Guide
4140 vs 4340: which alloy steel should a machined shaft or pin use?
4140 vs 4340 is the choice between a chromium-molybdenum steel and a nickel-chromium-molybdenum steel with the same carbon level, and for a machined shaft or pin it comes down to section size, the hardness you need, and how much toughness you are willing to pay for. 4140 is the default for most medium-load shafts, pins and spindles; 4340 earns its place when the section is thick, the hardness is high, or the part must not fail by fracture.
By Saola Manufacturing engineering team · Published · Updated
This guide is written for the engineer who has a high-load shaft on the screen and needs to decide whether the nickel in 4340 is worth it. It covers what the chemistry changes, how hardenability and toughness differ, which heat treat condition to put on the drawing, the mistakes that show up after machining, and what an RFQ should say so that a supplier buys the right bar and treats it the right way. Grade chemistry comes from the ASTM bar specifications; property statements are general engineering guidance and should be checked against the mill certificate and your own design data.
On this page
What is the chemical difference between 4140 and 4340?
Both grades carry the same carbon range, so they reach a similar peak hardness; the difference is that 4340 adds nickel and a little more molybdenum (with slightly less manganese and chromium), which push hardenability and toughness up. 4140 relies on chromium and molybdenum alone.
Carbon sets how hard martensite can get, and because the carbon window is identical, a fully hardened 4140 section and a fully hardened 4340 section end up close in surface hardness. What the extra alloy buys is the ability to form martensite deeper into a thick section, and a microstructure that resists cracking better at the same strength. The table lists the heat-analysis ranges for the two grades as standard alloy bar.
| Element | 4140, wt % | 4340, wt % | What it does in a shaft |
|---|---|---|---|
| Carbon | 0.38–0.43 | 0.38–0.43 | Sets the maximum as-quenched hardness |
| Manganese | 0.75–1.00 | 0.60–0.80 | Adds hardenability and ties up sulfur |
| Silicon | 0.15–0.35 | 0.15–0.35 | Deoxidizer, small strengthening effect |
| Chromium | 0.80–1.10 | 0.70–0.90 | Hardenability, temper resistance, forms hard nitrides |
| Molybdenum | 0.15–0.25 | 0.20–0.30 | Hardenability, reduces temper embrittlement |
| Nickel | not specified | 1.65–2.00 | Toughness, lower impact transition, deeper hardening |
| Phosphorus / sulfur | 0.035 / 0.040 max | 0.035 / 0.040 max | Kept low for toughness and fatigue |
Outside North America the same steels travel under other names, and an overseas supplier will often buy bar to a European, British or Japanese standard. The close equivalents of 4140 are 42CrMo4 (1.7225) in EN 10083-3, 709M40 in BS 970 (the old “EN19”) and SCM440 in JIS G4053. For 4340 the usual matches are 817M40 in BS 970 (the old “EN24”) and SNCM439 in JIS G4053, while the nearest EN 10083-3 grade, 34CrNiMo6, runs lower carbon and higher chromium than 4340. Close is not identical: if you will accept an equivalent, name it on the drawing; if you will not, write “no substitution without written approval”.
How much deeper does 4340 harden than 4140?
Considerably deeper: in the same quench, 4340 keeps a martensitic structure much further below the surface, so a thick 4340 shaft reaches near-uniform properties at its core where a 4140 shaft of the same diameter would be noticeably softer in the middle.
Hardenability is measured with the Jominy end-quench test in ASTM A255, which quenches one end of a standard bar and records hardness at steps along its length. A 4340 bar holds its hardness much further from the quenched end than 4140, and that distance translates into the section size that hardens through. The diameter at which each grade stops hardening through is not a fixed number: quench severity, agitation and the exact heat chemistry move it, so compare the end-quench curve for the actual heat (or the H-band below) with your section size rather than relying on a rule of thumb.
Why it matters for a shaft: bending and torsional stresses peak at the surface, so a shaft whose core is softer is not automatically weaker. The problem is the transition zone below the surface, at shoulders, keyways and cross holes, where a stress concentration meets steel that never fully hardened. If the drawing calls for a minimum hardness at mid-radius or at the core, or if the shaft is large, that requirement is often what forces 4340. If the shaft is small and loaded mainly at the surface, 4140 usually does the job.
When hardenability is critical, both grades are available as H-steels (4140H and 4340H) under ASTM A304, which are bought to a guaranteed Jominy band rather than a chemistry range alone. Naming the H-grade on the drawing narrows the heat-to-heat scatter that otherwise shows up as hardness variation between lots.
Is 4340 tougher than 4140 at the same hardness?
Yes, and the gap widens as hardness goes up: at moderate pre-hardened levels the two behave similarly, but at high hardness 4340 keeps markedly better impact toughness and resistance to crack growth. Nickel also lowers the temperature at which the steel turns brittle, which matters for parts that see cold service.
