UNS G43400 (aircraft quality E4340 = G43406) · ASTM A29 / SAE J404 band: C 0.38-0.43% – Mn 0.60-0.80% – Si 0.15-0.35% – Cr 0.70-0.90% – Ni 1.65-2.00% – Mo 0.20-0.30% – P 0.035% max – S 0.040% max – balance Fe. In aircraft quality E4340 the phosphorus and sulfur ceilings are tightened to 0.025% (Aircraft Materials and Smiths Advanced). THE DECISIVE ELEMENT IS NICKEL: the 1.65-2.00% nickel is absent from 4140 and is the reason this grade hardens deep. THE W.Nr. / EN ASSIGNMENT IS CONTRADICTORY BETWEEN SOURCES, so NO SINGLE NUMBER IS STATED on this card: the Virgamet page gives 40NiCrMo8-4 and 1.6565 in its title, but in its body matches 1.6565 to 40NiCrMo6 and 40NiCrMo8-4 to 1.6562, and names 40NiCrMo7 (UNI) / 40NCD7 (AFNOR) as the closest equivalent to AISI 4340; Otai gives 1.6511 / 36CrNiMo4 and 34CrNiMo6 / 1.6582; Flame Hardening gives 1.6565 together with ‘~EN24’. These carry different chemical bands. AN ORDER MUST BE PLACED AGAINST THE UNS G43400 / ASTM A29 CHEMISTRY AND AN AMS NUMBER, NOT AGAINST A W.Nr. THIS IS A MARTENSITIC QUENCH-AND-TEMPER STEEL: it hardens by austenitising, oil quenching and tempering. IT IS NOT STAINLESS. It does NOT precipitation harden; there is NO H900 / H1025 type ageing step.
Bought for parts that must be hard and tough right through to the centre despite a heavy section: aircraft landing gear components, drive and transmission shafts, heavy-section crankshafts and gears, high-strength fasteners, and ASTM A320 Grade L43 low-temperature bolting.
Forms
Round bar, flat bar, plate, sheet, pipe/tube, forging. All forms are supplied to order; the 4340 VAR product page states that material is supplied in the requested size and form, with certification.
Standards
AMS (verified, plain 4340 chemistry 0.80Cr – 1.8Ni – 0.25Mo – C 0.38-0.43%): 6415 (BARS, FORGINGS, MECHANICAL TUBING, FORGING STOCK; air melted; the baseline number) · 6414 (same forms; VACUUM CONSUMABLE ELECTRODE REMELTED / VAR) · 6409 (same forms; SPECIAL AIRCRAFT QUALITY CLEANLINESS; NORMALIZED AND TEMPERED) · 6484 (same forms; NORMALIZED AND TEMPERED) · 6359 (SHEET, STRIP, PLATE; aircraft quality) · 6454 (SHEET, STRIP, PLATE; VAR) · 6456 (WELDING WIRE – note: its carbon is 0.35-0.40% and its title says SAE 4340Mod; it is not plain 4340). Legacy military specification: AMS-S-5000 / MIL-S-5000. ASTM: A29 / A29M (general requirements) · A304 (alloy steel bars with hardenability requirements) · A322 (alloy steel bars, standard grades) · A331 (cold-finished bars) · A506 (hot-rolled sheet and strip) · A519 (seamless mechanical tubing) · A646 (premium quality blooms and billets for aerospace forgings; class 4340-7) · A711 (stock for forgings) · A752 (wire rod) · A829 (alloy structural steel plate) · A320 Grade L43 (low-temperature bolting). EN / ISO: the W.Nr. assignment is contradictory; see the identity field. THE AMS NUMBERS ARE NOT INTERCHANGEABLE. Six numbers carry the same chemical band but differ in MELTING ROUTE, CLEANLINESS LEVEL and DELIVERY CONDITION: 6415 is the air-melted baseline; 6414 imposes VACUUM ARC REMELTING (VAR);
Advantage
Its single most important practical advantage is that it holds the same tensile band in a far heavier section, and the numbers for that are read from the same condition-letter table of the same standard (AS 1444-1996): at a 250 mm section 4340 is given condition T at 850-1000 MPa tensile with a 570…
Welding
IT IS WELDABLE BUT HARDER TO WELD THAN 4140, because its hardenability is higher: the heat affected zone turns to martensite over a wider range of cooling rates.
