For welding very high carbon steels, which weld pass is most susceptible to cracking due to carbon pickup from the parent metal?

• A. Fill pass
• B. Interpass
• C. Root pass
• D. Cap pass

Answer: (C)

When welding very high carbon steels, carbon pickup from the parent metal into the weld pool is a key factor that can lead to cracking. The root pass is most susceptible because it is the first bead that fuses directly with the base metal, so it experiences the greatest dilution from the parent metal and the most opportunity for carbon to diffuse into the molten weld metal. That diffusion raises the carbon content in the solidifying weld, making it harder and more brittle. As the weld cools, this harder microstructure is prone to cracking, especially if hydrogen is present. The other passes sit on top of already solidified weld metal and are more influenced by the filler metal’s composition, so they experience less carbon pickup from the base metal.