Short answer: EN24 and EN24T are the same steel — BS 970 817M40, a 1.5% nickel-chromium-molybdenum alloy steel also known as 34CrNiMo6 and AISI 4340. The difference is heat treatment. "EN24" on its own usually means bar supplied annealed or as-rolled; EN24T (817M40T) means the bar has already been hardened and tempered to the BS 970 "T" strength band, typically 850–1000 MPa tensile and 248–302 HB.
In this guide: What is EN24 · Supply conditions · What “T” means · Bar size · Applications · Which to buy · FAQ
What is EN24 (817M40)?
EN24 is a through-hardening alloy steel with about 0.40% carbon, 1.5% nickel, 1.2% chromium and 0.25% molybdenum. That combination gives high strength, good toughness and deep hardenability, which is why it is used for axles, crankshafts, heavy-duty gears, connecting rods and high-tensile bolts. Full chemistry, mechanical properties and equivalents are on our EN24 / 817M40 grade page.
The three supply conditions
| As-rolled | Annealed | EN24T (hardened & tempered) | |
|---|---|---|---|
| How it is made | Hot-rolled at the mill and cooled in air | Reheated and cooled very slowly in a furnace | Heated to about 830–850 °C, quenched in oil, then tempered at about 550–650 °C |
| Structure | Uneven — EN24 partly hardens even in air, so hardness varies through the bar | Soft and uniform | Tempered martensite — strong and tough |
| Hardness | Variable, not guaranteed | Low — typically up to about 248 HB | 248–302 HB (T band) |
| Tensile strength | Not guaranteed | Not guaranteed (soft) | 850–1000 MPa |
| Machining | Awkward because hardness is uneven | Easiest | Machinable, but slower and harder on tools |
| What the buyer does next | Forges, or heat-treats before use | Rough-machines, heat-treats, then finish-machines | Machines and uses — no further heat treatment |
What does the "T" in EN24T mean?
BS 970 uses a letter to name the strength band a hardened-and-tempered bar is supplied to. The steel is the same; only the tempering changes:
| Condition | Tensile strength (MPa) |
|---|---|
| T | 850–1000 |
| U | 925–1075 |
| V | 1000–1150 |
| W | 1075–1225 |
| X | 1150–1300 |
| Y | 1225–1375 |
| Z | 1550 minimum |
T is the most commonly specified band. So "EN24T", "817M40T" and "EN24 in the T condition" all mean the same thing: EN24 hardened and tempered to 850–1000 MPa.
Why bar size matters for EN24T
Oil quenching only hardens so deep. On large diameters the core cools more slowly and cannot reach the same strength as the surface, so the standard limits the size ("limiting ruling section") up to which each strength band can be guaranteed, and lower minimum properties apply to bigger sections. When you ask for EN24T, always give the finished diameter — it decides whether the T properties can be guaranteed for that size.
Applications: where EN24 and EN24T are used
The finished part is the same either way — what changes is when the heat treatment happens. The typical pattern:
| Part / application | Usually bought as | Why |
|---|---|---|
| High-tensile bolts, studs and fasteners | EN24T | Threaded and machined straight from bar; strength must be guaranteed on the certificate |
| Machine-tool spindles, arbors and tool holders | EN24T | Machined and fitted without heat treatment, so no distortion to correct afterwards |
| Hydraulic and heavy-duty shafts, pins and bushes | EN24T | Good strength and toughness as supplied; short lead time for one-off or small batches |
| Parts to be nitrided | EN24T | The hardened-and-tempered core is the right base for nitriding a hard wear surface |
| Gears, pinions and splined shafts | Annealed | Heavy machining is done soft, then the part is hardened and tempered (or induction hardened) to the drawing |
| Large axles and drive shafts | Annealed | Machined close to size first; heat treatment after roughing gives better properties at the core of big sections |
| Crankshafts, connecting rods and other forgings | As-rolled | Forging stock is reheated and hot-worked anyway, then heat-treated after forging |
| Heavy engineering and mining components | Annealed or EN24T | Depends on size: EN24T where the section allows, annealed plus customer heat treatment for larger diameters |
Rule of thumb: if the part is mostly machining and will be fitted as-is, buy EN24T. If it needs heavy machining, forging or a specific hardness band other than T, buy annealed (or as-rolled for forging) and heat-treat to your drawing.
Annealed or EN24T — which should you buy?
- Buy annealed if you have your own heat-treatment arrangement, or the part needs heavy machining. Machine it soft, get it hardened and tempered, then finish-machine. Lowest material cost, but you carry the heat-treatment cost and the risk of distortion.
- Buy EN24T if you want to machine the part and fit it with no heat treatment in between. You pay more per kg, but you skip the heat-treatment step and receive guaranteed mechanical properties on the mill test certificate.
- As-rolled suits forging shops, who reheat the bar anyway, and buyers who will heat-treat before any machining.
What to check on the mill test certificate
For annealed or as-rolled EN24, the certificate should show the chemical analysis and heat number. For EN24T it should also show the tested mechanical properties — tensile strength, 0.2% proof stress, elongation, impact value and hardness — so you can confirm the bar is within the T band.
EN24 vs EN24T: frequently asked questions
Is EN24 the same as EN24T?
Yes, it is the same steel (BS 970 817M40). EN24T is EN24 that has already been hardened and tempered to the “T” condition, typically 850–1000 MPa tensile strength and 248–302 HB.
What hardness is EN24T?
EN24T is typically 248–302 HB (about 25–32 HRC), with a tensile strength of 850–1000 MPa. Always confirm the actual values on the mill test certificate.
Can annealed EN24 be hardened after machining?
Yes. Annealed EN24 is the easiest condition to machine, and parts are commonly rough-machined, then hardened and tempered to the required strength, then finish-machined or ground.
What are the equivalents of EN24?
EN24 (817M40) is close to 34CrNiMo6 / 1.6582 (EN 10083) and AISI/SAE 4340. Chemistry ranges differ slightly between standards, so specify the standard your drawing calls for.
Do you stock EN24T?
We stock EN24 round bar annealed (Ø16–100 mm) and as-rolled (Ø105–205 mm) at Darukhana, Mumbai. EN24T is supplied against orders of 3–5 tons, with mechanical properties on the mill test certificate.
What is the minimum order quantity for EN24 round bar?
The minimum order for stock material is 100 kg. Prices are ex-Darukhana; loading, transport and cartage are extra at actuals.
How we supply EN24
- Ready stock at Darukhana, Mumbai: EN24 round bar Ø16–100 mm in the annealed condition, and Ø105–205 mm as-rolled.
- EN24T (hardened & tempered): supplied against orders of 3–5 tons, with mechanical properties on the mill test certificate.
- Minimum order 100 kg for stock material. Cutting is available if the balance piece is also taken; cutting is charged extra.
- Prices are ex-Darukhana; loading, transport and cartage are extra at actuals. Every dispatch comes with a mill test certificate.
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Values above are typical figures for guidance under BS 970 / EN 10083 and are not a guarantee; actual properties depend on size, heat treatment and heat. Please confirm suitability for your application with our team.