Failure mode
Sensitization & Weld Decay
A welded stainless part passes delivery and then corrodes in a thin band alongside the weld months later. The area looks rougher or rust-stained, and in severe cases it fails intergranularly while the base metal away from the weld is untouched.
At a glance
| Grades affected | 304 · 304L · 316 · 316L |
|---|---|
| Where it shows up | Stainless Steel Sheet & Plate · Stainless Steel Tube & Pipe · Stainless Steel Fittings & Flanges |
| Measured by | Sensitization is detected by the practice in ASTM A262 — the oxalic acid etch screen followed by the appropriate test (for example the Streicher or Huey test). |
| Write into the order | Name the L grade · State the welding procedure · Ask for the certificate |
Mechanism
Why it happens
Four causes account for most of it. They are not equally common, and they are not equally within the mill's control.
Carbide precipitation
When austenitic stainless cools slowly through roughly 450–850 °C, chromium carbides form at the grain boundaries. Each carbide pulls chromium out of the surrounding metal, leaving a chromium-depleted band that no longer forms a passive film.
Slow cooling after welding
The heat-affected zone is where it happens — the weld puts the parent metal into the sensitizing range and a slow cool keeps it there.
Standard carbon grade welded
304 and 316 hold up to 0.08% carbon, enough to sensitize. The L grades cap carbon at 0.030% so the carbide population never forms.
Service temperature
Sustained operation in the 450–850 °C band sensitizes even an unwelded part, which is why furnace and exhaust parts need a stabilized or L grade.
Verification
How it is measured
Sensitization is detected by the practice in ASTM A262 — the oxalic acid etch screen followed by the appropriate test (for example the Streicher or Huey test). Intergranular attack shows as grain-boundary etching. The specification to write is the grade and the standard, not a promise of "no weld decay".
Specification
How to specify around it
Most of these cost nothing to write into an order and are difficult to enforce afterwards. This is the part worth copying into your next RFQ.
1
Name the L grade
Specify 304L or 316L, not 304 or 316, for anything that will be welded. The carbon difference is the entire fix.
2
State the welding procedure
Low heat input and controlled interpass temperature keep the parent metal out of the sensitizing range for the shortest time.
3
Ask for the certificate
A 3.1 certificate naming the heat and the carbon analysis confirms the low-carbon grade was actually delivered.
4
Consider stabilized grades
321 or 347, with titanium or niobium, are the answer for sustained elevated-temperature service where an L grade is not enough.
For welded stainless orders we ask whether the part will be welded and whether it runs hot, and we say up front if a standard-carbon grade is the wrong call. The certificate type is agreed at quotation so the carbon analysis is checkable against the order.
We do not claim this never happens. Metal is rolled, shipped across an ocean and unloaded by someone else. What we do is ask the question before the order runs, put the answer in writing, and be the single party that answers for it afterwards.