Limits
1) IT IS NOT STAINLESS. Its chromium is 0.70-0.90% and no passive layer forms; corrosion protection is mandatory. Assuming it is stainless because its nickel is high is a common mistake with this grade.
2) HIGH STRENGTH MEANS HIGH HYDROGEN RISK.
Compiled from manufacturer data sheets · confirm against the current specification before ordering
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Standards by Product FormWelding, Heat Treatment and MachiningTempering Temperature and Hardness · 4140, 4340 and 300MFrequently Asked Questions
AISI 4340 is a high strength engineering steel, alloyed with the nickel-chromium-molybdenum trio, that hardens through the depth of the section. It is the most widely used grade of the alloy steel group in the aerospace and defence industries; its UNS designation is G43406. It is also known as E4340 and simply 4340.
The 1.65-2.00% nickel in its composition provides toughness, the 0.70-0.90% chromium provides hardenability and wear resistance, and the 0.20-0.30% molybdenum provides strength at elevated temperature and resistance to temper embrittlement. This combination gives the material high fatigue strength, which is why it is chosen for critical parts working under repeated load.
Its vacuum melted quality is defined under AMS 6414 and is used in aerospace applications where cleanliness is critical. AMS 6415 covers bar and forged products, while AMS 6484 covers normalised and tempered bar. Heat treatment sequence: normalise at 900 °C for 1 hour, austenitise at 815 °C for 1 hour, oil quench, then double temper at 245 °C for 2 hours each.
It is widely used in parts working under high stress, in landing gear components, shafts, fasteners and structural parts. It can be supplied as round, hexagonal and flat bar, plate, forging stock and welding wire.
Chemical Composition · AISI 4340
DEFENCE METAL
C — Carbon
0.38-0.43%
Mn — Manganese
0.65-0.85%
Si — Silicon
0.15-0.30%
Cr — Chromium
0.70-0.90%
Ni — Nickel
1.65-2.00%
Mo — Molybdenum
0.20-0.30%
P + S
max 0.025%
Fe — Iron
Balance
Mechanical Properties · AISI 4340
DEFENCE METAL
Annealed condition
max 235 HB
Hardened + tempered
Rm 1793 MPa (260 ksi)
Akma Rp0.2
1496 MPa (217 ksi)
Elongation (4D)
10%
Reduction of area
30%
Standards and Equivalents · AISI 4340
DEFENCE METAL
Trade name
AISI 4340
UNS
G43406
AMS
6414 · 6415 · 6484
Available forms
Round bar · Flat bar · Plate · Sheet · Tube · Forgings All forms are supplied to order.
Standards by Product Form
STANDARDS BY PRODUCT FORM
DEFENCE METAL
Product form
Standards
Round bar · flat bar · forgings · rings · mechanical tubing · forging stock
AMS 6415 / 6414 / 6409 / 6484 (these cover forging stock) · ASTM A711 (stock for forgings) · ASTM A646 / A646M (premium quality blooms and billets for aerospace forgings; class 4340-7)
Wire rod
ASTM A752 (alloy steel wire rod). No AMS WIRE number other than welding wire, verified across four sources, was found for 4340.
Welding filler metal
AMS 6456 – ‘Steel, Welding Wire 0.80Cr 1.8Ni 0.25Mo (0.35-0.40C) (SAE 4340Mod) Vacuum Melted’. CAUTION: its carbon is 0.35-0.40% and its title says 4340Mod; it is not written into a base-metal order.
