Welding Methods
Total Page：16
File Type：pdf, Size：1020Kb
Welding Methods Learning Outcome When you complete this module you will be able to: Describe the methods, equipment and applications for oxyacetylene and electric arc welding. Learning Objectives Here is what you will be able to do when you complete each objective: 1. Describe forge welding. 2. Describe oxyacetylene welding and cutting. 3. Describe metal arc welding. 4. Describe braze welding. 1 WELD 6018 INTRODUCTION Welding has a dominant place in metal industries. It is common practice to shape and prepare metal parts, and then join them by welding to produce engineering structures. It is the purpose of this module to classify the various processes by which welds are applied, and to outline their applications. The engineer, draftsman-designer, and the operator are thus required to have some knowledge of welding, although need never acquire any practical ability in this field. AJ_2_0_1.jpg Welding consists of joining two or more pieces of metal by the application of heat P and sometimes pressure. The heat may be derived from a number of different sources and the processes by which heat is applied are divided into two general groups. FORGE WELDING Forge welding involves methods in which the joint surfaces are heated to the temperature range where the metal becomes sufficiently plastic, and pressure is then applied. Forge welding is the oldest form of welding. The parts to be joined are heated in a forge or furnace to the proper temperature range and then subjected to a pressure by hammering or rolling by hand or machine. This results in the parts being united. This process was used for a wide variety of applications, from the miscellaneous work of the blacksmith to the joining of steel plates in the fabrication of boiler drums and other pressure vessels. However, forge welding has been replaced by electric welding processes. FUSION WELDING In the fusion welding process, the joint surfaces are heated above the melting temperature of the base metal, and fusion is obtained without the application of pressure. 2 WELD 6018 Oxyacetylene Welding Process In this method, which is also referred to as gas welding, acetylene gas is burned with oxygen in a special torch to produce an intensely hot flame. This flame is used to melt the edges of the pieces of metal to be welded (the base metal). A filler metal, in the form of a welding rod, is melted into the molten pool of base metal at the same time. The welding rod used is the same material as the base metal and they all join together to form a single piece. This process is of particular advantage in making welds of any shape, or in any position, on comparatively thin sections, especially at sharp edges and corners. It is not, however, limited to thin sections, but may be satisfactorily used to produce welds in thick metal parts. For joining thin parts, a filler metal may or may not be required, but for thicker sections a filler rod of suitable composition must be used. 1. Oxyacetylene Welding Equipment The welding torch consists essentially of a mixing chamber with an oxygen connection and an acetylene connection at one end. Each connection is fitted with a needle valve, one to control the oxygen and one to control the acetylene. At the other end of the mixing chamber, a welding tip is attached. Different sizes of tips may be used to weld different metal thicknesses. A typical welding torch is illustrated in Fig. 1. AJ_2_0_14.jpg P Figure 1 Welding Torch (Airco) The type of torch in general use is the Equal Pressure Torch. Torch identification numbers vary around the world, but in North America the system adopted is a simple one of numbers that indicate the pressures to be used. A number two tip requires 14 kPa (2 psi) of oxygen pressure and 14 kPa of acetylene pressure. Likewise, a slightly larger tip showing a five stamped on it requires about 35 kPa (5 psi) of oxygen and acetylene pressure. 3 WELD 6018 The other equipment includes: 1. An acetylene regulator fitted to the acetylene cylinder. 2. An oxygen regulator fitted to the oxygen cylinder. 3. A torch handle with needle valves for control of the flame. 4. Cutting tips for cutting metal with the flame. 5. A cutting attachment. 6. Welding tips, with gas mixers, to suit the size of tip. 7. “Dualine” hose, in two colors, green side for oxygen and red side for acetylene. 8. A tip cleaner. 9. Dark glass goggles. 10. A sparklighter. 