AISI 431 – UNS S43100
High-strength Cr-Ni martensitic stainless steel
Martensitic with 15–17% Cr and 1.25–2.5% Ni, which has the best corrosion resistance among conventional martensitic grades and good toughness in the quenched and tempered condition. Used in shafts and fasteners that require mechanical strength with exposure to water and mild marine environments.
Designations and equivalents
| System | Designation |
|---|---|
| UNS | S43100 |
| W.Nr./EN | 1.4057 (X17CrNi16-2) |
| DIN | X17CrNi16-2 |
| JIS | SUS431 |
| Trade names | Type 431 |
| Others | GB 14Cr17Ni2 (formerly 1Cr17Ni2); AMS 5628 |
Standards by product form
| Form | Standard |
|---|---|
| Bar | ASTM A276 / A479 (431); EN 10088-3; AMS 5628 |
| Forging | ASTM A473 (431) |
| Plate | EN 10088-2 (1.4057) |
| Wire | ASTM A580 |
Tolerances and HS codes
- Forged bars (round and square): machining allowance from 2 mm (Ø 6–16 mm) to 24 mm (Ø 400–500 mm) on the diameter. E.g. Ø 100–125 mm finished: +7 mm, tolerance +6 / −0 (length up to 3,500 mm).
- Forged discs: allowance of 4 to 9 mm on the diameter and 3 to 11 mm on the height, plus-only tolerance.
- Cutting and machining: band saw −0 / +5 to +15 mm on length; waterjet +10 % of thickness on the diameter (min. 2 mm); machined parts −0 / +2 to +5 mm on the diameter. See the full tables (INF 002 and INF 003, the same for every alloy).
| Form | NCM (Brazil, 8 digits) | HS (international, 6 digits) | HTS (USA, 10 digits)* | CN (European Union, 8 digits)* |
|---|---|---|---|---|
| Forged bar | 7222.30.00 | 7222.30 | 7222.30.0024 | 7222 30 51 |
| Hot-rolled bar | 7222.11.00 | 7222.11 | 7222.11.0059 | 7222 11 11 |
| Plate (> 10 mm, width ≥ 600 mm) | 7219.21.00 | 7219.21 | 7219.21.0020 | 7219 21 10 |
| Flange and fitting | 7307.21.00 | 7307.21 | 7307.21.5000 | 7307 21 00 |
Tariff class: stainless steel (by chemical composition, under the Chapter 72 Notes and Section XV Note 5).
*: US and EU codes for reference. NCM per current legislation (General Rules of Interpretation, Notes to Chapters 72, 74 and 75 and Section XV Note 5). Confirm the final code with your customs broker according to the item size and finish. HS = first 6 digits of the NCM.
Chemical composition (%)
ASTM A276 (S43100)
| Element | Range (%) |
|---|---|
| C | ≤ 0.20 |
| Mn | ≤ 1.00 |
| Si | ≤ 1.00 |
| P | ≤ 0.040 |
| S | ≤ 0.030 |
| Cr | 15.0 – 17.0 |
| Ni | 1.25 – 2.50 |
| Fe | balance |
Mechanical properties
Quenched and tempered QT800 (EN 10088-3, 1.4057, d ≤ 160 mm); annealed per ASTM A276
| Property | Value |
|---|---|
| Yield strength (0.2%) | ≥ 600 MPa (QT800) |
| Tensile strength | 800 – 950 MPa (QT800) |
| Elongation | ≥ 14% |
| Charpy V impact (20 °C) | ≥ 25 J |
| Hardness, annealed (A276) | ≤ 277 HB |
| Max. hardness, hardened | ≈ 44 HRC (low temper) |
Typical physical properties
| Property | Value |
|---|---|
| Density | 7.70 g/cm³ |
| Melting range | 1,430 – 1,510 °C |
| Modulus of elasticity | 200 GPa |
| Thermal conductivity (100 °C) | 20.2 W/m·K |
| Thermal expansion (20–100 °C) | 10.2 µm/m·K |
| Electrical resistivity | 72 µΩ·cm |
Heat treatment, welding and machining
| Item | Usual practice |
|---|---|
| Service temperature | Structural use up to ≈ 400 °C (above that it loses strength); avoid below 0 °C in impact service, because of the drop in toughness. |
| Heat treatment | Subcritical annealing 620–660 °C (air) or full annealing 800–850 °C (furnace). Hardening: 950–1,020 °C, oil or air. Tempering 560–700 °C for toughness (QT800 ≈ 600–650 °C) or 250–370 °C for maximum hardness; avoid 400–560 °C. Double tempering recommended. |
| Welding | Difficult (air hardening). Preheat 200–300 °C, interpass ≥ 200 °C, post-weld tempering 620–660 °C. Consumables AWS E410/ER410 or ER410NiMo (strength) or E309L/ER309L (no post-weld treatment, lower strength). |
| Machining | Fair; machine in the annealed or QT800 condition, with a sharp tool and constant feed (work hardens less than austenitic grades). |
Filler metal for welding
The wire composition is usually not identical to the material: each page explains why.
