UNS K44220 · 300M · 4340M · E4340 Mod · SAE 434M · BS S155. NO VERIFIED W.Nr./EN NUMBER EXISTS; order against UNS K44220 and an AMS number. 300M is the SILICON- AND VANADIUM-MODIFIED derivative of AISI 4340: the Cr-Ni-Mo skeleton of 4340 is kept, silicon is RAISED from the 0.15-0.35% band to 1.45-1.80%, 0.05-0.10% vanadium is added and molybdenum is raised from 0.20-0.30% to 0.30-0.50%. THE COMPOSITION DEPENDS ON THE AMS NUMBER and the difference is IN THE CARBON: AMS 6417 C 0.38-0.43% · AMS 6419 C 0.40-0.45% · AMS 6257 C 0.40-0.44%. The other elements are the same in all three per the SAE title records: nominal 1.6Si – 0.82Cr – 1.8Ni – 0.40Mo – 0.08V. In aircraft quality the phosphorus and sulphur ceilings are tight (SSA and Lork: P 0.010% max, S 0.008% max) and ALL THREE AMS NUMBERS CARRY A CONSUMABLE ELECTRODE VACUUM REMELTED (VAR) REQUIREMENT. IT IS A MARTENSITIC QUENCH-AND-TEMPER STEEL: it hardens by austenitising, oil quenching and DOUBLE TEMPERING. It does NOT precipitation harden; there is no H900 / H1025 type aging step. IT IS NOT STAINLESS.
Bought for primary structural parts that must reach 1860-1930 MPa tensile strength in thick section, where weight is critical and failure loses the aircraft. The dominant application is AIRCRAFT LANDING GEAR: shock strut outer cylinders, inner pistons, axles, pins and attachment lugs.
Forms
Round bar · flat bar · plate · sheet · tube · forging. All forms are supplied to order. The AMS coverage extends ONLY to bars, forgings, forging stock and MECHANICAL tubing; no verified AMS number covers plate, sheet or strip in this alloy (see the standards map).
Standards
AMS (all three verified, and all three carry a CONSUMABLE ELECTRODE VACUUM REMELTED requirement): AMS 6417 — ‘Steel, Bars, Forgings, and Tubing, 1.6Si – 0.82Cr – 1.8Ni – 0.40Mo – 0.08V (0.38 – 0.43C), Consumable Electrode Vacuum Remelted’; scope bars, forgings, mechanical tubing and forging stock; current revision K-2021. · AMS 6419 — the same title with a CARBON BAND of 0.40-0.45%; current revision K-2021. · AMS 6257 — ‘Steel Bars, Forgings, Forging Stock, and Mechanical Tubing 1.6Si – 0.82Cr – 1.8Ni – 0.40Mo – 0.08V (0.40 – 0.44C) Consumable Electrode Vacuum Remelted NORMALIZED AND TEMPERED’; current revision G-2022. This number is a DELIVERY CONDITION specification: the material is supplied normalized and tempered. Military specifications it replaced (per SSA): AMS 6417 ← MIL-S-83135 · AMS 6257 ← MIL-S-8844 Rev D Class 3. Other military and OEM specifications: MIL-S-8844 Class 3 · BMS 7-26 Class 1 (Boeing) · DMS 1935 · CE-0896 · GM-1012 · LC-05-1190 · BS S155. ASTM: no ASTM number belonging to this alloy could be CONFIRMED across four independent sources, so none is given. AMS 6417 AND AMS 6419 ARE 300M, NOT PLAIN 4340. This is the most common confusion on the 4340 side. Both SAE title records define the alloy as ‘1.6Si – 0.82Cr – 1.8Ni – 0.40Mo – 0.08V’; plain 4340 carries 0.15-0.35% silicon and no vanadium.
Advantage
Its single most important practical advantage is the TEMPERING RESISTANCE that silicon brings, and it has two concrete consequences. FIRST, THE STRENGTH CEILING: the AMS 6417 floor is 1862 MPa tensile / 1517 MPa yield and the AMS 6419 floor is 1931 MPa tensile / 1586 MPa yield;
Welding
IT IS WELDABLE, BUT WELDING IS USUALLY AVOIDED ON AIRCRAFT PARTS. Supreme Steels states plainly: ‘Welding is possible with preheat and post-weld heat treatment’, ‘improper welding reduces toughness and strength’, and for critical components ‘shops often avoid welding and use forged or machined…
Limits
1) IT IS NOT STAINLESS. Chromium is 0.70-0.95% and no passive film forms. Dynamic Metals states that 300M requires protective measures in corrosive environments and that its stress corrosion cracking resistance is poor. PROTECTION by plating, cadmium or its alternatives, or paint IS MANDATORY.
Compiled from manufacturer data sheets · confirm against the current specification before ordering
On this page · click to jump
What 300M IsStandards by Product FormSpecification Architecture: The Real Difference Between AMS 6417, AMS 6419 and AMS 6257Product Forms With NO StandardChemical CompositionMechanical PropertiesPhysical PropertiesHeat Treatment and Thermal StabilityWeldingMachining, Grinding and Nital Etch InspectionCorrosion, Stress Corrosion and Hydrogen Embrittlement300M vs 4340 vs AerMet 100 vs AerMet 340 vs MaragingFrequently Asked QuestionsCommon Datasheet Errors
300M is an ultra high strength steel, essentially AISI 4340 enriched with silicon and vanadium and produced by vacuum arc remelting (VAR). Within the alloy steel group it is regarded as the classic material for landing gear applications. It is also known as AISI E4340 Modified.
What separates the material from 4340 is that it raises strength without raising carbon content. This is achieved with 1.45-1.80% silicon and 0.05-0.10% vanadium; silicon raises temper resistance while vanadium refines the grain structure. The result is a strength reaching 1931 MPa together with acceptable toughness and ductility.
Vacuum arc remelting holds phosphorus and sulphur to a combined level below 0.010% and lowers inclusion content, which directly improves fatigue life. Heat treatment: normalise at 927 °C for 1 hour, austenitise at 871 °C for 1 hour, oil quench, then double temper at 302 °C for 2 hours.
It is used in landing gear components, aircraft structural parts and critical components working under high stress. It is supplied as bar, tube and welding wire. Dual certification to the British aerospace standard BS S155 is available.
What 300M Is — and Why It Is Not Just “Modified 4340”
300M (UNS K44220, catalogued in aerospace listings as 4340 Mod or E4340 Mod) is a low-alloy, quench-and-tempered, ultra-high-strength aircraft structural steel. The one-sentence identity is this: 300M is 4340 with its silicon raised roughly sevenfold, vanadium added, and vacuum remelting made mandatory — and all three changes serve one purpose: to let the steel be tempered into the 1931–2070 MPa (280–300 ksi) band without embrittling.
Getting this right matters commercially, because the customer who buys 300M rarely wants “stronger 4340”. They want to be able to temper at a particular temperature. The table below shows where the difference actually lies.
