Instructions / Assembly

43
GMAW
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Sensitization
Two problems are associated with welds in the austenitic stain-
less steels: 1) sensitization of the weld heat affected zone, and
2) hot cracking of weld metal.
Sensitization leads to intergranular corrosion in the heat affected
zone as shown in Figure 32. Sensitization is caused by chromium
c
arbide formation and precipitation at grain boundaries in the
heat affected zone when heated in the 800 - 1600°F
(427 - 871°C) temperature range. Since most carbon is found
near grain boundaries, chromium carbide formation removes
some chromium from solution near the grain boundaries, there-
b
y reducing the corrosion resistance of these local areas. This
problem can be remedied by using low carbon base material
and filler material to reduce the amount of carbon available to
combine with chromium. Welds should be made without
preheat and with minimum heat input to shorten the time in the
sensitization temperature range.
The degree of carbide precipitation increases with:
1
. Higher carbon content (for example, because 301 and 302
grades have a maximum carbon content of 0.15% they are
more susceptible to carbon precipitation than grade 304 which
has a maximum carbon content of only 0.08%).
2. Time at the critical mid-range temperatures – a few seconds
at 1200°F (649°C) can do more damage than several minutes at
8
50°F (454°C) or 1450°F (788°C).
Welding naturally produces a temperature gradient in the steel.
I
t ranges from melting temperature at the weld to room
temperature some 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
s
everely corrosive conditions, lines of damaging corrosion
appear alongside each weld.
FIGURE 32: Intergranular Corrosion in the Heat Affected Zone
Control of Carbide Precipitation
The amount of carbide precipitation is reduced by promoting
rapid cooling. Fortunately, copper chill bars, skip welding and
other techniques needed to control distortion in sheet metal
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 tempera-
ture range, either ELC (extra low carbon) or stabilized grades are
needed.
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 ELC electrode, e.g., 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.