ABSTRACT. It is common for individual weld joints to be fabricated using a combination of electrode types and welding processes. While this situation arises most often as a result of repair welding, it also can arise due to scheduled fabrication sequencing, which requires a change from one electrode and/or process to another within the same weld joint. When weld metals deriving their properties from different metallurgical mechanisms are intermixed in the same joint, the resulting properties of the combination have caused some concern. This work is the first in a series that examines the intermixing of conventional carbon-manganese weld metals with various self-shielded flux cored arc weld metals. In this case, two different shielded metal arc weld metals are combined with various self-shielded flux cored arc weld metals. The effects of dilution from the underlying self-shielded flux cored root layers on the mechanical properties of shielded metal arc weld metal are examined. Variations in both tensile and Charpy V-notch impact properties have been documented. The effect on tensile results is limited to relatively minor changes in ductility. Reductions in Charpy V-notch impact energies were noted in all cases. The results are evaluated in terms of the chemical composition gradients and weld microstructure variations that result from dilution. Possible mechanisms are discussed. Introduction Welding is an integral part of most modern construction and manufacturing operations. On any given project, welding operations frequently involve a range of welding consumables and processes. It is often necessary to repair flaws that form as a result of service or defects created during manufacture. Repair welds are often made with welding consumables and processes different from those used in the original joint construction. For example, shielded metal arc welding (SMAW) repairs in submerged arc welding (SAW) deposits, gas metal arc welding (GMAW) deposits, gas-shielded flux cored arc welding (FCAW-G) deposits or selfshielded flux cored arc welding (FCAW-S) deposits are common. For many applications, the use of different welding consumables and processes in a single weld joint is planned in the normal fabrication sequence. Often, it is not practical or costeffective to fabricate a welded joint using a single consumable and process. For example, many line pipe welds are produced using SMAW for the root pass and FCAW-S for the fill passes. Fabrication of large components or structures often involves shop welding using GMAW and FCAW-G, followed by FCAW-S or SMAW in the field. The process of fitting and tack welding is often accomplished using SMAW, with the remainder of the structure welded using processes that achieve higher deposition rates. These are examples of applications in which intermixing of different weld metals can occur in a single weld joint. These examples indicate that, while not occurring in every case, intermixing does occur rather frequently as a normal part of the fabrication process. By contrast, most welding consumables are optimized without considering dilution effects from either the underlying base metal or a weld metal of different chemical composition. Welding consumable specifications standardize on the type of base material to be used for certification and conformance testing. Accordingly, electrodes are designed to produce weld metal mechanical properties through optimization of the alloy design and the use of slag/metal reactions to create the proper composition and microstructure. Depending on the anticipated service requirements and the operating characteristics desired, the alloy levels and slag systems are optimized in different ways. The introduction of elements through dilution/ intermixing at levels not originally intended can alter the weld metal properties and resulting weld performance. For example, when welding high carbon or microalloyed base metal, high concentrations of carbon or microalloying elements in the weld metal can occur through dilution. Similar dilution effects occur when one welding consumable is deposited over another of significantly different composition. Further dilution may also result in a loss of alloy elements in the weld, if the base material or underlying weld metal possesses a “leaner” composition than the welding consumable used to fabricate the balance of the weld joint. In either case, a shift in chemical composition away from optimum can occur. The effect of such unexpected variations in chemical composition can be undesirable changes in the mechanical properties of the weld metal. While a great deal of study has been devoted to dilution effects from base metals, only limited study has been conducted on the effects of intermixing weld metals deposited by different processes/electrode types. Most arc welding processes rely on a protective slag and/or a shielding gas to protect the weld metal from the atmosphere during welding. In this respect, the FCAW-S process is unique. FCAW-S consumables produce very little shielding gas and rely on the addition of large M. A. QUINTANA is with The Lincoln Electric Company, Cleveland, Ohio. M. Q. JOHNSON is with Edison Welding Institute, Columbus, Ohio. Paper presented at the AWS Annual Meeting, April 13–17, 1997, Los angeles, Calif. KEY WORDS Carbon-Manganese Weld Metal Charpy V-Notch Dilution Electrodes FCAW Multipass Welds Primary Metal Retransformed Metal SMAW amounts of deoxidizers (primarily aluminum) to react with oxygen and nitrogen from the atmosphere during metal transfer. FCAW-S welds typically contain between 0.8 and 1.6 wt-% aluminum. Since aluminum is a strong ferrite former, it can be balanced with the addition of an austenite stabilizer such as carbon, manganese or nickel to avoid primary solidification and stabilization of δ-ferrite at high temperatures. As a result, many FCAW-S weld deposits can contain substantially higher carbon (up to 0.45 wt-%), lower manganese (as low as 0.5 wt-%), lower oxygen (as low as 30 ppm) and significantly higher nitrogen (up to 700 ppm) than found in weld metals produced by other arc welding processes. Because of the unique chemical composition of FCAW-S weld metal, intermixing FCAW-S weld metal with conventional arc weld metals can produce unexpected results. Thus, fabrication or repair procedures that result in hybrid mixtures of different weld metal types should be carefully considered (Refs. 1, 2). A review of literature involving intermixing of FCAW-S welds and welds produced using other processes revealed only two studies. In the first study, Keeler and Garland (Ref. 3) report a large decrease in toughness in the root region when FCAW-S was deposited as a root pass and high toughness SAW was used for the fill passes. The average Charpy Vnotch (CVN) energy absorbed at –25°C (–13°F) was reduced from 100 J (74 ft-lb) when the root was deposited using a SMAW consumable to 35 J (26 ft-lb) when FCAW-S was used. Similarly, crack opening displacement (COD) results at –10°C (14°F) fell from an average of 0.71 mm (0.028 in.) for the SMAW root to an average of 0.14 mm (0.006 in.) for the FCAW-S root, with failure initiating in the intermixed region. Similarly, a second study (Ref. 4) showed that the –20°C (–4°F) CVN toughness of E7018 SMAW repairs made in E70T-4 weld metal ranged from 68 J (50 ft-lb) when only one pass (high dilution of SMA weld metal with E70T-4) was used to 83 J (61 ft-lb) when three SMA passes (lower dilution with E70T-4) were used to make a simulated repair. Experimental Approach Accordingly, a study was undertaken to determine
Posted on: Thu, 03 Oct 2013 04:20:19 +0000
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