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Due to its long-term cost effectiveness and inherent corrosion resistance, austenitic stainless steel has become a staple material across many industries—petrochemical, food processing and transportation to name a few.

Also known as 300 series, austenitic stainless steel poses distinct challenges when TIG welded, the greatest of which are carbide precipitation and distortion. Key to preventing these pitfalls are factors including: good heat control, correct travel speeds and adequate gas coverage.

Austenitic Stainless Steel Basics
By its very nature, austenitic stainless steel is a poor conductor of heat. The presence of nickel (6-22%), along with chromium (16-26%), enhances its corrosion and/or stain resistance, but these and other elements—often titanium or molybdenum—also cause it to react to heat differently than other materials. Effectively, austenitic stainless steel conducts heat at around half the rate of mild steel, but has a much higher rate of thermal expansion when welded.

Good heat control, gas coverage and travel speeds can help ensure TIG welding success on stainless steel.

Typically, austenitic stainless steel requires a DC power source and pointed tungsten (any type except pure) to TIG weld it. Like aluminum, it should be free of oil, paint and/or dirt prior to welding to achieve optimal results. Unlike aluminum, however, austenitic stainless steel does not require wire brushing prior to welding. Instead, the material should be wire brushed between welding passes with a stainless steel wire brush designated for this purpose; doing so helps remove potential interpass oxides.

The use of filler rod is recommended on applications with a base material thicker than 18 gauge and will be contingent upon joint design. For example, outside corners may not require filler rod, but an inside joint will. Most TIG applications require overmatching of the filler rod. That is, a filler rod with higher strength properties should be used. For example, on 304 series austenitic stainless steel, an ER308 rod should be used. Typically, austenitic stainless steel filler rods are available in diameters ranging from .035 to 5/32 (.9-to 4.0-mm) and chosen according to the joint design, welding parameters and applications.

What is Carbide Precipitation?
One of the biggest pitfalls to avoid when TIG welding austenitic stainless steel is carbide precipitation.

Carbide precipitation occurs when the chrome and carbon in the austenitic stainless steel is drawn out of the material and reacts to the atmosphere. It occurs between 800 to 1400 degrees Fahrenheit (426 to 760 degrees Celcius), so care should be taken to keep the weld zone below that range or in an inert atmosphere (via argon shielding gas).

Three main culprits are responsible for carbide precipitation: heat, travel speed and gas. Specifically, too hot of a TIG weld, too slow of travel speed and/or inadequate shielding gas coverage can individually, or in combination, cause the problem.

Heat and Travel Speed
The best defense against carbide precipitation is practice and a few key guidelines. One, remember the rule of amperages: use one amp of welding current for every thousandths of an inch of material thickness. Two, maintaining an appropriate travel speed helps prevent an excess amount of heat from entering the TIG weld.

Aside from practice, one way to monitor travel speed is to look for what is dubbed the “devil’s eye.” The “devil’s eye” is a fluid dot in the center of the weld puddle that is formed by foreign (but not worrisome) elements that continuously dance around in the center of the weld puddle. The presence of the “devil’s eye” is insurance that not only is travel speed appropriate, but also that other factors of the TIG weld (torch angle, filler rod position, penetration and root opening) are all optimal.

Gas Coverage
Using the appropriate type and amount of shielding gas is another important way to prevent carbide precipitation. Typically, pure argon provides the best results when welding thinner austenitic stainless steel, but the addition of small percentages of hydrogen is not uncommon when faster travel speeds are desired, especially on thicker pieces or in automated application.

Be sure to use the correct filler metal for your type of stainless steel.

The use of a gas lens is highly recommended when TIG welding austenitic stainless steel. A gas lens is a copper and brass component with layered stainless steel mesh screens that replaces the collet body in a standard GTAW torch. The gas lens helps distribute gas more evenly around the tungsten, arc and weld puddle and provides good cooling action.

Full penetration welds require back purging—covering the back of the weld with shielding gas. Back purging ensures that the underside of the weld is protected from atmospheric elements and can be done with commercial apparatuses or individually manufactured methods.

Finally, remember to maintain adequate post-flow. The best practice is to maintain one second of post-flow for every 10 amps of welding current used during welding.

Avoiding Distortion and Cracking
Because it is prone to greater thermal expansion than other materials, austenitic stainless steel tends to distort easily. Too high of current setting and/or too slow of travel speeds contribute to this problem. Thermal expansion occurs because the HAZ (heat affected zone) on austenitic stainless steel is more localized than other materials. When the weld cools, there is slow thermal transfer to the surrounding material that leads to buckling.

Joint design and clamping are good defenses against distortion. The key to joint design is creating a joint that limits the number of weld passes (especially on 1/4-inch and above austenitic stainless steel), and with it the amount of heat input. One way to limit these passes is to create a joint design consisting of a V-groove, modified V-groove, U-groove or J-groove.

Another way to prevent distortion is to clamp or fixture the piece of austenitic stainless steel. Doing so is especially important on gauge material, as these pieces are more prone to buckling.

Hand-in-hand with distortion, not surprisingly, comes the potential for cracking, especially in the weld initiation and crater area. One way to prevent cracking is to use run-on/run-off tabs.

These tabs need to match the base material and can be used on automated or hand-held TIG applications. They provide an area to ‘run-on’ or ‘run-off’ the weld by eliminating arc starting and stopping on the actual weld joint. They also aid in the complete filling of the crater area to help prevent cracking and are easily ground or cut off the weld after it cools.

Remember: even with the right type and amount of gas, good heat input and proper travel speeds, training and practice is still the best defense against the pitfalls of TIG welding austenitic stainless steels.