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SA-welding-tipsexplain what to do, when and why.SOLUTIONS with Effective, Powerful AdviceSA-welding-tips are useful information bits about Submerged Arc Welding (SAW), which is the process where the arc is concealed and shielded from air under a blanket of granulated flux that evolves a protective atmosphere and provides a cover of slag under which the weld bead cools down. The flux cover prevents fumes, sparks, spatter and radiation from coming out from the weld location, and therefore is less harsh an environment for operators than comparable processes with visible arcs of the same energy. The process, one more SA-welding-tip, is also protected against occasional air drafts, much more than gas shielded methods, so that it can be performed outdoors. Furthermore the flux can provide elements capable of controlling and improving the chemistry, by scavenging inclusions and oxides, and also of modifying the alloy with metallic additions if needed. Flux must be compatible with base and filler metal as the success of the operation is determined by mutual influences. SA-welding-tips should recognize that, because of these capabilities the selection of the proper flux for a given application may be more difficult than the choice of consumables for other processes because of subtle interactions that must be taken into account to obtain a weld metal of the correct quality.
A short note on Filler Metal for Submerged Arc Welding was published in issue 29 of Practical Welding Letter for January 2006. To read the note click on PWL#029. These SA-welding-tips are intended to help those readers not specifically informed about all the technicalities, in the proper selection and conduction of this process. First among the SA-welding-tips is a reminder of the main characteristics. The arc, providing the heat input needed for welding, is struck between a continuously supplied (through a feeder) bare metal wire or strip consumable electrode, and the work. The heat of the arc melts the electrode, some of the flux and a part of the base metal, providing a weld puddle to fill the joint. Other SA-welding-tips include the notion that it is mostly used for relatively thick, long, flat joints, in mechanical and semiautomatic operation, or for surfacing of large areas for proper corrosion or wear protection. The materials welded include carbon and low alloy steels, stainless steels and nickel alloys. It is not used for aluminum alloys, copper alloys and titanium.
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Power supplies provide direct current of the preferred polarity, or alternating current as suitable, at high amperage levels. Here are some additional important SA-welding-tips. Once the electrode diameter is selected, the parameters to set are the power source (if alternating or direct current and in this case also the polarity), the voltage, the electrode extension (the length of electrode between the contact tube and the arc), the current (directly in CC systems or as a function of wire feed speed in CV systems) and the travel speed. Type, width and depth of flux layer are also important factors. The current running in the circuit, as measured by an ampermeter, is determined by those selections. For establishing the correct electrode extension, the following SA-welding-tip should be remembered. The total current is split into two portions whose amounts are not known. One part of the total current preheats, by resistance heating (proportional to the resistance times the square of the current), the portion of wire called electrode extension, while the other part maintains the arc determining heat input and penetration. Regarding the characteristic curve (displaying the voltage at every current setting, or static volt-ampere curve) all power sources are described as having either a drooping shape (associated with constant current (CC) of either single or alternating polarity) or a flat shape (associated with constant voltage (CV) or potential). Please remark the following SA-welding-tips. With CV sources, after the required arc length is established by preselected voltage, any occasional arc length change (causing a corresponding change in voltage) is promptly recovered through a current change that melts more or less wire (electrode) to restore the original arc length. Direct current constant voltage (CV) power supplies are recommended for use with relatively thin steel at high welding speed, because constant voltage (CV) direct current provides smoother and more regular beads than alternating current. Alternating current, a further SA-welding-tip, is preferred at elevated currents (over 1000 Amperes) because it reduces the effect of magnetic arc blow (erratic arc) that becomes important with direct current sources. Constant current (CC) power supplies are not self regulating, they control the current level. The variable electrode feed speed is set by a voltage sensing device that measures the voltage across the arc (depending on arc length). The control then attempts to keep a constant arc length by varying the wire feed speed to restore the original arc length. In other words, all constant current machines require voltage sensors to drive a variable speed electrode feeder. These devices add to the equipment and maintenance costs, an important SA-welding-tip. Some direct current power sources can be switched between constant voltage