UNS S43100 · W.Nr. 1.4057 · X17CrNi16-2 · BS 431S29. This is a MARTENSITIC stainless steel: it transforms to martensite on austenitising and quenching and is then TEMPERED. It does NOT precipitation harden; there is NO H900 / H1025 type ageing step. It is the HIGHEST CHROMIUM hardenable grade of the family (15-17%, against 12-14% in 410, 416 and 420). The nickel here is not there for strength but FOR THE STRUCTURE: 16% chromium on its own makes the structure ferritic, and a ferritic structure does not harden by heat treatment; 1.25-2.50% nickel keeps the austenite field open and lets martensite form on quenching. EN 10088-3 for 1.4057: C 0.12-0.22% – Si 1.00% max – Mn 1.50% max – P 0.040% max – S 0.030% max – Cr 15.0-17.0% – Ni 1.50-2.50%. ASTM A276 / the Carpenter type analysis: C 0.20% max (THERE IS NO FLOOR) – Mn 1.00% max – P 0.040% max – S 0.030% max – Si 1.00% max – Cr 15.00-17.00% – Ni 1.25-2.50%. EN sets a carbon FLOOR (0.12%) and a higher nickel floor (1.50%); ASTM sets neither. Aubert & Duval give a nominal C 0.16% – Cr 17.00% – Ni 2.00% for their own APX grade.
Bought for parts that need high strength together with the best corrosion resistance available inside the martensitic family: pump and propeller shafts, valve stems, marine hardware and fasteners, aircraft fittings and attachments, turbine parts, plastics processing moulds and screws, chemical and…
Forms
Round bar, flat bar, plate, sheet, pipe, forging, wire. All forms are supplied to order.
Standards
AMS: 5628 – ‘Steel, Corrosion-Resistant, Bars, Wire, Forgings, and Tubing, 16Cr – 2.5Ni (SAE 51431)’. ASTM: A276 / SA-276 (bars and shapes). EN: 1.4057 · EN 10088-3 (bars, wire, sections; +A, +QT800 and +QT900 conditions). Welding procedure: ASME Section IX P-No 6. NOT AN AMS 5628 TRAP BUT AN AMS 5682 TRAP: some stockist pages list this grade under the heading ‘Alloy 431 / AMS 5682’, while the body of those same pages quotes only AMS 5628. That AMS 5682 belongs to 431 COULD NOT BE VERIFIED IN ANY SOURCE;
Advantage
IT HAS THE HIGHEST CORROSION RESISTANCE OF THE HARDENABLE MARTENSITIC GRADES, and it delivers that together with a tensile strength in the 1345-1580 MPa class.
Welding
IT CAN BE WELDED BUT IT IS DIFFICULT, and it is not done WITHOUT preheat and a postweld treatment. PREHEAT: 200-316 °C. Carpenter gives 204-316 °C (400-600 °F), the Engineering.com specification summary 204-316 °C, AZoM 200-300 °C and Abrams 100-300 °C. POSTWELD TREATMENT: about 649-650 °C.
Limits
1) FORBIDDEN TEMPERING BAND: 400-600 °C. Aubert & Duval state it most plainly: ‘we do not recommend using this steel in the tempering range from 400 °C to 600 °C’ – the reason given is unfavourable impact strength. Carpenter states that 371-565 °C (700-1050 °F) ‘will adversely affect impact strength and corrosion properties’;
Compiled from manufacturer data sheets · confirm against the current specification before ordering
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Standards by Product FormWelding, Heat Treatment and MachiningWhen to Choose 431Frequently Asked Questions
AISI 431 is an alloy in the martensitic stainless steel class offering high durability and wear resistance. AISI 431 steel contains chromium, nickel and manganese as alloying elements, which give it high hardness, better corrosion resistance and high tensile strength.
Corrosion resistance: The corrosion resistance of this stainless grade is at a very good level for many environments. The material has far better resistance to rusting than X20Cr13 (AISI 420) and can frequently be specified in environments where both hardness and resistance to rusting are required.
Weldability: Welding grade 1.4057 is not generally recommended, although the material can be welded in certain cases.
