Magicians never give away their secrets, and metal fabricators tend to be the same. Handling weld distortion is one of those challenges where some businesses will just ask you to trust them, but that’s not us. We think the best way of convincing you we know what we’re doing is to talk about it, so here’s how we deal with distortion.
Conduction moves welding heat through metal while expansion makes it grow. That’s a problem when welding because pieces tend to grow at different rates. Then once they’re joined and left to cool each piece, and the filler metal, wants to shrink back. But because they’re now joined each piece is constrained in how it can contract, so the shrinkage happens in a different orientation to the initial expansion. And as if by magic, your right-angled bracket is now twisted in three different directions.
Some metals and fabrications experience this worse than others. The 300 grades of austentic stainless steel are a particular challenge because they combine a high thermal expansion with low thermal conductivity. Likewise, joining different metals often leads to distortion headaches. The same goes for welding pieces of dissimilar section or thickness.
So what’s to be done? Well you can’t beat physics, but experienced welders, (the kind we employ,) have some tricks up their sleeves that help.
7 Tips & Tricks to Beat Weld Distortion
1. Consider the design
Before striking an arc, study the design to visualize how it’s going to move. Try to locate joins where the distortion has the least effect, typically around the neutral axis. This lessens the leverage the shrinkage forces can exert.
2. Plan the welding sequence
It’s human nature to start at one side of a fabrication and work across to the other side, but that only exacerbates distortion. Especially on a large structure it’s best to alternate either side of the neutral axis as this evens out the shrinkage effects.
3. Minimize heat input
The more heat put in to the metal the more it expands and subsequently contracts. The keys are to minimize the size of every weld, make the welds quickly, and use tack welds to join the. Allied to this, it often helps to use a technique called “backstep” welding. This refers to moving the torch right to left, but working left to right along the seam, so only welding a short length at a time.
4. Fixture the parts
Ideally, parts being welded should be clamped in place beforehand to control how heat moves them. In the metal fabrication business that’s not always possible, but welders use clamps as much as possible.
5. Use pre-setting
This means anticipating how the pieces will distort and pre-bend or position them to take this into account. Sometimes welders make a test piece to find out just what shrinkage effects to expect before pre-setting the actual job.
6. Stress relieve
After welding it’s usually possible to remove some distortion by annealing the whole fabrication. Of course, this assumes a big enough oven or furnace is available. A related technique is to shot-peen the weld. This helps remove stress but it can hide defects so it’s not a preferred method. It’s always better to avoid distortion in the first place.
7. Sharing our knowledge
Managing distortion is one of the biggest challenges in fabrication welding, but it’s not magic. It just takes experience combined with an understanding of the basic physics. If you’re talking to a fabricator who’s shy about how he does it, talk to us instead.