UNS K92580 · a Carpenter Technology alloy (AerMet 100) · NO verified W.Nr./EN number exists; order against UNS K92580 and an AMS number. NOMINAL COMPOSITION (Carpenter): C 0.23% – Ni 11.10% – Co 13.40% – Cr 3.10% – Mo 1.20% – balance Fe. SPECIFICATION BAND (Aircraft Materials and Titanium Industries, up to AMS 6532 revision J): C 0.21-0.25% – Ni 11.0-12.0% – Co 13.0-14.0% – Cr 2.90-3.30% – Mo 1.1-1.3%. Revision K-2024 of AMS 6532 NARROWS the carbon band to 0.23-0.25%; state the revision letter on the order. HARDENING MECHANISM: this is a SECONDARY HARDENING Ni-Co-Cr-Mo steel. It hardens by solution treatment + cryogenic treatment + aging, and this is a GENUINE precipitation hardening: the SAE title record for AMS 6532 calls the material ‘Precipitation Hardenable’. THE DIFFERENCE FROM MARAGING STEELS IS CARBON: maraging steels carry 0.03% C maximum and harden by precipitation of INTERMETALLIC phases such as Ni3Ti and Fe2Mo; AerMet 100 carries 0.23% C and hardens through nanoscale M2C CARBIDES precipitated at 482 °C. The 2023 Materials (MDPI) study calls the material a ‘high Co-Ni secondary hardening steel’ and measures needle-shaped hexagonal M2C carbides coherent with the martensitic matrix after 1-5 h of aging, which transform to an orthogonal structure and coarsen on longer aging. The same study reports 2.3-5.4 vol% REVERTED AUSTENITE at lath and block boundaries and finds that fracture toughness is governed primarily by that phase. IT IS NOT STAINLESS.
Bought for primary load-carrying parts that must deliver a yield strength above 1700 MPa AND a fracture toughness above 115 MPa·m^0.5 at the same time, and whose failure loses the aircraft or the vehicle: landing gear components, jet engine shafts, drive shafts, actuators, high-strength fasteners,…
Forms
Round bar · flat bar · plate · sheet · tube · forging · wire · welding wire · hollow bar. All forms are supplied to order. Carpenter’s own product list gives the same forms (bar, hollow bar, rounds, billet, plate, strip, sheet, wire, weld wire), but the AMS coverage is limited to bars, forgings and forging stock — see the standards map.
Standards
AMS (verified): AMS 6532 — ‘Steel, Bars and Forgings, and Forging Stock 3.1Cr – 11.5Ni – 13.5Co – 1.2Mo (0.23 – 0.25C) Vacuum Melted, Normalized and Overaged Precipitation Hardenable’. Scope: bars, forgings of 645 cm² (100 in²) cross-sectional area and under, and forging stock of ANY size. It is the 280 ksi (1931 MPa) tensile class. The current revision is L-2024. · AMS 6478 — the 290 ksi (1999 MPa) tensile class of the same alloy; title ‘Steel, Bars and Forgings 3.1Cr 11.5Ni 13.5Co 1.2Mo (0.21 – 0.25C) Vacuum Melted, Annealed Heat Treatable to 290 ksi’. Forms: bars, forgings, forging stock. ITS STATUS IS DISPUTED; see the standards note. Military and customer specifications: MIL-HDBK-5 (design data) · McDonnell Douglas MMS 217 · MIL-STD-2154 Type 1 Class A (this is an ULTRASONIC INSPECTION specification, not a material specification). ASTM: no ASTM number covering this alloy could be confirmed across four independent sources, so none is given. AMS 6532 AND AMS 6478 ARE DIFFERENT STRENGTH CLASSES OF THE SAME ALLOY and are not interchangeable: 6532 is the 280 ksi (1931 MPa) class, 6478 the 290 ksi (1999 MPa) class, with a 245 ksi (1689 MPa) yield floor.
Advantage
Its single most important practical advantage is that it keeps its fracture toughness at yield strengths above 1700 MPa. Carpenter gives typical longitudinal values of 1724 MPa yield / 1965 MPa tensile together with K_Ic 126 MPa·m^0.5 (115 ksi·in^0.5), Charpy V 41 J (30 ft-lb), 14% elongation and…
Welding
IT IS WELDABLE. The SSA Corp data sheet states ‘Weldable requiring no preheating’, that is, NO PREHEAT IS REQUIRED; the AMS 6478 title record likewise defines the alloy for applications requiring ‘high strength, toughness, and weldability’.
Limits
1) IT IS NOT STAINLESS. Chromium is 3.10% and no passive film forms. Carpenter rates general corrosion resistance as ‘Humidity Restricted’ and adds that ‘corrosion testing is recommended’; Titanium Industries states plainly that ‘AerMet 100 is not corrosion resistant, so it must be sealed if used in moist environments’;
Compiled from manufacturer data sheets · confirm against the current specification before ordering
On this page · click to jump
What AerMet 100 IsStandards by Product FormSpecification Architecture: AMS 6532 vs AMS 6478, and the Revision TrapProduct Forms With NO StandardChemical CompositionMechanical PropertiesPhysical PropertiesHeat Treatment and the Secondary-Hardening MechanismWeldingMachining, Forging and NitridingCorrosion, Stress Corrosion, Hydrogen Embrittlement and PlatingAerMet 100 vs 300M vs Maraging 250/350 vs 4340Frequently Asked QuestionsCommon Datasheet Errors
AerMet 100 is an ultra high strength steel, rich in cobalt and nickel and hardened by ageing. Within the alloy steel group it is one of the grades with the highest strength level; its UNS designation is K92580.
The distinguishing property of this material is that it offers a tensile strength above 1931 MPa together with exceptional fracture toughness and resistance to stress corrosion cracking. In conventional high strength steels toughness falls as strength rises, whereas AerMet 100 largely removes that trade-off.
The 13-14% cobalt and 11-12% nickel in its composition form finely dispersed precipitates during ageing, while the 2.9-3.3% chromium and 1.1-1.3% molybdenum support hardenability and corrosion resistance. Carbon is held low at 0.21-0.25% so that ductility is preserved. Heat treatment: solution treat at 885 °C for 1 hour and air cool, deep freeze at -73 °C for 1 hour, then age at 482 °C for 5 hours.
It is used in landing gear, jet engine shafts, drive shafts, structural components, fasteners and armour applications. It is supplied as bar and forging stock.
Round bar · Flat bar · Plate · Sheet · Tube · Forgings All forms are supplied to order.
What AerMet 100 Is — and What a “Secondary-Hardening Steel” Means
AerMet 100 (UNS K92580, specified as AMS 6532, and called A100 or Alloy 100 in some programme documents) is a nickel-cobalt-molybdenum, secondary-hardening, ultra-high-strength martensitic steel. The one-sentence identity is this: AerMet 100 delivers the same tensile strength as 300M (~1965 MPa / 285 ksi) with roughly twice the fracture toughness — and that combination is the alloy’s entire reason for existing.
