GRADE 431 STAINLESS STEEL (UNS S43100 / EN 1.4057) — TECHNICAL DATASHEET AND MECHANICAL SPECIFICATION

Grade 431 is a heat-treatable, nickel-bearing martensitic stainless steel engineered to deliver a unique combination of high tensile strength, torsional stiffness, and elevated impact toughness. Among the conventional martensitic grades of the 400-series, Grade 431 possesses the highest corrosion resistance due to its high chromium content and the stabilizing addition of nickel.

Characterized by a dual-phase microstructure consisting of a quenched and tempered martensitic matrix with a minor fraction of delta-ferrite (δ-ferrite), manufacturers precisely balance the chromium equivalent ($Cr_{eq}$) and nickel equivalent ($Ni_{eq}$) during melting to limit boundary segregation. For high-integrity aerospace and marine shafting, the chemistry is highly restricted to produce an essentially ferrite-free structure that optimizes transverse properties and guards against stress-corrosion cracking.

Grade 431 is globally codified across primary regulatory bodies for seamless component tracking and material verification within critical engineering assemblies.

The alloy composition is structured to balance through-hardening mechanics with localized passive film integrity across standard and customized variants.

* Note: Under EN 10088-3, specific requests for weldability permit a sulfur limit of 0.008%–0.030%, while controlled machinability allows a range of 0.015%–0.030%.

For materials engineers, structural designers, and quality control officials requiring full transient stress data logs, dynamic alignment graphs, and certified mill test layouts, the complete reference manual should be accessed.

The physical constants of Grade 431 are determined by its body-centered tetragonal martensitic structure, providing a lower expansion profile and higher thermal conductivity than austenitic grades.

Temperature-dependent variations for thermal expansion and conductivity constants are outlined below:

Prior thermomechanical history dictates room-temperature yields. In the annealed state, the material features an average hardness of ≤ 285–295 HBW.

The data below details mechanical boundaries evaluated for standard 1-inch bars, austenitized at 980–1065°C, oil quenched, and held at tempering targets for one hour:

Lacking specified molybdenum or nitrogen additions, the Pitting Resistance Equivalent Number ($\text{PREN} = \%Cr$) tracks uniformly between 15.0 and 17.0, granting Grade 431 the highest general passivity window among the standard martensitic 400-series grades.

• Forging and Hot Forming Parameters: Forgeability is medium. Pre-heat slowly and uniformly to 850°C (1562°F), then raise rapidly to the active window of 1150–1180°C (2102–2156°F). Deforming operations must cease immediately if the temperature drops below 900°C, as hot ductility falls off sharply, causing internal hot tearing. Due to strong air-hardening mechanics, finished forgings must never cool in open air; bury immediately in dry lime or ash insulation to avoid quench cracking fields.
• Sub-Critical Process Annealing: High hardenability renders full annealing highly impractical. Softening for machine processing requires sub-critical tempering: heat uniformly to 620–660°C (ASTM guidelines) or 680–800°C (EN standards), followed by slow cooling to yield a stable ferrite-spheroidal carbide matrix ($270\text{--}295\text{ HBW}$).
• Machining Constraints: In the soft annealed condition, the material is prone to adhesive galling and tearing, prompting built-up edge formation on tool profiles. Once hardened above 30 HRC, machining causes rapid tool wear and extreme local heat generation. Optimize by requesting "Ugima-treated" bar stock, which utilizes modified oxide inclusions to act as integrated lubricants, expanding carbide tool feeds and speeds up to three times.
• Weld Zone Cold-Cracking Prevention: Weldability is poor due to high carbon (0.12%–0.22%). The HAZ transforms completely into hard, brittle untempered martensite upon cooling. Fabricators must implement a strict thermal sequence: Preheat to 200–300°C → maintain 200°C min interpass boundaries → utilize hydrogen-free shielding gas → cool slowly to ambient for full martensitic transformation → run an immediate post-weld subcritical anneal at 650°C. For structural flexibility, pair with ER316L austenitic fillers; the deposit absorbs contraction strain plastics without post-weld heat treatment, though the joint cannot be thermally hardened.
• Surface Nitrogen Alterations: Parts pre-hardened to Condition T accept plasma nitriding, achieving a surface skin hardness > 65 HRC. However, nitriding precipitates chromium nitride ($CrN$) lattices, depleting free chromium from the surrounding matrix. This reduces corrosion limits; do not specify nitriding for items exposed to acidic or marine halide streams.