Stainless steel AISI 431 - X17CrNi16-2 - Z15CN16-02 - FE PM42 - 1.4057 - APX

A nickel-alloyed martensitic steel

Martensitic stainless steels are a distinct type of ... These steels are mainly alloyed with chromium and carbon.

Role and benefits of nickel in its composition

AISI 431 contains between 1.5 and 2.5% nickel. The presence ... nickel in AISI 431 improves corrosion resistance by widening the austenitic domain at high ... temperature. This makes it possible to increase the chromium content without reducing its toughness or its corrosion resistance. The alloy therefore offers better performance in environments prone to localized corrosion, such as marine or moderately acidic conditions.

Nickel also contributes to the toughness of martensitic steel ... which makes it more resistant to mechanical stresses.

Finally, its high chromium content forms a robust ... in marine environments where salt water can be particularly aggressive.

Heat treatment and its influence on properties

The heat-treatment process includes three steps ... to reduce brittleness while maintaining hardness).

Quenching and tempering process

• Austenitizing must be carried out at temperatures ... between 980 °C and 1065 °C, transforming the steel structure into austenite.
• It is followed by an air or oil quench to cool ... which rather requires an oil quench to avoid the formation of softer phases.
• Tempering between 370 °C and 565 °C must be avoided ... to the detriment of toughness and corrosion resistance.

Chart 1 – Ductility and toughness vs tempering temperature

On this second chart, it can be seen that intermediate tempers (≈ 370–480 °C) cause a sharp drop in impact energy without any real gain in ductility: this is the temper embrittlement zone to avoid. High tempers (590–650 °C), on the other hand, restore good ductility and high toughness, offering the best compromise for shafts and critical parts.

Chart 2 – Strength and hardness vs tempering temperature

As shown on this chart, AISI 431 retains a ... high ... better suited to highly loaded structural parts.

Mechanical properties depending on delivery condition

In general, the yield strength of AISI 431 is at least 600 MPa, and its tensile strength is between 800 and 1050 MPa. Its minimum elongation is 10%, and its hardness ranges between 229 and 440 HB.

Detailed table of AISI 431 mechanical properties

This table shows the classic strength / duc... as the tensile strength increases, the elongation A5 and the impact energy KV2 decrease.

Welding, forging, machining, and polishing behavior

AISI 431 is difficult to weld and precautions ... must be taken. A gas containing hydrogen or nitrogen must never be used for welding ... this grade. Preheating between 200 and 300 °C is required and, during welding, the alloy must not cool below 200 °C.

AISI 431 has average forgeability: during ... the forging process, the material must first be heated slowly to 850 °C, then quickly to 1180 °C. ... can be performed between about 950 and 1150 °C. The material must then be cooled slowly, preferably in ... a furnace or dry sand. It is widely used for shafts, bolts, and valve parts, which are frequently produced by die forging or open-die forging.

Finally, it has very good polishing capability and ... and poor machinability. It is “ gummy ” in the annealed condition and hard in the quenched condition. It is prone to galling and its ... machinability is comparable to that of 304 but clearly inferior to that of a sulfurized stainless steel such as 416.

Corrosion behavior

AISI 431 is one of the best-performing martensitic stainless ... provided it is machined correctly and, ideally, polished and passivated.

In chloride environments, the point to watch is pitting corrosion ... and you may need to switch to 316L, duplex, or a more highly alloyed PH stainless steel.

Overall, corrosion resistance depends strongly ... on heat treatment (especially at grain boundaries and in welded HAZs).

In the presence of H2S or acidic environments (oil ... and gas), 431 has limitations: it may be suitable for secondary parts in mildly aggressive areas, but grades ... specifically qualified to NACE (13Cr, duplex, nickel alloys) are preferred for pressurized parts.

Industrial applications

Aerospace industry  : this stainless steel ... is highly valued by the aerospace industry due to its exceptional tensile strength, toughness ... and corrosion resistance. These properties make it suitable for critical components such as cold-section parts ... turbine parts and fasteners that must withstand high stresses and harsh environmental conditions.

Marine industry  : marine fasteners, propeller shafts (its main application), and pump components.

Automotive industry  : gears and fuel injection components.