AISI 431
AISI 431 Stainless Steel: The Industrial Strength of a High Strength Martensitic Stainless Alloy
AISI 431 stainless steel is a nickel-alloyed, high strength representative of the martensitic stainless group. This alloy shows superior performance to similar martensitic classes such as AISI 410 and AISI 420 in terms of both mechanical strength and corrosion resistance. While the nickel addition increases the impact strength of the steel, the chromium content provides an above-average corrosion resistance. For these reasons AISI 431 is preferred across a broad industrial spectrum, from aerospace to automotive and from petrochemical equipment to marine components.
This article addresses the chemical composition of AISI 431, its heat treatment behaviour, its mechanical properties and its classifications according to standards at a technical level; at the same time it speaks to the industrial reader in the context of the expressions encountered in purchasing processes and the areas of use.
AISI 431 Chemical Composition and Microstructural Properties
AISI 431 is a martensitic stainless steel in which chromium and nickel are present together. Its difference from other martensitic grades is its nickel content of approximately 1.25–2.0%. This nickel contributes to both toughening and corrosion resistance, giving the material a more balanced behaviour.
Approximate chemical composition:
- Carbon (C): 0.16% – 0.25%
- Chromium (Cr): 15.0% – 17.0%
- Nickel (Ni): 1.25% – 2.50%
- Manganese (Mn): Max. 1.0%
- Silicon (Si): Max. 1.0%
- Phosphorus (P): Max. 0.04%
- Sulphur (S): Max. 0.03%
This composition allows the steel both to reach high hardness after quenching and to be more resistant to the risk of cracking after heat treatment. The nickel content is the most distinctive property setting AISI 431 apart from the other martensitic classes.
AISI 431 Heat Treatment and Mechanical Properties
One of the most important advantages of AISI 431 is that it offers versatile mechanical properties that can be optimised by heat treatment. The quenching temperature is in the range of 980–1050 °C. A martensitic structure is obtained after rapid cooling in water or oil. Tempering is then applied between 150–400 °C to establish the desired balance between toughness and hardness.
Typical mechanical properties after heat treatment:
- Tensile Strength (UTS): 850 – 1200 MPa
- Yield Strength (0.2% offset): 550 – 950 MPa
- Hardness: 28 – 45 HRC (depending on the tempering degree)
- Elongation: 10% – 18%
- Resistance to impact: High (especially in the annealed and tempered condition)
These values make AISI 431 a material preferred in parts requiring both high strength and acceptable ductility.
ASTM, AMS and Other International Standards
AISI 431 has been defined by many international standards bodies and its use technically regulated. These norms are of great importance for technical alignment between producers and users.
ASTM Standards:
- ASTM A276 – Stainless steel bars and section products
- ASTM A479 – Bar products for pressure vessel applications
- ASTM A314 – Industrial type stainless bars
- ASTM A182 – Forged flanges and fittings
AMS Standards:
- AMS 5628 – Annealed or hardened AISI 431 bar, forging and ring products
- AMS 5625 – Special quality 431 stainless steel for aircraft components
EN/DIN Equivalent:
- DIN 1.4057 / X17CrNi16-2
- EN 10088-3 semi-finished products compliant with the standard
These standards play a determining role in terms of quality assurance, especially in the aerospace, marine and energy sectors. In supply contracts, expressions such as “AMS 5628 certified 431 bar” are used to document the source and the quality of the material.
Application Areas and Industrial Use
AISI 431 is widely used in parts requiring both structural strength and corrosion resistance. Its application areas concentrate in environments where mechanical loading and environmental effects are present together.
- Marine and Offshore Industry: Thanks to its high chromium content, AISI 431 is resistant to salt water corrosion at a medium level. This property provides an important advantage in applications such as sea water pump shafts, propeller shafts, spindles and connection parts.
- Aerospace: Because of its high strength and hardness properties it is preferred in landing gear components, engine parts and fasteners. The term “431 stainless steel aerospace grade” is a typical expression of this area of use.
- Chemical and Petrochemical: Although its resistance to acids and alkalis is limited, it is suitable for valve parts, shaft systems and gaskets in mildly corrosive environments. ASTM A479 431 material is widespread in this type of application.
- Automotive and Machinery Industry: It is used in applications such as parts operating under high torque, gears, crankshafts and fasteners requiring high strength.
- Food Industry: Thanks to its medium resistance to corrosion, AISI 431 finds a place in components such as meat processing machines, blade systems and conveyor shafts.
Advantages and Disadvantages of AISI 431
Advantages:
- High mechanical strength
- Medium level corrosion resistance (especially in the tempered condition)
- Configurability by heat treatment
- Good polishability (decorative and functional surfaces)
- Standards compliance and widespread availability
Disadvantages:
- Lower general corrosion resistance compared with austenitic stainless steels
- Its weldability is low because of the high carbon content
- It carries a risk of cracking in chlorinated environments
- Its low temperature strength is limited; it may become brittle in very cold environments
Conclusion: The Place of AISI 431 in Supply and Application Decisions
AISI 431 is one of the stainless steel grades that provides an optimum balance between high strength, heat treatment capability and medium level corrosion resistance. This alloy can be preferred with confidence in engineering projects where both structural strength and environmental resistance requirements are met together.
From a purchasing point of view, technical terms such as “DIN 1.4057 stainless bar”, “431 round bar compliant with the AMS 5628 standard” and “431 stainless shaft steel” offer a clear and standard language when requesting material from a supplier. It is critical for correct material decisions that users take criteria such as service conditions, surface finishing needs and welded fabrication into account during product selection.
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| Chemical Properties | Min. | Max. | Mechanical Properties | Value | |
|---|---|---|---|---|---|
| C | - | 0.20 | Tensile Strength (MPa) | 862 | |
| Mn | - | 1.00 | Proof Stress (MPa) | - | |
| Si | - | 1.00 | Elongation A50 mm | 20 | |
| P | - | 0.04 | Hardness Brinell | 285 Max HB | |
| S | - | 0.03 | Density | 7.80 g/cm3 | |
| Cr | 15 | 17 | Melting Point | 1698 °C | |
| Ni | 1.25 | 2.50 | Modulus of Elasticity | 200 kN/mm² | |
| | | Electrical Resistivity | 0.72 x 10-6 Ω.m | ||
| | | Thermal Conductivity | 20.2 W/m.K | ||
| | | Thermal Expansion | 10.2-12.1 x 10-6/K |
We supply this material
We supply AISI 431 (1.4057) — to the AMS 5628 standard, in the dimensions and forms required. For stock availability and pricing:
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