Low-temperature bolting
ASTM A320 Grade L43 – base alloy 4340; minimum tempering temperature 593 °C (1100 °F); Charpy requirement at -101 °C (-150 °F). The 4140-based class is L7, NOT L43.
Sour service
NACE MR0175 / ISO 15156-2 – carbon and low alloy steels are acceptable without further testing when hardness does not exceed 22 HRC. The working hardness at which 4340 is normally used is above that ceiling; for sour service the grade choice must be made at the start.
Post-plating hydrogen embrittlement relief
SAE AMS 2759/9 (Hydrogen Embrittlement Relief / Baking of Steel Parts) – typically 190-218 °C for 2 to 24 hours, started within 1 to 4 hours of exposure.
Welding procedure group
NO ASME SECTION IX P-NUMBER IS STATED: 4340 is not listed as an ASME BPVC base metal and no P-Number could be verified across four independent sources.
In every row the AMS numbers come FIRST and ASTM follows. Where one product form has more than one AMS number, those are NOT alternatives: 6415 is air melted, 6414 is VAR, 6409 is special cleanliness plus normalized and tempered, and 6484 is normalized and tempered. The last row is there on purpose: it gathers in one place the numbers that circulate under the 4340 name but belong to other alloys. For wire (other than welding wire) and for pipe, no AMS number specific to 4340 and verified across four sources was found.
Two points stand out when ordering AISI 4340. First, the same material is sold in two cleanliness classes — air melted and vacuum arc remelted (VAR): AMS 6415 is air melted, while AMS 6414 is VAR premium quality, significantly cleaner with markedly better transverse toughness and fatigue life. Second, AMS 6484 and AMS 6409 are not a different alloy but the same material in the normalized and tempered delivery condition.
Important caution: 4340 is frequently quoted as equivalent to 34CrNiMo6 (1.6582). This is an approximate commercial substitute, not a chemical equivalent. 34CrNiMo6 has lower carbon, roughly twice the chromium and less nickel. For general engineering parts in the 850–1000 MPa class this rarely causes problems, but it is not acceptable on an aerospace or API order calling for an AMS or ASTM certificate. Where a true chemical match is required, specify 40NiCrMo7 or 1.6562 / 40NiCrMo8-4.
Welding, Heat Treatment and Machining
HEAT TREATMENT — SCHEMATIC
DEFENCE METAL
1 · HOT WORKING / FORGING
Step
1 · HOT WORKING / FORGING
Summary
Not a heat treatment but a precondition. The forging finish temperature and the cooling that follows decide whether a normalize is needed.
Temperature
The sources diverge at the upper limit. Flame Hardening 850-1050 °C · Otai and Compraco (theworldmaterial) 1065-1230 °C (1950-2250 °F) · AZoM gives 427-1233 °C (1800-2250 °F); the 427 °C lower bound is a unit or transcription error and HAS NOT BEEN USED.
Time
Until the whole section is at temperature. No numerical time was confirmed across four independent sources, so none is stated.
Cooling
Flame Hardening: slow FURNACE cooling after forging.
Resulting hardness
As-forged hardness depends on section and cooling rate; no binding hardness is stated.
DEFENCE METAL
2 · NORMALIZING
Step
2 · NORMALIZING
Summary
Homogenises the structure after forging or rolling. Recommended before hardening.
Temperature
840-930 °C. Carpenter 871-927 °C (1600-1700 °F) · Flame Hardening 850-880 °C · Virgamet 840-870 °C · Compraco 845-900 °C. The bands diverge at the upper end; NO AVERAGE HAS BEEN TAKEN.
Time
No numerical time was confirmed across four independent sources, so none is stated.
Cooling
AIR cool (Carpenter, Flame Hardening, Virgamet and Compraco all agree).
Resulting hardness
Compraco gives 363 HB, 1282 MPa tensile and 862 MPa yield for a 870 °C normalize. That figure derives from the ASM database and could not be confirmed across four independent sources, so it HAS NOT been put into the hardness and strength table.
