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AISI 405 / (1.4002) / UNS S40500

Corrosion resistance: AISI 405 stainless steel is a ferritic stainless steel with improved welding properties, formulated for primary forming into corrosion resistant wrought products. Its corrosion resistance is moderate.​‌​​‌​

Weldability: The aluminium content of the alloy reduces the tendency to harden adjacent to the weld bead, which minimises susceptibility to cracking. Its suitability for welding is good.

Machinability: It has a fairly high thermal conductivity among the wrought ferritic stainless steels. In addition it has a fairly low embodied energy and a moderately low base cost. Its properties are suited to the annealed condition. Machinability is moderate.​‌​​‌​

Heat treatment: It is a 12% chromium steel designed to be used in the as-welded condition. Unlike other 12% chromium steels it does not harden excessively on air cooling. Although AISI 405 is chosen for applications requiring greater high temperature capability and greater resistance to oxidation, it is generally more expensive than AISI 430 because of its higher cost.

Applications: Application areas are generally annealing boxes, steam nozzles, quenching racks, partitions and other fabrications. It cannot be annealed after welding.​‌​​‌​

1.4002 is an excellent choice for applications requiring high temperature capability and resistance to oxidation. It has limitations in terms of corrosion resistance, however, so austenitic steels may be preferred if the material is to be used in aggressive chemical environments.

What AISI 405 Is — and Why the Aluminium Is There​‌​​‌​

AISI 405 (UNS S40500 / W.Nr. 1.4002 / DIN X6CrAl13) is a ferritic stainless steel: nominally 12–13 % chromium, low carbon, and — the entire reason the alloy exists — 0.10–0.30 % aluminium. It carries no nickel (the ASTM ceiling is ≤0.60 %), it is magnetic, and it cannot be hardened by heat treatment.

The only way to understand 405 is to understand what goes wrong with 410. A steel at about 12–13 % chromium transforms partly to austenite at high temperature. On cooling, that austenite transforms to martensite — so the part hardens and embrittles without anyone asking it to. In welding this happens exactly in the heat-affected zone: the narrow band beside the fusion line goes above 1000 °C, cools in air, and is left as hard, crack-prone martensite. That is what happens when 410 is welded, and it is why welded 410 fabrication demands preheat and post-weld annealing.​‌​​‌​

The 405 solution is to shift the chemistry so that austenite cannot form at all. Aluminium is a strong ferrite former — like chromium, silicon and molybdenum it widens the ferrite field. A small addition of 0.10–0.30 % effectively closes the austenite field in a steel of about 12–13 % chromium. No austenite on heating means no martensite on cooling. The result, in the producer’s own words: “Unlike other grades in the 12 % chromium category, 405 is not susceptible to extensive hardening through air cooling from high temperature.” That is the commercial reason 405 exists, in one sentence: it is the 12Cr stainless made for parts that cannot be annealed after welding.

How to read that table: the difference between 405 and 410 is not only aluminium — the carbon differs too. The carbon of 410 is a BAND, 0.08–0.15 %: 410 must contain carbon, because it needs it to harden. In 405 carbon is only a ceiling (≤0.08 %). Aluminium and low carbon work together: one narrows the austenite field, the other lowers the hardenability of whatever little austenite does form.

Standards by Product Form​‌​​‌​

Next to the austenitics, 405 has narrow specification coverage. Plate, bar and tube are covered solidly; on the pipe, fitting, flange and fastener side there are real gaps, and quoting without knowing them is risky.​‌​​‌​

ASME Code Acceptance​‌​​‌​

The code status of 405 is not published as clearly as that of the austenitics, and this section has to be honest about it.

