How long after the welding should the head risers be checked?

Checking the head risers after welding is a crucial step to ensure the integrity and quality of the weld. The correct answer, which indicates a time frame of 7 minutes, is rooted in practical welding procedures. This duration allows enough time for the weld to cool down to a point where any potential issues can be identified without the heat affecting the material or the observations made.

In the context of thermite welding, 7 minutes is typically sufficient for the weld to reach a temperature where it no longer poses a burn hazard but is still within a time frame that allows for the detection of defects that might emerge as the weld sets. Evaluating the head risers too soon could lead to inaccurate assessments since the material may still be in a malleable state. Conversely, waiting too long could risk missing the window during which some critical inspections should occur before the weld cools completely.

Other choices reflect different times that may either be too short, risking exposure to hot material, or too long, potentially missing the optimal time for inspection. Thus, 7 minutes is a well-considered balance for the necessary checks after a thermite weld has been completed.