A welded stainless part that passes delivery and then corrodes in a thin band alongside the weld months later is usually one thing: sensitization — also called weld decay. The fix is almost never a better welder; it is a grade and a procedure written into the order before the metal is cut.
Sensitization is a chemistry problem, not a workmanship problem. When austenitic stainless cools slowly through roughly 450–850 °C, chromium carbides precipitate at the grain boundaries. Each carbide pulls chromium out of the surrounding metal, leaving a chromium-depleted band that can no longer maintain the passive oxide film. That band is where the corrosion starts — and because it sits right next to the weld, it looks like the weld itself failed.
The carbon limit is the whole fix
The carbon content decides whether sensitization can happen. The standard grades 304 and 316 hold up to 0.08% carbon, enough to form carbides on slow cooling. The L grades — 304L and 316L — cap carbon at 0.030%, below the threshold where the carbide population forms. That single number is why "write the L" is the first rule of weld-safe stainless.
| Grade | Carbon, max | Weld behavior |
|---|---|---|
| 304 / 316 | 0.08% | sensitizes on slow cooling |
| 304L / 316L | 0.030% | weld-safe without post-weld anneal |
| 321 / 347 (stabilized) | — | Ti / Nb ties up the carbon |
The stabilized grades 321 (titanium) and 347 (niobium) take a different route: the alloying element ties up the carbon so it cannot form chromium carbides. They are the answer for sustained elevated-temperature service — furnace and exhaust parts — where an L grade alone is not enough because the part runs in the sensitizing range continuously.
What the weld procedure controls
The grade is not the whole story. Heat input and interpass temperature decide how long the parent metal sits in the sensitizing range. Low heat input and a controlled interpass temperature keep the heat-affected zone out of the danger band for the shortest time — which is why a weld-safe grade still needs a weld procedure that does not defeat it.
Three things have to line up:
- Name the L grade, not the standard grade, for anything welded.
- State the weld procedure — low heat input, controlled interpass temperature.
- Ask for the certificate naming the heat and the carbon analysis, so the low-carbon grade is confirmed rather than assumed.
How it is detected
Sensitization is detected by the practice in ASTM A262 — an oxalic acid etch screen followed by the appropriate test (the Streicher or Huey test, depending on the grade). Intergranular attack shows as grain-boundary etching under the screen. This is a specification and acceptance test, not a shop-floor guess: the certificate names the method, and the result is checkable against the order.
What gets specified wrong
The most common mistake is ordering "316" for a welded part because it sounds better than 304, then welding it and finding intergranular corrosion alongside the weld. Plain 316 has a 0.08% carbon limit, not the 0.030% of 316L, and is not the weld-safe grade. If the part is welded, write 316L, not 316.
The second mistake is assuming a post-weld anneal fixes everything. Annealing does re-dissolve the carbides, but on a large fabricated assembly it is expensive, it distorts the part, and it is often skipped. Specifying the L grade is cheaper and more reliable than planning to anneal.
The third mistake is welding a standard-carbon grade and never knowing the difference until the part corrodes in service. Sensitization is silent until the corrosion shows; the certificate is the only way to catch it before it ships.
Bottom line
Weld decay is a specification problem before it is a quality problem. If a part will be welded, specify the L grade — 304L or 316L — and write the carbon limit into the order. The certificate then names the heat and the carbon analysis, so the weld-safe grade is what actually gets delivered.
For the weld-relevant failure mode in full, see sensitization and weld decay. If you are quoting a welded stainless assembly, send the drawing and whether it runs hot — we will confirm the grade and the certificate before the order runs.