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welding design process selection
Metalurgi Pengelasan: Metalurgi Pengelasan: Rancangan Pengelasan Rancangan Pengelasan dan Pemilihan Proses dan Pemilihan Proses Ir. Tri Prakosa, M. Eng. Proses Manufaktur II, Januari 2010 1 Introduction Introduction Heating the workpieces to a temperature sufficiently high to produce a weld involves important metallurgical and physical changes in the materials being welded. The strength, toughness, and ductility of a welded joint depend on many factors. For example, the rate of heat application and the thermal properties of metals are important in that they control the magnitude and distribution of temperature in a joint during welding. Institut Teknologi Bandung2 Introduction, con’t Introduction, con’t The microstructure and grain size of the welded joint depends on the magnitude of heat applied and temperature rise, the degree of prior cold work of the metals, and the rate of cooling after the weld is made. Weld quality depends on factors such as the geometry of the weld bead and the presence of cracks, residual stresses, inclusions, and oxide films. Their control is essential to reliable welds that have acceptable mechanical properties. Institut Teknologi Bandung3 THE WELDED JOINT THE WELDED JOINT Institut Teknologi Bandung4 The welded joint The welded joint Three distinct zones can be identified: 1. The base metal, that is, the metal to be joined. 2. The heat-affected zone (HAZ). 3. The weld metal, that is, the region that has melted during welding. The metallurgy and properties of the second and third zones depend strongly on the metals joined, the welding process, filler metals used, if any, and process variables. Institut Teknologi Bandung5 The welded joint, con’t The welded joint, con’t A joint produced without a filler metal is called autogenous, and the weld zone is composed of the resolidified base metal. A joint made with a filler metal has a central zone called the weld metal and is composed of a mixture of the base and filler metals. Institut Teknologi Bandung6 Typical fusion weld joint Typical fusion weld joint Institut Teknologi Bandung7 1. The base metal, that is, the metal to be joined. 2. The heat-affected zone (HAZ). 3. The weld metal, that is, the region that has melted during welding. Contoh karakteristik daerah fusi dari lasan pada pengelasan busur gas oxyfuel. Solidification of the weld metal Solidification of the weld metal After applying heat and introducing filler metal, if any, into the weld area, the molten weld joint is allowed to cool to ambient temperature. The solidification process is similar to that in casting and begins with the formation of columnar (dendritic) grains. These grains are relatively long and form parallel to the heat flow Institut Teknologi Bandung8 Solidification of the weld metal Solidification of the weld metal Because metals are much better heat conductors than the surrounding air, the grains lie to the plane of the two plates or sheets being welded (Figure a). The grains in a shallow weld are shown in Figure b. Institut Teknologi Bandung9 Struktur butir pada (a) lasan dalam (b) lasan dangkal. Perhatikan bahwa butir pada lasan yang mengalami pendinginan orientasinya tegak lurus permukaan logam dasar. Pada lasan yang baik, garis pendinginan yang diperlihatkan sebagai garis pada bagian tengah lasan dalam yang diperlihatkan pada (a) mempunyai migrasi butir yang menghasilkan kekuatan seragam pada manik/kampuh lasan. Solidification of the weld metal Solidification of the weld metal Grain structure and size depend on the specific alloy, the welding process, and the filler metal used. The weld metal is basically a cast structure and, because it has cooled slowly, it generally has coarse grains. Consequently, this structure has generally low strength, toughness, and ductility. Institut Teknologi Bandung10 [...]... the weld area prior to welding Changing the joint design and type of electrode Providing adequate shielding gas Incomplete penetration occurs when the depth of the welded joint is insufficient Penetration can be improved by: { { { { Increasing the heat input Lowering travel speed during welding Changing the joint design Ensuring that the surfaces to be joined fit properly 29 Institut Teknologi Bandung... within the base metal itself It has a microstructure different from that of the base metal before welding, because it has been subjected to elevated temperatures for a period of time during welding The portions of the base metal that are far enough away from the heat source do not undergo any changes during welding 14 Institut Teknologi Bandung Heat-affected zone The properties and microstructure of... Porosity, con’t Porosity in welds can be reduced by the following methods: { Proper selection of electrodes and filler metals { Improving welding techniques, such as preheating the weld area or increasing the rate of heat input { Proper cleaning and preventing contaminants from entering the weld zone { Slowing the welding speed to allow time for gas to escape 25 Institut Teknologi Bandung Slag inclusions... metal However, the proper selection of filler-metal composition or heat treatments following welding can improve the joint's mechanical properties The results depend on the particular alloy, its composition, and the thermal cycling to which the joint is subjected Cooling rates may, for example, be controlled and reduced by preheating the general weld area prior to welding 11 Institut Teknologi... inadequate or careless application of established welding technologies or substandard operator training The major discontinuities that affect weld quality are described as follow 23 Institut Teknologi Bandung Porosity Porosity in welds is caused by trapped gases released during melting of the weld area and trapped during solidification, chemical reactions during welding, or contaminants Most welded joints... alloy was developed prior to welding 17 Institut Teknologi Bandung Heat-affected zone They may have been developed by cold working, solidsolution strengthening, precipitation hardening, or by various heat treatments Of these strengthening methods, the simplest to analyze is base metal that has been cold worked, say, by cold rolling or forging The heat applied during welding recrystallizes the elongated... inclusions are compounds such as oxides, fluxes, and electrode-coating materials that are trapped in the weld zone If shielding gases are not effective during welding, contamination from the environment may also contribute to such inclusions Welding conditions are important, and with proper techniques the molten slag will float to the surface of the molten weld metal and not be entrapped 26 Institut... adequate shielding gas { Redesigning the joint to permit sufficient space for proper manipulation of the puddle of molten weld metal 27 Institut Teknologi Bandung Incomplete fusion and penetration Incomplete fusion (or lack of fusion) produces poor weld beads, such as those shown here Kualitas kampuh rendah sebagai akibat fusi yang tidak lengkap/penuh Sumber: American Welding Society 28 Institut... of heat during welding on the HAZ for joints made with dissimilar metals, and for alloys strengthened by other methods, are complex and beyond the scope of this lecture 21 Institut Teknologi Bandung WELD QUALITY 22 Institut Teknologi Bandung Weld Quality Because of a history of thermal cycling and attendant microstructural changes, a welded joint may develop certain discontinuities Welding discontinuities... an undercut can act as a stress raiser and reduce the fatigue strength of the joint-and may lead to premature failure Overlap (Figure b) is a surface discontinuity generally caused by poor welding practice and selection of the wrong materials A proper weld is shown in Figure c 32 Institut Teknologi Bandung Cracks Cracks may occur in various locations and directions in the weld area The types . melted during welding. The metallurgy and properties of the second and third zones depend strongly on the metals joined, the welding process, filler. weld metal Grain structure and size depend on the specific alloy, the welding process, and the filler metal used. The weld metal is basically a cast