Typical applications
- Pump and propeller shafts
- Valve stems and internals
- Bolts, nuts and studs
- Aircraft components
- Compressor and agitator parts
Alternatives: 410/420 if corrosion is mild and cost matters; F6NM (13-4) for better weldability and toughness; 17-4PH for higher strength with similar or better corrosion resistance; duplex 2205 for chloride corrosion.
AISI 431: complete guide
What AISI 431 stainless steel is
431 belongs to the family of martensitic and ferritic stainless steels. Chromium of 15 to 17% is the highest among conventional martensitic grades and gives the best corrosion resistance of the group; nickel of 1.25 to 2.5% balances that chromium and ensures that the steel transforms to martensite on quenching.
Welding
It is a steel that is difficult to weld, because it hardens in air. It requires preheating to 200 – 300 °C, a minimum interpass of 200 °C, slow cooling and post-weld tempering at 620 – 660 °C, with ER410NiMo consumables or, when joint strength is not critical, austenitic E309L. Whenever possible, weld before the final heat treatment.
Corrosion and limits of use
With a PREN of 15 to 17, 431 resists atmosphere, fresh water and mild marine environments at a level close to the austenitic 301 and 302, provided it is quenched and tempered, polished and passivated. It is not indicated for concentrated chlorides, stagnant seawater or acids. Under load, the practical limit is about 400 °C, and below 0 °C toughness must be verified by impact testing.
Applications
Pump and ship propeller shafts, valve stems and components, high-strength bolts, nuts and pins, aircraft components and machine parts that work in humid or mild marine environments.
Supply forms and Dustre services
Round bars and blocks forged in open die, cut to size on a band saw, with rough or finish machining, heat treatment (quenching and tempering, annealing, solution annealing and aging) in our own furnaces and hardness, tensile and impact testing. All material is supplied with a 3.1 inspection certificate and heat traceability.
Frequently asked questions
What is the chemical composition of AISI 431?
Per ASTM A276 (S43100): C ≤ 0.20; Mn ≤ 1.00; Si ≤ 1.00; P ≤ 0.040; S ≤ 0.030; Cr 15.0 – 17.0.
Which standards cover AISI 431?
Bar: ASTM A276 / A479 (431); EN 10088-3; AMS 5628. Forging: ASTM A473 (431). Plate: EN 10088-2 (1.4057). Wire: ASTM A580.
Up to what temperature can AISI 431 be used?
Structural use up to ≈ 400 °C (above that it loses strength); avoid below 0 °C in impact service, because of the drop in toughness.
Which materials can replace AISI 431?
410/420 if corrosion is mild and cost matters; F6NM (13-4) for better weldability and toughness; 17-4PH for higher strength with similar or better corrosion resistance; duplex 2205 for chloride corrosion.
