Weman Welding Processes Handbook

2. Auflage 2011
ISBN: 978-0-85709-518-3
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 280 Seiten

Reihe: Woodhead Publishing Series in Welding and Other Joining Technologies

The first edition of Welding processes handbook established itself as a standard introduction and guide to the main welding technologies and their applications. This new edition has been substantially revised and extended to reflect the latest developments.After an initial introduction, the book first reviews gas welding before discussing the fundamentals of arc welding, including arc physics and power sources. It then discusses the range of arc welding techniques including TIG, plasma, MIG/MAG, MMA and submerged arc welding. Further chapters cover a range of other important welding technologies such as resistance and laser welding, as well as the use of welding techniques for cutting, surface cladding and hardfacing, soldering and brazing. A final group of chapters discuss more general issues such as mechanisation, safety, residual stress and distortion, welding design, costs and quality assurance, as well as the welding of steel and aluminium.The new edition of Welding processes handbook confirms its reputation as a concise, authoritative and practical introduction to welding and its applications for both students and engineers. It is designed to meet the requirements of Module 1: Welding processes and equipment of the International Institute of Welding (IIW) guidelines for the training of welding personnel at IWE, IWT, IWS and IWP level. - This new edition has been substantially revised and extended to reflect the latest developments in the main welding technologies and their applications - Reviews gas welding and discusses the fundamentals of arc welding, including arc physics and power sources, before covering the range of arc welding techniques, including TIG, plasma, MIG/MAG, MMA and submerged arc welding - Examines a range of important welding technologies, such as resistance and laser welding and the use of welding techniques for cutting, surface cladding and hardfacing, soldering and brazing

Professor Klas Weman currently works for WEMAB AB in Sweden. He has previously worked for ESAB Welding Equipment AB and in the Welding Technology Department of the Royal Institute of Technology in Stockholm, Sweden.

Autoren/Hrsg.

Weman, K.