Impact toughness is usually reported as Charpy V-notch energy per ASTM E23, and if your application depends on it, the drawing or purchase order must say so, with the test temperature, because neither the grade name nor a hardness range guarantees an impact value. A shaft that sees shock loading, a reversing drive, a press pin or a crane component is where toughness becomes the deciding property rather than strength.
Both grades share one trap. Tempering in a band of roughly 260–370 °C (500–700 °F) is, as typical guidance, avoided because it causes tempered martensite embrittlement, a measurable drop in impact energy even though hardness looks normal. Depending on the grade and the quench, a hardness target in the upper 40s HRC can put the temper inside that band, so check the target with the heat treater before release. It is also one reason to specify a hardness range and let the heat treater choose the temper, rather than writing a tempering temperature on the drawing.
For aerospace and other fracture-critical work, 4340 is also bought to SAE AMS specifications (AMS 6415 for air-melted and AMS 6414 for vacuum-remelted material), where cleanliness is controlled more tightly than in commercial bar. If your shaft needs that level of control, the specification belongs on the drawing, not in an email.
4140 vs 4340: when is 4340 worth it for a high-load shaft?
4340 is worth it when at least one of three things is true: the section is too large for 4140 to harden through, the required hardness is high enough that 4140 toughness becomes marginal, or the consequence of a brittle fracture is unacceptable. If none applies, 4140 is the cheaper and more available choice.
Run the decision in this order:
- Section size. Look at the largest diameter that must meet a hardness or strength requirement at depth. Small shafts and pins, the kind turned from bar in volume, rarely need 4340 for hardenability alone.
- Hardness target. As typical commercial practice, 4140 is widely stocked pre-hardened at about 28–32 HRC, and in that range it machines well and is tough enough for most shafts. Once the requirement climbs toward the high 30s and 40s on the Rockwell C scale, 4340 starts to pay for itself in toughness.
- Failure mode. If the part fails safely by wear or yielding, 4140 is usually adequate. If it would fail by sudden fracture in a machine, a vehicle or a lifting device, the extra toughness margin of 4340 is cheap insurance.
- Service temperature. Parts that must keep impact toughness in cold outdoor service favor 4340 because of its nickel content.
- Surface treatment. If the shaft will be nitrided for wear, 4140 is often the better candidate: its chromium and molybdenum form hard nitrides, and the base steel costs less.
Cost and supply point the other way. 4340 carries nickel, costs more per kilogram, and is stocked in fewer sizes and conditions in most markets. Upgrading every shaft to 4340 “to be safe” adds material cost without improving a part that was never limited by toughness, and it can stretch sourcing for an overseas supplier who has to order a less common bar.
Which heat treat condition should the drawing call out?
Call out the condition you need at final inspection, as a hardness range with a test method, plus the bar specification; do not write only “4140 HT” or “heat treat as required”. The three common routes are pre-hardened bar machined to finish, annealed bar hardened after machining, and hardened bar finished by hard turning or grinding.
Each route has a different effect on cost and accuracy. Pre-hardened bar machined to finish avoids heat treat distortion entirely and suits most moderate-strength shafts. Machining annealed bar and then hardening it allows higher hardness but means the part moves during the quench, so critical diameters need grinding stock. Hardening and then hard turning or grinding gives the highest accuracy at high hardness, at the highest cost.
Examples of complete material callouts:
MATERIAL: AISI 4140 STEEL BAR PER ASTM A322, QUENCHED AND TEMPERED, 28–32 HRC PER ASTM E18. MILL CERTIFICATE WITH HEAT CHEMISTRY REQUIRED.MATERIAL: AISI 4340 STEEL BAR PER ASTM A322. HEAT TREAT AFTER ROUGH MACHINING TO 40–44 HRC PER ASTM E18, MEASURED ON THE FINISHED SURFACE.MATERIAL: QUENCHED AND TEMPERED ALLOY STEEL BAR PER ASTM A434, CLASS BC, GRADE 4140.
The third form uses ASTM A434, which covers quenched and tempered alloy bar sold to minimum tensile properties: hot-wrought bar in strength classes BB, BC and BD, and cold-finished bar in BB and BC. It is a good way to buy shaft stock when you care about tensile and yield strength more than about a hardness number. Whatever form you use, state where hardness is measured (surface, mid-radius or core), because a large 4140 shaft can pass at the surface and fail at mid-radius. Hardness testing on a small round part also needs a curvature correction, and the Rockwell method in ASTM E18 provides one for convex cylindrical surfaces.
What goes wrong with 4140 and 4340 parts after machining?
Most failures in these steels come from the sequence, not the grade: decarburized surfaces left on the part, quench cracks at sharp corners, distortion that eats the tolerance, and hydrogen embrittlement after plating. Each one is prevented by a line on the drawing.
- Decarburization. Hot-rolled bar and parts hardened in an uncontrolled atmosphere lose carbon at the surface, leaving a soft skin exactly where fatigue cracks start. Either machine the skin off after heat treatment or require a protective atmosphere, and say on the drawing that no decarburization is allowed on functional surfaces.
