If you’re new to TIG welding and noticed serious deformation after welding end caps onto rectangular hollow section (RHS) steel, you’re not alone. This exact scenario comes up often with beginners, and the good news is that the cause is usually straightforward.
In the case discussed here 3 mm steel, end caps welded all the way around at about 100 amps, the consensus is clear: too much heat was put into the part too quickly, with nowhere for it to go.
Understanding why the metal moved, not just that it moved, is the key to preventing it in the future.
Why 3 mm Steel Deforms So Easily With TIG?
TIG welding delivers very focused, localized heat. On thinner materials like 3 mm steel, that heat builds up fast, especially when welding closed shapes like RHS.
Once an end cap is partially welded, the remaining sides are being welded onto a part that’s already hot and constrained. As the metal expands and then contracts during cooling, it pulls the corners inward, turning once-square edges into subtle arcs.
At 100 amps, you’re well within a usable range for 3 mm steel, but amperage alone doesn’t tell the full story.
Heat input is affected by how long the arc stays in one place, how fast you travel, and whether the part is allowed to cool between welds. Running all four sides consecutively stacks heat on top of heat, almost guaranteeing distortion.
Travel Speed Matters More Than the Number on the Machine
One of the most common beginner assumptions is that lowering amps alone will fix distortion. While dropping amperage helps, travel speed is often the bigger factor.
When you move slowly, the surrounding metal has more time to absorb heat, enlarging the heat affected zone and increasing distortion.
Several experienced welders pointed out that it’s not just “less heat” that matters it’s how quickly you move that heat along. A slightly hotter setting combined with a faster, controlled travel speed can actually reduce distortion because the puddle spends less time heating any one area.
That said, this only works if your puddle control is solid. If moving faster causes you to lose the puddle or miss filler timing, it’s better to reduce amps and slow down slightly while practicing, rather than fighting instability.
Arc Length and Puddle Control Play a Hidden Role
Another factor that often goes unnoticed early on is arc length. A long arc spreads heat over a wider area and reduces control, which contributes to excessive edge melting and undercut.
Tightening up your arc keeping the tungsten close and steady concentrates heat exactly where you need it and minimize collateral damage to the edges of the cap.
Similarly, large or inconsistent puddles are a sign that heat is running away from you. Small, tight puddles give you more control and reduce how much base metal gets overheated.
Welding Sequence Can Make or Break the Part
Even with perfect settings, welding sequence matters. Welding one side after another without breaks allows heat to accumulate inside the tube and at the joint. Trapped hot air expands, stressing the joint internally while the metal itself is already near its limit.
A more distortion-friendly approach is to weld in stages. Many welders recommend tacking all corners first, then welding opposing sides rather than moving around the perimeter in order. Allowing short cooling periods even a few minutes can make a noticeable difference, especially when foot-pedal control isn’t available.
Was 100 Amps “Too High”? Technically Yes, but Context Matters
For continuous welds on 3 mm steel end caps, 100 amps is on the high side for a beginner, particularly without a pedal. Most experienced TIG welders would either start closer to 70-80 amps or use 100 amps briefly to initiate the puddle and then back off.
Without variable amperage control during the weld, heat management becomes entirely dependent on speed, pauses, and sequencing. That makes distortion much more likely when learning.
What to Change on the Next Attempt
Instead of chasing one single fix, focus on heat management as a system. Improvements usually come from a combination of:
- Slightly lower amperage or faster travel
- Shorter arc length and tighter puddle control
- Welding in stages instead of continuously
- Letting the part cool between sides
Just as importantly, practicing on scrap lets you feel how quickly heat builds and how the puddle responds as the part gets hotter.
Final Thoughts
The deformation you experienced isn’t a failure it’s a rite of passage in TIG welding. Almost everyone who learns TIG on thin or closed-section steel runs into this exact issue early on. The fact that you noticed it and asked the right question means you’re already on the right path.
With practice, you’ll start to feel when the metal is getting too hot long before it moves. At that point, controlling distortion becomes second nature not something you fight after the weld is done.