UNS S44004 · W.Nr. 1.4125 · X105CrMo17. 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 CARBON grade of the family and of the standard stainless steels: C 0.95-1.20%. That is about eight times the carbon ceiling of 410 (0.15%) and it is the single reason the grade reaches 58-62 HRC. EN 10088-3 for 1.4125: C 0.95-1.20% – Si 1.00% max – Mn 1.00% max – P 0.040% max – S 0.030% max – Cr 16.0-18.0% – Mo 0.40-0.80%. ASTM A276 / the Carpenter type analysis: C 0.95-1.20% – Cr 16.00-18.00% – Mn 1.00% max – Si 1.00% max – Mo 0.75% max – P 0.04% max – S 0.03% max. THE DECISIVE DIFFERENCE BETWEEN EN AND ASTM IS THE MOLYBDENUM: EN 10088-3 specifies a FLOOR for molybdenum (0.40% minimum), while ASTM gives only a CEILING (0.75% maximum) and sets no floor. An order placed as ‘UNS S44004’ or ‘ASTM A276 440C’ alone can be met with a heat containing next to no molybdenum.
Bought where wear resistance and high hardness come first, the environment is mild and the part is not welded: rolling element bearing races and balls (AMS 5880 is a bearing specification outright), valve seats and balls, nozzles, industrial knives and cutters, surgical instruments, knife edges,…
Forms
Round bar, flat bar, plate. Supplied annealed, hardened or semi-finished. All forms are supplied to order.
Standards
AMS (all three verified, plain S44004): 5630 (bars, wire, forgings and forging stock) · 5618 (bars, wire and forgings; with a CONSUMABLE ELECTRODE VACUUM MELTED requirement) · 5880 (bars, wire, forgings and forging stock; for BEARING APPLICATIONS). ASTM: A276 / SA-276 (bars and shapes). EN: 1.4125 · EN 10088-3 (bars, wire, sections; +A condition). Welding procedure: ASME Section IX P-No 6 (although this grade is not welded in practice). THE THREE AMS NUMBERS ARE NOT THE SAME THING: AMS 5630 is general bar, wire and forging; AMS 5618 calls for the SAME chemistry but specifies the MELTING ROUTE (consumable electrode vacuum melted), so AMS 5630 material IS NOT ACCEPTED against AMS 5618;
Advantage
IT REACHES THE HIGHEST HARDNESS OF ANY STANDARD STAINLESS STEEL. In numbers: the as-quenched hardness is 59-62 HRC (Lucefin 60 HRC, INTOCO 60-62 HRC, SB Specialty Metals 59 HRC);
Welding
IT IS NOT WELDED IN PRACTICE. Carpenter states that ‘because of its high-hardness capability, this steel is seldom welded’. Where it cannot be avoided, three sources give the same cycle: a PREHEAT of about 260 °C (500 °F), maintained throughout; Rolled Alloys requires welding with high heat inputs;
Limits
1) FORBIDDEN TEMPERING BAND: 425-593 °C. Carpenter requires that the grade not be tempered ABOVE 427 °C (800 °F) where maximum corrosion resistance is wanted; Abrams writes that 425-565 °C reduces impact strength and corrosion resistance;
Compiled from manufacturer data sheets · confirm against the current specification before ordering
On this page · click to jump
Standards by Product FormWelding, Heat Treatment and MachiningWhere 440C BelongsFrequently Asked Questions
AISI 440C is a martensitic stainless steel with a high carbon content. It is the hardest member of the martensitic steel family and one of the few stainless grades that can be taken above 60 HRC by heat treatment.
The high carbon level gives exceptional hardness and wear resistance after heat treatment, while the chromium content provides a moderate level of corrosion resistance. The real strength of the material is that it holds its surface hardness and performs well in abrasive environments.
Compared with the 420 series in the same group the relationship is clear: as carbon rises, hardness rises while corrosion resistance and toughness fall. 440C sits at the hard end of that scale and is chosen when a hardness beyond the reach of 420C is required. Because of its brittleness, elongation values are low and it is not suitable for impact loading.
