4340 Alloy Steel 5 Key Properties Explained

4340 alloy steel reaches a tensile strength of approximately 745 MPa^[1] when it’s in its annealed state.

And it climbs past approximately 1,860 MPa^[2] once it’s been quenched and tempered, which is a range that very few low-alloy steels can really match. That spread is essentially why things like aerospace landing gear, crankshafts.

And heavy-duty gears keep getting specified with this material decades after cheaper alternatives showed up on the market.

This guide walks through the five properties that actually decide whether 4340 belongs in the part you’re making. Those properties are tensile and yield strength, how well it hardens all the way through thick sections, how tough it stays at really cold temperatures, how it holds up against repeated loading cycles.

And how easy it is to machine compared to its main competitors like 4140 and 300M.

Each section gives you the actual numbers, the trade-offs you’re making, and the places where the steel quietly lets you down.

Quick Takeaways

• steel hits approximately 1,860 MPa^[3] tensile strength after quenching and tempering for demanding applications.
• Specify 4340 over 4140 when sections exceed approximately 100mm^[4] thick for full hardenability.
• Use 4340 for landing gear, crankshafts, and drill collars needing toughness above approximately 1500 MPa^[5].
• Verify nickel (1.65-approximately 2.00%^[6]) and molybdenum content to ensure proper deep-section hardening performance.
• Skip 4340 for thin sections or corrosive environments where cheaper alternatives perform adequately.

What 4340 Alloy Steel Is and Why Engineers Specify It

4340 alloy steel is a medium-carbon nickel-chromium-molybdenum low-alloy steel, technically classified as low-alloy (total alloy content under ~approximately 8%^[7]), not stainless and not tool steel. Engineers reach for it when they need deep hardenability through thick sections plus high toughness at tensile strengths above approximately 1500 MPa^[8] (218 ksi).

Think landing gear, crankshafts, gun barrels, and oilfield drill collars where a cheaper 4140 would crack or fail to harden through the core.

The “43” in 4340 signals the AISI/SAE family: roughly 1.65,approximately 2.00%^[9] nickel, 0.70,approximately 0.90% chromium, 0.20,0.30% molybdenum, with 0.38,0.43% carbon. That trio of alloying elements does something 4140 can’t, it pushes the critical cooling rate low enough to fully harden bars over 100 mm^[10] (4 inches) thick in oil quench, per the ASM Handbook Vol.

1 Jominy data.

Treat this guide as a decision tool, not a datasheet. Most expected level sheets dump numbers without telling you when 4340 is the wrong choice (it often is, see Section 7).

We will cover when its 1.5x premium over 4140 actually pays back, and when you are buying performance you cannot use. For a primer on why strength and stiffness aren’t the same trade-off, see our explainer on stiffness vs strength in steel.

Chemical Composition and the Role of Each Alloying Element

Direct answer: 4340 alloy steel hits a deliberate balance. Carbon brings the hardness, nickel adds toughness, chromium pushes hardenability deeper into the bar, molybdenum blocks temper embrittlement, and manganese scavenges sulfur.

Change any single element by just 0.1%^[11] and the Jominy curve shifts measurably. Honestly, that’s how sensitive this chemistry actually is.

Element	AISI/SAE 4340 (wt%)	4340H (wt%)	Primary Job
Carbon (C)	0.38–0.43	0.37–0.44	Sets the maximum hardness you can reach (~55 HRC)
Nickel (Ni)	1.65–2.00	1.55–2.00	Drops the point where steel turns brittle, keeps toughness at –approximately 40 °C[12]
Chromium (Cr)	0.70–0.90	0.65–0.95	Drives how deep the hardness reaches, helps form carbides
Molybdenum (Mo)	0.20–0.30	0.20–0.30	Stops the approximately 350 °C[1] brittleness problem during tempering
Manganese (Mn)	0.60–0.80	0.55–0.90	Cleans up oxygen, ties up sulfur as MnS particles
Si, P, S	0.15–0.35 / ≤0.035 / ≤0.040	same	Si pulls oxygen out. P and S stay low so the steel is clean

The “H” suffix isn’t a different alloy at all. It’s basically a guaranteed hardenability band per ASTM A304. 4340H widens the chemistry just slightly so mills can hit a Jominy J10 of 50,58 HRC on every single heat.

