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AISI 316 / (1.4401) / UNS S31600 / AMS 5648 / AMS 5524 / AMS 5573

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Corrosion resistance: Under normal conditions the corrosion resistance of AISI 316 is far better than that of AISI 304. In hot environments where acid is present, hairline cracking and pitting can form in the internal structure of the material, and above 60 °C stress-induced cracking is possible. It is resistant to 1000 mg/L of free chlorine in service water at standard temperature and to 500 mg/L above 60 °C.

Temperature capability: 316 has good oxidation resistance in environments reaching 870 °C intermittently and 925 °C continuously. If the environment is wet and the temperature is between 425 and 860 °C, however, corrosion resistance falls. Because the carbon (C) content of 316L is low, carbide precipitation does not occur in these environments.​‌​​‌​

Weldability: It shows excellent capability with all welding methods — electrode, gas shielded, wire and so on. The most suitable electrode or wire grade for 316 is 316, and for 316L it is 316L. When welding thin sections in grade 316, post-weld annealing is not required, but it must be applied on thick sections. For 316L, post-weld annealing is not required even on thick sections (6 mm and above, for example). 316Ti should be preferred over 316 for thick section welds.

Machinability: It has good machinability. Where corrosion resistance is not important, stainless steel products in grades 303 or 430F can be specified.​‌​​‌​

Heat treatment: It cannot be hardened by heat treatment.

Applications: It is used in food equipment handling highly acidic products, in laboratory benches and equipment, in medical devices, on the exterior facades of architectural structures by the sea (panels, rails and so on), in boat and ship fittings, in chemical transport containers, in heat exchangers, and in bolts, nuts, springs and screws.​‌​​‌​

AISI 316 has a wide range of applications because it provides high corrosion resistance, temperature capability and machinability. Heat treatment requirements should be such that the austenitic structure and corrosion resistance of the steel are preserved.

Annealing is widely used to relieve internal stresses and improve machinability, while stress relieving is applied particularly after welding. Hardening is not applied because of the austenitic structure of AISI 316. This steel is an excellent choice for high temperature and chemical environments and is a widely used material in industrial applications.​‌​​‌​

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What AISI 316 Is — and the Real Difference Between 316 and 316L

AISI 316 (UNS S31600 / W.Nr. 1.4401 / DIN X5CrNiMo17-12-2) is the molybdenum-bearing branch of the 18-8 austenitic family: nominally 17 % Cr – 11 % Ni – 2.1 % Mo. The only thing separating it from 304 is molybdenum, and molybdenum does one job: it raises the resistance of the passive film to local breakdown in chloride environments. The price is real. In oxidising acids — nitric acid above all — 316 is WORSE than 304, and because its chromium is lower (16–18 % against 18–20 %), it is also slightly worse in high-temperature oxidation. 316 is not a “better 304”; it is a 304 re-tuned for a different environment.​‌​​‌​

Three distinctions confuse buyers most often, and all three are readable on the mill certificate: (1) the carbon difference between 316 and 316L, (2) the molybdenum difference between 1.4401 and 1.4436 in Europe, (3) the molybdenum ceiling difference between ASTM and EN. All three change price and code acceptance.

316H is a further branch: its carbon is deliberately held high for creep service and it occupies its own UNS row in the ASTM A240 table. The 316H composition band could not be independently verified in this study, so we publish no numbers for it — if you are ordering 316H, confirm the band directly from the current A240 table.

1.4401 and 1.4436 are NOT the same thing in Europe​‌​​‌​

When you order “316” from a European mill, two different grades can arrive and both are legitimately labelled 316. The difference is molybdenum:

The rule: shipping 1.4401 against an order for 1.4436 is a non-conformance. The reverse leaves you with material that is chemically better but specification non-compliant, and many buyers reject it as a deviation. Do not substitute one for the other.

Standards by Product Form​‌​​‌​

316 has one of the broadest specification coverages of any austenitic stainless — but not every product form uses the same chemistry table, and the mechanical minima change with product form.​‌​​‌​

ASME Code Acceptance and MAXIMUM CODE TEMPERATURES​‌​​‌​

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This section carries the most expensive single difference between 316 and 316L, and almost no distributor page states it correctly.

