Summary: The following article is a part of National Board
Classic Series and it was published in the National Board BULLETIN. (3 pages)
The National Board has been frequently asked to give some guidance to gas metal
arc welding (GMAW or commonly called MIG) when welding in the low voltage short
circuiting (GMAW-S) mode.
GMAW-S is normally a solid wire (ASME SFA-5.18) gas shielded welding type
process which uses semiautomatic or automatic equipment. It is similar to the
spray arc or globular arc transfer GMAW processes, the flux cored arc welding
(FCAW) process, and is closely related to the submerged arc welding (SAW)
process. Although, the GMAW-S has unique features.
All of the related processes are very high energy processes which transfer weld
metal across a continuous electric arc and generally carry a large molten weld
puddle. GMAW-S is a low energy process which also generates its heat from an
electric arc but the weld metal is transferred only partially across an arc and
partially when the filler metal touches the base metal and the arc is short
circuited. This short circuiting occurs 20 to 200 times per second which
results in a small molten weld puddle.
When the arc is short circuited, the molten weld puddle is able to freeze more
quickly than when welding with a continuous arc. This gives GMAW-S a unique
ability to weld out of position, to weld thin base metals and to weld open butt
root passes without backing and without "blowing through." One bad
characteristic, however, is that the quick freeze puddle has a tendency to
"cold lap" when not carefully deposited with the correct technique by a skilled
welder. This bad characteristic is often why the process is written out of
Because of the same bad characteristic, ASME Section IX applied special
variables when the GMAW-S process is used. When qualifying a welding procedure,
the variable QW-403.10 limits the base metal thickness qualified to a maximum
of 1.1 times the test coupon thickness for procedure qualification test coupons
less than l/2 inch thick. The variable QW-404.32 also limits the deposited weld
metal thickness range to a maximum of 1.1 times the deposited thickness of the
procedure qualification test coupon for deposits less than 1/2 inch thick.
The welding technique employed when welding the GMAW-S process is also unique.
There are techniques for groove root pass and for fillet welding that will
produce soundly deposited welds. But the technique is so different from the
spray, globular or pulsed modes of transfer, that Section IX applies a special
variable for the GMAW-S process for welder performance qualification. When
qualifying a welder, the variable QW-409.2 becomes an essential variable which
requires requalification of the welder if he changes from spray arc, globular
arc or pulsating arc to the low voltage short circuiting arc, or vice versa.
The variable QW-404.32 also limits the deposited weld metal thickness for
performance qualification as noted above.
The quick freeze puddle which has a tendency to cause cold lap also affects the
performance qualification testing requirements as outlined in Section IX,
QW-304. This variable alternatively allows welders to be qualified by
radiographic examination in lieu of the mechanical tests prescribed in QW-302
for some few processes. A welder may be qualified by radiographic examination
when welding with most modes of the GMAW process, but when qualifying with the
GMAW process using the short circuiting arc mode, performance qualification by
radiography is not permitted. The reason for this is that cold laps are such
tight defects that they are very difficult to detect by radiography.
The ASME Code has essential variables directly related to the GMAW-S mode of
welding, but has no clear definition of "What is GMAW-S?" Industry can easily
determine when they are in the GMAW-S mode by checking the electrical arc
characteristics on an oscilloscope. This instrument will clearly indicate when
the arc is short circuiting. But the average Authorized Inspector and small
shop does not have an oscilloscope, so some characteristics for which to look
are presented in the following paragraphs.
The GMAW process can change from the various arc modes to the short circuiting
mode and vice versa by a change in amperage, voltage, shielding gas, filler
wire diameter or any combination of these factors. The GMAW-S process is
usually characterized by fine wire, that is, the 0.030 inch through 0.045 inch
diameter filler metal. But GMAW-S has been used in production on larger
diameter filler metals. The process is commonly used for welding of carbon, low
alloy and stainless steels, but a wide variety of other metals can be welded
using GMAW-S. The GMAW-S process is usually shielded with carbon/dioxide (CO2
) or a mixture of argon and carbon/dioxide. Unfortunately, globular arc and
spray arc may also use similar shielding gas. The high percentage argon,
argon-oxygen, helium and their mixtures are used with the spray arc mode and
not the short circuiting arc mode.
The voltage range used is generally the best guide to GMAW-S. A low voltage is
one of the reasons that GMAW-S does short circuit. If the shielding gas is CO2
or an argon CO2
mixture, the filler metal diameter is 1/16 inch diameter or less and the
voltage is 22 volts or less, the GMAW process is most likely in the short
circuiting mode. The short circuiting mode is actually one wherein the short
circuiting mode and globular mode are both occurring at random. As the voltage
increases from 17 volts to 22 volts the arc short circuits less frequently, and
the arc increases its globular transfer. Voltages above 25 usually indicate a
true globular arc or spray arc mode.
Some characteristics to look for are the ability of the GMAW-S process to weld
"out-of-position" and to weld open root groove welding without the use of
backing. The pulsed arc mode is easily distinguished from GMAW-S. The spray arc
or globular arc generally are not used out-of-position and are not used for
open root groove welding without the use of backing.
The process was originated as a sheet metal joining process which did not
require skilled welders to operate. This theory was not true. A well-trained
skilled welder is required to properly deposit sound weld metal with the GMAW-S
process. The use of the process has expanded to plate applications 1/2: inch
thick and greater in both groove and fillet weld applications. It is used to
handle out-of-position welding, root pass welding without backing, root pass
welding to keep the inside clean and flux free, and for clean smooth tack
In the final analysis, it is the ASME Code user who is responsible for
determining the mode of arc transfer being applied to Code welds. The Code user
must apply the Code variables and restrictions when using the GMAW-S process
and must document the variables, including the mode of arc transfer on the WPS,
PQR and WPQ forms.
This explanation of the ASME Code variables is the opinion of the National
Board and not an official ASME Code definition. The National Board does,
however, encourage those with expertise in this area to work with the
appropriate ASME Code committee to help clarify and define this process.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.