Selecting a shielding gas for joining stainless steel
The right choice is critical for success
The use of stainless steel has grown significantly over the past decade in North America. As its use has grown, so has the number of different consumables required to join it. Because stainless steels are used in applications that are different from carbon steels, it follows that their welding requirements differ also.
The two major alloy types within this materials class are the austenitic (300-series chromium and nickel alloys) and ferritic (400-series chromium only) grades. Each has unique properties—and welding requirements.
The shielding gas used with gas metal arc welding (GMAW), flux-cored arc welding (FCAW), or gas tungsten arc welding (GTAW) can have a significant impact on the weld properties and quality, as well as on the productivity of the welding operation. A shielding gas's many functions range from protecting the molten weld pool from the atmosphere to controlling the type of metal transfer and arc stability obtained during welding. To make the right shielding gas choice, you need to understand if it will give you the welding results you need.
In addition to a specific weld chemistry and soundness at acceptable levels of productivity, weld color and shape (or bead appearance) are important considerations in successful joining of austenitic alloys.
Gas blends used to join stainless steels frequently contain argon because of its inert nature, its ability to support easy arc starting, and its spray-type metal transfer method. For some welding processes, helium may be added to conduct more heat to the base metal to increase weld penetration and improve the weld puddle fluidity. This can lead to higher travel speeds, less distortion, and an overall reduction in weld cost.
In GMAW and FCAW, oxygen or CO2 is added to the shielding gas to improve arc stability and weld puddle fluidity. In special cases, nitrogen or hydrogen may be added in controlled amounts to refine weld properties and improve bead appearance in austenitic stainless steel. These three-part blends are the best choice for some stainless steel joining applications.
To select a shielding gas, you need to determine the materials to be joined, the welding process, and the filler metal composition needed. Then look at the available shielding gas blends to determine the best fit for your final product requirements.
GMAW of Austenitic Stainless Steels
Two-part Blends. The two-part blends fabricators of common stainless steels use traditionally are mixtures of argon with either oxygen or CO2. They are suitable for conventional or pulsed spray transfer.
If extra-low weld metal carbon content is required for maximum corrosion resistance, argon/oxygen (1 to 2 percent) blends can produce a spraylike metal transfer. These welds have a tough oxide coating that might require postweld cleaning to remove.
Argon/CO2 blends produce less surface oxide, good bead shape and wetting, broad penetration, and consistent quality. Because the amount of CO2 in the gas mix influences weld carbon content, blends with lower CO2 content (2 to 5 percent) generally are preferred by users. Travel speed increases of 25 percent usually can be achieved by using argon/-5 percent CO2 for applications in which argon/1 percent oxygen has been used.
Three-part Blends. Most three-part gas blends work well in short-circuiting, spray, and pulsed spray transfer and provide excellent welding characteristics and high productivity. Argon with additions of 25 to 35 percent helium and 1 to 2 percent CO2 can provide a 20 percent or more increase in travel speed over most two-part blends, better control of distortion in thin material, and superior bead shape and color. This blend is an appropriate choice for joining stainless to carbon steel (309 filler) using pulsed spray transfer.
For 300-series material, optimal bead color and shape can be obtained with argon/CO2-hydrogen blends. The reducing atmosphere produced by the 1 to 2 percent hydrogen addition minimizes oxide on the bead surface and helps enhance fluidity and penetration.
For years the industry-standard gas blend for short-circuit welding of stainless steel was a helium-based (85 to 90 percent) trimix with small additions of argon (5 to 10 percent) and CO2 (2 to 5 percent). Combining argon with 2 to 5 percent CO2 and 2 to 5 percent nitrogen produces good bead shape and color in short-circuit transfer while improving travel speed and productivity by about 20 percent. This blend's performance is nearly equivalent to other trimixes' for the spray and pulsed spray transfer welding of most austenitic stainless steels.
Mixtures containing nitrogen should be used with care when joining stainless steel to carbon steel to ensure an appropriate microstructure is obtained.
GMAW of Ferritic Stainless Steels
Ferritic stainless steels are used when some increased corrosion resistance is needed but the cost of chromium/nickel austenitic grades cannot be justified. For example, ferritic stainless steel has become the material of choice for automotive exhaust systems. The muffler, resonator, and tailpipe are areas particularly vulnerable to corrosion from inside (fuel combustion condensate) and outside (salt exposure).
