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AISI 415 / (1.4313) / UNS S41500

Corrosion resistance: AISI 415 has corrosion resistance that can be considered good, thanks to its low carbon structure, its nickel content of around 4% and its molybdenum content of around 0.6%. This grade should not be specified in severely corrosive environments.​‌​​‌​

Weldability: Grade 1.4313 can be welded without difficulty thanks to its low carbon structure. Repeating the heat treatment after welding is recommended, however, for homogeneous strength throughout the material.

Machinability: In this grade, where mechanical strength is the priority and the material is supplied heat treated, machinability is not very good.​‌​​‌​

Heat treatment: This grade can be heat treated and is generally given a pre-heat treatment after production. The most common heat treatments are QT650, QT780 and QT900. The numbers 650, 780 and 900 indicate the minimum tensile strength of the grade in MPa. It is generally produced as QT780 and QT900.

Applications: Because of its superior mechanical properties it is used in the chemical and petrochemical industries, in plant engineering, in the pump industry and in turbine runners. 1.4313 is also used in power engineering and in valve manufacture. Valves, blades, pump covers and pump casings, for example, are produced from 1.4313.​‌​​‌​

AISI 415 is an alloy in the martensitic stainless steel class with properties such as high hardness, wear resistance and high temperature capability. It is limited in terms of corrosion resistance and weldability, however. It is therefore suitable for cutting tools, mechanical parts requiring wear resistance and high temperature applications, while materials such as the austenitic steels should be preferred for use in chemical environments or areas carrying a high corrosion risk.

What AISI 415 Is — and Why It Is Called a “Soft Martensitic” Steel​‌​​‌​

AISI 415 (UNS S41500 / W.Nr. 1.4313 / DIN X3CrNiMo13-4) is the low-carbon, nickel- and molybdenum-bearing 13Cr martensitic stainless steel sold as ASTM A182 F6NM in wrought form and as CA6NM (UNS J91540) in cast form. Nominal composition is 13 % Cr – 4 % Ni – 0.5–1 % Mo with C ≤0.05 %. One fact separates it from every other alloy in the family: the carbon that makes martensite hard has been deliberately removed, and the austenite stability that makes martensite form at all has been put back with nickel.

In a classic martensitic stainless (410, 420, 440C) hardness comes from carbon. More carbon, more hardness — but the same carbon forms chromium carbide (Cr₂₃C₆) and consumes chromium at the grain boundaries. The result is lost toughness, weld cracking and sensitisation. In 415 carbon is held below 0.05 %. A 13Cr alloy that lean would stay ferritic — it would not transform to martensite on quenching at all. That is what the nickel is for: 3.5–5.5 % Ni widens the gamma field, makes the alloy fully austenitic at the austenitising temperature, and gives on cooling a low-carbon, therefore ductile and tough, lath martensite. Molybdenum is added to raise pitting resistance and temper resistance.​‌​​‌​

Honest positioning against the siblings​‌​​‌​

Standards by Product Form​‌​​‌​

This is where 415/F6NM is strongest: it has full specification coverage on both the wrought and the cast side. That sharply distinguishes it from siblings such as 416.​‌​​‌​

Code Acceptance and Temperature Limits — Be Careful Here​‌​​‌​

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415 is not a high-temperature alloy, and its design temperature limit is a metallurgical limit, not an arbitrary code number. The reason comes straight out of the heat treatment: the properties of the part are produced by tempering at roughly 560–620 °C. As the service temperature approaches the tempering temperature, the part keeps tempering in service — strength falls, and the loss is irreversible.

Product Forms With NO Standard

415 is the best in its family on this heading, but the gaps are still real. Three that a sales engineer must know:​‌​​‌​

Chemical Composition​‌​​‌​

The point to watch here is that ASTM and EN are not the same material. The nickel band, the molybdenum band and the nitrogen requirement all differ. Dual-certified material exists, but it is not automatic.

Mechanical Properties

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415 has three separate mechanical property systems and they are not interchangeable: standard ASTM, ASTM softened for NACE, and the EN QT classes. Taking a number from one table and pasting it into another is the most common purchasing error with this grade.​‌​​‌​

Physical Properties

Published data in this section conflicts seriously, and saying so is more useful than hiding it. Thermal conductivity in particular circulates as two values a factor of two apart.​‌​​‌​

Heat Treatment and Thermal Stability — the Heart of the Alloy​‌​​‌​

Understanding 415 means understanding DOUBLE TEMPERING. It is the alloy’s most important and most misunderstood feature.