Three Differences Between 4340 and 300M — and the Reason for Each
DEFENCE METAL
Silicon: 0.25 % → 1.45–1.80 %
This is the whole point. Silicon retards the nucleation and growth of cementite (Fe₃C) during tempering. In 4340, tempering in the 260–370 °C band forms films of cementite at prior austenite boundaries and lath interfaces; those films create the toughness trough known as tempered martensite embrittlement (TME), which is why 4340 is not tempered there. Silicon pushes that reaction to higher temperature. The result: 300M can be tempered in the ~300 °C window 4340 cannot enter, and it keeps its hardness there while delivering acceptable toughness
Vanadium: none → 0.05–0.10 %
Leaves undissolved V carbides/carbonitrides during austenitising. These act as grain-boundary pinning particles and limit austenite grain growth at 871 °C. Finer prior austenite grain = shorter martensite lath packets = higher toughness and higher resistance to crack initiation. Vanadium also contributes some secondary hardening on tempering
Carbon: 0.40 % → 0.40–0.45 %
A small but deliberate increase over 4340. It raises the intrinsic hardness of the martensite; this is what lets 300M reach 53 HRC minimum in the tempered condition. Molybdenum is also raised from 0.25 % to 0.30–0.50 % for hardenability and resistance to temper embrittlement
AMS 6417, AMS 6419 and AMS 6257 all carry “Consumable Electrode Vacuum Remelted” in their titles. This is not a recommendation, it is a specification requirement. At the 300 ksi level a sulphide stringer is no longer a defect — it is a crack nucleus
Where it sits in the family — honest positioning
The most common mistake in selling 300M is positioning it as a “high-performance alloy”. The more accurate framing is: 300M is the cheapest, most available and best-understood member of the ultra-high-strength class — and one of the lowest in fracture toughness within that class.
300M’s Place in the Ultra-High-Strength Steel Family
DEFENCE METAL
4340 / 4340 VAR
Same Ni-Cr-Mo backbone, no silicon and no vanadium. Typically used at 1790–1860 MPa (260–270 ksi). In the producer’s own comparison study: 269 ksi tensile with KIc 70 ksi√in. Cheaper, easier to machine, easier to temper — but it cannot be taken to 280 ksi. That is precisely why 300M exists
300M (this page)
1931–2070 MPa (280–300 ksi). The producer datasheet gives KIc 66–77 MPa√m (60–70 ksi√in); the same producer’s alloy comparison table gives 50 ksi√in — the two figures conflict and both are published. Landing gear, flap tracks, high-strength bolts
Same tensile class (1965 MPa / 285 ksi) but KIc 126 MPa√m (115 ksi√in) — roughly twice the toughness of 300M at the same strength. It is not a carbide-hardened martensite but a Ni-Co-Mo secondary-hardening steel. It is weldable. The price: cobalt, double vacuum melting and a single producer — one distributor source puts it at 50–70 % more expensive
AerMet 310 / AerMet 340
Higher strength, far lower toughness. Producer table: AerMet 310 315 ksi / 65 ksi√in; AerMet 340 352 ksi / 31.5 ksi√in. AerMet 340 is more brittle than 300M. The “AerMet” name is not a toughness guarantee
Carbon-free, intermetallic-hardened Fe-Ni martensites. Producer table: NiMark 250 → 258.6 ksi / 91.5 ksi√in (less strong than 300M but far tougher), Maraging 350 → 343.6 ksi / 38.5 ksi√in. Maraging wins on dimensional stability and weldability; 300M wins on price and fatigue performance
The one-sentence buying rule: if the part is sized by strength and flaw tolerance is managed by inspection, 300M is the right choice. If the part is sized by damage tolerance — if the design criterion is “must fly with a defined crack present” — 300M is usually the wrong choice and the answer is AerMet 100.
Standards by Product Form
STANDARDS BY PRODUCT FORM
DEFENCE METAL
Product form
Standards
Round bar · flat bar · forging · forging stock · MECHANICAL tubing
AMS 6417 (C 0.38-0.43% · VAR required) · AMS 6419 (C 0.40-0.45% · VAR required) · AMS 6257 (C 0.40-0.44% · VAR required · delivered NORMALIZED AND TEMPERED) · MIL-S-8844 Class 3 · BMS 7-26 Class 1 · DMS 1935 · BS S155
Plate · sheet · strip
NO STANDARD EXISTS. None of the three AMS numbers covers flat product; these forms are supplied by producer-customer agreement against the AMS 6417 chemistry.
Pressure tubing
NO STANDARD EXISTS. The ‘tubing’ in AMS 6417/6419/6257 is MECHANICAL tubing, not pressure tubing; this alloy is not a pressure vessel material.
Fitting · flange
NO STANDARD EXISTS.
Bloom · billet (for aerospace forging)
ASTM A646 (premium quality alloy steel blooms and billets) appears in the SSA and AZoM lists; it COULD NOT BE CONFIRMED ACROSS FOUR INDEPENDENT SOURCES that this number covers 300M, so it is not written on the card.
Product forms were read from the Defence Metal 300M page; the standard assignments were verified separately against the SAE title records. ALL THREE AMS numbers carry a consumable electrode vacuum remelted (VAR) requirement; no verified AMS number was found for air-melted 300M.
300M is an aerospace material; it lives in the AMS world, not the ASTM/ASME world. That is the single most important thing to keep in mind when reading the table below: do not look for a 300M specification for pipe, flanges, fittings or plate — none exists and none is coming.
Standards by Product Form · 300M (UNS K44220)
DEFENCE METAL
Bar
AMS 6417 (C 0.38–0.43 %) · AMS 6419 (C 0.40–0.45 %) · AMS 6257 (C 0.40–0.44 %, supplied normalized and tempered). All three cover bars, forgings, mechanical tubing and forging stock
Forgings
The same three specifications. Also ASTM A579 (superstrength alloy steel forgings) Grade 32 — matched to 300M in distributor listings, but not independently verified from the ASTM scope page
Forging stock / billet
Current revisions of AMS 6257 are titled “Bars, Forgings, Forging Stock, and Mechanical Tubing”; AMS 6419’s scope likewise includes forging stock
Mechanical tubing
Covered by AMS 6417 / 6419 / 6257 — all three carry “Tubing” in the title. Note: this is mechanical tubing, not pressure pipe. There is no 300M specification for pressure-containing fluid service
Plate · sheet · strip
None. None of the three AMS specifications covers plate or sheet. Some distributor pages offer “300M plate/sheet” — those are unspecified products sold to a company agreement
Seamless / welded pressure pipe
None — and none should be expected
Fittings · flanges
None — 300M is not a pressure-boundary material
Bolts · fasteners
The material is bought to AMS 6417/6419, but the fastener itself is made to a separate NAS/MS/manufacturer specification. No dedicated 300M fastener material specification was found
Welding wire · covered electrode
There is no AWS classification. Some distributor pages list “300M welding wire” — that is a matching-chemistry, unclassified producer product. See the welding section below
Cleanliness / inspection specification
AMS 2300 — “premium aircraft-quality” magnetic particle inspection. Very frequently invoked alongside 300M orders; it is a quality/cleanliness specification, not a material specification
Military
MIL-S-8844 (Steel Bar, Reforging Stock and Mechanical Tubing, Low Alloy, Premium Quality) — cancelled and superseded by SAE AMS-6414 and SAE AMS-6257. MIL-S-83135 is also cited
There is NO EN or DIN standard covering 300M. German supplier indexes list W.Nr. 1.6928 as a “SiNiCrMo” heat-treatable steel, and one supplier index cross-references it directly to K44220 — but that is a supplier-level, single-source match
IMPORTANT CORRECTION — a widespread catalogue error. Many sources give the European equivalent of 300M as “35NiCrMoV12-5”. That is wrong. 35NiCrMoV12-5 carries Werkstoff number 1.6959 and its composition is C 0.30–0.40 % · Si 0.15–0.35 % · Mn 0.40–0.70 % · Cr 2.50–3.50 % · Ni 1.00–1.40 % · Mo 0.35–0.60 % · V 0.08–0.20 %. Its silicon is less than a fifth of 300M’s and its chromium more than triple — it lacks the one element that makes 300M what it is. Published quench-and-temper strengths for it are in the 1200–1700 N/mm² band, not 300M’s 1931–2070 MPa. Do not offer these two steels as equivalents.