Machinability: Even in its as-supplied condition the material is heat treated, and its surface hardness — even as it comes from the mill — is around a minimum of 30-32 HRC. It is therefore a hard, high strength material, and machining it can be somewhat more difficult than machining other stainless steels.
Heat treatment: 431 stainless steel contains both chromium and nickel. Grade 431 is a stainless steel whose resistance to fracture can be made very high by heat treatment.
Applications: Grade 431 stainless steels are used in the manufacture of bolts and nuts, in propeller shafts, pump shafts, mixer shafts, marine equipment and laboratory equipment.
AISI 431 belongs to the martensitic stainless steel class and is an ideal material for applications requiring high hardness, wear resistance, high tensile strength and corrosion resistance.
Chemical Composition
DEFENCE METAL
C
Max. 0.20
Mn
Max. 1.00
Si
Max. 1.00
P
Max. 0.04
S
Max. 0.03
Cr
Min. 15 · Max. 17
Ni
Min. 1.25 · Max. 2.50
Mechanical Properties
DEFENCE METAL
Tensile Strength (MPa)
862
Proof Stress (MPa)
–
Elongation A50 mm
20
Hardness Brinell
285 Max HB
Density
7.80 g/cm3
Melting Point
1698 °C
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
Standards and Equivalents · AISI 431
DEFENCE METAL
Trade name
AISI 431
UNS
S43100
W.Nr (DIN/EN)
1.4057
AMS
5628
ASTM
A276 · A314 · A484 · A580
Available forms
Round bar · Flat bar · Plate · Sheet · Tube · Forgings All forms are supplied to order.
Standards by Product Form
STANDARDS BY PRODUCT FORM
DEFENCE METAL
Product form
Standards
Round bar, flat bar (sections)
AMS 5628 (16Cr – 2.5Ni, SAE 51431; bars, wire, forgings and tubing) · ASTM A276 / ASME SA-276 (bars and shapes) · EN 10088-3 (1.4057, +A / +QT800 / +QT900)
Wire
AMS 5628 also covers wire. ASTM A580 (wire) was found in 3 sources and ASTM A493 (cold heading wire) in 1; neither has been written onto the card.
Forgings
AMS 5628 also covers forgings. ASTM A314 (billets and bars for forging) was found in 2 sources, and ASTM A473 (forgings) and A579 (superstrength forgings) in 1 each.
Tubing and pipe
The AMS 5628 title also covers TUBING (the SAE title record reads ‘Bars, Wire, Forgings, and Tubing’). No separate ASTM pipe specification could be verified against four sources; ASTM A511 was found in a single source.
Plate and sheet
No plate or sheet specification for 431 could be verified against four sources. An order must be tied to a specification agreed between buyer and seller.
Welding filler metal
Two routes: matching martensitic AWS A5.4 E410 / A5.9 ER410, or austenitic 1.4430 (316L) or 1.4370 (307) (AGST). No shielding gas containing hydrogen or nitrogen is used (AGST). Preheat 200-316 °C, postweld treatment at about 649-650 °C.
Welding procedure group
ASME Section IX P-No 6 (martensitic stainless)
AMS 5628 covers four product forms (bars, wire, forgings, tubing) under a single number; that comes from the aerospace fastener history of 431. AMS 5682 DOES NOT BELONG to this grade; it appears in the headings of some stockist pages while the body of those same pages gives only AMS 5628. It must not be written into an order. Every number for which four sources could not be reached is written inside its row together with how many sources it was found in.
AISI 431 is a martensitic stainless steel with 15–17% chromium and 1.25–2.5% nickel. The nickel addition — unusual for a straight-chromium martensitic grade — widens the austenite field so the alloy still hardens fully at the chromium level needed for corrosion resistance, which is why 431 is the most corrosion resistant of the conventionally hardenable stainless steels.
