Welding of Stainless Steels

The Welding of Stainless Steels
by Pierre-Jean Cunat
Materials and Applications Series, Volume 3
T H E W E L D I N G O F S T A I N L E S S S T E E L S
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T H E W E L D I N G O F S T A I N L E S S S T E E L S
The Welding of Stainless Steels
Second Edition, 2007
(Materials and Applications Series, Volume 3)
© Euro Inox 2001, 2007
Publisher
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Author
Pierre-Jean Cunat, Joinville-le-Pont, France
Acknowledgement
The paragraphs marked (*) in the text
are extracted from “Working with Stainless Steels”,
Paris (SIRPE) 1998
Cover Photograph
ESAB AB, Göteborg (S)
Contents
1 General information on stainless steels 2
2 Stainless steel welding processes 3
3 Weldability of stainless steels 23
4 Selecting shielding gases for welding
of stainless steels 24
5 Selecting welding consumables for
welding of stainless steels 25
6 Joint preparation in arc welding 26
7 Finishing treatments for welds 28
8 Safe practices 30
9 Glossary: terms and definitions 32
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1
T H E W E L D I N G O F S T A I N L E S S S T E E L S
2
1 General Information on Stainless Steels
1.3 Austenitic - Ferritic Duplex
Stainless Steels:
Fe-Cr-Ni (Mo)-N
The microstructure of duplex stainless
steels consists of a mixture of austenite
and ferrite. They exhibit characteristics
of both phases with higher strength and
ductility. Nitrogen is added to provide
strength and also aids in weldability. They
are magnetic and non-hardenable by heat
treatment.
Range of compositions: C ≤ 0.03% -
21% ≤ Cr ≤ 26% - 3.5% ≤ Ni ≤ 8% -
(Mo ≤ 4.5%) - N ≤ 0.35%
1.4 Martensitic Stainless Steels:
Fe-Cr-(Mo-Ni-V)
These alloys can be heat treated to a wide
range of useful hardness levels.
The martensic structure obtained is magnetic.
Range of compositions: C ≤ 1.2% -
11.5% ≤ Cr ≤ 17% -
(Mo ≤ 1.8% - Ni ≤ 6% - V ≤ 0.2%)
1.1 Austenitic Stainless Steels:
Fe-Cr-Ni (Mo)
These alloys are the most popular grades
of stainless steels because of their excellent
formability, corrosion resistance, and
weldability. All are non-magnetic in the
annealed condition.
Range of compositions: C ≤ 0.10% -
16% ≤ Cr ≤ 28% - 3.5% ≤ Ni ≤ 32% -
(Mo ≤7%)
1.2 Ferritic Stainless Steels:
Fe-Cr-(Mo)
Ferritic stainless steels have a low carbon
content, with chromium (and molybdenum)
as major alloying elements.
They are non-hardenable by heat treatment
and are always magnetic.
Range of compositions: C ≤ 0.08% -
10.5% ≤ Cr ≤ 30% - (Mo ≤ 4.5%)
T H E W E L D I N G O F S T A I N L E S S S T E E L S
3
2 Stainless Steel Welding Processes
2.1 Electric Arc Processes
2.1.1 Processes with a Refractory
Metal Electrode
2.1.1.1 Gas Tungsten Arc Welding:
GTAW (*)
The GTAW process, also known as the TIG
(Tungsten Inert Gas) or WIG (Wolfram Inert
Gas) process, is illustrated in the above figure.
The energy necessary for melting the
metal is supplied by an electric arc struck
and maintained between a tungsten or
tungsten alloy electrode and the workpiece,
under an inert or slightly reducing
atmosphere. Stainless steels are always
welded in the DCEN (Direct Current
Electrode Negative) or DCSP (Direct Current
Straight Polarity) mode. In these conditions,
it is the workpiece that is struck by
the electrons, enhancing penetration, while
the electrode, which is generally made
from thoriated tungsten (2% ThO2), undergoes
very little wear. If a filler metal is
employed, it is in the form of either bare
rods or coiled wire for automatic welding.
The inert gas flow which protects the arc
zone from the ambient air, enables a very
stable arc to be maintained. Depending on
the base material, shielding gases consist
mainly of mixtures of argon (Ar), helium
(He) and hydrogen (H2) (see section 4
‘Selection of shielding gases for welding
stainless steel’).
70 à 90°
20°
Direction of travel
Shielding
gas inlet
Copper support
+
Backing gas
Filler metal
Arc
Shielding gas
Welding
power
supply
Torch
Ceramic nozzle
Refractory metal electrode
Principle of manual
gas tungsten-arc
welding
Welding
power
supply
Shielding gas
Shielding gas
Water
Water
Plasma-forming gas
Plasma-forming gas
Cathode
(thoriated
W)
Direction of travel
Weld pool
Plasma stream
Solidified weld Workpiece
H. F.
T H E W E L D I N G O F S T A I N L E S S S T E E L S
4
• an excellent metallurgical quality, with a
precise control of penetration and weld
shape in all positions;
• sound and pore-free welds
• very low electrode wear
• easy apprenticeship
The common workpiece thickness range is
0.5 mm to 3.5 / 4.0 mm.
The main advantages of this process when
used on stainless steels can be summarised
as follows:
• a concentrated heat source, leading to
a narrow fusion zone;
• a very stable arc and calm welding pool
of small size. Spatter is absent and
because no flux is required in the
process, oxidation residues are eliminated
so that any final cleaning operation
is very much simplified;
2.1.1.2 Plasma Arc Welding: PAW (*)
Plasma welding is similar to Gas Tungsten
Arc Welding (GTAW). The significant difference
is that the arc plasma is constricted by
a nozzle to produce a high-energy plasma
stream in which temperatures between
10 000 and 20 000°C are attained.
Welding processes generally employ a
‘transferred arc’ configuration, where the
constricted arc is formed between the electrode
and the workpiece, whereas other
applications more often use a ‘non-transferred’
constricted arc.