Specification Architecture: The Real Difference Between AMS 6417, AMS 6419 and AMS 6257
All three specifications define the same nominal alloy — 1.6Si – 0.82Cr – 1.8Ni – 0.40Mo – 0.08V — and all three require vacuum remelting. The differences are in the carbon band and the supplied condition, and those differences are the most common source of ordering errors.
Three AMS Specifications · What Differs
DEFENCE METAL
AMS 6417
Title: “Steel, Bars, Forgings, and Tubing, 1.6Si – 0.82Cr – 1.8Ni – 0.40Mo – 0.08V (0.38–0.43C), Consumable Electrode Vacuum Remelted.”The low-carbon band. One distributor source states the band as 0.39–0.43 % — [conflict]; the SAE title says 0.38–0.43 %. Typical acceptance values quoted: 1862 MPa (270 ksi) tensile · 1517 MPa (220 ksi) yield · 8 % elongation · 30 % reduction of area (single-source distributor compilation)
AMS 6419
Title: identical, but (0.40–0.45C). The high-carbon band is the high-strength band. The specification’s own application note: parts up to 89 mm (3.5 in) in section, required to through-harden to a minimum of 53 HRC. Minimum properties: 1931 MPa (280 ksi) tensile · 1586 MPa (230 ksi) yield · 7 % elongation in 4D · 25 % reduction of area (longitudinal)
AMS 6257
Title: same chemistry, (0.40–0.44C), but “Normalized and Tempered” — the supplied condition differs. This is stock intended to be machined or forged; final strength comes from the customer’s own heat treatment
Which document does which job
AMS 6419 = finished high-strength part. AMS 6417 = slightly lower carbon, slightly tougher, 270 ksi class. AMS 6257 = raw material in a machinable supplied condition. Saying “300M” is not enough; put the specification number and revision in the order text
Stress-corrosion warning — inside the specification itself
AMS 6419 states in its own application paragraph that certain design and processing procedures may make these products susceptible to stress-corrosion cracking after heat treatment, and refers to SAE ARP1110 for practices that minimise it. This is a warning inside the material specification and it should be treated as such
Macro examination
For bars, billets, tube rounds and forging stock, AMS 6419 requires macrostructure examination of full transverse cross-sections etched in hot hydrochloric acid (referencing ASTM A604). Decarburisation limits are tabulated, and ground, turned or polished surfaces must be free from decarburisation
Product Forms With NO Standard — the Commercially Valuable Section
This is the section your sales engineers should memorise. With 300M the specification gap is not a “not yet written” gap as it is with some nickel alloys; it is a deliberate scope decision. 300M is a forged structural steel and the AMS system has locked it into that role.
Specification Gaps for K44220
DEFENCE METAL
Plate · sheet · strip
No specification. The scope of AMS 6417/6419/6257 is limited to bars, forgings, forging stock and mechanical tubing. The honest answer to a “300M plate” enquiry is: chemistry can be certified to AMS 6419, but product form and mechanical acceptance are by agreement. Historic plate applications such as rocket motor cases exist, but they ran on programme specifications
Cold-drawn wire · spring wire
No specification. 300M’s 0.40–0.45 % carbon and 53 HRC target make it unsuited to cold drawing in any case. If high-strength wire is wanted, you are in the wrong alloy
No specification, and there should not be one. 300M’s stress-corrosion and hydrogen embrittlement behaviour make it unsuitable as a pressure-boundary material. 300M does not appear in ASME Section VIII, Section I, B31.1 or B31.3
Castings
There is no cast equivalent of 300M. If an ultra-high-strength casting is wanted, a different family must be used, and the fact that it is not 300M must be written into the order acknowledgement
Welding consumables
No AWS classification. Do not look for an ER/E number for 300M. “300M welding wire” is sold; it is a matching-chemistry producer product and is generally used under specific programme approval rather than as a general repair route
Additive manufacturing powder
No separate powder specification was found. 300M’s high carbon and silicon make it a demanding composition for layerwise processing from a cold-cracking standpoint
Requests for an “ASTM equivalent” for piping components
This is the most frequent enquiry and the honest answer is “there is none”. The ASTM A579 Grade 32 and ASTM A646 300M-8 attributions appear in distributor listings but were not independently verified from ASTM scope pages, and both are forging/bar documents, not fitting documents
1.45–1.80 % — about seven times 4340. The defining element of the alloy. Producer typical value 1.65 %
Manganese (Mn)
0.60–0.90 %. Producer typical value 0.75 %
Chromium (Cr)
0.70–0.95 %. Producer typical value 0.80 %. This is for hardenability, not corrosion resistance — 0.8 % chromium forms no passive film
Nickel (Ni)
1.65–2.00 %. Producer typical value 1.80 %. Toughness and hardenability
Molybdenum (Mo)
0.30–0.50 % (some distributor compilations cap it at 0.45 % — conflict; the SAE title nominal is 0.40 %). Producer typical value 0.40 %. Hardenability and resistance to temper embrittlement
Vanadium (V)
0.05–0.10 %. Producer typical value 0.07 %. Grain refinement. Some older compilations print “V 0.05 % min” or “≤0.050 %” — the latter is wrong; vanadium is a deliberate addition, not an impurity ceiling
Phosphorus (P) and Sulphur (S)
In aircraft quality typically ≤0.010 % each (single-source distributor compilation). Some general-purpose datasheets print P ≤0.035 %, S ≤0.040 % — those are not realistic for aerospace 300M and were most likely copied from a generic 4340 table
Copper (Cu)
≤0.35 % (single-source distributor compilation)
Iron (Fe)
Balance
Two traps when reading the composition.First: a widely mirrored datasheet shows vanadium as “≤0.050 %”, carbon as “0.40–0.460 %” and phosphorus as “0.035 %”. That table looks like a generic 4340 table with a vanadium row pasted in and does not match the AMS bands. Second: 300M’s chemistry is identical across the three AMS specifications apart from carbon; a certificate that does not state the carbon band is not telling you which specification the material was made to.
Mechanical Properties
STRENGTH VALUES
DEFENCE METAL
Condition
Hardness
Yield MPa
Tensile MPa
Elongation
AMS 6417 (C 0.38-0.43%) · hardened and tempered · longitudinal
Hardened and tempered · SUPPLIER TYPICAL BAND
42-48
1450-1550
1860-2030
10-15%
Hardened and tempered · SUPPLIER TYPICAL BAND (second source)
—
1520-1620
1930-2000
—
Tempered at 316 °C (600 °F)
—
—
2069
—
The rows show the SPECIFICATION MINIMUM and the PRODUCER / SUPPLIER TYPICAL VALUE separately. They must not be mixed: the specification minimum is the order floor, the typical value is the expected result. NO AVERAGE WAS TAKEN. In the chart, a value given as a range is drawn at its lower bound; the full range is in the table. The AMS 6417 floor is identical in five independent sources (Aircraft Materials, the SSA data sheet, the SSA product page, Lork and Phi-Motion); it is the most strongly verified row in the table. The supplier typical hardness band (Supreme Steels 42-48 HRC) differs from the specification floor (52 HRC) because the tempering temperatures differ. See the contradictions list.
