A welder is tasked with repairing a structural component made of A36 carbon steel. The ambient temperature is 5 °C. The repair involves a fillet weld joining a 25 mm thick plate to a 20 mm thick section. What preheating temperature range is most appropriate for this application?

Question: A welder is tasked with repairing a structural component made of A36 carbon steel. The ambient temperature is 5 °C. The repair involves a fillet weld joining a 25 mm thick plate to a 20 mm thick section. What preheating temperature range is most appropriate for this application?

Answer options:

• No preheat required
• 50 °C to 75 °C ✅ 100 °C to 125 °C
• 150 °C to 175 °C

Correct answer: 100 °C to 125 °C

Explanation: For A36 carbon steel with combined thickness exceeding 32 mm (25 mm + 20 mm = 45 mm, but effective thickness calculation for fillet welds can be complex; best to consider the thicker single plate, 25mm), and an ambient temperature of 5 °C, a preheat temperature of 100 °C to 125 °C helps prevent hydrogen-induced cracking and ensures slower cooling rates. Not preheating or using too low a preheat temperature would increase the risk of cracking.