In general, orbital welding gives better results than manual welding when joining two pieces of tubing without using a fitting. With orbital welding (also known as Gas Tungsten Arc Welding, or GTAW) the welding electrode moves in a circle around the joint, guided by a machine rather than by hand.
For a good weld, though, you also need materials with the right chemical makeup. One of the chief factors in stainless steel, for instance, is the sulfur content. The level of sulfur can make the surface tension of the weld puddle change, affecting the heat flow into the puddle. This characteristic is sometimes called the Marangoni Effect.
Standard 316L stainless usually has sulfur levels in the range of 0.015 percent to 0.025 percent by weight, making it a good choice. As the sulfur content decreases, so does the weldability. That's because the lower sulfur content requires more heat input for a set depth of penetration.
When the sulfur level drops below 0.005 percent by weight, it changes the surface tension of the weld and allows the heat to flow away from the center of the weld puddle.
When the heat flows away, weld penetration decreases and the OD width of the weld bead increases. In some cases, the weld can be 50 percent wider when the sulfur content is low. To compensate, the weld heat must be increased, sometimes as much as 40 percent. But that introduces other problems, like reducing the corrosion resistance of the weld and causing more grain growth in the heated zone.
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Too much carbon content can cause problems too. Carbide precipitation can occur at carbon levels above 0.03 percent by weight in stainless, reducing corrosion resistance. There's a tradeoff involved here. The L grade meets carbon content requirements in 304 and 316 stainless steels, but at a cost of an approximately 15 percent loss of strength.
Manganese is released as a vapor during welding and is redeposited downstream of the weld. This is a matter of controversy but is a consideration in only the most corrosive of systems. The cost of material and availability is a major factor. Most surface deposited manganese can be removed by a hot (180°F) DI water rinse for approximately one hour.
Passivation removes the iron particles from the surface of stainless steel and forms a thin chromium oxide film, an inexpensive and effective corrosion resistant barrier in many applications. Passivation can be done before or after installation.
Electropolishing provides for very good surface finishes with a much thicker chromium oxide barrier. Electropolishing is common in UHP systems where clean up time and ultra low contamination are primary concerns. In many cases, it is also a more effective corrosion resistant barrier, but it comes at a large cost.
Swagelok Orbital Welding Training (Next Class: October 22-26, 2018)
There are a number of types and classes of materials available for orbital welding, and there is just as large a group of subclasses. You may find yourself ordering 316L, VIM/VAR, ultra low manganese, electropolished to a 5Ra max., hydrogen bright annealed OD with electrically etched mill markings when all you thought you needed was ordinary stainless steel.
The following considerations are some of the most important for selecting a material:
The material of choice, by far, in most systems is 304L stainless steel or 316L stainless steel. Other choices include carbon steels, ferrous alloys, other stainless steels, nickel alloys, refractory and reactive metals. Whenever possible use tubing, not pipe, for your critical systems. Typically, tubing is readily available with the requirements necessary for orbital welding in these systems.
You don't need a degree in metallurgy to navigate all the choices for orbital welding. Just call Swagelok Northern California at 510-933-6200 or contact us through our website. We'll help you find the right mix of materials for your job.
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