The warning that belongs on the page: the ASME ceiling of 816 °C is a code permission, not a recommendation. 316 sensitises when held in the 427–816 °C band, and that entire band sits inside the code permission: a 316 vessel operating at 600 °C and code-compliant becomes susceptible to intergranular corrosion over its service life. The code limits strength, not corrosion.

Product Forms Whose Scope Is Narrower Than Assumed​‌​​‌​

There is almost no product form for which 316 has “no standard”. But there are places where the scope is narrower than buyers assume, and the sales engineer needs to know them.

Chemical Composition

ASTM A240 / ASME SA-240 — weight %​‌​​‌​

S31600 (Type 316): C ≤0.08 · Mn ≤2.00 · P ≤0.045 · S ≤0.030 · Si ≤0.75 · Cr 16.0–18.0 · Ni 10.0–14.0 · Mo 2.00–3.00 · N ≤0.10 · Fe balance.

S31603 (316L): identical, with one change: C ≤0.030.
S31635 (316Ti): identical (C ≤0.08), plus Ti: 5×(C+N) min, 0.70 max.​‌​​‌​

EN 10088 — weight %

Typical mill analyses (the producer’s nominal table, not a specification): 1.4401: C 0.04 · Cr 17.2 · Ni 10.1 · Mo 2.1 % · 1.4436: C 0.04 · Cr 16.9 · Ni 10.7 · Mo 2.6 % · 1.4404 (316L): C 0.02 · Cr 17.2 · Ni 10.1 · Mo 2.1 % · 1.4571 (316Ti): C 0.04 · Cr 16.8 · Ni 10.9 · Mo 2.1 % + Ti. Note this: the nominal chemistry of 1.4401 and 1.4404 is identical apart from carbon — the entire difference between them is carbon.

Mechanical Properties​‌​​‌​

ASTM A240 minima — the contractual numbers​‌​​‌​

Annealed plate and sheet, ASTM A240 / SA-240: 316 (S31600) tensile ≥515 MPa (75 ksi), yield (0.2 %) ≥205 MPa (30 ksi), elongation (50 mm) ≥40 %, hardness ≤217 HBW / ≤95 HRB. 316L (S31603) tensile ≥485 MPa, yield ≥170 MPa, elongation ≥40 %, ≤217 HBW. 316Ti (S31635) is identical to 316: 515 / 205 MPa, elongation ≥40 %. 317 515 / 205 MPa (elongation ≥35 %), 317L 515 / 205 MPa (elongation ≥40 %).

EN 10088-2 · flat product​‌​​‌​

1.4401 (cold and hot rolled): Rp0.2 240 MPa · Rp1.0 270 MPa · Rm 530–680 MPa · A 40 %. 1.4404 (316L): Rp0.2 240 MPa cold rolled, 220 MPa hot rolled (a 20 MPa drop), Rm 530–680 MPa, A 40 %. 1.4436: Rp0.2 240 MPa, Rm 550–700 MPa. 1.4571 (316Ti): Rp0.2 240 MPa, Rm 540–690 MPa.

Careful — the most common calculation error: the EN flat-product yield is 240 MPa and the ASTM minimum is 205 MPa. The 17 % gap is not a material difference, it is a standards difference. Design to the EN number and buy to ASTM and your calculation sits above the real minimum.​‌​​‌​

EN 10088-3 · bar (varies with diameter — there is no single number)

How to read that table: in cold drawn thin bar the yield rises to 400 MPa — double the annealed value. That is not an alloy difference, it is cold work. Above 16 mm the cold-work effect is gone and the yield returns to 200 MPa. The sentence “316 bar yields at 400 MPa” is true only for ≤10 mm.

Typical values (NOT minima — do not use them as acceptance criteria)​‌​​‌​

Annealed, room temperature: 316 yield 292 MPa, tensile 568 MPa, elongation 68 %, reduction of area 81 %. 316L yield 302 MPa, tensile 608 MPa, elongation 56.8 %. 317L yield 319 MPa, tensile 610 MPa. Charpy V-notch (annealed 316 at 23 °C): 88–134 J (65–100 ft-lb). The austenitic structure keeps high impact resistance even at cryogenic temperatures — this is its single biggest advantage over ferritic and martensitic stainless steels.

