​‌​​‌​

AISI 304 / (1.4301) / UNS S30400 / AMS 5511 / AMS 5513

​‌​​‌​

Corrosion resistance: The corrosion resistance of AISI 304 and AISI 304L in normal atmospheric environments is very good. 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 200 mg/L of free chlorine in service water at standard temperature and to 150 mg/L above 60 °C.

Temperature capability: 1.4301 (304) 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 304L 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 304 is 308, and for 304L it is 308L. When welding thin sections in grade 304, post-weld annealing is not required, but it must be applied on thick sections. For 304L, post-weld annealing is not required even on thick sections (6 mm and above, for example).

Machinability: Grade 1.4301 (304) is the most frequently used and most versatile stainless steel in the world. It has excellent formability and weldability. The austenitic structure of 304 allows deep drawing without intermediate annealing, which is why it is chosen for products requiring deep drawing such as sinks, gas flues and saucepans.​‌​​‌​

Heat treatment: Because of its low carbon (C) content it cannot be hardened by heat treatment.

Applications: Food processing equipment; kitchen, boat and vehicle equipment (sinks, fittings, pans, handrails and so on); architectural facade cladding (panels, rails and so on); chemical transport containers; heat exchangers; bolts, nuts and screws; and springs.​‌​​‌​

AISI 304 is a versatile, highly corrosion resistant, machinable and durable stainless steel alloy.

What AISI 304 Is — and the Real Difference Between 304 and 304L​‌​​‌​

AISI 304 (UNS S30400 / W.Nr. 1.4301 / EN name X5CrNi18-10) is the most produced stainless steel in the world and the reference point of the entire austenitic family. Nominally 18 % chromium – 8 % nickel — which is where the trade name “18-8” comes from. It is face-centred cubic (FCC) austenitic: it cannot be hardened by heat treatment, it forms superbly, it welds easily, it is non-magnetic and it stays ductile down to cryogenic temperatures.

The real value of 304 is not one outstanding property but the absence of failure anywhere. 316 is better in chloride, 321 more stable hot, 430 cheaper, 303 faster to machine — but none is simultaneously formable, weldable, code-covered and stocked in every product form. 304 is the engineering definition of “good enough”.​‌​​‌​

304 or 304L — there is no one-line answer, but there is a one-question answer

The only difference is carbon: ASTM A240 gives C ≤0.07 % for 304 and C ≤0.030 % for 304L (older editions and many datasheets still show 0.08 % for 304). That small difference changes two things at once, and the two pull in opposite directions.​‌​​‌​

The other variants in the family​‌​​‌​

Standards by Product Form​‌​​‌​

This table is exactly where the commercial strength of 304 lies: practically every product form has a standard that names it. When ordering, write the specification number together with the product form, not just the grade name — the same grade is sold under different documents as sheet, bar, pipe and forging.​‌​​‌​

ASME Code Acceptance and Maximum Code Temperatures​‌​​‌​

​‌​​‌​

The numbers in this section are CODE LIMITS, not material capability. Confusing the two is how a datasheet sentence like “304 withstands up to 925 °C” becomes a design temperature. That sentence is about oxidation; the code is about allowable stress.

Product Forms Whose Scope Is Narrower Than Assumed

304 appears to exist everywhere; that is also what causes the commercial accident. The rows below are the points where the assumption “304 suits everything anyway” breaks.​‌​​‌​

Chemical Composition​‌​​‌​

There is no single chemistry table for 304 — there are three, and all three differ. This is the most overlooked commercial detail of the grade.

Mechanical Properties

​‌​​‌​

​‌​​‌​

In austenitic stainless, “strength” is not one number. The same 304 plate yields at 205 MPa annealed and exceeds 900 MPa after 30 % cold work. Always state which condition a table describes.

304 cannot be hardened by heat treatment. Hardness and strength come only from cold deformation, and that same deformation increases susceptibility to chloride stress corrosion cracking. If high strength and chloride must coexist, the answer is not cold work but a change of material — duplex 2205 / F53 class grades, or precipitation hardening stainless such as 17-4 PH.​‌​​‌​

Physical Properties

Warning: 304 physical data circulates in two publishing traditions — the US/ASTM tradition and the European/EN tradition — and some values genuinely differ. Both are given below; do not average them.​‌​​‌​

Heat Treatment and Thermal Stability​‌​​‌​

Welding

304 is sold on its weldability and the reputation is deserved — but “welds easily” does not mean “welds carelessly”. Three things are managed: heat input, filler choice and ferrite number.​‌​​‌​

Machining​‌​​‌​

304 is a difficult material to machine, and the reason is not hardness — annealed 304 is only about 170 HB. The problem is work hardening: deformation in the cutting zone hardens the surface instantly, and on the next pass the tool has to cut the hard skin it created itself.

