( لکه قوس )Arc strikes
Continuing our series of alternate acceptance criteria for various forms of discontinuities, we will look at the arc strike. For the proper application of alternate weld acceptance criteria, refer to our July issue.
Arc strikes are usually created in one of two ways. One, the welder inadvertently touches the energized electrode to the steel he or she is welding. An arc is created and a small surface weld is formed. The other typical arc strike occurs the welder misses the root when starting the weld, either when dropping the helmet or through some other hand motion. Once the arc is started, the welder can now see that he or she missed the root, then drags the electrode into the root to begin the weld.
Neither form of arc strike is attractive. We do not concern ourselves with appearances, however, concentrating upon the effect of the arc strike upon the strength and performance of the joint or structure.
The arc strike is similar to a very small weld, one made without preheat and certainly smaller than the minimum weld size required to reduce the risk of cracking. The arc strike cools very rapidly, leaving a very small but very hard surface and heat-affected zone (HAZ). A high-hardness HAZ increases the risk of brittle fracture, hence an arc strike may serve as a crack initiator. The significance of a crack found at the surface also exceeds that of a crack below the surface.
Crack propagation requires a combination of high stress, adequate crack size and low toughness in the steel. Fatigue from cyclic loading certainly increases the likelihood of crack propagation.