Ball bearings, valve components, die parts, cutting tools and knives, and precision mechanism parts exposed to wear in aerospace are the main applications. It can be supplied in the heat treated and in the annealed condition.
Chemical Composition · AISI 440C (1.4125)
DEFENCE METAL
C — Carbon
0.95 – 1.20%
Cr — Chromium
16.0 – 18.0%
Mo — Molybdenum
0.75% max
Mn — Manganese
1.0% max
Si — Silicon
1.0% max
Fe — Iron
Balance
Heat Treatment and Mechanical Properties · AISI 440C
DEFENCE METAL
Austenitising
1010 – 1065 °C
Quenching medium
Air or oil
Tempering
150 – 250 °C
Hardness
58 – 62 HRC
Tensile strength (Rm)
760 – 1950 MPa (depending on condition)
Yield strength (Rp0.2)
450 – 1700 MPa
Elongation
2 – 5%
Wear resistance
Very high
Standards and Equivalents · AISI 440C
DEFENCE METAL
Trade name
AISI 440C
UNS
S44004
W.Nr (DIN/EN)
1.4125
AMS
5618 · 5630 · 5880
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 5630 (bars, wire, forgings and forging stock) · AMS 5618 (the same forms, with a CONSUMABLE ELECTRODE VACUUM MELTED requirement) · AMS 5880 (the same forms, for BEARING APPLICATIONS) · ASTM A276 / ASME SA-276 · EN 10088-3 (1.4125, +A)
Wire
AMS 5630, 5618 and 5880 also cover wire. ASTM A580 (wire) and A493 (cold heading wire) were each found in a single source and have not been written onto the card.
Forgings and forging stock
AMS 5630, 5618 and 5880 cover forgings and forging stock. ASTM A473 (forgings) and A314 (billets and bars for forging) were each found in 2 sources.
Plate
No plate specification for 440C could be verified against four sources. An order must be tied to a specification agreed between buyer and seller.
Bearing balls and races
AMS 5880 is for BEARING APPLICATIONS outright. AMS 7445 is a specification for 440A, 440B and 440C BALLS and is not a substitute for bar.
Surgical instruments
That ASTM F899 covers 440C was found in only 1 source (BGH); four sources were not reached and it has not been written onto the card.
Welding
THIS GRADE IS NOT WELDED. There is no practical matching filler metal. Where it cannot be avoided: a preheat of about 260 °C, high heat input, and a 6-8 hour anneal at 732-760 °C immediately after welding followed by slow furnace cooling. Procedure group: ASME Section IX P-No 6.
The three AMS numbers call for the same chemistry but DO NOT SPECIFY the same thing: 5630 is general, 5618 specifies the melting route (consumable electrode vacuum melting) and 5880 specifies the bearing application. One is not accepted in place of another. AMS 5631 and AMS 5632 DO NOT BELONG to this grade; they are for 440A and for 440A/440F/440FSe respectively. AMS 7445 is a ball specification. Every number for which four sources could not be reached is written inside its row together with how many sources it was found in.
With 0.95–1.20% carbon, AISI 440C attains the highest hardness of any standard stainless grade. That high carbon defines both the strengths and the limits of the alloy: it is the source of its bearing and cutting performance, and equally the reason it cannot be welded and why its corrosion resistance depends on heat treatment.
Standards by Product Form · AISI 440C (S44004 / 1.4125)
There is no current dedicated ASTM product standard. ASTM A176 historically covered this form but was withdrawn in 2015. Current practice: ASTM A480 / A480M general requirements plus mill chemistry to the UNS analysis
Seamless / welded pipe · tube
NOT covered by any recognised ASTM tube or pipe standard — A268, A269, A312 and A249 do not include high-carbon martensitic grades
Fittings
—
Welding wire · electrode
There is no dedicated AWS filler classification for 440C (no ER440C exists)
WARNING — catalogue traps. (1) Listings citing “ASTM A176 440C plate” are out of date; the standard was withdrawn in 2015. (2) Offers of “ASTM 440C seamless tube” rest on non-standard custom production, not on a real ASTM tube specification. (3) AMS 5630, AMS 5618 and AMS 5880 are not interchangeable: 5630 is air melt, 5618 is vacuum induction melt, and 5880 is specifically for bearing applications. (4) ASTM A895 does not cover 440C — it is a free-machining plate, sheet and strip specification.
