The handling of most molten metals in an air atmosphere requires protection from the detrimental effects of oxygen, nitrogen, and hydrogen (from water vapor or humidity), all of which can cause property degradation by adsorption or outright flaws like porosity or nonmetallic inclusions. The need for molten metal to be protected or shielded from such adverse interactions is as great for fusion welding as it is for bulk melt processing (e.g., steelmaking), only on a much smaller scale. Need for protection is most important during the transfer of molten metal from the tip of a consumable electrode or filler wire into the molten weld pool, because exposure occurs for a large surface area/volume ratio compared to the molten weld pool. Failure to provide suitable protection can result in unsound deposits due to the presence of porosity (primarily from nitrogen, but also from hydrogen from dissociated water), or less-than-optimal ductility and toughness due to gas-metal reactions that lead to embrittlement by dissociation or nonmetallic inclusion formation.
There are, in principle, two extreme approaches to protection that can be taken:
(1) Virtual total exclusion of air from the environment of the weld by employing a vacuum, and
(2) without any attempt at shielding, incorporation of a sufficient amount of elements whose affinity for oxygen and nitrogen (and, to a lesser extent, hydrogen”) is greater than the base metal.
In practice, the total exclusion of air from the welding environment is usually not possible, except at that time in the future that welding is performed in the infinite vacuum of space
Despite the use of shielding gases, whether generated within the process from ingredients in consumable electrodes (e.g., in SMAW, FCAW, SAW, or ESW) or supplied from an external source (e.g., in GTAW, PAW, GMAW, and EGW), or vacuum (e.g., for EBW), some residual oxygen and nitrogen inevitably is present in base metal@) and filler metal to be dissolved and diluted into the molten weld pool. For this reason, deoxidizing and denitriding elements (e.g., Mn, Si, Al, Ti, Zr, Ca, Mg and rare-earth metals, or REMs) are used in many processes, at least to some extent, depending on the process. For SMAW, FCAW, SAW, and ESW, use of deoxidizers/denitriders predominates over use of shielding gases, while for the inert gas-shielded processes, like GTAW, PAW, GMAW, and EGW, use of deoxidizers/denitriders is usually limited to small additions of Si, Mn, Al, or Ti, and so on. Such elements used in fillers, as well as in flux-bearing, slag-generating consumables (e.g., covered electrodes, flux-cored wires, granular SAW flux, or molten slag in ESW), are used to “kill” oxygen and nitrogen by tying it up as a mild compound floated away in the resulting slag.
In practice, a quantitative means for assessing the relative degree of protection versus killing in a particular process or welding consumable is the nitrogen scale. On this scale, residual nitrogen is used in this assessment. Reasons for the use of residual nitrogen in the weld metal as opposed to residual oxygen to assess shielding versus killing include the following:
First, residual oxygen in the weld metal can come from too many internal sources (such as oxide inclusions in the filler or base metal, oxides in the flux, or oxidizing shielding gases like CO, as opposed to just air), which is not the case for nitrogen.
Second, a substantial quantity of deoxidation products (oxides) always separate out of the weld metal before solidification, as opposed to nitrides, which tend to stay in the weld metal,” so residual oxygen gives no real indication of protection from air.
Third, the more oxides that form, the more they tend to agglomerate and float out due to their lower density compared to the weld metal, thereby leaving residual oxygen low even when exposure to oxygen could have been high.
Reference: Principles of welding, Robert Messler
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