Standards by Product Form · AISI 431 (S43100 / 1.4057)
DEFENCE METAL
Bar · Shapes
ASTM A276 / A276M · AMS 5628
Billet (for forging)
ASTM A314
Forgings · Rings · Flanges
ASTM A473 · AMS 5628
Bar for boilers / pressure vessels
ASTM A479 / A479M
Wire
ASTM A580 / A580M · AMS 5628
Sheet · Plate · Strip
NOT covered by ASTM A240 — see the warning below
Seamless / welded pipe · tube · fittings
— (no verifiable pipe or tube standard was found for 431)
Welding wire
AWS A5.9 ER410 or ER410NiMo (matching); austenitic alternative ER309 / ER312
WARNING — common catalogue error: numerous trading sites advertise “ASTM A240 Grade 431 plate”. This is wrong: S43100 does not appear in A240’s grade roster (the martensitic and ferritic grades A240 actually covers are 405, the 409 family, 410, 410S, 429, 430, 439, 434, 436 and the 444 family — neither 420 nor 431). The genuine product standards for 431 are on the bar and forging side: A276, A314, A473, A479, A580 and AMS 5628. Treat any “A240 431 plate” offer as a catalogue error.
Composition (ASTM A276, S43100): C ≤0.20% · Mn ≤1.00% · Si ≤1.00% · P ≤0.040% · S ≤0.030% · Cr 15.00–17.00% · Ni 1.25–2.50%. Annealed (Condition A) minimums: tensile ≥ 490 MPa, yield ≥ 215 MPa, elongation ≥ 35%, hardness ≤ 285 HBW. Hardened (AMS 5628): tensile ≥ 1379 MPa, yield ≥ 1034 MPa, elongation ≥ 10%, reduction of area ≥ 40%. EN 10088-3 classes: +QT800 → Rp0.2 ≥ 600, Rm 800–950 N/mm²; +QT900 → Rp0.2 ≥ 700, Rm 900–1050 N/mm². (The “850 / 880” classes quoted in some sources could not be verified in any independent EN datasheet; the standard classes are +QT800 and +QT900.)
Welding, Heat Treatment and Machining
HEAT TREATMENT — SCHEMATIC
DEFENCE METAL
1 · ANNEALING – sub-critical process anneal and softening anneal
Step
1 · ANNEALING – sub-critical process anneal and softening anneal
Summary
Two different practices circulate under one heading. The grade AIR HARDENS; a genuine softening calls for controlled furnace cooling.
Temperature
620-800 °C. AZoM 620-660 °C · Carpenter 650-677 °C · Lucefin 680-750 °C · Abrams, AGST and Rodacciai 680-800 °C. No single figure has been written; an envelope is given.
Time
No numerical time could be confirmed across four independent sources.
Cooling
AZoM and Carpenter cool IN AIR (a sub-critical process anneal). Abrams says ‘slow and controlled, in the furnace’, AGST ‘furnace or air’, and Aubert & Duval air cooling from 680 °C. Where softening is the aim, controlled FURNACE cooling is chosen.
Resulting hardness
EN 10088-3 +A ceiling: 295 HB max and 950 MPa tensile max. SOURCES THAT DIVERGE: Rodacciai 295 HB max for sizes up to 100 mm · AGST 295 HB · Stainless.eu 330 HBW max · Abrams 331 HB max in the delivery condition · AZoM 285 HB max · Aubert & Duval 300 HB after 680 °C. NO AVERAGE HAS BEEN TAKEN.
DEFENCE METAL
2 · AUSTENITISING + QUENCH (hardening)
Step
2 · AUSTENITISING + QUENCH (hardening)
Summary
The step that produces the hardness. Chromium and carbon go into solid solution and martensite forms on cooling. The grade hardens even in air.
Temperature
980-1065 °C. Carpenter 982-1066 °C · AZoM 980-1065 °C · Lucefin 980-1030 °C · Stainless.eu 1000-1050 °C · Aubert & Duval a single value of 1020 °C. A SOURCE THAT DIVERGES: Abrams gives 1150-1180 °C; that figure is the FORGING temperature (AGST quotes the same figure in the context of ‘heated quickly to around 1180 °C’ for forging) and MUST NOT BE USED as a hardening temperature. NO AVERAGE HAS BEEN TAKEN.
Time
No numerical time could be confirmed across four independent sources. Named value: AZoM gives about half an hour.
Cooling
OIL, AIR, POLYMER or WATER. Carpenter says OIL; AZoM ‘oil or air’; Lucefin ‘oil / polymer’ or air; Abrams ‘oil, air, water’; Aubert & Duval ‘oil or water, with gas pressure quenching as an option depending on the shape’.
