316L (UNS S31603) is a low-carbon austenitic stainless steel whose ASTM A240 composition runs approximately 16.0–18.0% chromium, 10.0–14.0% nickel, 2.00–3.00% molybdenum, carbon held to 0.030% max, with manganese ≤2.00%, silicon ≤0.75%, phosphorus ≤0.045% and sulfur ≤0.030% — iron making up the balance. Chromium builds the passive film, molybdenum strengthens chloride pitting resistance, nickel stabilizes the austenite, and the ultra-low carbon cap suppresses sensitization during welding or slow cooling through the sensitization range. Read the table below as specification limits for a heat, not as the certified chemistry of any particular bar, plate, or MIM part — that comes from the mill test report or the supplier's powder-lot data.
This page breaks the composition into the element table with limits, the metallurgical job each element performs, and the pitting resistance equivalent (PREN) calculation that turns Cr–Mo–N chemistry into a corrosion-screening number. It closes with the MIM context: why carbon and oxygen control during powder processing and sintering decides whether a sintered 316L part actually delivers the chemistry printed on the drawing. For how these limits relate to mechanical properties, see the 316 yield strength page.
316L Composition Table (ASTM A240 Limits)
The table below reports the specified composition of 316L in weight percent under ASTM A240 (plate, sheet, and strip product forms). Two cautions apply. First, these are specification limits — a compliant heat can sit anywhere inside them, which is why chloride-service parts are qualified against heat-specific chemistry rather than nominal ranges. Second, ASTM A276 (bars and shapes) and other UNS S31603 product tables are not always identical to A240: commonly published A276/UNS S31603 chemistries allow silicon up to about 1.00%, versus the frequently cited A240 limit of 0.75%, and phosphorus limits vary slightly across editions and product forms. The applicable edition and product form control.
| Element | ASTM A240 Limit (wt %) | Primary Role |
|---|---|---|
| Carbon (C) | ≤ 0.030 | Suppresses sensitization; the L in 316L |
| Chromium (Cr) | 16.0–18.0 | Forms the passive film |
| Nickel (Ni) | 10.0–14.0 | Stabilizes austenite; ductility, toughness |
| Molybdenum (Mo) | 2.00–3.00 | Chloride pitting and crevice resistance |
| Manganese (Mn) | ≤ 2.00 | Austenite former; steelmaking practice |
| Silicon (Si) | ≤ 0.75 | Deoxidation |
| Phosphorus (P) | ≤ 0.045 | Controlled residual |
| Sulfur (S) | ≤ 0.030 | Controlled residual; sulfide inclusions can initiate pits |
| Nitrogen (N) | commonly ≤ 0.10 | Austenite stability; strong pitting contribution |
| Iron (Fe) | balance | Matrix |
Nitrogen appears at ≤0.10% in many 316L product tables where the applicable standard edition lists it, even though condensed element lists often omit it — ASTM A240 tables do not universally carry nitrogen as an explicit required limit. It earns its row: nitrogen stabilizes austenite and contributes strongly to pitting resistance — the 16× coefficient in the PREN formula below — so ignoring it understates the corrosion ranking of nitrogen-bearing heats.
What Each Element Does
Chromium (16.0–18.0%) is the element that makes the steel stainless: it forms and sustains the chromium-rich passive film responsible for corrosion resistance. Film performance depends on surface condition, environment, and electrochemical history, so chromium content alone does not settle service behavior.
Molybdenum (2.00–3.00%) improves resistance to localized corrosion, especially chloride-associated pitting and crevice attack. The effect is alloy- and environment-dependent — it is not an unconditional corrosion guarantee, and it does not prevent all forms of corrosion.
Nickel (10.0–14.0%) stabilizes the austenitic structure and contributes ductility and toughness. Note that nickel is not the principal element behind 316L's molybdenum-associated pitting resistance — crediting nickel for chloride performance is a common specification error.
Carbon is intentionally limited to 0.030% maximum. Lower carbon reduces chromium-carbide precipitation at grain boundaries during welding or thermal exposure, which lowers sensitization risk — but it does not make sensitization impossible under every thermal history. Manganese (≤2.00%) is an austenite-forming process alloying element that accommodates steelmaking practice; its sulfide inclusions can locally reduce pitting resistance, which keeps sulfur and inclusion control on the list of things that matter. Silicon (≤0.75% in A240 chemistry) supports deoxidation and is not a primary corrosion-resistance addition.
PREN: Turning Chemistry into a Pitting Screen
A widely used pitting resistance equivalent is PREN = %Cr + 3.3×(%Mo) + 16×(%N), with element percentages entered as numerical weight-percent values. Using nominal midpoint chemistry — Cr 17.0%, Mo 2.5%, N 0.05% — gives PREN = 17.0 + 3.3(2.5) + 16(0.05) ≈ 25. Working only from the specified A240 ranges with N = 0 gives a broad span of roughly 22.6–27.9 from minimum to maximum Cr and Mo. Treat both numbers as estimates, not certification values.