Elevated-temperature tensile data (typical)​‌​​‌​

316: at 20 °C yield 292 / tensile 568 MPa (68 % elongation) · at 427 °C 183 / 493 MPa (47 %) · at 538 °C 161 / 472 MPa (55 %) · at 649 °C 156 / 349 MPa — here elongation falls to 24 % and reduction of area to 32 %, so ductility bottoms out · at 760 °C tensile 212 MPa · at 871 °C tensile 124 MPa.

316L for comparison: at 427 °C yield 184 / tensile 455 MPa; at 538 °C 179 / 444 MPa; at 649 °C 174 / 374 MPa; at 871 °C 116 / 185 MPa. So the yield values of 316 and 316L are very close, but from 427 °C upward 316L is distinctly weaker in tensile (455 against 493 MPa). The reason the code cuts 316L at 454 °C is creep, and the tables above are short-term tensile data only — do not use them in place of creep data.​‌​​‌​

Strength by cold work — 316 cannot be hardened but is not weak

Representative values after 60 % cold reduction: 316 yield 1,036 MPa / tensile 1,170 MPa / elongation 3.5 %; 316L yield 1,144 MPa / tensile 1,341 MPa / elongation 5.8 %; 317 yield 1,044 / tensile 1,182 MPa; 317L yield 1,026 / tensile 1,269 MPa. The conclusion: 316 does not harden by heat treatment, but cold work raises its yield more than fivefold. The price is ductility — elongation falls from 40 % to 3.5 %. Cold-worked 316 also has higher magnetic permeability and, because the part now carries residual stress, it is more exposed to chloride stress corrosion cracking.​‌​​‌​

Physical Properties

A note for the designer: the thermal expansion of 316 is about 1.4× that of carbon steel and its thermal conductivity about one third. Together these magnify distortion and residual stress in welding. The low conductivity also means the heat of cutting goes into the tool — that is the physical reason 316 machines badly.

Heat Treatment and Thermal Stability​‌​​‌​

One heat treatment only: solution annealing

316 cannot be hardened by heat treatment. The austenitic structure does not transformation-harden; the only route to higher strength is cold work. The only heat treatment applied is solution annealing.​‌​​‌​

The sensitisation band — the real weakness of 316​‌​​‌​

Held in the 427–816 °C (800–1500 °F) band, 316 precipitates chromium carbides at the grain boundaries. Because the carbide consumes chromium, the zone immediately beside the boundary becomes chromium-depleted, and that narrow band dissolves far faster than the bulk in a corrosive environment — intergranular corrosion. This is not a thermal embrittlement; it is purely a corrosion event and it is invisible in mechanical testing.

The measured result (ASTM A262 comparative tests): in Practice B, 316 base metal corrodes at 36 mpy (0.9 mm/yr) and shows intergranular attack; welded 316 at 41 mpy (1.0 mm/yr). In the same test 316L base runs 26 mpy (0.7 mm/yr) and welded 23 mpy (0.6 mm/yr). In Practice A, 316 gives a ditched structure — UNACCEPTABLE, while 316L gives a step structure — acceptable. In Practice E U-bend testing, the 316 weld develops fissures — UNACCEPTABLE, while 316L shows none. Those three lines are the difference between 316 and 316L, measured.​‌​​‌​

Who is affected: 316 (C ≤0.08 %) sensitises in the weld HAZ, during stress relief in the 427–816 °C band, and in continuous service in that band. 316L (C ≤0.030 %) is practically immune for welding — there is not enough carbon to form the carbide; but it is NOT immune in service: prolonged exposure at 427–816 °C is harmful to 316L too. In 316Ti, titanium ties up the carbon as TiC and leaves the chromium free — the same result without lowering the carbon, and with higher strength at temperature than 316L.