Composition (ASTM A276): C 0.95–1.20% · Cr 16.00–18.00% · Mn ≤1.00% · Si ≤1.00% · P ≤0.040% · S ≤0.030% · Mo ≤0.75%. AMS 5630 / 5618 narrow this and add: Mo 0.40–0.65% · Ni ≤0.75% · Cu ≤0.50%. The 440-family carbon split: 440A 0.60–0.75% · 440B 0.75–0.95% · 440C 0.95–1.20%. Mechanicals: annealed, tensile ~758 MPa, yield ~448 MPa, elongation ~14%, hardness ≤ 269 HBW; hardened and tempered at about 300 °C, tensile ~1965 MPa, yield ~1896 MPa, hardness 58–60 HRC.
Welding, Heat Treatment and Machining
HEAT TREATMENT — SCHEMATIC
DEFENCE METAL
1 · ANNEALING – softening (machining is done in this condition)
Step
1 · ANNEALING – softening (machining is done in this condition)
Summary
To spheroidise the carbides and make the structure machinable. In this grade annealing is not optional but the mandatory first step of the process route.
Temperature
780-900 °C. BGH 780-840 °C · Lucefin 780-840 °C · Rolled Alloys and Carpenter 843-871 °C · West Yorkshire Steel 840-875 °C · Abrams 850-900 °C · INTOCO full anneal 845-900 °C · SB Specialty Metals 899 °C. A PROCESS ANNEAL IS ALSO GIVEN: INTOCO 675-760 °C (below the critical temperature, an intermediate softening).
Time
No numerical time could be confirmed across four independent sources. Named value: SB Specialty Metals says two hours at 899 °C.
Cooling
VERY SLOW FURNACE COOLING IS MANDATORY. Carpenter says ‘cool very slowly’; Rolled Alloys and West Yorkshire say ‘slow cool in furnace’; Abrams says in the furnace to 600 °C and then in air; SB Specialty Metals says at 14 °C per hour maximum to 649 °C and then in air. Cooled quickly, the grade air hardens and becomes unmachinable.
Resulting hardness
EN 10088-3 +A ceiling: 285 HB max (t <= 100 mm). SOURCES THAT DIVERGE: BGH, Lucefin and Abrams 285 HB max · Penn Stainless (the ASTM A276 table) 269 HBW max · Rolled Alloys about 223 HB · SB Specialty Metals 217-255 HB · INTOCO 23-25 HRC. NO AVERAGE HAS BEEN TAKEN.
In a grade with this much carbon, going straight to the austenitising temperature risks thermal shock cracking. Three sources give a staged heat.
Temperature
INTOCO preheat 760-800 °C · New Jersey Steel Baron equalise at 760 °C · SB Specialty Metals two stages: 593-677 °C first, then 760-788 °C. Carpenter, Rolled Alloys and West Yorkshire do not write this step separately.
Time
New Jersey Steel Baron gives 15 minutes at 760 °C. No numerical time could be confirmed across four independent sources.
Cooling
None; the part goes straight on to the austenitising temperature.
Resulting hardness
This step has no hardness result; its purpose is to even out the temperature distribution.
DEFENCE METAL
3 · AUSTENITISING + QUENCH (hardening)
Step
3 · AUSTENITISING + QUENCH (hardening)
Summary
The step that produces the hardness. Part of the carbon goes into solid solution and the rest stays in the structure as chromium carbide, which is what gives the wear resistance.
Temperature
1010-1070 °C. Carpenter 1010-1066 °C · Rolled Alloys 1010-1066 °C · Abrams 1010-1066 °C · BGH 1010-1070 °C · West Yorkshire Steel 1010-1070 °C · INTOCO 1010-1065 °C · Penn Stainless 1010-1065 °C. SOURCES THAT DIVERGE: Lucefin 1000-1050 °C · SB Specialty Metals 1010-1038 °C · New Jersey Steel Baron a single value of 1038 °C. NO AVERAGE HAS BEEN TAKEN.