Resulting hardness
Lucefin gives the as-quenched hardness as about 45 HRC. Stainless.eu describes the hardened condition at 46 HRC. Abrams gives the attainable working hardness as 32-47 HRC (300-447 HB).
DEFENCE METAL
3 · TEMPERING – LOW BAND (180-400 °C), the highest strength
Step
3 · TEMPERING – LOW BAND (180-400 °C), the highest strength
Summary
The step in which the highest strength and hardness are kept. It stays BELOW the forbidden band.
Temperature
About 180-400 °C. Stainless.eu gives 180 °C as the lowest tempering temperature. Measured points: Lucefin at 200, 300 and 350 °C · AZoM at 204 °C · Aubert & Duval at 400 °C.
Time
No numerical time could be confirmed across four independent sources. Named value: Abrams says at least 1 hour.
Cooling
Air.
Resulting hardness
AZoM measures 1345 MPa tensile / 1055 MPa yield / 20% elongation / 388 HB at 204 °C. Lucefin measures 1580 MPa tensile / 1290 MPa yield / 14% elongation / 20 J KV at 200 °C. Aubert & Duval give 1400 MPa tensile / 1050 MPa yield / 13% elongation at 400 °C – but that temperature sits on the edge of the forbidden band, and the same maker does not recommend that band.
DEFENCE METAL
4 · TEMPERING – HIGH BAND (600-700 °C), the service condition and +QT800 / +QT900
Step
4 · TEMPERING – HIGH BAND (600-700 °C), the service condition and +QT800 / +QT900
Summary
This is the tempering band actually used in industry. It is chosen ABOVE THE FORBIDDEN BAND.
Temperature
600-700 °C. Abrams 600-650 °C · AZoM 650 °C · Aubert & Duval 630 °C · Rodacciai 650-700 °C for +QT800 · Carpenter 649 °C for the postweld treatment.
Time
Abrams says at least 1 hour and Carpenter says ‘soak at heat at least one hour’. Two sources agree, but four were not reached.
Cooling
Air (Abrams, Rodacciai, AGST).
Resulting hardness
EN 10088-3 +QT800 (t <= 160 mm): 600 MPa yield min, 800-950 MPa tensile, 12% elongation min, 20 J KV longitudinal min. Rodacciai +QT900 (up to 60 mm): 700 MPa yield min, 900-1050 MPa tensile, 12% elongation min, 16 J KV min. AZoM measures 960 MPa tensile / 695 MPa yield / 20% elongation / 277 HB at 650 °C. Aubert & Duval give 1000 MPa tensile / 750 MPa yield / 15% elongation and 40 J/cm2 KCU at 630 °C.
DEFENCE METAL
FORBIDDEN TEMPERING BAND – 400-600 °C
Step
FORBIDDEN TEMPERING BAND – 400-600 °C
What happens
Impact toughness drops, and Carpenter adds that the corrosion properties are adversely affected as well. In this grade the band matters for a further reason: the service band (600-700 °C) starts just ABOVE it, so a furnace deviation downwards runs straight into the fault.
As named in the source
Aubert & Duval for 431 / APX, 400-600 °C: ‘we do not recommend using this steel in the tempering range from 400 °C to 600 °C’, the reason given being unfavourable impact strength · Carpenter for 431, 371-565 °C (700-1050 °F): ‘will adversely affect impact strength and corrosion properties’ · AZoM for 431, 425-600 °C: ‘tempering of these steels at 425 to 600 °C should be avoided, owing to the loss of impact toughness’ · Stainless.eu for 1.4057: temper from 180 °C, avoiding the 400-600 °C zone. Because the sources diverge at the ends, no single figure has been written and the 400-600 °C envelope is used; the Carpenter lower end of 371 °C falls below even that envelope.
