Why do some materials need to be preheated before welding?

The primary purpose of preheating before welding is to slow the cooling rate of the weldment, preventing cold cracking while also reducing weld stress and the risk of microstructural hardening.

Avoiding Cold Cracking
Preheating prolongs the diffusion time of hydrogen atoms in the weld metal, reducing the likelihood of hydrogen accumulation and cracking. A slower cooling rate gives hydrogen atoms more opportunities to escape, preventing hydrogen-induced cracking.

Reducing Microstructural Hardening
The austenite transformation rate slows at high temperatures, preventing the formation of a hardened martensite structure. For example, controlling the cooling rate between 800°C and 500°C allows austenite to gradually transform into ferrite and pearlite, improving crack resistance.

Reducing Welding Stress
Preheating reduces the temperature gradient between the weld zone and the base metal, reducing thermal stress and constraint. Local or global preheating can significantly reduce temperature nonuniformity, lower strain rates, and prevent crack formation.

Improving Process Stability
Contaminants are more easily removed at high temperatures, reducing the risk of weld defects. For example, contaminants are more completely decomposed at 400°F, improving weld quality.