304 and 316L are the two most common austenitic stainless steels for CNC machined parts. They look identical, machine similarly, and are often confused. The 2–3% molybdenum addition in 316L is what gives it better chloride resistance, but it also costs ~30–50% more per kg. The decision is straightforward once you know your service environment, your weld requirement, and your cost ceiling. This page gives you the data and a decision matrix to make it in five minutes.
Both grades are austenitic (non-magnetic, FCC crystal structure), non-hardenable by heat treatment, and ductile. The difference is the 2–3% molybdenum addition in 316L, which makes it more resistant to pitting and crevice corrosion in chloride environments. The L in 316L stands for “low carbon” (≤ 0.03% C), which prevents sensitization (chromium-carbide precipitation) during welding.
| Property | 304 (1.4301) | 316L (1.4404) | What this means in practice |
|---|---|---|---|
| Nominal composition | 18% Cr / 8% Ni (called “18-8”) | 16–18% Cr / 10–14% Ni / 2–3% Mo | 316L has Mo; that is the whole story |
| PREN (Pitting Resistance Equivalent) | ~18 | ~25 | 316L is meaningfully more resistant to pitting in chloride |
| CPT in 3.5% NaCl | ~22 °C (no pitting above this temp) | ~30–40 °C | 316L handles warm seawater; 304 does not |
| Magnetic | No (can become slightly magnetic after cold work) | No (same caveat) | Both are austenitic; both can be checked with a magnet on cold-headed parts |
| Cost (relative) | 1.0× | ~1.3–1.5× | 316L is consistently 30–50% more expensive per kg of bar or plate |
| Typical applications | Kitchen equipment, architectural trim, general industrial, food & beverage (non-coastal), automotive trim | Marine hardware, chemical processing, pharmaceutical, medical implants (with ASTM F138), coastal architecture, food processing (salt, brine) | If chloride exposure is the main concern, 316L; otherwise 304 is fine and cheaper |
| Weldability | Good; risk of sensitization in heavy sections | Excellent; L grade prevents sensitization | For welded assemblies, 316L is the safer default |
| Machinability rating (relative to 304 = 100%) | 100% (baseline) | ~70–80% | 316L work-hardens faster; expect more tool wear and lower feeds |
Both grades are defined by ASTM (US) and EN (Europe) standards. The composition limits are slightly different between the two systems but the alloys are functionally interchangeable.
| Element | 304 (UNS S30400) per ASTM A240 | 316L (UNS S31603) per ASTM A240 | EN 1.4301 / 1.4404 (EN 10088-2) |
|---|---|---|---|
| Carbon (C) | ≤ 0.07% (304); ≤ 0.03% (304L) | ≤ 0.03% | 1.4301: ≤ 0.07% C; 1.4404: ≤ 0.03% C |
| Chromium (Cr) | 17.5–19.5% | 16.0–18.0% | 1.4301: 17.5–19.5%; 1.4404: 16.5–18.5% |
| Nickel (Ni) | 8.0–10.5% | 10.0–14.0% | 1.4301: 8.0–10.5%; 1.4404: 10.0–13.0% |
| Molybdenum (Mo) | — | 2.0–3.0% | 1.4404: 2.0–2.5% |
| Manganese (Mn) | ≤ 2.0% | ≤ 2.0% | ≤ 2.0% both |
| Silicon (Si) | ≤ 0.75% | ≤ 0.75% | ≤ 1.0% both |
| Phosphorus (P) | ≤ 0.045% | ≤ 0.045% | ≤ 0.045% both |
| Sulfur (S) | ≤ 0.030% | ≤ 0.030% | ≤ 0.015% (EN tighter) |
| Nitrogen (N) | ≤ 0.10% | ≤ 0.10% | ≤ 0.10% both |
| Iron (Fe) | Balance | Balance | Balance |
For most CNC machined parts, the mechanical properties of 304 and 316L are very close. The 5–10% higher strength of 316L comes from the Mo addition, but it is rarely a decisive factor.