Mechanism warning
This is NOT the 475 °C EMBRITTLEMENT of ferritic stainless steels. In martensitic Cr-Ni steels the mechanism is temper embrittlement together with chromium carbide precipitation at the grain boundaries. NUMERICAL EVIDENCE (Lucefin, dia. 10 mm, oil quenched from 1000 °C): the impact energy reads 20 J at 200 °C, 15 J at 300 °C, 20 J at 350 °C, 15 J at 400 °C, 20 J at 450 °C and 15 J at 500 °C – that is, it stays LOW and UNSTABLE at 15-20 J right across the band. For comparison, Aubert & Duval give a KCU of 40 J/cm2 after tempering at 630 °C, where the material behaves tough. A SECOND WARNING: the tensile strength falls steadily across this band (Lucefin: 1440 MPa at 400 °C, 1360 MPa at 500 °C), so a hardness reading alone DOES NOT TELL YOU whether you are inside the band.
DEFENCE METAL
Lucefin measurement – 1.4057 / X17CrNi16-2, dia. 10 mm round, oil quenched from 1000 °C, then tempered
Title
Lucefin measurement – 1.4057 / X17CrNi16-2, dia. 10 mm round, oil quenched from 1000 °C, then tempered
Reading
The table is a single measurement series (Lucefin) and is labelled as such; figures from different sources have not been mixed into it. Three things can be read from it. FIRST: the strength halves steadily from 200 to 700 °C (1580 down to 800 MPa), with no sharp jump between steps. SECOND: the impact energy is stuck in a 15-20 J band between 200 and 500 °C and does not rise – that is the impact-side signature of the forbidden band; Lucefin gives no KV values for the rows at 550 °C and above. THIRD: the +QT800 and +QT900 conditions actually used in industry (EN 10088-3: 800-950 and 900-1050 MPa tensile respectively) correspond to the 600-700 °C rows of the table; the standard conditions are therefore defined deliberately ABOVE the forbidden band. A CHECK AGAINST OTHER SOURCES: independent measurements give the same trend but not the same numbers – AZoM 1345 MPa at 204 °C and 960 MPa at 650 °C; Aubert & Duval 1400 MPa at 400 °C and 1000 MPa at 630 °C. NO AVERAGE HAS BEEN TAKEN.
The diagram is schematic; the time axis is NOT to scale. No published TTT/CCT curve was used, so no curve is drawn. THIS ALLOY IS MARTENSITIC: it hardens by quenching and tempering, it does NOT precipitation harden. There is NO ageing step of the H900 / H1025 / H1075 / H1150 type. Each of the steps below has been verified separately. In this grade the tempering band USED IN INDUSTRY is the high band (600-700 °C), not the low one. The two conditions defined by EN 10088-3 (+QT800 and +QT900) fall in that band. The forbidden band and the service band ARE ADJACENT: the forbidden band ends at 600 °C and the service band starts at 600 °C. Tight furnace temperature control is therefore mandatory. THE HARDENING TEMPERATURE TRAP: one source (Abrams) writes 1150-1180 °C; that is the FORGING temperature. Five independent sources give 980-1065 °C. A part quenched from 1150 °C coarsens in grain size and loses toughness. The grade air hardens. A material described as ‘annealed’ may therefore not actually be soft unless it was cooled in a controlled furnace; the annealed hardness ceilings vary between 285 HB and 331 HB across the sources. No published TTT/CCT curve could be verified against four sources, so NO CURVE IS DRAWN in this diagram.
Welding
431 can be welded but it is not the preferred joining method — its high hardenability makes HAZ cracking a real risk. Where welding is required, GTAW, SMAW and GMAW are used. Preheat: US sources give 204–316 °C and European practice 100–300 °C; in practice 200–300 °C, confirmed against the project welding procedure. Interpass must not fall below preheat.Post-weld heat treatment is mandatory — sources give figures across the 650–760 °C band with controlled cooling, to temper the as-welded martensite and restore toughness and corrosion resistance. Filler: ER410 / E410 for a matching deposit (most crack-sensitive), ER410NiMo / E410NiMo as the more forgiving option; where preheat and PWHT cannot be applied, austenitic E308L / E309 / E312 gives a ductile joint. Keep the PWHT temperature out of the embrittlement band described below.
