Why is preheating sometimes recommended before brazing?

Preheating is sometimes recommended before brazing primarily to reduce thermal shock and improve the flow of the filler metal. When a workpiece is brazed, it can experience rapid temperature changes, especially if the parts are significantly colder than the melting point of the filler metal. Thermal shock can lead to cracking or distortion of the base metals, which compromises the integrity of the joint.

By preheating the workpieces, the temperature differential is minimized, which mitigates the risk of thermal shock. This controlled heating ensures that the base metals reach a more uniform temperature, allowing for even expansion and reducing residual stresses. Additionally, preheating can enhance the flow characteristics of the filler metal; a warmer surface often allows the filler to spread and penetrate into the joint more effectively, promoting a stronger bond and a more reliable brazed joint.

The other options, while relevant to certain aspects of brazing and metalworking, do not directly address the primary reasons for preheating in the context of reducing thermal shock and improving filler metal flow.