First, the transformation temperatures​‌​​‌​

Why double tempering is MANDATORY — the mechanism​‌​​‌​

A 415 part that received only one temper can be in service with the worst possible microstructure. Here is why:

Step 1 — as quenched. The part is untempered lath martensite throughout. Hard (around 40 HRC), brittle, full of residual stress. Unusable in this condition.
Step 2 — first temper, typically 650–690 °C. Two things happen at once. The martensite tempers (carbides precipitate, stress relaxes, toughness rises) and, because Ac1 is only 600–620 °C, the temperature enters the critical range: AUSTENITE re-forms in the microstructure. That austenite is enriched in nickel. Nickel-enriched austenite has an Ms below room temperature — so it does not transform to martensite on cooling; it stays as stable thin films at room temperature. The literature calls this reversed austenite, and it is the real source of the toughness: soft, ductile films that blunt crack tips.
Step 3 — and here is the trap. If the first tempering temperature is too high (measurement: austenite content peaks around 630–640 °C and then destabilises), the austenite that forms is not sufficiently nickel-enriched, its Ms stays above room temperature, and on cooling it transforms into brand-new, UNTEMPERED martensite. That undoes the work of the temper: hardness comes out higher than expected and toughness drops.
Step 4 — second temper, typically 590–620 °C. Performed BELOW Ac1. It forms no new austenite; it tempers the fresh martensite accidentally created by the first temper and stabilises the remaining reversed austenite films. That is the second temper’s only job, and it cannot be skipped.​‌​​‌​

Welding

Welding is the reason 415 exists. The unweldability of 410 created this alloy, and the result is genuinely striking: hydraulic turbine runners are repaired in place, in the field, by multi-pass welding — unthinkable with a steel that hardens to 40 HRC.​‌​​‌​

Machining​‌​​‌​

An honest warning: no published, verifiable cutting-speed table could be found for this grade. The table below gives verified qualitative statements and the engineering consequences of the material’s hardness. For numerical parameters, take your tool supplier’s 13Cr-4Ni / F6NM recommendation.

Corrosion — Where It Is Good, Where It FAILS

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415 is a mid-range corrosion alloy, and both halves of that sentence matter. It is clearly better than 410 and carbon steel; it is clearly worse than 316 and duplex. Treat it as a corrosion alloy and you will be disappointed; treat it as a corrosion-resistant STRUCTURAL steel and it is excellent.​‌​​‌​

Let the number speak: PREN

The pitting resistance equivalent number is PREN = %Cr + 3.3 × %Mo. For the ASTM S41500 composition band: lower end 11.5 + 3.3 × 0.50 = 13.2; upper end 14.0 + 3.3 × 1.00 = 17.3. The EN 1.4313 band is narrower still: 13.0 to 16.3. For comparison: 316 is about 24, and F53 super duplex is above 40. That single number tells the honest story of where 415 belongs in chloride service: 415 is not a seawater alloy and never was.​‌​​‌​

Where 415 IS good

Fresh water — its home ground. Hydraulic turbine runners are the application this alloy was designed for. In fresh water 13Cr-4Ni stays passive, resists cavitation erosion far better than carbon steel or 410, and when it is damaged it can be repaired in place by welding. No other alloy combines those three properties.
Sour service (H₂S-bearing oil and gas). Accepted under NACE MR0175 / ISO 15156 subject to hardness ≤23 HRC and solution annealing plus double tempering. Valve bodies, wellhead components and flanges are sold on this basis.
Mild and moderate process fluids. Crude oil, refinery streams, condensate, industrial water circuits.
Low temperature. With 42 J Charpy at −60 °C, it is a serious candidate for cold climate and non-cryogenic low-temperature service. You would not expect that from a martensitic stainless, and it is 415’s most valuable property.​‌​​‌​

Where 415 FAILS — read this list before quoting

1. Seawater and high chlorides. At PREN 13–17, pitting and crevice corrosion are inevitable. One manufacturer page states it plainly: “not suitable for chloride environments”. Do not use 415 in stagnant seawater, chlorinated water or salt-spray environments. That is what F53 and F55 are for.
2. Crevice geometries. Under flange faces, under gaskets, under rings, under deposits. Even if bulk chloride is low, it concentrates locally inside the crevice and the alloy’s limited PREN does not cope.
3. Non-oxidising acids. Hydrochloric acid, dilute sulphuric acid. The passive film of a 13Cr steel does not survive in them.
4. Sour service with the hardness limit exceeded. This is the scenario that looks compliant on paper and breaks in the field. Above 23 HRC the risk of sulphide stress cracking (SSC) is real, and the failure is sudden, brittle and unannounced. A typical 415 bar is ~28 HRC — so the default material is NOT compliant. Sour service is a separately ordered condition.
5. Welded joints where PWHT was skipped. The untempered martensite in the heat-affected zone both violates the hardness requirement and is open to hydrogen cracking. Unless a qualified alternative such as temper-bead welding is used, PWHT is not negotiable.
6. Stress relief performed above Ac1. The part partially re-austenitises and produces untempered martensite on cooling. Hardness rises unexpectedly, toughness falls and there is no external sign of any of it. Furnace records matter as much as the material certificate on a sour-service part.
7. Galvanic couples. 415 is anodic to copper alloys and graphite gaskets, and sits on the anodic side against austenitic stainless and nickel alloys too. Do not couple a small 415 part to a large austenitic surface.
8. Long-term service above 300 °C. Not a corrosion problem — a tempering problem. Strength falls quietly.​‌​​‌​