DEFENCE METAL
3 · ANNEALING
Step
3 · ANNEALING
Summary
Softening for machinability. The sources give the subcritical and the supercritical anneal separately.
Temperature
SUBCRITICAL (softening): Flame Hardening 650-700 °C · Virgamet 660-700 °C. SUPERCRITICAL (full anneal): AZoM 844 °C (1550 °F) · Otai 844 °C · Vulcan and Flame Hardening 800-850 °C · Compraco 830-860 °C. The two anneals are NOT the same thing and the order must state which one is required.
Time
Until the whole section is at temperature. No numerical time was confirmed across four independent sources, so none is stated.
In the annealed condition, 235 HB maximum (Aircraft Materials and Rickard). Rickard also gives about 860 MPa (125 ksi) tensile for the annealed condition; that figure could not be confirmed across four independent sources and has not been put into the table.
DEFENCE METAL
4 · AUSTENITISING + QUENCH (hardening)
Step
4 · AUSTENITISING + QUENCH (hardening)
Summary
This is the step that produces the hardness. The nickel in 4340 makes martensite formation possible even in heavy sections.
Temperature
800-875 °C. Carpenter 802-857 °C (1475-1575 °F) · Flame Hardening 830-860 °C · Virgamet 810-840 °C · AZoM and Otai 830 °C (1525 °F) · Compraco 800-845 °C. DIVERGING SOURCE: Industeel gives 875 °C and reports AC1 at 705 °C and AC3 at 760 °C. NO AVERAGE HAS BEEN TAKEN.
Time
Compraco gives a minimum of 15 minutes per 25 mm of thickness. No single numerical time was confirmed across four independent sources, so no binding time is stated.
Cooling
OIL. Carpenter, Flame Hardening, Virgamet, AZoM, Otai and Compraco all specify oil. Compraco says the oil should be kept below 65 °C, or a salt bath at 200-210 °C used. Industeel says oil or water depending on thickness and shape.
Resulting hardness
No as-quenched, untempered hardness could be confirmed across four independent sources, so none is stated. In this condition the material is brittle and IS NOT USED WITHOUT TEMPERING.
DEFENCE METAL
5 · TEMPERING
Step
5 · TEMPERING
Summary
MANDATORY after quenching. The temperature is chosen for the target strength, and the 250-450 °C band is not used.
Temperature
Carpenter 204-649 °C (400-1200 °F) · Compraco 200-650 °C (400-1200 °F) · Flame Hardening 540-680 °C · Virgamet 550-650 °C. See the FORBIDDEN BAND box: Industeel states that 250-450 °C is to be avoided.
Time
Flame Hardening and Vulcan: soak at heat for AT LEAST 1 hour. Industeel gives 600 °C for a maximum of 2 hours for stress relief.
Cooling
Air cool (Flame Hardening). Total Materia gives accelerated cooling from above 600 °C as the countermeasure against reversible temper embrittlement.
Resulting hardness
See the tempering table.
DEFENCE METAL
Tempering table
Note
The table shows the relationship between tempering temperature and strength. EVERY ROW IS GIVEN WITH ITS SOURCE, and the rows come from different specimen and melting conditions: the Carpenter rows are for VACUUM ARC REMELTED (VAC-ARC) Escalloy 4340, the Compraco rows are ASM-database typical values. The two sources give different numbers at the same temperature; NO AVERAGE HAS BEEN TAKEN. This table is NOT an ordering specification; an order must be tied to an AS 1444 condition letter or to a specification such as ASTM A320 L43.
DEFENCE METAL
FORBIDDEN TEMPERING BAND – 250-450 °C
Step
FORBIDDEN TEMPERING BAND – 250-450 °C
What happens
Impact toughness drops. Two mechanisms cover the region: tempered martensite embrittlement (TME, irreversible) and temper embrittlement (TE, reversible).
