GRADE GUIDE · EN24
EN24 (817M40, close to AISI 4340) is the step up from 4140: nickel added to the chrome-moly base buys deeper hardenability and better toughness at high strength. Usually supplied in the T condition (~850 MPa tensile), it is the bar for heavily loaded shafts, gears and axles.
Also listed as: 817M40 · ≈ AISI 4340 · 34CrNiMo6 / 1.6582 (near) · EN24T
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The nickel is the point: it lets large sections through-harden evenly and keeps the core tough when the strength is pushed up — which is why crankshafts, heavy gear blanks, high-tensile couplings and aircraft-derived parts specify EN24 or its American twin 4340 rather than plain chromoly. In the standard T condition it arrives ready to machine at around 248–302 HB.
The old British EN numbering survives stubbornly in Gulf trading — EN24T rolls off the tongue in every machine shop — while certificates may read 817M40, 1.6582 (34CrNiMo6, the near-equivalent) or 4340. Off-cut bar ends from shaft work are the natural listing; for the heaviest sections, EN24's even properties through the core are exactly what you are paying for.
Large sections and high demanded strength: the nickel keeps big diameters uniformly hardened and tough where 4140's properties fade toward the core. For modest shafts, 4140 does the job cheaper.
A specified heat-treated condition — hardened and tempered into the 850–1,000 MPa tensile band. Other letters (U, V, W…) mark higher tempers; the certificate states which condition the bar left the mill in.
Reluctantly — high carbon equivalent means serious preheat, low-hydrogen practice and post-weld tempering. Treat welding as a repair procedure, not a fabrication method, on this grade.