In which situation is preheating of the base metal advisable?

Preheating of the base metal is advisable when working with metals of very different thicknesses. This practice helps to reduce thermal stress and the risk of cracking by ensuring that the heat is distributed more evenly throughout the material. When two pieces of metal with significantly different thicknesses are welded together, the thinner section can heat up and cool down more quickly than the thicker section. This difference in heating can lead to warping or cracking in the thinner material, while the thicker section may not reach the necessary temperature for effective welding.

By preheating the base metal, the overall temperature of both materials is increased, which minimizes the potential for uneven temperature gradients during the welding process. This allows for better fusion and strength in the weld, ensuring a more reliable and durable joint.