Frequently Asked Questions

My supplier gave me a certificate saying “S41500, NACE MR0175 compliant”. Is that enough for my sour-service valve?​‌​​‌​

Probably not, and the reason lies in what the word “compliant” is doing in that sentence.
NACE MR0175 / ISO 15156 accepts S41500 for sour service conditionally. The conditions are not in the name of the material but in its condition: solution annealing plus DOUBLE tempering, and a measured hardness of ≤23 HRC. If a certificate only shows that the chemistry falls inside the S41500 band, it has told you nothing.
One number shows why this is serious: a typical 415 bar measures about 28 HRC, i.e. five points above the ceiling. And the hardness ceiling permitted by standard ASTM A182 F6NM is 295 HBW, roughly 31 HRC. In other words a perfect, fully conforming F6NM part can be unusable in sour service. That is not an inconsistency — A182 was written for general service, MR0175 for H₂S.
For the same reason the mechanical minimums of sour-service F6NM are reduced: yield 517 MPa instead of 620 MPa, tensile 655 MPa instead of 790 MPa. The 23 HRC ceiling forces a higher temper, which costs strength. A specification demanding both standard F6NM strength and NACE compliance may be contradicting itself.
What to ask for: (1) heat-treatment records showing two separate tempering cycles with temperature and hold time; (2) measured HRC values per lot, with the measurement location stated; (3) where possible, a ceiling of carbon ≤0.02 % and nitrogen ≤100 ppm — work on industrial heats shows these two elements are the deciding variables in meeting 23 HRC; one heat at 0.027 % C and 180 ppm N could be brought under the limit by optimised double tempering, while a second heat at 0.03 % C and 280 ppm N stayed above the limit despite double tempering; (4) a surface hardness check on machined faces, because heavy metal removal can leave a locally hardened layer. Hardness is not a material property but a process outcome — and in sour service what you are buying is the process.

The customer drawing says CA6NM and we have a 1.4317 casting on the shelf. Can we ship it?​‌​​‌​

Be careful — these two are listed as “equivalents” but they are not the same thing, and the difference is exactly where the customer’s money goes.
ASTM CA6NM makes molybdenum MANDATORY: 0.40–1.00 %. EN 1.4317 / GX4CrNi13-4 does not: it gives only a 0.70 % maximum, with no minimum. So a 1.4317 casting with no molybdenum — or with 0.05 % — fully conforms to its standard and is not CA6NM.
The measurable consequence is pitting resistance. Using PREN = %Cr + 3.3 × %Mo: a molybdenum-free 1.4317 (Cr 12–13 %) gives PREN 12–13. A molybdenum-bearing CA6NM (Cr 13 %, Mo 0.7 %) gives PREN ~15.3. About a 20 % difference — and in a chloride-bearing environment that difference decides whether pitting initiates at all.
The practical answer: if the actual foundry analysis of the 1.4317 casting in your stock shows molybdenum above 0.40 % and chromium inside the CA6NM band, the part is effectively of CA6NM chemistry and you can demonstrate that with the heat analysis certificate. Check the mechanical values separately too: A743 CA6NM requires 755 MPa tensile / 550 MPa yield. What you must not do is match the two grade names from an equivalence table and ship without checking the analysis. Equivalence tables show similarity, not conformity.

Our turbine runner has a crack. Can we weld it in place without dismantling? What about PWHT?​‌​​‌​

Yes — and this is precisely why 415 is so widespread.
A hydraulic turbine runner can weigh tens of tonnes, and dismantling, transporting and fitting it into a furnace is often economically impossible. 415 was designed for exactly this: because carbon is below 0.05 %, the martensite that forms in the weld heat-affected zone is not hard and brittle but relatively soft and ductile. The preheat requirement drops to as little as 10 °C — in heavy cast sections, 100–140 °C. Keep interpass temperature below 345 °C.
Filler is ER410NiMo (GTAW/GMAW) or E410NiMo-15/-16 (SMAW) — so the weld metal is itself a 13Cr-4Ni martensite and behaves like the base metal.
The PWHT question has two answers, and which you choose defines the project.
Route 1 — full PWHT. Double temper: 663–691 °C air cool, then 593–621 °C air cool. This gives the best microstructure. But doing it uniformly on a large runner with local heating is difficult, and done badly it generates its own distortion and residual stress.
Route 2 — temper-bead welding. Beads are placed in a controlled sequence and size so that each new bead tempers the heat-affected zone of the previous one. It produces an acceptable microstructure without furnace PWHT, and it is what makes field turbine repair economically viable. But it is a qualified technique, proven by procedure qualification with macro sections and hardness traverses — it is not improvised.
Whichever route you take, the absolute rule is the same: the second temper, or any stress-relief operation, must NOT exceed Ac1 (about 600–620 °C). If you do, you partially re-austenitise the part and produce untempered martensite on cooling — the exact opposite of what the repair was for. That temperature is the one red line in 415 repair.

