What minimum temperature must the weld be left at until it can be finish ground if it's between ambient temperature and 400 degrees F?

The correct answer is based on the principles of thermite welding and how high temperatures affect the final properties of the weld. In thermite welding, the molten metal produced during the reaction must cool and reach a safe temperature before finishing operations like grinding can be performed.

Leaving the weld at or below 400 degrees Fahrenheit ensures that the heat does not negatively influence the integrity or structural properties of the weld. If the temperature were to be above that threshold, it could lead to thermal stresses or alterations in the metal's structure that could compromise the weld's quality.

Thus, ensuring the workpiece is allowed to cool down to 400 degrees F or below before any finishing work aligns with best practices for maintaining the structural integrity of the weld. This approach helps to ensure a robust, high-quality finish without risking any deformation or weakening of the joint.