| Property | 304 (annealed) | 316L (annealed) | Notes |
|---|---|---|---|
| Tensile strength (Rm) | 515–720 MPa (per ASTM A240) | 485–690 MPa (per ASTM A240) | 304 actually slightly stronger; both more than adequate for structural use |
| Yield strength (Rp0.2) | ~205 MPa (min) | ~170 MPa (min) | 316L has lower yield — actually a slight benefit for deep drawing |
| Elongation (A50) | ~40% min | ~40% min | Both highly ductile; no practical difference for machining |
| Hardness (Brinell HB) | ~201 HB max (per ASTM A240) | ~217 HB max (per ASTM A240) | 316L slightly harder; negligible for tool selection |
| Density | 8.0 g/cm³ | 8.0 g/cm³ | Identical (within 0.5%) |
| Modulus of elasticity (E) | ~193 GPa | ~193 GPa | Identical |
| Magnetic permeability | ~1.02 (annealed) | ~1.02 (annealed) | Both non-magnetic in annealed condition; can rise to 1.5–3 after cold work |
The molybdenum in 316L is what you are paying for. Mo improves resistance to pitting and crevice corrosion in chloride environments — salt water, salt spray, de-icing salts, chloride-bearing process chemicals, body fluids, and sterilizing agents.
| Environment | 304 expected behavior | 316L expected behavior | Recommendation |
|---|---|---|---|
| Indoor, dry, ambient | Excellent — no corrosion in service | Excellent — no corrosion in service | Either; 304 wins on cost |
| Outdoor, urban / industrial atmosphere | Good — may show tea-staining after years | Excellent — no staining | 316L preferred for aesthetic parts; 304 OK for hidden structural |
| Coastal / marine atmosphere (salt spray) | Pitting within 1–3 years on exposed surfaces | Excellent — no pitting in decades of exposure | 316L |
| Immersion in seawater (warm, > 25 °C) | Rapid pitting and crevice corrosion | Limited service life; consider 904L, duplex | 316L minimum; duplex or super-austenitic for sustained service |
| Food processing — dry, salt-free | Excellent — standard “food grade” | Excellent — also “food grade” | Either; 304 is the de-facto standard |
| Food processing — brine, salt, vinegar | Pitting risk; surface staining | Excellent — standard for brined food equipment | 316L |
| Medical implants (per ASTM F138) | Not approved for implants | 316L (specific composition) is approved for surgical implants | 316L per ASTM F138 (not generic 316L) |
| Chemical processing — sulfuric, phosphoric acid | Poor; rapid attack | Limited; consider Hastelloy or higher-Ni grades | Neither for hot concentrated acid; 316L for dilute / cold |
| Pharmaceutical / biotech clean-in-place (CIP with NaOH, HNO3) | Acceptable; some pitting risk in high-Cl CIP | Standard for pharma process equipment | 316L |
| Body fluids, implants, surgical instruments | Not approved | 316L per ASTM F138 / F139 (implant grade) | 316L implant grade |
The Critical Pitting Temperature (CPT) is the standard metric for comparing stainless grades in chloride service. It is measured per ASTM G48 (ferric chloride pitting test) or ASTM G150 (electrochemical CPT test), in a 3.5% NaCl solution simulating seawater.
| Grade | Typical CPT in 3.5% NaCl (ASTM G150) | Typical CCT in 3.5% NaCl (ASTM G48) | PREN |
|---|---|---|---|
| 304 / 304L | ≈ 22 °C (some sources show 20–25 °C) | < 0 °C | ~18 |
| 316L | ≈ 30–40 °C (commonly cited 32 °C; some sources 40 °C) | ≈ 5–10 °C | ~25 |
| 904L | ≈ 55–60 °C | ≈ 25 °C | ~36 |
| 2205 duplex | ≈ 70–80 °C | ≈ 35–45 °C | ~35 |
| 2507 super-duplex | ≈ 85–90 °C | ≈ 55–60 °C | ~42 |
What this means in practice: in a room-temperature chloride solution, both 304 and 316L are fine. Once the temperature crosses the CCT (crevice corrosion temperature), 304 starts to pit in tight crevices (gasket faces, threaded connections, salt deposits under gaskets). 316L delays that to a higher temperature, but it is not immune — for sustained hot chloride service, you need duplex.