Principle of keyhole
plasma welding
T H E W E L D I N G O F S T A I N L E S S S T E E L S
5
Since the plasma jet is extremely narrow,
it cannot provide adequate protection for
the weld pool, so that it is necessary to
add a larger diameter annular stream of
shielding gas.
The gases used both for this purpose
and for forming the plasma are similar to
those employed in GTAW, namely pure
argon (Ar), Ar – hydrogen (H2) up to 20%,
Ar-helium (He) – H2. The hydrogen-containing
mixtures are recommended for welding
austenitic stainless steels, but like in the
case of GTAW, are be to proscribed for ferritic,
martensitic and duplex grades. For the
latter materials, it is recommended to add
nitrogen to maintain the appropriate proportions
of austenite and ferrite in the
weld (see section 4 ‘Selecting shielding
gases for welding stainless steels’).
In manual plasma welding, where the torch
is hand-held, the so-called ‘micro-plasma’
and ‘mini-plasma’ processes are employed
for currents between 0.1 and 15 amperes
and the ‘non-emergent jet’ technique for
currents between about 15 and 100
amperes.
In automatic welding, where the torch is
mounted on a carriage, the so-called ‘keyhole’
process is employed. By increasing
the welding current (above 100 amperes)
and plasma gas flow, a very powerful
plasma beam is created which can achieve
full penetration in the workpiece. During
welding, the hole progressively cuts through
the metal with the weld pool flowing in
behind to form the weld.
The major advantage of the PAW process
over GTAW is the remarkable stability of
the arc leading to:
• a ‘rigid’ arc which enables better control
of power input;
• greater tolerance to variations in
nozzle- workpiece distance, without
significant modification to the weld
morphology;
• a narrow heat-affected zone (HAZ) and
generally faster welding speeds;
• greater tolerance to faulty preparation,
particularly in the case of keyhole
welding.
The common workpiece thickness range is:
• 0.1 mm to 1.0 mm for micro-plasma and
mini-plasma processes
• 1.0 mm to 3.5 mm for the non-emergent
jet technique
• 3.5 mm to 10.0 mm for the keyhole
process (in a single pass).
T H E W E L D I N G O F S T A I N L E S S S T E E L S
6
2.1.2 Processes with a Fusible
Electrode
2.1.2.1 Gas Metal Arc Welding: GMAW (*)
In the GMAW process, also known as to
MIG (Metal Inert Gas) process, the welding
heat is produced by an arc struck between
a continuously fed metal wire electrode
and the workpiece.
Contrary to the GTAW and PAW processes,
the electrode is consumable, an arc being
struck between the fusible filler wire and
the workpiece under a shielding gas.
The essential characteristics of this process
are:
• the use of very high current densities in
the electrode wire (>90A/mm2), about
10 times higher than in the covered
electrode (SMAW) process;
• rapid melting of the electrode wire
(melting rate of about 8 m/min) due to
the high temperature of the arc, making
it necessary to use an automatic wire
feed system, supplied by 12 kg. spools;
• stainless steels are always welded in
the DCEP (Direct Current Electrode
Positive) or DCRP (Direct Current Reverse
Polarity) mode, the positive pole of the
generator being connected to the electrode;
• the welding torch is generally held
manually (so-called ‘semi-automatic’
process), but for high welding powers
it is fixed to a carriage (‘automatic’
process).
Welding
power
supply
Spool of filler wire
Shielding gas regulator
Shielding gas supply Direction of
travel
Workpiece
Nozzle
Contact tube
Torch
220/380V
Feed rolls
Control
console
Command cable
Current conductor
Metallic sheath
Solid electrode wire
Shielding gas inlet
Electrode feed unit including:
Wire feeder: wire drive motor
and feed rolls
Control console: Gas electrovalve,
Command relays and electronic controllers
Principle of gas metal
arc welding
T H E W E L D I N G O F S T A I N L E S S S T E E L S
7
The mechanism of metal transfer in the arc
is an important process parameter, three
principle modes being distinguished:
• The short-circuiting or dip transfer
mode, in which the metal melts to form
large droplets whose diameter is often
greater than that of the electrode wire.
As the droplet forms at the end of the
electrode, it makes contact with the
weld pool and creates a short circuit,
with a sudden increase in current. The
surface tension causes a pinching effect
which separates the droplet from
the electrode. The frequency of this
phenomenon is of the order of 20 to
100 Hz, corresponding to cycle times
between 0.01 and 0.05 seconds.
• The globular transfer or gravity transfer
mode. As in the previous case, melting
occurs in the form of large droplets,
which break away when their mass is
sufficient to overcome surface tension
forces and due to the greater arc
length, fall freely before coming into
contact with the weld pool.
• The spray transfer mode involves current
densities above a certain transition
level, of the order of 200 A /mm2.
The electrode melts to give a stream of
fine droplets. As the current density
increases further, the electrode tip
becomes conical and the stream of
even finer droplets is released axially.
GMAW requires a shielding gas to prevent
oxidation in the welding arc (see section 4
‘Selecting shielding gases for welding
stainless steels’). Argon with 2% oxygen
(O2) gives a stable arc and is suitable for
most applications. Argon with 3% carbon
dioxide (CO2) gives about the same result.
The welding speed and penetration can
sometimes be increased when helium (He)
and hydrogen (H2) are added to the argon
+ O2 or argon + CO2 shielding gas. Gases
higher in CO2 (MAG process) tend to produce
significant carbon pickup by the weld
pool together with chromium oxidation.
It is for this reason that they are not recommended.
The bead size and extent of penetration, will
vary according to the workpiece grade (ferritics,
austenitics, etc.), the type of joint, the
transfer mode and the skill of the welder.
For single V joints and square butt joints
welded in one run, the common workpiece
thickness range is 1.0 mm to 5.0 mm.
Note: The GMAW process is frequently referred to as
MIG welding. Confusion often arises between MIG and
MAG welding processes. In fact, in the MIG process,
the oxidising nature of the shielding gas (see section
‘Selecting gases for welding stainless steels’) is negligible,
whereas it is deliberately enhanced in the MAG
process. However, in the GMAW / MIG process, a low
percentage of oxygen (O2) or carbon dioxide (CO2) is
often needed in the shielding gas (argon) to improve
both arc stability and wetting by the molten metal.
