Magicians never give away their secrets, and metal fabricators tend to be the same. One of those secrets is how to handle distortion in welding. Some businesses will just ask you to trust them, but that’s not how we work. 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.
Conductivity and Expansion in Dissimilar Materials
Inexperienced welders are sometimes surprised to find that the two pieces they’d so carefully positioned prior to welding have twisted and turned once joined together. This is the effect of weld distortion. It’s virtually impossible to stop distortion occurring, but by understanding what causes it we can minimize its impact.
Distortion is primarily a result of thermal expansion but it’s also related to how quickly heat disperses through a material – its thermal conductivity. It’s common knowledge that some metals expand more than others, but what’s not so widely appreciated is that thermal conductivity can vary widely too.
A comparison of aluminum and copper should help explain. Copper is an excellent conductor of heat, transferring it twice as effectively as aluminum. However, aluminum grows more per °F rise in temperature. Join the two together, apply some heat and watch as this difference makes them buckle and twist.
In fairness, it’s not often anyone wants to weld copper to aluminum. (It can be done, although brazing is probably a better approach.) But the same issues exist with different grades of steel. Austentic grades of stainless for example have higher thermal expansion coefficients than low carbon steels. Thus if you weld 304 stainless to a piece of mild steel, the stainless is going to expand more. Then, when the filler metal cools and solidifies the stainless will try to return to its original size. Now though it’s locked to the mild steel, so one of them is going to bend.
Heat Flow and Weld Distortion
Distortion isn’t only a problem when you’re welding dissimilar metals. It can be just as much of a nuisance when welding pieces of the same grade.
If the pieces are different size and thickness, heat flow and expansion will also be different. As you probably know from experience, it takes longer, (meaning you have to put more heat in,) to warm a big block of steel than to get a piece of thin sheet to the same temperature.
So if you’re welding pieces of different dimensions, as with austentic and low carbon steel, they will expand differently before filler metal freezing locks them together. Once that’s happened, cooling back to room temperature is going to result in twisting and deformation.
Distortion can even happen if you’re welding two identically-sized pieces. This again is down to how the pieces contract once locked together. Picture creating a right angle between two long plates: start welding at one end and each will warm and grow. Then, as they cool you’re likely to find some curvature.
Another factor to consider is the filler metal. Assuming you’ve chosen a filler that’s stronger than the base metal, the filler will probably have a different expansion coefficient. Again, when that cools it’s likely to result in distortion.
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
- Consider the Design
The best place to address weld distortion is during design, starting with material selection. As discussed above, specifying welds between different metals with different dimensions or expansion and conductivity coefficients is asking for trouble. Even if you can’t use the same grade of metal, try to match the expansion coefficients of what you do use.
Then there’s the part geometry and the locations of the welds. Study the design to visualize how it’s going to move as it warms and cools. Put welds where distortion will have least effect. This is typically around the neutral axis, which lessens the leverage shrinkage forces can exert.
Remember that welds rarely need be continuous. A stitched weld can be almost as strong. And don’t overlook opportunities to add stiffening ribs rather than relying just on the weld for strength.
- 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 makes distortion worse. Especially on a large structure it’s best to alternate either side of the neutral axis as this evens out the shrinkage effects.
Ideally, the designer will produce a Weld Procedure Specification (WPS) that instructs the welder how to go about making the weld. This may include guidance about how many passes are needed on a heavy weld. Fewer passes with larger electrodes will put less heat into the weld and so result in less distortion.
- 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 pieces. 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.
Less experienced welders sometimes fall prey to “overwelding”. As the word suggests, this refers to applying too much filler (and therefore too much heat.) A heavily convexed bead doesn’t do much for strength but does increase the shrinkage forces.
- 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 feasible, but welders use clamps as much as possible. Make sure clamps are strong enough for the task.
Also under fixturing, let’s consider joint preparation. Beveling is a way to reduce the volume of filler needed, so reducing shrinkage forces. (See point 3 above.)
- Use Pre-Setting
This means anticipating how the pieces will distort and pre-bending or positioning them to take this into account. The WPS may define the pre-setting needed. Alternatively, welding a test piece is a great way of finding out just what shrinkage effects should be expected before pre-setting the actual job. The downside of course is that it takes time and material. But if it avoids scrap it may be worth doing.
- Pre-heat Base Metal Pieces
Pre-heating removes a variable by allowing each piece being welded to expand before the arc is struck and filler metal applied. Everything will still shrink as it cools, but preheating reduces some of the variability. All but thin sheet materials benefit from pre-heating but it’s especially important with higher carbon content (that’s greater than 0.25%) steel.
Stress-relieving is only considered after you’ve tried everything else and you’re still battling distortion. There are two approaches:
- Shot-peen. While this is a good way of reducing surface stress it must be used cautiously with welds. The risk is that the surface deformation will hide and even accentuate weld cracking.
- Anneal. This entails placing the whole fabrication in an oven or furnace and allowing it to soak. Of course, this assumes a big enough oven or furnace is available. It’s always better to avoid distortion in the first place.
Sharing our knowledge
Managing distortion is one of the biggest challenges in welding, but producing distortion-free fabrications doesn’t require a magic wand. 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.