- Quench cracks. Sharp keyway corners, thread roots with no relief, and cross holes near a shoulder concentrate the stress of the quench. Specify fillet radii at shoulders and in keyway corners, and break edges before hardening.
- Distortion. Long, slender shafts bow when hardened. Leave grinding stock on bearing journals, or use pre-hardened bar if the strength target allows.
- Hydrogen embrittlement. Electroplating a hardened 4140 or 4340 part can charge it with hydrogen. ASTM B850 selects the post-plating bake by the actual tensile strength of the steel and places it before any conversion coating, so the drawing must state the hardness and the bake requirement together.
- Machining hard bar. As typical shop practice, bar above about 40 HRC is finish turned with hard-turning tooling or ground, at slower rates than pre-hardened stock. If the hardness only matters on a journal, consider induction hardening that zone instead of through hardening the whole part.
Welding is the other frequent surprise. Both grades are hardenable enough to crack in the heat-affected zone without preheat and post-weld treatment, and 4340 more so. If a shaft must be welded to a hub or flange, say so at the RFQ stage, because it can change the grade and the heat treatment sequence.
What should the RFQ say for a 4140 or 4340 shaft or pin?
The RFQ should name the grade and bar specification, the heat treat condition as a hardness range with its test location, any impact or tensile requirement, the finish, and whether an equivalent grade is acceptable. A supplier that has all five can quote the bar, the heat treatment and the machining sequence without guessing.
- Grade and specification, including whether an H-grade, an A434 class or an AMS specification is required, and which equivalents are acceptable.
- Condition and hardness: annealed, pre-hardened or hardened after machining, the hardness range, the test method and where it is measured.
- Mechanical tests: tensile or Charpy requirements, with the test temperature for impact.
- Finish: plating or oiling, and the hydrogen embrittlement bake if the part is hardened and plated.
- Paperwork: a mill certificate with heat chemistry, and a heat treat record if you need one.
On the Saola side, plainly: neither 4140 nor 4340 appears in the list of bar grades the CNC turning page names as running today, and heat treatment is not run at our lead CNC plant. Bushings, spacers, caps, inserts and shafts turned from bar on its CNC lathes are the typical turning work, and the tolerances on your drawing are confirmed feature by feature in the quote. If your drawing calls for 4140 or 4340, send it with the condition stated, and the reply will say directly whether the part is a fit rather than quote around the gap. On finishing, zinc plating and oil-dip rust protection are on the surface finishing menu; black oxide and nitriding are not. Material certification and first-article inspection reports are available as described on the quality page.
The RFQ package guide covers the rest of what to send. When the drawing is ready, send it for a quote.
Frequently asked
Is 4140 stronger than 4340?
Not inherently. With the same carbon range they reach similar strength in small sections. 4340 pulls ahead in large sections, where it hardens deeper, and at high hardness, where it stays tougher.
Can I swap 4340 for 4140 on an existing drawing without other changes?
The geometry rarely needs to change, but the heat treatment usually does, because 4340 responds differently to the same quench and temper. Re-qualify the hardness and any impact requirement on a first article before releasing the change.
Is 4140 the same as 42CrMo4 or EN19?
They are close equivalents with slightly different composition windows, not the same grade. If the drawing will accept 42CrMo4, 709M40 or SCM440, write that explicitly so the supplier can buy locally available bar without a deviation.
Should a pin be through hardened or case hardened?
A pin that sees bending or shock is usually better through hardened to a moderate level with a tough core. A pin that mainly wears against a bushing may do better with a hard case from induction hardening or nitriding over a pre-hardened core.
Does 4340 rust less than 4140?
No meaningful difference. Neither is corrosion resistant, so plan on a plating, an oil film or another coating, and remember the hydrogen embrittlement bake if the part is hardened and electroplated.
Sources
- ASTM A322-24, Standard Specification for Steel Bars, Alloy, Standard Grades, ASTM International, 2024
- ASTM A29/A29M-23, Standard Specification for General Requirements for Steel Bars, Carbon and Alloy, Hot-Wrought, ASTM International, 2023
- ASTM A434/A434M-24, Standard Specification for Steel Bars, Alloy, Hot-Wrought or Cold-Finished, Quenched and Tempered, ASTM International, 2024
- ASTM A255-20a, Standard Test Methods for Determining Hardenability of Steel, ASTM International, 2020
- ASTM A304-20, Standard Specification for Carbon and Alloy Steel Bars Subject to End-Quench Hardenability Requirements, ASTM International, 2020
- ASTM E18-25, Standard Test Methods for Rockwell Hardness of Metallic Materials, ASTM International, 2025
- ASTM E23-25, Standard Test Methods for Notched Bar Impact Testing of Metallic Materials, ASTM International, 2025
- ASTM B850-98(2022), Standard Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement, ASTM International, 2022