Stainless steel grades look like a wall of nearly identical numbers until you learn the selection is not a lookup — it is a convergence. Three questions collapse the table to one or two candidates: what is the medium, how hot does it run, and will it be welded. Our printable stainless steel grades chart carries the composition and PREN columns for the daily-driver grades; this field guide is the reasoning that turns the chart into a pick.
Key takeaways
- Three questions pick the grade: chlorides in the medium? sustained temperature? welded construction?
- 304 is the general-duty default; 316 adds 2–3% molybdenum for chloride-bearing media; 316L caps carbon at 0.03% for welded service.
- 310S is the high-temperature specialist — continuous service is commonly cited to about 1150 °C in oxidizing conditions.
- 321 and 347 are stabilized grades: titanium or niobium ties up carbon so welded parts survive the sensitizing range.
- 904L and duplex grades are the chloride-heat answers beyond 316’s comfort — matched to PREN, not habit.
The three questions
Does the medium carry chlorides? Chlorides attack the passive film and drive pitting; the screening number is PREN — %Cr + 3.3 × %Mo + 16 × %N. From the grades chart: 304 sits at roughly 18–20, 316/316L at about 23–26. Below the chloride threshold, 304 is honest; above it, the molybdenum grades earn their surcharge. Where pitting risk lives in detail: pitting corrosion in stainless steel fittings.
How hot does it run? Sustained heat changes the candidate list entirely — 304 and 316 are fine in ordinary process heat, but long exposure in the 425–870 °C band precipitates chromium carbides at grain boundaries (sensitization), and genuinely hot service wants 310S or the stabilized grades.
Will it be welded? Welding moves the heat-affected zone through that sensitizing band, and the fix is either low carbon — the L grades at 0.03% carbon maximum — or stabilization, which is what 321 and 347 are for. The grade-versus-weld trade-offs are worked through in 316 vs 316L stainless steel.
Grade by grade, one line each
- 304 — the default: 18% Cr, 8% Ni, no molybdenum; panels, supports and mild media. Cheapest honest answer when chlorides are absent.
- 316 — 304’s recipe plus 2–3% molybdenum; the chloride-bearing workhorse: seawater-adjacent process, deicing salt atmospheres, chemical duty.
- 316L — 316 with carbon capped at 0.03%; the welded-construction version, protecting the heat-affected zone from carbide precipitation. Corrosion behavior matches 316; the difference is the weld.
- 310S — the high-temperature specialist, 25% Cr and 20% Ni; continuous service is commonly cited up to about 1150 °C in oxidizing conditions, lower where sulfur or low oxygen rules. Furnace parts, burner hardware, hot ducting.
- 321 — 304 stabilized with titanium at five times the carbon-plus-nitrogen content; TiC forms instead of chromium carbide, so welded parts survive the sensitizing range. The classic welded high-temperature grade — with one caveat: it can suffer knife-line attack in the narrow weld zone, which 347 avoids.
- 347 — 304 stabilized with niobium at ten times the carbon; the same protection as 321 with the knife-line caveat closed; preferred where the weld itself sees the heat.
- 904L — the superaustenitic step-up: roughly 20% Cr, 25% Ni, 4.5% Mo with copper; PREN around 36. Hot chlorides and acids that leave 316 pitted — seawater systems, sulfuric handling — are its territory, quoted per project rather than from the wall chart.
