SAE 4140 steel

Identity and order definition

voestalpine Australia's quenched-and-tempered 4140, condition U or T/U, lists 740 MPa minimum proof strength at 10–100 mm and 665 MPa at 101–250 mm. The 251 mm and above row labels 665 MPa an aim. Verify the applicable section and acceptance requirements for the order. voestalpine 4140 data, pp. 1–2

SAE 4140 is a chromium-molybdenum alloy steel, associated with UNS G41400. Alro's 4140 Q&T product page lists C 0.38–0.43%, Mn 0.75–1.00%, Cr 0.80–1.10%, Si 0.15–0.30% and Mo 0.15–0.25%. Treat these as the cited product's chemistry information, rather than a complete acceptance specification for every form. Alro 4140 hot-rolled Q&T; voestalpine product identification

SAE's current public J404 record identifies J404_200901, revised January 27, 2009, as Chemical Compositions of SAE Alloy Steels. Its scope distinguishes heat chemistry from product/check-analysis variations under J409. A chemistry designation therefore does not by itself guarantee a finished component's strength, toughness, hardness or dimensions. SAE J404 publisher record

Supply condition and section size

Annealed stock intended for machining and later heat treatment is a different purchase from stock already quenched and tempered to agreed mechanical properties. Alro explicitly says its Q&T properties depend on the heat-treatment/tempering route and asks purchasers to confirm specific requirements. Alro condition guidance

One bounded example is voestalpine High Performance Metals Australia's linked 4140 sheet, which describes Q&T supply to condition U or T/U:

The final row is not presented as the same guarantee as the smaller-section minima. Confirm the exact size interpretation, test location and contractual requirements with the supplier. Do not interpolate missing acceptance terms at a size boundary. The sheet's Charpy entries omit a test-temperature qualification, so no general impact/service-temperature rating is inferred here. voestalpine linked datasheet, pp. 1–2

These are named supplier-product values, not universal SAE minima. Atlas's older April 2006 sheet assigns its smaller-section condition U range through 180 mm, illustrating why supplier tables must retain their dates and product basis. Do not combine one source's size range with another's current guarantees. Atlas 4140, p. 1

Hardenability is distinct from hardness

ASTM's active A304-20 covers steel bars subject to end-quench hardenability requirements and identifies these compositions with an H suffix. Its public description makes the ordered end-quench limits a requirement. A 4140H order therefore adds a hardenability basis; it does not state that the delivered bar has one final working hardness. ASTM A304-20, scope and abstract

A hardness reading describes the tested location and condition. Hardenability concerns response to cooling, which matters when trying to obtain a required condition through a section. For a large shaft or a component with abrupt section changes, establish core as well as surface requirements. Neither a small test coupon nor a surface indentation alone establishes uniform final properties throughout the component.

If hardenability control is necessary, specify the relevant standard, grade and required band or permitted ordering method. Retain the heat's test evidence. Avoid assuming an ordinary chemistry certificate proves an ordered H-band requirement, or that a hardenability band replaces the finished part's heat-treatment and inspection requirements.

Heat treatment and surface processes

Atlas labels its tempering diagram typical values for a 30 mm section. It also describes surface hardening by nitriding and induction/flame treatment. These are processing options for its 4140 product, not a guarantee that a tempering-curve value applies at the center of an arbitrary block. Atlas 4140, pp. 2–3

If wear resistance requires a hardened surface, define case depth, surface hardness and core requirements separately. A nitrided or locally hardened case is not equivalent to through-hardening the entire section. Qualification should also address cracks, final finish and the transition between treated and untreated regions where relevant.

Welding, machining and exposure

Alro warns that its 4140 Q&T needs appropriate pre/post-weld thermal treatment to avoid cold cracking; Atlas does not recommend welding its supplied product and provides exceptional-case guidance. Both statements require an application-specific decision, rather than treating 4140 like an unrestricted mild-steel weldment. Alro welding guidance; Atlas, p. 3

Before weld repair, establish the original treatment, joint restraint, hydrogen controls and final property requirements. Confirm whether the required thermal route is compatible with the finished part. For machining, distinguish black, peeled, ground and other finishes in the order; the grade number supplies no fit tolerance. Define corrosion protection separately: Alro notes that the chromium content does not confer stainless-steel corrosion resistance. Alro product guidance

Bounded calculation example

Assume an ideal 100 mm diameter solid bar with uniform axial loading. Area is π × 100² / 4 = 7854 mm². Using the cited supplier's 10–100 mm 740 MPa minimum proof strength gives a simple gross-section proof-force product of 7854 × 740 = 5.812 MN. Substituting the next band's 665 MPa gives 5.223 MN, a difference of about 0.589 MN.

These products are illustrative arithmetic, not allowable loads. They omit buckling, holes, eccentricity, connections, fatigue, toughness and design factors. The point is to preserve the section and supply-condition basis, not to assign every 100 mm 4140 bar that strength.

Source versions

SAE identifies J404_200901; ASTM identifies A304-20. Atlas explicitly prints Revised April 2006. Voestalpine labels the download “4140 2026,” but the inspected PDF has no explicit printed revision date; its filename is not treated as independent edition evidence. Alro is a live product page. Confirm binding properties and the required standard editions when ordering.

See also grades, tempers and conditions.