Product Forms With NO Standard — the Commercially Valuable Section

This is where the real sales knowledge for 405 lives. Buyers think of 405 as if it were an austenitic and ask for every product form; the coverage is narrow.​‌​​‌​

Chemical Composition​‌​​‌​

ASTM A240 / ASME SA-240 — weight %

S40500 (Type 405): C ≤0.08 · Mn ≤1.00 · P ≤0.040 · S ≤0.030 · Si ≤1.00 · Cr 11.5–14.5 · Ni ≤0.60 · Al 0.10–0.30 · Fe balance.​‌​​‌​

Mechanical Properties​‌​​‌​

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The minima CHANGE WITH PRODUCT FORM — this is the most common error

There is no single “yield value” for 405. The same grade carries different minima depending on which specification it was ordered to:​‌​​‌​

Typical values — three sources, three different numbers [CONFLICT]​‌​​‌​

The published typical values for 405 are inconsistent between sources, and showing that is more honest than hiding it. Three data sets in circulation for the annealed condition:

Toughness — the quiet limit on 405

The real weakness of ferritic stainless steels is not strength but TOUGHNESS. Unlike the austenitic structure, the ferritic structure shows a ductile-to-brittle transition temperature (DBTT): below a certain temperature the material fails in a brittle manner. Austenitic 316 still carries 88–134 J of impact energy at cryogenic temperatures, while ferritics fall far below that under their transition temperature.​‌​​‌​

Let us be honest: no numerical DBTT or Charpy table for 405 could be independently verified in this study, so we publish no number. But the qualitative rule is established practice and the engineering conclusion is clear: ferritic stainless steels are used in thin sections. As the section thickens, both stress triaxiality and weld grain coarsening increase, and together they push the transition temperature up. For a thick, notched pressure part that may take impact at low temperature, 405 is the wrong material. Any thickness limit must come from the specification you order to and the customer’s impact testing requirement — do not invent a millimetre figure.

Physical Properties​‌​​‌​

The physical properties of 405 differ sharply from those of austenitic stainless, and the differences affect design directly. The ferritic structure is far closer to carbon steel than to the austenitics.

Heat Treatment and Thermal Stability

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One real operation: annealing

405 cannot be hardened by heat treatment. The ferritic structure does not transform to martensite; preventing exactly that is the whole purpose of the aluminium. The operation applied is annealing, and its aim is not hardness but ductility and the removal of internal stress.​‌​​‌​

The damaging phase windows — and HOW MUCH 405 is actually exposed to them​‌​​‌​

Two classic embrittlement mechanisms are published for ferritic stainless steels. Both depend on chromium, and 405 sits at the BOTTOM of the band. This is the distinction datasheets miss most often when they copy and paste.

Welding

The entire commercial value of 405 lives here, so this section has to be detailed. 405 is welded by shielded fusion and resistance methods. Oxyacetylene welding is not suitable.​‌​​‌​

Machining and Forming​‌​​‌​

Unlike the austenitics, 405 is an easy material to machine. The reason is directly microstructural: the ferritic structure does not work-harden as the austenitic structure does, its thermal conductivity is roughly twice as high (heat does not build up in the tool), and the chip breaks more readily.

Corrosion — Where It Works, Where It FAILS

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Realistic positioning

The honest sentence is this: 405 is a marginal stainless steel. In the publisher’s own words, “AISI Type 405 / 1.4002 stainless steel has marginal stainless steel corrosion resistance” and it is “resistant only to mild atmospheric and fresh water“. The reason is simple: corrosion resistance comes from chromium, and in 405 the chromium is only 11.5–14.5 % — below the 16–18 % of 316 and below the 16–18 % of 430. Aluminium contributes nothing to corrosion resistance; its role is metallurgical.​‌​​‌​

WHERE IT FAILS​‌​​‌​

Honest Comparison — 405, 410, 409 or 430​‌​​‌​

Frequently Asked Questions​‌​​‌​

What exactly is the difference between 405 and 410?