Typical levels are 2% O2 or 3% CO2. Higher levels of
O2 or CO2 give excessive oxidation of chromium (Cr),
manganese (Mn) and silicon (Si) and excessive pickup
of carbon (C) in the weld pool. For example, the
carbon content (% C) in the weld metal, which is
0.025% for 2% CO2 containing shielding gas, could
reach 0.04% for 4% CO2.
T H E W E L D I N G O F S T A I N L E S S S T E E L S
8
Example of a flux-cored
electrode wire
2.1.2.2 Flux Cored Arc Welding: FCAW (*)
A variant of the GMAW process is the FCAW
(Flux Cored Arc Welding) process, in which
the electrode wire is composed of a stainless
steel sheath filled with a solid flux,
whose role is similar to that of the electrode
covering in the manual SMAW
process. The core provides deoxidizers and
slag forming materials and may provide
shielding gases in the case of self-shielded
FCAW electrodes.
The FCAW technique combines the advantages
of the SMAW method with the high
productivity of an automatic or semi-automatic
process due to the possibility of continuously
feeding the cored wire.
Compared to a conventional solid electrode,
the flux provides a slag cover and
enhances productivity.
Thus, for a current of about 200 amperes,
the deposition rate is about 100 g / min.
for a solid 1.6 mm diameter wire containing
20% Cr and 10% Ni, compared to about
170 g / min for a flux-cored wire of the
same diameter. This large difference is due
to the fact that in the flux-cored wire, only
the metal sheath conducts electricity, since
the core, composed of a mixture of mineral
and metal powders, possibly bound
in an alkali silicate, has a high electrical
resistivity.
Both FCAW and GMAW have similar bead
sizes. For single V joints and square butt
joints welded in one run, the common
workpiece thickness range is 1.0 mm to
5.0 mm.
Metal sheath Core = Powdered metal, flux
and slag forming materials
Core
Core
T H E W E L D I N G O F S T A I N L E S S S T E E L S
9
transfer of metal droplets, the effective
shielding of the weld pool and its wettability.
The metallurgical role involves chemical
exchanges between the weld pool and the
slag, i.e. refining of the weld metal.
The covering contains a certain amount of
calcium carbonate (CaCO3) which dissociates
in the arc at about 900°C, to form
CaO and CO2, the latter ensuring protection
of the arc zone. The following section gives
a short description of the most frequently
used covered-electrodes:
• Rutile (titania) electrodes: Slag formation
is the main shielding mechanism in
rutile based electrodes. Rutile electrodes
are easy to handle, ensure low
spatter and produce welds with smooth
Principle of the shielded
metal arc welding
process
2.1.2.3 Shielded Metal Arc Welding
(covered Electrode): SMAW (*)
Although the SMAW process, also known
as the MMA (Manual Metal Arc) process is
very old, since the first applications were
reported by Kjelberg in 1907, it remains
widely employed due to its great flexibility
and simplicity of use.
The electrode consists of a metal core covered
with a layer of flux. The core is usually
a solid stainless steel wire rod. The covering,
which plays on essential role in the
process, is extruded onto the core, and
gives each electrode its specific ‘personality’.
It serves three main functions: electrical,
physical and metallurgical. The electrical
function is related to initiation and
stabilisation of the arc, while the physical
action concerns the viscosity and surface
tension of the slag, which control the
Solidified slag
Mixing zone
Weld pool
Solidified weld
Covering = slag forming, flux
and gas forming materials
Molten metal droplets
Liquid slag
Direction of travel
covering
covering
Metal core
Workpiece
Covered electrode
T H E W E L D I N G O F S T A I N L E S S S T E E L S
10
surfaces. The slag formed during the
welding operation is easy to remove
• Basic (lime) electrodes: Limestone is
the main constituent of basic covered
electrodes due to its favourable arc-stabilizing
and metallurgical characteristics.
It also evolves carbon dioxide
which provides a gas shield. However, a
major disadvantage of limestone is its
high melting point. This is counteracted
by additions of fluorspar (CaF2) which
helps to lower the slag melting point.
Basic coverings will absorb moisture
if they are left in the open air for any
length of time, and special care should
be taken to keep the electrode dry.
2.1.2.4 Submerged Arc Welding: SAW (*)
The typical drying time is one hour at a
temperature of approximately 150°C to
250°C.
Rutile covered electrodes can be employed
in both the AC and DC modes whereas
basic (lime covered) electrodes are used
essentially in the DCEP mode.
The common workpiece thickness range is:
1.0 mm to 2.5 mm for single run
processes
3.0 mm to 10.0 mm for a multipass
technique
+
+ +
--
Direction of
travel
Weld pool
Solidified weld
Workpiece
Granular flux
Mixing zone
Welding
power
supply
Coil of
filler wire
Contact tube
Solidified slag
Feed rolls
Filler wire
Principle of the
submerged arc welding
process
T H E W E L D I N G O F S T A I N L E S S S T E E L S
11
In the SAW process the welding heat is
generated by the passage of a heavy electric
current between one or several continuous
wires and the workpiece under a
powdered flux which forms a protective
molten slag covering.
The process may be either fully or semiautomatic,
although in the case of stainless
steels most work is done with fully
automatic equipment.
In the automatic process, the welding current
can be very high, up to 2000 amperes
per wire, leading to a large power input
and consequently a heavy dilution of the
base metal by the filler material.
The process is suitable for butt and fillet
welding in the flat position and horizontal
– vertical fillet welding. The power supply
is generally of the DCEP reverse polarity
type, and more rarely AC, when several
wires are employed simultaneously in
order to avoid arc blow phenomena. For
both DC and AC generators, the electrode
wire pay out speed must be equal to the
melting rate in order to obtain a perfectly
stable arc. This is achieved by the use of
feed rolls commanded by a motor reducing
gear system with servo–controlled speed.
