Nitrogen as a Backing Gas 1

[apiguy]

Hello all, another situation to solve.

We have a new Thermal Oxidizer being constructed. The vent header piping material is SA358 Type 254 SMO. The original WPS specified Argon as a backing gas. When I arrived to inspect the project the welders were using nitrogen as a backing gas. The heat-tint is dark blue on the root surface and HAZ approximately 1 1/4" wide into the base metal. Does anyone have any suggestions regarding the effects of the use of nitrogen on the corrosion resistance of 254 SMO material?

 

[Norphlet]

The Procedure handbook of Arc Welding by Lincoln Electric Company may answer some of your questions.

“The primary purpose of a shielding gas is to protect the molten weld metal from contamination by the oxygen and nitrogen in air. The factors, in addition to cost, that affect the suitability of a gas include the influence of the gas on the arcing and metal transfer characteristics during welding, weld penetration, width of fusion and surface shape, welding speed, and the tendency to undercut.”

“Among the inert gases – helium, argon, neon, krypton, and xenon – the only ones plentiful enough for practical use in welding are helium and argon. Although pure inert gases protect metal at any temperature from reaction with constituents of the air, controlled quantities of reactive gases mixed with inert gases improve the arc action and metal transfer characteristics. Oxygen, nitrogen, and carbon dioxide are reactive gases. Carbon dioxide can be used alone or mixed with one of the inert gases. Nitrogen is occasionally used alone, but is usually mixed with argon, as a shielding (backing) gas to weld copper. The most extensive use of nitrogen is in Europe, where helium is relatively unavailable.”

“The chemical behavior (for shielding gas) of carbon dioxide is oxidizing.”
“The chemical behavior of air is reaction.”
“The chemical behavior (for shielding gas) of nitrogen is reducing.”
“The chemical behavior (for shielding gas) of argon is inert.”

The molecular weight of Helium, Nitrogen, and Argon are 4.003, 28.013, and 39.948 respectively. The heavier a gas, the more effective it is for arc shielding. Argon is 30% heavier than nitrogen. This may relate to shielding gas (torch position) only and may not relate to backing gas.

In addition to chemical reactions, heat input (amperage, electrode or filler diameter, travel speed, layers) may have something to do with the blue color at HAZ.

 

[Norphlet]

"Metals and How to Weld them"
by
T. B. Jefferson
Publisher, Welding Engineer Publications, Inc.
Editor, The welding Encyclopedia
and
Gorham Woods
Metallurgist
The James F. Lincoln Arc Welding Foundation, 1962

"Metals and How to Weld them" has writings regarding to nitrogen as follows.

"Nitrogen is generally found combined with other elements as a nitrate. Nitrates usually have a detrimental effect. Nitrogen is useful, however, for the stabilization of austenite and for the lessening of grain growth in some stainless steels."

"Nitrogen is found in all steel and may be as high as 0.030% in Bessemer steel. In this percentage it has no noticeable effect on weld metal properties."

"Temperature, F Color"
400 Faint straw
440 Straw
475 Deep straw
520 Bronze
540 Peacock
590 Full blue
640 Light blue

 

[mcprofusion]

You indicated that the WPS stipulated the use of Argon as a backing gas. Then why are they using Nitrogen, if the WPS has not been changed.

The presence of Nitrogen in a weld can significantly affect its mechanical properties. For metals that neither dissolve not react with nitrogen, such as copper and nickel, nitrogen can be used as the shielding gas during welding. However, metals that either dissolve nitrogen or from nitrides (or both), such as Fe, Ti, Mn, and Cr, the protection of the weld metal from nitrogen should be considered.

In Fe, Nitrogen will form bubbles and results in the formation of gas porosity in the weld metal, during solification.

Nitrogen is also an austenite stablelizer in austenetic stainless steels.; the amount of ferrite in an austenetic stainless steel can be significantly reduced by increasing nitrogen content. The ferrite content significantly affects the solidification cracking of the weld metal.

Nitrogen can also form brittle nitrides that will act as ideal sites for crack initiation. Therefore, such nitrides can significantly reduce both ductility and impact toughness of the weld metal.

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I see this simple and clear...save yourself the aggravation and possible weld failures that may not even appear until much later "Why use a different gas for backing ?" You cant go wrong with Argon, use Argon. Even if i fully trusted the use of Nitrogen as my backing gas; why would i want to carry to bottles around ?
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