Mold sand should remain on the weld until it cools to what temperature?

The correct answer is based on the importance of ensuring that the weld joint remains undisturbed during the cooling process to prevent defects in the weld. Mold sand provides insulation, which helps the weld maintain its structural integrity as it cools.

When the temperature reaches about 900 degrees Fahrenheit, it can be considered safe for the removal of the mold sand. At this temperature, the weld has typically solidified sufficiently to handle external forces without risk of cracking or distortion. Removing the mold sand too early, when the weld is still above this temperature, could expose the weld to stresses that might lead to issues such as warping or incomplete fusion.

This careful attention to temperature during the cooling phase is crucial for producing strong, reliable welds. Thus, maintaining the mold sand until the temperature drops to around 900 degrees Fahrenheit is a standard practice to ensure quality in thermite welding.