How quickly must the welding charge be ignited after removing the preheating torch?

In thermite welding, it is important to ignite the welding charge promptly after removing the preheating torch to ensure optimal conditions for the welding process. The reason for this timing is primarily related to the heat retention and the stability of the joint being welded.

After the preheating torch is removed, the temperature of the surfaces needs to be maintained above a certain threshold to enable the thermite reaction to occur effectively. Delaying ignition beyond a specific time frame can result in a drop in temperature, potentially leading to a poorly welded joint or insufficient fusion between the metals.

Selecting 15 seconds as the appropriate window allows just enough time to transition from the preheating phase to the ignition of the welding charge while maintaining the integrity of the heat in the weld area. This timing balances the need for thorough preheating with the requirement to initiate the thermite reaction before the workpiece cools down.