When should you specify 304L instead of 304?
The only real difference is carbon. When the lower carbon of 304L matters, and when plain 304 is fine.
Short answer: specify 304L when thick sections will be welded and not annealed afterwards, or when the service can attack the grain boundaries of a weld. For thin parts, parts that are not welded, or parts that are annealed after welding, plain 304 is normally adequate. Many mills now supply material certified to both 304 and 304L, which satisfies either specification.
What actually differs
| 304 (S30400) | 304L (S30403) | |
|---|---|---|
| Carbon, % max | 0.07 | 0.030 |
| Nickel, % | 8.00 to 10.50 | 8.00 to 12.00 |
| Chromium, % | 17.50 to 19.50 | 17.50 to 19.50 |
| Yield strength, annealed, MPa min | 205 | 170 |
| Tensile strength, annealed, MPa min | 515 | 485 |
Scroll the table horizontally to see all columns.
Why carbon matters in welding
Holding stainless steel for a period in the sensitising temperature range, which happens in the heat-affected zone beside a weld, can let chromium carbides form at the grain boundaries. That depletes chromium next to the boundary and can leave the weld zone open to intergranular attack in some environments. A lower carbon content reduces this, which is the point of the L grade.
The trade-off
The lower carbon gives a slightly lower minimum strength. If the design relies on the higher minimum yield strength of 304, or the part will be heat treated after welding, discuss it before choosing L.
Grades covered
Products covered
Specifying for a live project?
Send the service conditions and we will tell you what we would specify, and what we hold.