As named in the source
The Industeel 4340 data sheet states it directly: ‘avoid temperature between 250-450 °C’. Herring (The Heat Treat Doctor): TME 250-400 °C (480-750 °F), the mechanism being cementite precipitation on prior-austenite and interlath boundaries together with impurity segregation, IRREVERSIBLE; TE 375-575 °C (705-1070 °F), the mechanism being segregation of P, Sn, As and Sb to grain boundaries, REVERSIBLE by re-tempering above 575 °C with rapid cooling. Thermal Processing Magazine: TE 375-575 °C, most prominent in low-alloy Cr-Ni grades. Total Materia: irreversible 250-400 °C, reversible 450-650 °C; countered by accelerated cooling from above 600 °C and by 0.2-0.3% molybdenum.
Neden 4340 icin daha kritik
Herring writes that the highest embrittlement effect is observed in Cr-Ni and Cr-Mo steels and that chromium, nickel and manganese increase it. 4340 contains both chromium and nickel, so the same band is a more serious constraint for it than for nickel-free 4140. Molybdenum (0.20-0.30%) reduces the effect but does not remove it.
Peer-reviewed source note
The 4140 and 4340 tempering study published in Materials Testing in 2022 compares tempering at 300, 450, 550 and 650 °C and reports that the impact value rose with tempering temperature; that is, NO separate collapse in toughness inside the 250-450 °C band WAS REPORTED in that study. It is recorded here because it is a contrary finding.
Servis uyarisi
This band is not only a HEAT TREATMENT prohibition: long SERVICE inside 250-450 °C 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 tempering. It does NOT precipitation harden; there is NO ageing step of the H900 / H1025 / H1150 type. The five steps below were each verified separately. 4340 DOES NOT PRECIPITATION HARDEN. There is NO ageing step of the ‘H900 / H1025’ kind for this material; those steps belong to precipitation hardening stainless steels. A subcritical anneal (650-700 °C) and a full anneal (800-860 °C) are NOT the same thing. The order must state which one is required. The quench medium is oil. Industeel also lists water depending on thickness, but because of the high hardenability of 4340 a water quench markedly raises the risk of cracking and distortion. Quenched but untempered 4340 is NOT used. Tempering is the mandatory final step of the cycle. The Carpenter rows in the tempering table are for vacuum arc remelted material; they cannot be applied directly to air melted material. The melting route is not part of the heat treatment but it is part of the order: AMS 6414 and AMS 6454 impose a VAR requirement. A contrary peer-reviewed finding on the 250-450 °C band is also recorded; the card does not hide it.
Welding
GTAW, GMAW, SMAW, electron beam, laser and resistance welding are all applicable, but because of its high hardenability 4340 is never welded without preheat and post-weld heat treatment. Preheat and interpass temperature lie between 205 and 315 °C. Weld in the annealed or normalized and tempered condition; welding a quenched-and-tempered part leaves untempered martensite in the heat-affected zone. After welding, either a full heat treatment (austenitise, quench, temper) or at minimum a stress relief below the part’s tempering temperature is mandatory. Low-hydrogen practice is essential — 4340 is highly susceptible to hydrogen-induced cold cracking. The matching aerospace filler is AMS 6456 wire.
Heat treatment
Hardening is by martensitic transformation followed by tempering; nickel, chromium and molybdenum raise hardenability while carbon provides the hardness. Normalizing is carried out at 870–925 °C, air cooled. For hardening, austenitise between 802 and 857 °C (practical target 830–860 °C) and quench in oil — do not water quench, the cracking risk is high. Temper between 204 and 649 °C according to the strength required, for at least one hour per 25 mm of section. The defining characteristic of 4340 is its high hardenability: it will through-harden sections up to about 100 mm.
