Why are deoxidized filler rods used for welding mild steel?

• A. to add alloying elements to the weld metal
• B. to prevent undercut
• C. to prevent the electrode for oxidizing
• D. to control porosity

Answer: (D)

Porosity control is the key idea here. When welding mild steel, the molten weld metal can trap gases or react with oxygen, forming oxides that show up as porosity and weaken the weld. Deoxidized filler rods include elements like silicon and manganese that scavenge oxygen in the molten metal, forming stable oxides that stay in the slag or are otherwise immobilized. This prevents gas pockets from forming in the weld and leads to a sounder, more reliable weld joint. They do add some alloying elements, but their primary purpose is to remove oxygen and reduce porosity. Other effects, such as undercut or electrode oxidation, are influenced by technique and broader filler-metal characteristics, not the main reason for using deoxidized rods.