Welding stainless steel to carbon steel is one of the most frequent - and most misunderstood - jobs on the shop floor. Get the filler wrong and you get cracks. Get the technique wrong and you get corrosion. Here is how to do it properly.
Step 1 - Choose the Right Filler
Standard 308 / 316 fillers will dilute with iron from the carbon steel and can crack. The correct choices are:
- ER309L - for routine joints and corrosion-service applications
- ER312 - for high-restraint, unknown, or difficult joints where cracking risk is high
Step 2 - Clean Both Sides
Grind at least 25 mm on either side of the joint until you reach bare, sound metal. Contamination is the number one cause of dissimilar joint failure.
Step 3 - Control Dilution
Use a short arc, small stringer beads, and low heat input. Excess penetration into the carbon steel dilutes the weld pool and can push the ferrite balance out of the safe zone.
Step 4 - Manage Heat
- No preheat needed for most mild steel ↔ austenitic stainless joints
- Interpass temperature ≤ 150 °C
- Do not use post-weld heat treatment unless the code demands it - PWHT on austenitic dissimilar joints can cause sigma phase embrittlement
Step 5 - Root Pass and Fill
For a groove weld:
- Root pass with ER309L or ER312 (ER312 for higher restraint)
- Fill and cap with the same wire, or continue with ER309L
- Keep bead profile flat, avoid heavy weaving
Step 6 - Post-Weld Cleanup
Wire-brush only with a stainless steel brush (never carbon steel - it seeds rust points). For corrosion service, consider pickling and passivation.
When to Choose ER312 Specifically
Use ER312 whenever the joint has:
- High restraint or thick sections
- Unknown or partially unknown base metal
- Hardenable base metal (tool, spring, high-C steel)
- High risk of cracking due to service loads
Dayang Welding supplies AWS A5.9 ER309L and ER312 to fabricators worldwide with full traceability.
