Instructions / Assembly

44
www.lincolnelectric.com
GMAW
T
he low carbon content in ELC grades leaves more chromium
t
o provide resistance to intergranular corrosion.
S
tabilized Grades
(
321, 347, 348)
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
r
esistance. 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 affini-
ty 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
w
hen service temperatures reach the sensitizing range. They
are welded with the niobium stabilized electrodes, i.e., 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 processing equipment. Types 316 and 316L
are welded with AWS E316L-XX electrodes.
The 316L and 317L are ELC grades that must be welded with
ELC type electrodes to maintain resistance to carbide precipita-
tion. Both 317 and 317L are generally welded with E317 or
E317L electrodes respectively. They can be welded with AWS
E316-XX electrode, but the welds are slightly lower in
molybdenum content than the base metal with a corresponding
lower corrosion resistance.
When hot oxidizing acids are encountered in service, E316, E316L,
E317 or E317L welds may have poor corrosion resistance in the
as-welded condition. In such cases, E309 or E309Cb electrodes
may be better. As an alternative, the following heat treatment
will restore corrosion resistance to the weld:
1. For 316 or 317 – full anneal at 1950 - 2050°F (1066 - 1121°C).
2. For 316L and 317L – stress relieve at 1600
°F (871°C).
High Temperature Grades (302B, 304H, 309, 309S, 310, 310S)
These high alloy grades maintain strength at high temperatures
and have good scaling resistance. They are primarily used in
industrial equipment at high service temperatures – sometimes
over 2000°F (1093°C).
AWS E310-XX electrodes are needed to match the high temper-
ature properties and scaling resistance of grades 310 and 310S.
Both 302B and 309 grades are generally welded with E309-XX
electrodes. 304H is generally welded with E308H-XX electrodes.
E310-XX electrodes can be used on light plate. E310-XX welds
on heavy plate tend to be more crack sensitive than E309-XX
weld metals.
Free Machining Grades (303, 303Se)
Production welding of these grades
is not recommended
because the sulfur or selenium and phosphorus cause severe
porosity and hot short cracking.
I
f welding is necessary, special E312-XX or E309-XX electrodes
a
re recommended because their high ferrite reduces cracking
tendencies. Use techniques that reduce admixture of base
metal into the weld metal and produce convex bead shapes.
Hot Cracking
Hot cracking is caused by low melting materials such as metallic
compounds of sulfur and phosphorous which tend to penetrate
grain boundaries. When these compounds are present in the
w
eld or heat affected zone, they will penetrate grain boundaries
and cracks will appear as the weld cools and shrinkage stress
develops.
Hot cracking can be prevented by adjusting the composition of
the base material and filler material to obtain a microstructure
with a small amount of ferrite in the austenite matrix. The ferrite
provides ferrite-austenite grain boundaries which are able to
control the sulfur and phosphorous compounds so they do not
permit hot cracking. This problem could be avoided by reducing
the sulfur and phosphorus to very low amounts, but this would
increase significantly the cost of making the steel.
Normally, a ferrite level of 4 FN minimum is recommended to
avoid hot cracking. Ferrite is best determined by measurement
with a magnetic instrument calibrated to AWS A4.2 or ISO 8249.
It can also be estimated from the composition of the base
material and filler material with the use of any of several constitution
diagrams. The oldest of these is the 1948 Schaeffler Diagram.
The Cr equivalent (%Cr + %Mo + 1.5 x % Si + 0.5 x %Cb) is
plotted on the horizontal axis and the nickel equivalent
(%Ni + 30 x %C + 0.5 x %Mn) on the vertical axis. Despite long
use, the Schaeffler Diagram is now outdated because it does
not consider nitrogen effects and because it has not proven
possible to establish agreement among several measurers as to
the ferrite percent in a given weld metal.
An improvement on the Schaeffler Diagram is the 1973
WRC-DeLong Diagram, which can be used to estimate ferrite
level. The main differences are that the DeLong Diagram
includes nitrogen (N) in the Ni equivalent (%Ni + 30 x %C x 30 x
%N + 0.5 x %Mn) and shows Ferrite Numbers in addition to
“percent ferrite.” Ferrite Numbers at low levels may approximate
“percent ferrite.” The most recent diagram, the WRC-1992
Diagram,
Figure 33 on page 45, is considered to be the most
accurate predicting diagram at present. The WRC-1992
Diagram has replaced the WRC-DeLong Diagram in the ASME
Code with publication of the 1994-95 Winter Addendum. Its Ni
equivalent (%Ni + 35 x %C + 20 x %N + 0.25 Cu) and chromium
equivalent (%Cr + %Mo + 0.7 x %Cb) differ from those of
Schaeffler and WRC-DeLong.
Ferrite Number may be estimated by drawing a horizontal line
across the diagram from the nickel equivalent number and a
vertical line from the chromium equivalent number. The Ferrite
Number is indicated by the diagonal line which passes through
the intersection of the horizontal and vertical lines.