Arc Machines 307 - Manual (Page 156)

Arc Machines, Inc. Model 307 Orbital Tube Welder Training
Document No. 740096 Chapter 16. Page 3.
Rev. A
Nitrogen
Nitrogen is not an inert gas. It is sometimes used as an I.D. purge gas but is
typically supplied in cylinders with levels of oxygen contamination unsuitable
for high-purity applications.
Heat tint/discoloration
Discoloration, or heat tint, resulting from oxygen and/or moisture in the purge
gas forms in colors ranging from a pale straw color to dark brown, or dark
brown and black (sugared), and from a pale gray or bluish haze to a deep in-
tense blue, with the deeper colors indicative of higher oxygen concentrations.
Oxidation (heat tint) is formed when elements and compounds in the base
metal diffuse or vaporize from the base metal in the heat of welding, form ox-
ides, and are deposited on the surface of the weld and heat-affected zone
(HAZ). Sometimes deposits, particularly of manganese which vaporizes at
lower temperatures than the other elements, are seen on the downstream side
of the weld only. The light blue color that results is sometimes referred to as
a "blue halo."
A direct relationship has been demonstrated between oxygen levels in the
purge gas, heat tint, and loss of corrosion resistance of stainless steel and
duplex materials after welding. Welds on reactive materials, such as titanium,
also show severe detrimental effects on mechanical strength resulting from
oxidation and benefits are obtained from improved purity of the purge gas and
effective shielding.
DISCOLORATION/HEAT TINT
Left: A semiconductor quality weld on electropolished 316L stainless steel. There is no
discernable discoloration of the weld or HAZ. Right: A badly oxidized or "sugared" weld
on 316L stainless steel. Even slight amounts of discoloration have been shown to have a
negative effect on the corrosion resistance of stainless steel.