The gap between minimum and typical is unusually large here and it is the most misunderstood aspect of buying 300M. The specification minimum is an acceptance criterion; the producer typical is what is actually achieved. The design allowable is a third thing again and must come from a source such as MMPDS/MIL-HDBK-5.
300M Mechanical Properties · Minimum (specification) vs Typical (producer)
Reduction of area falls from 30 % average to 15 % average as cross-sectional area increases. Transverse ductility degrades seriously in heavy sections — design around forging flow direction
AMS 6419 application note: through-hardening to a minimum of 53 HRC. The AMS 6417 distributor compilation gives ≥52 HRC
Fracture toughness KIc
[CONFLICT — both numbers come from the same producer] The product datasheet gives 60–70 ksi√in (66–77 MPa√m) per ASTM E399 for the 575 °F temper. The same producer’s alloy comparison table gives 50 ksi√in at 287 ksi tensile. A distributor comparison also prints 60–70 MPa√m. The safe range is 50–70 ksi√in; do not publish a single figure
Modulus of elasticity (E)
205 GPa (29,700 ksi)
Shear modulus (G)
80 GPa (11,600 ksi)
Poisson’s ratio
0.28
Charpy impact
No verified catalogue value was found. At the 280 ksi level Charpy is not the right design measure in any case — use KIc
Fatigue
One of 300M’s genuine practical strengths, and the real payoff of vacuum melting. However no verified catalogue S-N curve or endurance limit was found — rather than publishing a number, talk about surface condition, shot peening and residual stress
Why you should publish a band rather than a number. 300M’s strength is extremely sensitive to tempering temperature. The producer datasheet gives a tempering range of 260–316 °C (500–600 °F); AMS 6419 locks it at 302 ± 6 °C (575 ± 10 °F). Across that ~55 °C window tensile strength slides from roughly 300 ksi down towards 270 ksi. “300M is 300 ksi” is not a material property; it is a heat-treatment outcome.
Physical Properties
300M Physical Properties
DEFENCE METAL
Density
7.84 g/cm³ (0.283 lb/in³) — producer value. Some compilations give 7.87 g/cm³ (0.284 lb/in³); the difference is trivial but it is a conflict
Thermal conductivity
37.5 W/m·K (260 Btu·in/hr·ft²·°F) — producer value. This is a low conductivity and it is the physical reason heat stays at the surface during grinding
~1422 °C (2590 °F) — single-source general compilation; no liquidus/solidus distinction is given
Electrical resistivity
No verified value was found
Magnetic behaviour
Ferromagnetic. This is what makes magnetic particle inspection (AMS 2300) possible — the primary volumetric/surface inspection method for 300M
Heat Treatment and Thermal Stability — the Complete Route
HEAT TREATMENT — SCHEMATIC
DEFENCE METAL
1 · ANNEALING (for machinability)
Step
1 · ANNEALING (for machinability)
Summary
Softening before machining. It is not part of the hardening cycle.
Temperature
844 °C (1550 °F) — AZoM. Four independent sources could not be assembled for this figure; it is given with its single source named.
Time
No numerical time was confirmed across four independent sources, so none is given.
Cooling
Slow FURNACE cooling (AZoM).
Resulting hardness
No annealed hardness was confirmed across four independent sources, so none is given.
DEFENCE METAL
2 · NORMALIZING
Step
2 · NORMALIZING
Summary
Homogenises the structure after forging. It is the first stage of the AMS 6417 cycle.
Temperature
927 °C ± 14 °C (1700 °F ± 25 °F) — SSA, Phi-Motion and AZoM give the same figure; Aircraft Materials and Michlin give 1700 °F. Five sources agree.
Time
Aircraft Materials gives 1 hour. Being a single source, no binding time is written.
Cooling
AIR cooling (SSA, Phi-Motion, AZoM, Aircraft Materials).
Resulting hardness
Material to AMS 6257 is delivered tempered after this stage. No normalized hardness was confirmed across four independent sources.
DEFENCE METAL
3 · AUSTENITISING + OIL QUENCH (hardening)
Step
3 · AUSTENITISING + OIL QUENCH (hardening)
Summary
This is the stage that gives the hardness.
Temperature
871 °C ± 14 °C (1600 °F ± 25 °F). SSA, Phi-Motion, Michlin and Aircraft Materials give 1600 °F; AZoM gives 872 °C; Supreme Steels gives ‘near 871 °C’. Six sources agree.
Time
Aircraft Materials gives 1 hour. Being a single source, no binding time is written.
Cooling
OIL. SSA, Phi-Motion, AZoM, Michlin, Aircraft Materials and Supreme Steels all quench in oil.
Resulting hardness
No as-quenched untempered hardness was confirmed across four independent sources. In this condition the material is brittle and IS NOT USED UNTEMPERED.
DEFENCE METAL
4 · DOUBLE TEMPERING (mandatory)
Step
4 · DOUBLE TEMPERING (mandatory)
Summary
Mandatory after quenching, and done TWICE. The temperature is chosen for the target strength.
Temperature
SPECIFICATION BAND: 204-649 °C (400-1200 °F) — SSA, Phi-Motion and AZoM give the same band. AEROSPACE PRACTICE SITS AT THE LOWER END of that band: Aircraft Materials 575 °F (≈302 °C), Michlin 500-600 °F (260-316 °C), AZoM 600 °F (316 °C) for 300 ksi. See the forbidden band box.
Time
Aircraft Materials gives 2 hours per temper, applied twice.
Cooling
No cooling medium was confirmed across four independent sources, so none is given.
Resulting hardness
Michlin: up to 52 HRC in sections of 89 mm (3.5 in) and under. AZoM and SSA state 52 HRC minimum.
DEFENCE METAL
5 · STRESS RELIEF (after grinding and welding)
Step
5 · STRESS RELIEF (after grinding and welding)
Summary
Applied after grinding or welding.
Temperature
288 °C (550 °F) — AZoM.
Time
No time was confirmed across four independent sources, so none is given.
Cooling
Not given.
Resulting hardness
Because it stays below the tempering temperature, no hardness loss is expected.
DEFENCE METAL
Tempering table
Note
The table shows the relationship between tempering temperature and strength. EACH ROW IS NAMED WITH ITS SOURCE. This table is NOT an order specification; the order must be tied to AMS 6417, AMS 6419 or AMS 6257. NO AVERAGE WAS TAKEN.
DEFENCE METAL
TEMPERING FORBIDDEN BAND — APPROXIMATELY 350-500 °C
Step
TEMPERING FORBIDDEN BAND — APPROXIMATELY 350-500 °C
What happens
Tempered martensite embrittlement (TME). Impact toughness and fracture toughness fall. In 300M this band is HIGHER than in 4340; the cause is silicon.
