Lincoln Electric Welder - Manual (Page 7)

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distance from the weld. A narrow
zone on each side of the weld
remains in the sensitizing
temperature range for sufficient time
for precipitation to occur. If used in
severely corrosive conditions, lines of
damaging corrosion appear
alongside each weld.
Control of Carbide Precipitation
The amount of carbide precipitation
is reduced by promoting rapid
cooling. Fortunately, the copper chill
bars, skip welding and other
techniques needed to control
distortion in sheet metal (see pg 34)
help reduce carbide precipitation.
Annealing the weldment at 1900°F
(1038°C) or higher, followed by water
quench, eliminates carbide
precipitation, but this is an expensive
and often impractical procedure.
Therefore, when weldments operate
in severe corrosive applications or
within the sensitizing temperature
range, either ELC or stablilized
grades are needed.
Another remedy is to use stabilized
stainless steel base metal and filler
materials which contain elements
that will react with carbon, leaving all
the chromium in solution to provide
corrosion resistance. Type 321 con-
tains titanium and Type 347 contains
niobium (columbium) and tantalum,
all of which are stronger carbide
formers than chromium.
ELC – Extra Low Carbon –
Grades (304L, 308L)
The 0.04% maximum carbon
content of ELC grades helps
eliminate damaging carbide
precipitation caused by welding.
These grades are most often used
for weldments which operate in
severe corrosive conditions at
temperatures under 800°F (427°C).
ELC steels are generally welded with
the ELC electrode, AWS E308L-XX.
Although the stabilized electrodes
AWS E347-XX produce welds of
equal resistance to carbide
precipitation and similar mechanical
properties, the ELC electrode welds
tend to be less crack sensitive on
heavy sections and have better low
temperature notch toughness.
The low carbon content in ELC
grades leaves more chromium to
provide resistance to intergranular
corrosion.
Stabilized Grades (321, 347, 348)
Stabilized grades contain small
amounts of titanium (321), niobium
(columbium) (347), or a combination
of niobium and tantalum (347, 348).
These elements have a stronger
affinity for carbon then does
chromium, so they combine with the
carbon leaving the chromium to
provide corrosion resistance.
These grades are most often used in
severe corrosive conditions when
service temperatures reach the
sensitizing range. They are welded
with the niobium stabilized
electrodes, AWS E347-XX.
Type 321 electrodes are not
generally made because titanium is
lost in the arc. AWS E347-XX is
usually quite satisfactory for joining
type 321 base metal.
Molybdenum Grades
(316, 316L, 317, 317L, D319)
Molybdenum in stainless steel
increases the localized corrosion
resistance to many chemicals. These
steels are particularly effective in
combatting pitting corrosion. Their
most frequent use is in industrial
FIGURE 1