What Is 4340 Steel? When Should You Choose It?
4340 is a heat-treatable nickel–chromium–molybdenum low-alloy steel. Consider it when a part needs high strength and toughness through a demanding section, especially after quenching and tempering. Its main advantage over 4140 is greater hardenability; its final strength still depends on the supplied condition, section size and heat treatment.
What makes 4340 an alloy steel?
Often called AISI or SAE 4340, and commonly identified as UNS G43400, this steel contains about 0.4% carbon plus deliberate additions of nickel, chromium and molybdenum. Those additions distinguish it from plain-carbon steel. They help the steel develop a hardened structure at cooling rates that would leave a less hardenable steel softer.
The number identifies a material family, not one finished property set. An annealed bar intended for machining is a different starting point from quenched-and-tempered stock ready for machining at its supplied strength.
Swipe within the table to read all columns.
| Element | Published range | Why it matters |
|---|---|---|
| Carbon | 0.37–0.44% | Provides the carbon needed for substantial quench-hardening response. |
| Nickel | 1.55–2.00% | Contributes to the alloy’s combination of hardenability and toughness. |
| Chromium | 0.65–0.95% | Helps the section respond to hardening. |
| Molybdenum | 0.20–0.35% | Contributes to hardenability and the response to tempering. |
| Manganese | 0.55–0.90% | Forms part of the full chemistry affecting transformation during cooling. |
| Silicon | 0.10–0.35% | A deoxidizing addition; standard 4340 is distinct from high-silicon 300M. |
Source: Atlas 4340 datasheet, page 1. These supplier ranges relate to the product specifications stated there; they are not a replacement for the chemistry limits on an SAE, AMS or customer order. Check all required elements and the actual heat analysis against the governing specification.
Hardenability describes depth, not a hardness reading
Hardness measures resistance to indentation at a test location. Hardenability describes how readily a steel develops a hardened structure below the surface under a given cooling condition.
This distinction explains 4340’s value in larger sections: the center cools more slowly than the surface. Greater hardenability can help the core reach the required structure. It does not guarantee uniform hardness through every diameter or shape.
Quenching can form hard martensite. Tempering then changes that structure and adjusts the balance of hardness, strength and toughness. A highly hardenable steel can still be supplied in a soft condition.
How strong and hard is 4340?
Read the condition before the number. Annealed 4340 is used as a machinable starting material for later heat treatment. Quenched-and-tempered 4340 can be supplied at a specified property level. “Prehardened” stock still needs an actual hardness or strength range on the order.
The examples below come from one producer’s heat-treated property table. They show how strength, hardness and a stated section limit travel together. Strength is reported in megapascals (MPa).
Swipe to see all property columns.
| Condition | Limiting ruling section | Minimum 0.2% proof strength | Tensile strength range | Brinell hardness |
|---|---|---|---|---|
| U | 100 mm | 740 MPa | 930–1,080 MPa | 269–331 HB |
| V | 63 mm | 835 MPa | 1,000–1,150 MPa | 293–352 HB |
| X* | 30 mm | 1,005 MPa | 1,150–1,300 MPa | 341–401 HB |
Source: voestalpine 4340 datasheet, page 1. *Condition X is typically ordered annealed for machining and subsequently heat-treated to the specified properties. The producer lists normal supply in conditions U and V. These examples do not establish every supplier’s offering or a universal diameter limit.
The ruling section is the section controlling the heat-treatment response; it is not automatically the smallest finished diameter. A thick hub joined to a slender shaft must be assessed in the geometry that undergoes heat treatment.
0.2% proof strength describes the stress associated with a specified small permanent strain. Tensile strength is the maximum engineering stress reached in a tensile test. These values serve different design checks and are not interchangeable with hardness.
A range is not a guarantee of its midpoint. If a drawing requires at least 1,200 MPa tensile strength, a published range of 1,150–1,300 MPa does not by itself meet that minimum. Agree on the required lower limit, section, treatment and test evidence before approving the stock.