Leseproben

2 Gas welding
2.1 Introduction
Oxy-acetylene gas welding is one of the oldest methods of welding and, for many years, was the most widely used welding technique. Its use is a lot less common today. Nevertheless, it is a versatile method, using simple and relatively cheap equipment. It is suitable for repair and construction work, for welding pipes/tubes and structures with a wall thickness of 0.5–6 mm and in materials particularly prone to cracking, such as cast iron. It is also used for welding non-ferrous metals and for cladding and hardfacing. In addition to welding, the technique is often used for cutting, and is also very useful for heating and straightening materials. The heat is generated by the combustion of acetylene in oxygen, which gives a flame temperature of about 3100 °C. This is lower than the temperature of an electric arc, and also produces a less concentrated heat. The flame is directed onto the surfaces of the joint, which melt, after which filler material can be added as necessary. The melt pool is protected from air by the reducing outer zones of the flame. The flame should therefore be removed slowly when the weld is completed. The less concentrated flame results in slower cooling, which is an advantage when welding steels that have a tendency to harden, although it does make the method relatively slow, with higher heat input and the added risk of thermal stresses and distortion. 2.2 Equipment
A set of equipment for gas welding consists essentially of gas bottles, pressure regulators, gas hoses, flashback arresters and welding torches (Figure 2.1). Figure 2.1 A gas welding set. Welding gases and their storage Gas bottles for combustible gases must be stored outdoors or in a well-ventilated area. Special warning signs must be displayed on the outside of the storage area. Acetylene and oxygen bottles must be kept well apart. Acetylene Acetylene (C2H2) is the main fuel gas for gas welding. Its main properties compared to other fuel gases are listed in Table 2.1. It consists of 92.3 % carbon and 7.7 % hydrogen by weight. Its combustion in oxygen produces a higher combustion temperature than that of any other hydrocarbon gas. In addition, its flame is the most concentrated in comparison with other gases. TABLE 2.1 Important characteristics of fuel gases. It is important to be aware of the risks of using acetylene. Acetylene ignites very easily, and produces an explosive mixture in air over a wide range of concentrations (2.3–82 %). As a result it is important to check carefully that there are no leaks from gas bottles or hoses. Acetylene is chemically unstable under pressure, even without the presence of air. Under certain conditions, it can explosively decompose to its constituents (carbon and hydrogen). To enable the gas to be stored safely, the bottles are filled with a porous mass, saturated with acetone, which absorbs the gas when it is filled. The pressure in the bottles is 2 MPa. However, explosive decomposition can occur in pipes or hoses leading from the bottle if the pressure in them exceeds 1.5 MPa. Oxygen Oxygen is stored as a compressed gas or liquid. In bottles, it is usually stored at a pressure of 20 MPa. Those using oxygen in large quantities usually receive the gas in liquid form. It is important to make sure that all connections are clean and tight, in order to avoid leakage. If pure oxygen is directed at something flammable, a fire can start very easily. In particular, oil or grease must never be applied to connections. Pressure regulators The purpose of the pressure regulator is to reduce the high and variable pressure in the bottle to a suitable working pressure for the welding torch. It keeps the gas flow rate constant throughout the life of the bottle charge, despite any variations in back pressure caused by the heating of the welding torch. Gas hoses Gas hoses are colour-coded: red for acetylene and blue for oxygen. In addition, in order to protect against mistakes, the acetylene connection has a left-hand thread, while the oxygen connection has a right-hand thread. Backfire and flashback A backfire means that the flame burns backwards into the torch with a popping sound. It occurs if the combustion speed of the flame exceeds the speed at which the gas is being supplied, so that the flame front moves backwards. In the case a of sustained backfire, which gives a whistling sound, all gas valves must immediately be shut off. A flashback is caused by the reverse flow of gases upstream into the hoses, e.g. by oxygen having entered the acetylene hose and thus forming an explosive mixture. A flashback arrester fitted at the regulator prevents a flashback from going any further back and reaching the acetylene bottle where it would trigger an explosive decomposition. Figure 2.2 Flashback. Welding torches A typical welding torch is shown in Figure 2.3. One can distinguish between two types of welding torches: injector torches for low pressure acetylene and medium pressure torches for high pressure acetylene. Figure 2.3 Gas welding torch. In high pressure torches, the acetylene and oxygen flows are powered by the pressure in their storage bottles, and mix in the mixing chamber section of the torch. In low-pressure torches, the oxygen flows into the torch through a central jet, producing an injection effect that draws in acetylene from the surrounding connection. From here, the gases continue to the mixing section in the welding torch prior to combustion. 2.3 Gas flames
The basic requirement for a good weld is that the size and type of the flame should be suited to the type of work. The size of the flame depends on the size of the torch nozzle and on the pressure of the gases flowing through it. This pressure should be maintained within certain limits. If it exceeds the normal pressure, there will be a considerable jet effect and the flame will become ‘hard’. Below the correct pressure, the jet effect will be reduced and the flame will be ‘soft’. There are three different types of flames, depending on their chemical effect on the melt pool: neutral, carburising and oxidising. Neutral flame The neutral flame (see Figure 2.4) is used most. It is easily recognised by the three clearly distinguished combustion zones. The innermost zone, the cone, is a mixing zone and glows white. Acetylene burns here to form carbon monoxide and hydrogen which produce a colourless tongue around the cone. The second zone chemically reduces any metal oxides and keeps the melt pool clean. The outer, blue zone of the flame is where carbon monoxide and hydrogen burn with oxygen from the air, forming the final combustion products of carbon dioxide and water vapour. It prevents oxygen in the air from coming into contact with the molten metal, and so acts as a shielding gas. Figure 2.4 A neutral welding flame. Carbon monoxide and hydrogen are formed in the innermost reaction zone. They produce a reducing zone (in the middle), with combustion continuing in the outer zone with oxygen from the surrounding air. The carburising flame If the proportion of acetylene in a neutral flame is increased, there is insufficient oxygen to burn the surplus acetylene in the core zone. The acetylene therefore continues to the second zone, where it appears as a highly luminous yellow-white flame (see Figure 2.5). To some extent, the length of second zone indicates the amount of excess acetylene. Figure 2.5 Carburising flame. The oxidising flame If the quantity of oxygen is increased, the flame changes to an oxidising flame. The core length is reduced, and the flame takes on a violet tinge with low luminosity (see Figure 2.6). Figure 2.6 Oxidising flame. 2.4 Welding techniques
Two different methods of welding are used when gas welding: forehand and backhand. The flame in forehand welding is directed away from the finished weld, while in backhand welding it is directed towards it (Figure 2.7). Figure 2.7 Forehand welding (left) and backhand welding (right). Thin sheet metal (less than 3 mm) is normally welded using forehand welding. Steel over 3 mm thick should be backhand welded as it will give the necessary deep penetration and help gases and slag to escape from the large melt pool. Backhand welding is also faster than forehand welding. This means the workpiece...