Many alloys, including 409 and 439, commonly are joined using GMAW with either solid or metal-cored electrodes to produce corrosion-resistant parts.
Gas Blend. Control of weld carbon content, required by several automotive fabrication standards, can be achieved by properly matching the filler wire with the shielding gas. Ferritic stainless steels have been joined using argon/oxygen blends for spray and pulsed spray transfer welding to prevent carbon pickup in the weld metal. This approach minimizes weld carbon increases, but it can reduce weld travel speed.
Depending on the maximum allowable carbon level, argon blends with 5 to 10 percent CO2 generally produce acceptable weld chemistry and microstructure in ferritic stainless steel welds. These blends can improve welding speed and productivity in most applications.
While these gas mixtures can be used with either solid or metal-cored wires, weld chemistry can be more easily tailored to provide better control over welding heat input and bead penetration when metal-cored wires are used. This is important when joining the thin-gauge materials typically found in automotive applications. While three-part blends might improve performance further, two-part blends of argon and CO2 will meet or exceed most requirements for ferritic stainless steels.
Productivity Increase With Shielding Gas and Wire Change. A major automotive parts manufacturer evaluated several metal-cored wire/shielding gas combinations to produce a critical flexible coupling for automotive exhaust systems. The required circumferential weld joined a mixture of 409, 304, 321, and INCONEL® alloy base materials. The major objective was to reduce defects to improve part acceptance rates and welding speeds. Cost reduction was essential with improved quality.
A 0.045-inch-diameter 18 chromium/columbium metal-cored wire used with several argon/CO2 blends was compared to the currently used 0.035-in.-diameter 308 low carbon, high silicon solid wire with argon/oxygen shielding. Current production rates were measured at three units per minute. With the metal-cored wire-argon/CO2 combination, the production rate increased to more than five units per minute.
Metal-cored wire, with its higher deposition rate for the same current level, provided excellent wetting characteristics and a broad arc shape, resulting in a flat bead shape with acceptable penetration. Little or no postweld machining of overweld was required to meet the original equipment manufacturer's (OEM's) specifications.
FCAW of Austenitic and Ferritic Stainless Steels
Flux-cored wires are developed for use with specific shielding gas compositions. For austenitic stainless steels, the choice is either 100 percent CO2 or 75 percent argon/25 percent CO2. The slag covering on the weld limits carbon absorption, so shielding gases with high CO2 content can be used.
The chosen gas blend typically depends on the welding position and operating conditions. An argon/CO2 blend generally provides the widest range of operation and the best operator appeal. With FCAW, argon-25 percent CO2 offers good control for out-of-position welding and a reduced distortion rate compared with 100 percent CO2 shielding.
GTAW of Austenitic and Ferritic Stainless Steels
Shielding gases for GTAW include pure argon, argon/helium, and argon/hydrogen (for austenitic grades only) blends. Here, too, shielding gas selection is based on productivity, distortion requirements, and desired color match.
Argon is used most widely because it offers excellent arc starting and low heat conductivity, making it suitable for joining thin materials. It is suitable for manual welding, because torch-to-work changes have little impact on operating voltage, and melt-through in thin material is limited.
The addition of up to 50 percent helium or hydrogen (up to 5 percent for manual welding) increases the shielding gas's heat conductivity, allowing faster welding speeds (increases of 50 to 100 percent) and higher productivity. In addition, the higher travel speeds help reduce distortion.
If color match is critical when joining 300-series stainless steel, the reducing effect of hydrogen from an argon/hydrogen blend helps decrease the residual oxide on the weld deposit, minimizing or eliminating the need for postweld cleaning. As the thickness of the base metal increases, the hydrogen or helium content of the argon blend should increase to achieve the desired weld appearance and shape.
The Critical Selection
Shielding gas selection is critical to the successful joining of stainless steels. In most ferritic and austenitic stainless steel welding applications, argon/CO2 blends can help improve productivity and performance, especially if conventional and pulsed spray transfer techniques are used. When bead appearance is important, three-part gas blends can provide good results while helping to reduce overall costs.
Careful matching of shielding gas to the welding operation is an important step toward achieving world-class weld quality and productivity.
Kevin Lyttle is manager of welding R&D and Garth Stapon is marketing manager, metal fabrication, Praxair Inc., 39 Old Ridgebury Road, Danbury, CT 06810-5113, 203-837-2702, fax 203-837-2454, www.praxair.com.
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