Stainless steels or, more precisely, corrosion-resisting steels
are a family of iron-base alloys having excellent resistance
to corrosion. These steels do not rust and strongly resist
attack by a great many liquids, gases, and chemicals. Many of
the stainless steels have good low-temperature toughness and
ductility. Most of them exhibit good strength properties and
resistance to scaling at high temperatures. All stainless
steels contain iron as the main element and chromium in
amounts ranging from about 11% to 30%. Chromium provides the
basic corrosion resistance to stainless steels. There are
about 15 types of straight chromium stainless steels.
Nickel is added to certain of the stainless steels, which are
known as chromium-nickel stainless steel. The addition of
nickel reduces the thermal conductivity and decreases the
electrical conductivity. The chromium-nickel steels belong to
AISI/SAE 300 series of stainless steels. They are
nonmagnetic and have austenitic microstructure. These
stainless steels contain small amounts of carbon because this
element has tendency to make chromium carbides, which are not
corrosion resistant. Carbon is undesirable particularly in
the 18% chromium, 8% nickel group.
Manganese is added to some of the chromium-nickel alloys.
Usually these steels contain slightly less nickel since the
chromium-nickel-manganese alloys were developed originally to
conserve nickel. In these alloys, a small portion of nickel
is replaced by manganese, generally in a two-to-one
relationship. The AISI/SAE 200 series of stainless
steels are the chromium-nickel-manganese series. These steels
have an austenitic microstructure and they are nonmagnetic.
Molybdenum is also included in some stainless steel alloys.
Molybdenum is added to improve the creep resistance of the
steel at elevated temperatures. It will also increase
resistance to pitting and corrosion in many applications.
Stainless steels can be welded using several different
procedures such as shielded metal arc welding, gas tungsten
arc welding, and gas metal arc welding.
These steels are slightly more difficult to weld than mild
carbon steels. The physical properties of stainless steel are
different from mild steel and this makes it weld differently.
These differences are:
- Lower melting temperature,
- Lower coefficient of thermal conductivity,
- Higher coefficient of thermal expansion,
- Higher electrical resistance.
Austenitic Type. Manganese steels are not hardenable by
heat treatment and are nonmagnetic in the annealed condition.
They may become slightly magnetic when cold worked or welded.
This helps to identify this class of stainless steels. All of
the austenitic stainless steels are weldable with most of the
welding processes, with the exception of Type 303,
which contains high sulphur and Type 303Se, which
contains selenium to improve machinability.
The austenitic stainless steels have about 45% higher thermal
coefficient of expansion, higher electrical resistance, and
lower thermal conductivity than mild-carbon steels. High
travel speed welding is recommended, which will reduce heat
input and carbide precipitation, and minimize distortion.
The melting point of austenitic stainless steels is slightly
lower than melting point of mild-carbon steel. Because of
lower melting temperature and lower thermal conductivity,
welding current is usually lower. The higher thermal expansion
dictates that special precautions should be taken with regard
to warping and distortion. Tack welds should be twice as often
as normal. Any of the distortion reducing techniques such as
back-step welding, skip welding, and wandering sequence should
be used. On thin materials it is very difficult to completely
avoid buckling and distortion.
Ferritic Stainless Steels. The ferritic stainless
steels are not hardenable by heat treatment and are magnetic.
All of the ferritic types are considered weldable with the
majority of the welding processes except for the free
machining grade 430F, which contains high sulphur
content. The coefficient of thermal expansion is lower than
the austenitic types and is about the same as mild steel.
Welding processes that tend to increase carbon pickup are not
recommended. This would include the oxy-fuel gas process,
carbon arc process, and gas metal arc welding with CO2
shielding gas.
The lower chromium types show tendencies toward hardening with
a resulting martensitic type structure at grain boundaries of
the weld area. This lowers the ductility, toughness, and
corrosion resistance at the weld. For heavier sections preheat
of 200°C is beneficial. To restore full corrosion
resistance and improve ductility after welding, annealing at
760-820°C, followed by a water or air quench, is
recommended. Large grain size will still prevail, however,
and toughness may be impaired. Toughness can be improved only
by cold working the weld.
If heat treating after welding is not possible and service
demands impact resistance, an austenitic stainless steel
filler metal should be used. Otherwise, the filler metal is
selected to match the base metal.
Martensitic Stainless Steels. The martensitic stainless
steels are hardenable by heat treatment and are magnetic. The
low-carbon type can be welded without special precautions.
The types with over 0.15% carbon tend to be air hardenable
and, therefore, preheat and postheat of weldments are
required. A preheat temperature range of 230-290°C is
recommended. Postheating should immediately follow welding and
be in the range of 650-760°C, followed by slow cooling.
If preheat and postheat are not possible, an austenitic
stainless steel filler metal should be used. Type 416Se
is the free-machining composition and should not be welded.
Welding processes that tend to increase carbon pickup are not
recommended. Increased carbon content increases crack
sensitivity in the weld area.
Welding filler metals
The selection of the filler metal alloy for welding the
stainless steels is based on the composition of the stainless
steel. The various stainless steel filler metal alloys are
normally available as covered electrodes and as bare solid
wires. Recently flux-cored electrode wires have been
developed for welding stainless steels.
Filler metal alloy for welding the various stainless steel
base metals are: Cr-Ni-Mn (AISI No. 308);
Cr-Ni-Austenitic (AISI No. 309, 310, 316, 317, 347);
Cr-Martensitic (AISI No. 410, 430); Cr-Ferritic
(AISI No. 410, 430, 309, 502). It is possible to weld several
different stainless base metals with the same filler metal
alloy.
Welding procedures
For shielded metal arc welding, there are two basic
types of electrode coatings. These are the lime type indicated
by the suffix 15 and the titanium type designated by the
suffix 16. The lime type electrodes are used only with direct
current electrode positive (reverse polarity). The
titanium-coated electrode with the suffix 16 can be used with
alternating current and with direct current electrode
positive. Both coatings are of the low-hydrogen type and both
are used in all positions. However, the type 16 is smoother,
has more welder appeal, and operates better in the flat
position. The lime type electrodes are more crack resistant
and are slightly better for out-of-position welding. The
width of weaving should be limited to two-and-one-half (2,5)
times the diameter of the electrode core wire.
Covered electrodes for shielded metal arc welding must be
stored at normal room temperatures in dry area. These
electrode coatings, of low hydrogen type, are susceptible
to moisture pickup. Once the electrode box has been opened,
the electrodes should be kept in a dry box until used.
The gas tungsten arc welding process is widely used
for thinner sections of stainless steel. The 2% tungsten is
recommended and the electrode should be ground to a taper.
Argon is normally used for gas shielding; however,
argon-helium mixtures are sometimes used for automatic
applications.
The gas metal arc welding process is widely used for
thicker materials since it is a faster welding process. The
spray transfer mode is used for flat position welding and
this requires the use of argon for shielding with 2% or 5%
oxygen or special mixtures. The oxygen helps producing better
wetting action on the edges of the weld. The short-circuiting
transfer can also be used on thinner materials. In this case,
CO2 shielding or the 25% CO2 plus 75%
argon mixture is used. The argon-oxygen mixture can also be
used with small-diameter electrode wires. With extra
low-carbon electrode wires and CO2 shielding the
amount of carbon pickup will increase slightly. This should
be related to the service life of the weldment. If corrosion
resistance is a major factor, the CO2 gas or the
CO2-argon mixture should not be used.
For all welding operations, the weld area should be cleaned
and free from all foreign material, oil, paint, dirt, etc.
The welding arc should be as short as possible when using any
of the arc processes.
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