Machining
Machinability rating is roughly 50–57 (B1112 = 100). The best condition for machining is annealed or normalized and tempered (≤255 HB). It can be machined between 302 and 352 HB but tool life falls markedly; above 45 HRC only hard turning with CBN or grinding is practical. Use thick CVD-coated carbide inserts with a positive rake and a sharp edge. Work hardening is moderate, but a blunt tool leaves a hardened layer on the surface — keep a steady feed and never let the tool dwell. Heat build-up in the cutting zone can cause local tempering or re-hardening (white layer); in grinding the risk of burn and grinding cracks is high, so use a light depth of cut and plenty of coolant.
Tempering Temperature and Hardness · 4140, 4340 and 300M
STRENGTH VALUES
DEFENCE METAL
Condition
Hardness
Yield MPa
Tensile MPa
Elongation
Condition T
248-302 HB
635 min
850-1000
13% min
Condition U
269-331 HB
740 min
930-1080
12% min
Condition V
293-352 HB
835 min
1000-1150
12% min
Condition W
311-375 HB
925 min
1080-1230
11% min
Condition X
341-401 HB
1005 min
1150-1300
10% min
Annealed delivery condition
235 HB max
–
–
–
ASTM A320 Grade L43 · low-temperature bolting, up to 100 mm (4 in) diameter
321 HBW / 35 HRC max
725 min
860 min
16% min
The akma_mpa and cekme_mpa columns carry ONLY the MPa value; the ksi equivalents are in the akma_ksi and cekme_ksi columns. The AS 1444 rows define the CONDITION LETTER together with the SECTION: a given condition letter is guaranteed only up to the section stated. The ASTM A320 L43 row is a BOLTING specification and includes a hardness CEILING and a low-temperature impact requirement. For typical values against tempering temperature see the tempering table in the heat treatment diagram; that table is a producer measurement, not a specification. In the chart, a value given as a range is drawn at its lower bound; the full range is in the table. The AS 1444 rows and the ASTM A320 row CANNOT be compared with each other: one is a structural quench-and-temper table, the other a bolting specification. An AS 1444 condition letter on its own is not a strength; the condition letter and the section must be written together. In ASTM A320 L43 the minimum tempering temperature is 593 °C. That is well above the 250-450 °C forbidden band, so the specification effectively rules the band out. Tensile and yield figures for the annealed condition have NOT been put in the table: the values found are copies deriving from the same ASM database, and the four-independent-source requirement is not met. The mechanical minima of the AMS numbers themselves are not in the table; the AMS texts are paywalled and could not be verified across four independent sources.
Typical tempering temperature versus hardness for an oil-quenched thin section. Sources vary by about ±3 HRC and values fall as section thickness increases:
Tempering Temperature · Typical Hardness (oil quenched, thin section)
DEFENCE METAL
204 °C
~52 HRC — tensile ≈ 1900–1930 MPa
260 °C
~49 HRC — tensile ≈ 1830 MPa
316–371 °C
~45–47 HRC — avoid tempering in this range (embrittlement band)
427 °C
~41 HRC
482 °C
~38 HRC
538 °C
~35 HRC
593 °C
~30 HRC — tensile ≈ 1140 MPa
649 °C
~25 HRC
There are two distinct embrittlement traps: tempering between roughly 260 and 370 °C causes tempered martensite embrittlement (rule of thumb: temper either at 204–260 °C or at 385 °C and above); and slow cooling through the 375–575 °C band causes classic temper embrittlement — take the part out of the tempering furnace and cool in air or faster.
The high-silicon, vanadium-bearing vacuum-melted version of 4340; markedly better toughness at 1860–1930 MPa
Interchangeable?
4340 must never be substituted for 300M — at that strength level 4340 is brittle. Not without engineering approval
Frequently Asked Questions
What is the real difference between 4340 and 4140?
The carbon content is the same in both (0.38–0.43%). The difference is in the alloying: 4140 contains no nickel, 4340 contains 1.65–2.00%. Nickel raises hardenability and toughness rather than hardness. If you need a given hardness in a thin section, 4140 is sufficient and cheaper. If you need through-hardening in a heavy section, or higher impact toughness at the same hardness, choose 4340 — it holds roughly 2–5 HRC more at the same tempering temperature. Using 4340 for a thin-section part is money wasted.