For welding stainless steels, a ‘lime/fluoride’
type flux is most widely used, its typical
composition being:
25% ≤ CaO + Mg O ≤ 40%, SiO2 ≤ 15%,
20% ≤ CaF2 ≤ 35%.
Two forms exist, produced either by melting
or bonding. Fused fluxes are produced
by heating to temperatures of the order of
1600 – 1700°C, and are converted to powder
form either by atomisation on leaving
the melting furnace, or by crushing and
screening the solidified bulk material.
Bonded fluxes are produced from raw
materials of appropriate grain size, bonded
together with an alkali silicate binder. The
mixture obtained is dried, then mechanically
treated, to obtain the desired final
particle size.
Only part of the flux is fused during welding
and the unfused material is picked up,
usually by a suction hose and returned to
a hopper for further use. The fused flux
solidifies behind the welding zone and on
cooling contracts and can be readily
detached.
For thicker material, welds are usually
made in one or two passes, i.e. a single
run on a manual backing weld, or one run
from either side of the plate, but a multipass
technique may also be employed. In
thinner material, welds can be made in one
run with the aid of a grooved backing strip.
Since the SAW process is used mainly for
thick austenitic stainless steel sheet, particular
care must be taken to avoid the formation
of sigma phase due to the use of
high welding energies. This is especially
the case for 25% Cr – 20% Ni alloys, but
also for 18% Cr – 9% Ni grades with high
ferrite contents. In multipass welding,
where the temperature range 650 – 900°C
is crossed several times, the risk of sigma
T H E W E L D I N G O F S T A I N L E S S S T E E L S
12
phase formation is increased. Subsequent
solution annealing at 1050°C is then highly
recommended.
In the as-delivered condition, the fluxes are
perfectly dry. In order to prevent moisture
pick-up, it is recommended to store the
flux at a temperature about 10°C higher
than that of the workshop, in an atmosphere
whose relative humidity does not
exceed 50%.
If moisture pick-up is feared or suspected,
it is useful to bake the powder at 300°C for
at least two hours.
The SAW process is generally used for joining
heavy workpieces in the thickness
range 10 – 80 mm, after the root run has
been completed using another welding
process. The bottom bead may also be
made with the aid of a grooved backing
strip.
2.1.2.5 Stud Welding: SW
Stud welding is a method of attaching a
metal stud to a workpiece generally in the
form of a sheet or a plate.
There are two distinct stud welding methods:
arc welding (ARC) and capacitor discharge
(CD).
1. Arc Stud Welding (ARC) involves the
same basic principles and metallurgical
aspects as any other arc welding procedure.
The stud is placed in contact with
the workpiece using a hand tool called
the stud gun, and an arc is struck which
melts the stud base and an area of the
workpiece. Before welding, a ceramic
ferrule is placed in position over the end
of the stud to shield the arc and to confine
the weld metal.
The stud is then forced into the weld
pool and held in place until the molten
metal solidifies and forms a homogeneous
joint. The cycle is completed in
less than a second, producing a full
strength joint. The expandable ferrule is
broken away to expose a smooth and
complete fillet at the stud base.
2. Capacitor discharge (CD) stud welding
involves the same basic principles and
metallurgical aspects as any other arc
welding procedure. When the weld gun
is activated, a special precision weld tip
initiates a controlled electric arc from
the welder capacitor bank which melts
the end of the stud and a portion of the
workpiece. The stud is held in place as
the molten metal solidifies, instantly
creating a high quality fusion weld.
Since the entire weld cycle is completed
in several milliseconds, welds can be
made to thin sheet without pronounced
distortion, burn-through or discoloration
and with small diameter fasteners
( 9 mm and less). CD welding also permits
stud welding of dissimilar metallic
alloys.
T H E W E L D I N G O F S T A I N L E S S S T E E L S
13
1
2
3
4
Stud and ceramic
ferrule against the
workpiece
Stud against the
workpiece
Stored energy
discharged and stud
moves downward
Stud forced into
molten metal
Stud rises and arc
struck
Control times out
and stud plunges
into molten steel
Metal solidifies
and weld is
completed in a
fraction of a second
Metal solidifies
and weld is
completed in a
fraction of a second
Arc stud welding Capacitor discharge stud welding
ARC or CD Process?
The arc process is generally used for stud
diameters of 6 mm and above and when
welding to thicker base materials or in
structural applications.
The CD technique is generally used for stud
diameters up to 9 mm and is employed primarily
when welding to thin sheet metal.
Stainless Steel Studs
Most stainless steels can be stud welded.
With the exception of free machining
grades, austenitic stainless steels studs are
most commonly used for stud welding.
Stainless steel studs are currently welded
to stainless steels and can also be welded
to mild steel. In this case, it is essential
that the carbon content of the base metal
does not exceed 0.20%.
T H E W E L D I N G O F S T A I N L E S S S T E E L S
14
Principle of resistance
spot welding
2.2 Resistance and Induction
Processes
2.2.1 Resistance Spot Welding:
RSW (*)
This process is still extensively used and is
particularly suited to the welding of thin
stainless steel sheets. Melting is induced
by resistance heating due to the passage
of an electric current through the workpiece
materials at the joint.
Five different stages are generally distinguished
in the spot welding process,
namely:
• Positioning of the sheets to be joined
• Lowering of the upper electrode and
application of the clamping force
• Welding with a low voltage alternating
current, producing a heat energy
W (joules) = R (ohms) x I2 (amperes) x
t (seconds)
• Holding of the clamping force or application
of an additional forging force
and finally
• Raising of the upper electrode before
proceeding to the next cycle.
With regard to the electrode materials, in
the case of stainless steels, the best combination
of low resistivity and high
mechanical strength is obtained with copper
– cobalt – beryllium alloys. The electrode
tips are generally in the form of a
truncated cone with an open angle of 120°.
Formation of the weld nugget depends on
the welding current and its duration and
on the clamping force applied by the electrodes.
