Chapter 8 Fusion Welding PDF

CHAPTER 8 JOINING PROCESSES: FUSION WELDING BMFS 2613 MANUFACTURING PROCESS 1  Introduction Oxyfuel Gas Welding Arc Welding Processes: Nonconsumable Electrodes C...

CHAPTER 8 JOINING PROCESSES: FUSION WELDING BMFS 2613 MANUFACTURING PROCESS 1  Introduction Oxyfuel Gas Welding Arc Welding Processes: Nonconsumable Electrodes Chapter Outline Arc Welding Processes: Consumable Electrodes Electrodes for Arc Welding The Weld Joint, Quality and Testing 2 Introduction Fusion welding is defined as melting together and coalescing materials by means of heat. Filler metals, which are metals added to the weld area during welding, also may be used. Welds made without the use of filler metals are known as autogenous welds. 3 4  Oxyfuel-gas welding (OFW) is a general term to describe any welding process that uses a fuel gas combined with oxygen to produce a flame, which is the source of the heat Oxyfuel-gas required to melt the metals at the joint. Welding The most common gas-welding process uses acetylene; the process is known as oxyacetylene-gas welding (OAW). It is typically used for structural metal fabrication and repair work. 5 Oxyacetylene Welding 6  Acetylene (C2H2) Most popular fuel among OFW group because it is capable of higher temperatures than any other Up to 3300C (6300F) Two stage reaction of acetylene and oxygen: First stage reaction (inner cone of flame) C2H2 + O2 → 2CO + H2 + heat Second stage reaction (outer envelope) 2CO + H2 + 1.5O2 → 2CO2 + H2O + heat 7 Types of Flames The proportion of acetylene and oxygen in the gas mixture is an important factor in oxyfuel-gas welding. At a ratio of 1:1, the flame is considered to be neutral (Figure 30.1a). With a higher oxygen supply, the flame can be harmful (especially for steels), because it oxidizes the metal; for this reason, a flame with excess oxygen is known as an oxidizing flame (Figure 30.1b). If the oxygen is insufficient for full combustion, the flame is known as a reducing, or carburizing flame (Figure 30.1c) 8 FIGURE 30.1 Three basic types of oxyacetylene flames used in oxyfuel–gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; and (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene. (d) The principle of the oxyfuel–gas welding process.  Filler Metals Filler metals are used to supply additional metal to the weld zone during welding, and are available as filler rods or wire (Figure 30.1d) and may be bare or coated with flux. The purpose of the flux is to retard oxidation of the surfaces of the parts being welded, by generating gaseous shield around the weld zone. The slag develop protects the molten puddle of metal against oxidation as the weld cools. 10 FIGURE 30.2 (a) General view of, and (b) cross-section of, a torch used in oxyacetylene welding. The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light. Then the oxygen valve is opened and the flame adjusted. (c) Basic equipment used in oxyfuel–gas welding. To ensure correct connections, all threads on acetylene fittings are left handed, whereas those for oxygen are right handed. Oxygen regulators usually are painted green and acetylene regulators red.  In arc welding, the heat required is obtained from electrical energy. The process involves either a nonconsumable or consumable electrode. An AC or DC power supply produces an arc between the tip of the electrode and the workpiece to be welded. The arc generates temperatures of about 30,000°C, much higher than Arc Welding Processes those developed in oxyfuel-gas welding. 12  Arc Welding Processes: Nonconsumable Electrode In nonconsumable electrode arc welding processes, the electrode is typically a tungsten electrode (Figure 30.5). Externally supplied shielding gas is necessary in order to prevent oxidation of the weld zone. FIGURE 30.5 (a) The gas tungsten-arc welding process, formerly known as TIG (for tungsten inert gas) welding. (b) Equipment for gas tungsten-arc welding operations.  Current and Polarity Typically, direct current is used. The direction of current flow is called polarity, is important. In straight polarity (direct current electrode negative, DCEN), the workpiece is positive, and the electrode is negative. DCEN generally produces welds that are narrow and deep (Figure 30.6a). In reverse polarity (direct current electrode positive, DCEP), the workpiece is negative and the electrode is positive. Weld penetration is less, and the weld zone is shallower and wider (Figure 30.6b). DCEP is preferred for sheet metals and for joints with very wide gaps. In the AC current, the arc pulsates rapidly, suitable for welding thick sections and for using larger diameter electrodes (Figure 30.6c). 15 FIGURE 30.6 The effect of polarity and current type on weld beads: (a) DC current with straight polarity; (b) DC current with reverse polarity; and (c) AC current.  Gas Tungsten Arc Welding (GTAW) In gas tungsten arc welding (GTAW), also known as TIG (tungsten inert gas) welding, the filler metal is supplied from a filler wire. Because the tungsten electrode is not consumed in this operation, a constant and stable arc gap is maintained at a constant current level. The filler metals are similar (compatible) to the metals to be welded, and the flux is not used. The shielding gas is usually argon or helium, or a mixture of the two gasses. Welding with GTAW may be done without filler metals. 