A pressure vessel made of SA-516 Grade 70 carbon steel requires a repair weld. The wall thickness is 50 mm. According to typical Canadian welding codes relevant to pressure vessels, what is the minimum preheat temperature generally required for this material and thickness?

Question: A pressure vessel made of SA-516 Grade 70 carbon steel requires a repair weld. The wall thickness is 50 mm. According to typical Canadian welding codes relevant to pressure vessels, what is the minimum preheat temperature generally required for this material and thickness?

Answer options:

• 50 °C
• 75 °C ✅ 100 °C
• 200 °C

Correct answer: 100 °C

Explanation: For carbon steels like SA-516 Grade 70 with significant thicknesses (e.g., above 38 mm), a preheat temperature of at least 100 °C (212 °F) is commonly required by Canadian codes like CSA B51 or ASME Section VIII Div. 1 (referenced in CSA B51) to reduce hydrogen cracking and improve ductility. Lower temperatures would be insufficient for this thickness, and 200 °C might be excessive unless specifically dictated by a Welding Procedure Specification (WPS) or higher carbon equivalent.