Introduction to Welding
No industrial structure is complete without welding. Welding allows engineers to fuse different materials together coherently, or give unique shapes to structures that are not possible otherwise. From aircrafts to ships and from bridges to pressure vessels, all the industries need some kind of welding. The most accepted technical definition of welding is that it is a localized coalescence of metals or non-metals produced either by heating the materials to the required welding temperatures, with or without the application of pressure, or by the application of pressure alone, and with or without the use of filler materials. Less formally, welding is the process of joining of two metals or non-metals by using high temperature (and in some cases, pressure) so that the two materials coalesce. The term 'coalescence' means the fusion or growing together of the grain structure of the materials being welded. The definition includes the terms metals or non-metals because materials such as plastics ceramics, and so forth, are not metals and they can be welded with the current welding technology.
The Welding Process
Welding processes differ greatly in the manner in which heat, pressure, or both heat and pressure are applied and in the type of equipment used. Welding processes can be classified on the basis of the source of heat and the type of interaction (liquid / liquid or solid / solid). The end product, after the fusion of the two metals / non-metals is called as 'weldmet'. Below are the principal types of welding processes; do note that in all more than 30 sub-typesexist.
Arc Welding: One of the most common welding processes is electrode arc welding. The main types, or systems, of arc welding in this family include Carbon arc welding, Metal Insert Gas (MIG) welding, Shielded Metal Arc Welding (SMAW), Tungsten Inert Gas (TIG) Welding and a few others like Flux Cored Arc Welding, Submerged Arc Welding, etc. All these arc welding processesuse the same three components. The first and most obvious is electricity, which creates the arc. The second one is some kind of filler material, and the third component is flux that welds the joints together.
Gas Welding: Gas welding is a system of welding that uses one of various gases and oxygen to ignite a torch. The most popular choice of gas for gas welding is acetylene, followed by oxyacetylene and oxyhydrogen. The different gases used for the welding process lend the weldmet a different finish, since all these gases have a different flash point. Choosing one for gas welding depends on the type of project, cost, and flame control. Gas welding is used to weld ferrous and nonferrous metals together, and where cost is an important consideration.
Resistance Welding: Resistance welding is one of the oldest welding processes. It is based on the principle that when current passes through an electric resistance, it produces heat, and that the amount of heat produced depends on factors like resistance of the material, current strength and its duration, and the surface conditions.Resistance welding reduces the chance of weldmet deformation and results in a better quality weld. Robotic resistance welding, an Industry 4.0 innovation, increases throughput and accuracy
Solid State Welding: In this process, two work pieces are joined together using pressure. Heat may sometimes be used to accelerate the diffusion process at mating surfaces; but pressure is the main welding agent. It is also therefore called as 'pressure welding'. Coalescence results due to inter molecular diffusion process in which the interface molecules of work pieces flow from high concentration region to low concentration region due to applied pressure. Pressure welding does not affect the mechanical or physical properties of parent material. It is therefore used for industrial welding for heat sensitive material.
Apart from these four, there are other types of welding processes like thermo-chemical welding, and radiant energy welding. However, they are used less frequently in industrial and heavy engineering applications than the welding processes described above.
Heavy engineering, which involve structures like dams, bridges, offshore oil rigs and super highwaysneed to consider various aspects like load, wind, vibration, fatigue stresses, and so on. Since the cost of these projects run into crores of rupees, it is just not feasible to use a 'trial and error' method in building them. The welding process is a basic part of most mechanical work.There is no real need to critically analyse weldmet and weld joints for simple structures that are to be used in light, routine-type service like ornamental iron, fence posts, gates, etc. However, joint weld analysis is an extremely important consideration in the sturdiness and safety of critically important and costly heavy engineering structures. Pressure reactors, aircraft frames, bridge joints, etc. need to be analysed thoroughly for proper welding. As the structures grow complicated, the quality of the weld makes a tremendous impact on the longevity of the structures.
Irrespective of the process used, all welded joints contain discontinuities defined as an interruption in the typical structure of the material such as a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. While not defects, it is a challenge to keep these discontinuities to a minimum in order to maintain the quality of the welded joint. Porosity consisting of spherical or cylindrical cavities that are formed as gases entrapped in the liquid weld metal escape while the metal solidifies, is another challenge. Reducing fatigue cracks that may originate at the weld toes are another important consideration for longevity of the welded joints. Welding results in residual stresses and distortion of the workpieces and produces metallurgical changes. Weld joints also require stress-relief heat treatment to increase their life. It is equally important not to 'overweld'a part. Apart from being costly, overwelding can produce a welded joint that cannot withstand the designed forces or vibration. Overwelded joints are not as flexible, and the resulting joint stresses are focused alongside the weld, and can result in cracks just alongside the welds.All these challenges make weld analysis of joints a critical task
Welding Analysis Solutions
Weld joint analysis entails a thorough evaluation of the welded parts for defects. It is said that any chain is as strong as the weakest links, and complicated engineering structures are no exception. Due to inherent limitations, weld joints are one of the weakest links in any structure. Take the case of an offshore oil rig. The foundation of the rig has to bear the effect of wind, waves and water current from virtually any direction 24 / 7, 365 days a year. In addition, it has to support about 30.000 ton weight of the platform. Imagine the consequences if the structure on which this load rests is not welded properly at the joints. To tackle such critical cases, each component of the structure needs to be evaluated carefully and properly evaluated for the safety of the oil rig. One of the most important Numerical Method called Finite Element Analysis (FEA)is the most popular tools used to determine the strength of the welded components.It is the preferred choice to perform welding simulations and to predict weld residual stresses in different types of joints and materials, and under different conditions. Altair HyperLife™ is one such popular and proven software for weld joint analysis. Engineering service companies use Altair modelling and simulation software Altair modelling and simulation software, ESI SYSWELD and other such software to provide weld joint services for mission critical industrial structures. Because analysis of weld joints is one of the most important factors affecting a structure's life and safety, a competent and proven software solution should be used. Altair, ESI SYSWELD and other welding analysis solutions are being increasingly used and trusted by companies in India and even overseas countries like Singapore.