17  Plasma Arc Welding In plasma arc welding (PAW), a concentrated plasma arc is produced and directed toward the weld area. The arc is stable and reaches temperatures as high as 33,000°C. A plasma is an ionized hot gas composed of nearly equal numbers of electrons and ions. The plasma is initiated between the tungsten electrode and the orifice by a low current pilot arc. When a filler metal is used, it is fed into the arc, as is done in GTAW. Arc and weld zone shielding is supplied by means of an outer shielding ring and the use of gasses, such as argon, helium or mixtures.  In transferred arc method (Figure 30.7a), the workpiece being welded is part of the electrical circuit. The arc transfers from the Two electrode to the workpiece, hence the term transferred. methods of In the nontransferred method (Figure 30.7b), plasma arc the arc occurs between the electrode and welding the nozzle, and the heat is carried to the workpiece by the plasma gas. This thermal- transfer mechanism is similar to that for an oxyfuel flame. 19 FIGURE 30.7 Two types of plasma-arc welding processes: (a) transferred and (b) nontransferred; deep and narrow welds can be made by these processes at high welding speeds.  Arc Welding Processes: Consumable Electrode Shielded Metal Arc Welding (SMAW) Shielded metal arc welding (SMAW) is one of the oldest, simplest, and most versatile joining processes; consequently, about 50% of all industrial and maintenance welding is performed by this process. The electric arc is generated by touching the tip of a coated electrode against the workpiece, and withdrawing it quickly to a distance sufficient to maintain the arc. The electrodes are in the shapes of thin, long round rods (referred to as stick welding) that are held manually. 21 FIGURE 30.8 Schematic illustration of the shielded metal-arc welding process; about 50% of all large-scale industrial-welding operations use this process. The heat generated melts a portion of the electrode tip, its coating, and the base metal. The molten metal consists of a mixture of the base metal (the workpiece), the electrode metal, and substances from the coating on the electrode; this mixtures forms the weld when it solidifies. The electrode coating deoxidizes the weld area and provides a shielding gas, to protect it from oxygen in the environment.  Submerged Arc Welding (SAW) In submerged arc welding (SAW), the weld arc is shielded by a granular flux, consisting of lime, silica, manganese oxide, calcium fluoride, and other compounds. The flux is fed into the weld zone from a hopper by gravity flow through a nozzle. The thick layer of flux completely covers the molten metal, and prevents spatter and sparks, and suppresses the intense ultraviolet radiation and fumes characteristic of the SMAW process. The flux also acts as a thermal insulator, by promoting deep penetration of heat onto the workpiece. 23 FIGURE 30.10 Schematic illustration of the submerged-arc welding process and equipment; the unfused flux is recovered and reused.  The consumable electrode is Submerged Arc Welding a coil of bare round wire 1.5 (SAW) to 10 mm in diameter, and is fed automatically through a tube (welding gun). Because the flux is gravity fed, SAW process is largely limited to welds in a flat or horizontal position having a backup piece. Circular welds can be made on pipes and cylinders, provided that they are rotated during welding. 25  Gas Metal Arc Welding (GMAW) In gas metal arc welding (GMAW), also known as metal inert gas (MIG) welding, the weld area is shielded by an effectively inert atmosphere of argon, helium, carbon dioxide, or various other gas mixtures (Fig. 30.11a). The consumable bare wire is automatically fed through a nozzle into the weld arc by a wire feed drive motor (Fig. 30.11b). In addition to using inert shielding gases, deoxidizers usually are present in the electrode metal itself, in order to prevent oxidation of the molten metal puddle. Multiple weld layers also can be deposited at the joint. FIGURE 30.11 (a) Schematic illustration of the gas metal-arc welding process, formerly known as MIG (for metal inert-gas) welding. (b) Basic equipment used in gas metal-arc welding operations.  Metal can be transferred by three methods in GMAW 28  GMAW The temperatures generated in GMAW are relatively low. This method is suitable only for thin sheets and sections of less than 6mm; otherwise incomplete fusion may result. Pulsed arc systems are used for thin ferrous and nonferrous metals. The GMAW process is suitable for welding most ferrous and nonferrous metals. It is used extensively in the metal fabrication industry. Simple in nature; training the operators is easy. Can be automated. 29  Flux-cored Arc Welding (FCAW) The flux-cored arc welding (FCAW) process is similar to gas metal arc welding, except that the electrode is tubular in shape and is filled with flux, hence the term flux-cored. Cored electrodes produces a more stable arc, improve the weld contour, and produce better mechanical properties of the joint. The flux in these electrodes is much more flexible than the brittle coating used in SMAW electrodes, thus the tubular electrode can be provided in long coiled lengths. 30 FIGURE 30.12 Schematic illustration of the flux-cored arc-welding process; this operation is similar to gas metal-arc welding, shown in Fig. 30.11.  