It is a proprietary alloy developed by a single producer. Publicly available compilations place its development in the late 1980s and early 1990s, when the US Navy and McDonnell Douglas were looking for something tougher than 300M for F/A-18 E/F landing gear; it is covered by US patents 5,087,415 and 5,268,044 and was qualified in the mid-1990s under AMS 6532. This historical summary rests on a single compiled source and has not been verified against primary documents, but it is consistent with the alloy’s design intent.
Secondary hardening — stating the mechanism correctly
AerMet 100 does not get its strength from the intrinsic hardness of carbon martensite the way 300M or 4340 do. Its carbon is deliberately kept low (0.21–0.25 %). The as-quenched structure is a relatively soft, ductile lath martensite. Most of the strength arrives later, in a separate ageing (not annealing) step, from nanometre-scale M₂C carbides nucleated on the dislocations inside that martensite. This is called secondary hardening, and it is conceptually similar to maraging steels (Ni₃Mo, Ni₃Ti intermetallics) — except that the precipitate is a carbide, not an intermetallic.
Three Different Hardening Mechanisms — a Distinction Not to Blur
Strength comes from the supersaturated carbon in as-quenched martensite. Tempering lowers strength; its only purpose is to bring brittleness down to a manageable level. Carbon is high (0.40–0.45 %), and that translates directly into not being weldable
AerMet 100 · M₂C secondary hardening
The as-quenched structure is soft. Strength comes from M₂C carbides (Cr- and Mo-based) formed during ageing at 482 °C for 5 hours. Ageing raises strength. Carbon is low (0.23 %), and that translates into weldability
Essentially carbon-free (≤0.03 %). Strength comes from Ni₃Mo, Ni₃Ti and Fe₂Mo intermetallics. Dimensional stability is the best of the group (no quench), but it trails AerMet 100 on fatigue performance and price
The role of cobalt — frequently misstated
Cobalt forms no carbide of its own and is not a direct strengthener. Its function is to raise molybdenum’s activity in the matrix during ageing and to retard dislocation recovery. The M₂C precipitates therefore nucleate finer and more numerous, on a denser dislocation network. Without cobalt the same chemistry does not give this strength-toughness balance — and it is also why the alloy is expensive
Where it sits in the family — honest positioning
Selling AerMet 100 as “the best ultra-high-strength steel” is wrong. The accurate framing is: AerMet 100 is the best point on the strength-toughness curve; it is neither the strongest nor the toughest.
AerMet 100’s Place in the Ultra-High-Strength Steel Family
Same tensile class, far lower toughness. Producer comparison table: 287 ksi tensile / KIc 50 ksi√in, against 287 ksi / 120 ksi√in for AerMet 100. Cheaper, more available, not weldable
4340 / 4340 VAR
269 ksi / 70 ksi√in. A markedly lower strength class. The right choice where cost dominates and 260–270 ksi is enough
AerMet 100 (this page)
287 ksi / 120 ksi√in in the producer comparison table. The product datasheet gives longitudinal 285 ksi (1965 MPa) with KIc 115 ksi√in (126 MPa√m). Weldable, outstanding SCC resistance, double vacuum melted
352 ksi / 31.5 ksi√in. The strongest and most brittle member of the family, and less tough even than 300M. Published product data: yield 2160 MPa, tensile 2430 MPa, elongation 11.3 %, notched Charpy 14.6 J
258.6 ksi / 91.5 ksi√in and 343.6 ksi / 38.5 ksi√in. Maraging 250 is very tough but in a lower strength class; Maraging 350 is strong but brittle. AerMet 100 fills the gap between them
The one-sentence buying rule: if your design criterion is damage tolerance — if the part has to fly, rotate or carry load with a defined crack present — AerMet 100 is the right answer. If your criterion is strength alone and flaws are managed by inspection, 300M does the same job for significantly less money.
Standards by Product Form
STANDARDS BY PRODUCT FORM
DEFENCE METAL
Product form
Standards
Round bar · flat bar · forging (645 cm² / 100 in² cross-sectional area and under)
AMS 6532 (280 ksi class · current revision L-2024) · AMS 6478 (290 ksi class · status disputed)
Forging stock (any size)
AMS 6532 (from revision J the title carries ‘and Forging Stock’) · AMS 6478
Plate · sheet · strip
NO AMS EXISTS. Carpenter produces these forms but no verified AMS number covers them; they are supplied by producer-customer agreement against the AMS 6532 chemistry.
Wire · welding wire
NO AMS EXISTS. Carpenter produces weld wire; no verified AMS number covers that wire.
Hollow bar · structural tubing
NO AMS EXISTS. Neither AMS 6532 nor AMS 6478 covers tubing.
Inspection (all forms)
MIL-STD-2154 Type 1 Class A — this is an ULTRASONIC INSPECTION specification, NOT a material specification; it must not be written on the order as a material standard.
Product forms were read from the Defence Metal AerMet 100 page; the standard assignments were verified separately against SAE title records and producer data sheets. No verified ASTM number was found for this alloy, so the ASTM column is left empty.
AerMet 100 is an aerospace material and its specification coverage is narrow. The most important rows in the table below are the ones where no standard exists — because the producer also sells this alloy as plate, sheet, strip and wire, and none of those has an AMS specification.
Standards by Product Form · AerMet 100 (UNS K92580)
DEFENCE METAL
Bar · forgings
AMS 6532 — current revision title: “Steel, Bars and Forgings, and Forging Stock, 3.1Cr – 11.5Ni – 13.5Co – 1.2Mo (0.21–0.25C), Vacuum Melted, Normalized and Overaged, Precipitation Hardenable.”This is the alloy’s primary specification
AMS 6532 revisions J and later carry “and Forging Stock” in the title. Earlier revisions cover bars and forgings only — put the revision number in the order text
Plate · sheet · strip
NO AMS specification. The producer makes and sells these forms; acceptance criteria are by producer/customer agreement. See the gap section below
Wire · weld wire
NO AMS specification. The producer lists wire and weld wire forms; no AWS classification was found
Hollow bar · structural tubing
NO AMS specification. The producer makes hollow bar and lists structural tubing as an application — that is an application description, not a product-form specification
NONE — and none should be expected. AerMet 100 is not a pressure-boundary material
Castings
NO cast equivalent
Inspection specification
MIL-STD-2154, Type 1 Class A (ultrasonic inspection of wrought metals) — frequently invoked on AerMet 100 orders. It is an inspection specification, not a material specification
Design data
MIL-HDBK-5 / MMPDS — the alloy appears in this design allowables compilation
OEM
MMS 217 (McDonnell Douglas; some listings add an “EXC SONIC” note)
ASME code acceptance
NONE. AerMet 100 does not appear in ASME Section I, Section VIII, B31.1 or B31.3. There is no maximum code temperature because there is no code coverage
Europe / Werkstoff
No verified EN name or W.Nr. equivalent was found. It is a proprietary alloy and has not entered European standardisation
Ordering practice. A correct AerMet 100 order line contains three things: (1) specification and revision (AMS 6532 or AMS 6478), (2) supplied condition (normalized and overaged / annealed — that is, machinable), and (3) who is responsible for the final heat treatment. AerMet 100 is normally supplied un-aged and strength is developed by the customer’s heat-treatment cycle. That is an important practical difference from 300M.
