Many options abound. With careful forethought regarding the gas delivery system, you will enhance your cost position without sacrificing the structural and metallurgical properties of GTA welded stainless steel.

Characteristics of Stainless Steel and Shielding Gases In selecting and designing a gas delivery system, it is important to understand which properties of stainless are critical in the welded condition. Stainless steel offers several benefits over mild steel, such as corrosion resistance in aggressive media due to a minimum of 12 to 26% by weight chromium (Cr) in iron (Fe), which forms a continuous layer of chromium oxide on the surface. This oxide layer provides a protective film for the base material, which maintains its serviceability in a variety of applications. However, in marine environments, the regenerating nature of the chromium oxide layer is hindered when the oxygen content is depleted in and around joint designs that create inherent crevices. Corrosion or "stain" resistance makes the 300 series stainless an excellent choice for food and petrochemical applications. Nickel concentrations of 3 to 22% by weight stabilize the austenite grain structure to provide good toughness characteristics at both elevated and cryogenic temperatures. Carbon (C), manganese (Mn), and nitrogen (N) also stabilize the austenite structure. Changes During Welding As stainless steel is welded, however, it undergoes melting and resolidification that leads to the possibility of other structures forming due to alloying elements. Depending on the weld cycle, ferrite may form due to the content of chromium, molybdenum (Mo), and niobium (Nb). Small amounts of ferrite in an austenitic stainless help prevent weld cracks. Fig. 5 (right) —The Shaeffler ferrite prediction diagram. It is very important to control the amount of ferrite in the weld because at cryogenic temperatures, ferrite loses its toughness. Secondly, at elevated temperatures, ferrite becomes brittle. There are two commonly used methods for predicting the resulting ferrite number (FN). The Shaeffler diagram (Fig. 5) and the Welding Research Council WRC–1992 diagram (Fig. 6) compare the sum weighted nickel equivalent to the weighted chromium equivalent in the resultant weld. Fig. 6 (left) —Ferrite prediction diagram (Welding Research Council, WRC–1992). The nickel equivalent represents the austenite-forming alloys on the vertical axis. An exception is that the WRC–1992 diagram accounts for nitrogen content. The chromium equivalent represents the ferrite-forming alloys on the horizontal axis. If the chemical composition is not known, a magnet or Severn gauge can be used based on the relationship between magnetic response and ferrite content —the higher the magnetic response the higher the ferrite content. Molybdenum, titanium (Ti), and niobium are also added to the basic iron-chromium-nickel alloy because they have a greater affinity to combine with carbon than does chromium. This in turn maintains the concentration of chromium available to replenish the oxide layer to prevent pitting in sulfur and other corrosive environments. Considerations for Weld Procedures Even though the austenitic 200 and 300 series stainless steels are considered the most weldable, variables such as joint design, service condition, material thickness, and the correct shielding gas must be considered to ensure the desired characteristics are retained. A successful weld procedure should also take into account the effects of the shielding gas ionization potential, thermal conductivity, reactivity, density, and total cost of ownership. GTAW Process and Its Shielding Gas Fig. 7 — Schematic of basic GTAW torch. Figure 7 illustrates the basic setup for GTA welding. The GTAW power supply is typically set up for direct current electrode negative (DCEN), meaning the negative terminal is connected to the torch and the positive to the workpiece. This establishes the current flow toward the part being welded. As the power supply is engaged, an electric arc is established with the workpiece. The shielding gas is then ionized to create a plasma of flowing electrons. The ionization or electron donating process occurs with a gas once sufficient energy in the form of electron volts is applied by the electric arc to remove the electron furthest away from the atom core. A gas with low ionization potential offers superior arc starts and arc stability. Figure 8 lists the ionization potential of several gases. Gases at the other end of the spectrum, such as helium, require a higher arc voltage to free the electrons and maintain the plasma, therefore offering a "hotter" arc. This increases the heat input for joining thicker and higher thermally conductive materials. A successful weld procedure should take into account the effects of shielding gas ionization potential, thermal conductivity, reactivity, density, and total cost of ownership. Gas Ionization Potential However, gas ionization potential and its contribution to heat input must be considered when welding the 300 series stainless because of the material's high coefficient of thermal expansion, which may cause part distortion. Argon offers smooth arc starting characteristics due to its low ionization potential. A mixture of argon and helium is used on thicker material because it offers the benefits of each gas. Fig. 8 ‹ Ionization potential of several gases (source: Mark Winter, Department of Chemistry, University of Sheffield, England). Once the arc is established, the plasma gas begins to distribute the heat radially toward the workpiece. Gases with a low thermal conductivity exhibit a narrow arc with a high inner core temperature that produces a deep funnel penetration profile. Figure 9 illustrates in ascending order the thermal conductivity of various gases. Gases at the other end of the spectrum conduct more heat radially, thereby producing a wider but shallower penetration profile. In the ionized state, gases have the potential to react with the molten weld pool chemically or via diffusion into the adjacent austenitic microstructure. Hydrogen, a diatomic molecule, is commonly mixed with argon in concentrations from zero to five percent to enhance weld characteristics. Hydrogen's reducing, electron-donating characteristic allows it to react with the cooling weld pool to minimize the amount of oxide formation. This creates a cleaner weld and improves wettability. Fig. 9 ‹ Thermal conductivity of various gases. (Source: Tsidoro Martinez, Department of Thermodynamics, Ciudad Universitaria, Madrid, Spain.) Maintain Weld Chemistry Careful control of the weld procedure must be taken to prevent cracking and embrittlement due to trapped hydrogen. Small concentrations of nitrogen may be added to argon in an effort to stabilize the austenite structure, but proper weld chemistry must be maintained to ensure the minimum concentration of ferrite formation to prevent cracking. Argon and helium are considered to be chemically inert and not reactive with the cooling weld pool. The effects of gas density can most easily be seen in the shielding gas flow rate required to prevent atmospheric contamination. Hydrogen and helium are both lighter than air and require flow rates two to three times higher than argon, which is heavier than air. It is because of this and the cost per cubic foot that industry has spent considerable development time in helium recovery and alternative shielding gas technologies.