One of the great benefits of working with steel is the ability to weld it without losing material strength. Since we are dealing with high temperatures when welding, the thermal effects can cause great problems with distortion. Most of the time this is an undesirable side effect and in many cases can be eliminated or at least minimized.
There are 13 basic methods of minimizing weld distortion in weldments. They are:
- Reduce the volume of the weld
- Minimize the heat input
- Reduce the number of passes needed for the joint
- Fixture the part
- Anticipate the distortion
- Remove the heat in the process
- Use symmetry about the neutral axis
- Mirror Identical Parts
- Adding heat after cooling
- Torsional or Buckling distortion
- Fully tack the weldment first
As an engineer, one of my roles was to inspect weldments that were out of tolerance. The second largest cause of problems were weldments that deformed when the weld was applied. (Parts welded in backwards or in the wrong place was number 1). Many of these parts needed to be scrapped which lead to stoppage in production and increased rework. Both are undesirable and avoidable.
The most important part of understanding and predicting distortion is to evaluate what happens in the welding process. When the liquid weld material at approximately 6500°F is added to the cooler parent material, the liquid material will form a shell around it still containing the liquid material inside. As the weld material cools, the shell becomes thicker and thicker until all the material becomes solid. As the cooling process continues, the weld material will contract and create new stresses in the parent material. (As a side note, when fully cooled, the weld material will be at its yield strength in tension.)
For example, if we have two plates arranged in a “T” joint and we weld a fillet on each side, we would expect the cooling weld material to deform the cross part of the “T” into more of a rounded top. Even if the cross part is held in position until the weldment is fully cooled, it will curve slightly when released. This is commonly known as elastic distortion (see suggestion 6)
#1 Reduce the volume of the weld
It sounds simple enough, but simply making sure that your welds are sized for the desired load can remove a significant amount of distortion. Also, when joining plates, there is a variety of angles and spacing to choose from. Calculate the area required for each and choose the one with the least area.
#2 Minimize the heat input
This relates to #1 in that it is not only the size of the weld, but also the parent material that is being welded that causes distortion. Reducing the voltage or current can minimize distortion. Also, increasing the travel speed can minimize distortion.
Heating up the entire weld area can be an easy method to eliminate distortion since the contraction of the weld will be the same as the parent material. This prevents any residual tension or compression stresses from forming.
Like preheating, stress relieving can be used to eliminate distortion. If the weldment is put back in the fixture and heated, it will relieve any residual stresses from the welding process. It is important that during the stress relieving process, the weldment is constrained to its desired condition. Simply heat treating it without constraints may make it deform more. Also, be careful not to stress relieve at a temperature high enough to change the temper in the steel.
#5 Reduce the number of passes needed for the joint
A number of small passes will deform more than several large passes. This is mainly a consideration with thick materials that require complete joint penetration.
#6 Fixture the part
Holding the part in place during the entire welding and cool down phase will force the weldment to maintain its locked shape. However, when released, there will be a slight amount of elastic distortion. Elastic distortion is where a relatively small force can produce a large deflection. This principle is illustrated by the picture of the ruler where the force applied by the finger is very small compared to the deflection it produces. If a weldment is welded in a fixture and allowed to cool, it may spring to a slightly deflected state when released. In most cases, the part could be put back into the fixture and conform to the desired shape.
#7 Anticipate the distortion
If you know that the distortion is going to happen, let it. When fixturing the weldment components, purposely put them together so at the end of the welding process, the distortion makes it straight. In the T example above, slightly bending the cross plate in the center could produce a mostly straight cross plate.
#8 Remove the heat in the process
Adding large metal blocks near the weld on both sides will reduce the amount of distortion because it will act as a heat sink by pulling heat from the weld and redistribute it in the parent material allowing for even cool down. Copper is probably the best material for this. It has a higher specific heat than aluminum and will not become welded to the steel. Try several configurations to find the one that works best.
#9 Use symmetry about the neutral axis
Welding on or symmetrical about the neutral axis can eliminate distortion. Make sure that all the weldment components are tacked together thoroughly before beginning. If the welding cannot be symmetrical, perhaps the weld size could be increased or decreased to make it symmetrical. The figure to the left shows that balancing the area of the weld from the neutral axis can be used to keep the weldment from bending. Look for opportunities where welds can be removed if they do not allow for symmetrical welding. Keep in mind that this may also be accomplished by adding only a few components at a time and welding in stages. This process will take longer to produce, but will lead to far more usable weldments.
#10 Mirror Identical Parts
Perhaps welding at the neutral axis is just not possible. Try tacking two (or more) parts together so that the welding is done symmetrically around the neutral axis. There are three main drawbacks to this process. First, the weldment must be cool to the touch before they can be separated. Second, the tacks holding the components together must be broken and ground smooth. Finally, this process doesn’t fit into most lean manufacturing processes unless there are multiple weldments that are produced at the same time.
#11 Adding heat after cooling
Using a rosebud nozzle on a torch, heat can be applied to the weldment in certain specific places to straighten a weldment. I’ll caution you that while this does work, there is no real method to it. In my experience, trial and error is the best method to tackle this. If what was heated gives you worse results, try heating the opposite side. Document your findings so that the process can be repeatable. An interesting alternate use of heat can be to make a straight beam curved without applying a load.
#12 Torsional or Buckling distortion
Local sections of a weldment may suffer from local buckling or large amounts of distortion. For these two types of distortion, the best solution is the increase the torsional resistance or critical buckling strength (See Omer Blodgett’s Design of Weldments Section 3.6 and 2.12) In the case of a tall I-beam with a thin web, welding may cause the web to locally buckle in a wavy configuration or twist. welding many vertical stiffeners can reinforce the web and prevent localized buckling
#13 Fully tack the weldment first
If work is started on one end and moves consistently toward the other end, the components will end up pulling away from each other. Tacking the part from end to end will prevent them from pulling apart. Also, sequencing the weld location can minimize the heat input.
In conclusion, weld distortion is predictable and there are ways to minimize it. The more you study and apply the principles mentioned here, the more that weld deformation and distortion can be designed out before a prototype is even built. Applying these principles will save money in scrapped product and fixtures and hours of unproductive time spent evaluating out of tolerance parts.