Specification Architecture: AMS 6532 vs AMS 6478, and the Revision Trap
Both specifications define the same chemistry — 3.1Cr – 11.5Ni – 13.5Co – 1.2Mo (0.21–0.25C), vacuum melted — and both cover bars and forgings. The difference is the target strength class and the way the supplied condition is described. AMS 6532’s title has also changed across revisions, which is a genuine trap when working from legacy drawings.
Two Specifications · What Differs
DEFENCE METAL
AMS 6532 · earlier revisions (B, D)
Title: “… Vacuum Melted, Annealed, Heat Treatable to 280 ksi (1931 MPa) Tensile Strength.” That is: supplied annealed, heat treatable to 280 ksi
AMS 6532 · later revisions (E onward)
Title: “… Vacuum Melted, Normalized and Overaged, Precipitation Hardenable.”The description of the supplied condition changed: “annealed” became “normalized and overaged”, and “heat treatable to 280 ksi” became “precipitation hardenable”. The alloy did not change; the way the specification describes the supplied condition did
AMS 6532 · J onward
“and Forging Stock” was added to the scope
AMS 6478
Title: “… Vacuum Melted, Annealed, Heat Treatable to 290 ksi (1999 MPa) Tensile Strength.” It differs from AMS 6532 by its 290 ksi target rather than 280 ksi. Bars and forgings
Practical consequence
Saying “AMS 6532” is not enough. If a legacy drawing says “AMS 6532, annealed, heat treatable to 280 ksi” and your material is certified to a current revision as “normalized and overaged”, the document language will not match even though the material is correct. Explain this in the order acknowledgement
Melting
Both titles say “Vacuum Melted”. Producer practice is the double vacuum route of vacuum induction melting (VIM) + vacuum arc remelting (VAR); the specification title does not spell those two out separately. A customer who requires double vacuum melting must state it explicitly in the order
Why double vacuum melting is essential. AerMet 100’s commercial promise is toughness, and the enemy of toughness is inclusions. Vacuum induction melting (VIM) lowers oxygen, nitrogen and hydrogen and holds the chemistry in a narrow band; vacuum arc remelting (VAR) then gives directional solidification, reducing macrosegregation and residual oxide inclusions. 115 ksi√in fracture toughness cannot be obtained from a dirty melt — an inclusion acts as a crack nucleus and lowers KIc directly. In AerMet 100, melting practice is not a quality enhancement; it is part of the alloy’s definition.
Product Forms With NO Standard — the Commercially Valuable Section
This is the section your sales engineers should memorise. In AerMet 100 the gap does not arise from the alloy being obscure; it arises because specification scope was kept narrow: AMS 6532 and AMS 6478 cover only BARS and FORGINGS (and forging stock in later revisions). The producer sells considerably more than that.
Specification Gaps for K92580
DEFENCE METAL
Plate · sheet · strip
NO AMS specification — but the product EXISTS. The producer explicitly lists plate, sheet and strip. This is the most common commercial situation: the customer asks for “AerMet 100 plate to AMS 6532”. The honest answer: chemistry can be certified to the AMS 6532 band, but mechanical acceptance and product-form tolerances are by agreement. Plate is not within AMS 6532’s scope, and that belongs in the order acknowledgement
Wire · spring wire
NO AMS specification. The producer lists a wire form. Drawing and annealing condition are entirely by agreement
Weld wire
No AWS classification was found. The producer lists a weld wire form — a matching-chemistry producer product. Because AerMet 100 is genuinely weldable this form is actually used, but do not look for an ER/E number; there is none
Hollow bar and structural tubing
NO AMS specification. The producer makes hollow bar and lists structural tubing among applications. That is not a tube specification — it means trepanned or pierced-and-forged bar
Pressure pipe · fittings · flanges · valves
NONE, and there should be none. AerMet 100 is not stainless, has no code coverage and is not used as a pressure-boundary material
Castings
NO cast equivalent. The alloy’s entire value comes from wrought structure and clean melting
Fasteners
Material is bought to AMS 6532/6478, but the fastener itself is made to a separate NAS/MS/manufacturer specification. No dedicated AerMet 100 fastener material specification was found — although fasteners do appear on the producer’s own application list
Additive manufacturing powder
No separate powder specification was found. Additive manufacturing research on AerMet 100 has been published, but no qualified powder/process specification was verified
Requests for a European equivalent
There is no verified EN designation or Werkstoff number. It is a proprietary alloy; rather than asking for an “EN equivalent”, order by AMS number
Chemical Composition
AerMet 100 Chemical Composition · weight %
DEFENCE METAL
Carbon (C)
0.21–0.25 % (AMS title) · producer nominal 0.23 %. Keeping it low is deliberate: strength comes from M₂C precipitation, not from carbon martensite. It is also the reason the alloy is weldable
Cobalt (Co)
13–14 % · producer nominal 13.40 %. The most expensive and most defining element in the alloy. It forms no carbide; it raises molybdenum activity and retards dislocation recovery
Nickel (Ni)
11–12 % · producer nominal 11.10 %. Matrix toughness and low-temperature toughness; it also enables the reverted austenite that forms during ageing
Chromium (Cr)
2.9–3.3 % · producer nominal 3.10 %. One of the principal constituents of the M₂C carbide.This level is NOT enough for stainlessness — see the corrosion section
Molybdenum (Mo)
1.1–1.3 % · producer nominal 1.20 %. The other principal M₂C constituent and the element at the centre of secondary hardening
Titanium (Ti)
≤0.05 % (given as a maximum in the producer’s nominal table). This is a ceiling, not an addition — titanium carbonitrides form coarse, brittle inclusions and lower toughness directly
Iron (Fe)
Balance
Melting
Vacuum melting is a specification requirement (“Vacuum Melted”). Producer practice is the VIM + VAR double vacuum route
Two points when reading the composition.First: many distributor pages publish only the nominal values (0.23 C / 13.40 Co / 11.10 Ni / 3.10 Cr / 1.20 Mo) and present them as if they were bands. The specification band is different, and a certificate shows a band. Second: some pages list AerMet 100 under a “stainless steel” heading — that is metallurgically wrong. 3.1 % chromium forms no passive film.
