What is thermal arc blow?

• A. When the arc becomes too hot
• B. When the hot spot on the plate lags behind during vertical welding
• C. When gas coverage is excessive
• D. When the weld pool solidifies too quickly

Answer: (B)

Thermal arc blow describes the arc being deflected by the magnetic effects created by the current in the work and the heat distribution in the metal. In vertical welding, the heat in the plate forms a hot region that does not respond instantaneously to the moving arc. That hot spot lags behind as you weld upward, and the magnetic interaction between the arc current and this lagging hot region pulls the arc toward the hotter area. That shift of the arc path is what’s meant by arc blow in this thermal, heat-driven context. So the best description is that the hot spot on the plate lags behind during vertical welding, causing the arc to blow toward it. It’s not simply about the arc getting hotter, gas coverage, or how fast the weld pool solidifies.