Some simple steps can be used to achieve better quality gas metal arc welds
By Irving M. Rathwell
Welders often ask the question, "What is the difference between GMAW (gas metal arc welding) and MIG (metal inert gas)?" The term "metal inert gas" primarily means the shielding gas will not combine with other elements in the weld pool. The American Welding Society (AWS) has adopted the term "gas metal arc welding" because the shielding gases utilized now are not necessarily inert. In many cases, the components of the shielding gas cause a chemical reaction that promote a combination in the weld pool that will improve strength, penetration, fusion and weldability.
The reason "MIG" is still commonly used to describe the gas metal arc welding process is because most people find it much easier to say. The same holds true for gas tungsten arc welding. Even though the proper AWS term is GTAW, most people prefer to call the process "TIG." This is not correct, but it is easier to say.
Advantages such as the capability of welding all commercial metals and alloys, high welding speeds, all-position welding and ease in automation have combined to make GMAW a widely used welding process. However, the process also has limitations.
Primary problems that have arisen with the gas metal arc welding (GMAW) process have been incomplete fusion and/or penetration. These problems have sometimes made manufacturers nervous when the process has been listed by one of their vendors. Perhaps leading to this perception of an inherent difficulty is the American Welding Society's D1.1, Structural Welding Code ‹ Steel. D1.1 allows the use of prequalified welding procedures for almost every process, except with the short circuiting method of transfer with GMAW.
So how can the negatives of GMAW be eliminated, or at least reduced to a minimum? Several changes can greatly improve penetration and fusion, and one doesn't need to be a rocket scientist to bring them about. Often, only common sense is needed to achieve improved and more acceptable welds.
The first step to producing sound welds should be to take a look at what needs to be welded, material type and the end use of the welded product.
When welding heavier thicknesses of steel (3Ž8 in. and above) a good start is made with precleaning and preheating. Because GMAW is such a fast-fill, fast-freeze process, penetration can be a problem in the root pass when the steel is fairly cool. As the steel conducts heat away from the weld pool, the weld metal contracts or shrinks at a rapid rate, causing undue stress. The stressed steel is pulled in opposite directions, causing the weld to crack. A preheat temperature of about 150300°F should suffice on mild or medium carbon steel.
As with most processes, an increase in voltage can aid in fusion, but if the voltage is above the range for recommended amperage/voltage parameters, porosity and/or underbead cracking can occur.
Assume a 1Ž2-in.-thick A-36 steel in a beveled groove joint is being welded. The joint is to be back ground and welded on the second side. The plates have been properly beveled, fitted, tacked, cleaned and preheated. Assuming full penetration is desired, what type of welding wire and shielding gas should be used?
If this is to be a statically loaded connection that will undergo minimal stress, perhaps what is in order is an AWS ER70S-3 electrode with carbon dioxide (CO2) as the shielding gas. The CO2 shielding gas greatly aids penetration and is much less expensive than many other gases. This sounds great, but, remember, for every advantage there is usually a disadvantage. While we usually can count on CO2 to produce very strong welds, it can also produce less attractive welds. Spatter and a coarse-looking weld can be a problem if aesthetics are a factor in the completed weld. If the weld must not only be strong but also look good, a wiser selection may be AWS ER70S-6 with a shielding gas mixture of 75% argon and 25% carbon dioxide.
A mixture of ER70S-6 with 75% Ar and 25% CO2 is not only suitable for statically loaded structures but also serves quite well for dynamically loaded structures and, with the proper procedure, can even be used to weld martensitic steels. This combination of electrode and shielding gas will provide excellent low-hydrogen characteristics.
The Effects of Hydrogen
Many welders are familiar with the term "low hydrogen," but really have no understanding as to why low hydrogen is desirable. During World War II, many of the Navy's rapid-fire gun mounts came apart due to underbead cracking. The underbead cracking was primarily caused by extremely high temperatures during the welding process. The high temperatures initiate the release of hydrogen into the molten weld pool, especially with the electrode used at that time. This hydrogen gas would then be trapped in the weld heat-affected zone (HAZ). Entrapment of gas in the grain boundaries would eventually propagate into intergranular cracking under the weld. Ultimately, failure of the weld would be accelerated due to fatigue. The same problem holds true when welding martensitic steels such as 4130 or 4140.
Proper preheat, interpass temperature maintenance and postweld heat treatment, in conjunction with the proper electrode/shielding gas combination, can all but eliminate this problem. Heavy, argon-based shielding gases produce aesthetically pleasing welds with low spatter. The only drawbacks are the cost is higher and closer attention must be paid to the welder's technique to attain proper fusion and penetration.
Shielding Gas Pressure
No matter how well a joint is prepared, or how well the filler metal and shielding gases are matched to the material, no matter what the preheat and whether interpass temperature monitoring is utilized, a weld is doomed to failure if improper shielding gas pressures are used. Usually about 2535 ft3/h is sufficient. If gas pressure is too low, a lack of shielding will occur and the weld will be riddled with porosity.
If a breeze or windy conditions exist, shielding gas will be blown away. Increasing pressure will not always solve the problem. If shielding gas pressure is too high, the gas will become turbulent, drawing oxides and nitrides into the weld, causing porosity. The solution ‹ without changing processes ‹ is to set up wind barricades.
Another reason for a lack of shielding gas is spatter build-up, either in the nozzle or in the gas diffuser of the gun itself. Spatter buildup will prevent the gas from exiting the gun and providing shielding in the first place. If the shielding gas hose coming from the tank is cut or leaking from a loose connection, pressure is lost without necessarily registering as being low on the flow meter.
The biggest ‹ or at least most prevalent ‹ problem with gas metal arc welding is caused by novice or uninformed welders performing the welding operations. In most instances, the cure for GMAW-related welding problems is to train the welder.
While its true many variations exist for gas metal arc welding, keep in mind the basic variables hold true no matter which shielding gases, techniques and power supplies are used.
Irving M. Rathwell (firstname.lastname@example.org) is Technical Specialist, Materials Joining Institute, Lorain County Community College, Elyria, Ohio.