Mechanical Properties
STRENGTH BY AGEING CONDITION
DEFENCE METAL
Condition
Hardness
Yield MPa
Tensile MPa
Elongation
Overage annealed (677 °C / 16 h) — machining condition
40 max
—
—
—
Solution treated + cryogenic, not aged
51.0-53.0
—
—
—
Full cycle (885 °C + −73 °C + 482 °C / 5 h) — AMS 6532 MINIMUM, longitudinal
53 min
1620
1931
10%
Full cycle — AMS 6532 MINIMUM, transverse
—
1620
1931
8%
Full cycle — PRODUCER TYPICAL, longitudinal
53.0-54.0
1724
1965
14%
Full cycle (482 °C / 5 h) — PEER-REVIEWED MEASUREMENT
—
1732-1749
1949-1973
—
Full cycle (482 °C / 7 h) — PEER-REVIEWED MEASUREMENT
—
1727
1975-1989
—
AMS 6478 class (290 ksi) — SPECIFICATION FLOOR
—
1689
1999
—
The rows are NOT the minimums of a single specification. SPECIFICATION MINIMUM, PRODUCER TYPICAL VALUE and PEER-REVIEWED MEASUREMENT are given in separate rows, each named with its source. No averaging was done; the scatter between sources is left as a range. Order to the specification minimum. In the chart, a value given as a range is drawn at its lower bound; the full range is in the table. Sources disagree on the AMS 6532 yield floor: Aircraft Materials gives 1620 MPa (235 ksi), ZYTC 1720 MPa (250 ksi). No average was taken; the disagreement is recorded in the contradictions list. Producer typical values are NOT specification minimums and do not replace an order specification.
AerMet 100 is entirely contained in this table. The reason to buy the alloy is not a single number but high tensile strength and high fracture toughness at the same time. The values below are for the standard heat treatment (885 °C solution + −73 °C cryogenic + 482 °C / 5 hours ageing).
65 % — an exceptional figure for this strength class
Charpy V-notch · longitudinal
41 J (30 ft-lb)
Fracture toughness KIc · longitudinal
126 MPa√m (115 ksi√in) — the number the alloy exists for
Transverse values
Yield/tensile unchanged (1724 / 1965 MPa) · Elongation 13 % · Reduction of area 55 % · Charpy 34 J (25 ft-lb) · KIc110 MPa√m (100 ksi√in). The transverse toughness penalty is small for this class and is one of the alloy’s strengths
468 °C (875 °F): 54.5–55.5 HRC · 482 °C (900 °F): 53.0–54.0 HRC · 496 °C (925 °F): 51.0–52.5 HRC
Low-temperature toughness
One distributor compilation states that approximately 41 J is retained at −73 °C — single source, not verified against the producer datasheet
Fatigue
The producer publishes S-N curves showing the effect of notch severity (Kt = 1, 2 and 3) out to 10⁷ cycles. Numerical endurance values must be read off those graphs and are not published on this page — use MMPDS for design
Independent research data — it confirms the producer table
An independent microstructural study reports, for AerMet 100 aged at 482 °C for 5–7 hours, yield 1727–1749 MPa, tensile 1949–1989 MPa and KIc 116.5–122 MPa√m. That agrees well with the producer’s 1724 / 1965 MPa and 126 MPa√m and shows the datasheet is not optimistic. The same study states explicitly that the best strength-toughness balance lies in the 482 °C / 5–7 hour window.
Why fracture toughness matters so much
Critical crack size scales roughly with the square of KIc. The gap between AerMet 100 and 300M in the producer comparison table — 120 against 50 ksi√in at the same 287 ksi tensile — means a roughly sixfold larger tolerable crack. That directly changes inspection intervals, overhaul periods and the aircraft’s ability to fly damaged. It is the single technical reason landing gear design moved from 300M to AerMet 100.
Physical Properties
AerMet 100 Physical Properties
DEFENCE METAL
Density
7.94 g/cm³ (0.287 lb/in³) — producer value. Some distributor pages give 0.285 lb/in³ — a conflict; the difference is small, but do not publish two values on the same page
24–93 °C (75–200 °F): 5.49–5.55 × 10⁻⁶ in/in·°F · 24–482 °C (75–900 °F): 6.29–6.34 × 10⁻⁶ · 24–538 °C (75–1000 °F): 6.28–6.43 × 10⁻⁶. The producer publishes values for both the annealed and heat-treated conditions
Critical temperatures
Ac₁ = 574 °C (1065 °F) · Ac₃ = 829 °C (1525 °F). Ac₁ at 574 °C matters: the 482 °C ageing temperature is below it, so the austenite that appears during ageing is a local enrichment effect rather than a bulk transformation
Magnetic properties · overage annealed (677 °C / 16 h)
Maximum permeability 180 · Remanence 11,600 G · Coercivity 52.0 Oe
Magnetic properties · solution treated and aged
Maximum permeability 150 · Remanence 10,100 G · Coercivity 39.5 Oe
Thermal conductivity · specific heat
Not found in the producer datasheet — do not publish these two values
Maximum service temperature
~427 °C (800 °F) — producer/distributor statement. This is not a code limit (AerMet 100 has no code coverage); it is a practical ceiling imposed by the need to stay below the 482 °C ageing temperature
Heat Treatment and the Secondary-Hardening Mechanism — the Complete Route
HEAT TREATMENT — SCHEMATIC
1 · SOLUTION TREATMENT
885 °C ± 14 °C (1625 °F ± 25 °F) 1 hour
2 · COOL
Controlled cooling: from 885 °C to 66 °C (150 °F) in 1-2 hours. OIL above 50 mm (2 in) diameter or 25 mm (1 in) plate thickness; air is sufficient below that. WATER QUENCHING IS NOT RECOMMENDED (Carpenter).
3 · AGEING
see the table below
DEFENCE METAL
Solution treatment
Step
1 · SOLUTION TREATMENT
Temperature
885 °C ± 14 °C (1625 °F ± 25 °F)
Time
1 hour
Cooling
Controlled cooling: from 885 °C to 66 °C (150 °F) in 1-2 hours. OIL above 50 mm (2 in) diameter or 25 mm (1 in) plate thickness; air is sufficient below that. WATER QUENCHING IS NOT RECOMMENDED (Carpenter).
Carpenter requires this step ‘to obtain full toughness capability’. Skipping it leaves untransformed austenite.
DEFENCE METAL
Standard aging (AMS 6532 / AMS 6478 cycle)
Step
Standard aging (AMS 6532 / AMS 6478 cycle)
Temperature
482 °C ± 6 °C (900 °F ± 10 °F)
Time
5 hours
Cooling
air
Resulting hardness
53.0-54.0 HRC (Carpenter and SSA) · 53-56 HRC (ZYTC) · the AMS 6532 floor is 53 HRC minimum
DEFENCE METAL
Longer aging measured in a peer-reviewed study
Step
Longer aging measured in a peer-reviewed study
Temperature
482 °C
Time
7 hours
Cooling
air
Resulting hardness
Materials (MDPI) 2023: 1727 MPa yield, 1975-1989 MPa tensile, K_IC 119-122 MPa·m^0.5. The same study states that ‘the strength and toughness matching of AerMet 100 steel is achieved by tempering at 482 °C for 5~7 h’.
DEFENCE METAL
0a · NORMALIZING
Step
0a · NORMALIZING
Summary
Homogenises the structure after forging or rolling.
Temperature
899 °C (1650 °F)
Time
1 hour
Cooling
air cool to room temperature
Resulting hardness
No hardness figure was confirmed across four independent sources, so none is given.