Two operational rules: (1) Sections heavier than 11.1 mm (7/16 in) usually require annealing after welding — in a heavy section the cooling rate is not fast enough through the critical band. (2) Apply a low-temperature stress relief to 316 and you walk straight into the sensitisation band; in corrosive service, stress relief on 316 is either a full solution anneal or nothing at all.​‌​​‌​

High-temperature oxidation

316 shows good oxidation resistance and a low scaling rate in air up to 871–899 °C (1600–1650 °F). But be honest: in this duty 316 is generally somewhat inferior to 304, because 304 carries more chromium (18 % against 16 % in 316). Molybdenum buys nothing here. For high-temperature oxidation the right answers are 310 and 314, not 316.​‌​​‌​

Welding

The austenitic stainless steels are the most weldable of the stainless steels, and 316 is the typical case. All fusion and resistance processes are used routinely: GTAW (TIG), GMAW (MIG), SMAW, submerged arc, flux-cored, plasma, resistance. Oxyacetylene welding is not recommended.​‌​​‌​

Machining​‌​​‌​

316 machines poorly compared with carbon steel, for three physical reasons: (1) work hardening — the cut surface hardens and the second pass enters harder material; (2) low thermal conductivity — the heat goes into the tool, not the part; (3) continuous ductile chips — they do not break, they wrap. Molybdenum adds a slightly gummier behaviour than 304 on top of that.

Corrosion — Where It Works, Where It FAILS

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What molybdenum actually buys​‌​​‌​

PREN (pitting resistance equivalent number) is commonly calculated as Cr + 3.3×Mo + 16×N. With typical analyses:

304 (Cr 18.0 · no Mo · N 0.06) → PREN 19.0. 316 (Cr 16.5 · Mo 2.1 · N 0.05) → PREN 24.2. 317 (Cr 18.5 · Mo 3.1) → PREN 29.7. 904L (Cr 20.5 · Mo 4.5) → PREN 36.2. For duplex 2205 independent sources give 34–36; the mill’s own table gives PRE 24 for 1.4401 and 25 for 1.4436. Warning: PREN is not a single number. The nitrogen coefficient appears in the literature as 16 or 30, and adding tungsten changes the formula again. Never put PREN values computed with different formulas side by side.​‌​​‌​

Pitting and crevice corrosion — the critical temperatures

This is where datasheets mislead most often: you cannot publish a single number for “the critical pitting temperature of 316”, because CPT depends on the test medium. Two different data sets, both correct:​‌​​‌​

Chloride limits — the practical numbers​‌​​‌​

The practical limits published by the mill: 304 / 304L ≈100 ppm chloride, 316 / 316L ≈2,000 ppm, 317 / 317L ≈5,000 ppm. Seawater carries about 19,000 ppm chloride, and in the mill’s own words 316 and 317 are not recommended for seawater. These limits move down with temperature, pH, stagnation and the presence of crevices: in a stagnant, warm, crevice-rich system you will see failures well below 2,000 ppm.

Stress corrosion cracking (SCC) — 316 is NOT exempt​‌​​‌​

This is the most dangerous misconception about 316. Molybdenum raises pitting resistance, but does essentially nothing against chloride-induced stress corrosion cracking: austenitic stainless steels are structurally exposed to SCC, and 316 is exposed.

Three conditions must coexist for SCC: (1) halide ions (generally chloride), (2) residual or applied tensile stress, (3) temperature above roughly 49 °C (120 °F). With all three present, 316 cracks. One independent source puts the threshold for 316L at 500 ppm chloride at about 55 °C; another says 60 °C; a mill datasheet uses 50 °C. The numbers cluster in the 49–60 °C band — and none of them says 316 does not crack.​‌​​‌​

Behaviour in acids — the honest table​‌​​‌​

Sulphuric acid — the section that must be read by concentration. The mill’s own words: “316 and 317 alloys are considerably more resistant than any of the other chromium-nickel types to solutions of sulphuric acid. At temperatures as high as 49 °C (120 °F), 316 and 317 alloys are resistant to concentrations of this acid up to 5 percent.” That sentence sets two limits at once: 5 % concentration and 49 °C. Outside that envelope 316 is consumed rapidly — the boiling 10 % figure above is the proof. Any datasheet that writes “316 resists sulphuric acid” without conditions is wrong. For sulphuric acid the right address is 904L or the nickel alloys.​‌​​‌​

Nitric acid — where 316 is WORSE than 304. The mill’s own words: “One known exception is highly oxidising acids such as nitric acid to which the molybdenum-bearing stainless steels are less resistant.” Specifying 316 for nitric acid service is a mistake; 304 or 304L is better. Molybdenum weakens the passive film under oxidising conditions — the unavoidable price of the 316 design, and the refutation of “316 is always better than 304”.

