٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University STR654: INSPECTION, MAINTENANCE and REPAIR of STEEL STRUCTURES Lecture 3b Sherif A. Mourad Professor of Steel Structures and Bridges Faculty of Engineering, Cairo University WELDING PROCESSES Weldability of steel. Welding positions. Arc welding techniques. Other welding techniques. ١ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Weldability of steel Weldability is the capacity of a metal to be welded under the fabrication conditions imposed into a specific, suitably designed structure and to perform satisfactorily in the intended service. Weldability is improved by low-carbon, fine- grain size and restricted thickness. Weldability of steel ٢ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Welding Positions Flat position Weld metal can be deposited faster. Typically used for shop welds, where manipulating devices can be used to rotate the work in a flat position. Over head position Electrode diameters below 5 mm are to be utilized to avoid metal run down. Horizontal-vertical position. Welding Position ٣ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Arc Welding Shielded Metal Arc Welding Submerged Arc Welding Gas Metal Arc Welding Flux Cored Arc welding Gas Tungsten Arc Welding Arc Welding Plasma Arc Welding Electrogas Welding Electroslag Welding ٤ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Arc Welding Shielded Metal Arc Welding Shielded Metal arc Welding Process ٥ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Shielded Metal Arc Welding Definition: Heating Source is electrical arc between the consumable electrode and the work piece No Pressure is applied Shielded Metal Arc Welding Equipment: Welding power source. Electrode holder. Ground clamp. Welding cables & connectors. Accessories (chipping hammer, wire brush ,…) Protective equipment (helmet, glove, …) ٦ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Shielded Metal Arc Welding Shielding is provided by the electrode covering Mainly gas, very little slag→ small pool, all positions Mainly slag, very little gases → high amperage, high deposition rate, i.e. flat position Shielded Metal Arc Welding Process Advantages: The equipment is relatively simple, not expensive and portable The filler metal and the means of its protection as well as the weld metal from harmful oxidation during welding are provided by the covered electrode Auxiliary gas shielding or granular flux is not required ٧ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Shielded Metal Arc Welding The process is less sensitive to wind and draft than the other gas shielded arc welding processes It can be used in areas of limited access The process is suitable for most commonly used metals and alloys Shielded Metal Arc Welding Process Disadvantages: Low deposition rate because of the limited electrode length Slag removal required It could not be automated ٨ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Gas Metal Arc Welding Gas Metal Arc Welding Process Gas Metal Arc Welding Definition: It uses an arc between a continuous filler metal electrode and the weld pool Externally applied gas or flux cored into the wire or both for shielding Without the application of pressure ٩ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Gas Metal Arc Welding Advantages: The only consumable electrode process that welds all commercially available alloys No restrictions due to limited electrode length All positions High deposition rate Gas Metal Arc Welding High speed of welding Long weld bead Deeper penetration Minimal Post weld cleaning ١٠ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Gas Metal Arc Welding Thus the process is considered well suited for: High production and automated welding applications Robotics use Gas Metal Arc Welding Limitations: Complex welding equipment More difficult in hand - to - reach places Air drafted High level of radiated heat and arc intensity ١١ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Gas Metal Arc Welding Gas Metal arc Welding Terminology Gas Metal Arc Welding Welding ampere varies with wire feed speed (WFS) or melting rate (MR) as follows: WFS = aI + bLI 2 a > Constant dependent on polarity, composition of the electrode (for the electrode heating) b > Constant for electrical resistance heating L > Electrode extension or stick out I > Welding current ١٢ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Flux Cored Arc Welding Gas Shielded Flux Cored Arc Welding Flux Cored Arc Welding Self-Shielded Flux Cored Arc Welding ١٣ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Advantages of FCAW High-quality weld metal deposit. Excellent weld appearance-smooth, uniform welds. Excellent contour of horizontal fillet welds. Advantages of FCAW Many steels weldable over a wide thickness range. High operating factor - easily mechanized. High deposition rate - high current density. ١٤ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Advantages of FCAW Relatively high electrode deposit efficiency. Economical engineering joint designs. Visible arc - easy to use. Advantages of FCAW Less pre-cleaning required than GMAW. Reduced distortion over SMAW. Up to 4 times greater deposition rate than SMAW. ١٥ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Advantages of FCAW Use of self-shielded electrodes eliminates need for flux handling or gas apparatus, and is more tolerant to windy conditions present in outdoor construction (see disadvantage "6" below for gas shields). Advantages of FCAW Higher tolerance for contaminants that may cause weld cracking. Resistant to under-bead cracking. ١٦ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Limitations of FCAW FCAW is presently limited to welding ferrous metals and nickel base alloys. The process produces a slag covering which must be removed. Limitations of FCAW FCAW electrode wire is more expensive on a weight basis than solid electrode wires, except for some high alloy steels. ١٧ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Limitations of FCAW The equipment is more expensive and complex than that required for SMAW however, increased productivity usually compensates for this. The wire feeder and power source must be fairly close to the point of welding. Limitations of FCAW For the gas shielded version, the external shield may be adversely affected by breezes and drafts. Except in very high winds this is not a problem with self- shielded electrodes because the shield is generated at the end of the electrode, which is exactly where it is required. ١٨ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Limitations of FCAW Equipment is more complex than that for SMAW, so more maintenance is required. More smoke and fumes are generated (compared to GMAW and SAW). Gas Tungsten Arc welding Gas Tungsten Arc Welding Process ١٩ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Gas Tungsten Arc welding Advantages High quality welds. No spatter. With / Without filler. Root pass control. Gas Tungsten Arc welding Relatively inexpensive. Welding variables control. All Metals Independent control of heat source and filler metal addition. ٢٠ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Gas Tungsten Arc welding Limitations Low deposition rates. More welder coordination. Less economic. Gas Tungsten Arc welding Possible draft. Tungsten inclusions. Low tolerance for contamination. Coolant leakage ٢١ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Submerged Arc welding Submerged Arc Welding Process Submerged Arc welding Definition: Heating with an arc between a bare consumable metal electrode and the work piece. Arc and molten metal are submerged in a blanket of granular fusible flux on the work Pressure is not used Sometimes additional filler metal is obtained from a welding rode or metal granules ٢٢ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Submerged Arc welding Fused fluxes: Raw materials are powdered, dry mixed – melted in electric furnace - cooled - crushed and screened for size Submerged Arc welding Advantages: Good chemical homogeneity not hygnoscopic, easily recycled without chemical changes ٢٣ STR654 Lecture 3b ٢٠٢٠ Sherif A. Mourad - Faculty of Engineering, November Cairo University Submerged Arc welding Main disadvantage is when adding deoxidizer or Ferro-alloys during fabrication losses and segregation occurs Submerged Arc welding Bonded Fluxes: Raw materials are powdered, dry mixed and bonded with K or Na-Silicate. Then wet mix is palletized and backed at high temperature (lower than fused fluxes). Pellets are crushed and screened