22-11-2012, 02:21 PM
GAS METAL ARC WELDING OF STAINLESS STEEL
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Introduction
As you learn more about GMAW, it will
become apparent that this is a sophisticated
process. Welders who have used
“stick” welding (Shielded Metal Arc
Welding or SMAW) are sometimes of the
opinion that the GMAW process is simpler;
but to deposit a high quality bead
requires as much knowledge as, or more
than, with the SMAW process. The reason
for this is the number of variables that
affect the arc and the degree of control
the operator has over those variables.
The purpose of this manual is to make you
a better welder by increasing your knowledge
of how the GMAW process works.
A more knowledgeable welder can be
more productive by working smarter, not
harder. Figure 1 shows why your company
is interested in educating you in welding
stainless steel. Your labor and overhead
account for about 80% of the cost of
depositing weld metal. Any knowledge you
gain from this course not only helps you,
but also helps to make your company more
competitive in a very tough marketplace.
If you should have any questions in the
future that this manual or your supervisor
cannot answer, please feel free to have him
contact your Praxair regional engineering
staff for further assistance.
This training program was written to give
you a better understanding of the MIG
welding process. MIG is an acronym for
Metal Inert Gas, which is not technically
correct for stainless steels because shielding
gases for these materials contain
an active gas such as oxygen or carbon
dioxide. The correct term according to the
American Welding Society
Austenitic Stainless Steel –
Austenitic stainless steels contain from
16% to 26% chromium, up to 35% nickel,
and have very low carbon content. Some of
these steels are also alloyed with a minor
amount of molybdenum, columbium and
titanium. Austenitic stainless steels are
used where corrosion can be severe. They
are easily weldable. All alloys of this type
are non-magnetic due to their face centered
cubic structure at room temperature.
Austenitic grades include the 200 and 300
series of stainless steels. The 304 and 316
grades are very commonly used in welded
fabrications. Over 80% of today’s stainless
steel welding applications are done with
these types of grades.
Martensitic Stainless Steel –
Martensitic alloys contain from 12% to
17% chromium, up to 4% nickel and
.1% to 1.0% carbon. Some alloys will also
have minor additions of molybdenum,
vanadium, columbium, aluminum and
copper. These alloys are used where high
mechanical strength, hardness and corrosion
resistance are required. They are not
easily weldable.
How are Stainless Steels Formulated?
In order to understand how stainless steels
resist corrosion, let’s look at the basic
metallurgy of these materials. All metals
are crystals, meaning that the atoms are
arranged in an ordered matrix. An easy
way to visualize a metal is to think of
layers of balls with each ball in the layer
touching its four neighbors (see figure 2).
Copper
Copper can be used as a strengthening
agent in alloys that respond to precipitation
hardening. Upon cooling or the
application of some other heat treatment
cycle, copper forms a precipitate in the
matrix, making deformation more difficult
and increasing the strength of the material.
Manganese
Manganese is added in small amounts as a
deoxidizer, desulfurizer and strengthener.
Manganese additions to austenitic stainless
grades of steel also reduce the crack sensitivity
of the weld metal. Manganese reacts
with some of the available free oxygen to
form manganese oxide (MnO). It will also
combine with any free sulfur to form manganese
sulfide (MnS). Sulfur is detrimental
because it solidifies at low temperatures
and can locate at grain boundaries where
it dramatically reduces the strength of the
weld metal. After combining with oxygen
and sulfur, manganese, a weak carbide former,
will form manganese carbide (Mn3C)
which helps to strengthen the matrix.