DEFENCE METAL
0b · OVERAGE ANNEAL (machining condition)
Step
0b · OVERAGE ANNEAL (machining condition)
Summary
Machining is done in this condition. Carpenter calls this step an ‘overage anneal’.
Temperature
677 °C (1250 °F)
Time
16 hours
Cooling
air
Resulting hardness
40 HRC maximum
DEFENCE METAL
0c · STRESS RELIEF (optional)
Step
0c · STRESS RELIEF (optional)
Summary
Applied before mechanical straightening or after rough machining.
Temperature
177-204 °C (350-400 °F) before straightening · 427 °C (800 °F) after rough machining
Time
5 hours (177-204 °C) · 1-3 hours (427 °C)
Cooling
air
Resulting hardness
No hardness change is given.
DEFENCE METAL
AGING LOWER LIMIT — 468 °C (875 °F)
Step
AGING LOWER LIMIT — 468 °C (875 °F)
What happens
The Carpenter data sheet states plainly: ‘Never be aged at temperature below 875 F (468 C)’. This is not an embrittlement band but a LOWER LIMIT: M2C precipitation does not complete and the target strength is not reached.
Temper gevrekligi notu
It could NOT be confirmed across four independent sources that the temper embrittlement band seen in low-alloy quench-and-temper steels (4340, 300M) also applies to AerMet 100. This alloy is in any case heat treated at 482 °C, which is the secondary hardening peak region. An unverified forbidden band is NOT written.
The diagram is schematic; the time axis is NOT to scale. No published TTT/CCT curve confirmed across four independent sources was used, so no curve is drawn. THIS ALLOY GENUINELY PRECIPITATION HARDENS: the SAE title record for AMS 6532 calls the material ‘Precipitation Hardenable’. But THE MECHANISM IS NOT MARAGING: the hardness comes from nanoscale M2C CARBIDES precipitated at 482 °C (Materials/MDPI 2023 · Vacuum · Journal of Alloys and Compounds), not from intermetallic phases. The CRYOGENIC STEP between solution treatment and aging is mandatory and cannot be skipped. The cycle Carpenter gives is one piece: normalize → overage anneal (machining) → solution treat → cryogenic → age. No intermediate step may be skipped. The aging window is ± 6 °C. That is a far tighter tolerance than 4340 or 300M tempering, and furnace calibration is a specification requirement. The 2023 Materials (MDPI) study used an oil quench; Carpenter quenches in air or oil according to section thickness. Both practices exclude water.
AerMet 100’s heat treatment is conceptually different from 300M’s. In 300M the quench gives strength and tempering takes some back. In AerMet 100 the quench leaves a soft structure and ageing creates the strength. That difference governs everything from machinability to weldability.
899 °C (1650 °F), 1 hour, air cool to room temperature. To homogenise the structure after forging
0b · Overage anneal (for machinability)
677 °C (1250 °F), 16 hours. Result: 40 HRC maximum. Rough machining is done in this condition — this is what “Normalized and Overaged” in the current AMS 6532 title refers to
1 · Solution treatment (austenitize)
885 ± 14 °C (1625 ± 25 °F), 1 hour.A neutral-atmosphere furnace, salt bath or vacuum is MANDATORY — the producer states explicitly that AerMet 100 is subject to decarburisation during hardening
2 · Quench
Cool from solution temperature to 66 °C (150 °F) in 1–2 hours.Sections over 50 mm (2 in) diameter and plate over 25 mm (1 in) are oil quenched; smaller sections may be air cooled. Water quenching is NOT recommended.The 1–2 hour window is a requirement, not a tolerance
3 · CRYOGENIC TREATMENT
−73 °C (−100 °F), 1 hour. Its purpose is to transform the retained austenite left after quenching into martensite. The producer specifies it to maximise toughness. One distributor compilation states that skipping it lowers toughness by about 15 % — single source, not verified against the producer datasheet
4 · AGEING
482 ± 6 °C (900 ± 10 °F), 5 hours.Never below 468 °C (875 °F). M₂C precipitation occurs here and all of the strength comes from this step
The M₂C mechanism — what actually happens during ageing
An independent atomic-scale study measured, step by step, what happens at 482 °C — and that is the answer to the question why exactly five hours:
~1 hour: after an incubation period of roughly an hour, needle-shaped hexagonal M₂C begins to nucleate on the dislocations inside the martensite, with a mean radius of the order of 0.2 nm. 3 hours: precipitate length grows to 3–5 nm; the structure is still hexagonal. 5–7 hours: the hexagonal M₂C transforms to an orthorhombic structure and atomic-scale lamellar precipitates appear. This is the window where strength and toughness are jointly optimal. 8 hours and beyond:significant coarsening sets in as the mechanism shifts from nucleation to growth. Tensile strength falls sharply, because the carbides coarsen and the dislocation density collapses at the same time.
Reverted austenite — the second mechanism
A second thing happens during ageing: as the carbides form, nickel and chromium diffuse into the neighbouring martensite and create a locally austenite-stabilising composition. The result is thin austenite films along lath boundaries. Measured values: ~2.3 % by volume at 1 hour, ~2.6 % at 5 hours, ~2.9 % at 7 hours, ~5.4 % at 20 hours. Film thickness runs 5 nm at 1 hour, 15 nm at 7 hours, ~50 nm at 20 hours.
These austenite films contribute to toughness (they blunt the crack tip and absorb energy by transformation). But the same study gives a critical warning: beyond 8 hours, even though austenite keeps increasing, the rate of KIc improvement slows, because the fall in dislocation density outweighs the toughening contribution. So “age longer and it gets tougher” is not true.482 °C / 5 hours is an optimised point, not an arbitrary one.
The price of shifting the ageing temperature
The producer publishes how hardness, strength and toughness change across the 468–496 °C (875–925 °F) band: 54.5–55.5 HRC at 468 °C, 53.0–54.0 HRC at 482 °C, 51.0–52.5 HRC at 496 °C. Strength falls as ageing temperature rises, while fracture toughness and impact energy peak around 482 °C. The instruction never to go below 468 °C (875 °F) is explicit — below that, M₂C does not form fully and the alloy ends up neither hard nor tough.
Decarburisation — the step that gets skipped
The producer warns explicitly: AerMet 100 is subject to decarburisation during hardening. In an alloy with 0.23 % carbon, losing carbon from the surface leaves a soft layer that cannot form M₂C and therefore will not age — and that layer is exactly the surface that governs fatigue life. This is why solution treatment must be done in a neutral atmosphere, a salt bath or vacuum. AerMet 100 processed in an open-atmosphere furnace may still meet the mechanical values on its certificate and still underperform in fatigue.
Welding — AerMet 100 Is Weldable (and This Is Where It Parts Company With 300M)
COMPARISON
Three ultra-high-strength aerospace steels are compared on ONE question: how much fracture toughness remains at a yield strength around 1700 MPa, and what does it cost? Compositions come from SAE AMS title records and producer data sheets; strength values are given as specification minimum or producer typical, EACH NAMED WITH ITS SOURCE. THE THREE STEELS HARDEN BY DIFFERENT MECHANISMS and the table shows this on a separate row. NO AVERAGING WAS DONE.