Time
New Jersey Steel Baron gives 30 minutes and SB Specialty Metals 30-45 minutes. No single numerical time could be confirmed across four independent sources.
Cooling
WARM OIL, AIR or PRESSURISED GAS. Carpenter, Rolled Alloys, Abrams, West Yorkshire and Penn Stainless say ‘quench in warm oil or cool in air’; INTOCO gives a nitrogen gas quench at 2-6 bar; New Jersey Steel Baron gives aluminium plates and compressed air down to below 50 °C; SB Specialty Metals gives air, positive pressure vacuum or interrupted oil down to 66 °C. No source recommends a water quench.
4 · DEEP COOLING – CONVERTING RETAINED AUSTENITE (a step peculiar to this grade)
Step
4 · DEEP COOLING – CONVERTING RETAINED AUSTENITE (a step peculiar to this grade)
Summary
To turn the austenite left after quenching into martensite. Its purpose is both to raise the hardness and to give DIMENSIONAL STABILITY. It is done BEFORE tempering.
Temperature
-70 to -80 °C (INTOCO) · -73 °C, that is -100 °F (Carpenter Micro-Melt) · -74 °C, that is -100 °F (New Jersey Steel Baron, a dry ice and kerosene bath). DEEP CRYOGENIC TREATMENT (DCT) is a separate practice: Abrams gives -196 °C for 24 hours.
Time
Carpenter Micro-Melt 1 hour · New Jersey Steel Baron 4-6 hours depending on thickness · Abrams (DCT) 24 hours. Three sources give three different times; NO AVERAGE HAS BEEN TAKEN.
Cooling
The part is warmed to room temperature afterwards and only then tempered (Carpenter Micro-Melt).
Resulting hardness
Carpenter Micro-Melt: about 60 HRC without the refrigeration and about 61-62 HRC with it. Abrams reports that deep cryogenic treatment raises the hardness by up to 7%. INTOCO gives DIMENSIONAL STABILITY as the reason for the treatment.
DEFENCE METAL
5 · TEMPERING – LOW BAND (150-370 °C), the service condition
Step
5 · TEMPERING – LOW BAND (150-370 °C), the service condition
Summary
This is the NORMAL service condition of the grade. The aim is to take out the stress without giving up the hardness. A DOUBLE TEMPER is recommended.
Temperature
150-370 °C. Carpenter 149-177 °C (300/350 °F) for at least one hour · INTOCO 165 °C for maximum hardness and 350 °C for maximum toughness · New Jersey Steel Baron rows at 149, 204, 260 and 316 °C · West Yorkshire Steel rows at 150-350 °C · Penn Stainless 150-370 °C · Lucefin rows at 100-400 °C.
Time
Carpenter says at least one hour; New Jersey Steel Baron gives 2 hours twice (a DOUBLE TEMPER); INTOCO recommends a double temper. Two sources call for a double temper; four were not reached.
Cooling
Air.
Resulting hardness
150-175 °C: 60 HRC (West Yorkshire, Carpenter, New Jersey Steel Baron, INTOCO, Lucefin). 200-204 °C: 59 HRC (Lucefin, New Jersey Steel Baron, West Yorkshire). 250-260 °C: 57 HRC (Lucefin, New Jersey Steel Baron, West Yorkshire). 300-316 °C: 56-57 HRC (New Jersey Steel Baron 56, West Yorkshire 56, Rolled Alloys 56, Lucefin 57). 350 °C: Lucefin 57 HRC, West Yorkshire 56 HRC, INTOCO 52-54 HRC for maximum toughness. That last divergence is recorded under conflicts.
DEFENCE METAL
6 · TEMPERING – HIGH BAND: NOT USED
Step
6 · TEMPERING – HIGH BAND: NOT USED
Summary
The other grades of the family have a usable high tempering band above the forbidden band. 440C DOES NOT.
Temperature
The 425-593 °C band is forbidden (see below). Above the band the hardness collapses: Lucefin measures 46 HRC at 550 °C and 41 HRC at 600 °C. At that point there is no case left for having chosen 440C.