For welded assemblies, the carbon content is what matters. In the heat-affected zone (HAZ) of a weld, standard 304 or 316 (with C around 0.05–0.07%) can sensitize — chromium carbides precipitate at the grain boundaries at 450–850 °C, leaving the surrounding matrix chromium-depleted and susceptible to intergranular corrosion.
| Scenario | Recommendation | Why |
|---|---|---|
| Thin sheet, single-pass weld, no post-weld heat treat | 304L or 316L | Low C avoids sensitization even in the HAZ |
| Thick section, multi-pass weld | 316L + low-heat-input process (TIG) | Multi-pass welds re-heat the HAZ; L grade is essential |
| Weld + post-weld solution anneal (1040 °C, water quench) | Standard 304 / 316 acceptable | Anneal dissolves the carbides; mechanical properties restored |
| No welding, just machining | Either standard grade (cheaper) | L grade only matters for welded assemblies |
As of mid-2026, the nickel-molybdenum alloy surcharge and spot pricing make 316L consistently 30–50% more expensive per kg than 304 in bar, plate, and tube form. The exact delta moves with the LME nickel price and the Mo price.
| Form | 304 indicative price (mid-2026, USD/kg) | 316L indicative price (mid-2026, USD/kg) | Premium |
|---|---|---|---|
| Round bar, 10–50 mm dia | ~$3.5–4.5 | ~$5.0–6.5 | ~40–50% |
| Plate, 3–10 mm | ~$4.0–5.0 | ~$5.5–7.0 | ~35–45% |
| Tube, seamless | ~$6.0–8.0 | ~$8.5–11.0 | ~35–45% |
| Wire / small bar for Swiss turning | ~$5.0–6.5 | ~$7.0–9.0 | ~35–40% |
For a machined part, the material cost is usually 20–40% of the total part cost (the rest is machining, inspection, overhead). So the 40% material premium on 316L translates to ~8–15% on the finished part price — noticeable but rarely a deal-breaker if the application requires it.
Use this matrix to pick between 304 and 316L in five minutes. If the answer is “not sure” on a row, go with 316L — the premium is small relative to the cost of a corrosion failure in service.
| Question | If YES | If NO |
|---|---|---|
| Does the part see salt water, salt spray, or coastal atmosphere? | 316L | Continue |
| Does the part see chloride-bearing process chemicals, brine, or CIP solutions? | 316L | Continue |
| Is the part a medical implant or surgical instrument? | 316L per ASTM F138 (or F139 for wire) | Continue |
| Is the part a pharmaceutical or biotech process component? | 316L | Continue |
| Will the part be welded into an assembly (especially thick section or multi-pass)? | 316L (or 304L if no chloride) | Continue |
| Is the part purely structural / decorative / indoor / dry? | 304 (cheaper) | — |
| Is the part food-contact but not brined / salted? | 304 is the de-facto standard | — |
| # | Mistake | What goes wrong | Fix |
|---|---|---|---|
| 1 | Specifying 304 for a marine part because “it’s stainless” | Pitting within 1–3 years; tea-staining; eventual through-wall leak | Default to 316L for any coastal, marine, or salt-spray exposure |
| 2 | Specifying standard 304 for a welded assembly in a corrosive environment | Intergranular corrosion in the HAZ; weld fails before the parent material | Use 304L or 316L for welded assemblies |
| 3 | Using 316L where the surface finish is rough (Ra > 1.6 µm) | The grade advantage is lost; rough surface nucleates pitting | Specify electropolishing or Ra ≤ 0.8 µm for chloride service |
| 4 | Specifying 316L “just in case” for a dry indoor part | Pays 30–50% material premium for no benefit | 304 is fine for dry indoor; use 316L only when the environment requires it |
| 5 | Using generic 316L for medical implants | Implant-grade 316L has tighter limits on Ni, Cr, Mo, N (per ASTM F138) | Specify “316L per ASTM F138” for implants, not generic 316L |
