The Evolution of Shielding Gas
By NATHAN MOYER
Reproduced with the permission of Welding Journal and the American Welding Society
The advent of gas-shielded arc welding processes can be traced back to the 1920s. However, because of limited research, these processes did not become commercially viable until the 1940s and 1950s. The basic push for research initially was World War II.
Over the last 50 years, the industrial gas industry has made significant contributions to the welding industry through the production and purification of different gases and gas mixtures. Today, there are still many hours of research being done on new gas blends and the effects they have on welding.
It started with the gas tungsten arc welding (GTAW) process. At the beginning of World War II, the aircraft industry looked for a better way to construct aircraft. The GTAW process, shielded with helium, was the result. This step got the ball rolling for more research and development on the gas metal arc welding (GMAW) and flux cored arc welding (FCAW) processes. Helium (He) and carbon dioxide (CO2) were the main options the welder had at the time due to limited gas manufacturing capabilities.
The fundamental purpose of shielding gas is to keep the nitrogen and oxygen in the atmosphere out of the molten weld pool as it cools. The shielding gas of choice during the development of the GMAW process was CO2. This gas was chosen as a result of analyzing the gas produced from the flux of a shielded metal arc welding (SMAW) electrode. Researchers found CO2 to be the predominate gas in the shielding surrounding the molten pool during the SMAW process. In a few cases, this gas is still the gas of choice for GMAW and FCAW. GTAW process needs were different from GMAW. This process needed a truly inert shielding gas, and helium was the only inert gas available at the time until argon (Ar) came along.
Argon Makes an Impact
Known in the industrial gas business as "the big A," argon is an inert gas. This means it does not oxidize and has no effect on the chemical composition of the weld metal. Pure argon cannot be used for GMAW of steels since the arc becomes too unstable. An oxidizing gas component is therefore used to stabilize the arc and to ensure a smooth metal transfer during welding. This oxidizing component may be either CO2, oxygen (O), or a combination of these gases. The amount of the oxidizing component added will depend on the steel type and application.
The electric arc in gas shielded arc welding can be divided into three parts: the arc plasma, the cathode area, and the anode area. In GMAW, when the electrode constitutes the positive (the anode), the cathode area is on the workpiece as one or more cathode spots. The oxidizing additive is necessary to stabilize these cathode spots; otherwise the arc will tend to flicker around on the surface of the workpiece, forming spatter and an irregular weld bead. There are often advantages in using only CO2 with argon. One is the slight improvement in weld geometry and appearance over Ar-O mixtures. This occurs because of differences in weld pool fluidity, surface tension, and oxides in the molten metal. With CO2 instead of O, there is also less oxidation and slag formation, which can have an effect on the appearance of the weld as well as the need for cleaning the weld. Another advantage is improved joint penetration, especially in the sidewall. This is mainly a factor of the higher arc voltage and energy employed when welding with CO2 in the mixture.
Throughout the industrial gas market, many different combinations of the Ar-CO2-O mixture are available. There are several goals of these three-part blends: reducing spatter, ability to weld thinner material, and a wider "sweet spot" window where the parameters can be set on a welding machine. Some argue these goals can all be achieved with Ar-CO2 and proper parameter settings, but some end users who swear by these three-part blends state they will never go back to using traditional Ar-CO2 mixes.
Establishing a Standard
Recently, with so many different mixes and so many different manufacturers of these mixes, the American Welding Society recognized a need to regulate the quality and consistency of shielding gases. Therefore, in 1997, AWS A5.32/A5.32M-97, Specification for Welding Shielding Gases, was produced. This established specifications for purity and moisture of raw components, i.e., argon, carbon dioxide, oxygen, and helium. The standard also established mix tolerances of components and methods for testing and recording these specifications. This provided an end user with a way to know what is being bought and assurance a purchase is what it claims to be.
Developments in Shielding Gases
The latest developments in shielding gas technology have included some new additions, namely, helium, hydrogen, nitrogen, and nitric oxide. Of course, as stated previously, helium has been used for many years in welding. However, it is now being used in some new applications. Helium, like argon, is an inert gas that can be used together with argon and a few percent of CO2 or O for gas metal arc welding of stainless steel. In its pure state, or mixed with argon, it is used as a shielding gas for GTA and GMA aluminum welding. Compared with argon, helium provides better side wall penetration and higher welding speeds by generating a more energy-rich arc. The process is more sensitive to arc length variations when helium is the shielding gas, and the arc is more difficult to strike in GTA welding. Helium and helium mixtures can be used as a root protection gas in installations where it is necessary for the gas to rise in order to force out trapped air. Helium rises because it has a lower density than air.
Hydrogen (H) can be added to shielding gases for GTA welding of austenitic stainless steels in order to reduce oxide formation. The addition also means more heat in the arc and a more constricted arc, which improves penetration. It also gives a smoother transition between weld bead and base metal. For root protection purposes, hydrogen addition is beneficial due to its reducing effect of oxygen. Nitrogen with 10% hydrogen is commonly used for root protection. It is not recommended for root protection of austenitic-ferritic (duplex) steels. Here, argon or high-purity nitrogen should be used.
Nitrogen (N) is used as an additive in shielding gases for GTA welding of super-austenitic and super-duplex stainless steels. These steels are alloyed with up to 0.5% nitrogen to increase mechanical properties and resistance to pitting. If the shielding gas contains a few percent of nitrogen, nitrogen losses in the weld metal can be prevented. As stated earlier, nitrogen with 10% hydrogen is a common root protection gas that delivers a good reducing effect. Pure nitrogen will further increase pitting resistance at the root side when welding super-austenitic and super-duplex stainless steels.
The addition of nitric oxide (NO) to shielding gases reduces ozone emissions in the welding zone. This technology was first developed by AGA Gas, Inc., in an effort to reduce the ozone in the welder's atmosphere. The name MISON was attached to this family of NO-containing gases produced by AGA Gas. Reduction of ozone can significantly enhance the quality of the welding environment and reduce the incidence of mucous irritation. There are also possible beneficial effects on concentration, productivity, and consistency in welding quality. When trials were done with this gas, it was discovered NO also served to stabilize the arc to good effect when welding high-alloyed stainless steels and aluminum.
Research continues to look for ways to reduce spatter, increase deposition rates, and improve weldability through shielding gases. We've come a long way in the past 50 years; just think what the next 50 years will bring.
NATHAN MOYER (nathan.moyer- @us.lindegas.com) is Regional Welding Specialist, AGA Gas Member of the Linde Gas Group, Cleveland, Ohio.