EN 19

What is EN 19?

EN 19 is a one percent chromium-molybdenum medium-carbon alloy steel. It is supplied hardened and tempered to a specified condition. It is a through-hardening engineering steel for medium and heavy sections in which plain carbon steel would harden only near the surface. Chromium and molybdenum increase hardenability rather than hardness. Molybdenum also suppresses temper embrittlement and preserves a useful tempering range.

EN 19 and SAE 4140 are near-equivalent grades that are commonly treated as interchangeable. They remain separate specifications. EN 19 has a higher minimum molybdenum content, a wider carbon range, and less restrictive sulphur and phosphorus limits. It is purchased to a mechanical property condition. SAE 4140 is purchased to a chemical analysis.

709M40 is the BS 970 designation for EN 19. The 700 to 799 range identifies the chromium-molybdenum alloy series. The letter M indicates supply to a mechanical property specification. The number 40 represents the mean carbon content multiplied by one hundred.

Laxcon Steels lists EN 19 in its grades reference as an alloy steel grade. The same steel is written EN19, 709M40, 42CrMo4, 4140 and 1.7225.

What is the chemical composition of EN 19?

Composition limits in weight percent are carbon 0.36 to 0.44, manganese 0.7 to 1.0, sulphur 0.05 maximum, phosphorus 0.05 maximum, silicon 0.1 to 0.35, chromium 0.9 to 1.2 and molybdenum 0.25 to 0.35.

What is EN 19 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 19

BS 970 specifies several mechanical property conditions for EN 19 rather than one strength value. Each condition defines a tensile strength range with corresponding minimum yield strength, elongation, impact value, and Brinell hardness. Each set of values applies only up to its limiting ruling section. Condition T has a tensile strength of 850 to 1000 N/mm squared. It is the condition commonly specified for bar and is the basis of the designation EN 19 T.

The following BS 970 property bands are published by UK stockholders working to the standard. Each row remains subject to the limiting ruling section for its condition.

The limiting ruling section determines whether the specified condition applies at the centre of a bar. The centre of a large bar cools more slowly than the centre of a small bar. A heat treatment that produces condition T in a 60 mm bar can leave a 200 mm bar below the required properties. The standard therefore assigns a maximum diameter to each property band.

Heat treatment and hardenability

Annealed EN 19 is heated slowly to 860 to 890 degrees Celsius. It is soaked until the section reaches a uniform temperature and then quenched in oil. Tempering follows at a temperature selected from the tempering curve for the required condition. The section is soaked for two hours per 25 mm of ruling section and cooled in air.

Tempering between roughly 250 and 375 degrees Celsius seriously reduces the impact value. This effect is known as tempered martensite embrittlement. Stockholder heat-treatment guidance identifies this range as unsuitable. A component tempered within the range can satisfy its hardness specification while failing a Charpy test.

Chromium and molybdenum delay the diffusion-controlled transformations that compete with martensite formation during quenching. Martensite can therefore form at cooling rates that plain carbon steel cannot attain at the centre of the same section. Molybdenum also suppresses temper embrittlement during long holds and slow cooling through the critical range. This permits higher tempering temperatures and improved toughness at a given strength.

Comparison with SAE 4140

EN 19 and SAE 4140 are one percent chromium, molybdenum-bearing, 0.4 percent carbon through-hardening steels. Components specified in one grade are routinely produced from the other, subject to confirmation of the applicable specification.

The following composition limits are expressed in weight percent and come from the Laxcon Steels grade reference. Sulphur and phosphorus values are maximums.

The minimum molybdenum content of EN 19 is 0.25 percent, which is the upper limit for SAE 4140. A bar can therefore comply with SAE 4140 while remaining below the EN 19 minimum. The EN 19 chromium range is also higher by 0.1 percent at both limits. SAE 4140 has tighter sulphur and phosphorus limits. EN 19 is consequently slightly more hardenable, while SAE 4140 has lower permitted sulphur and phosphorus. Substitution requires a certificate that satisfies both composition ranges and cannot be applied as a general rule.

Grade designations

EN 19 is the designation from BS 970:1955. A later edition designated the same steel as 709M40. Its continental designation is 42CrMo4, with material number 1.7225. It is standardised within the EN 10083-3 family of quenched and tempered alloy steels. The nearest SAE grades are 4140 and 4142. SAE 4142 has the same chemistry with a higher carbon range. The grade reference also lists EN 19 A, EN 19 B, and EN 19 C. All three reduce the manganese range to 0.5 to 0.8 percent and lower the molybdenum minimum to 0.20 percent, which removes the margin over SAE 4140 described above. EN 19 B and EN 19 C divide the carbon range, at 0.35 to 0.40 and 0.40 to 0.45 percent respectively, while EN 19 A widens it to 0.35 to 0.45 percent.

Machining and welding

EN 19 is normally machined in the annealed or pre-treated condition when the required tolerance permits. A fully treated bar places greater demands on tooling because of its hardenability. Machining after heat treatment requires sharp tool geometry, rigid work holding, and moderate cutting speeds. Heavy interrupted cuts on treated bar can break inserts.

Welding requires a controlled procedure. The International Institute of Welding carbon equivalent, calculated from the midpoints of the composition limits above, is about 0.81. This is roughly twice the conventional value of 0.45 at which preheating, low-hydrogen consumables, and interpass control become obligatory. The heat-affected zone transforms to hard martensite during cooling. It can crack without preheating and a post-weld temper. Even with these measures, the weld zone no longer retains the purchased condition of the bar.

Applications

EN 19 is used for shafts, axles, spindles, connecting rods, gears, gear shafts, tool holders, die holders, drill collars, downhole components, machine-tool structures, high-tensile bolting, and general heavy machined components. ASTM A193 Grade B7 specifies chromium-molybdenum steel of this type in the quenched and tempered condition for high-temperature bolting in flanged joints. Permitted grades include 4140, 4142, and 4145, together with their hardenability-controlled H versions.

Where EN 19 is not the right choice

EN 19 is unsuitable when the section exceeds its hardenability limit. In very large diameters, the centre can fail to reach the condition achieved at the surface. EN 24 is used for sections beyond this range. It adds 1.3 to 1.8 percent nickel to a similar chromium-molybdenum base. It hardens through sections that EN 19 cannot reach and provides better toughness at the same strength.

EN 19 is also unsuitable when the component does not require increased hardenability. In a light section, a plain carbon grade such as EN 8 can achieve the same central properties at lower cost. The alloy content then provides no additional required property.

EN 19 has no corrosion resistance. Wet or humid service therefore requires coating or plating. Where the environment controls the material requirement, martensitic stainless steel is the relevant steel family. At the high end of the tempering range, EN 19 becomes notch sensitive. It is also sensitive to hydrogen introduced during pickling or electroplating. Surface-treatment procedures and post-plating bake-out must therefore form part of the specification.

Other alloy steel grades