AISI 431

What is AISI 431?

AISI 431 is a nickel-bearing martensitic stainless steel. It contains 15.0 to 17.0 percent chromium, 1.25 to 2.50 percent nickel and up to 0.20 percent carbon. Atlas Steels describes it as having the best corrosion resistance among the martensitic grades. Nickel contributes to this property by permitting a higher chromium content. Chromium stabilises ferrite, so increasing the chromium content of a plain 12 percent chromium martensitic steel can prevent transformation to martensite during quenching. Nickel stabilises austenite and restores the balance required for hardening with up to 17 percent chromium.

AISI 431 has high tensile and torque strength, good toughness and a maximum attainable hardness of approximately 40 HRC. It resists a wider range of corrosive media than AISI 410 or AISI 420. Its overall corrosion resistance is approximately equal to or slightly below that of AISI 304.

AISI 431 is not readily cold worked because it has high yield strength before hardening. Atlas Steels does not recommend cold heading, bending, deep drawing or spinning. The grade is used for turned and ground shafting rather than formed parts.

Laxcon Steels lists AISI 431 in its grades reference as a martensitic grade. The same steel is written 431, SUS 431, 1.4057 and S43100.

What is the chemical composition of AISI 431?

Composition limits in weight percent are carbon 0.2 maximum, manganese 1.0 maximum, sulphur 0.03 maximum, phosphorus 0.04 maximum, silicon 1.0 maximum, chromium 15.0 to 17.0 and nickel 1.25 to 2.5.

What is AISI 431 equivalent to in other standards?

A widely reproduced SAE table gives S41623 for 431; that is the UNS number of 416Se and was rejected. 431 is S43100.

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

What is the PREN of AISI 431?

Computed by Laxcon Steels from the composition above as PREN = Cr + 3.3Mo + 16N, the relationship published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014). The published relationship is given for austenitic and duplex stainless steels, so this figure extends it: read it against other martensitic grades rather than across families. The first figure uses the minimum specified chromium, molybdenum and nitrogen, so it is the floor for a conforming heat rather than a typical value; the second uses the maxima, so the two together are the width of the band one compliant grade allows. PREN ranks resistance to chloride pitting and nothing else: it does not predict crevice corrosion, stress corrosion cracking or service life, and it is not a substitute for a corrosion test. The pitting resistance reference carries the full table and the caveats.

Mechanical properties of AISI 431

ASTM A276 does not specify tensile or yield strength for AISI 431 in annealed Condition A. It specifies a maximum hardness of 285 HB. It also requires the material to reach at least 40 HRC after heating to a minimum of 1,020 degrees Celsius and quenching in oil. Atlas Steels states that annealed Condition A is rarely stocked. Its published typical annealed values are approximately 862 MPa tensile strength, 655 MPa 0.2 percent proof stress and 20 percent elongation.

AISI 431 is normally supplied in the hardened and tempered condition. Condition T under AS 1444 and BS 970 specifies a tensile strength of 850 to 1,000 MPa, a minimum proof stress of 635 MPa, a minimum elongation of 11 percent and a hardness range of 248 to 302 HB. The tensile value is a specified range rather than a minimum. Machining and fatigue behaviour depend on the shaft remaining within this range.

Lower tempering temperatures produce higher strengths. Atlas Steels reports approximately 1,320 MPa tensile strength, 1,020 MPa proof stress, 20 percent elongation, 380 HB and 75 J Izod after tempering at 300 degrees Celsius. Tempering at 600 degrees Celsius produces approximately 1,030 MPa tensile strength, 800 MPa proof stress, 20 percent elongation and 310 HB. Elongation remains near 20 percent across this tempering range. AISI 431 therefore retains more ductility than the plain chromium martensitic grades AISI 410 and AISI 420.

Heat treatment of AISI 431

A full anneal is not practical because AISI 431 hardens during slow cooling. Process annealing consists of heating to 620 to 660 degrees Celsius and cooling in air. This treatment softens the material between operations but does not produce a fully annealed structure.

Hardening requires heating to 980 to 1,065 degrees Celsius, holding for approximately half an hour and quenching in air or oil. Preheating at 760 to 790 degrees Celsius is useful for complex parts and previously hardened material. The tempering temperature is selected according to the required mechanical properties. The range from 425 to 600 degrees Celsius should be avoided because it reduces impact toughness. Atlas Steels reports that this effect is less pronounced in AISI 431 than in most other martensitic grades.

Corrosion and heat resistance

AISI 431 has a pitting resistance equivalent number of 15.0 at the specified minimum composition and 17.0 at the specified maximum composition. This is approximately equal to the range of ferritic AISI 430. Atlas Steels rates the grade as resistant to a wide variety of corrosive media. Its overall corrosion resistance is approximately equal to or slightly below that of AISI 304.

AISI 431 has reasonable resistance to salt water in cold southern waters but is unlikely to perform successfully in warmer tropical waters. This temperature dependence limits its use in seawater. AISI 431 propeller and pump shafting is common in temperate marine service and uncommon in tropical service. A smooth surface finish and a hardened and tempered condition provide the best performance.

AISI 431 resists scaling in intermittent service at temperatures up to approximately 925 degrees Celsius and in continuous service at temperatures up to approximately 870 degrees Celsius. The practical service limit is the tempering temperature of the part. Exposure above that temperature continues the tempering process and reduces strength.

Machining, welding, applications and designations

AISI 431 has reasonable machinability in the annealed or lightly tempered condition. Machining becomes difficult above approximately 30 HRC. Improved-machinability versions are available for high-volume work. Welding is difficult because of cracking risk. A preheat of 200 to 300 degrees Celsius is recommended. AISI 410 filler can provide matching properties. Austenitic 308L, 309 or 310 fillers provide more ductile welds when matching properties are not required. Either method requires post-weld heat treatment at 650 degrees Celsius.

Typical applications include propeller shafting, pump shafts, valve spindles, nuts and bolts, beater bars and marine hardware. The European designation is material number 1.4057 with the name X17CrNi16-2. The UNS designation is S43100. The Japanese designation is SUS431, and purchasers also use SS431. Laxcon supplies AISI 431 for these applications and designations.

Where AISI 431 is not the right choice

AISI 431 is unsuitable for warm seawater. Its resistance in cold water does not extend to tropical service. A shaft selected from a temperate-water precedent can undergo pitting and crevice corrosion in a hot climate. Duplex 2205 has a PREN range of 34.1 to 37.8, and super duplex 2507 has a range of 37.7 to 47.6. Both exceed the AISI 431 range of 15.0 to 17.0 and also provide high yield strength.

AISI 431 is unsuitable for formed components. Its high yield strength in the supplied condition prevents ready cold working. Cold heading, bending, deep drawing and spinning are not recommended. Components that require these operations need an austenitic grade. The grade is also a poor choice for welded fabrication. Welding presents a cracking risk and requires preheating and post-weld heat treatment. A joint made with matching filler must be heat treated as a complete assembly.

Tempering between 425 and 600 degrees Celsius is unsuitable where impact toughness is required. Sub-zero service is also unsuitable because martensitic steels undergo a ductile to brittle transition. The 440 series can provide hardness above approximately 40 HRC because of its higher carbon content, but with lower corrosion resistance. The precipitation hardening grades 17-4 PH and 15-5 PH are alternatives when higher strength is required without heat-treatment distortion. These grades age at low temperature and are commonly used for long shafting.

Other martensitic grades