Electrodes for Arc welding Electrodes for consumable arc welding processes are classified according to the following properties: Strength of the deposited weld metal Current (AC or DC) Type of coating Electrodes are identified by numbers and letters (Table 30.3), or by color code if the numbers and letters are too small to imprint. Typical coated electrode dimensions are in the range of 150 – 460 mm in length and 1.5 – 8 mm in diameter. 32 33  Basic functions of electrode coatings: Stabilizes the arc Generate gasses, to act as a shield Electrode against the surrounding atmosphere Control the rate at which the electrode Coatings melts Act as flux, to protect the weld against the formation of oxides, nitrides, and other inclusions and, with the resulting slag, to protect the molten weld pool Add alloying elements, to the weld zone to enhance the properties of the joint 34 The Weld Joint, Quality and Testing Three distinct zones can be identified in a typical weld joint: Base metal (unaffected base metal) Heat affected zone (HAZ) Weld metal (fusion zone) 35  Solidification of the Weld Metal Weld Metal (Fusion Zone) Heat affected zone (HAZ) Melt and resolidified. Within the base metal. Formation of columnar (dendritic) Subjected to elevated temperature grains. that change the microstructural Cast structure. and mechanical properties (do not melt during welding). Has low strength, toughness and Recrystalizes into new grain ductility as compared to the base structures. metal. Weakest zone in welding. Unaffected base metal receives heat that not critically changes the properties of the metal. 36  Weld Quality - discontinuities/imperfections Porosity (spheres shapes) Slag Inclusions (compound trapped in the weld zone) Caused by: Gases released during melting of the weld Caused by: area but trapped during solidification Ineffective shielding gas Chemical reactions during welding Contamination from environment Contaminants Slag entrapment Can be reduced by: Can be prevented by: Proper selection of electrodes and filler Cleaning the weld bead surface by means of metals. a wire brush. Improved welding techniques, such as Providing sufficient shielding gas. preheating the weld area or increasing the Redesigning the joint to permit sufficient rate of heat input. space for proper manipulation of the Proper cleaning and the prevention of puddle of molten weld metal. contaminants from entering the weld zone. Reduced welding speeds, to allow time for gas to escape. 37  Incomplete Fusion Incomplete Penetration Produces poor weld beads. Occurs when the depth of the Can be improved by: welded joint is insufficient. Raising the temperature of the base Can be improved by: metal Increasing the heat input Cleaning the weld area prior to Reducing the travel speed welding Modifying the joint design Modifying the joint design and Ensuring that the surfaces to be changing the type of electrode used joined fit each other properly Providing sufficient shielding gas  Weld Profile – effects the Underfilling – the joint is not filled with the proper strength and appearance of amount of weld metal the weld Undercutting – melting away of the base metal and the subsequent generation of a groove in the shape of a sharp recess or notch Overlap – surface discontinuity caused by poor welding practise or by selection of improper materials.  May occur in various Can be prevented by: locations and directions in Modifying the joint the weld area. design to minimize Typical types of cracks are Cracks longitudinal, transverse, stresses developed from shrinkage during crater, underbead and toe cooling. cracks. Change the Caused by: parameters, Temperature gradients procedures, and causing thermal sequence of the stresses in the weld welding operations. zone. Preheat the Variations in the components to be composition of the welded. weld zone causing Avoid rapid cooling of different rates of the welded contraction during components. 40 cooling. FIGURE 30.23 Types of cracks developed in welded joints; the cracks are caused by thermal stresses, similar to the development of hot tears in castings, as shown in Fig. 10.12.  Because of localized heating and cooling during welding, the expansion and contraction of the weld area causes residual stresses in the workpiece. Residual stresses can lead to the Residual following defects: Distortion, warping, and buckling of Stress and the welded part Stress corrosion cracking Distortion Further distortion if a portion of the welded structure is subsequently removed, such as by machining ang sawing Reduced fatigue life of the welded structure FIGURE 30.25 Distortion of parts after welding; distortion is caused by differential thermal expansion and contraction of different regions of the welded assembly.  Testing of Welds – Destructive Testing The quality of welding is established by testing. Several standards are available for test procedures. Destructive testing techniques include: Tension test Tension shear test Bend test Fracture toughness test Creep and corrosion test Hardness test FIGURE 30.28 (a) Specimens for longitudinal tension-shear testing and for transfer tension-shear testing. (b) Wraparound bend-test method. (c) Three-point transverse bending of welded specimens.  Nondestructive Testing Nondestructive testing techniques for welded joints generally consist of the following: Visual inspection Radiographic (X-rays) Magnetic particle Liquid Penetrant Ultrasonic 46 THANK YOU 47