What is the difference between AMS 6414 and AMS 6415?
Both cover the same chemistry and the same product forms (bar, forgings, tubing). AMS 6415 is air melted; AMS 6414 is vacuum arc remelted (VAR) premium quality — far cleaner, with phosphorus and sulphur ≤ 0.010% and markedly better transverse toughness and fatigue life. Flight-critical parts call for 6414; 6415 is adequate for general high-strength engineering parts.
I am going to plate a high-strength 4340 part — anything to watch out for?
Yes. Above roughly 1200 MPa / 40 HRC, 4340 is highly susceptible to hydrogen embrittlement. A stress-relief bake after electroplating or acid cleaning is mandatory, with the cycle determined in accordance with ASTM B850. Note also that 4340 is not stainless — it has no corrosion resistance and must be protected by paint or plating, and in the high-strength condition the risk of stress-corrosion cracking in chloride environments is high.
COMPARISON
FOUR SEPARATE CRITERIA, EACH READ FROM ONE SOURCE FAMILY. (1) CHEMISTRY: the band for all three grades is taken from ASTM A29 / SAE J404, so nickel, chromium and molybdenum are compared under the same specification logic. (2) DEPTH OF HARDENABILITY: for 4340 and 4140 the condition letter – section – tensile band tables that THE SAME PUBLISHER (Flame Hardening Services) derives from THE SAME STANDARD (AS 1444-1996) in THE SAME FORMAT are used. The criterion is: ‘up to what section is the same tensile band held’. Because that mapping holds the section and the test rule constant, it is the only comparable number. (3) LOW-TEMPERATURE TOUGHNESS: the class assignment within the same ASTM A320 family (L7 = 4140, L43 = 4340). (4) MELTING AND CLEANLINESS: the numbers verified from SAE AMS title records. Hardness or tensile figures collected from different sources have NOT been placed side by side.
DEFENCE METAL
Grade
UNS
W.-Nr.
Carbon
Nickel
Chromium
Molybdenum
Sertlesebilirlik derinligi
Toughness
Ams ergitme
Typical use
AISI 4340
G43400 (aircraft quality E4340 = G43406)
The W.Nr. assignment is CONTRADICTORY between sources (1.6565 / 40NiCrMo8-4, 1.6511 / 36CrNiMo4 and 1.6582 / 34CrNiMo6 are given by different sources); no single number is stated on the card
0.38-0.43%
1.65-2.00%
0.70-0.90%
0.20-0.30%
In the AS 1444 condition table, condition T holds at a 250 mm section: 850-1000 MPa tensile, 635 MPa minimum yield, 248-302 HB. The top end is condition X: 1150-1300 MPa at a 30 mm section.
Its low-temperature toughness is the highest in this group: ASTM A320 Grade L43 sets a 27 J average Charpy requirement at -101 °C on a 4340 base.
THERE ARE VAR-REQUIRED AMS NUMBERS: 6414 (bars, forgings, tubing, forging stock) and 6454 (sheet, strip, plate). AMS 6409 additionally carries a special aircraft quality cleanliness requirement.
Aircraft landing gear, drive and transmission shafts, heavy-section crankshafts and gears, high-strength fasteners, ASTM A320 L43 bolting.
AISI 4140
G41400 (aircraft quality E4140 = G41406)
1.7225 / 42CrMo4 (close equivalent; the ASTM and EN bands do not fully overlap on chromium and manganese)
0.38-0.43%
NOT SPECIFIED (residual element)
0.80-1.10%
0.15-0.25%
In the AS 1444 condition table only conditions R and S hold at a 250 mm section: 700-930 MPa tensile. The top end is condition W: 1080-1230 MPa at a 20-30 mm section. The same trend appears on the EN 10083-3 +QT side: 900 MPa minimum yield at 16 mm, 500 MPa minimum yield at 250 mm.