Heat treatment
An air- or oil-hardening martensitic alloy; the roughly 2% nickel lowers the Ms/Mf temperatures and lets thin sections harden fully in air. Austenitising: 982–1066 °C (European sources give 950–1050 °C), oil quench for heavy sections, air cool for thin ones. Tempering: at least 1 hour, air cool. Tempering between 371 and 565 °C (700–1050 °F) is strictly forbidden — both impact toughness and corrosion resistance fall sharply in this band. This is the most critical heat-treatment caution for the grade. In European practice +QT800 uses 750–800 °C plus a second temper at 600–700 °C, and +QT900 uses 600–650 °C. Soft anneal: 650–677 °C, air cool (~270 HB) — a full furnace anneal is not recommended because the alloy air-hardens on slow cooling. Delivery condition: annealed (Condition A) for machining stock, or hardened and tempered (Condition T / +QT800 / +QT900) for finished parts — the buyer must state which.
Machining
In the annealed condition machinability is comparable to SAE 3150 / 6150 alloy steel. It machines readily up to about 30 HRC annealed or softly tempered; above that it becomes markedly difficult. The material is prone to galling and built-up edge — prefer carbide tooling, which allows 2–3× the cutting speed and 50–100% higher feed than HSS. Rigid clamping and a sharp cutting edge are essential. Harden after machining.
When to Choose 431
COMPARISON
One standard set: the chemistry bands come from EN 10088-3:2005 Table 9 and from the ASTM specifications; the attainable hardness and the weldability come from the manufacturers’ own data sheets. All five grades are MARTENSITIC and none of them precipitation hardens.
DEFENCE METAL
Grade
UNS
W.-Nr.
EN designation
Carbon
Chromium
Nickel
Molybdenum
Sulphur
Maximum hardness
Weldability
Note
AISI 410
S41000
1.4006
X12Cr13
0.08-0.15%
11.5-13.5%
0.75% max
–
0.030% max
38-47 HRC tempered; practical working ceiling about 43-45 HRC
Conditional – a preheat of 177-204 °C is MANDATORY and a postweld anneal is required
The reference grade of the family. Carbon ceiling 0.15%.
AISI 415
S41500
1.4313
X3CrNiMo13-4
EN: 0.05% max · ASTM: 0.05% max
EN: 12.0-14.0% · ASTM: 11.5-14.0%
EN: 3.5-4.5% · ASTM: 3.5-5.5%
EN: 0.30-0.70% · ASTM: 0.50-1.00%
EN: 0.015% max · ASTM: 0.030% max
+QT900: 285-346 HB, about 30-37 HRC. THE LOWEST ATTAINABLE HARDNESS IN THE FAMILY.
GOOD – 100-160 °C preheat, 580-620 °C postweld temper, ER410NiMo filler. THE ONLY GENUINELY WELDABLE GRADE IN THE FAMILY.
Soft martensitic. Bought for toughness and welding, not for hardness. The only grade with a specification impact floor.
AISI 416
S41600
1.4005
X12CrS13
EN: 0.06-0.15% · ASTM: 0.15% max
12.0-14.0%
–
0.60% max
0.15-0.35% (EN 10088-3) – ADDED ON PURPOSE
26-32 HRC in Condition T (Rolled Alloys, Swiss Steel); at a low temper Lucefin measures 1490 MPa tensile at 200 °C
NOT SUITABLE – the sulfur forms MnS inclusions and causes hot cracking
410 plus sulfur. Corrosion resistance and weldability have been given up for machinability.
AISI 431
S43100
1.4057
X17CrNi16-2
EN: 0.12-0.22% · ASTM: 0.20% max
15.0-17.0% – THE HIGHEST CHROMIUM IN THE FAMILY
EN: 1.50-2.50% · ASTM: 1.25-2.50%
–
0.030% max
Working hardness 32-47 HRC (Abrams); 1345 MPa tensile / 388 HB tempered at 204 °C (AZoM), 1580 MPa at 200 °C (Lucefin)
Difficult – needs a 200-300 °C preheat and a postweld treatment at about 650 °C; corrosion resistance falls after welding
The highest corrosion resistance among the hardenable martensitics. The nickel is what stops 16% chromium making the structure ferritic.