There are two differences, and both serve the same purpose: avoiding martensite. The first is aluminium. 405 contains 0.10–0.30 % Al; 410 contains none. Aluminium is a ferrite former: it suppresses austenite formation at high temperature, and if no austenite forms, no martensite forms on cooling. The second is carbon. The carbon of 410 is a mandatory BAND of 0.08–0.15 % — 410 has to want carbon, because its hardenability comes from it. In 405 carbon is only a ceiling (≤0.08 %) and lower is preferred. The practical consequence: 410 hardens when air-cooled from high temperature and leaves hard, crack-prone martensite in the weld HAZ, which is why welded 410 fabrication demands preheat and post-weld annealing. 405 does not — in the producer’s words, “unlike other grades in the 12 % chromium category, 405 is not susceptible to extensive hardening through air cooling from high temperature“. The price is strength: 410 can be hardened, 405 cannot, and its annealed yield minimum is 170 MPa instead of 205 MPa. Decision rule: if the part needs hardness, 410; if the part will be welded and cannot be annealed, 405.​‌​​‌​

Is 405 magnetic? Was stainless steel not supposed to be non-magnetic?

405 is magnetic, and that is not a defect but the definition of its class. The sentence “stainless steel is non-magnetic” applies only to the austenitic stainless steels and only in the annealed condition. Stainless steel splits into four main families and three of them are magnetic: ferritic (405, 409, 430), martensitic (410, 420, 440C) and duplex. Only the austenitic family (304, 316, 321) is practically non-magnetic when annealed — and even that is conditional, because cold deformation and the delta ferrite in weld metal raise permeability. Magnetism is a consequence of CRYSTAL STRUCTURE, not of chromium or of stainlessness: the body-centred cubic structure of ferrite is ferromagnetic, the face-centred cubic structure of austenite is not. So the test “a magnet stuck to it, therefore it is not stainless” is metallurgically wrong, and on ferritic grades such as 405 it is completely misleading. But the converse warning is also needed: in any application sensitive to magnetic fields, 405 is eliminated from the start.​‌​​‌​

Do I have to anneal 405 after welding?

No — and that is the entire reason the alloy exists. 405 was developed for “fabrications that cannot be annealed after welding”, and the published application list says exactly that. Because aluminium prevents HAZ martensite, the mandatory justification that exists for 410 does not exist for 405. But there are two nuances. (1) Post-weld annealing maximises ductility — so while it is not mandatory, calling it useless would also be wrong. It can be worth considering after forming or on heavily restrained joints. The published cycle is soak at 1500–1600 °F, then raise rapidly to 1900–2050 °F; for low-chromium ferritics it is separately published that heating at 788–843 °C (1450–1550 °F) assures a wholly ferritic structure and that cooling should stop no lower than 593 °C to control distortion. (2) The real welding problem in 405 is not martensite but grain coarsening in the HAZ, and annealing does not reverse it. The only defence against grain coarsening is procedural: low heat input, narrow beads, fast travel, single pass where possible. That is the one thing post-weld annealing cannot fix.​‌​​‌​

How good is the corrosion resistance of 405, really?

Low. The publisher’s own words are “marginal stainless steel corrosion resistance” and “resistant only to mild atmospheric and fresh water“. Even so, many distributor pages claim “good corrosion resistance” for 405 — because it is easy to copy a 12Cr page from a 304 page. The truth: corrosion resistance comes from chromium, and in 405 chromium is 11.5–14.5 %. Where it works: nitric acid, organic acids, alkalis, mild atmosphere, fresh water and oxidation in air up to 704 °C. Where it ends: sulphuric, hydrochloric, hydrofluoric and phosphoric acid, and seawater. But in one area 405 beats the austenitics outright: chloride stress corrosion cracking. The ferritic structure is structurally closed to that mechanism, whereas austenitic 316 cracks in 4–24 hours in boiling magnesium chloride. So writing 405 off as a “bad stainless” is also wrong: it has a narrow but real envelope, and inside that envelope it is cheap, weldable and immune to SCC.​‌​​‌​

Common Datasheet Errors and Traps

1) Classifying 405 as “martensitic”. There are supplier pages in the market whose product title reads “Martensitic Stainless Steel Tube S40500”. 405 IS FERRITIC; the sole purpose of the aluminium is to prevent martensite from forming. This error comes from mistaking 405 for 410 and it sets up the welding procedure wrongly from the start.​‌​​‌​

2) Publishing a hardening cycle. The same datasheets both say “this alloy does not respond appreciably to heat treatment” and print a cycle of 982–1010 °C plus an oil quench. That is an internal contradiction and it leads buyers to expect hardness from 405. 405 cannot be hardened.

