Historical Development of Welding
Welding is a process that allows the permanent connection of the solid parts to each other and that realizes the continuity of the material when it is applied. The welding in its most common involves the addition of localized heat that allows the fusion of the material. This material can be the same material component parts that are joined, but can also be a foreign material to them, said filler material: the first is known as autogenous welds in the second heterogeneous welding or brazing. Welding creates a permanent link that differs from other permanent connections (such as riveting or bonding) do not realize that the continuity of the material. With some types of gas welding, if done correctly and according to certain principles, you are guaranteed almost total continuity in the characteristics of the material of the parts together.
In its broadest sense refers to the union by the welding heat input of different materials together, or similar materials, as is done commonly for example the welding of plastics. The glass can be "welded". But by definition is between the weld metal, and those dealing with here.
Development of welding:
Since the Middle Ages joined heating to heat steel parts yellow-white on the forge and then hammering to make them uniform. However, for the welding process with characteristics consistent and reproducible, it was necessary to arrive at 1901 with the oxyacetylene welding, in which the parties were united by fusion of the leaflets. In this welding process the energy required for fusion of the pieces was supplied by the combustion of a gas (in this case acetylene) with pure oxygen. Reaching temperatures high enough (and above the melting point of iron) was no longer necessary for the operation of hammering the pieces together, to the advantage of simplicity and repeatability of the operation.
At the beginning of the twentieth century were developed electric generators powerful enough to generate an arc having a power sufficient to melt iron. The first welding process that was developed using the energy of the electric arc electrode process was not protected, now completely abandoned in favor of proceeding with coated electrodes, in which the coating plays a variety of essential functions for the production of a joint of good features. To date (2006) the SMAW welding process is the most widely used.
During the Second World War was felt the need to produce welded joints of good quality with a much higher productivity than that which could be coated on the electrode, then the United States had begun the study of processes with continuous wire, and particular submerged arc, which allowed for productivity and reproducibility absolutely larger than those of processes for SMAW. Were developed in the postwar period (the fifties) processes for MIG and MAG have a productivity comparable to the submerged arc, but with greater flexibility. Was developed in parallel with the TIG process, allowing very precise control welding and the characteristics of a continuous operation, which was not permitted by the electrode coated.
Finally, in the seventies were developed processes for concentrated energy, ie electron beams and lasers, which permit the area of material modified by welding. Currently studies are underway for the diffusion bonding, which does not lead to melting the material to be welded, but pressure to a temperature high enough because the atoms in the crystal lattice spread across the surface of separation of parts, so come to realize at relatively low temperatures.
Main applications of welding:
The welding is used mainly for construction of vessels subject to significant efforts (mainly due to pressure) (pressure vessels) or for construction of supporting structures of varying complexity (carpentry). Outside of these two applications, which already cover a very large jobs, welding is used in the construction of vehicles, both maritime and aerial and terrestrial. The initial development of the welding was out of this range of applications, in particular the need for the shipbuilding industry, which required the joining of metal sheets too thick for the nails with considerable resistance and weigh as much as possible, limited.
The main feature of the weld is to create monolithic structures, ie structures that do not show discontinuity in the presence of features of the joints. This peculiarity of the weld is of great importance both when required mechanical strength is required when uniform is a uniform resistance to external aggression (eg corrosion). Given these characteristics, the weld has significant applications in various fields of engineering:
· Mechanical Engineering: construction of mechanical structures of complex shape and subjected to significant efforts
· Civil Engineering: construction of metal structures of buildings and bridges to support
· Chemical engineering: construction of vessels (pressurized or not), boxes of pumps, valves and boxes of piping systems
· Engineering nuclear reactor pressure vessels, piping, containment and security structures
· Transport engineering: construction of land vehicles and naval
· Aeronautical Engineering: aircraft structures.
However, there are special cases in which the welding is used for unions "partially" continue as in the case of the "spot".
The welding process:
Each type of welding is done through different processes and machinery. You can still describe a generic process that unites the different welding processes. To achieve a weld of two parts is necessary first to prepare the edges of the joint through what is called crimping. So the joint is heated to different temperatures depending on the process used. When the joint is heated to melt the edges joining it with the material of the joint or with the help of a filler material to be homogeneous it comes to gas welding. However, if the joint when heated below the melting temperature is fused to it a material contribution to it and mixed with lower melting point is about heterogeneous welding or brazing.