As named in the source
Horn and Ritchie, Metallurgical Transactions A 9A, August 1978: TME appears at about 275 °C in 4340 and at 400-450 °C in 300-M with 1.59% Si; silicon pushes the replacement of epsilon-carbide by cementite to higher temperatures. · Metals (MDPI) 2021, 11(9), 1349: for 4340 ‘TME typically manifests after tempering between 200 and 400 °C’, for 300-M ‘a higher temperature regime of 350 to 500 °C is associated with TME’; silicon delays the decomposition of retained austenite. · Thermal Processing Magazine: the general TME band is 260-370 °C and silicon or molybdenum retard cementite precipitation, ‘raising the critical temperature for embrittlement’. · Total Materia: TME is usually seen as a toughness minimum at 300-350 °C, and in steels with 1-2% silicon the carbide is still present after tempering at 400 °C.
Kaynaklar ayrisiyor
The two peer-reviewed sources differ on the LIMITS of the band: Horn and Ritchie give 400-450 °C, Metals 2021 gives 350-500 °C. NO AVERAGE WAS TAKEN; the ENCLOSING band (approximately 350-500 °C) is written on the card.
Pratik sonuc
Aerospace tempering is done BELOW this band, around 290-316 °C. The 427 °C row in the Supreme Steels table is INSIDE the band and is not used on parts with a toughness requirement.
Service warning
This band is not only a HEAT TREATMENT prohibition: prolonged SERVICE in the 350-500 °C band runs the same mechanism.
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 IS A MARTENSITIC QUENCH-AND-TEMPER STEEL: it hardens by austenitising, oil quenching and DOUBLE TEMPERING. It does NOT precipitation harden; there is no H900 / H1025 type AGING STEP. Each stage below was verified separately. Silicon does two things at once in this steel: it raises the tempering resistance, opening the strength ceiling, and by the same mechanism it carries the embrittlement band upward. These are not two separate phenomena. Double tempering is not an option in this alloy but the standard practice named by every source. Material to AMS 6257 is delivered NORMALIZED AND TEMPERED; the final hardening cycle is run at the part maker.
This section is the whole of 300M. The chemistry only delivers the promised strength-toughness balance when the route below is followed. A customer who skips a step or shifts a temperature buys 300M and gets 4340 performance, or worse.
300M Heat Treatment Route · AMS 6419 Requirement
DEFENCE METAL
1 · Normalize
927 ± 14 °C (1700 ± 25 °F), hold 60 ± 5 minutes, air cool. Its purpose is to erase the banded, heterogeneous structure left by forging or rolling and establish a uniform prior austenite grain. It is not optional
2 · Austenitize (harden)
871 ± 14 °C (1600 ± 25 °F), hold 60 ± 5 minutes. This is a relatively low austenitising temperature and that is deliberate: part of the vanadium carbide population is meant to stay undissolved and pin the grain boundaries. Higher temperature = coarser austenite grain = lower toughness
3 · Quench
In oil. Water quenching is not used — in a steel with 0.42 % C and 1.65 % Si, water raises quench-cracking risk to an unacceptable level. The part must be tempered as soon as it has finished cooling; as-quenched, untempered 300M can crack on the rack under its own residual stress
4 · DOUBLE TEMPER
302 ± 6 °C (575 ± 10 °F), each cycle 2 hours ± 0.2 hour, air cool between cycles. These are two separate cycles, not one four-hour cycle. The producer datasheet gives the range as 260–316 °C (500–600 °F) for 4 hours — [conflict: the specification says 2+2 hours, the producer says 4 hours]; state which governs in the order text
~844 °C (1550 °F), slow furnace cool — single-source general compilation
Why double tempering is mandatory
During the first tempering cycle some of the carbon in the martensite precipitates as carbide, and the retained austenite — now less stable — transforms to fresh, untempered martensite on cooling. A part that stops after one temper therefore contains islands of untempered martensite: hard, brittle and ready to initiate a crack. The second cycle exists solely to temper that fresh martensite. This is why air cooling to room temperature between cycles is essential — the transformation has to happen first. A heat-treatment certificate that says “tempered four hours” does not satisfy the double-temper requirement.
Tempered martensite embrittlement and the role of silicon
In a silicon-free steel such as 4340, tempering in roughly the 260–370 °C band makes toughness fall as temperature rises — an unusual and dangerous behaviour. The mechanism is the decomposition of interlath retained austenite into films of cementite in that band; those films form a ready-made path for a crack. 300M’s 1.45–1.80 % silicon retards exactly that reaction. Silicon has negligible solubility in cementite, so cementite can only grow by diffusing silicon out of its way, and that is slow. The consequence: the embrittlement window shifts upward and 300M can be safely tempered at ~300 °C where 4340 cannot. This is the sole reason 300M is usable at 280 ksi.
Practical consequence: tempering 300M above 370 °C drops strength quickly and removes the reason for buying the alloy; tempering below 260 °C leaves residual stress and brittleness unmanageable. The window is narrow, and furnace calibration is a quality issue rather than a preference.
Service temperature
300M’s service temperature is below its own tempering temperature. A part tempered at ~300 °C loses strength as its service temperature approaches that value. For landing gear and flap track applications this is not a constraint; but for parts near brakes and surfaces heated by friction it is a serious design limit. 300M is not a high-temperature steel.
Welding — 300M Is Not Welded
Do not soften this section. 300M is not welded in primary structure. This is not a preference or a “difficult but possible” situation; in aerospace practice it is a material to which structural welding is not applied.
Why It Is Not Welded
DEFENCE METAL
Carbon equivalent
0.40–0.45 % C, plus Mn, Cr, Mo and Ni. The heat-affected zone inevitably transforms to untempered martensite, producing a completely brittle band that can reach beyond 65 HRC
Cold (delayed) cracking
High-carbon martensite + hydrogen from the weld + high residual stress = the classic delayed cracking triangle. 300M supplies all three corners simultaneously
The post-weld heat treatment dead end
The only way to genuinely fix the HAZ is full re-heat-treatment (normalize + austenitize + oil quench + double temper). On a landing gear leg this is effectively impossible: distortion, loss of dimension and the cost of full re-inspection are unacceptable. A stress relief alone is not enough
Specification silence
AMS 6419 says nothing at all about welding. When an aerospace material specification is silent on welding, that does not mean it is permitted — it means welding is out of scope
Filler metal
There is no AWS classification for 300M. Matching-chemistry wire is sold but unclassified; using an undermatching (softer) filler locks the joint below parent strength and destroys the reason the part is made of 300M
What is actually done
300M parts are forged in one piece and finish-machined. Where joining is required, mechanical attachment is used (bolts, pins, interference fits). Worn surfaces are restored not by welding but by hard chromium plating, HVOF spray, or shot peening plus regrinding
If weldability is genuinely required, change the alloy. Weldable options in the same strength class are AerMet 100 (the producer states it is weldable without preheat) and the maraging steels (very low carbon means no brittle carbon martensite forms in the HAZ). This is the most concrete and least disputed difference between 300M and AerMet 100.
Machining, Grinding and Nital Etch Inspection
After heat treatment 300M sits at about 53 HRC. Machining at that hardness is feasible, but the real risk is not cutting — it is grinding, and grinding damage is one of the documented leading causes of 300M-class parts fracturing after entering service.
