What is a typical preheat temperature range used when welding thick steel sections with GMAW?

• A. 0–50°C
• B. 200–300°C
• C. 150–200°C or higher depending on alloy
• D. -50 to -100°C

Answer: (C)

Preheating helps slow the metal’s cooling when welding thick steel with GMAW, which reduces the risk of hydrogen-induced cracking and improves weld quality on heavy sections. Because thicker plates and different alloys behave differently, a common starting range is about 150–200°C, with the exact value adjusted upward for tougher alloys or higher carbon content. This ensures enough heat to promote proper fusion and control of stresses; too little preheat can still crack or warp the weld in thick sections, while excessive preheat can cause distortion or unwanted metallurgical changes. Very low or cryogenic temperatures aren’t used for thick-steel welding, and while some specific cases may call for higher preheat (even into the 200–300°C range), the typical guideline is around 150–200°C or higher depending on the alloy.