# SAE 4340 steel

Nickel-chromium-molybdenum alloy steel: chemistry, product quality, section-dependent heat treatment and the limits of strength and hardenability data.

Reviewed: 2026-11-01

Tags: materials, steel, alloy-steel, heat-treatment

## Identity and specification boundaries

Atlas's **April 2006 hardened-and-tempered 4340** bar table lists **835 MPa minimum 0.2% proof stress** up to **60 mm**, approximately **condition V**, and **740 MPa above 60 through 100 mm**, approximately **U**. A separate voestalpine PDF uses a **63 mm ruling-section limit for V**. Keep the supplier and section definition with the number. [Atlas 4340 supply table](https://atlassteels.com.au/documents/Atlas4340.pdf), [voestalpine treatment and properties](https://www.voestalpine.com/highperformancemetals/australia/app/uploads/sites/265/2024/08/Datasheet-4340-2026-v2.pdf)

4340 is a medium-carbon nickel-chromium-molybdenum alloy steel used for strength and toughness after suitable heat treatment. Eaton lists heavy-duty shafting, axle shafts and mine-drilling parts among its applications. Those examples identify candidate uses; they do not establish an allowable stress or qualify a finished component. [Eaton hot-rolled 4340](https://www.eatonsteel.com/4340-hot-rolled-steel-bar.html)

SAE J404 covers alloy-steel chemical compositions. Its public scope distinguishes ladle analysis from check-analysis variations and explains that these are SAE grades because AISI no longer issues grade designations. The cited record is **J404_200901, revised January 27, 2009**. A chemistry designation is only one part of an order: add the product specification and edition, form, supplied condition, dimensions and required tests. The public scope is not a substitute for the full standard's composition tables. [SAE J404 scope and record](https://saemobilus.sae.org/standards/j404_200901-chemical-compositions-sae-alloy-steels)

Do not assume “4340,” “4340H,” “E4340,” “VAR” and a particular aerospace specification are interchangeable descriptions. Each additional designation can establish a different requirement. A supplier's cross-reference to EN24 or another national grade should trigger comparison of the actual specifications, not automatic substitution.

## Chemistry: bounded supplier example

Eaton publishes the following composition for its hot-rolled 4340 bar, in mass percent:

| Element | Published range or limit |
| --- | --- |
| Carbon | 0.38–0.43 |
| Manganese | 0.60–0.80 |
| Silicon | 0.15–0.35 |
| Nickel | 1.65–2.00 |
| Chromium | 0.70–0.90 |
| Molybdenum | 0.20–0.30 |
| Phosphorus | 0.035 maximum |
| Sulfur | 0.040 maximum |

This is that supplier's product table, not an assertion that every international 4340-related specification uses identical ranges. [Eaton chemistry table](https://www.eatonsteel.com/4340-hot-rolled-steel-bar.html)

Check the certificate against the ordered heat or check-analysis basis.

## Product quality and supplied condition

Universal Stainless describes its 4340 product as **AOD + VAR or VIM + VAR**, offering round and flat bar, billet and ingot. It lists capabilities including AMS 6409, 6414, 6415 and 6484, with sizes depending on specification and order. These are capabilities of that producer's product, not attributes of every commercial 4340 bar. A specification appearing on a website does not prove that a particular delivered heat was certified to it. [Universal Stainless 4340](https://www.univstainless.com/4340/)

Record rolling, thermal condition and any peeled finish separately. Require evidence for annealed or quenched-and-tempered delivery.