Machining · 300M
DEFENCE METAL
Tooling
Coated carbide, or cubic boron nitride (CBN) for hard turning. Tools must be kept extremely sharp — a dull tool rubs instead of cutting and generates surface heat
Cutting speed and feed
Lower speeds and feeds than for mild steels, with moderate to heavy depths of cut. No verified numerical speed/feed table for 300M was found; rather than inventing figures, use the tooling manufacturer’s data for hardened alloy steel at 50–55 HRC
Coolant
High-pressure, high-volume flood coolant. The aim is not only tool life but keeping heat out of the part
Workholding
Rigid clamping. High cutting forces and chatter leave local heating and residual tensile stress in the surface
Condition before machining
Roughing is normally done in the normalized and tempered condition (the AMS 6257 supplied condition) or on annealed stock; only finishing is left for after hardening
Stress relief
For complex parts, a low-temperature stress-relief bake after rough machining may be needed for dimensional stability. The temperature must stay below the part’s tempering temperature — going above it lowers strength
Grinding damage — 300M’s most insidious failure mode
Grinding applies very high power density to a very small area. 300M’s thermal conductivity is a low 37.5 W/m·K; the heat generated does not diffuse into the part but stays in the first few tens of microns of the surface. Two distinct forms of damage appear, and neither is visible to the naked eye:
1 · Rehardened (untempered) martensite. The surface locally exceeds Ac₁ (~727 °C) and is instantly quenched by the coolant. The result is an untempered, very hard, very brittle skin with an over-tempered soft band immediately beneath it. 2 · Over-tempering (grinding burn). The surface stays below Ac₁ but exceeds the tempering temperature; hardness drops locally. In a documented landing gear pin investigation the heat-affected region measured 52 HRC (540 HV100) against a core of 55–56 HRC (592–615 HV100). In both cases the surface is left in residual TENSILE stress, and the microstructural mismatch initiates a fatigue crack. In the same investigation the intergranular damage region measured 0.28 mm deep and 1.88 mm wide.
Nital (temper) etch inspection — why it is mandatory
Grinding damage need not appear as a surface crack; it may only have changed the microstructure. The accepted way to find it is etch inspection: the surface is etched in a 2 % nital (nitric acid in alcohol) solution, and regions in different tempering states appear in different shades. Rehardened areas appear light; over-tempered areas appear dark. The governing document is SAE AMS 2649 — “Etch Inspection of High Strength Steel Parts”.
Damage under the plating is invisible — critical warning.Magnetic particle inspection carried out after hard chromium plating cannot reliably find cracks beneath the coating; documented landing gear investigations state this explicitly. In those same investigations damage could only be mapped after the plating was stripped and the part etched in a 2 % nital bath, supplemented by Barkhausen noise inspection. This is why etch inspection is performed immediately after grinding and BEFORE plating. An overhaul flow that reverses that order buries the damage in the system.
Documented case data (same material class, landing gear pin): in one event the pin had seen 23,535 flight cycles, of which 2,309 since rework, and striation analysis showed the crack had initiated 4,150 cycles before the last overhaul — meaning it was present and undetected during maintenance. In a second event the pin had seen 4,710 cycles since rework, but only 797 cycles passed from crack initiation to fracture — meaning the crack formed after the grinding operation in the last overhaul. Grinding damage shortens cyclic life by orders of magnitude.
Corrosion, Stress Corrosion and Hydrogen Embrittlement
Where it fails — say this first
300M is not stainless and it has no corrosion resistance. Its 0.70–0.95 % chromium is there for hardenability; it forms no passive film. Bare 300M rusts in humid air. In salt environments pitting starts quickly, and every pit in a 280 ksi material is a crack nucleus. This is why every 300M part in service is coated — without exception.
Stress corrosion cracking (SCC)
All ultra-high-strength low-alloy steels are susceptible to chloride stress corrosion cracking above roughly 1380 MPa (200 ksi) yield, and susceptibility rises with strength. Because 300M sits at the top of that strength range, it is among the most susceptible members of the class. This is not an academic caution: AMS 6419’s own application paragraph states that certain design and processing procedures may make the product susceptible to SCC after heat treatment, and directs the user to SAE ARP1110 for mitigating practices.
No verified catalogue KISCC value for 300M was found, and publishing a single figure would mislead — the value depends strongly on tempering temperature, strength level and surface condition. For comparison: the KISCC read from the producer’s 3.5 % NaCl chart for AerMet 100 is ~66 ksi√in (~72 MPa√m) (single source, read from a graph) — which means AerMet 100’s KISCC is of the same order as 300M’s total KIc. Framing the comparison this way is more honest than inventing a 300M number.
Hydrogen embrittlement — critical for 300M
This is the one topic everyone who handles 300M must understand. Hydrogen embrittlement is the entry of atomic hydrogen into the steel lattice, its accumulation in regions of triaxial tensile stress (notch roots, crack tips, residual-stress concentrations), and the resulting fracture with no plastic warning at all. The fracture is delayed: the part is sound at assembly and breaks hours or days later under steady load.
Hydrogen Management in 300M · Working Rules
DEFENCE METAL
Threshold
Industry practice treats steels above ~1380 MPa (200 ksi) tensile as susceptible to hydrogen embrittlement. 300M sits at 1.4 times that threshold — susceptibility is not in question
Where the hydrogen comes from
Electrolytic plating (cadmium, chromium, zinc-nickel), acid cleaning/pickling, electrolytic degreasing, phosphating, and in service the corrosion reaction itself. Hard chromium and cadmium plating are the two highest-risk processes
Bake-out (embrittlement relief bake)
190–205 °C (375–400 °F). It must be started within 4 hours of completing plating — every hour of delay lets hydrogen migrate to stress concentrations. Duration is set by part strength and section; in the 300M class it is typically of the order of 23 hours
It is a PROCESS test, not a material test. A notched specimen is held at a defined percentage of its notched fracture strength under sustained load for 200 hours. Any fracture means the process is embrittling. Specimens are processed with the plating lot
F519’s critical limit for 300M
Standard F519 specimens are made of 4340. The standard itself acknowledges that components with ultimate strengths above 260–280 ksi may not be represented by that baseline. At 300M strength levels and above, specimens made from the production material are recommended — and this is the step most supply chains skip
Does the bake reduce strength
190–205 °C is below 300M’s ~302 °C tempering temperature, so the bake does not meaningfully affect strength. This is not a coincidence — 300M’s high tempering temperature is what opens the bake-out window and is the reason the alloy can be plated at all
Coating selection. The historic standard is cadmium: low hydrogen uptake, excellent galvanic protection, good lubricity. But cadmium is toxic and increasingly restricted, and is being displaced by zinc-nickel and IVD aluminium. Hard chromium is used on wear surfaces but carries the highest hydrogen risk and leaves a microcracked layer that lowers fatigue life — which is why surfaces to be chromium plated are first shot peened into compressive residual stress. The plate-grind-inspect sequence is the single most critical process decision in a 300M part’s life.
Fatigue
300M’s fatigue behaviour is the real payoff of vacuum remelting: reducing sulphide and oxide inclusions removes crack initiation sites. Against that, at the 280 ksi level fatigue is acutely surface-sensitive: a notch, a grinding mark, a corrosion pit or residual tensile stress will cut life by large factors. This is why shot peening is close to mandatory on 300M parts and why the plating, grinding and inspection sequence is managed so carefully. No verified catalogue endurance limit was found; use an MMPDS-type source for design.
