Controlling Weld Distortion in Fabrication

Every weld shrinks as it cools, and that shrinkage pulls on the part. A frame can come out short, a flange can tilt, and a long beam can bow. Each weld can pass inspection, and the part can still be scrap because it no longer meets the print. Fixing distortion afterward is slow and costly, so a good fabricator plans for it before the first tack. You know how to fit and tack a joint. Now you will learn to predict what it will do.

What You Will Be Able to Do

• Name the types of distortion and explain what causes each one.
• Estimate shrinkage on a real weldment.
• Use weld size, balance and sequence to reduce movement.
• Choose between presetting, prebending and restraint, and know the risk of each.
• Know when straightening needs a qualified person and a procedure.

Why Metal Moves

Welding heats a small area while the metal around it stays cold. The hot metal tries to expand, but the cold metal holds it back. When the weld cools, it tries to shrink, and the cold metal resists that as well. If the pull is stronger than the metal can resist, the part changes shape for good.

The numbers are large. According to TWI, a major welding research institute, carbon-manganese steel weld metal shrinks about 3 percent as it solidifies. The weld and the heat-affected zone (HAZ) shrink about 7 percent more as they cool to room temperature. Whatever does not turn into movement stays locked in as residual stress, which is stress left in the metal with no outside load on it.

That gives you the basic trade-off. Stop the metal from moving, and you raise the stress. Let it move, and the part changes shape. You cannot get rid of shrinkage, so you control where it goes.

The Types of Distortion

Angular distortion comes from uneven shrinkage through the thickness. A single-V joint has far more weld metal at the top than at the root, so the top shrinks more and the plates fold upward.

Bowing depends on the neutral axis, an imaginary line through the balance point of a section. A weld away from that line works like a bowstring and pulls the member into a curve. Welds placed evenly on both sides of it pull against each other.

Materials differ too. Stainless steel expands more than carbon steel when heated, so it distorts more with the same heat. Thin sheet can also buckle, and a buckled panel may snap from one shape to another when you push on it.

Estimating Shrinkage

Shrinkage is hard to predict exactly. TWI gives these rules of thumb for steel, which are good enough for planning. Treat them as starting points, and use your shop's own results when you have them.

The fillet figure assumes the leg is no more than three-quarters of the plate thickness. Larger legs shrink more, and the butt weld range depends on the number of passes.

Worked Example: Allowing for Shrinkage

A 10 foot plate gets 6 stiffeners running across it, each welded on both sides. That makes 12 fillets, and each one pulls across its own weld, which is along the length of the plate.

• Shrinkage = 12 x 1/32 inch = 3/8 inch
• The plate comes out about 3/8 inch short of its starting length.

If the print holds that length to plus or minus 1/8 inch and you cut the plate to the exact number, the part fails. Your options are to cut it long and trim after welding, or to add the allowance to the cut length if your lead approves it.

Now take a butt seam running the full length of a 20 foot plate. The rule gives 1/8 inch for every 10 feet along the weld, so expect about 1/4 inch of shortening. A plate that has several long seams can also bow.

Control Starts With Weld Size

The cheapest way to reduce distortion is to deposit less metal. Fillet size matters more than people expect, because the metal grows with the square of the leg.

Going just 1/16 inch over a 1/4 inch fillet adds more than half again as much metal. So:

• Weld to the size on the print. A bigger weld is not a better weld.
• Keep the fit tight. A wide gap fills with extra weld metal. TWI lists poor fit-up as a direct cause of added distortion.
• Use intermittent welds only if the print calls for them. You cannot change a continuous weld to a skip weld on your own. The engineer sets that.

Balancing the Heat

Next, set the shrinkage forces against each other. The joint design usually comes from the engineer, but you control the order of welding.

• Weld both sides. A double-V joint or fillets on both sides of a tee balance the pull, if you alternate sides.
• Stay balanced around the neutral axis. On a beam, weld the top and bottom in pairs and do not finish one side first. Two welders working opposite sides at the same time is a good way to do this.
• Weld from the center out. Work from the stiffest or middle area toward the free ends, so that shrinkage has somewhere to go.
• Tack well. Tacks hold the fit, so spacing them evenly stops the joint closing up ahead of the arc. On box sections, more tacks help reduce twist.

Welding Sequences

In a backstep sequence, the overall progress goes one way, say left to right, but each short bead is welded right to left. The heat from each bead does not build up ahead of the arc in the same way, and shrinkage is spread out.

Use a sequence only within your Welding Procedure Specification (WPS). Your sequence still has to meet interpass limits, and more stops mean more craters to fill. If a shop drawing gives a sequence, follow it. It may be there to control stress at a critical joint.

