Low Voltage Short Circuiting-GMAW
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 purchase specifications.
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 welding.
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 of the ASME Boiler and Pressure Vessel Code for current requirements.