EN 24

What is EN 24?

EN 24 is a nickel-chromium-molybdenum medium-carbon alloy steel supplied in the hardened and tempered condition. It is used when section size or service duty exceeds the capacity of chromium-molybdenum steels. Nickel increases hardenability and improves toughness at a given strength.

EN 24 and SAE 4340 are near-equivalent grades, and both designations may appear on the same drawings. Their composition limits are not identical. EN 24 has a wider carbon range, a higher chromium maximum and a lower nickel minimum. It is purchased to a mechanical property condition rather than solely to a chemical analysis.

The BS 970 designation for the grade is 817M40. The 800 series identifies nickel-chromium-molybdenum alloy steels. The letter M identifies supply to a mechanical property specification. The number 40 represents the mean carbon content multiplied by one hundred.

Laxcon Steels lists EN 24 in its grades reference as an alloy steel grade. The same steel is written EN24, 817M40, 34CrNiMo6, 4340 and 1.6582.

What is the chemical composition of EN 24?

Composition limits in weight percent are carbon 0.35 to 0.45, manganese 0.45 to 0.7, sulphur 0.05 maximum, phosphorus 0.05 maximum, silicon 0.1 to 0.35, chromium 0.9 to 1.4, nickel 1.3 to 1.8 and molybdenum 0.2 to 0.35.

What is EN 24 equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

Mechanical properties of EN 24

EN 24 is specified by condition letter. Condition T is the common stock condition. Conditions X, Y and Z cover the high-strength range. These conditions combine higher strength with more demanding processing and design requirements.

The following BS 970 property bands are published by UK stockholders working to the standard. Each band applies only up to the maximum diameter associated with that condition.

From condition U to condition Z, tensile strength increases by roughly half while impact energy falls to about one fifth of its initial value. The specified condition therefore determines the balance between strength and toughness.

Heat treatment and hardenability of EN 24

Hardening uses slow heating to 830 to 850 degrees Celsius, soaking until the section is uniform, and oil quenching. The required condition determines the tempering temperature from a tempering curve. Tempering requires soaking for two hours per 25 mm of ruling section, followed by air cooling. Tempering between roughly 250 and 375 degrees Celsius seriously reduces impact value and is avoided. Conditions on either side of this range are produced with different tempering temperatures.

EN 24 contains 1.3 to 1.8 percent nickel in a chromium-molybdenum base. This addition delays transformation sufficiently for martensite to form at the centre of sections that chromium-molybdenum steel cannot through-harden. This hardenability distinguishes EN 24 from EN 19 in heavy shafts. Nickel also increases toughness in the tempered structure. At 1000 N/mm squared, EN 24 therefore has greater impact energy than an equivalent grade tempered to the same strength without nickel.

EN 24 and SAE 4340 compared

The following table gives the weight percent limits from the Laxcon Steels grade reference. The sulphur and phosphorus values are maximum limits.

SAE 4340 has a nickel minimum of 1.65 percent, which is above the midpoint of the EN 24 range. EN 24 has a chromium maximum of 1.4 percent, compared with 0.9 percent for SAE 4340. The grades achieve similar hardenability through different composition ranges. SAE 4340 has narrower limits for carbon, manganese, sulphur and phosphorus. Material satisfies both designations only when its certificate falls within both composition windows.

Designations for EN 24

EN 24 is the grade number in BS 970:1955. The later BS 970 designation is 817M40. The continental designation is 34CrNiMo6, with material number 1.6582, within the EN 10083-3 family of quenched and tempered alloy steels. The nearest SAE grade is 4340. In EN 24 T, the letter T identifies the BS 970 condition and is not part of the grade name.

Machining and welding behaviour of EN 24

EN 24 is machined in the pre-treated state in lower conditions and ground in higher conditions. From condition W upward, hardness makes milling and turning specialist operations. Splines, keyways, threads and similar features are normally cut before final heat treatment.

EN 24 is more difficult to weld than EN 19. Using the International Institute of Welding expression and the midpoints of the listed composition ranges, its carbon equivalent is about 0.88. This value is close to twice the 0.45 level at which preheating and low-hydrogen practice become obligatory. It also exceeds the values of the lower through-hardening grades. Welding is excluded in the higher-strength conditions because the heat-affected zone no longer retains the specified condition, even when the weld is correctly executed.

Applications of EN 24

EN 24 is used for heavily loaded rotating and reciprocating parts. Applications include crankshafts, connecting rods, main shafts, drive shafts, heavy gears, pinions, differential shafts, aircraft and vehicle structural components, drilling and downhole tooling, high-tensile studs and bolts, die blocks and holders. The grade is used for large sections in which chromium-molybdenum steel would remain soft at the centre. It is also used where both high strength and toughness are required.

Where EN 24 is not the right choice

EN 24 is unsuitable when a light section does not require its hardenability. A small part quenched and tempered in EN 19, or in EN 8 for genuinely light duty, can reach the same properties at the centre. In such cases, EN 24 adds cost without adding a property.

Conditions X, Y and Z require controls associated with notch sensitivity. Fillet radii, thread roots, keyway corners and tool marks can govern component life more strongly than tensile strength. The steel is also susceptible to hydrogen embrittlement. Acid pickling, electroplating and some phosphating processes can introduce hydrogen. Above roughly 1000 N/mm squared actual tensile strength, a post-coating bake is the accepted control. ASTM B850 specifies temperature and duration classes. A coated part shipped without this bake can fail under static load after hours or days and without warning.

EN 24 has no corrosion resistance, so wet service requires a coating. It is not a case-hardening steel. EN 36C or SAE 8620 are carburising grades for applications that require a hard, wear-resistant surface over a tough core.

Other alloy steel grades