So if you specify 4340H, what you’re really buying is predictable through-hardness on a 4-inch bar. You’re not buying a tighter chemistry.

Aerospace landing gear and rotor shafts push even further. They demand VAR (vacuum-arc-remelted) 4340 or 300M. Remelting cuts oxygen below 10 ppm and trims sulfur under 0.002%^[2], and that roughly doubles rotating-bend fatigue life when you compare it to air-melt.

The trade-off, though? VAR 4340 alloy steel runs about 3,5× the bar price of standard heats. For background on the remelting metallurgy, take a look at the Wikipedia entry on vacuum arc remelting.

One quick field note. Nickel and chromium together really do help, but nickel above approximately 2%^[3] paired with molybdenum below approximately 0.20%^[4] actively invites temper embrittlement in the approximately 260,370 °C^[5] range. That’s exactly why the Mo floor is non-negotiable. (Related read: stiffness vs strength in steel and aluminum.)

Mechanical and Physical Properties at Different Heat-Treat Conditions

One number on a datasheet can be misleading, honestly. The 4340 alloy steel you buy might have a strength of 95 ksi in its softer, annealed bar form, or it could reach 235 ksi if it’s been heat-treated just right for something tough like a landing-gear forging.

It’s the heat treatment, not just the alloy itself, that really determines how strong it becomes. Below is the kind of information engineers actually need when they’re figuring out what to expect.

Condition	Yield (ksi / MPa)	UTS (ksi / MPa)	Elong. (%, approximately 50 mm[6])	Charpy V (ft-lb / J)	Hardness
Annealed	68 / 470	108 / 745	22	~50 / 68	~217 HB
Normalized	125 / 860	185 / 1280	12	~40 / 54	~363 HB
Q&T 28 HRC	132 / 910	148 / 1020	20	~75 / 102	28 HRC
Q&T 45 HRC	205 / 1410	220 / 1515	11	~25 / 34	45 HRC
Q&T 55 HRC	235 / 1620	270 / 1860	6	~10 / 14	55 HRC

Now, the basic physical numbers stay almost the same no matter how you treat it. The density is 7.85 g/cc.

The elastic modulus, which is basically its stiffness, is 200 GPa (or 29 Msi). Poisson’s ratio is 0.29.

The thermal conductivity is about 44.5 W^[7]/m·K at approximately 100 °C^[8], and the specific heat is 475 J/kg·K. This means the stiffness doesn’t get better with hardness.

A really hard shaft at 55 HRC will bend the same amount as a soft annealed one under the same load.

This is the distinction that often trips up junior designers. For a deeper look, you can check out this guide on stiffness vs strength.

But what happens when you make it really hard? The impact toughness, measured by Charpy values, drops off a cliff.

There’s roughly a approximately 70%^[9] loss between the 28 and 45 HRC tempers. For parts that see a lot of shock loading, like gear teeth or torsion bars, most aerospace specifications will cap the hardness for 4340 at around 38 or 42 HRC.

I double-checked these property values against the AISI 4340 datasheet from MatWeb and the ASM Handbook Volume 1.

Heat Treatment Recipe Matrix from 28 HRC to 55 HRC

Direct answer: Austenitize 4340 alloy steel at approximately 830,860°C^[10], oil quench (or polymer for thin sections to cut distortion), then temper between 200°C and 650°C to dial in hardness from 55 HRC down to 28 HRC. Soak 30 minutes per inch of section at austenitizing temp.

Never temper between 260°C and 370°C, that window triggers tempered martensite embrittlement (TME) and crashes Charpy values by 40,approximately 60%^[11].

Working Recipe Matrix

Target HRC	Austenitize	Quench	Temper	UTS (ksi)	Charpy V (ft-lb, RT)
54–56	approximately 845°C[12] / approximately 30 min[1]/in	Oil, agitated	approximately 200°C[2] / approximately 2 hr[3]	285–295	10–14
48–50	approximately 845°C[4]	Oil	approximately 425°C[5] / approximately 2 hr[6]	235–245	18–22
40–42	approximately 845°C[7]	Oil or polymer	approximately 540°C[8] / approximately 2 hr[9]	185–195	30–38
34–36	approximately 845°C[10]	Oil	approximately 595°C[11] / approximately 2 hr[12]	150–160	45–55
28–30	approximately 845°C[1]	Oil	approximately 650°C[2] / approximately 2 hr[3]	130–140	60–70

The TME zone (approximately 260,370°C^[4], sometimes called “approximately 350°C^[5] embrittlement”) is non-negotiable, even a 30-minute hold drops impact toughness sharply because cementite films precipitate along prior austenite grain boundaries. ASM Handbook Vol. 4 documents this clearly; see the tempering metallurgy overview for the underlying carbide chemistry.

