The short answer
Order EN19 for most shafts, gears, pins and bolts up to about 100 mm diameter. Order EN24 when the part is bigger than that, takes heavy shock loads, or the drawing calls for a strength above Condition T.
What is in them
| Element | EN19 | EN24 |
|---|---|---|
| Carbon | 0.36 – 0.44 | 0.36 – 0.44 |
| Manganese | 0.70 – 1.00 | 0.45 – 0.70 |
| Chromium | 0.90 – 1.20 | 1.00 – 1.40 |
| Molybdenum | 0.25 – 0.35 | 0.20 – 0.35 |
| Nickel | – | 1.30 – 1.70 |
The nickel in EN24 is what separates them. It lets the steel harden further into a thick section and makes it tougher, so it is less likely to crack under impact.
Strength in Condition T
Condition T is the hardened and tempered state most stockists supply. In that condition both grades are specified at 850 to 1000 MPa tensile strength and 248 to 302 HB. A 40 mm bar of either will do the same job.
Section size is where they part ways. Quenching only hardens so deep, so the specified properties of EN19T hold up to about 100 mm ruling section, while EN24T holds them up to about 250 mm. On a large shaft, EN19 can be noticeably softer in the middle than at the surface.
EN24 is also the usual choice when a drawing calls for Condition V or W, the higher-strength conditions above T.
Equivalent grades
| Standard | EN19 | EN24 |
|---|---|---|
| BS 970 | 709M40 | 817M40 |
| EN / DIN | 42CrMo4 (1.7225) | 34CrNiMo6 (1.6582) |
| AISI / SAE | 4140 | 4340 |
Machining, hardening and welding
Both machine well in Condition T. Both can be induction or flame hardened, or nitrided, for a hard wearing surface over a tough core.
Neither is a welding steel. If a weld cannot be avoided it needs preheat, slow cooling and stress relief, and it is best kept off load-bearing sections.
Cost and availability
EN24 costs more because of the nickel, and its price moves with the nickel market. EN19 is usually the cheaper and more widely stocked of the two, so it is the sensible default when the part does not need what EN24 adds.
Which one for your part
- Shafts, axles and spindles up to about 100 mm: EN19
- Gears and couplings in normal service: EN19
- High-tensile bolts and studs: EN19
- Shafts and pins over 100 mm: EN24
- Parts that take repeated shock, such as crusher, mining and earthmoving pins: EN24
- Drawings that call for Condition V or above: EN24
For gears and cams that need a very hard surface, a case-hardening steel such as EN36B may suit better than either.