The Art of TIG Welding Stainless Steel
When TIG welding stainless steel there are a few points to keep in mind to achieve a cosmetically appealing and sound weld. Because stainless steel does not adequately dissipate heat, maintaining proper heat input when welding is critical. Too much heat can lead to warping, embrittlement or rust. As little as five amps too much can damage stainless steel’s properties. There are, however, several ways to control heat input:
Good fit-up: Adding filler metal to fill gaps puts more heat into the part, so good fit-up is important. It’s impossible to add a lot of filler metal and keep energy out of the part.
The correct filler metal: The filler metal diameter should be thinner than the base metal. If it’s thicker than the base metal, too much heat is needed to melt the filler metal. The filler metal should also match the base metal alloys in order to maintain consistent mechanical and corrosion properties.
Choose the right tungsten size: You can’t weld precisely on 1/16-in. material with a 1/8-in. tungsten. Use the right tungsten diameter based on your amperage.
Use the correct tungsten geometry: The tungsten’s shape plays a role in the weld’s width and penetration. In TIG welding stainless steel, the sharper the tungsten, the wider and less penetrating the bead will be. On a sharper point, (ground to a taper length that is more than 2½ times the electrode diameter), the arc tends to fan out, creating a wider heat affected zone. With a blunter point (less than 2.5 times the electrode diameter), the arc comes straight down with less flaring for a deeper, thinner bead and thinner heat affected zone.
Use a fingertip or foot control: You need to be able to start the arc and adjust the amperage from the beginning to the end of the weld. Set the welder to the desired amperage, which should be just a bit more power than you’ll need. If your welder is fairly accurate, you’ll only need to adjust the fingertip or foot control a little bit to adjust welding output.
Start with low amperage and allow the puddle to form: Then back off two or three amps and add filler.
Maintain the correct puddle size: The weld puddle should be the thickness of the base metal. If the puddle grows too large, turn down the heat. Eliminate craters by easing down the current at the end of the weld and adding filler metal until the puddle solidifies. Use your torch’s fingertip or foot pedal control or your welder’s sequencer. Keep the gas flowing and directed at the puddle until the orange color fades. The post flow also cools the puddle and the tungsten. Moving the torch too fast can blow gas away from the tungsten, turn it black and make it more difficult to start next time.
Use pulsing: To control heat input, use a welder with DC pulsing capabilities. In pulsing, the current transitions between a high peak amperage and a low background amperage that maintains the arc but allows the puddle to cool. The peak current provides good penetration, but the background current allows the weld puddle to cool slightly, preventing warping, embrittlement and carbide precipitation.
Pulses per Second (PPS): Is simply how many times the machine will complete one pulsing cycle in a time span of one second. Increasing the number of pulses per second produces a smoother ripple effect in the weld bead, narrows the weld bead. Reducing the number of pulses per second widens the weld bead. Pulsing also helps agitate the puddle and release any porosity or gas trapped in the weld. For TIG welding stainless steel, use a pulse rate of 100 to 500 PPS. Start at 100 and work upward. Higher pulsing (generally above 100 pulses per second) increases puddle agitation, which in turn produces a better grain molecular structure within the weld. High speed pulsing also constricts and focuses the arc. This increases arc stability, penetration and travel speeds, and it produces a smaller heat-affected zone.
TIG Welding Stainless Steel : Finish the Weld
Stainless steels, in particular, 304 and similar materials, are widely used in food, dairy, drug and processing equipment. To prevent bacterial growth, all fractures, cracks and crevices in the weld should be removed, and exposed surfaces be ground and polished to match the parent metal. If welds are made in pre-finished stainless steel, the weld beads should be held to a minimum size to avoid excessive and expensive finishing costs. The chrome-nickel grades are more difficult to grind than the straight chromium grades, so weld metal deposits should be as flat as possible. Heat from grinding should be held to a minimum also to avoid distortion of thin gauge materials. If the grinding wheels or belts were used previously on carbon steel, chemical cleaning should follow to remove any iron particles that might have become embedded in the stainless steel surface.
A technique of butt-welding polished sheets from the reverse or unpolished side has been successful. Sheets are first sheared from the back side so that any “shear drag” is on the polished side. Full penetration of the joint is achieved with a minimum of welding alloy penetrating the polished side. Relatively light grinding can then be used to prepare the weld on the polished side for final polishing and blending with the surrounding area.
TIG welded stainless steel provided by a good TIG Welding Service produces a smooth uniform beads that are easy to grind, polish or finish.