DEFENCE METAL
Grade
AMS
Mechanism
Anahtar element
Carbon
Typical yield MPa
Typical tensile MPa
Toughness note
Corrosion
Note
AISI 4340 (UNS G43400)
AMS 6415 (air melted) · AMS 6414 (VAR)
Martensitic quench and temper: austenitise + oil quench + temper. It does not precipitation harden.
Ni 1.65-2.00% (deep hardening)
0.38-0.43%
1035-1530 (depending on tempering temperature)
1140-1980 (depending on tempering temperature)
Varies with tempering temperature; the 250-450 °C band is not used.
Not stainless; protection is mandatory.
The reference point. Both 300M and AerMet 100 were developed to pass the limits of this grade.
Dynamic Metals reports fracture toughness in the 60-70 MPa·m^0.5 band (single source).
Not stainless; protection is mandatory.
Silicon raises the tempering resistance; the embrittlement band of 4340 shifts to a higher temperature in 300M (Horn and Ritchie 1978; Metals/MDPI 2021).
AerMet 100 (UNS K92580)
AMS 6532 (280 ksi) · AMS 6478 (290 ksi)
SECONDARY HARDENING: solution treatment + cryogenic + 482 °C aging. It is a GENUINE precipitation hardening; the precipitating phase is the M2C CARBIDE, not an intermetallic.
K_Ic 115-126 MPa·m^0.5 (common band of five sources; Carpenter 126, peer-reviewed measurement 116.5-120.5).
Not stainless; Carpenter rates it ‘Humidity Restricted’ and protection is mandatory. Stress corrosion cracking resistance is a separate property and it is high.
Dynamic Metals reports a cost 50-70% above 300M. It also imposes a narrow 482 °C ± 6 °C aging window and a mandatory cryogenic step.
DEFENCE METAL
Additional information
Mechanism difference
THE THREE ARE NOT IN THE SAME CLASS. 4340 and 300M are QUENCH-AND-TEMPER steels: hardness comes from the quench and tempering takes it back. AerMet 100 is a SECONDARY HARDENING steel: at 51-53 HRC as quenched, its hardness RISES to 53-54 HRC when AGED at 482 °C. That is why the heat treatment of AerMet 100 is called aging, not tempering, and why the rule ‘higher tempering means lower strength’, valid for 4340 and 300M, does not apply to this alloy.
Ortak sinir
NONE OF THE THREE IS STAINLESS. All three require a coating, paint or other corrosion protection system; even AerMet 100, which has the highest chromium (3.1%), forms no passive film.
Secim kurali
If a toughness requirement is met by 60-70 MPa·m^0.5, 300M is the correct choice. If the specification calls for more than 115 MPa·m^0.5 at a yield above 1700 MPa, AerMet 100 is needed. 4340 is the reference grade with the lowest strength ceiling and the easiest supply and machining.
The 4340 values in the table are taken from the AISI 4340 card in this same card set and were verified with the same source discipline. Four independent sources could not be assembled for the 300M fracture toughness; because it rests on a single source the figure is given as ‘Dynamic Metals reports’ and not as a settled value.
The producer’s statement is unambiguous: AerMet 100 is weldable without preheating. This is the most concrete and least disputed difference between it and 300M or 4340, and it follows directly from the low carbon content.
Why It Welds — and What to Watch
DEFENCE METAL
Low carbon
0.21–0.25 % C against 300M’s 0.40–0.45 %. The martensite formed in the heat-affected zone is low-carbon lath martensite, not a hard, brittle carbon martensite. The risk of delayed cold cracking is far below the 300M class
Preheat
Producer statement: not required. That removes the need for preheat equipment and process control even in heavy sections
Filler metal
The producer lists a weld wire product form, so matching-chemistry filler exists. No AWS classification was found; filler is ordered by the producer’s product name
Post-weld processing — the critical point
Weld metal and HAZ have been locally re-solutioned by the weld thermal cycle. For full strength the logical route is post-weld ageing (482 °C / 5 hours). The producer datasheet does not separately define a post-weld heat treatment cycle — that step is set by the programme or fabricator procedure, and you should not invent a cycle on a web page
Welding already-aged material
If an aged part is welded, an overaged (softened) band forms in the HAZ. Without re-ageing the joint stays below parent strength. Weld sequence has to be planned together with the design
Hydrogen
Being weldable does not mean being insensitive to hydrogen. At the 285 ksi level, low-hydrogen processes and consumables, dry shielding gas and clean surfaces are mandatory
An honest caveat. The word “weldable” comes from the producer and is correct, but it does not mean landing gear may be welded freely. In primary structure, welding is always subject to programme approval, a qualified procedure and a post-weld heat-treatment plan. What AerMet 100 brings is that welding is metallurgically possible at all — with 300M it is not.
Machining, Forging and Nitriding
Machining · AerMet 100
DEFENCE METAL
Machinability
Producer statement: somewhat more difficult to machine than 4340 at 38 HRC. That follows directly from the cobalt content and the fine carbide structure
Tooling and cutting speed
Carbide tools are recommended at 280–350 SFM (≈85–107 m/min) — the producer’s own figure. This is one of the few points in this family where verified numerical data exists
Condition for machining
Roughing is done in the overage annealed condition (677 °C / 16 hours, 40 HRC maximum). Finishing is done after ageing at 53–54 HRC
Stress relief after rough machining
427 °C (800 °F), 1–3 hours — producer recommendation, for dimensional stability
Stress relief before straightening
177/204 °C (350/400 °F), 5 hours
Grinding
No detailed grinding parameters were found in the producer datasheet. Nevertheless, as with 300M, grinding damage in ultra-high-strength steels (rehardening, overageing, residual tensile stress) is a real risk; AMS 2649 governs etch inspection of high-strength steel parts and applies here
Forging
Primary breakdown to a maximum of 1232 °C (2250 °F); finish below 899 °C (1650 °F). These are the producer’s own figures. Keeping the finishing temperature low is about preserving the fine grain structure — the alloy’s toughness depends on it.
Nitriding
AerMet 100 can be nitrided, which is an alternative to chromium plating for wear surfaces. An independent plasma nitriding study ran treatments at 440–500 °C for 6 hours and measured: hardened layer thickness of ~62 µm at 440 °C and ~105 µm at 500 °C; phases of α′-Fe (nitrogen-containing martensite) and γ′-Fe₄N, with weak ε-Fe₂₋₃N peaks at 440 °C; and volumetric wear rate reduced by more than 88 % at 460–500 °C.
The critical metallurgical point: the nitriding temperature range overlaps the ageing temperature (482 °C). The same study found that the specimen nitrided at 440 °C had a core hardness roughly 90 HV higher than the one nitrided at 500 °C, and that it even exceeded the hardness of the untreated as-quenched specimen — meaning nitriding is simultaneously an ageing cycle. The practical consequence: nitriding temperature and time also set the core strength of the part; it cannot be planned independently as a “surface treatment” and must be treated as part of the heat-treatment plan.
