Why is preheating carbon steel before welding commonly practiced, and what effect does it have on the weld?

• A. To remove moisture and contaminants from the surface.
• B. To heat the workpieces to a higher ambient temperature.
• C. To slow cooling and reduce hydrogen cracking risk, improving ductility and controlling hardness in the heat-affected zone.
• D. To improve appearance of the weld bead.

Answer: (C)

Preheating carbon steel before welding slows the cooling of the weld and the heat-affected zone. This slower cooling gives hydrogen time to diffuse out of the weld region and surrounding metal, reducing the risk of hydrogen-induced cracking. It also lowers the peak hardness that can form in the heat-affected zone, helping maintain ductility and reducing brittleness. Slower cooling also helps lessen residual stresses from the thermal gradient. While drying the surface can be a minor benefit, the main purpose is controlling the cooling rate to prevent cracking and keep the weld area tougher. It’s not about raising ambient temperature or improving bead appearance.