The short answer is yes, you can weld 316 stainless steel with 308 filler metal in many cases, but the choice depends on the application's specific requirements, environmental conditions, and performance expectations. Below, we break down the technical rationale, limitations, and best practices for this combination.
Key Differences Between 316 and 308
To understand the compatibility of these materials, first compare their compositions:
| Alloy | Cr | Ni | Mo | C | Primary Use |
|---|---|---|---|---|---|
| 316 | 16–18% | 10–14% | 2–3% | ≤0.08% | High corrosion resistance (marine, chemical). |
| 308 | 19–21% | 10–12% | 0% | ≤0.08% | Welding 304/304L stainless steel. |
Critical Note:
Molybdenum (Mo): The absence of molybdenum in 308 filler is the most significant difference. Mo enhances resistance to pitting and crevice corrosion in chloride-rich environments (e.g., seawater).
When Is 308 Filler Acceptable for Welding 316?
Non-Critical Applications:
For non-structural or non-corrosive environments (e.g., indoor furniture, decorative pieces), 308 filler can suffice.
The weld will have adequate strength and ductility for general purposes.
Budget or Availability Constraints:
If 316L filler is unavailable, 308 is a common substitute for temporary repairs or low-cost projects.
Thermal Cycling Avoidance:
308's lower thermal expansion coefficient (closer to 316 than 316L) may reduce distortion in thin sections.
Limitations and Risks
Reduced Corrosion Resistance:
The weld zone will lack molybdenum, making it vulnerable to pitting in chloride-rich environments (e.g., coastal areas, chemical plants).
Example: A 316 pipeline welded with 308 filler may corrode faster at the weld in seawater.
Galvanic Corrosion Risk:
Dissimilar compositions between the weld (308) and base metal (316) can create galvanic couples in conductive media, accelerating corrosion.
Code Compliance:
Standards like ASME BPVC or AWS D1.6 may require matching filler metals for pressure vessels or critical infrastructure.
Best Practices for Using 308 Filler on 316
Post-Weld Passivation:
Use nitric acid or citric acid passivation to restore the weld's oxide layer and improve corrosion resistance.
Avoid Chloride Exposure:
Limit use to environments with minimal chlorides, acids, or sulfides.
Consider Overmatching Fillers:
For critical joints, use 316L filler (ER316L) to retain Mo content, or 309L filler (higher Cr/Ni) for better corrosion resistance.
Alternatives to 308 Filler
ER316L:
Matches 316's composition (with Mo), ideal for corrosive environments.
Lower carbon content minimizes carbide precipitation in heat-affected zones.
ER309L:
Higher chromium and nickel (23% Cr, 13% Ni) bridges dissimilar metals (e.g., 316 to carbon steel) and offers better oxidation resistance.
Industry Perspectives
Oil & Gas: Avoid 308 filler for subsea or sour service (H₂S) due to corrosion risks; 316L or duplex fillers are preferred.
Food/Pharma: Acceptable for non-immersed equipment if sanitized regularly.
Architecture: Suitable for indoor structures but risky for coastal façades.
Conclusion: Proceed with Caution
While welding 316 with 308 filler is technically feasible, it trades cost savings for compromised corrosion resistance. For non-critical, dry, or low-risk applications, this combination works. However, in harsh environments or regulated industries, always opt for 316L filler to ensure longevity and compliance.
Before proceeding, consult project specifications, evaluate environmental exposure, and prioritize performance over short-term savings. When in doubt, involve a welding engineer to validate material choices-because a failed weld in a critical system costs far more than the price of the right filler metal.
