Which statement about welding low carbon steels is true?

• A. Common arc welding methods, although preheating may be required with mild steels because hardening is a possibility
• B. Filler metals with an as-welded tensile strength in the 430 MPa to 490 MPa range
• C. If welded at below freezing temps, rapid cooling may cause hardening and cracking of the parent metal near weld; preheat
• D. All of the above

Answer: (D)

Welding low carbon steels hinges on controlling heat input to avoid hardening and cracking, choosing filler metal with compatible strength, and using preheat in cold conditions. Common arc welding methods can be used on mild steels, but preheating is often recommended because it slows the cooling after welding, reducing the tendency for the weld area to harden and for hydrogen-related cracking to occur. The filler metal you select should have an as-welded tensile strength in a range that matches the base metal so the joint behaves similarly in service; for low carbon steels this often falls around the 430–490 MPa range, which aligns with typical mild steel strength levels. When welding at subzero temperatures, rapid cooling can cause the metal near the weld to harden and crack; preheating helps manage the heat flow and hydrogen diffusion, mitigating that risk. Taken together, these points are all true, so the correct choice reflects that all of the above.