Time
–
Cooling
–
Resulting hardness
Lucefin: 432 HB (46 HRC) at 550 °C and 381 HB (41 HRC) at 600 °C. SB Specialty Metals gives 51 HRC at 538 °C; that divergence is recorded under conflicts.
DEFENCE METAL
FORBIDDEN TEMPERING BAND – 425-593 °C
Step
FORBIDDEN TEMPERING BAND – 425-593 °C
What happens
Impact toughness and corrosion resistance fall. What makes this band distinctive in this grade is that THE HARDNESS DOES NOT FALL – IT RISES. A hardness reading therefore does not show whether the band was entered.
As named in the source
Carpenter for 440C: where maximum corrosion resistance is wanted it is not tempered ABOVE about 427 °C (800 °F) · Abrams for 440C, 425-565 °C (797-1049 °F): reduces impact strength and corrosion resistance · SB Specialty Metals for 440C, 427-593 °C (800-1100 °F): ‘tempering between 800-1100 °F should be avoided due to a decrease in both toughness and corrosion resistance’ · West Yorkshire Steel for 440C: tempering above 400 °C is not recommended as it can reduce corrosion resistance. Because the sources diverge at the ends, no single figure has been written and the 425-593 °C envelope is used; the West Yorkshire limit of 400 °C falls below even that envelope.
Mechanism warning
This is NOT the 475 °C EMBRITTLEMENT of ferritic stainless steels. In high carbon martensitic stainless steels the mechanism is temper embrittlement together with chromium carbide precipitation at the grain boundaries; every carbide that precipitates further reduces the free chromium in the matrix and lowers the corrosion resistance. NUMERICAL EVIDENCE – SECONDARY HARDENING: in the Lucefin measurement (oil quenched from 1020 °C, dia. 16 mm) the hardness reads 595 HB (57 HRC) at 300, 350 and 400 °C and RISES to 615 HB (58 HRC) at 450 and 500 °C, then collapses to 432 HB (46 HRC) at 550 °C. A second, independent piece of evidence: SB Specialty Metals gives 54 HRC at 316 °C and 55 HRC at 427 °C – a rise in the same direction. THE PRACTICAL CONSEQUENCE: a 440C part tempered at 450-500 °C PASSES a hardness test; what has been lost is toughness and corrosion resistance, and that does not show up in a hardness reading.
DEFENCE METAL
440C tempering temperature against hardness (the sources are named on every row)
Title
440C tempering temperature against hardness (the sources are named on every row)
Reading
The table is NOT a single source’s series; the sources giving each figure are named on every row and figures from different sources have NOT been averaged. Three things can be read from it. FIRST: from 150 to 350 °C the hardness only falls from 60 to 56-57 HRC; that band is the grade’s service band. SECOND: in the 425-500 °C band the hardness DOES NOT FALL but rises to 58 HRC (Lucefin) – this is secondary hardening and it is a trap, because toughness and corrosion resistance fall in the same band. THIRD: above 550 °C the hardness collapses (46 HRC) and none of the grade’s advantages is left. A NOTE ON REFRIGERATION: the New Jersey Steel Baron rows belong to specimens that WERE deep cooled and the SB Specialty Metals rows to specimens that were not; that is why the SB low-band figures (56 HRC at 204 °C) sit below the others, and the difference is stated in the row note.
The diagram is schematic; the time axis is NOT to scale. No published TTT/CCT curve was used, so no curve is drawn (the BGH document states that it contains a tempering graph for hardening from 1040 °C; because its numerical points could not be verified against four sources, NO CURVE HAS BEEN DRAWN HERE). 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. The cycle of this grade has ONE STEP MORE than those of the rest of the family: DEEP COOLING comes between the quench and the temper. The cycle of this grade has ONE STEP MORE than those of the rest of the family: DEEP COOLING comes between the quench and the temper. There is no such step in 410, 415, 416 or 431. THE PREHEAT STEP (760-800 °C) is written separately only for this grade. The reason is the carbon: a high carbon part taken straight to 1040 °C can crack from thermal shock. The cooling rate during annealing is critical in this grade. SB Specialty Metals specifies 14 °C per hour maximum down to 649 °C; material cooled quickly air hardens and cannot be machined. Deep cooling and DEEP CRYOGENIC TREATMENT (DCT) are not the same thing: the first is 1-6 hours at -70 to -80 °C, the second 24 hours at -196 °C. They are given on separate rows. A double temper is recommended by two sources (INTOCO and New Jersey Steel Baron); because four sources were not reached, it has not been written as mandatory but recorded as a recommendation. The BGH document states that it holds a tempering graph for hardening from 1040 °C; because its numerical points could not be verified against four sources, NO CURVE IS DRAWN in this diagram.
