817M40 / EN24 Alloy Steel
The British counterpart of 4340: a nickel-chromium-molybdenum steel supplied hardened and tempered to condition T (850 to 1000 MPa) and above — for drawings written to BS 970 or EN24.
What 817M40 (EN24) is.
817M40, formerly EN24, is the UK designation for a 0.40 percent carbon nickel-chromium-molybdenum steel close to AISI 4340 in strength and hardenability. It is supplied hardened and tempered to BS 970 condition letters, most often T (850 to 1000 MPa, about 123 to 145 ksi), with U, V or W for higher strength. Drawings from British and Commonwealth OEMs still call for it, and 4340 to the same strength condition is the normal substitute.
We machine 817M40 components, studs and shafts from certified bar in the condition specified, or from 4340 heat treated to match when the customer accepts the equivalent.
Chemical composition.
| Element | Symbol | Min % | Max % |
|---|---|---|---|
| Carbon | C | 0.36 | 0.44 |
| Manganese | Mn | 0.45 | 0.70 |
| Phosphorus | P | — | 0.035 |
| Sulfur | S | — | 0.040 |
| Silicon | Si | 0.10 | 0.35 |
| Nickel | Ni | 1.30 | 1.70 |
| Chromium | Cr | 1.00 | 1.40 |
| Molybdenum | Mo | 0.20 | 0.35 |
Per BS 970 for 817M40, confirmed by UK stockholder datasheets (Hillfoot, Steel Express). The older EN24 ranges of BS 970:1955 were slightly wider.
Mechanical properties.
| Condition (BS 970) | Tensile strength (MPa / ksi) | 0.2 % proof stress, min (MPa / ksi) | Elongation, min (%) | Hardness (HB) |
|---|---|---|---|---|
| T | 850–1000 (123–145) | 680 (99) to 150 mm · 654 (95) 150–250 mm | 13 | 248–302 |
| U | 925–1075 (134–156) | 755 (110) | 12 | 269–331 |
| V | 1000–1150 (145–167) | 850 (123) | 12 | 293–352 |
Conditions T, U and V confirmed by two UK stockholder datasheets (West Yorkshire Steel, Hillfoot); condition T proof stress also by Steel Express. Harder conditions W to Z are produced in smaller sections; ask our estimating team. The limiting ruling section for each condition depends on bar size.
Specifications that apply.
How 817M40 (EN24) ships from Houston.
- Machined components from certified bar
- Studs, threaded rod and U-bolts
- Hex bolts and cap screws
- Nuts, washers and spacers
- Pins, shafts and bushings
817M40 (EN24) fasteners we make.
Every item is manufactured to your drawing or standard in 817M40 (EN24), imperial or metric, with an EN 10204 3.1 certificate.
What is 817M40?
817M40 is the BS 970 designation that replaced EN24: about 0.40 percent carbon with nickel, chromium and molybdenum for deep hardenability and high strength with good toughness. British practice specifies the delivered condition by letter, T through Z, each a tensile range with Z a minimum only, rather than by class as ASTM A434 does. Condition T is the everyday order; U, V and W follow for more strength.
817M40 and 4340
817M40 and 4340 have similar strength and hardenability, and 4340 quenched and tempered to the same tensile range is the usual substitute. The chemistry is not identical: 4340 carries less chromium (0.70–0.90 %) and more nickel (1.65–2.00 %). Where a customer's approvals name 817M40 explicitly we source certified bar to BS 970 so the paperwork matches.
Testing, certification and marking
- Hardness tested per lot; tensile on request or where the specification requires it
- Manufacturer's identification and heat number on every piece where size allows
- EN 10204 3.1 material test report with heat number and chemistry
- PMI verification on request
AISI 4340 Alloy Steel (UNS G43400)
The deep-hardening high-strength steel — 4340 reaches 155 ksi and beyond in large sections — for crankshafts, drilling tools, aircraft parts and A320 L43 bolting.
ASTM A322 bar · ASTM A434 Q&T barAISI 4140 Alloy Steel (UNS G41400)
The workhorse Cr-Mo steel behind B7, L7 and Grade 7 — the bar stock for studs, shafts, couplings and any part that needs 100 to 150 ksi and good toughness.
AMS 6427 (aircraft quality) · AMS 6411 (vacuum-arc remelted)4330V Modified Alloy Steel
A 4340-class steel with lower carbon and a vanadium addition for toughness: 220 to 240 ksi with good impact resistance — the aerospace and oil-tool grade for highly stressed components.