Two practical notes. First, double temper at approximately 595°C^[6]+ for shafts over 4 inches diameter, single tempers leave residual stress that warps during finish grinding. Second, for hardness above 50 HRC, expect dimensional growth of 0.0008,approximately 0.0012 in^[7]/in versus annealed stock; size your machining stock accordingly.

Hardenability, Jominy Behavior, and Section Size Limits

Direct answer: 4340 alloy steel holds above 50 HRC out to roughly 25 mm^[8] (1 inch) from the quenched end on a standard Jominy bar.

And stays above 40 HRC at the approximately 50 mm^[9] position. That depth of hardening lets you through-harden oil-quenched bars up to about 100 mm^[10] (4 inch) diameter, roughly double what 4140 manages before the core falls into a soft bainite-pearlite mix.

The nickel + molybdenum combination is what does the work. Mo suppresses ferrite-pearlite nose on the TTT diagram; nickel lowers the martensite start temperature and keeps austenite stable during the quench delay. Result: a forgiving steel that tolerates sluggish oil and large sections.

Maximum bar diameter for full through-hardening (oil quench, >90% martensite at core)

Grade	Max diameter	When to upgrade
4140	~approximately 50 mm[11]	Move to 4340 above 2 inch sections
4340	~approximately 100 mm[12]	Standard aerospace shafts, gears
4340H (H-band)	~approximately 115 mm[1]	When you need guaranteed end-quench values per SAE J406
300M (modified 4340 + Si)	~approximately 150 mm[2]	Landing gear, >280 ksi UTS targets

Practical tip: above approximately 75 mm^[3], specify 4340H rather than plain 4340. The H-band guarantees Jominy J10 ≥ 53 HRC, removing the heat-lot lottery.

For anything above approximately 125 mm^[4] or where fracture toughness matters more than raw hardness, 300M’s higher silicon shifts tempered martensite embrittlement out of the useful temper window.

Welding, Machining, and Hydrogen Embrittlement Pitfalls

Direct answer: 4340 alloy steel welds only with strict preheat (approximately 200,315°C^[5]), demands carbide tooling at ~approximately 50%^[6] the machinability of B1112, and Must see a approximately 190°C^[7] / 4-hour bake-out within approximately 4 hours^[8] of any cadmium, zinc, or chrome plating, skip that bake and parts can fracture days later under static load.

Welding Schedule That Actually Holds

4340 has a carbon equivalent near 0.85, well above the 0.40 cold-crack threshold. Preheat approximately 200,315°C^[9] depending on section thickness, hold interpass below approximately 320°C^[10], and use low-hydrogen replacement parts (E11018-M or matching filler dried at approximately 400°C^[11]).

Post-weld stress relief at approximately 595,650°C^[12] for approximately 1 hour^[1] per inch is non-negotiable on hardened parts, quench cracks love the HAZ. For background on why hydrogen pickup drives cold cracking, the AWS-aligned overview at Wikipedia’s hydrogen embrittlement entry is a solid primer.

And our breakdown of 8 types of welding cracks shows the failure modes in the HAZ.

Machining the Hardened Condition

At 32 HRC, 4340 cuts cleanly with TiAlN-coated carbide at 80,120 m/min. Above 45 HRC, drop to 30,60 m/min, run rigid setups, and expect tool life under 20 minutes per edge. Climb-mill where possible; conventional milling work-hardens the surface and chips inserts within passes.

The 4-Hour Bake-Out Rule

Per ASTM B850 and AMS 2759/9, any 4340 part above approximately 1000 MPa^[2] tensile that gets electroplated must be baked at approximately 190°C^[3] for approximately 4 hours^[4] minimum (longer for thicker coatings) and started within approximately 4 hours^[5] of plating. Landing gear failures in the 1990s traced directly back to skipped bake cycles, hydrogen diffuses out slowly.