Welding
440C is not recommended for welding. Several independent sources state that it is “seldom welded” or that “welding is generally not recommended”, because 0.95–1.20% carbon transforms into untempered, brittle martensite in the HAZ on cooling, with the attendant cracking risk. Where welding is unavoidable, hold preheat at about 260 °C throughout, and immediately on completion apply a full anneal at 732–760 °C for 6–8 hours with slow furnace cooling. Sources disagree on filler: some recommend E/ER309 or 310 for a softer, more ductile deposit (sacrificing the hardness match), others a 420-type filler that matches base-metal hardness but is itself crack-prone. Presenting a single “correct” filler would be misleading — the choice belongs to the part and the procedure.
Heat treatment
Martensitic hardening plus carbide. Austenitising: 1010–1065 °C; quench in warm oil, or air for thin sections. Deep-freeze (cryogenic) treatment matters particularly for this grade: the high carbon leaves significant retained austenite after quenching, which caps attainable hardness. Manufacturer data shows that refrigeration at −73 °C (−100 °F) for 1 hour raises hardness from about 60 HRC to 61–62 HRC. Tempering: 150–175 °C for 1 hour → about 60 HRC; hardness then falls gradually as temper temperature rises (150 °C→60, 200 °C→59, 250 °C→57, 300–350 °C→56 HRC). The 400–565 °C band is forbidden — it costs both toughness (temper embrittlement) and corrosion resistance (chromium carbide re-precipitation at grain boundaries). Full anneal: 843–871 °C with very slow furnace cooling → 223–269 HBW; this is the standard delivery condition. Sub-critical anneal: 732–760 °C, air cool.
Machining
Machine in the dead-soft annealed condition — the manufacturers agree on this point. Chips are tough and stringy, so chip breakers or curlers are needed. Carbide tooling allows 2–3× the cutting speed of HSS and 50–100% higher feed. Finish machining is done before hardening; once hardened, only grinding, honing and EDM are practical. Overheating during grinding reduces both hardness and corrosion resistance — a critical point in bearing and blade finish grinding.
Where 440C Belongs — and What “Stainless” Means Here
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).
The most misunderstood property of 440C is its corrosion resistance. That resistance is not inherent to the alloy; it is a consequence of the heat-treat condition:
Corrosion-Resistance Mechanism · AISI 440C
DEFENCE METAL
In the annealed condition
Carbon ties up much of the chromium as coarse chromium carbides; free chromium in the matrix approaches or falls below the ~10.5% threshold needed for a passive film. Annealed 440C is not particularly corrosion resistant
Hardening (austenitising)
Redissolves the carbides and returns chromium to the matrix
Low temper (≤ ~400 °C)
Keeps the chromium in the matrix — corrosion resistance is at its best here
Tempering at 400–565 °C
Carbides re-precipitate at grain boundaries; toughness and corrosion resistance both fall
Passivation + polishing
Required for full performance: a clean, scale-free, preferably polished surface
Chlorides / salt spray
Restricted. At best (hardened + low-tempered + passivated) the resistance is described as “approaching Type 304 in many environments”
Selection Guide · 440C and Its Neighbours
DEFENCE METAL
440A / 440B
C 0.60–0.75% / 0.75–0.95%. Lower hardness and wear resistance, somewhat better toughness
Precipitation-hardening; a much lower hardness ceiling but better toughness, weldability and corrosion resistance
52100 bearing steel
Not stainless (insufficient chromium for passivation). Cheaper, with no corrosion protection. Published fatigue research reports 440C bearings performing roughly twice as well as 52100 for fatigue life
Not recommended above about 400–425 °C — the limit is temper embrittlement and corrosion loss, not oxidation
Frequently Asked Questions
What is the maximum achievable hardness of 440C, and how do I get it?