11. Flux coated brazing rods. 12. Low carbon steel filler rods. The oxygen for the process is supplied in steel cylinders at a pressure of about 15 400 kPa. The acetylene is also supplied in cylinders, but as it is unstable in its free state, it is carried in an absorbed state in acetone, which is held in the cylinder by a porous filler. In this way it can be stored under a pressure of 1720 kPa. Both the oxygen and the acetylene cylinders must be fitted with regulators to reduce the pressure of each gas to a suitable amount for the welding process. For example, the oxygen pressure necessary for welding a certain thickness of metal might be 28 kPa and the regulator would have to reduce the cylinder pressure from 15 400 kPa to 28 kPa. Similarly, the acetylene pressure would have to be reduced from 1720 kPa to about 28 kPa. The regulators are actually pressure reducing valves and are fitted with two pressure gages. One gage shows the cylinder pressure and the other gage shows the operating pressure. Fig. 2 illustrates a complete oxyacetylene outfit and necessary equipment for operation. 4 WELD 6018 AJ_2_0_19.jpg P AJ_2_0_20.mov V Figure 2 Oxyacetylene Outfit (Airco) 2. Oxyacetylene Cutting When iron or steel is heated to a temperature of 870°C, it will burn when brought into contact with oxygen. If a blast of pure oxygen in the form of a thin jet is directed on the hot iron or steel, the metal will be quickly burned away in the form of a narrow cut. A common misconception is that the steel melts away due to the heat, but this is not the case. The steel actually burns, combining chemically with oxygen. To cut steel plate, normal pressure would be about 20 kPa for acetylene and about AJ_2_0_21.mov 280 kPa for oxygen. The oxygen pressure would be increased or decreased to suit V the thickness of the plate. An oxyacetylene welding outfit with a cutting torch attachment is illustrated in Fig. 3. 5 WELD 6018 AJ_2_0_3.jpg G Figure 3 Oxyacetylene Welding Outfit Cast iron cannot be cut successfully and neatly by this method since it does not burn progressively. To cut stainless steel neatly with an oxyacetylene torch, a waster plate of ordinary low carbon steel must be clamped to the stainless steel plate and the two burned together. Nonferrous metals such as copper, brass, aluminum, and their alloys do not burn evenly and progressively and are not normally cut with standard oxyacetylene equipment. 6 WELD 6018 Metal Arc Welding Process Arc welding is a fusion welding process wherein the pieces of metal are joined together by means of an electric arc. The heat is obtained from an electric arc formed between the base metal and an electrode. The temperature produced by the arc ranges from 3000°C to 8300°C, resulting in a molten pool forming on the work at the arc location. By manipulation of the electrode, the molten pool is made to travel along the joint as desired. The covered metal electrode (filler rod) melts and carries metal across the arc from the electrode to the work. Very small globules are deposited in the molten pool on the work. The electrode is consumed and the metal is deposited at a rate dependent on the value of the current. In metal arc welding, the weld is shielded from the atmosphere during welding by the decomposition of the electrode covering. A shielded metal arc weld being produced is illustrated in Fig. 4. AJ_2_0_16.jpg P AJ_2_0_17.jpg P Figure 4 Shielded Metal Arc This method of welding has a wide application for high-speed welding on thin AJ_2_0_22.mov sections. It is used in the fabrication of steel barrels and tanks, and also for V welding pipe and pressure vessels. The procedure is adaptable for welding of sections in flat, vertical, or overhead positions. 1. Arc Welding Equipment AJ_2_0_2.jpg Either alternating or direct current may be used for arc welding, depending on the P type of electrodes. The most satisfactory source of direct power is a portable motor-generator set. The generator is of the variable voltage type, which adjusts its voltage to the arc demands. 7 WELD 6018 Alternating current is supplied from single-phase transformers, either with variable voltage taps or with a variable inductance in each arc circuit to obtain desired current values. Another source of power is the rectifier, which can convert ac into dc. Fig. 5 shows a simplified diagram of an arc welding circuit. The electric current passes through the metal being welded and returns to the welding power source through another cable, thus completing the circuit. AJ_2_0_9.jpg P Figure 5 Arc Welding Circuit 2. Electrodes Carbon electrodes, bare wire, and covered electrodes with a covering principally of combustible materials, generally require direct current. Covered electrodes with a covering principally of minerals are, in general, satisfactory for direct or alternating current. The combustible electrode type gives protection to the arc largely by producing a AJ_2_0_4.jpg gaseous envelope around the arc which excludes oxygen and nitrogen.