300M vs 4340 vs AerMet 100 vs AerMet 340 vs Maraging — an Honest Comparison
COMPARISON
300M and AISI 4340 are compared along ONE heat treatment route (normalize → austenitise → oil quench → temper) and on ONE phenomenon: how does the silicon addition change the tempering behaviour? Compositions come from SAE AMS title records and from the actual heats measured in peer-reviewed publications; the embrittlement band temperatures come from peer-reviewed publications. NO AVERAGING WAS DONE.
DEFENCE METAL
Grade
Silicon
Vanadium
Molybdenum
Carbon
Ams ornegi
Temperleme uygulamasi
Tme bandi
Note
AISI 4340 (UNS G43400)
0.15-0.35% (ASTM A29 / SAE J404 band). Heat measured by Horn and Ritchie: 0.26%
none
0.20-0.30%
0.38-0.43%
AMS 6415 (air melted) · AMS 6414 (VAR)
204-649 °C specification band; single tempering is common
About 275 °C (Horn and Ritchie 1978) · 200-400 °C (Metals/MDPI 2021)
The embrittlement band sits at a LOW temperature, which makes it hard to temper low and take high strength.
300M (4340M · UNS K44220)
1.45-1.80% (AMS nominal 1.6%). Heat measured by Horn and Ritchie: 1.59%
AMS 6417 · AMS 6419 · AMS 6257 (all three VAR required)
204-649 °C specification band; DOUBLE TEMPERING mandatory; aerospace practice 290-316 °C
400-450 °C (Horn and Ritchie 1978) · 350-500 °C (Metals/MDPI 2021)
The embrittlement band has moved UP by about 150 °C. That makes it possible to temper at 290-316 °C and still take 1862-1931 MPa tensile: the tempering temperature is no longer inside the band.
DEFENCE METAL
Additional information
Silicon mechanism
Horn and Ritchie (1978): silicon and aluminium ‘are known to retard the replacement of epsilon-carbide by cementite to higher tempering temperatures’, which delays the mechanical destabilisation of interlath retained austenite films and pushes the embrittlement window upward. Metals (MDPI) 2021: ‘silicon delays the decomposition of retained austenite to higher temperatures and/or longer times’. Total Materia: in steels with 1-2% silicon the carbide is still present after tempering at 400 °C. Thermal Processing Magazine: silicon retards cementite precipitation and raises the critical temperature for embrittlement.
Strength difference
The AMS 6417 floor is 1862 MPa tensile / 1517 MPa yield and the AMS 6419 floor is 1931 MPa tensile / 1586 MPa yield. The AMS numbers for plain 4340 carry no such floor; the strength of 4340 is set by the tempering temperature.
Melting practice difference
All three AMS numbers for 300M carry a CONSUMABLE ELECTRODE VACUUM REMELTED (VAR) requirement. On the 4340 side both an air-melted number (AMS 6415) and a VAR number (AMS 6414) exist; that is, in 300M VAR is not an option but a requirement.
Ortak sinir
NEITHER IS STAINLESS and both require corrosion protection. In both, the service temperature must stay below the tempering temperature. In both, as-quenched material is not used untempered.
The 4340 composition band and AMS numbers in the table are taken from the AISI 4340 card in this same card set. The two peer-reviewed sources differ on the limits of the TME band; both are written separately in the table, EACH NAMED WITH ITS SOURCE, and no average was taken.
The strength and toughness numbers in the table below come from a single producer’s own comparison study, which means they are mutually consistent and usable for ranking. Note that the 300M KIc in this table (50 ksi√in) conflicts with the same producer’s product datasheet value (60–70 ksi√in).
Strength and Fracture Toughness · Producer Comparison Data
DEFENCE METAL
AerMet 100 (482 °C age)
Tensile 287 ksi · KIc120 ksi√in
AerMet 310
Tensile 315 ksi · KIc65 ksi√in
AerMet 340
Tensile 352 ksi · KIc31.5 ksi√in
Maraging 250 class
Tensile 258.6 ksi · KIc91.5 ksi√in
Maraging 300 class
Tensile 291.0 ksi · KIc67.7 ksi√in
Maraging 350
Tensile 343.6 ksi · KIc38.5 ksi√in
4340
Tensile 269 ksi · KIc70 ksi√in
300M
Tensile 287 ksi · KIc50 ksi√in
Three honest conclusions from the table. 1 · At the same tensile strength (287 ksi), the difference between AerMet 100 and 300M is toughness — 120 against 50 ksi√in. Because critical crack size scales with the square of KIc, that is roughly a sixfold larger tolerable crack. That is exactly why landing gear moved from 300M to AerMet 100. 2 · 4340 at 269 ksi appears TOUGHER than 300M at 287 ksi (70 against 50 ksi√in). 300M’s advantage is not toughness; it is the ability to reach that strength level at all. The statement “300M is better than 4340 in every respect” is false. 3 · AerMet 340 is markedly more brittle than 300M (31.5 against 50 ksi√in). A brand name is not a toughness guarantee; the strength-toughness trade runs the same way in every family.
Selection Guide · Which One, When
DEFENCE METAL
Cost dominates, 260–270 ksi is enough
4340 / 4340 VAR. Cheaper, easier to temper, easier to machine, tougher
280–300 ksi required, flaw tolerance managed by inspection
300M. This is exactly what 300M was designed for
280–300 ksi required, damage tolerance is the design criterion
AerMet 100. Same strength, roughly double the toughness, better SCC resistance. The price: cost and supply
Dimensional stability is critical (dies, precision mechanisms)
Maraging. There is no quench; distortion on ageing is far below that of conventional hardening
Corrosion resistance also required
None of them. This entire family must be coated. If corrosion resistance is genuinely 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
Above 320 ksi required
AerMet 310 / AerMet 340 / Maraging 350.Accept up front that toughness will collapse and design accordingly
Frequently Asked Questions
The customer asks for “300M, AMS 6417” but the drawing calls for 280 ksi. Can we supply?
Be careful — those two requirements are in tension. AMS 6417 and AMS 6419 are two different carbon bands of the same alloy: 6417 → C 0.38–0.43 %, 6419 → C 0.40–0.45 %. In distributor compilations AMS 6417’s typical acceptance values are 270 ksi tensile / 220 ksi yield, while AMS 6419’s are 280 ksi tensile / 230 ksi yield. What happens in practice: heats are frequently made in the 0.40–0.43 % band and satisfy both specifications simultaneously — one supplier explicitly states it stocks material at a minimum of 0.40 % C for exactly this reason. So dual-certified material exists and is common. But the responsibility is yours: write into the order acknowledgement which specification will be certified to which mechanical values. The word “300M” on its own commits to nothing. If the customer needs 280 ksi, the correct answer is AMS 6419; even where 280 ksi is achievable on an AMS 6417 certificate, the specification minimum is not 280 ksi, and in a dispute that difference works against you.
Our 300M part is worn. Can we weld-build it up and re-machine?