- PREN is a screening index for comparing alloys — it is not a percentage, a corrosion rate, or a guaranteed service limit.
- It ignores chloride concentration, temperature, pH, crevice geometry, surface finish, fabrication contamination, and weld heat input; a higher PREN does not always win in every environment.
- Coefficients vary across publications — some omit nitrogen — so state the formula whenever a PREN number is reported.
- Nominal PREN calculated from spec limits is not the certified PREN of a heat; corrosion-critical work uses the mill test report or powder-lot chemistry.
In practice, PREN is best used to rank candidate alloys at the shortlisting stage (316L against 304L, duplex, or higher-molybdenum grades), then set aside in favor of application-specific corrosion testing for the final call.
MIM Context: Carbon and Oxygen Control in Sintering
Composition control in metal injection molding does not stop at the powder certificate. The feedstock, binder system, debinding schedule, and sintering atmosphere all touch the final chemistry of a sintered 316L part. Carbon is the headline risk: incomplete binder removal before sintering can leave residual carbon that picks up as carbon enrichment, while atmosphere chemistry sets whether the surface loses or gains carbon. Either drift pushes the part away from the ≤0.030% carbon limit that defines the L grade — and the corrosion benefit that comes with it. Oxygen behaves similarly, with oxide residues affecting densification and surface chemistry.
This is why MIM buyers should ask a supplier not only for powder-lot chemistry but for the finished-part carbon and oxygen verification behind it. Emitech runs 316L within a material family that spans 316L/304/17-4PH/420/430L stainless plus low-alloy steels and titanium, processed on 17 MIM injection machines, 13 debinding furnaces, and 22 sintering furnaces (4 continuous + 18 vacuum) — full details are on the MIM capabilities page, and heat-specific chemistry for a current part is quoted on request. For the alloy-selection view across all MIM stainless grades, see the MIM materials guide; for the welding behavior that low carbon exists to protect, see Can You Weld MIM Parts.
Frequently Asked Questions
Q: What is the ASTM A240 chemical composition limit for 316L stainless steel?
Carbon ≤0.030%, chromium 16.0–18.0%, nickel 10.0–14.0%, molybdenum 2.00–3.00%, manganese ≤2.00%, silicon ≤0.75%, phosphorus ≤0.045%, sulfur ≤0.030%, nitrogen ≤0.10% where the applicable product standard lists it, iron balance — all in weight percent. These are specification limits for a compliant heat, not the certified chemistry of a specific product; heat-specific values come from the mill test report.
Q: Do ASTM A240 and ASTM A276 specify the same 316L chemistry?
Not always. Commonly published A276/UNS S31603 tables allow silicon up to about 1.00%, versus the frequently cited A240 limit of 0.75%, and phosphorus limits vary slightly across standards editions and product forms. The differences are small but real, so check the applicable edition and product form before writing a purchase specification.
Q: Does the 0.030% carbon cap completely prevent sensitization?
No — it reduces the risk, it does not eliminate it. Lower carbon means less chromium-carbide precipitation at grain boundaries during welding or slow cooling through the sensitization range, but sensitization is not impossible under every thermal history. Heavy sections, high heat input, or repeated repairs can still degrade corrosion resistance; qualify the actual fabrication sequence for critical service.
Q: How is PREN calculated for 316L, and should nitrogen be included?
The widely used expression is PREN = %Cr + 3.3×(%Mo) + 16×(%N). Nitrogen should be included when it is known, because its 16× coefficient makes even 0.05% shift the result. Nominal midpoint chemistry gives roughly 25; the A240 Cr and Mo ranges with nitrogen set to zero span about 22.6–27.9, a theoretical N = 0 boundary rather than the practical full PREN range of nitrogen-bearing heats. State the formula used, since coefficients vary across publications, and do not use nominal PREN as a substitute for heat-specific certification.
Q: Can I calculate PREN from nominal composition instead of a mill test report?
For screening and alloy ranking, yes — that is exactly what nominal PREN is for. For qualification of a chloride-service part, no: nominal ranges are broad enough (≈22.6–27.9 with N = 0 for 316L) to change decisions, and PREN ignores chloride concentration, temperature, crevice geometry, and surface condition. Use nominal chemistry to shortlist, then the certified heat chemistry — and application testing — for the final call.
Composition is where 316L specification decisions start, but it is not where they end: the same nominal chemistry delivers different service results depending on product form, thermal history, surface condition, and — in MIM parts — on sintering-process carbon and oxygen control. If you are specifying 316L parts and want feedstock and process details behind the chemistry, send the drawing through the contact page: Emitech reviews MIM suitability and returns a quotation within 24 hours.
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