Presetting, Prebending and Restraint

Sometimes you need to go further. TWI describes three ways to do it.

Presetting leaves the parts free to move. It is simple, needs no special equipment, and leaves lower residual stress. The hard part is knowing how much to offset, so run a trial.

Worked Example: Presetting a Flange

A trial tee made of a 12 inch wide flange plate comes out with the flange tilted 1.5 degrees on each side of the web. To work out how far each edge moved, use the 6 inch distance from the web to the edge:

• Rise at the edge = 6 x tan(1.5) = 6 x 0.026 = about 0.16 inch

Before welding the real part, block up the flange so that each edge starts about 0.16 inch the other way. Shrinkage then pulls it back to flat. Check the next part, and adjust your preset based on what you measure.

Restraint is the most common method. Clamps, jigs and strongbacks hold the work until it cools. A strongback is a heavy bar or angle fixed across a joint to keep it in line. TWI's warning matters here: the more firmly you hold a joint, the higher the residual stress, and that stress can crack the weld. Wedged strongbacks let the joint shrink across its width, so they greatly reduce the cracking risk compared with fully welded ones.

Warning: Restraint and crack-prone steel are a bad mix. On thick, high-strength or high-carbon steel, a rigidly held joint can crack as it cools. Follow the preheat and sequence on your WPS, and ask before you weld any strongback or temporary attachment to the work. Temporary welds usually have to meet the same quality rules and be removed properly afterward.

Keep clamps on until the weldment has cooled. Taking them off a hot part lets it move. Even when it is cool, expect some spring when you release the clamps, because locked-in stress is still there.

Straightening After Welding

If a part still moves out of tolerance, it may be possible to straighten it. There are two main ways:

• Mechanical straightening. A press or jack bends it back.
• Heat straightening. Heat is applied to a small pattern of spots or wedges, and the part moves as they cool.

Treat straightening as a repair. Too much heat can damage the steel, and each steel has its own limit. Quenched and tempered steels are especially sensitive. On code work, straightening needs an approved method, and the limits come from the code and the engineer. It is done by, or under, a person qualified to do it. Do not take a torch to a bowed beam on your own, and do not cool heated steel with water unless the procedure allows it.

Planning a Weldment

Before you strike an arc on anything with a tight tolerance, walk through these steps:

1. Read the tolerances. Find out which dimensions really matter.
2. Estimate the shrinkage. Add up the welds across each critical dimension.
3. Check the fit. Fix gaps now, while it is cheap.
4. Choose a sequence. Balance your welds, and put it in writing if more than one person is welding.
5. Set your controls. Decide on any preset, clamps or fixtures.
6. Measure as you go. Check after the first welds, not at the end.
7. Record the result. Write down what moved and by how much, so that the next job starts better.

Common Mistakes

• Oversizing fillets. It adds metal, heat and shrinkage.
• Finishing one side before starting the other. The part pulls toward the side you welded first.
• Cutting to exact length when many welds cross the dimension. The part comes out short.
• Clamping crack-prone steel as tightly as you can. Stress goes up and the weld can crack.
• Removing clamps while the part is hot. It moves after you let go.
• Straightening without approval. You can damage the steel and void the job.

Check Your Understanding

A single-V butt joint on two flat plates comes out with the plates tilted up on the weld side. What type of distortion is this, and what could you do next time?

It is angular distortion, caused by more shrinkage at the top of the joint. You could preset the plates the other way, or use a double-V joint if the print allows it.

You are welding a long box beam and it keeps bowing toward the first side you weld. What should you change?

Balance the welds around the center of the section. Alternate sides, or have two welders work opposite sides at the same time.

A panel has 8 fillets running across its length. About how much shorter will it be?

About 8 x 1/32 inch, or 1/4 inch. That is a planning figure, so check it against your shop's results.

Your lead suggests welding a heavy strongback across a joint in thick, high-strength steel. What is the concern, and what is a better option?

A fully welded strongback adds stress and raises the risk of cracking. Wedged strongbacks let the joint shrink across its width, and the preheat and sequence on the WPS still apply.

Key Takeaways

• Shrinkage cannot be avoided, so plan where it goes.
• Stopping movement raises residual stress, and residual stress can crack welds.
• Weld to size, because a small increase in leg size adds a lot of metal.
• Balance welds around the neutral axis, and use backstep or skip sequences on long seams.
• Presetting leaves low stress but needs a trial. Restraint is common but raises cracking risk.
• Straightening is a controlled repair, so it needs approval and a qualified person.