Corrosion, Stress Corrosion, Hydrogen Embrittlement and Plating
Where it fails — say this first
AerMet 100 is NOT stainless. Its 2.9–3.3 % chromium is there as a constituent of the M₂C carbide; it is not enough to form a passive film (that takes roughly 11 % chromium, and a significant fraction of AerMet 100’s chromium is already tied up in carbide). The producer rates the alloy “humidity restricted” on its own four-level corrosion scale and states that it must be sealed (coated) if used in a moist environment. Bare AerMet 100 rusts.
This is the most commonly misunderstood point on this page. Some distributor pages list AerMet 100 under a “stainless steel” heading — that is wrong. AerMet 100’s advantage is not general corrosion resistance but STRESS CORROSION CRACKING resistance. Those are two different things and must not be conflated.
Stress corrosion cracking (SCC)
The producer publishes a KISCC of about 66 ksi√in (≈72 MPa√m) in 3.5 % NaCl (read from a graph, single source). State what that number means: the total KIc for 300M in the same producer’s comparison table is 50 ksi√in. In other words, the threshold at which AerMet 100 begins to crack in salt water is higher than 300M’s total fracture toughness in dry air.That is the most honest and most striking way to frame the comparison.
Why it is so resistant. In ultra-high-strength steels, SCC and hydrogen embrittlement are effectively the same mechanism: hydrogen accumulating at grain boundaries and crack tips and lowering cohesion. AerMet 100’s advantage is twofold: (1) the fine, dense M₂C precipitate dispersion provides a large population of reversible hydrogen traps, so hydrogen is distributed rather than concentrated at critical sites; (2) the reverted austenite films blunt the crack tip. This mechanistic explanation is the common reading in the literature; the producer datasheet does not phrase it that way.
Hydrogen embrittlement and plating
Outstanding SCC resistance is not immunity to hydrogen. At 1965 MPa (285 ksi) tensile, AerMet 100 sits well above the ~1380 MPa (200 ksi) threshold above which industry treats steels as susceptible. Every electrolytically plated AerMet 100 part requires a hydrogen relief bake.
Plating and Hydrogen Management in AerMet 100
DEFENCE METAL
Why it is coated
It has no corrosion resistance. The producer states it must be sealed if used in a moist environment. Coating is a requirement, not an option
Typical coatings
In landing gear practice: cadmium (the historic standard), zinc-nickel and IVD aluminium; hard chromium on wear surfaces. No coating specification named by the producer specifically for AerMet 100 was verified — the programme specification governs
The bake window — a critical constraint
General aerospace practice bakes after plating at 190–205 °C (375–400 °F). AerMet 100’s ageing temperature is 482 °C, so the bake temperature is far below it and does not affect strength. The constraint runs the other way: the bake temperature must never be allowed to approach the ageing temperature
Duration
Set by part strength and section. No verified duration figure specific to AerMet 100 was found; the governing process specification must be used
ASTM F519 — and a specific problem for AerMet 100
F519 qualifies a process: a notched specimen is held at a defined percentage of its notched fracture strength under sustained load for 200 hours. Standard specimens are made of 4340, and the standard itself acknowledges that parts above 260–280 ksi may not be represented by that baseline. AerMet 100 is precisely above that band — specimens made from the production material are recommended. The practical difficulty: according to practitioner sources, AerMet 100 specimens in F519 dimensions are not readily available commercially and generally have to be machined in-house
The producer and distributors cite use to ~427 °C (800 °F). The reason is the ageing temperature: running a part aged 5 hours at 482 °C anywhere near that temperature means overageing it in service — M₂C carbides coarsen, reverted austenite increases, and strength falls permanently. 427 °C is not a code limit (AerMet 100 has no ASME code coverage); it is a metallurgical ceiling. Consider peak local temperature, not continuous service temperature.
AerMet 100 vs 300M vs Maraging 250/350 vs 4340 — an Honest Comparison
The numbers in the table below come from a single producer’s own comparison study and are therefore mutually consistent. Note that the same producer’s 300M product datasheet gives 60–70 ksi√in for KIc while this table says 50 ksi√in — that is a conflict, and both values are published.
Strength and Fracture Toughness · Producer Comparison Data
Four Alloys · The Other Differences That Drive the Buying Decision
DEFENCE METAL
Welding
AerMet 100: weldable, no preheat required.Maraging: weldable (very low carbon). 300M and 4340: not welded in primary structure
Dimensional stability
Maraging is the best — no quench, and ageing shrinkage of the order of 0.05 % linear. AerMet 100 is quenched, so distortion is something to manage
Corrosion resistance
None of them is stainless; all four must be coated. AerMet 100 is rated “humidity restricted” on the producer’s scale
Stress corrosion resistance
AerMet 100 is clearly the best (KISCC ≈66 ksi√in in 3.5 % NaCl). 300M and 4340, in the producer’s own words, must be protected with chromium or cadmium coating
Cost and availability
4340 < 300M < Maraging < AerMet 100. A single-source distributor comparison puts AerMet 100 at 50–70 % more expensive than 300M with longer lead times, because of cobalt content and double vacuum melting
Supplied condition
AerMet 100 is normally supplied un-aged (machinable) with final heat treatment at the customer. 300M can be bought either heat treated or normalized and tempered
Elevated temperature
AerMet 100 is cited to ~427 °C; 300M’s ceiling is below its own ~302 °C tempering temperature. Neither is a genuine high-temperature material — above 500 °C move to alloy 718 or Waspaloy
Three honest conclusions from the table. 1 · AerMet 100’s advantage is not strength but the strength-toughness COMBINATION. Maraging 350 is stronger (343.6 ksi) but far more brittle (38.5 ksi√in). AerMet 340 is stronger still (352 ksi) and more brittle still (31.5 ksi√in). 2 · Maraging 250 offers nearly comparable toughness at a lower strength level (258.6 ksi / 91.5 ksi√in). If the strength class can be relaxed, maraging is a serious alternative and is superior on dimensional stability. 3 · The producer’s claim of “three times the fracture toughness of 300M” does not quite match its own data. Its comparison table gives 120 against 50 ksi√in (2.4×), while its own 300M product datasheet gives 60–70 ksi√in (1.6–1.9×). Publish the numbers, not the marketing sentence.
Frequently Asked Questions
A customer asks for “AerMet 100 plate to AMS 6532”. Can we supply it?
Be careful — the request contains a contradiction. The scope of AMS 6532 is bars, forgings and (in later revisions) forging stock. The title is explicit: “Steel, Bars and Forgings, and Forging Stock…”. Plate is not within AMS 6532’s scope. The same is true of sheet, strip, wire and weld wire — the producer makes and sells all of these forms, and none of them has an AMS specification. The honest answer has three parts:(1) chemistry can be certified to the AMS 6532 band (3.1Cr – 11.5Ni – 13.5Co – 1.2Mo, C 0.21–0.25 %); (2) melting is still vacuum melting and that does not change; (3) but product-form tolerances, mechanical acceptance values and inspection scope are by agreement and that belongs in the order acknowledgement. What you must not do is issue a certificate stamped “AMS 6532” on plate. Such a document claims specification compliance for a product form outside the specification’s scope and is indefensible in a dispute. The correct wording is: “Chemistry conforms to AMS 6532 Table 1; product form is outside the specification’s scope.”
