The principle purpose of the coating in SMAW is to keep air excluded from the molten weld metal. Atmospheric air consists primarily of Nitrogen (~80%) and Oxygen (~20%) with varying amounts of water and small percentages of other gases. The molten metal has a high affinity for nitrogen, oxygen, and hydrogen (from water). Absorbed nitrogen can result in embrittlement, oxygen can result in severe oxidation and inclusion of oxides, and hydrogen can result in embrittlement. When the SMAW coating “burns” it provides a shielding gas free of elemental oxygen and nitrogen. This shielding gas surrounds the arc and covers the molten the puddle thus excluding air from the arc and the molten puddle preventing nitrogen absorption and metal oxidation.
In SAW the granular flux used with the process provides the shielding of the molten pool and a coating on the wire would be superfluous. Further SAW is a continuous wire feed process that uses coiled wire. As you should be well aware, the coating used with SMAW does not take kindly to bending; cracking and flaking off the wire core. Thus SAW wire would have to be supplied in straight lengths if the coating used was the same as is that used with SMAW and this would be highly impractical.
In GTAW the inert gas (typically argon) that is used provides the require shielding – surrounding the arc and flooding the molten pool. The inert gas excludes the atmospheric air. Generally, the use of fluxed filler metals would provide no additional benefit and would result in weld discontinuities.
However, flux cored and flux coated filler metals are available for use with GTAW where gas backing of open root welds is not possible for stainless steels. These fluxed filler metals produce a slag covering on the back side of the weld that protects the backside/inside of such a weld which is not adequately shielded by the inert gas from the torch. Such filler metals are recommended for the root pass only and standard bare wire would be used for all subsequent passes. Weld discontinuities become a real problem if used for fill passes.
Hope this helps.
John
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of VIshal Sapkale
Sent: Thursday, October 28, 2010 1:21 AM
To: materials-welding@googlegroups.com
Subject: [MW:7900] About electrode
Dear all,
i hv simple que.
we used coated electrode for SMAW so why don't we use coated filler wire for SAW & GTAW
Regards,
VIshal
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