R1
R2
R3
R4
R5
I
}
F F F'
F
F
Welding transformer
Sheets to be joined
Secondary Primary
Automatic
switch
Contact
resistance
Holding
F'= F
Welding Clamping Positioning Forging
F'> F
R4 et R5 : Resistances of the
workpiece sheets
(depend on their resistivities)
R2 : Contact resistance between
the 2 sheets (depends on
their surface conditions
and the clamping force F)
R1 et R3 : Contact resistances between
the electrodes and the sheets
W (joules) = R (ohms) x
I (amperes) x
t (seconds)
R = R1 + R2 + R3 + R4 + R5
2
T H E W E L D I N G O F S T A I N L E S S S T E E L S
15
The welding parameters recommended for
18% Cr – 9% Ni austenitic stainless steel
and stabilised 17% Cr ferritic grades are
indicated in the following table.
The parameters given in the above table
must be optimised to allow for the surface
condition (pickled, glazed, bright annealed,
polished), which has a strong influence on
the interface resistance, which in turn plays
a decisive role in nugget formation.
Contrary to other fusion welding processes,
in resistance spot welding, the melt pool
cannot be controlled visually. The only
defects perceptible to the eye are an
excessive electrode indentation and surface
spatter. However, a simple albeit
destructive inspection method is the socalled
‘peel test’, which gives a rapid indication
of the quality of the spot weld. In
this test, one of the welded sheets is
peeled off the other so that ‘buttons’ of
metal tend to be pulled from one or other
of the sheets.
Sheet Thickness Electrode Tip Electrode Welding Welding Time
(mm) Diameter Clamping Force Current (no. of Periods)
(mm) (daN) (A)
18% Cr – 9% Ni Austenitic Grades
0.5 3.0 170 3500 3
0.8 4.5 300 6000 4
2.0 6.0 650 11000 8
Stabilised 17% Cr Ferritic Grades
0.5 3.0 150 4000 3
0.8 4.5 250 7550 4
T H E W E L D I N G O F S T A I N L E S S S T E E L S
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Principle of resistance
seam welding
2.2.2 Resistance Seam Welding:
RSEW (*)
The principle of resistance seam welding is
similar to that of spot welding, except that
the process is continuous. The major difference
is in the type of electrodes, which
are two copper-alloy wheels equipped with
an appropriate drive system. The wheel
edges usually have either a double chamfer
or a convex profile. Compared to spot
welding, where the principal process
parameters are the welding current, the
heating time and the hold time, additional
factors to be considered in seam welding
are the use of a modulated or pulsed current
and the welding speed.
The welding parameters recommended for
Fe-Cr-Ni austenitic grades are indicated in
the following table.
Welding transformer
Primary
Secondary
Upper electrode wheel
Direction of travel
Workpieces
Lower electrode wheel
Upper electrode wheel
Workpieces
Lower electrode
Wheel
Discontinuous
seam welding
ContInuous
seam welding
(overlapping
weld nuggets)
Direction of travel
Sheet Wheel Clamping Welding Off Time Welding Welding
Thickness Thickness Force Time (periods) Current Speed
(mm) (mm) (daN) (periods) (Amp) (cm/min)
0.5 3.0 320 3 2 7900 140
0.8 4.5 460 3 3 10600 120
1.5 6.5 80 3 4 15000 100
2.0 8.0 1200 4 5 16700 95
3.0 9.5 1500 5 7 17000 95
T H E W E L D I N G O F S T A I N L E S S S T E E L S
17
Basic Projection Designs
In both spot and seam welding, the major
advantages of electrical resistance heating
are the limited modification of the
microstructure in the heat affected zones,
the virtual absence of surface oxidation if
the sheets are correctly cooled (by streaming
cold water) and the very small distortion
of the sheets after welding.
2.2.3 Projection Welding: PW (*)
In this process, small prepared projections
on one of the two workpiece surfaces are
melted and collapse when current is supplied
through flat copper-alloy electrodes.
The projections are formed by embossing
(sheet metal parts) or machining (solid
metal parts) usually on the thicker or higher
electrical conductivity workpiece of the
joint. The projections are shaped and positioned
to concentrate the current and a
large number of spot welds can be made
simultaneously. Lower currents and pressures
are used than for sport welding in
order to avoid collapse of the projections
before the opposite workpiece surface
has melted. The welding time is about the
same for single or multiple projections of
the same design.
Projection welding is especially useful for
producing several weld spots simultaneously
between two workpieces.
Various designs of weld fasteners are available
for annular projection welding applications;
e.g. shoulder-studs, bolts, pins,
nuts and pads.
e e
e e
D D
H H
e : thickness of the sections : 0.3 mm - 3.0 mm
H : projection height : 0.4 mm - 1.5 mm
D : projection diameter : 1.4 mm - 7.0 mm
T H E W E L D I N G O F S T A I N L E S S S T E E L S
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Principle of the
electroslag welding
process
2.2.4 Electroslag Welding: ESW
The electroslag welding process was developed
at the E.O. Paton Welding Institute
(Ukraine) in the early 1950 s.
Electroslag welding is a single pass
process used to produce butt joints in the
vertical position. Joints thicker than 15mm
(with no upper thickness limit) can be
welded in one pass, and a simple squareedged
joint preparation is required. The
process is similar to a vertical casting operation
since the molten weld metal is contained
by the two workpiece plates and by
a pair of cooled copper shoes.
Except during startup of the electroslag
operation, there is no arc. The continuously
fed electrodes are melted off by electrical
resistance heating as they pass through
a conductive molten slag layer (slag bath).
The slag bath also melts the adjacent
workpiece plate edges and shields the
molten metal from the atmosphere. The
temperature of the bath is of the order of
1900°C.
To start the electroslag process, a layer of
flux is placed in the bottom of the joint
and an arc is struck between the electrodes
and the starting block or starting pad to
provide a molten slag bath.
As welding proceeds, the copper shoes
and the wire feed unit are moved up the
joint at speeds of the order of 30 mm/min.
The metal deposition rate is about
350 g/min. The electrode wire compositions
normally match those of the base
metal. The most popular electrode sizes are
1.6 mm, 2.4 mm and 3.2 mm in diameter.
The metallurgical structure of electroslag
joints is unlike other fusion welds. The
slow cooling and solidification can lead to
a coarse grain structure. It is for this reason
that the process is recommended only
for austenitic grades.