and constant current selectable modes. Alternating current power supplies may be conventional welding transformers rated as constant current sources. Open circuit voltage should be in excess of 80 V to assure reignition of the arc at every polarity reversal. Certain types of power sources provide versatility of usage being suitable also for other welding processes. Newer solid state power supplies for alternating current provide square wave output for both current and voltage, solving the problem of arc extinction (as polarity alternates). These sources have constant voltage (CV)characteristics that permits the use of constant speed feeders, without voltage sensing control. Flux Classification SA-welding-tips While fluxes can be classified according to the methods of production (Fused or Bonded) more important is their action on the weld metal. Neutral fluxes do not change the composition of weld metal as produced by the fusion together of base metal and electrode. They contain little amounts of deoxidizers (manganese and silicon) or none at all. These fluxes are used on deoxidized, clean steel or for multiple passes on thick plates. Active fluxes contain certain amounts of deoxidizers (manganese and silicon) to help eliminating porosity and weld cracking in contaminated steel (by oxygen, nitrogen or sulfur). They are used for single pass welds. Mechanical properties may be affected by manganese and silicon content in the weld metal. Welding parameters, especially voltage, affect the flux to wire ratio by weight, one more SA-welding-tip, which in turn influences composition and properties. Because of the possible consequences, weld parameters must be strictly controlled when using active flux, much more than with neutral flux. A further classification uses the basicity index (BI) that attempts to estimate the resulting oxygen content that will be found in the weld metal. Basic fluxes tend to have lower oxygen content and better impact properties, especially in large multipass welds. Acid fluxes, preferred for single pass deposition because of easier welding, have higher tolerance for contamination that could produce porosity. A note on how to keep flux in place for root weld support in thin groove welds is available in the FAQ Page (Opens a new page). Selection of consumables should be based on the base metal and its condition (clean or contaminated), and on the requirements for the final weld. There is no AWS classification for fluxes or flux-electrode combination for stainless steels or for nickel alloys. It must be noted that consumable manufacturers offer many more products that are not yet officially classified under proprietary brands. The following SA-welding-tips give the Standards that specify Filler Materials for SAW: ANSI/AWS A5.17/A5.17M-97 AWS A5.23/A5.23M-97 ANSI/AWS A5.9-93 AWS A5.14/A5.14M-97 ISO 14171:2002 Parameter selection SA-welding-tips Welding Current affects penetration, dilution and weld deposition rate. Direct Current Reverse Polarity (Electrode Positive) is generally used. The principles that apply to heat partition between electrode and work were explained for other processes (See Tig Welding Tips) and are repeated here: In DCEP (Electrode Positive): 30% of the heat is concentrated in the workpiece 70% of the heat in concentrated in the electrode Shallow penetration Wide weld area In DCEN (Electrode Negative): 70% of the heat is concentrated in the workpiece 30% of the heat is concentrated in the electrode Some penetration and more buildup Narrow weld area It may be misleading to compare the overall performance of SAW in both polarities at the same current level, because of the different partition of the heat generated as explained above. One should remember that in order to sustain the higher current level in DCEP a larger size electrode might be required so that the comparison becomes even more difficult if not impossible. Straight Polarity (Electrode Negative) tends to give narrow beads with less penetration and more buildup therefore contributing to reduce base metal dilution, preferred sometimes, but not always, for surfacing applications. Straight Polarity (Electrode Negative) is also preferred for poor fitup or larger than optimum root gap, and at comparable current gives higher deposition rate. At comparable current level Reverse Polarity (Electrode Positive) will give lower weld deposition rate, wider beads with increased penetration. Electrode size affects current density. At a given current level smaller size electrode has higher current density, producing narrower beads at higher penetration than larger size wires. The smaller electrode will also permit higher deposition rate. In cases where it may be important to limit the heat input rate, this may be obtained by increasing the weld travel speed, by reducing current or both. Summing up, the most important of all SA-welding-tips is the following one. Because of complexity and interactions of all parameters involved there is no better method of determining their suitability for any given application than performing welding tests and examining visual aspect, shape, and dimensions of the weld bead in metallographic sections of test pieces.
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To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics. For anyone of the following subjects click on the underlined item Tig Welding Tips
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