Stainless Steel 431: High-Strength Martensitic Grade for Demanding Service

What Is Stainless Steel 431

Stainless steel 431 is a martensitic grade with a mixed martensitic-ferritic microstructure and 15 to 17 percent chromium plus 1.25 to 2.5 percent nickel. The nickel addition improves corrosion resistance and toughness while the martensitic phase provides high strength, filling the gap between basic martensitic grades such as 410 and austenitic grades such as 304.

Chemical Composition

Key limits per ASTM A240 are carbon 0.20 percent maximum, chromium 15.00 to 17.00 percent, nickel 1.25 to 2.50 percent, and manganese 1.00 percent maximum. The higher chromium and nickel contents give better corrosion resistance than 410 or 420.

Mechanical Properties

In the heat-treated condition 431 has a minimum tensile strength of 725 MPa, a minimum yield strength of 515 MPa, and a minimum elongation of 15 percent, with hardness up to 285 HB annealed and up to 45 HRC after hardening. The combination of strength and fatigue resistance suits high-stress rotating and reciprocating components.

Heat Treatment

Annealing at 800 to 900 degrees C with slow cooling softens the steel for fabrication. Quenching from 950 to 1050 degrees C in water forms additional martensite for strength, and tempering at 200 to 600 degrees C tailors the properties: tempering near 300 degrees C gives 40 to 45 HRC for valve stems, while tempering near 600 degrees C reduces hardness to about 30 HRC and boosts toughness for drive shafts.

Welding

Welding 431 requires care to preserve the structure and avoid martensite embrittlement in the heat-affected zone. Preheat to 150 to 250 degrees C, use low heat input with TIG preferred, and temper after welding at 250 to 300 degrees C to soften the heat-affected zone. Filler selection should stabilize the weld zone, for example with a higher-nickel austenitic filler.

Applications

431 is used for automotive drive shafts, aircraft components such as landing gear parts and control rods, pump shafts, valve stems, fasteners, and high-strength hardware requiring corrosion resistance to road salt, moisture, and mild acids. Equivalent grades include EN 1.4057, JIS SUS431, and DIN X2CrNi17-2.

Frequently Asked Questions

Q: What is the composition of 431?

A: 15 to 17 percent chromium and 1.25 to 2.5 percent nickel.

Q: What is the tensile strength of 431?

A: 725 MPa minimum.

Q: How does 431 compare with 410?

A: Higher chromium and nickel give better corrosion resistance.

Q: Can 431 be welded?

A: Yes, with preheat, low heat input, and post-weld tempering.

Q: What hardness can 431 reach?

A: Up to about 45 HRC.

Q: What applications use 431?

A: Drive shafts, aircraft components, valve stems, and fasteners.

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