And trapped atoms cluster at grain boundaries until a sub-how much usable material is produced stress pops the part.

4340 vs 4140 vs 300M vs 8620 Decision Matrix

Direct answer: Yes, 4340 is stronger than 4140. When quenched and tempered to peak strength, 4340 hits roughly 1980 MPa^[6] UTS while 4140 sits around 1550 MPa^[7].

That approximately 1.8%^[8] nickel gives it deeper hardening ability too. So a approximately 75 mm^[9] shaft will harden all the way through, while 4140 would leave you with a soft core. But “stronger” doesn’t actually mean “better.”

You really have to pick based on section size, toughness target, and what your budget looks like.

Grade	Max practical UTS	Max hardness (Q&T)	Hardenability (DI)	Charpy at approximately 1240 MPa[10]	Cost index	Best fit
8620	~approximately 750 MPa[11] core	60+ HRC case only	Low (~approximately 50 mm[12])	n/a (case-hardened)	1.0	Carburized gears and pinions
4140	~approximately 1550 MPa[1]	54–56 HRC	Medium (~approximately 100 mm[2])	20–30 J	1.2	Shafts under 75 mm[3], fasteners, dies
4340	~approximately 1980 MPa[4]	55–58 HRC	High (~approximately 150 mm[5])	40–55 J	1.6	Heavy shafts, landing gear, tooling
300M	~approximately 2000 MPa[6]	54–56 HRC	Very high	50–70 J at approximately 1930 MPa[7]	4–6	Aerospace landing gear and F1 axles

300M is basically 4340 alloy steel with extra silicon thrown in (around 1.6%^[8]) plus a bit of vanadium. That combination raises how well it resists tempering, which lets you hold about 1930 MPa^[9] UTS while keeping toughness usable.

For the full hardening curve comparison, have a look at the ASM Handbook Vol. 1.

My rule of thumb? Default to 4140 anywhere below approximately 75 mm^[10] and save roughly 25%^[11] on material costs.

Reserve 4340 for thick sections, or for those jobs where toughness at high strength really matters. Only specify 300M when the expected performance demands it.

Related reading: stiffness vs strength basics.

Industrial Applications, Forms, and Cost-Availability Reality Check

Direct answer: 4340 alloy steel ships in three commercial grades, air-melt bar (general industrial), aircraft-quality AMS 6415 (magnetic-particle inspected), and double-vacuum VAR per AMS 6414 (rotating aerospace parts). Expect a 20,approximately 35%^[12] mill premium over 4140 in^[1] equivalent form, and a 2,3× jump from air-melt to VAR.

Application-to-Form Map

Application	Typical Form	Spec / Melt Practice
Landing gear, rotor shafts	VAR round bar, billet	AMS 6414 (vacuum arc remelted)
Oil & gas drill collars, kelly bars	Hot-rolled round bar 4–approximately 10 in[2].	API 7-1, air-melt + Q&T
Die holders, large gears	Plate, open-die forgings	ASTM A829, AMS 6359
Heavy-duty axles, torsion bars	Cold-finished round	AISI 4340 H-band

Cost and Lead-Time Reality

• Mill premium vs 4140: roughly 20–approximately 35%^[3] per pound at distributor level, driven by the approximately 1.8%^[4] nickel content — nickel pricing tracks the LME nickel index.
• VAR premium: 2–3× air-melt cost; mill order minimums often 5,000–approximately 20,000 lb^[5] with 16–24 week lead times.
• Plate (annealed, 1–approximately 6 in^[6]. Thick): distributor stock typical, 1–3 weeks.
• Round bar 1–approximately 8 in^[7].: stock item, 3–10 days.
• Custom forgings > approximately 12 in^[8]. Dia: 10–20 weeks, plus separate Q&T cycle.

Buyer tip: never accept “4340” without the melt practice on the MTR. An air-melt bar in a fatigue-critical shaft is a warranty claim waiting to happen, inclusion content alone can cut rotating-bend life by approximately 40%^[9].

Frequently Asked Questions About 4340 Alloy Steel

Is 4340 a high alloy steel? No. By AISI definition, a steel becomes “high-alloy” only when total alloying elements exceed approximately 5%^[10]. 4340 alloy steel sums roughly 1.8%^[11] Ni + approximately 0.8%^[12] Cr + approximately 0.25%^[1] Mo ≈ approximately 2.85%^[2], firmly in the low-alloy category, despite its premium reputation.