Is there a faster-machining 304? Yes. Mills produce improved-machinability 304 variants: controlled sulphur (far below free-machining levels), controlled inclusion shape, tight grain size and low residual elements. The ASTM chemistry is still S30400/S30403, so welding, forming and corrosion behaviour remain those of 304. They do not reach 303 speeds — but 303 cannot be welded and cannot see chloride. The real choice is usually between these two, not between “304 or 303”.

Corrosion — Where It Works, Where It FAILS​‌​​‌​

The protection of 304 comes from one thing: a passive oxide film a few nanometres thick, formed by chromium. That film repairs itself in the presence of oxygen — that is the strength of 304. But the film is weak against the chloride ion and cannot re-form where oxygen cannot reach. Every failure mode of 304 follows from those two sentences.​‌​​‌​

Where it works well

Atmospheric corrosion. Decades in urban and rural atmospheres. Architectural facades, railings, trim.
Potable water and food. Brewing, dairy and winemaking equipment; kitchen benches, sinks and cookware; food processing and storage.
A broad range of organic and inorganic chemicals. Particularly good in oxidising media such as nitric acid — they feed the passive film.
Cryogenic service. No ductile–brittle transition.
High purity service. Demineralised water, pharmaceutical purified water (with the right surface finish).​‌​​‌​

Where it FAILS

1. Chloride pitting. 304 has no molybdenum. Its PREN (Cr + 3.3Mo + 16N) is published between ~18 and 20 — the molybdenum contribution is zero. One mill’s measured values are striking: critical pitting temperature (CPT) <10 °C and critical crevice temperature (CCT) <0 °C. In other words, in a standard laboratory chloride solution 304 can pit below room temperature.
2. Chloride thresholds — sources diverge, and the divergence is instructive. Published values: ~200 mg/L at ambient and ~150 mg/L at 60 °C (one producer); ~400 mg/L at ambient, ~150 mg/L at 60 °C (another publication); and a more conservative producer: 100 ppm chloride is generally considered the limit for the 18-8 alloys. Three sources, a four-fold spread. The correct engineering posture is to take the most conservative value, because the next item can invalidate all of them.
3. Crevice corrosion — and the CONCENTRATION of chloride. The real danger is not the average chloride but the local one. Under a gasket, under a bolt head, under a deposit, at the foot of weld spatter, oxygen is consumed and chloride rises far above the bulk value. The same happens wherever evaporation occurs: a wet–dry cycling surface, a dripping condensate, a leak drying under insulation. “Our water is only 50 ppm chloride” guarantees nothing where crevices and evaporation exist.
4. Chloride stress corrosion cracking (SCC) — the most insidious failure of 304. It needs three things together: tensile stress + chloride + temperature. The engineering threshold is quoted as ~60 °C. But the threshold is not absolute: an independent safety authority report documents failures at much lower temperatures where chloride concentrates. The same report gives two more figures: laboratory threshold stresses of 80–100 MPa — some studies report values as low as 10 % of the 0.2 % proof stress — and that maintaining low chloride levels cannot ensure freedom from SCC. The source of stress is usually not the service load but weld residual stress or cold work, and weld residual stress approaches the yield strength.
5. SCC under insulation (CUI-SCC). Wetted insulation leaves austenitic stainless pipework particularly vulnerable: the insulation holds chloride, the wet–dry cycle concentrates it, and the pipe temperature sits in the risk band. Established countermeasures: wrapping with aluminium foil or thermally sprayed aluminium coating.
6. Seawater. Do not use it. 304 is not a seawater material. Nor, in truth, is 316; real seawater service belongs to 904L, super duplex F55 or the nickel alloys.
7. Reducing acids. Sulphuric and hydrochloric acid reduce the passive film. 304 has limited use in dilute and cold versions of them and cannot be used in concentrated or hot ones.
8. Intergranular corrosion when sensitized. 304 that has passed through 425–860 °C without re-annealing separates along its grain boundaries in a corrosive medium. This is the reason 304L exists.
9. Iron contamination. A carbon steel brush, grinding dust or weld spatter leaves free iron on the surface; that iron rusts and the stainless is blamed for rusting. The cure is segregated tooling, pickling and passivation.​‌​​‌​

Honest Comparison — 304 or Something Else​‌​​‌​

Frequently Asked Questions​‌​​‌​

Should we buy 304 or 304L? Our supplier already ships dual-certified plate, so the question is settled — isn’t it?

