Which preheat ranges are commonly used for carbon steel to minimize hydrogen cracking?

• A. 50–80 C
• B. Typical preheat ranges from about 100–150 C (200–300 F) for many carbon steels; higher preheat may be required for thicker sections or specific alloys, per code.
• C. -50 to -20 C
• D. 300–400 C

Answer: (B)

Preheating carbon steel before welding is used to slow the cooling rate and give diffusible hydrogen time to diffuse out, reducing the risk of hydrogen cracking in the weld and heat-affected zone. For many carbon steels, a preheat temperature around 100–150 C (200–300 F) provides a practical balance: it sufficiently slows cooling to minimize hydrogen cracking without causing excessive distortion or altering material properties. Codes often note that higher preheat may be needed for thicker sections or certain alloy details, but the typical range is this 100–150 C window.

The other options don’t fit as well: 50–80 C is usually not enough to meaningfully reduce hydrogen cracking in many carbon steels, especially heavier sections. Cryogenic temperatures (-50 to -20 C) are not used for this purpose and can worsen embrittlement in some steels. A preheat of 300–400 C is generally excessive for common carbon steels and can lead to overheating, distortion, and unwanted metallurgical changes.