When should crucibles be charged in relation to the preheating of the rail ends?

Charging crucibles prior to the preheating of the rail ends is essential for several reasons. First, doing so allows the thermite reaction to reach its optimal conditions before the rail ends are heated. The preheating process is meant to reduce thermal shock and ensure that the rails are at the right temperature to facilitate a strong weld. If the crucible is charged beforehand, it ensures that all components are ready and aligned with the planned sequence of events.

Additionally, charging the crucible first allows for any necessary adjustments or checks to be made to the mixture before the operation begins, ensuring that the thermite reaction proceeds smoothly without delays. This method also helps in managing the overall timing and efficiency of the welding process, ensuring the best possible results.

In contrast, charging after the preheating process may disrupt the welding timeline or lead to potential issues with the quality of the weld, as the conditions may not align as intended. Charging during or simultaneously could complicate the operation and affect concentration or safety, which is crucial in such high-temperature applications. Therefore, the optimal practice is to charge the crucibles prior to preheating the rail ends.