AISI 440C
S44004
1.4125
X105CrMo17
0.95-1.20% – THE HIGHEST CARBON IN THE FAMILY
16.0-18.0%
–
EN: 0.40-0.80% (THERE IS A FLOOR) · ASTM: 0.75% max (NO FLOOR)
0.030% max (EN 10088-3) / 0.015% max (Lucefin, Abrams)
59-62 HRC as quenched; 60 HRC tempered at 150-175 °C; 61-62 HRC with refrigeration at -73 °C (Carpenter). THE HIGHEST IN THE FAMILY AND AMONG STANDARD STAINLESS STEELS.
Not welded in practice – it needs a 260 °C preheat and a 6-8 hour anneal at 732-760 °C
A bearing and cutting grade. The 1% carbon ties chromium up as carbide; not all of the 16-18% Cr on paper works for corrosion resistance.
DEFENCE METAL
Additional information
Inverse relationship
As the carbon rises the attainable hardness rises and the toughness and weldability fall. The ladder is plain: 415 (0.05% C) is welded and stops at 30-37 HRC; 410 (0.15% C) is welded conditionally and reaches 43-45 HRC; 431 (0.12-0.22% C) is welded with difficulty and reaches 47 HRC; 440C (0.95-1.20% C) is not welded and reaches 60 HRC. 416 sits outside that ladder: its carbon is the same as 410’s and what separates it is the sulfur.
Nikelin isi
410, 416 and 440C carry no nickel; 415 carries 3.5-5.5% and 431 carries 1.25-2.50%. In those two grades the nickel does two different jobs. In 415 it makes the structure transform to martensite even though the carbon is very low (it would otherwise stay ferritic). In 431 it stops 16% chromium making the structure ferritic. The same element, for two different reasons.
Kukurdun isi
Sulfur is present ON PURPOSE only in 416: EN 10088-3 specifies a band of 0.15-0.35%. In the other four grades sulfur is an IMPURITY and is capped (0.015-0.030% max). The same element is a product feature in one grade and a defect in the others.
Cokelme uyarisi
NONE OF THE FIVE GRADES PRECIPITATION HARDENS. Ageing steps such as H900, H1025, H1075 and H1150 belong to PRECIPITATION HARDENING grades such as 17-4 PH, 15-5 PH, 13-8 PH and Custom 455, and have no counterpart in any of these five. Here the condition names are +QT650, +QT780, +QT900 or, on the ASTM side, Condition A / T / H.
The comparison rests on the EN 10088-3 and ASTM texts for the chemistry bands and on manufacturers’ data sheets for the hardness; figures from different test methods have not been gathered into one row. Because the ASTM and EN bands for 415 differ, both are written out separately in that row; the same applies to the carbon and molybdenum rows of 416, 431 and 440C. The molybdenum row for 440C is the most important trap in this table: EN 1.4125 specifies a FLOOR for molybdenum (0.40%), while ASTM S44004 gives only a CEILING (0.75%). A 440C bought against ASTM may contain almost no molybdenum. The carbon row for 431 is the second trap: EN 1.4057 sets a carbon FLOOR (0.12%), ASTM S43100 does not (only the 0.20% ceiling).
431 has one clear selling point: the best corrosion resistance among the martensitics, combined with high strength. That is why it is the standard grade for shafting in marine environments.
Precipitation-hardening. Similar corrosion resistance with a simpler, lower-distortion ageing treatment; advantageous on complex geometries
Duplex grades
Substantially better chloride and pitting resistance, but lower attainable strength than fully hardened 431
Seawater immersion
Genuinely contested: 431 is strong in marine atmospheres and the splash zone; for continuous immersion many buyers still prefer duplex or nickel-aluminium bronze
Service limits: an operating range of roughly −73 °C to 650 °C is quoted, with a continuous scaling limit around 704 °C. European sources give a more conservative range for mechanically loaded service (−40 °C to 400 °C) — the difference reflects scaling resistance versus load-bearing capacity. The grade is magnetic and may retain permanent magnetism after hardening, so demagnetisation is sometimes needed.
Frequently Asked Questions
Why is 431 preferred over 410 or 420 for marine pump and propeller shafting?
