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Slide_Shielded metal arc welding process.ppt
- 2. Shielded Metal ArcWelding (SMAW)
- 3. SMAW Electrode Classification Example E7018 •E indicates electrode • 70 indicates 70,000 psi tensile strength • 1 indicates use for welding in all positions • 8 indicates low hydrogen E7018-A1-H8R
- 4. •ANSI/AWS - 5.1: Specification for Covered Carbon Steel •ANSI/AWS - 5.5 : Specification for Low Alloy Steel •ANSI/AWS - 5.4 : Specification for Corrosion Resistant Steel AWS Website: http://www.aws.org
- 5. Coating Materials -PartialList Arc Stabilizers Titania TiO2 Gas-Forming Materials Wood Pulp Limestone CaCO3 Slag-Forming Materials Alumina Al2O3 TiO2 SiO2 Fe3O4 Slipping Agents to Aid Extrusion Clay Talc Glycerin Binding Agents Sodium Silicate Asbestos Starch Sugar Alloying and Deoxidizing Elements Si, Al, Ti, Mn, Ni, Cr
- 6.
- 7. SMAW Advantages • Easily implemented •Inexpensive • Flexible • Not as sensitive to part fit-up variances Shielded Metal Arc Welding
- 8. Advantages • Equipment relativelyeasy to use, inexpensive, portable • Filler metal and means for protecting the weld puddle are provided by the covered electrode • Less sensitive to drafts, dirty parts, poor fit-up • Can be used on carbon steels, low alloy steels, stainless steels, cast irons, copper, nickel, aluminum
- 9. Quality Issues • Discontinuities associatedwith manual welding process that utilize flux for pool shielding – Slag inclusions – Lack of fusion • Other possible effects on quality are porosity, and hydrogen cracking Shielded Metal Arc Welding
- 10. Limitations • Low Deposition Rates •Low Productivity • Operator Dependent Shileded Metal Arc Welding
- 11. Other Limitations • Heatof welding too high for lead, tin, zinc, and their alloys • Inadequate weld pool shielding for reactive metals such as titanium, zirconium, tantalum, columbium
Editor's Notes
- #1 Now let’s consider the Shielded Metal Arc Welding Process in more detail. This is typically a manual welding process where the heat source is an electric arc which is formed between a consumable electrode and the base material. The electrode is covered by a coating, which is extruded on the surface of the electrode. During welding, the electrode coating decomposes and melts, providing the protective atmosphere around the weld area and forming a protective slag over the weld pool.
- #2 The arc is struck between the tip of the electrode and the workpiece. The arc is moved over the work at the appropriate arc length and travel speed, melting and fusing a portion of the base metal and continuously adding filler metal. Electrode types and sizes, (with predetermined coating compositions) are selected to correspond to the required strength levels of base metal, the types of welding power supplies utilized and depth of penetration and amount of weld metal fill required.
- #3 This slide illustrates the American Welding Society Electrode Classification System found in Code A5.5-96. The E7018 electrode is probably one of the most commonly used. The E indicates that this is an electrode, the 70 indicates that the weld metal deposited has at least 70,000 pounds per square inch tensile strength, the 1 indicates that the electrode can be used in all positions, and the 8 indicates that it is a low hydrogen electrode. Additional information May be given by a series of suffixes separated by dashes. In the second example, the “A1” indicates Chemical composition for undiluted weld metal. “H8” indicates conformity to the diffusi The arc is moved over the work at the appropriate arc length and travel speed, melting and fusing a portion of the base metal and continuously adding filler metal. ble hydrogen test. (8 ml of H2 per 100g of deposited metal). Finally, the “R” indicates conformity to an absorbed moisture test (less than 0.4% Moisture Content).
- #4 Common electrode specification can become confusing to the user and manufacturer, depending upon the number of special requirements desired for an application. Some common American Welding Society SMAW specifications for electrode classifications are presented here. You are urged to examine these. They can be obtained from AWS at the website listed.
- #5 This is a partial list of materials which are added to the coating of the SMAW electrode. Each material has a function it performs, but not all the materials are present in each electrode. As an example, Titania is added because it helps stabilize the arc and keep it from going out after initiated. Wood pulp and limestone burn or decompose to make large volumes of gas which forces the oxygen rich atmosphere away from the weld thus protecting the hot weld metal. The slag forming elements form a tenacious slag over the hot weld metal protecting it from oxidation. The slipping agents make the material more pliable during the extrusion process and are mostly burned off or modified during rod baking. The binding agents help hold all the elements together until the rod is baked. And the alloying and deoxidizing agents are added for these particular reasons.
- #6 This is a more extensive list of materials used in SMAW Covered Electrodes.
- #7 This table lists several different electrodes and the specific elements present in the covering for these electrodes.
- #8 An additional advantage is the familiarity most welders have with the process. This process is usually the first one taught to welders. From a cost standpoint, initial investment in the process is low in comparison to other welding processes such as gas metal arc welding. SMAW’s flexibility is unprecedented in narrow access applications and, as the above photograph shows, even in underwater welding.
- #9 Shielded Metal Arc Welding (SMAW) is by far the most widely used arc welding process. It is very popular because of it’s many advantages. The equipment is relatively easy to use, inexpensive, and portable. The filler metal and means for protecting the weld puddle are provided by the covered electrode. It is a versatile process in that it can be used on carbon steels, low alloy steels, stainless steels, cast irons, copper, nickel, and aluminum.
- #10 Aspects of the SMAW process present disadvantages from a quality standpoint; these include a dependence on operator technique, as well as the starting and stopping of the arc to change electrodes. Slag entrapment and lack of fusion to the basemetal or previous passes can occur during welding as a result of improper torch manipulation by the welder. Improper cleaning can also cause slag inclusion defects. In addition, at each start and stop there is a possibility of porosity being formed since it takes some time for the slag to melt and form a protective gas over the molten weld pool.
- #11 SMAW has a low weld metal deposition rate compared to other processes. This is because each welding rod contains a finite amount of metal. As each electrode is used, welding must be stopped and a new rod inserted into the holder. A 12-inch electrode may be able to deposit a bead 6-8 inches long. The overall productivity of the process is impeded by: Frequent changing of electrodes, Interpass cleaning (grinding, brushing, etc.), Grinding of arc initiation points and stopping points, Slag inclusions which require removal of the defect and rewelding of the defective area.
- #12 There are other limitations of the SMAW process as well. The heat of the welding arc is too high for some lower melting metals. And the shielding of metals that react aggressively with the atmosphere is inadequate.