For most work it really is settled. But there are two situations where dual certification misleads you, and both are expensive.
First, the difference: the only divergence is carbon — ≤0.07 % for 304, ≤0.030 % for 304L. In the 425–860 °C band carbon migrates to the grain boundaries and precipitates as chromium carbide; because it takes its chromium from the adjacent matrix, the zone next to the boundary becomes chromium depleted and intergranular corrosion starts there. The heat-affected zone of a weld spends time in exactly that band. In 304L there is no carbon left to precipitate, so the risk effectively disappears. In exchange 304L is 35 MPa weaker in yield and 30 MPa in tensile (170/485 against 205/515 MPa).
Dual-certified plate combines the two: carbon at the 304L limit (≤0.030 %) while the mechanicals also meet the 304 minima. It is usually the right move.
First trap: high temperature. In ASME, plain 304 may be used above 538 °C (1000 °F) only if its carbon is ABOVE 0.04 %. Dual-certified material is by definition ≤0.030 % — so it does not substitute for “304” above 538 °C. Published code limits: 304 → 816 °C, 304L → 650 °C. Dual-certified plate carries 304L’s limits. If you genuinely need 304 hot, buy 304H (C 0.04–0.10 %), or read and confirm the carbon on the certificate.
Second trap: the design calculation. If the buyer sized the wall on 304’s 205 MPa yield and the supplier ships 304L-certified plate that also meets the 304 mechanicals, there is no problem. The problem appears when plate certified only to 304L arrives: it is assessed against 170 MPa and the calculation is short. Do this: if you want dual certification, write “dual certified 304/304L, ASTM A240” on the order — not “304L acceptable”. They are not the same thing.​‌​​‌​

Our 304 pipeline cracked under insulation. Our chloride analysis says 40 ppm. How is that possible?

This is classic chloride stress corrosion cracking under insulation, and the 40 ppm figure misled you — because it was measured in the wrong place.
SCC needs three things at once: tensile stress, chloride and temperature. You almost certainly had all three.
Stress. Forget the service load: weld residual stress approaches the yield strength and every circumferential weld leaves it behind. Laboratory threshold stresses have been measured at 80–100 MPa, and some studies report values as low as 10 % of the proof stress. The stress criterion is effectively satisfied on every welded pipe.
Chloride. The error here is conceptual. Your 40 ppm is the bulk chloride of the fluid. SCC cares about the LOCAL chloride at the metal surface. Insulation gets wet, and wet insulation creates a wet–dry cycle at the pipe surface; at every drying step chloride is left behind and concentrates. Within months the surface chloride can reach hundreds of times the bulk value. The independent safety authority report puts it plainly: maintaining a low chloride level cannot ensure freedom from SCC where chloride concentrates in crevices or under deposits.
Temperature. The quoted threshold is ~60 °C, but failures below it are documented. Look at the highest temperature reached at any stage — steam-out, a cleaning cycle, commissioning, an excursion. One hot cycle a week matters more than the annual average.
What to do. (1) Fix the insulation: wrap the pipe in aluminium foil or apply thermally sprayed aluminium — both are established, proven measures. (2) Keep water out: jacket permeability, air leaks, valve and flange boxes. (3) Reduce the stress: post-weld solution annealing on critical lines — but do NOT apply a partial stress relief in the 425–860 °C band, that produces sensitization. (4) Change the material: 316 raises the threshold but is not immune; for real immunity look to ferritic or duplex grades. What you must not do is repeat the analysis and conclude “chloride is low, that cannot be the cause”.​‌​​‌​

Our 304 kitchen bench has rusted. Isn’t stainless supposed to be stainless?

