Why arc strike occur in arc welding?

Answer 1

because you're making a short-circuit that's why!, whaddayaknow uh?

More correctly as the label "arc welding" implies an arc is made by striking the metal to be welded with the electrode and this establishes the arc needed to generate the heat needed to melt the rod or filler metal and weld the parent metals together.

The problem comes about when the arc is established outside of the area to be welded and this strike leaves a very hard area (due to the momentary strike that creates a hard node) that with any stress can become the start point for a crack or small fracture that with continued stress will grow until complete failure of the part. There are many examples of this occurring where, e.g. a single arc strike near a welded seam on a ship has started a crack that then moved to the welded seam and then followed the seam down the side, under the keel and the ship broke in half.

So an arc strike occurs most commonly due to an accidental strike by a welder outside of the weld area where it will not be consumed by the weld process and if left will expose the part to potential failure at some time in the future.

"Arc strike" is commonly used to describe a mark left on the metal caused by the misplaced arc being started outside the weld area.

Welder competence will play a big part in the ability to strike an arc in the weld zone where intended but regardless of the reason for the "arc strike" the answer is to remove the arc strike to prevent possible part failure at some time in the future.