If you are staring at a list of stainless steel grades and only remember one thing, make it this: the number tells you the alloy family, and the family — not the exact grade — is what decides most jobs. 304, 316, 430, 2205 look similar on a shelf; the differences that matter are whether the alloy is austenitic, ferritic or duplex, and what that means for weldability, magnetism, corrosion resistance and cost.
Stainless grades split into a handful of families, and each family has a character. This is the map most order forms skip, and it is the part that prevents expensive substitutions.
The three families that cover most orders
| Family | Crystal structure | Magnetic? | Examples | Character |
|---|---|---|---|---|
| Austenitic (300 series) | face-centred cubic | non-magnetic (annealed) | 304, 304L, 316L, 321, 310S | the workhorses; weldable, tough, corrosion-resistant |
| Ferritic (400 series) | body-centred cubic | magnetic | 430, 409, 439 | cheaper, magnetic, less weldable, good for non-welded parts |
| Duplex | mixed austenite + ferrite | weakly magnetic | 2205, 2507 | roughly double the strength of 304, high chloride resistance |
The 200 series (201, 202) is a manganese-substituted austenitic family that cuts nickel to reduce cost. It forms and looks like 304 but is not a like-for-like substitute — it work-hardens faster and has a lower corrosion ceiling, which is why it is the family most often quietly substituted into a job that was priced on 304.
Austenitic — 304 and its neighbours
304 is the default. Chromium 18–20%, nickel 8–10.5%, non-magnetic when annealed, welds well, resists ordinary atmospheres, food contact and most indoor duty. When a drawing just says "stainless steel," it means 304 unless someone paid extra.
304L is 304 with carbon pulled to 0.030% maximum. The "L" is for low carbon, and it exists for welding — it avoids the chromium-carbide precipitation at grain boundaries that leaves ordinary 304 vulnerable alongside a weld. For any welded assembly, write the L.
316L adds molybdenum (2–3%) on top of low carbon. That single addition is what buys chloride resistance, which is why 316L is the coastal, marine, pool and pharma grade. It is also the standard food-grade answer for equipment washed with chlorinated water.
321 is 304 with titanium added to tie up carbon, used where the part runs hot and welded — exhaust systems, heat exchangers. 310S is the high-temperature grade, rated for sustained service well above where 304 would scale.
Ferritic — 430 and when it is enough
430 is the 400-series workhorse: chromium around 16–18%, little or no nickel, magnetic, and noticeably cheaper than 304. It resists ordinary indoor atmospheres but not chloride attack, and it does not weld as forgivingly as 304. It is the right call for non-welded, non-structural parts — appliance trim, interior panels, some automotive trim — where magnetism is actually wanted and the budget matters.
The trap with 430 is rust. In a damp or coastal location 430 will show surface rust spots that 304 would not, and customers read those spots as a defective material when it was really the wrong grade for the location.
Duplex — when one alloy does the job of two
2205 is the most common duplex grade, with roughly twice the yield strength of 304. That extra strength means thinner wall for the same duty — less weight, less material cost — plus chloride resistance that sits above 316L. Duplex is the answer for pressure vessels, seawater systems, structural members in corrosive service and anywhere the job needs strength and corrosion resistance at the same time.
The trade-off is processing: duplex work-hardens and does not machine or form as easily as 304, and it needs care at the weld. It is not a drop-in — it is a material you choose deliberately.
How to read the number, fast
- First digit 2 → 200 series, manganese-substituted austenitic, cost-reduced 304 lookalike. Confirm before accepting it in place of 304.
- First digit 3 → 300 series, austenitic, the weldable corrosion-resistant default. The L suffix means low carbon for welding.
- First digit 4 → 400 series, ferritic or martensitic, magnetic, cheaper, for non-welded or hardened parts.
- Number starting 2 in the 2000s (2205, 2507) → duplex, high strength plus high chloride resistance.
What gets specified wrong
The most common mistake is accepting 201 where the drawing said 304. 201 looks like 304 and is cheaper, but it is not the same alloy — it has a lower corrosion ceiling and behaves differently in forming. If the price gap looks too good, check the mill certificate against the drawing's grade.
The second mistake is using plain 304 on a welded part. Without the L, a welded 304 assembly risks intergranular corrosion alongside the weld. The fix is one letter: 304L or 316L.
The third mistake is putting 430 outdoors near salt and then blaming the supplier when it spots. 430 is a fine indoor grade; it is simply the wrong grade for a coastal or damp location. The location, not the price, should decide the grade.
The decision in one table
| You need | Choose | Watch for |
|---|---|---|
| General purpose, dry indoor | 304 | write 304L if welded |
| Welded assembly | 304L or 316L | the L is mandatory |
| Coastal / chlorinated / marine | 316L | specify PREN if in doubt |
| High-temperature welded | 321, 310S | confirm the temperature |
| Non-welded, magnetic, cheap | 430 | keep it indoors and dry |
| Strength + corrosion, thin wall | 2205 | plan for harder processing |
Bottom line
The grade is a family decision first and a chemistry decision second. Start from the family — austenitic, ferritic or duplex — then pick the exact grade from what the part does: welded or not, wet or dry, strength-critical or cost-sensitive. When in doubt, 304L is the safe default and 316L is the safe upgrade for anything chlorinated.
For the stainless steel spec ranges — coil, sheet and plate, tube and pipe, bar and rod — see the category pages, each with the grade list and standards. If you are quoting a mixed stainless order, send the drawing and the service environment — we will pick the family and grade, and flag anywhere a cheaper grade was substituted in.