Measured hardness and estimated strength are also different evidence. In NIST TN 1858, Table 2, differently heat-treated 4340 reference lots measured 45.0 and 30.0 on the Rockwell C (HRC) scale. Their listed tensile strengths, 1,480 and 952 MPa, were estimated from hardness conversions rather than measured by tensile tests. They illustrate condition dependence; they are not design allowables for a production part.
When should you choose 4340 over another steel?
4340 earns consideration when the design needs a combination of core strength, toughness and hardening response that a simpler route cannot reliably deliver. Start with the part’s limiting requirement rather than the highest tensile number in a catalogue.
Swipe to compare the material routes and evidence needed.
| Requirement | Starting candidate | Where 4340 fits | Evidence that decides |
|---|---|---|---|
| A machined part whose strength and toughness targets are already achievable | 4140, in a defined condition. | Consider the extra alloy content when it provides a needed benefit in core response or toughness. | Comparable heat-treated properties, actual section and total processing cost. |
| A highly loaded section requiring strength below the surface | 4340, with the required final condition and hardenability control. | Its Ni–Cr–Mo alloy system makes it a useful candidate for deeper hardening. | Core properties, test location, heat-treatment capability and load requirements. |
| A carburized wear case on a lower-carbon core | 8620 or another carburizing steel. | Compare 4340 when the design instead calls for high strength through the section; surface treatments can be considered separately. | Case depth, surface hardness, core properties and contact loading. |
| An approved aerospace design requiring a different very-high-strength material | 300M or the specific alloy on the approved drawing. | 4340 remains an option only within the permitted material and process specification. | Approved material, melt route, design data and process qualification. |
Comparison basis: Atlas 4340, Atlas 8620H and Carpenter Technology 300M. These are different material routes, not results from a matched performance test.
A shaft example: identify what 4140 is missing
Suppose a shaft meets the specified surface hardness after heat treatment, but tests show that its thick center does not meet the required core property. That gives a concrete reason to compare 4340 and its greater hardenability, together with the proposed quench and tempering route.
If 4140 already meets the core, toughness and fatigue requirements, the alloy upgrade needs a different justification. A crack starting at a sharp fillet, machining damage or a corrosion pit also calls for a review of geometry and surface condition.
This is a selection example, not a reported customer test. Shafts, axles, gears and couplings are candidate applications; their names alone do not establish that 4340 is required.
300M and “nearest equivalents” need their own specifications
300M is a modified 4340-type steel with substantially more silicon and other chemistry differences; it is not simply ordinary 4340 tempered harder. Carpenter lists it as a separate alloy with its own specifications and vacuum arc remelting route.
Likewise, a supplier’s cross-reference to EN24 or a 40NiCrMo-type grade is a starting point for comparison. Check chemistry, product form, heat-treated condition and required inspection against the drawing before accepting a substitution.
Specify the heat-treatment response you need
A heat-treatment plan must account for the actual part. Heating, the quench and tempering affect structure, distortion and the properties remaining after final machining. A small test coupon that cools quickly may not represent the center of a heavy section.
What does the H in 4340H mean?
The H suffix identifies steel ordered to specified end-quench hardenability requirements. In a Jominy test, one end of a specimen is quenched and hardness is measured at increasing distances from that end. The resulting curve describes response across different cooling rates.
ASTM A304 covers bars with these hardenability requirements. State the applicable limits and specification on the order. An H designation does not mean the delivered bar is already hard, nor does it certify final-part tensile strength or toughness.
Set the route before final machining
Decide whether to machine prehardened stock or rough-machine softer stock and harden it later. Allow for straightness changes and finishing after heat treatment. Any later stress relief must be compatible with the previous temper and final property requirement.
Producer heat-treatment schedules are starting references for that product. They do not settle holding time, quench conditions or distortion for every geometry. Use the applicable process specification and a route demonstrated for the relevant section.
Check the critical location after processing
Identify where core hardness, tensile properties or toughness must be demonstrated. Add surface integrity, dimensional inspection and appropriate nondestructive testing where the drawing requires them.