The chart itself — reproduced below exactly as published, values not recomputed — deliberately stops at these six; 310S and 904L sit outside a wall chart and are quoted per project:
| Grade | Carbon max | Chromium | Nickel | Molybdenum | Stabilizer | Approx. PREN | Typical instrumentation use |
|---|---|---|---|---|---|---|---|
| 304 | 0.08% | 18–20% | 8–10.5% | — | — | 18–20 | General duty, panels, mild environments |
| 304L | 0.03% | 18–20% | 8–12% | — | — | 18–21 | Welded assemblies where low carbon helps |
| 316 | 0.08% | 16–18% | 10–14% | 2–3% | — | ≈23–26 | Chloride-bearing service: tube, fittings, valves |
| 316L | 0.03% | 16–18% | 10–14% | 2–3% | — | ≈23–26 | Welded pharma, chemical and process duty |
| 321 | 0.08% | 17–19% | 9–12% | — | Ti 5×(C+N) min | 18–21 | High-temperature runs, expansion duty |
| 347 | 0.08% | 17–19% | 9–13% | — | Nb 10×C min | 18–21 | Welded high-temperature equipment |
Full printable version: stainless steel grades chart
PREN quick compare
The PREN column is the fastest desk screen for chloride pitting — same formula as the grades chart, values reproduced not recomputed:
| Grade | Approx. PREN | Read |
|---|---|---|
| 304 / 304L | 18–20 | Chloride-free duty |
| 316 / 316L | 23–26 | Moderate chlorides |
| 904L | ~36 | Hot chlorides, acids |
| Duplex 2205 | ~35 | 316-plus strength and chloride margin |
PREN screens pitting only. It says nothing about stress corrosion cracking — chloride SCC is a nickel-and-stress story, which is why chloride stress corrosion cracking gets its own post — and nothing about high-temperature strength.
When to upgrade beyond 316
Three triggers, in order of how often they are missed. First, hot chlorides: when temperature and chlorides coexist, pitting margins shrink and SCC enters the picture — the threshold conversation belongs in the RFQ, and duplex or nickel alloys are the answers, not a bigger safety factor on 316. Second, sensitizing-range exposure with welding: choose the L or stabilized grades before the weld, not after the failure. Third, true high temperature: 310S and its stabilized cousins for the service band, because 304 and 316 were never the right tool there.
RFQ lines
medium— fluid, chloride content if known, contamination allowances.temperature band— normal and upset, metal temperature.welded construction— and where: HAZ exposure decides L versus stabilized.grade per line— quote 316L for the welded runs even when 316 carries the spec headline; the mill certificate closes the loop.
FAQ
Is 304 always cheaper than 316?
Structurally, yes — 316’s nickel and molybdenum content cost more, and that ordering never changes. The size of the gap floats: mills quote a base price plus a monthly alloy surcharge tied to LME nickel and molybdenum prices, and the surcharge spread between the grades widens dramatically when nickel rallies. Budget the structural difference, confirm the spot spread when you order.
What is the difference between 321 and 347?
Both are stabilized versions of 304 — titanium in 321, niobium in 347 — tying up carbon so chromium carbides cannot precipitate at welds in the sensitizing range. The practical difference: 321 can suffer knife-line attack in the narrow zone right beside the weld, where 347 does not. Where the weld itself sees sustained heat, 347 is the safer pick.
When do I need 310S?
When sustained metal temperature climbs past the austenitic workhorse range — continuous oxidizing service is commonly cited to about 1150 °C for 310S, with lower limits in sulfur-bearing or low-oxygen atmospheres. Furnaces, burners, hot ducting. For merely warm process lines, 310S is overkill; for its band, 304 and 316 are the wrong tools.
Is 904L just better 316?
It is a different animal for a different problem: a superaustenitic with about 25% nickel, 4.5% molybdenum and copper, carrying PREN around 36 against 316’s 23–26. In hot chlorides and acids — seawater systems, sulfuric handling — it survives where 316 pits. It costs accordingly and earns it there; in mild duty it is money spent on protection you do not need.
Does the L grade help with stress corrosion cracking?
No — that is the trap. The L grade’s low carbon prevents weld sensitization, which is intergranular corrosion, a different failure. Chloride stress corrosion cracking tracks nickel content, and 316L carries the same 10–14% nickel as 316. For chloride SCC the ladder runs duplex first, then nickel alloys — the suffix buys nothing.