Izod 27-54 J in the AS 1444 table; KV 35 J minimum in EN 10083-3 +QT. Its low-temperature bolting class is ASTM A320 L7.
THERE IS NO VAR-REQUIRED AMS NUMBER. Its highest cleanliness requirement is the ‘special aircraft-quality cleanliness’ wording that AMS 6529 places on normalized bar.
Shafts and spindles, gears, hydraulic cylinder rods, die holders, ASTM A193 B7 studs and A320 L7 bolting, drilling components.
AISI 8740
G87400
No W.Nr. equivalent verified across four independent sources was found
0.38-0.43%
0.40-0.70%
0.40-0.60%
0.20-0.30%
Its nickel and chromium sit between 4140 and 4340. No AS 1444 or equivalent section-versus-strength table could be verified across four independent sources, so NO NUMBER IS GIVEN IN THIS ROW.
Izod 41 J in the annealed condition (AZoM, ASM-database derived). No four-source figure was found for the quenched-and-tempered condition.
6322 (bars, forgings, rings) · 6323 (mechanical tubing) · 6325 and 6327 (bars, forgings) · 6358 (sheet, strip, plate) · 7452 (bolts and screws) · 7456 (studs) · 7496 (flash welded rings). No VAR-required number in this list was verified.
The one decisive chemical difference is NICKEL. In 4140 nickel is NOT specified; in 8740 it is 0.40-0.70%; in 4340 it is 1.65-2.00%. Carbon is 0.38-0.43% in all three, so THE MAXIMUM ATTAINABLE HARDNESS IS SIMILAR IN ALL THREE. Nickel forms no carbide; it strengthens the ferrite in solid solution and lowers the transformation temperature. The practical consequences are two: deeper hardening at the same carbon, and higher low-temperature toughness. THE DIFFERENCE IS NOT IN THE HARDNESS BUT IN HOW DEEP INTO THE SECTION THAT HARDNESS REACHES.
Derinlik sayisi
Read from the same table, the number is this: at a 250 mm section 4340 is guaranteed 850-1000 MPa tensile (condition T), while at the same section 4140 gets 700-930 MPa (conditions R and S). At a 30 mm section 4340 reaches 1150-1300 MPa (condition X) while 4140 stops at 1080-1230 MPa (condition W).
Ergitme farki
The second difference that decides an order is the melting route: 4340 can be supplied against VAR-required AMS numbers (6414 and 6454); 4140 HAS NO VAR-required AMS number, and none was verified in the 8740 list either. A specification that calls for remelted material can be met only with 4340.
Ortak sinir
ALL THREE ARE QUENCH-AND-TEMPER STEELS AND NONE OF THEM IS STAINLESS. Corrosion protection, the AMS 2759/9 hydrogen relief bake after plating and the 22 HRC limit of NACE MR0175 / ISO 15156-2 apply to all three alike. The 250-450 °C tempering band is also a problem for all three, and according to Herring it is most pronounced in the Cr-Ni-bearing 4340.
Temper farki
On temper embrittlement the ranking reverses: Herring writes that the highest embrittlement effect is observed in Cr-Ni and Cr-Mo steels and that chromium and nickel increase it. The nickel in 4340 is therefore a gain on the hardenability side and a constraint on the tempering band side.
The hardenability comparison is read from tables in which THE SAME PUBLISHER applies THE SAME STANDARD in the same format; that is safer than collecting numbers from different sources. One publisher (Vulcan) gives the SAME condition-versus-section table for 4140 and 4340 and does not separate the two grades; that contradiction is recorded, and the Flame Hardening tables were used for the comparison. NO NUMBER IS GIVEN for hardenability in the 8740 row; no section-versus-strength table could be found across four independent sources. The consequence of nickel is not ‘harder’ but ‘hard deeper in’; all three grades have carbon in the same band. On the temper embrittlement side nickel is a DISADVANTAGE; the comparison shows that reverse direction too.
