Welding Causes Distortion, But Why?

Every time we weld on a piece of metal or even heat it up with a torch, the shape of the metal changes as we apply thermal energy or heat. The metal expands based upon a physical property called the coefficient of thermal expansion. This coefficient is different for each metal and is described in a unit that tells us how much the material grows per degree of temperature increase, or contracts when cooling. There are also other factors that contribute to distortion such that the temperature increase may remain localized close to where the weld is made, but other materials may spread out the heat far from where the weld is being made warming up the entire part or unit. How much restraint or bracing is there to support the part can also be a factor too. How much heat also plays a role. A careful selection of welding process will also play a big factor.

If we look at the most simple case of distortion, it can be easier to understand. Imagine a flat steel bar 1/8 thick, 2" wide and 12" long. Something we might find in any typical shop. If we look at what happens to this bar if we make a 2" weld across it on the 2" width, we can see some of the effects.

The localized heating up of the metal while welding causes expansion in the area of the weld and adjacent to it in the heat affected zone. This heating temporarily causes the metal or bar to bend down, away from the weld. The cooler metals adjacent to the side of the weld that do not get heated resist that expansion force. These resistive forces help to permanently deform the material. As the metal begins to cool thermal contraction forces causes metal shrinkage in the welded part. Some of these contraction forces are dissipated as they distort the work metal. In some areas of the work, these resistive forces are not dissipated and are stored as residual or locked in stresses. These locked in stresses can cause problems if not accounted for. In our welded bar, we see it bend upward as it cools. If we made repeated welds on the bar, we would also begin to see its 12" length begin to shorten too. The 2" width will also shrink as we make welds also. If we weld a tee joint set at 90 degrees, we see angular distortion as the part cools and most of the time it will not remain square without bracing or restraint. Another form of distortion is the angular bending we see in tee joints.

There are some factors that we can control that will affect how much distortion we experience. How much restraint or external clamping forces can we apply to the part to keep it from distorting. Sometimes good simplenbracing can replace a bunch of good beefy clamps. Often times a large part has its overall mass working in its favor. Internal restraint due to the mass of the workpiece can also help prevent a lot of residual stresses or distortion we might see in a smaller part. How stiff is the material that were working with, can also greatly affect how much distortion we see. More rigid materials will resist distortion. The amount of heat input to the part also drastically affect the amount of distortion that we see in the finished unit. The rate at which we input the heat will also affect it as well. Also, the cooling rate can be a factor in the amount of distortion that we see. The interaction of all these different forces can be very complicated and not easily understood. We can take many measures to counteract these forces and reduce their effects, but predicting or calculating these is no easier for most, than predicting the next earthquake.

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