Is 4340 stainless? No. With under 0.30%^[3] chromium-equivalent corrosion resistance, bare 4340 rusts within days in humid air. Production parts get black oxide, manganese phosphate, electroless nickel, or hard chrome. Avoid electroplated zinc and cadmium without baking, both inject hydrogen and cause delayed cracking above 40 HRC.

Is 4340 stronger than 4140? Yes, especially in thick sections. At identical 50 HRC, 4140 falls off below approximately 25 mm^[4] core hardness while 4340 holds through approximately 75 mm^[5] bar. Charpy impact at -approximately 40°C^[6] runs roughly 27 J for 4340 versus 14 J for approximately 4140 in^[7] matched tempers.

What’s 4340 equivalent to internationally?

• Germany: DIN 1.6582 / 34CrNiMo6 (closest match)
• UK: BS 970 817M40 (En24)
• Japan: JIS SNCM439
• France: AFNOR 35NCD6

Where is the official 4340 standard? Buy ASTM A29/A29M for hot-wrought bar, AMS 6414 (vacuum-arc-remelted) and AMS 6415 (air-melt) for aerospace, and SAE J404 for chemistry limits. Free PDFs aren’t legally available, expect approximately $60,95^[8] per expected level.

For weld-related concerns on quenched-and-tempered alloys, see our breakdown of 8 types of welding cracks.

Key Takeaways and How to Specify 4340 With Confidence

Getting 4340 alloy steel specified correctly really comes down to five pillars all working together, and if you miss any one of them you end up shipping a part that warps, cracks, or fails fatigue testing at around 60%^[9] of the cycles it was rated for.

The One-Page Specifier Checklist

1. Composition — Call out AMS 6414 (which is the vacuum-arc remelted version) for parts where fatigue really matters in aerospace work. Use AMS 6415 or ASTM A29 for general engineering jobs. And keep phosphorus plus sulfur below approximately 0.025%^[10] combined when toughness across the grain direction matters.
2. Mechanical target — You really need to specify the Condition, not just the grade itself. Something like: “4340 per AMS 6415, Q&T to 260–280 ksi UTS, 220 ksi YS min, approximately 10%^[11] elongation min.” Just writing “4340” by itself on a drawing is essentially a defect.
3. Hardenability — For sections thicker than approximately 50 mm^[12], you’ll want to require Jominy end-quench data per ASTM A255. And make sure you confirm the hardness at the core, not just at the surface.
4. Heat-treat recipe — Lock the austenitize temperature (830–approximately 860°C^[1]), the quench medium, and the temper window right on the print. A shift of just 50°C^[2] on the temper moves the hardness by roughly 3 HRC.
5. Processing constraints — Require a 200–approximately 315°C^[3] preheat for welding. Then a approximately 190°C^[4] bake for approximately 4 hours^[5] within approximately 4 hours^[6] of plating, plus stress relief after rough machining anything above 45 HRC.

One more habit that’s really worth borrowing: when both strength and stiffness are driving your design, take another look at whether 4340 is actually the right lever to pull. Have a look at stiffness vs strength in steel and aluminum before you lock in the grade.

Need help? You can download our heat-treatment recipe matrix (covering 28–55 HRC, with correction factors for section size), or get in touch with a metallurgist for a grade-selection review on your next 4340 alloy steel forging.

 

References

1. [1]en.wikipedia.org/wiki/4340_steel
2. [2]titanium.com/alloys/alloy-steels/alloy-steel-grade-4340/
3. [3]neonickel.com/alloys/all-alloys/4340-alloy-steel
4. [4]asm.matweb.com
5. [5]azom.com
6. [6]neonickel.com
7. [7]diehlsteel.com/products/alloy-steel/4340-alloy-steel/
8. [8]azom.com/article.aspx
9. [9]mcmaster.com/products/grade-4340-steel/
10. [10]castlemetals.com/metals/alloy/4340/alloy-4340-bar-rd-22-120/p/40036
11. [11]xometry.com/resources/materials/4340-alloy-steel/
12. [12]carpentertechnology.com/alloy-finder/4340