For a forged component, specify the forging requirements as well as chemistry. A bar certificate alone cannot establish the quality of a finished forging. Critical parts may also require tighter control of material cleanliness and melting practice.
For broader property mechanisms and testing concepts, continue with the 4340 alloy steel properties guide. This selection guide focuses on how those properties affect a material and processing choice.
Machining, welding and coatings can change the decision
Match machining to the supplied hardness
4340 can be machined in an appropriately supplied quenched-and-tempered condition, so machining before hardening is not the only route. For more demanding final hardness, softer starting stock may make rough machining easier. Plan tool selection, workholding, heat-treatment allowance and final surface finish together.
Review welding before committing to the alloy
4340’s carbon content and high hardenability can produce a hard heat-affected zone and increase hydrogen-assisted cracking risk. Atlas advises against welding its supplied product as a routine fabrication choice. Specialized repair or joining routes require a procedure suited to the chemistry, condition, section and required final properties.
An AWS discussion of a 12-inch-diameter 4340-type repair shows why ordinary structural-steel preheat guidance cannot simply be transferred to this alloy. That application-specific discussion is not a universal welding schedule.
Consider preheat and interpass control, a low-hydrogen process, filler selection, cooling, any postweld treatment and inspection together. Laser welding also needs this metallurgical review; a narrow, attractive weld does not prove recovered strength or freedom from cracking.
Plan corrosion protection and hydrogen control together
4340 is not stainless steel and can rust. Choose a coating or another material route according to the environment and required service life. A high strength requirement does not make corrosion protection optional.
Some cleaning, pickling and electroplating operations can introduce hydrogen. For high-strength parts, the surface-process specification must address pretreatment, coating, any required embrittlement-relief treatment and acceptance evidence. Baking is a risk-reduction measure, not proof that every part is unaffected.
What an F519 result tells you. ASTM F519 evaluates hydrogen embrittlement associated with coating/plating processes and service environments using defined test specimens. It is not a test for ranking the intrinsic susceptibility of different steel grades, or a substitute for all finished-part acceptance checks. ASTM B850 separately addresses post-coating treatments to reduce hydrogen-embrittlement risk.
What should a 4340 purchase order include?
Use the drawing and governing specification to define the evidence needed for the actual part. A useful order connects the material, manufacturing route and final acceptance:
- Material and product specification. Name the required grade, standard and revision, plus bar, billet, forging or other product form.
- Size and condition. Include dimensions, tolerances, straightness, machining allowance, delivery condition and any planned later heat treatment.
- Final properties and locations. State the required proof/yield and tensile strengths, hardness range, ductility and any toughness requirements, with test temperature, orientation and sampling location as applicable.
- Section response and production route. Add hardenability limits, the critical section and heat-treatment verification. Specify cleanliness, melt route and forging requirements when the design calls for them.
- Surface processing and inspection. Identify welding, coating and hydrogen controls, surface-quality limits, dimensional checks and required nondestructive examinations.
- Traceability and substitution control. Link the delivered material to its heat, treatment batch and test report. Require review of proposed grade, condition or process substitutions.
Evaluating a laser process for a 4340 part?
Share the material specification, heat-treated condition, drawing and target result with Oceanplayer Laser. For welding, include the joint and acceptance criteria; for cleaning, include the surface treatment, contamination and finish that must be preserved.
Technical references
- Atlas Steels — 4340 datasheet, revised April 2006. Supplier chemistry, hardening response, applications and welding limitations.
- voestalpine — 4340 datasheet, 2026. Selected heat-treated condition values, ruling sections and supply condition.
- NIST Technical Note 1858, Table 2 and Section 2. Measured hardness and estimated tensile strength of reference 4340 lots.
- ASTM A304. Public scope for steel bars ordered to end-quench hardenability requirements.
- Atlas Steels — 8620H and Carpenter Technology — 300M. Alternative material routes and their different purposes.
- AWS Inspection Trends — “The Answer Is,” May 2024. Application-specific discussion of preheat for a 4340-type repair.
- ASTM F519 and ASTM B850. Public scopes for hydrogen-embrittlement evaluation and post-coating treatment guidance.