TIG Welding Stainless Steel : Gas Coverage
Using the appropriate type and amount of shielding gas is an important way to prevent carbide precipitation when TIG welding stainless steel. Typically, pure argon provides the best results when welding thinner austenitic stainless steel, but the addition of small percentages of helium is not uncommon when better penetration and faster travel speeds are desired, especially on thicker pieces. The average flow rate required is between 15 to 20 psi, anything greater will cause turbulence in the gas flow and weld puddle and result in a poor weld. The use of a gas lens is highly recommended when TIG welding 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 TIG 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 ensures that the underside of the weld is protected from atmospheric elements and can be done with commercial apparatuses or individually manufactured methods.
The use of Chill Bars. The successful welding of stainless steel by various welding methods depends to a large extent on the type of back-up bar or plate used. Experience has indicated that pure copper is the most satisfactory material for backing up a weld. The high heat conductivity of such a back-up bar or plate will prevent its sticking to the weld metal, while its chill-mold effect will assure a clean smooth weld metal surface. Copper back-up bars can be made by cutting pieces from copper plate or sheet. Chill bars serve the best purpose by controlling distortion on light gauge material, and also help to prevent excessive burn-through or melting of the base metal.
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 when TIG Welding Stainless Steel
Because stainless steel is prone to greater thermal expansion than other materials it tends to distort easily. Too high of a 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 with other materials. When the weld cools, there is slow thermal transfer to the surrounding material that can lead to buckling. By clamping, using a fixture or adequately spaced tack welds, especially on thin gauge material, you can reduce the chances of buckling. Along with distortion comes the potential for cracking, By using a joint design consisting of a V-grove, modified V-grove, U-grove of a J-grove that limits the number of weld passes and the amount of heat applied the chance of cracking will be reduced. Cracking will also occur in the weld initiation and crater area. One way to prevent cracking in this area is to use run-on/run-off tabs. These tabs need to match the base material for the best results. The tabs provide an area to ‘run-on’ or ‘run-off’ the weld thus eliminating arc starting and craters on the actual weld joint and can be easily ground or cut off-after the weld 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 stainless steel.