The heat needed for the implementation of the process is obtained with different systems:
· A flame produced by combustion of a gas with air or oxygen.
· An electrical arc is formed between two electrodes (one of them can be the same piece).
· Electrical resistance produced by the Joule effect to passing a current through the workpiece.
· Power laser or other means of energy input from non-flame.
To obtain a resistance welding, technically sound and free of imperfections, the area of merger must be protected from oxidation and the molten metal must be purified of dross. Welding to prevent oxidation in the atmosphere must be so free as possible of oxygen (inert) for this purpose in the area near the weld should be added substances such as gas, borax, carbonates and silicates, which create a "protective cloud "near the weld and allow the expulsion of waste. In oxy-acetylene welding is produced reducing atmosphere, while the arc welding is carried out in the atmosphere produced by burning the lining of the electrode or in the gas stream.
The filler metal may be in the form of rods or filaments, which are closer to the zone of fusion (and TIG welding torch, in English tungsten inert gas) or be the real electrode melts because of ' electric arc itself causes.
Heterogeneous welding or brazing:
The heterogeneity is commonly called brazing welding and allows merging parties to join the league just filler and while preserving the edges of the joint. As part of the brazing can be distinguished:
· Brazing: temperatures above 450 ° C but below the melting point of materials to be welded, the joint must be prepared to facilitate the penetration of the weld material by capillary
· Soldering: is at temperatures below 450 ° C and below the melting point of materials to be welded, the joint must be prepared to facilitate the penetration of the weld material by capillary
· brazing: filler alloys are alloys melt at temperatures even higher than those used in brazing but always lower than the melting point of materials to be welded, the joint is prepared similarly to preparation for a gas welding.
Also known as precise welding (spot welding in English) or nails, welding, often achieved by induction welding, is a type of resistance welding and is to match the material being welded parts and compress the two pieces by a machine. Subsequently, the passage of electricity heats the bodies to be welded up to the melting point in less than 15 seconds, thus combining the two materials from a nail inside resistant long-lasting. This kind of welding is adopted in many centers for pre-shaping to make double brackets produced in one step more rigid and therefore safe to handle.
The welding spot, except when the term is used loosely to mean the precise welding of the above, does not refer to a particular welding process, but rather to a particular application of the welding processes already mentioned. This is to generate welding points on the perimeter of the pieces to be joined, ie without creating a weld or a continuous weld without interruptions, creating what many locations at more or less regular distances between them. This procedure is often used to join parts subjected to low mechanical stress, or at least including the welding should not create a tight joint. It is typically used in non-automated welding processes such as basting useful to hold together the parts for subsequent seam welding.
In ultrasonic welding, high frequency (15 kHz to 40 kHz) low amplitude vibration is used to create heat by friction between the materials to be welded. The interface of the two parts is specially designed to concentrate the energy for maximum resistance welding. The ultrasonic welding can be used on almost all plastics. It is the fastest available heat sealing technology.
Inspection of welds:
The security implications related to the use of welding, especially in the field of pressure vessels and civil engineering criteria imposed on the guarantee of the reliability of the welds. This check occurs on two distinct phases:
· Screening of staff and the procedure (preventive control)
· Control of the welded joint (production control)
Control and qualification of welding procedures:
Welding, in the case involving security issues, should only be performed by qualified personnel using qualified procedures, standards of qualification procedures vary depending on the scope and depending on the material to be welded. In particular in Europe follow the Euronorms EN 15614-1 for the qualification of the arc furnace steel and nickel alloys and UNI EN 287 for the qualification of welders, while the United States apply to ASME Sect. IX.
Typically, to qualify a procedure must be carried out of the heels, which are controlled by non-destructive methods and by which they are taken for test-destructive tests (tensile, bending, impact, etc.).
Control of welded joints after production:
The welded joints, after the execution, undergo non-destructive, more or less extended, depending on the joint request of the reliability, also for a set number of feet of welding joints (depending on scope) are products other heels that will be subject to destructive tests (usually the most significant among those already incurred during the qualification procedure).
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