3) Misstating the role of aluminium. One widely circulated page gives the role of aluminium as “preventing grain growth during welding“. That is wrong. Aluminium is a ferrite former; by narrowing the austenite field it prevents martensite. Grain coarsening is still a problem in 405 and aluminium does not solve it.​‌​​‌​

4) Quoting the nickel ceiling as a single number. ASTM A240: Ni ≤0.60 % · the SA-268 route: ≤0.50 %. A heat at Ni 0.55 % passes one and fails the other. The number is unusable unless the governing document is stated.

5) Giving the yield without the product form. A240 (plate): 170 MPa · SA-268 (tube): 205 MPa. Hardness likewise: ≤179 HB against ≤207 HB. This is not a contradiction but a product form difference — although unstated, it looks like one and wrecks the calculation.​‌​​‌​

6) Copying the thermal expansion from an austenitic datasheet. Some 405 pages give expansion as 9.2 (68–212 °F) and 20.5 (68–1832 °F). 9.2 × 10⁻⁶ in/in/°F ≈ 16.6 × 10⁻⁶ /K, and that is the value for AUSTENITIC 316. The published figure for ferritic 405 is 10.8 × 10⁻⁶ /K — nearly a third lower. This is a copy-paste error that directly corrupts a design.

7) Printing numbers without units. One published 405 page gives thermal conductivity as “416” with no unit at all (most probably Btu·in/hr·ft²·°F). Never pass on a number without its unit. The verified value is 27 W/(m·K) (one source gives 30).​‌​​‌​

8) Carrying the 475 °C and sigma warnings over blindly. Those warnings were written for high-chromium ferritics (430, 444, 26-1). The literature’s own threshold for sigma is above 25 % Cr; 405 carries 11.5–14.5 %. Print the warning, but print the level with it — the dominant embrittlement mechanism for 405 is not these phases but grain coarsening in the weld.

9) Giving the 475 °C band as a single range. Three different bands are published: 399–566 °C, 250–550 °C, 375–525 °C. The common intersection is roughly 400–525 °C. A page that shows only one band has not seen the other two.​‌​​‌​

10) The phrase “good corrosion resistance”. The producer’s own words are “marginal” and “only mild atmospheric and fresh water”. A page claiming good corrosion resistance for 405 has most likely carried its text over from a 304 page.

11) Calling the filler “405”. The correct consumable is 405Cb — the electrode containing columbium instead of aluminium. Aluminium oxidises under the arc and does not survive in the pool. In practice 309 / 310 / 312 austenitic fillers are also used, in which case the weld metal expands about 1.5× as much as the parent metal and that must be allowed for under cyclic loading.​‌​​‌​

12) Contradictory annealing temperatures. Two independent sources give 649–760 °C, a third gives 700–800 °C. The first band is better supported; the second has most likely migrated from the general post-weld annealing practice for ferritics (750–800 °C).

13) Publishing typical mechanicals as single numbers. The two data sets in circulation give yields of 276 MPa and 200 MPa — a 38 % spread. Tensile: 448 and 470 MPa. Hardness: 131 HB and 170 HB. Design to the specification minimum, not to typical values.​‌​​‌​

14) Hiding the density and modulus conflicts. Density appears as 7.70 / 7.75 / 7.80 g/cm³ and the modulus of elasticity as 200 GPa (three sources) and 190 GPa (one source). The majority sits at 200 GPa; footnote the difference.

15) Lumping all chloride resistance under one heading. 405 resists chloride SCC but does not resist chloride pitting and crevice corrosion. A page that merges the two mechanisms into one line will show 405 as either better or worse than it is. The ferritic advantage exists only in SCC.​‌​​‌​

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