What is rusting is almost certainly not the 304 but the iron ON the 304. This is the most common and most easily cured of all stainless complaints.
What protects 304 is a passive oxide film a few nanometres thick that repairs itself in the presence of oxygen. But free iron smeared onto the surface is not stainless: with moisture it produces red-brown rust and stains the 304 underneath. The sources are almost always a carbon steel wire brush, grinding dust, a shared bench top, weld spatter or steel hand tools.
The second possibility is genuine pitting. 304 has no molybdenum and is weak against chloride, and a kitchen is full of it: salt, bleach-based cleaners, lemon, brine, hard-water marks. The worst case is a droplet drying on the surface, because evaporation concentrates chloride over and over. Measured values confirm it: CPT <10 °C, CCT <0 °C.
The third possibility is crevice corrosion: under the sealant, at the sink-to-bench joint, under screw heads — oxygen cannot reach there and the film cannot be repaired.
What to do. Remove the surface rust with a stainless wire brush or a non-abrasive pad, then passivate (commercial citric or nitric acid products). Do not leave bleach on the surface and rinse and dry after use — a drying droplet is far more damaging than the droplet itself. If the problem recurs and the environment really is chloride-rich (coastal, poolside, heavy salt use), the answer is material, not cleaning: 316 exists for exactly this scenario, thanks to molybdenum.​‌​​‌​

Common datasheet errors — check these before you order

1. Publishing a single “304 chemistry”. There are at least three different tables: A240 (plate) Cr 17.50–19.50 / Ni 8.00–10.50 / C ≤0.07 / Si ≤0.75; A276 (bar) Cr 18.0–20.0 / Ni 8.0–11.0 / C ≤0.08 / Si ≤1.00; EN 10088-2 Cr 17.5–19.5 / S ≤0.015. It is entirely possible for a heat to pass A240 and fail A276.
2. Still printing carbon as 0.08 %. Current editions of A240 give ≤0.07 %; A276 still gives ≤0.08 %. Both are correct — they are different documents. Know which one you ordered against.
3. A silent conflict in modulus and thermal expansion. Modulus is published as 193 and 200 GPa; thermal expansion as 17.2 and 16.0 × 10⁻⁶/K — both under the heading “304”. Do not average them; state which tradition you used — the 7 % expansion gap is a real difference on a long pipeline.
4. Treating “304 can be used to 925 °C” as a design temperature. That number is about oxidation and scaling. On the code side there is an 816 °C limit for ASME VIII Div. 1 and a requirement that carbon exceed 0.04 % above 538 °C. On the European pressure route, design values are typically given only to 400 °C. Three different numbers, three different meanings — label each one.
5. Assuming dual-certified 304/304L is 304 at high temperature. Its carbon is ≤0.030 %, so it carries 304L’s creep limits and cannot be used above 538 °C.
6. Giving the sensitization band as a single number. Published bands are 425–860, 425–815 and 480–820 °C. Publish the conservative envelope; do not average them.
7. Selling ASTM A554 tube as pressure pipe. A554 is ornamental and mechanical tube, not a pressure specification. Pressure lines take A312 / A358 / A409. This is the most dangerous error on the list.
8. Writing “304 casting”. The cast equivalents are CF-8 (≈304) and CF-3 (≈304L); they contain 5–20 % δ-ferrite and are not the same as wrought 304. Write CF-8 on a valve or pump body order.
9. Giving the chloride threshold as a single number. Published values range between 100 ppm, ~200 mg/L and ~400 mg/L — a factor of four. And all of them assume no crevices and no evaporation. Take the conservative value and publish the local-concentration warning.
10. Publishing PREN to a decimal place. Values between 18 and 20 circulate for 304, even in two brochures from the same producer. PREN is not measured; it is calculated from a typical chemistry — the decimal is meaningless.
11. Writing “304 is non-magnetic” without a condition. Correct version: it is non-magnetic in the annealed condition; after cold work strain-induced martensite makes it magnetic — two points on the same plate can behave differently.
12. Recommending “post-weld stress relief”. It is a carbon-steel reflex and it is harmful in austenitic stainless: a slow cycle at 425–860 °C produces sensitization. If something is needed, it is a full solution anneal plus rapid cooling.
13. Skipping weld cleaning. Heat tint is not harmless; the correct sequence is mechanical cleaning with stainless tooling → pickling → passivation.
14. Claiming “304 is suitable for seawater”. False. Seawater service is not safe even in 316; it requires 904L, super duplex or a nickel alloy.
15. Confusing 1.4306 with 1.4307. Both are sold as “304L” but their chromium and nickel bands differ. On a European order, write the W.Nr. explicitly.
16. Equating ASTM and EN elongation: ASTM ≥40 % · EN ≥45 %.​‌​​‌​

​‌​​‌​