+
–
Oscillation
Electrode
Molten slag
Weld pool
Water cooled copper shoe
Workpiece
Electrode guide and current
contact tube
Workpiece
Direction of travel
Water cooled copper shoe
Solidified weld metal
Surface of Solidified weld
Starting pad
T H E W E L D I N G O F S T A I N L E S S S T E E L S
19
2.2.5 Flash Welding: FW (*)
This technique is used essentially for long
products, e.g. rods, bars, tubes and
shaped sections. Although apparently similar
to upset welding, flash butt welding is
in fact quite different. Indeed, during upset
welding, it has been observed that when
the abutting surfaces are not in perfect
contact, the current passes only in a few
small areas, leading to intense local heating
and rapid melting, creating arcs which
violently expulse the molten metal out of
the joint due to the associated magnetic
fields (the flashing phenomenon).
The important process parameters are the
welding current and the voltage, which must
be sufficient to cause flashing, the instantaneous
flashing speed, which must be proportional
to the metal consumption and
compensated by the travel of the mobile
clamps, the duration of flashing and the
final upset forging stage. The roughness of
the initial faying surfaces must be such that
the contact points are sufficiently numerous
and well distributed to produce uniform
flashing across the whole of the joint area.
After upset forging, the joint profile should
show the characteristic three-finned profile
indicative of a successful welding operation.
The recommended welding parameters, as
a function of section area, are given for
austenitic grades in the following table.
Some typical applications are: wheel rims
(for bicycles) produced from flash welded
rings, rectangular frames (for windows and
doors),etc.
F F
Cracks Mobile
electrode
clamp
Fixed
electrode
clamp
Effect of welding parameters on the final weld profile Principle of flash welding
Thickness Section Area Initial Die Final Die Material Loss Flash Time
(mm) (mm2) Opening Opening (Flashing & (sec)
(mm) (mm) Forging) (mm)
2.0 40 13 5 8 2.2
5.0 570 25 7 18 6.0
10.0 1700 40 15 25 17.0
T H E W E L D I N G O F S T A I N L E S S S T E E L S
20
Principle of
HF induction welding
2.2.6 High Frequency Induction
Welding: HFIW (*)
High frequency induction welding is essentially
used for making tubes from strip. The
process is performed by a multiple roll
forming system. On leaving the last rolling
stand, the tube comprises a longitudinal
slit which is closed by welding. The joint is
formed by solid-solid contact, with intermediate
melting, as the strip edges are
brought together by a pair of horizontal
rolls (squeezing rolls).
Due to the skin effect, the induced HF current
(140 to 500 kHz) follows the path of
minimum impedance, concentrating the
heating at the edges.
In the case of ferritic stainless steels, this
high productivity process avoids the grain
coarsening phenomenon to which these
grades are susceptible.
In this case, welding powers between 150
and 300 kW are employed depending on
the tube diameter, the welding speed varying
with the machine from 50 to 90 m/min.
a
a
HF supply
Weld
Apex
Welding or squeesing rolls
Inductor
Impeder
Current flow lines
Tube
Impeder
(magnetic core)
Section a a
T H E W E L D I N G O F S T A I N L E S S S T E E L S
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2.3 Radiation Energy
Processes (*)
2.3.1 Laser Beam Welding: LBW
The laser effect (Light Amplification by
Stimulated Emission of Radiation) was discovered
in the optical wavelength range by
Maiman in 1958. The possibility immediately
appeared of using a laser beam as a
small area contact–free high intensity
power source for welding applications. The
continuous power levels available are particularly
high for carbon dioxide lasers,
although it must be remembered that the
effective welding power depends on the
reflectivity of the workpiece material for a
given incident wavelength.
The sources most widely used for welding
purposes are CO2 gas lasers and solid
state yttrium-aluminium garnet (YAG)
lasers. YAG lasers are preferred for welding
thin stainless steel sheets (< 1.5 mm) in the
pulsed mode. So-called CO2 lasers are better
adapted for the welding of thicker stainless
steel sheets or strips (1.5 to 6.0 mm).
As in high frequency induction welding
(HFIW), this process is widely used for the
production of longitudinally welded tubes.
With a power of about 6 kW, a 2 mm thick
strip of stabilised 17% Cr ferritic stainless
steel can be welded at a speed of about
7 m/min, and since the thermal cycle is
very brief, grain coarsening in the heat
affected zone is extremely limited.
Principle of CO2
(CO2 , N2 , He) laser used
for welding
Ar
Ontladingsbuis
Cooling tube
Gas inlet:
CO2, N2, He
Gas inlet:
CO2, N2, He
Plane or concave
mirror
Diameter
20 to
100 mm
Exciting electrode
Vacuum pump outlet
Perforated plane mirror
NaCl window
Argon
Shielding gas
(Argon)
Exciting electrode
Exciting
electrode
High voltage
supply
≅ 10 to 20kV
T H E W E L D I N G O F S T A I N L E S S S T E E L S
22
Principle of electron
beam welding
2.3.2 Electron Beam Welding: EBW
Electron beam welding uses energy from a
high velocity focussed beam of electrons
made to collide with the base material.
With high beam energy, a hole can be melted
through the material and penetrating
welds can be formed at speeds of the
order of 20 m/min.
EBW can produce deep and thin welds with
narrow heat affected zones. The depth to
width ratio is of the order of 20/1.
Welds are made in vacuum, which eliminates
contamination of the weld pool by
gases. The vacuum not only prevents weld
contamination but produces a stable
beam. The concentrated nature of the heat
source makes the process very suitable for
stainless steels. The available power can
be readily controlled and the same welding
machine can be applied to single pass
welding of stainless steel in thicknesses
from 0.5 mm to 40 mm.