No — and this is the clearest answer on this page. 300M’s 0.40–0.45 % carbon means the heat-affected zone will inevitably form untempered martensite. Combine that brittle zone with hydrogen from the weld and the weld’s own residual stress and all three conditions for delayed cold cracking are satisfied. The crack can appear hours or days after welding, with no overload event at all. The answer “we will preheat and post-weld heat treat” does not work, because the only way to genuinely restore the HAZ is full re-heat-treatment: 927 °C normalize + 871 °C austenitize + oil quench + double temper at 302 °C. On a landing gear leg that means distortion, loss of dimension and complete re-inspection — in practice more expensive than making a new part. What is actually done: worn surfaces are restored by hard chromium plating or HVOF spray, then ground, then etch-inspected to AMS 2649 with nital, and then hydrogen relief baked at 190–205 °C after plating. If the damage is too deep to recover by coating, the part is scrapped. In the 300M world that is normal; weld repair is not.
Can we print “300M equivalent: 1.6928 / 35NiCrMoV12-5” on our datasheet?
You can print half of it; the other half you should not. 35NiCrMoV12-5 is definitively wrong. That steel carries Werkstoff number 1.6959 and a composition of C 0.30–0.40 % · Si 0.15–0.35 % · Cr 2.50–3.50 % · Ni 1.00–1.40 % · Mo 0.35–0.60 % · V 0.08–0.20 %. Its silicon is less than a fifth of 300M’s — meaning the mechanism that makes 300M work (retarding cementite precipitation, shifting the tempering window upward) is simply absent. Its chromium is more than triple, giving completely different hardenability and tempering behaviour. Its published quench-and-temper strength band is 1200–1700 N/mm², not 300M’s 1931–2070 MPa. 1.6928 is defensible but single-source. German supplier indexes describe 1.6928 as a “SiNiCrMo” heat-treatable steel, and at least one supplier index cross-references it directly to UNS K44220. The composition family looks right. But no EN/DIN standard backs it — there is no European material standard covering 300M. The honest wording is: “European equivalent: no EN/DIN standard covers 300M; supplier indexes cross-reference W.Nr. 1.6928. Material is ordered to AMS 6417 / AMS 6419 / AMS 6257.”
What if we skip the bake after plating? The part passed its test anyway.
Passing a test proves nothing — the definition of hydrogen embrittlement is delayed fracture. The mechanism is this: during electrolytic plating some of the atomic hydrogen liberated at the cathode surface passes through the coating and enters the steel lattice. There it is mobile, and over time it migrates to the regions of highest triaxial tensile stress — notch roots, thread roots, fillet radii, the tips of existing microcracks. When local concentration reaches a critical level the material separates at that point with no plastic deformation. The part is sound at assembly; it fractures hours, days or sometimes weeks later under steady load. The fracture surface is intergranular and does not look like fatigue. This is why the bake is a specification requirement, not good practice:190–205 °C, started within 4 hours of completing plating, for a duration set by strength and section — in the 300M class typically of the order of 23 hours. The governing documents are AMS 2759/9, with ASTM F519 used to qualify the process. The four-hour window is not arbitrary: every hour of delay gives hydrogen more time to reach the damaging locations. One further warning: F519 baseline specimens are made of 4340, and the standard itself states that parts above 260–280 ksi may not be represented by that baseline. In the 300M and AerMet class, use specimens made from the production material; otherwise “it passed F519” documents a result that does not apply to your part.
Can we use 300M in a part that runs at around 400 °C?
No. 300M is not a high-temperature steel, and the reason lies directly in the heat-treatment route. 300M draws its strength from martensite tempered at around 302 °C. Running a part at or near its tempering temperature means continuing to temper it in service: carbides coarsen, dislocation density falls, and hardness and yield strength drop permanently. A 300M part running at 400 °C progressively invalidates its own strength certificate. The practical ceiling is well below the tempering temperature, and the designer must consider peak local temperature, not continuous service temperature — landing gear parts near brakes, friction-heated surfaces, fire zones. Alternatives: for structural service around 400 °C, AerMet 100 (the producer cites use to roughly 427 °C), or for higher temperatures an entirely different family — alloy 718 or Waspaloy. What you must not do is read 300M’s room-temperature strength table and assume it holds at elevated temperature.
Common Datasheet Errors — Check Before You Order
1 · “AMS 6417 = AISI 4340” — WRONG. At least one specification index lists AMS 6417 as 4340 and gives a composition of 1.90 % Ni, 0.25 % Si — that is 4340’s chemistry. SAE’s own title reads “1.6Si – 0.82Cr – 1.8Ni – 0.40Mo – 0.08V (0.38–0.43C)”. 1.6 % silicon plus vanadium makes it unambiguously 300M. 2 · “AMS 6257 is air melted” — WRONG. One distributor page separates AMS 6257 as “air melt” from AMS 6419 as “VAR”. SAE’s own titles state that both are “Consumable Electrode Vacuum Remelted.” AMS 6257 differs not in melting but in its normalized-and-tempered supplied condition. 3 · “35NiCrMoV12-5 / 1.6959 = 300M” — WRONG. That steel has 0.15–0.35 % silicon and 2.50–3.50 % chromium. It lacks 300M’s defining element and its quench-and-temper band is 1200–1700 N/mm². It is not an equivalent. 4 · Composition tables printing “V ≤0.050 %” — MISREAD. In 300M vanadium is not an impurity ceiling but a deliberate 0.05–0.10 % addition. The same tables usually also print P ≤0.035 % and S ≤0.040 %, which are not realistic for aerospace 300M (typically ≤0.010 %) and were most likely copied from a generic 4340 table. 5 · Publishing a single KIc number.The same producer gives 60–70 ksi√in on the product datasheet and 50 ksi√in in its alloy comparison table. Publish a band (50–70 ksi√in) and state the source and tempering condition. 6 · “4 hours tempering” instead of “double temper”. AMS 6419 requires two separate 2-hour cycles with air cooling to room temperature between them. The producer datasheet says 4 hours. These are not the same thing — tempering the fresh martensite formed in between is the second cycle’s only job. 7 · Offering “300M to MIL-S-8844”.MIL-S-8844 has been cancelled and superseded by SAE AMS-6414 and SAE AMS-6257. It may still appear on legacy drawings; do not use it as a current ordering document. 8 · Listing “300M plate” or “300M pipe”. The scope of the three AMS specifications is bars, forgings, forging stock and mechanical tubing. There is no 300M specification for plate, sheet or pressure pipe. If it is sold, it is sold to a company agreement, and that belongs in the order acknowledgement. 9 · Writing “weldable”.AMS 6419 says nothing about welding and 300M is not welded in primary structure. There is no AWS filler classification. Some pages list a “welding wire” product form — that is an unclassified producer product, not an approved welding route. 10 · Density conflict. The producer gives 7.84 g/cm³; general compilations give 7.87 g/cm³. The difference is trivial, but do not publish two values on the same page. 11 · The sentence “300M is 300 ksi”. 300 ksi is a heat-treatment outcome, not a material constant. The AMS 6419 minimum is 280 ksi; 300 ksi is the typical value obtained at the low end of the tempering window. Minimum, typical and design allowable are three different things — label each one. 12 · Showing the hydrogen bake as “optional”. For every electrolytically plated 300M part the bake is a specification requirement: 190–205 °C, started within 4 hours of plating. AMS 2759/9 and ASTM F519 are the governing documents. 13 · Skipping etch inspection after grinding.AMS 2649 governs etch inspection of high-strength steel parts. Magnetic particle inspection performed AFTER plating cannot reliably find grinding cracks beneath the coating — documented landing gear fractures came from exactly that gap.