WHERE IT FAILS — summary​‌​​‌​

Honest Comparison — 316 or Something Else​‌​​‌​

Frequently Asked Questions​‌​​‌​

Should I order 316 or 316L?

The decision rule in one sentence: if the part will be welded, cannot be solution annealed afterwards, and will see a corrosive environment — 316L. If the part will carry pressure above 454 °C and is designed under the ASME code — 316 (or 316Ti), because 316L’s code ceiling is 454 °C. If both conditions apply at once, the answer is 316Ti. In practice the best answer is dual-certified material: a heat held below 0.030 % carbon that also meets the 205 MPa yield and 515 MPa tensile minima is certified as both S31600 and S31603, combining 316L’s sensitisation resistance with 316’s mechanical minima. One caution on the code ceiling: to use it as 316, the designer must write 316 into the specification and use 316 stresses in the code calculation — two numbers on a certificate do not raise the code ceiling by themselves.​‌​​‌​

Can 316 be used in seawater?

The short answer: not for long-term service. Seawater carries about 19,000 ppm chloride, while the practical chloride limit for 316 is about 2,000 ppm — an order of magnitude apart. The mill’s own datasheet explicitly does not recommend 316 or 317 for seawater. So why does everyone call 316 “marine grade”? Because two different duties are being confused. Marine atmosphere (salt air, spray, occasional wetting with drying and washing in between) is not the same thing as permanent immersion. 316 performs well in the first, and the laboratory data supports it: in seacoast exposure at ambient temperature 316 showed no SCC. In the second — especially in stagnant seawater and wherever there is a crevice (under gaskets, under bolt heads, under biofouling) — 316 is not reliable: its G48 critical crevice temperature is −2.5 °C, so crevice attack is thermodynamically possible even in ice-cold water. For permanent seawater immersion the correct classes are super duplex, the 6Mo austenitics, or nickel alloys.​‌​​‌​

Is 316 magnetic?

Practically not, in the annealed condition — but “non-magnetic” is a simplification. The measured value for annealed 316 is magnetic permeability <1.02 (at 200 oersteds), which means a hand magnet will not stick. Three situations change that: (1) Cold deformation — bending, drawing, deep drawing, a turned surface; permeability rises with the amount of deformation. (2) Weld metal — 316 fillers are deliberately formulated to contain delta ferrite to prevent hot cracking, and delta ferrite is ferromagnetic, so the bead is more magnetic than the parent metal. (3) Castings — the CF-8M structure contains delta ferrite. Therefore the field test “a magnet stuck to it, so it is not stainless” is UNRELIABLE for 316. Verify the chemistry from the certificate or by PMI — especially the molybdenum, since that is the only real difference between 304 and 316.​‌​​‌​

Why is 316 worse than 304 in nitric acid?

Because molybdenum becomes a liability in oxidising media. The corrosion resistance of stainless comes from the chromium oxide passive film. In reducing environments (chloride waters, dilute sulphuric acid) that film breaks down locally, and molybdenum accelerates repassivation of the broken site and arrests pit growth — that is the entire reason 316 exists. In strongly oxidising media such as nitric acid the film is already very stable; what matters there is film quality, i.e. the chromium content. Molybdenum does not join that film and at high oxidising potential begins to dissolve itself. The mill’s sentence is unambiguous: “the molybdenum-bearing stainless steels are less resistant to highly oxidising acids such as nitric acid“. On top of that, 316’s chromium is two points lower than 304’s — both effects pull the same way. In a nitric acid plant, 316 is a more expensive material giving a worse result.​‌​​‌​

What is the maximum temperature for 316?