440C reaches roughly 60 HRC as-quenched, and 61–62 HRC with a supplementary cryogenic (deep-freeze) step. The sequence for maximum hardness is: austenitise at 1010–1065 °C, quench in warm oil (air for thin sections), follow immediately with a cold or cryogenic treatment at −73 °C for 1 hour to convert retained austenite to martensite, and finish with a low-temperature temper at 150–175 °C for at least one hour. Do not temper in the 400–565 °C band — even where it costs little hardness, it produces secondary embrittlement and pulls toughness and corrosion resistance down together. The route aerospace and medical bearing makers follow for top-end, consistent hardness is bearing-quality material (AMS 5880 or vacuum-melted AMS 5618) combined with cryogenic treatment.
Is annealed 440C bar corrosion resistant, or must it be hardened first?
Annealed 440C is only mildly corrosion resistant — noticeably less so than 304/316 and less than its own hardened form. The reason is that 0.95–1.20% carbon ties up a large share of the chromium as coarse carbides in the annealed microstructure, leaving less free chromium to form a stable passive film. Full corrosion performance requires three things: austenitising (which redissolves the carbides and returns chromium to the matrix), a low-temperature temper (below about 400 °C, which keeps it there) and passivation of a clean, scale-free surface. If your application needs corrosion resistance in service, specify hardened, tempered and passivated 440C rather than annealed bar — and avoid any subsequent thermal exposure above about 400 °C, which re-precipitates the carbides and undoes the gain.
What is the difference between “bearing-grade” and standard 440C, and is the premium worth it?
Standard 440C bar is typically supplied to ASTM A276 / A314 / A473 and AMS 5630 (air melted). Bearing-grade material is specified instead to AMS 5880 (“for bearing applications”) or to vacuum-induction-melted AMS 5618, and for aerospace and medical miniature bearings it is often further processed as a powder-metallurgy remelt. The premium buys cleaner steel — fewer and smaller non-metallic inclusions and less carbide segregation — which directly improves rolling-contact fatigue life and gives more consistent hardness after heat treatment, including the cryogenic step. For non-critical wear parts (valve seats, bushings, general tooling), standard AMS 5630 / A276 material is adequate and considerably cheaper; for rolling-element bearings, especially in aerospace, medical or high-reliability service, specify AMS 5880 or vacuum-melt / PM material.
STRENGTH VALUES
DEFENCE METAL
Condition
Hardness
Yield MPa
Tensile MPa
Elongation
EN 10088-3 · 1.4125 · +A (annealed bar, t <= 100 mm)
–
–
–
–
ASTM A276 · 440C · annealed
–
448
758
14%
Hardened and tempered at 204 °C
59 HRC
1900
2030
4%
As quenched (untempered)
59-62 HRC
–
–
–
Hardened and tempered at 150 °C
60 HRC
–
–
–
Hardened, deep cooled (-73 °C for 1 hour) and tempered
61-62 HRC
–
–
–
Hardened and tempered at 350 °C (maximum toughness)
Hardened and tempered at 450-500 °C (FORBIDDEN BAND)
58 HRC
–
–
–
Hardened and tempered at 550 °C
46 HRC
–
–
–
Hardened and tempered at 600 °C
41 HRC
–
–
–
SPECIFICATION LIMITS and MANUFACTURER MEASUREMENTS are given on separate rows and must not be mixed. THERE IS NO IMPACT REQUIREMENT for 440C on the ASTM side. That is the low toughness of the grade as it appears in the specification; an impact column has deliberately been left out of the table. In the condition tempered at 204 °C the elongation is 4% (Penn Stainless). That is less than a quarter of the specification floor for 415 (15%) and it shows where the grade must not be used. The annealed hardness ceiling varies between 217 HB and 285 HB across the sources; the EN 10088-3 and ASTM A276 ceilings (285 HB and 269 HBW) are not the same either. That divergence is recorded under conflicts. The 450-500 °C row is NOT A RECOMMENDATION; it is there to show that the hardness rises inside the forbidden band.
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