| 6 | Assuming the magnet test distinguishes 304 from 316L | Both are non-magnetic when annealed; both can become slightly magnetic after cold work | Use PMI (positive material identification) XRF or chemistry cert (EN 10204 3.1) to verify grade |
| 7 | Mixing 304 filler wire with 316L base metal (or vice versa) | Galvanic mismatch at the weld; preferential corrosion | Use filler that matches the lower-grade base metal (e.g. 304 filler for 304 base) |
Choose 316L whenever the part will see sustained chloride exposure — salt water, salt spray, marine atmosphere, de-icing salts, brine, vinegar or other chloride-bearing food products, pharmaceutical CIP solutions, or body fluids. Also choose 316L (or 304L) if the part will be welded into an assembly, especially in thick sections. For purely indoor, dry, structural, or decorative use, 304 is the right choice and saves 30–50% on material cost.
No — implant-grade 316L is defined by ASTM F138 (for bar and wire) or ASTM F139 (for sheet and strip). It has tighter limits on the alloy composition (notably on Ni, Cr, Mo, N) and stricter documentation requirements (full traceability, melt source controlled, surface finish controlled). Generic 316L per ASTM A240 is acceptable for pharmaceutical process equipment and surgical instruments that are not implanted, but for implants you must specify ASTM F138.
No, not reliably. Both are austenitic stainless steels, both are non-magnetic when properly annealed. After cold working (e.g. cold heading, deep drawing, machining), both can become slightly magnetic (permeability up to 1.5–3), but the effect is similar for both grades. The only reliable way to distinguish them on the shop floor is portable XRF (X-ray fluorescence) positive material identification (PMI) — it reads the Mo content in seconds. A 2–3% Mo reading = 316L; no Mo = 304.
Two reasons. First, molybdenum is more expensive than the iron and chromium it displaces. Second, 316L contains 2–3% more nickel than 304, and nickel is the single biggest cost driver in austenitic stainless pricing. The result: 316L is consistently 30–50% more expensive per kg of bar or plate than 304, with the delta moving as the LME nickel and Mo prices move. For a finished machined part, the premium is typically 8–15% on the part price (because material is only 20–40% of part cost).
316L does not rust in the traditional sense (it does not form red-brown iron oxide), but it can corrode — specifically by pitting and crevice corrosion in chloride environments. The Mo addition delays pitting but does not eliminate it. In hot seawater (> 30–40 °C) or under deposits that create crevices (gasket faces, salt buildup, threaded connections), 316L will eventually pit. For sustained hot-chloride service, you need a higher grade — 904L, duplex 2205, or super-duplex 2507.
Yes, but with filler selection rules. The standard practice is to use a filler that matches the lower-grade base metal — for 304 to 316L, use 308L filler (which is the standard 304 filler). Using 316L filler on a 304 base creates a galvanic mismatch at the weld — the 316L filler is nobler than the 304 base, so the 304 base preferentially corrodes. The exception is if the entire assembly is in chloride service, in which case some welders use 309LMo or 316L filler for added Mo in the weld; consult your welding procedure specification (WPS) for the specific case.
We are a CNC machining supplier working in 304, 304L, 316, and 316L (including ASTM F138 implant grade). Send us your service environment, weld requirements, and finish spec — we will recommend the right grade and quote it with a 3.1 mill cert.
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