There is no single answer, and the counter-question is “which limit?” There are four limits and each gives a different number. (1) ASME code ceiling: Section VIII Div. 1 allows 816 °C for 316 and 454 °C for 316L. That is a strength/creep limit. (2) Oxidation limit: good behaviour in air up to 871–899 °C — but in this duty 316 is slightly worse than 304. (3) Sensitisation limit: 427 °C. Held above this for long, 316 returns to a corrosive environment susceptible to intergranular corrosion. If the equipment will operate in corrosive service, this is the real ceiling — not 816 °C. (4) European pressure vessel design values: the mill publishes design values up to 400 °C. So in corrosive service the practical ceiling for 316 is around 400–427 °C; 816 °C is meaningful only where corrosion is not a factor. Never use a datasheet that states a single “maximum service temperature” without first asking which limit it is.​‌​​‌​

Common Datasheet Errors and Traps

1) “316 is a seawater stainless / marine grade.” The most common and most expensive myth. Seawater is ≈19,000 ppm chloride, the practical limit for 316 is ≈2,000 ppm, and the mill explicitly does not recommend seawater. “Marine grade” is true for marine atmosphere, not for permanent immersion.​‌​​‌​

2) “316 = 1.4401 = 1.4436.” In Europe 1.4401 (Mo 2.00–2.50 %) and 1.4436 (Mo 2.50–3.00 %) are separate grades whose bands do not overlap. The ASTM S31600 band (2.00–3.00 %) covers both, so the difference is invisible on the American side and is a non-conformance on the European side.

3) “316 and 316L are practically the same.” Largely true for general corrosion; completely false on code and sensitisation. ASME VIII-1 ceiling: 816 °C against 454 °C. ASTM A262 Practice A: 316 is ditched (unacceptable), 316L is step (acceptable).​‌​​‌​

4) Publishing CPT as a single number. For the same material: 15 °C in G48, 70 °C in 500 ppm chloride water. A CPT without a stated medium is noise, not information.

5) Ignoring crevice corrosion. Most pages discuss pitting and say nothing about crevices. Yet the G48 critical crevice temperature for 316 is −2.5 °C — and most field failures start under a gasket, on a flange face, at a pipe support.​‌​​‌​

6) “316 beats 304 in every acid.” False. 316 is worse in nitric acid — the mill says so in its own words. 304 is also ahead in high-temperature oxidation.

7) “316 resists sulphuric acid.” Only inside the envelope of 5 % concentration and 49 °C. In boiling 10 % H₂SO₄ the rate is 16.1 mm/yr — 16 mm of wall in a year.​‌​​‌​

8) Using the EN yield in place of the ASTM minimum. EN 10088-2 flat product is 240 MPa, ASTM A240 is 205 MPa. The 17 % gap is a standards difference, not a material difference.

9) “316 bar yields at 400 MPa.” In EN 10088-3 that value applies only to cold drawn bar ≤10 mm. Above 16 mm the same grade drops to 200 MPa. A bar yield quoted without a diameter is unusable.​‌​​‌​

10) “316 is non-magnetic.” Annealed permeability is <1.02, but cold work, the delta ferrite in weld metal and the cast structure all raise it. The magnet test is not a quality control method for 316.

11) “316 cannot be hardened, so it is weak.” It does not harden by heat treatment — correct. But 60 % cold reduction takes the yield to 1,036 MPa. The error is either calculating cold-worked 316 with annealed values, or putting a cold-worked part into chloride service while ignoring its residual stress.​‌​​‌​

12) Treating PREN as an absolute number. The nitrogen coefficient is 16 or 30 depending on the source; the mill publishes 24 for 1.4401 while an independent source publishes 26. Rank only within one formula.

13) “Let us stress relieve after welding.” For 316 in corrosive service that means walking into the sensitisation band (427–816 °C). Either a full solution anneal with cooling to black heat in under three minutes, or nothing.​‌​​‌​

14) Treating castings and wrought material as the same. The cast equivalent of 316 is CF-8M (for 316L, CF-3M); the cast structure contains delta ferrite and behaves differently both mechanically and in corrosion. There is no standard product called a “316 cast valve body”.

15) Ignoring the heavy-section rule. Sections above 11.1 mm (7/16 in) usually require annealing after welding, because the cooling rate is not fast enough through the critical band. A fabricator who does not know this rule will carry a procedure that works on thin sheet over to heavy plate and deliver sensitised equipment.​‌​​‌​

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