+
+–
–
Wehnelt
DC Supply to filament (cathode)
DC Supply to
wehnelt 1 to 3 kV
High voltage DC
power supply
15 to 150 kV
Electromagnetic
focusing lens
Electromagnetic
deflection coil
Cross section of an electron
beam weld
Simplified representation of an electron
beam machine
Vacuum system
Workpiece
Carriage
Anode
Electron beam
T H E W E L D I N G O F S T A I N L E S S S T E E L S
23
3 Weldability of Stainless Steels
3.1 Austenitic Stainless Steels :
Fe-Cr-Ni (Mo)-(N)
➤ Structures containing a few percent of
ferrite (usual case)
• Insensitive to hot cracking
• Good resistance to intergranular corrosion
for low carbon and stabilised
grades
• Excellent toughness and ductility
• Embrittlement can occur after long
exposures between 550 and 900°C due
to decomposition of the ferrite to form
sigma phase
➤ Fully austenitic structures (exceptional)
• Sensitive to hot cracking during solidification
• Good resistance to intergranular corrosion
for low carbon and stabilised
grades
• Excellent toughness and ductility
3.2 Ferritic Stainless Steels :
Fe-Cr-(Mo-Ni-V)
➤ Semi ferritic grades: 0.04% C - 17% Cr
• Sensitive to embrittlement by grain
coarsening above 1150°C
• Poor toughness and ductility
• Sensitive to intergranular corrosion
• Post-weld heat treatment at about 800°C
restores the mechanical properties and
intergranular corrosion resistance
➤ Ferritic grades: 0.02% C – 17-30% Cr –
(Stabilised Ti, Nb)
• Sensitive to embrittlement by grain
coarsening above 1150°C
• Satisfactory ductility and improved
toughness compared to semi-ferritic
grades
• Generally insensitive to intergranular
corrosion
3.3 Austenitic – Ferritic Duplex
Stainless Steels :
Fe-Cr-Ni (Mo)-N
• Insensitive to hot cracking
• Excellent toughness and good ductility
in the range from –40% C to 275°C
• Sensitive to embrittlement by sigma
phase when exposed between 500 and
900°C
3.4 Martensitic Stainless Steels :
Fe-Cr-(Mo-Ni-V)
• Sensitive to cold cracking, depending
on the carbon and hydrogen contents
and residual stress levels, below about
400°C (preheating and post heating are
generally recommended)
• High tensile strength and hardeness
Good toughness, particularly for low
carbon grades
T H E W E L D I N G O F S T A I N L E S S S T E E L S
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4 Selecting Shielding Gases for Welding of
Stainless Steels (1)
4.1 Influence of the Shielding
Gas on: GTAW, PAW, GMAW,
FCAW and LBW
The choice of shielding gas has a significant
influence on the following factors:
• Shielding Efficiency
(Controlled shielding gas atmosphere)
• Metallurgy, Mechanical Properties
(Loss of alloying elements, pickup of
atmospheric gases)
• Corrosion Resistance
(Loss of alloying elements, pickup of
atmospheric gases, surface oxidation)
4.2 Selection of Welding Gas
• Weld Geometry
(Bead and penetration profiles)
• Surface Appearance
(Oxidation, spatters)
• Arc Stability and Ignition
• Metal Transfer (if any)
• Environment
(Emission of fumes and gases)
The interaction between the welding
process and the shielding gas has been
described in greater detail in § 2.
(1) Hydrogen–containing mixtures must not be used for welding ferritic, martensitic or duplex
stainless steels
(2) For welding nitrogen–containing austenitic and duplex stainless steels, nitrogen can be
added to the shielding gas
Welding Process Shielding Gas Backing Gas
Plasma Gas
Ar
Ar + H2 (tot 20 %) – (1) Ar
GTAW Ar + He (tot 70 %) N2 (2)
Ar + He + H2 (1) N2 + 10 % H2(1)
Ar + N2 (2)
PAW Idem GTAW Idem GTAW
98 % Ar + 2 % O2
97 % Ar + 3 % CO2
GMAW 95 % Ar + 3 % CO2 + 2 % H2(1) Idem GTAW
83 % Ar + 15 % He + 2 % CO2
69 % Ar + 30 % He + 1 % O2
90 % He + 7,5 % Ar + 2,5 % CO2
FCAW No No
97 % Ar + 3 % CO2 Idem GTAW
80 % Ar + 20 % CO2
LBW He Idem GTAW
Ar
Ar: argon; H2: hydrogen; He: helium; N2: nitrogen; CO2: carbon dioxide
T H E W E L D I N G O F S T A I N L E S S S T E E L S
25
(1) AISI: American Iron and Steel Institute
(2) Covered electrodes for manual metal arc welding of stainless and heat resisting steels. There
are two basic flux coverings: basic (B) or lime (direct current) and rutile ( R ) or titania (direct
or alternating current)
(3) Wire electrodes, wires and rods for arc welding of stainless and heat-resisting steels: G for
G.M.A.W., W for G.T.A.W., P for P.A.W. or S for S.A.W.
