Arc strike is an arc initiated (accidental or intentional) on the base metal surface away from the weld zone. The arc strike is a critical discontinuity because there is a localized area where the base metal has been melted and then rapidly cooled due to the massive heat sink created by the surrounding base metal. This changes the structure of the base metal and causes it to become hard and brittle. This can lead to cracking which could lead to failure of the weldment.
All arc strikes should be removed by grinding. Grinding to a depth of 1/8 inch below the original surface should remove all traces of arc strikes and their hardened heat-affected zones. In locations where the arc strikes have been removed should have magnetic-particle inspection and hardness testing performed.