When welding mild steel to stainless steel, which filler is used for the root pass and subsequent passes?

When welding mild steel to stainless steel, which filler is used for the root pass and subsequent passes?

• A. 309 and then most commonly 308
• B. 308 and then most commonly 309
• C. 309L and then 308L
• D. 316 and then 347

Answer: (A)

When welding mild steel to stainless steel, the root pass needs a filler that can bond well to both base metals and handle the dilution from the carbon-steel side. A 309-type filler is ideal here because it has higher chromium and nickel contents that bridge the two different metals, providing good wetting and a sound metallurgical bond at the root. Using 309L helps keep the carbon content low enough to minimize carbide precipitation while still matching the stainless side. For the subsequent passes, you want a filler that reinforces the stainless weld metal and maintains corrosion resistance. 308L filler is commonly used for these passes because its composition is well suited to the stainless side (such as 304/304L), delivering a stable, corrosion-resistant austenitic weld metal and good compatibility with the root and filler passes. Other fillers would not provide the same effective bridge at the root or the same stainless-weld-metal characteristics for the rest of the joint, so the combination of 309 (root) followed by 308 (fill) is preferred.

When welding mild steel to stainless steel, the root pass needs a filler that can bond well to both base metals and handle the dilution from the carbon-steel side. A 309-type filler is ideal here because it has higher chromium and nickel contents that bridge the two different metals, providing good wetting and a sound metallurgical bond at the root. Using 309L helps keep the carbon content low enough to minimize carbide precipitation while still matching the stainless side.

For the subsequent passes, you want a filler that reinforces the stainless weld metal and maintains corrosion resistance. 308L filler is commonly used for these passes because its composition is well suited to the stainless side (such as 304/304L), delivering a stable, corrosion-resistant austenitic weld metal and good compatibility with the root and filler passes.

Other fillers would not provide the same effective bridge at the root or the same stainless-weld-metal characteristics for the rest of the joint, so the combination of 309 (root) followed by 308 (fill) is preferred.