431’s defining feature is its 1.25–2.5% nickel addition — an unusual ingredient for a straight-chromium martensitic grade. The nickel widens the austenite field so the alloy still hardens fully (in air or oil) at the chromium level needed for corrosion resistance, while directly improving resistance to general and pitting attack. Independent sources describe 431 as “the most corrosion resistant of the conventionally hardenable stainless steels”. Against nickel-free 410 or high-carbon 420, 431 holds up meaningfully better in humid and marine atmospheres while still reaching the high tensile and yield class (Condition T / +QT900) required for shaft torque and fatigue loading. Note that for permanent seawater immersion some sources rate 431 as only “approaching 304” and recommend duplex or nickel-aluminium bronze instead — 431 is at its strongest in marine atmospheres and the splash zone.
Why must I specify the tempered condition, and which range is forbidden?
431 is delivered either annealed (soft, machinable, low strength) or hardened and tempered to a defined class. The ASTM A276 table does not publish a fixed Condition T value for 431; hardened-condition mechanicals are set by AMS 5628 or by agreement with the purchaser. EN 10088-3 defines discrete classes instead: +QT800 (Rp0.2 ≥ 600, Rm 800–950 N/mm²) and +QT900 (Rp0.2 ≥ 700, Rm 900–1050 N/mm²). Ordering “431” alone leaves strength, hardness and ductility unconstrained. Equally critical is the 371–565 °C (700–1050 °F) temper-embrittlement band: material tempered or held there loses impact toughness and corrosion resistance together. Specify both the exact condition (for example +QT900) and confirm that the mill’s tempering temperature falls outside that window.
Can 17-4 PH substitute for 431?
Often yes — it is a common alternative and offers similar corrosion resistance. The key difference is the heat treatment: 431 is quench-and-temper hardened (oil or air from 982–1066 °C, then tempered), which is more prone to distortion on complex parts and requires care to dodge the 371–565 °C embrittlement band. 17-4 PH is precipitation-hardened — solution treat, then a single lower-temperature age (H900–H1150) — giving markedly tighter dimensional control and simpler processing for intricate geometries. 17-4 PH generally reaches higher strength at a given corrosion level and dominates in aerospace fittings, while 431 remains a strong, lower-cost choice for larger shafts and bar stock thanks to its simpler composition and established supply chain (A276 / A314 / A473 / AMS 5628). Decide on part geometry, distortion tolerance, the required strength class and cost.
STRENGTH VALUES
DEFENCE METAL
Condition
Hardness
Yield MPa
Tensile MPa
Elongation
EN 10088-3 · 1.4057 · +A (annealed bar)
–
–
950 max
–
EN 10088-3 · 1.4057 · +QT800 (t <= 160 mm)
–
600
800-950
12% min
Rodacciai · 1.4057 · +QT800 (up to 60 mm)
–
600
800-950
14% min
Rodacciai · 1.4057 · +QT900 (up to 60 mm)
–
700
900-1050
12% min
Stainless.eu · 1.4057 · QT800
–
650
800
7% min
Stainless.eu · 1.4057 · QT880
–
690
880-1080
12% min
Annealed – measurement
–
655
862
20%
Hardened and tempered at 204 °C – measurement
–
1055
1345
20%
Hardened and tempered at 650 °C – measurement
–
695
960
20%
Hardened and tempered at 200 °C – measurement
–
1290
1580
14%
Hardened and tempered at 400 °C – measurement (FORBIDDEN BAND)
–
1050
1400
13%
Hardened and tempered at 630 °C – measurement
–
750
1000
15%
Attainable working hardness
32-47 HRC (300-447 HB)
–
–
–
In the chart, a value given as a range is drawn at its lower bound; the full range is in the table. SPECIFICATION MINIMA and TYPICAL MEASUREMENTS are given on separate rows and must not be mixed. The same condition name (+QT800) carries DIFFERENT floors in two sources: EN 10088-3 and Rodacciai give 600 MPa yield with 12-14% elongation, while Stainless.eu gives 650 MPa yield but only 7% elongation. The condition name alone does not define an order. The 400 °C row lies inside the forbidden band, and the maker who supplies it (Aubert & Duval) writes in the same document that it does not recommend that band. The row is there to show what happens inside the band; it is not a recommendation. The annealed hardness ceiling varies between 285 HB and 331 HB across the sources and that divergence is recorded under conflicts.