Could we use 17-4 PH instead of 415? It gives higher strength and the prices are similar.​‌​​‌​

It depends on the application — but these two alloys do not solve the same problem, and “higher strength” is the wrong axis of comparison.
17-4 PH genuinely is stronger, and because it carries more chromium (15–17.5 %) its corrosion resistance is somewhat better too. For high-strength, medium-sized, single-piece machine elements (shafts, valve trim, fasteners) it is often the right choice.
There are three areas where 415 wins, and they are not negotiable.
First, toughness. In its sour-service version 415 can be supplied with 42 J average Charpy at −60 °C. A precipitation-hardening alloy cannot offer that at high-strength conditions. In cold climates, under impact loading, in thick section, the difference is serious.
Second, weldability and heavy section. 415 air hardens, its preheat requirement is minimal, and the post-weld tempering regime is the same as the base metal’s. 17-4 PH is weldable but requires post-weld solution treatment plus re-ageing, which is not practical on a multi-tonne part. If you have to repair a turbine runner or a large pump casing in the field, 17-4 PH is not even in the race.
Third, sour service. High-strength conditions of 17-4 PH such as H900 are not accepted for sour service under NACE MR0175 — they are simply too hard. 415 is accepted subject to the 23 HRC ceiling and double tempering. If H₂S is present, the discussion ends there.
Summary: small, high-strength, unwelded, sweet-service part → 17-4 PH. Large, welded, tough, low-temperature or sour-service part → 415. Comparing them on price and yield strength is looking at the wrong side of the question.

Common datasheet errors — check these before you place an order​‌​​‌​

1. “1.4313 is not suitable for welding” — WRONG, and the exact opposite of the truth. At least one widely mirrored European distributor page carries the sentence “Steel is not suitable for welding”. This is a generic boilerplate line written for martensitic stainless steels and pasted onto the wrong grade. 415/F6NM was developed by lowering carbon SPECIFICALLY SO THAT IT WOULD BE WELDABLE; AWS A5.9 ER410NiMo and AWS A5.4 E410NiMo are matching filler metals that exist for this grade. The world’s hydraulic turbine runners are made of this alloy and repaired in the field by welding.
2. Hardness scale confusion. One page prints the A240 S41500 hardness limit as “32 HRB maximum”. 32 HRB is roughly the hardness of aluminium and is meaningless here. The correct figure is 32 HRC (about 295 HBW). Sanity-check any table where yield exceeds tensile or a hardness looks absurd.
3. Is the NACE hardness ceiling 22 or 23 HRC? One distributor page says “NACE compliant below 22 HRC”; the F6NM producer’s own NACE specification and the academic literature give 23 HRC (about 275 HV). The majority and the primary sources are at 23 HRC. The 22 probably comes from the general martensitic rule of thumb. Confirm which number goes into your purchase specification from the current edition of the standard, and state it explicitly to the supplier.
4. Nitrogen: minimum or maximum? For EN 1.4313, sources split between N ≥0.020 (a minimum) and N ≤0.02 (a maximum). Those are opposite requirements. High nitrogen pushes hardness up in a sour-service part; state the nitrogen requirement explicitly in the order.
5. Thermal conductivity differs by a factor of two. 15 W/m·K (European/EN route) and 24–25 W/m·K (secondary databases) are published for the same material. If you are doing a thermal calculation, record in your report which value you used — a factor of two completely changes the design of a jacket heater or cooler.
6. ASTM tensile and EN yield are paired in the same table. ASTM A182 F6NM = 790/620/15 %; EN +QT780 = 780–980/620/15 %. They look similar, but in EN, Rm is a BAND, while in ASTM it is only a floor. A batch tempered too hard passes ASTM and is rejected under EN.
7. Standard F6NM minimums are confused with NACE F6NM minimums. Standard: 790 MPa / 620 MPa. Modified for NACE: 655 MPa / 517 MPa. The gap is not arbitrary: the 23 HRC ceiling forces a higher temper. A specification that demands both the high minimums and NACE compliance contains a contradiction that must be resolved before the order.
8. One temper is assumed to be enough. The ASTM A487 CA6NM text specifies a single temper. If you want sour service or critical toughness you must write DOUBLE tempering into the purchase specification separately — because the job of the second temper is to temper any untempered martensite created by the first, and if that job is skipped no inspection will reveal it.

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