The Weld Nugget

Is weld stress stressing you out? Not an uncommon situation. But before you reach out for a bottle of your favorite heartburn medication, take a deep breath, sit back, and think about the welding process. Welding is an inherently stress inducing process given the rapid rise and fall in weld temperature, often in a matter of milli-seconds. In addition to cooling rate, factors such as clamping methods, pre-weld residual stress, and material issues can add to an already stressful situation.

For starters, you can try and reduce cooling rate of the weld by providing a gradual reduction in energy. In pulsed welding, this can take the form of a separate cool down pulse or a taper down of the energy profile at the end of the welding pulse. In continuous energy welds, such as with most arc welds and CW laser welds, you can try to change the fixture design such that the fixtures do not cool the weld too rapidly. Changes could include fixture material or cutting down on cooling water. If the parts are coming in with significant residual stress from prior machining/forming/stamping operations, you can try and anneal/stress-relieve the parts to reduce residual stress. Keep in mind that any heat treatment can warp the parts and they may not fit together as well as they did before annealing; poor fitup can cause a whole new set of problems. Excessively rigid restraints during welding can also lead to high residual stress. Given a choice, one of the two parts being welded should be located firmly while the other one should have some liberty to adjust itself during the welding process. One way to do that is to tack weld the parts together and then securely hold one part while the other realigns during welding.

Rapid cooling rate can also produce undesirable microstructures in the weld zone as well as the HAZ (see previous newsletter at this link for a discussion on HAZ) which can further complicate the situation. This is especially true with high carbon content steels which can form harder phases at faster cooling rates. Pre-heating the parts, in additon to reducing the cooling rate, also allows hydrogen to escape from the weld, thus reducing the potential for hydrogen embrittlement. Slower cooling rate also reduces residual stresses. Keep in mind that rapid cooling rate is not always so bad. In instances where minor impurity elements can segregate along grain boundaries, a rapid cooling rate can actually prevent segregation and could help avoid cracking.