Atlas's historical sheet supplies 4340 hardened and tempered, with strength varying by diameter. Its table gives the following two rows:

| Atlas bar diameter | Tensile strength | Minimum 0.2% proof stress | Approximate condition |
| --- | --- | --- | --- |
| Up to 60 mm | 1000–1150 MPa | 835 MPa | V |
| Above 60 through 100 mm | 930–1080 MPa | 740 MPa | U |

These are **Atlas product data from April 2006**, not a current universal strength table. [Atlas 4340 supply conditions](https://atlassteels.com.au/documents/Atlas4340.pdf)

## Hardenability is not a hardness guarantee

Hardenability concerns the response through a section under a specified cooling history; it should not be confused with a single surface hardness measurement. ASTM A304-20 covers hot-worked steels with end-quench hardenability requirements, identified by an **H suffix**, and requires the end-quench requirements to conform to the purchase order. If a controlled hardenability response is necessary, order and verify it rather than assuming ordinary chemistry-controlled 4340 supplies it. [ASTM A304-20 scope and abstract](https://store.astm.org/standards/a304)

A Jominy distance is a position along a standardized end-quenched specimen, not a directly interchangeable depth below the surface of a production shaft. Atlas labels its chart as typical Jominy results and its tempering diagram as typical values for a **30 mm section**. Neither diagram independently demonstrates the core properties of a much larger forging. [Atlas hardenability and tempering diagrams](https://atlassteels.com.au/documents/Atlas4340.pdf)

For critical parts, agree on the limiting or ruling section, representative heat treatment, sampling location, orientation and acceptance tests. Surface hardness, tensile strength, proof strength and impact energy answer different questions. Do not replace a specified toughness test with a hardness-to-strength conversion or transfer an impact result without its specimen and test-temperature conditions.

## Heat treatment and finishing

The voestalpine Australia PDF linked as “Datasheet 4340 2026” recommends hardening at **815–845°C**, oil or polymer quenching, and immediate reheating to **455–650°C** for tempering, with at least two hours of tempering after reaching uniform temperature. Qualify the procedure for the actual geometry and furnace load. [voestalpine linked 4340 PDF](https://www.voestalpine.com/highperformancemetals/australia/app/uploads/sites/265/2024/08/Datasheet-4340-2026-v2.pdf)

Its table associates condition U with a 100 mm limiting ruling section, 930–1080 MPa tensile strength and 740 MPa minimum proof strength; condition V has a 63 mm limit, 1000–1150 MPa tensile strength and 835 MPa minimum proof strength.  For stress relief in the quenched-and-tempered condition, the PDF specifies 30–50°C below the previous tempering temperature. Retain the treatment history. [voestalpine treatment and property table](https://www.voestalpine.com/highperformancemetals/australia/app/uploads/sites/265/2024/08/Datasheet-4340-2026-v2.pdf)

Atlas warns that cold drawing retains hot-rolling surface defects, so a bright finish does not eliminate the need for machining allowance. It also discourages welding because of quench-cracking risk. Treat repair welding as a separately qualified process, with the actual condition and subsequent property requirements defined; do not infer weldability from the word “steel.” [Atlas machining and welding cautions](https://atlassteels.com.au/documents/Atlas4340.pdf)

## Bounded calculation and selection record

**Original estimating example:** a solid 50 mm round has cross-sectional area `π × 50² / 4 = 1963.5 mm²`. A concentric tensile load of 100 kN produces nominal stress `100,000 / 1963.5 = 50.9 MPa`. Dividing by the cited U-condition proof value gives `50.9 / 740 = 0.069`. This ratio is not a design safety factor or allowable-load approval. It omits threads, bending, fatigue, buckling, residual stress, environment and all required design margins, and assumes that the actual material has the accepted property.

Record the grade and product specification, producer/melt route where required, supplied and final conditions, ruling section, certificate and test requirements together. If corrosion, plating, surface hardening or another exposure governs the component, obtain the relevant process and service qualification separately; none of the strength tables establishes universal compatibility or fatigue life.

## Source editions

Atlas's PDF explicitly says **revised April 2006**. SAE's record is January 2009; ASTM's cited listing identifies **A304-20** as active. voestalpine labels its download **2026**; the PDF has no printed revision date. Its treatment table differs from the web page; the values above follow the PDF. Eaton and Universal Stainless's live pages show no technical revision date. Confirm current supply terms at quotation.