(4) Tubular cored electrodes for metal arc welding with or without a gas shield of stainless and
heat resisting steels
5 Suggested Welding Consumables for
Welding Stainless Steels
Base Material Welding Consumables
(1) EN 1600 EN 12072 EN 12073
Name Number Covered Wires and Rods Flux Cored
Electrodes Electrodes (3) (2) (4)
X5CrNi18-10 1.4301 304 E 19 9 G 19 9 L T 19 9 L
X2CrNi18-9 1.4307
304 L E 19 9 L G 19 9 L T 19 9 L
X2CrNi19-11 1.4306
X5CrNiTi18-10 1.4541 321 E 19 9 Nb G 19 9 Nb T 19 9 Nb
X5CrNiMo17-12-2 1.4401 316 E 19 12 2 G 19 12 3 L T 19 12 3 L
X2CrNiMo17-12-2 1.4404 316 L E 19 12 3 L G 19 12 3 L T 19 12 3 L
X6CrNiMoTi17-12-2 1.4571 316 Ti E 19 12 3 Nb G 19 12 3 Nb T 19 12 3 Nb
X2CrNiMo18-15-4 1.4438 317 L E 19 13 4 N L G 19 13 4 L T 13 13 4 N L
X10CrNi18-8 1.4310 301 E 19 9 G 19 9 L T 19 9 L
X2CrNiN18-7 1.4318 301 L E 19 9 L G 19 9 L T 19 9 L
X12CrNi23-13 1.4833 309 S E 22 12 G 22 12 H T 22 12 H
X8CrNi25-21 1.4845 310 S E 25 20 G 25 20 T 25 20
X25CrNiMo18-15-4 1.4438 317 L E 19 13 4 N L G 19 13 4 L T 13 13 4 N L
X2CrTi12 1.4512 409 E 19 9 L G 19 9 L T 13 Ti
X6Cr17 1.4016 430 E 17 or 19 9 L G 17 or 19 9 L T 17 or 19 9 L
X3CrTi17 1.4510 430 Ti / 439 E 23 12 L G 23 12 L T23 12 L
X2CrMoTi18-2 1.4521 444 E 19 12 3 L G 19 12 3 L T 19 12 3 L
X2CrTiNb18 1.4509 441 E 23 12 L G 23 12 L T 23 12 L
X6CrMo17-1 1.4113 434 E 19 12 3 L G 19 12 3 L T 19 12 3 Nb
X2CrNiN23-4 1.4362 – E 25 7 2 N L G 25 7 2 L T 22 9 3 N L
X2CrNiMoN22-5-3 1.4462 – E 25 7 2 N L G 25 7 2 L T 22 9 3 N L
X12Cr13 1.4006 410 E 13 or 19 9 L G 13 or 19 9 L T 13 or 19 9 L
X20Cr13 1.4021 – E 13 or 19 9 L G 13 or 19 9 L T 13 or 19 9 L
X30Cr13 1.4028 420 E 13 or 19 9 L G 13 or 19 9 L T13 or 19 9 L
T H E W E L D I N G O F S T A I N L E S S S T E E L S
26
6 Joint Preparation in Arc Welding
The principal basic types of joints used
in arc welding are the butt, lap, corner,
edge and T configurations. Selection of the
proper design for a particular application will
depend primarily on the following factors:
• The mechanical properties desired in
the weld
• The type of grade being welded
• The size, shape and appearance of the
assembly to be welded
• The cost of preparing the joint and
making the weld
No matter what type of joint is used, proper
cleaning of the workpieces prior to welding
is essential if welds of good appearance
and mechanical properties are to be
obtained. On small assemblies, manual
cleaning with a stainless steel wire brush,
stainless steel wool or a chemical solvent
is usually sufficient. For large assemblies or
for cleaning on a production basis, vapour
degreasing or tank cleaning may be more
economical. In any case, it is necessary to
completely remove all oxide, oil, grease,
dirt and other foreign matter from the
workpiece surfaces.
6.1 GTAW and PAW
The square-edge butt joint is the easiest to
prepare and can be welded with or without
filler metal depending on the thickness of
the two pieces being welded. Part positioning
for a square-edge butt joint should
always be tight enough to assure 100%
penetration. When welding light gauge
material without adding filler metal,
extreme care should be taken to avoid lack
of penetration or burn through.
The flange type butt joint should be used
in place of the square edge butt joint
where some reinforcement is desired. This
joint is practical only on relatively thin
material (1.5 to 2.0 mm).
The lap joint has the advantage of entirely
eliminating the need for edge preparation.
The only requirement for making a good
lap weld is that the sheets be in close contact
along the entire length of the joint to
be welded.
Corner joints are frequently used in the
fabrication of pans, boxes and all types of
containers. According to the thickness of
the base metal, filler metal may or may not
be required to provide adequate reinforcement
on all corner joints. Make sure that
the parts are in good contact along the
entire length of the seam.
All T joints require the addition of filler
metal to provide the necessary build up.
When 100 per cent penetration is required,
be sure that the intensity of the welding
current is adequate for the thickness of the
base material.
Edge joints are used solely on light gauge
material and require no filler metal addition.
The preparation is simple but this
configuration should not be used where
direct tensile loads are to be applied to the
finished joint, since this type of joint may
fail at the root under relatively low stresses.
T H E W E L D I N G O F S T A I N L E S S S T E E L S
27
6.2 GMAW
For GMAW welds, the root opening as well
as the V angles can frequently be reduced
from those normally employed in SMAW.
The amount of weld metal per unit length
can thus be reduced up to 30% by providing
designs which require less filler metal.
When designing GMAW welds for narrow
grooves, it is often necessary to employ a
high current density (spray transfer).
6.3 FCAW
In butt weld joints, the root openings and
V angles can be reduced, often enabling a
saving of the order of 40% in the amount
of filler metal used in the joint.
The optimum joint design will often be
determined by the ease with which slag
can be removed in multi-pass welds.
In fillet welding, smaller sizes can be
employed for the same strength. The deep
penetration capacity of flux cored wire
gives the same strength as the larger fillet
from a SMAW electrode which has low penetrating
power.
By comparison with SMAW electrodes,
FCAW wires offer significant cost savings in
a variety of ways, such as higher deposition
rates, narrower grooves and sometimes
two passes before stopping for slag
removal.
6.4 SAW
The groove openings are reduced compared
to those required by other arc
processes. The weld passes are heavier
than for SMAW electrodes. For open root
configurations, it is often desirable to provide
a flux backing held in place by a copper
chill bar or by a ceramic bar.
For all processes, bevelling is not required
for thicknesses of 3.0 mm and less, but
thicker base material should be bevelled to
form a “V”, “U” or “J” groove.
T H E W E L D I N G O F S T A I N L E S S S T E E L S
28
7 Finishing Treatments for Welds
The need for surface finishing applies primarily
to arc welds. Welds made by resistance
welding processes, with the exception
of flash butt welding, normally go into
service as welded, or after light cleaning.
On completion of an arc welding operation,
the weld area and surrounding base material
may be contaminated by weld spatter
and oxide films, depending upon the joint
type, material thickness and welding technique
employed.
For maximum corrosion resistance, careful
attention should be given to the finishing
operation in order to remove all surface
contaminants and irregularities that might
act as sites for corrosive attack in subsequent
service.
In certain applications, where considerations