman, Klas, 2003, "Welding processes handbook". New York: CRC Press LLC (Weman, 2003) 5. Proto Manufacturing, 2006, "X-ray Diffraction Residual Stress Measurement" Copyright2006 - Rev. A061101 (Proto, 2006) 6. TC 9-237,2005, SAFETY PRECAUTIONS IN WELDING OPERATIONS (TC,2005) 7. Figures Sources 1. www.haydencorp.com/images/weld1.gif Shielded Metal Arc Welding 2. www.haydencorp.com/images/weld3.gif Gas Metal Arc Welding 3. http://www.lincolnelectric.com/knowledge/articles/content/fcwawire.asp Flux-cored arc welding 4. www.handyharmancanada.com/.../Part%202.htm Read More