Can we skip the cryogenic step? Our furnace has no −73 °C capability.
Do not skip it — and the reason is metallurgical, not procedural. When AerMet 100 is quenched from 885 °C, not all of the structure transforms to martensite; some retained austenite remains. That austenite cannot precipitate carbide during the subsequent ageing step — so that volume contributes nothing to strength. Worse, unstable retained austenite can transform to martensite in service under load, leaving behind an untempered, un-aged, brittle volume. The cryogenic step exists solely to convert that austenite to martensite BEFORE ageing:−73 °C (−100 °F) for 1 hour. The producer specifies it to maximise toughness. A single-source distributor compilation states that skipping the step lowers toughness by about 15 % — that figure was not verified against the producer datasheet, but the direction is not in dispute. The practical answer: −73 °C is easily reached with a mechanical chiller or a dry ice / alcohol bath; liquid nitrogen is not needed. If you do not have the capability, send the heat treatment out.An AerMet 100 part with the cryogenic step skipped is 300M performance bought at AerMet 100 prices.
Is AerMet 100 stainless? Can we use it uncoated in a marine environment?
No and no. This is the most common misunderstanding on this page. AerMet 100 is not stainless. Its 2.9–3.3 % chromium is not enough to form a passive film — a self-passivating stainless needs roughly 11 % free chromium, and much of AerMet 100’s chromium is already tied up in M₂C carbide. The producer rates the alloy “humidity restricted” on its own corrosion scale and states it must be sealed in a moist environment. Bare AerMet 100 rusts. What gets conflated is this: AerMet 100’s advantage is not general corrosion resistance but stress corrosion cracking resistance. In 3.5 % NaCl, KISCC ≈66 ksi√in (≈72 MPa√m) — it begins to crack under stress in salt water only at a very high threshold. But it still corrodes, and every corrosion pit in a 285 ksi material is a fatigue crack initiation site. Correct practice: in marine or humid service AerMet 100 is always coated (cadmium, zinc-nickel, IVD aluminium; hard chromium on wear surfaces), and after electrolytic plating a hydrogen relief bake is applied. If a genuinely stainless ultra-high-strength material is needed, look at precipitation-hardening stainless steels such as 17-4 PH — but the strength class is far lower, and that is a trade, not an upgrade.
Does moving from 300M to AerMet 100 justify the cost?
It depends on what sizes your part — and do not decide before you have settled that question. When it does not justify it: if the part is sized by strength, meaning stress level is limited by yield strength and flaws are managed by inspection, there is no meaningful strength difference between AerMet 100 and 300M (both around 285 ksi). Per a single-source comparison you would pay 50–70 % more with longer lead times for nothing. When it does justify it: if the part is sized by damage tolerance. Critical crack size scales with the square of KIc; the producer comparison table’s 120 against 50 ksi√in means a roughly sixfold larger tolerable crack. That translates directly into longer inspection intervals, fewer overhauls, less scrap and lower life-cycle cost. Three further reasons:(1) AerMet 100 is weldable, 300M is not — repair and manufacturing flexibility differ. (2) In salt environments its KISCC ≈66 ksi√in is of the same order as 300M’s total KIc. (3) The transverse toughness penalty is small (115 longitudinal / 100 transverse ksi√in), which matters in large forgings. The honest summary: AerMet 100 is not a material upgrade; it is a RISK upgrade. Price the risk, not the alloy.
Common Datasheet Errors — Check Before You Order
1 · “AerMet 100 is a stainless steel” — WRONG. More than one distributor page files the alloy under a “stainless steel” heading. 2.9–3.3 % chromium forms no passive film, and the producer rates it “humidity restricted” and requires it to be sealed in moist environments. 2 · Unit conversion error: “285 ksi = 2069 MPa”.285 ksi = 1965 MPa; 2069 MPa = 300 ksi. This error appears in the producer’s own blog article and has been copied into many pages from there. Treat any page giving the tensile strength as 2069 MPa with suspicion. 3 · “Three times the fracture toughness of 300M” — does not match its own data. The producer’s own comparison table gives 120 against 50 ksi√in (2.4×), while its own 300M product datasheet gives 60–70 ksi√in (1.6–1.9×). Publish the number, not the multiplier. 4 · “AMS 6532 covers plate/sheet/wire” — WRONG. The title reads “Bars and Forgings, and Forging Stock”. There is no AMS specification for plate, sheet, strip, wire or weld wire, even though the producer sells those forms. 5 · Treating AMS 6532 and AMS 6478 as the same document.AMS 6532 → the 280 ksi class, AMS 6478 → the 290 ksi class. Same chemistry, different strength target. State which one governs in the order text. 6 · The revision trap. AMS 6532’s title changed across revisions: earlier revisions say “Annealed, Heat Treatable to 280 ksi”, later ones say “Normalized and Overaged, Precipitation Hardenable”. The alloy is the same; the description of the supplied condition changed. An auditor comparing an old drawing against a new certificate will stop here. 7 · Showing the cryogenic step as “optional”.−73 °C for 1 hour is part of the producer’s standard cycle and exists to transform retained austenite. Write the cycle as three steps (solution + cryogenic + age). 8 · Writing “water quenched”. The producer does NOT recommend water quenching. The correct statement is: cool from solution temperature to 66 °C in 1–2 hours; oil above 50 mm section, air cool for smaller sections. 9 · Omitting the decarburisation warning. The producer states explicitly that the alloy decarburises during hardening and requires a neutral atmosphere, salt bath or vacuum. A datasheet without that line is incomplete. 10 · Calling the ageing step an “anneal”.482 °C / 5 hours is not an ANNEAL but a PRECIPITATION HARDENING step and it increases strength. 677 °C / 16 hours is a genuine overage anneal and softens the material to 40 HRC maximum. Datasheets that confuse the two are common. 11 · Density conflict. The producer gives 0.287 lb/in³ (7.94 g/cm³); some distributors give 0.285 lb/in³. The difference is small, but do not publish two values on the same page. 12 · Using “AerMet 100 = A100 = Alloy 100” without explanation. These names point to the same UNS number (K92580), but AerMet is a registered trade name. Writing the specification and UNS number is always safer. 13 · Inventing a code temperature.AerMet 100 has NO ASME code coverage — it does not appear in Section I, Section VIII, B31.1 or B31.3. The cited ~427 °C (800 °F) is a metallurgical ceiling (the need to stay below the ageing temperature), not a code limit. Keep the two apart. 14 · Planning nitriding independently of heat treatment. The published plasma nitriding study uses 440–500 °C — a band that overlaps the ageing temperature — and the same study measured core hardness changing with nitriding temperature. Nitriding is not just a surface treatment; it is also an ageing cycle.
