Welding thin metals is the art of making many tack welds, then carefully welding longer beads to avoid heat distortion. Tube welding, in particular, requires a steady hand to maneuver the arc evenly around the entire circumference of the tube. Stainless steel welding requires careful prep work and a precise sense of timing during welding to avoid contaminating the weld. Welding thin stainless steel tube brings all of these challenges together. Completing these welds to specification requires an extraordinary level of skill and artistry from the welder—or a machine’s precision and timing.
Gas tungsten arc welding (GTAW) is the obvious choice for welding stainless steel tubing because it solves many of these challenges. It uses shielding gas to keep stainless steel welds pure, and the use of a tungsten electrode allows for fusion welding without the need for additional fill material. This means that extra heat is not needed to melt filler material, which helps prevent heat distortion in thin-wall tubing. The tungsten electrode also allows a fine degree of control over how heat affects the workpiece. For instance, the choice of tungsten grind angle affects arc geometry and width. The ability to exert greater control over these factors makes GTAW an excellent method for welding thin stainless steel tube.
The Challenges of Welding Thin Stainless Steel Tube
Thin materials can be roughly defined as material that is less than 2.5 millimeters (approximately 3/32 of an inch) in thickness. Thin-walled materials do not have a great deal of mass to disperse heat across. This leads to a buildup of heat in a localized area around the weld, potentially causing burn through or, more likely, causing the metal to warp, bend, or buckle during welding or afterward during cooling.
These problems are magnified in stainless steel. Stainless steel has a low thermal conductivity that doesn’t allow heat to quickly distribute throughout the metal. As a result, stainless steel is notorious for the ease with which it warps during welding. The thinner the stainless steel is, the easier it warps. Tube welding magnifies these issues. Manual welders must stop and reposition themselves as they weld up the circumference of the tube, leading to uneven heating and cooling of the material and making heat distortions more likely. GTAW minimizes heat issues, but this pattern of uneven heating, cooling, and heating can still cause heat distortion when welding thin stainless steel tube. Fortunately, the GTAW process is always improving, and new technologies can minimize or eliminate these issues.
Pulse GTAW Meets the Challenges of Stainless Steel
GTAW’s heat control provides a compelling reason to use GTAW for welding any thickness of stainless steel. GTAW is the least heat-intensive form of welding that is commonly used. Refinements in the GTAW process have improved the welder’s ability to control the geometry of the arc and the heat the arc inputs into the workpiece still further.
Pulsed GTAW welding changes the electric current between a high peak current and a lower/background current several times per second. The high current melts the metal, while the low current allows the weld to cool and solidify. This reduces the total amount of heat being put into the workpiece, which minimizes distortion without making any compromises in the fusion or formation of the weld. With high frequency pulse welding—above 2,000 hertz—the pulse can pinch the weld arc, narrowing the heat-affected zone while increasing penetration. All-in-all, pulse GTAW is an ideal method for stainless steel of any thickness, but its advantages especially shine when welding thin stainless steel. Tube-specific welding challenges, however, which arise from the circular shape of the workpiece, can be most easily addressed through automated GTAW welding.
Automated Orbital GTAW Is the Best Way to Weld Thin Tube
Welding the circumference of a cylinder has always been a challenge regardless of material thickness or metal type. The problem is one of accessibility, as there is no way for a welder to weld the full diameter in a single pass. A welder simply cannot reach all the way around a tube or pipe and must divide the circumference of the cylinder into sections to be welded one at a time.
This method creates opportunities for weld defects because of the following drawbacks:
- The potential for contamination of the weld is significantly higher when starting or stopping a weld. Gas coverage is just beginning prior to arc start, and if the arc is struck too soon, contamination can occur. If the gas is stopped before the weld has fully cooled, then the last molten sections can become contaminated.
- Starting and stopping the welding process creates patterns of localized heating while other parts of the weld are still cooling. This has the unfortunate side effect of multiplying instances of uneven heating across the entire diameter of the weld and enhancing the potential for heat distortion.
- Welding the circumference of tubing by sections is time-consuming. This provides cooling time that creates a more significant temperature differential between sections of the tube’s circumference, ultimately worsening any distortions that occur during welding.
GTAW orbital welding, unlike manual welding, does away with the need to weld by section, making the welding of thin stainless steel tube faster, easier, and more consistent. Once orbital welding joint preparation is complete, an automated, enclosed orbital weld head can be affixed to the workpiece and can weld the full circumference in a single contiguous pass. This removes many of the potential issues that arise from starting and stopping the welding process.
A low-profile, self-contained weld head also allows for the welding of thin stainless steel tube in the inaccessible environments that are common where stainless steel tubing is frequently used. The welding of closely spaced stainless steel tubes connected to a heat exchanger or high-purity biopharmaceutical process tubing, for instance, may take place in a tight, enclosed space without much room for maneuvering. In these situations, orbital welding allows the use of a low-profile weld head in conjunction with a remote weld pendant that allows the operator to start the weld from a comfortable distance. This is yet another reason that orbital GTAW is the best method for welding thin stainless steel tube in almost any application and environment.
Arc Machines, Inc. provides a range of weld heads for fusion welding of thin stainless steel tube, as well as tubing of other challenging metals and alloys. For product inquiries, contact sales@arcmachines.com, and for service contact service@arcmachines.com. To develop a custom solution, contact us to arrange a meeting.
