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Mild Steel EN Grades: What Do the Numbers Mean?

An EN grade is a steel specification from BS 970, the British Standard that's classified engineering steel by composition and strength since the 1940s. "EN" originally stood for "Emergency Number," a designation system developed during the Second World War to standardise steel specifications and speed up the selection of materials for manufacturing components like guns and aircraft parts. Mild steel EN grades like EN8, EN1A and EN24 are still used constantly in the trade today, even though the formal numbering has technically moved on since.

Where the EN System Comes From and Why It's Still Used Understanding engineering steel grades starts with knowing where the EN system came from. The original EN designation system was introduced during the Second World War as a way to bring order to Britain's steel specifications and make sure manufacturers across the country were working to consistent, comparable standards. BS 970 was revised in 1955 to bring together close to 200 steel grades, with letters added as sub-categories of existing grades, which is where designations like EN1A and EN3B come from.

Where the EN System Comes From and Why It's Still Used

Understanding engineering steel grades starts with knowing where the EN system came from. The original EN designation system was introduced during the Second World War as a way to bring order to Britain's steel specifications and make sure manufacturers across the country were working to consistent, comparable standards. BS 970 was revised in 1955 to bring together close to 200 steel grades, with letters added as sub-categories of existing grades, which is where designations like EN1A and EN3B come from.

By 1970, the old EN numbers were formally replaced with a new coding system built around composition rather than a simple sequential number, so EN8 officially became 080M40, EN24 became 817M40, and so on. In practice, the old EN names never really went away. Suppliers, fabricators and engineers still ask for

By 1970, the old EN numbers were formally replaced with a new coding system built around composition rather than a simple sequential number, so EN8 officially became 080M40, EN24 became 817M40, and so on. In practice, the old EN names never really went away. Suppliers, fabricators and engineers still ask for "EN8" or "EN24" out of habit and clarity, so both the old EN number and the modern equivalent tend to appear side by side on spec sheets and product listings, including BM Steel's own. For the fundamentals of mild steel itself, composition, general properties, and the structural S-grades, see our guide to what mild steel is; this piece focuses specifically on decoding the EN system.

How to Read an EN Grade

The modern designation that replaced the old EN numbers follows a consistent format: three digits, a letter, then two more digits, for example 080M40

1. The First Three Digits

These identify the steel's basic composition family. 080 denotes a plain carbon-manganese steel, the same broad family both EN1A and EN8 belong to, while 817 (as in EN24's modern equivalent, 817M40) denotes an alloy steel containing nickel, chromium and molybdenum.

2. The Letter

This describes how the steel was tested and supplied. "M" means the steel has been mechanically tested and certified to match its chemical composition, while "A" means it's supplied to a chemical formula as an untested batch. EN8 and its modern equivalent, 080M40, are the "M" (tested) version; EN3B's modern equivalent, 080A15, is the "A" (untested batch) version of a similar family.

3. The Final Two Digits

These give the carbon content, expressed as a percentage multiplied by 100. 080M40 has a carbon content of roughly 0.40%; 080A15 has around 0.15%, a difference that has a real effect on hardenability and strength even though both belong to the same broad composition family.

Once you can read those three parts, you can make sense of almost any EN-equivalent code you come across, not just the ones covered here.

Common EN Grades Decoded

With the numbering system covered, here's how it plays out across three grades you'll see constantly: EN1A, EN8 and EN24. Each sits in a different composition family and suits a different job, with its own trade-offs around machinability, strength and weldability worth knowing before you specify.

EN1A
Free-Cutting Steel

EN1A steel is known as free-cutting or free-machining steel. Its modern equivalent is 230M07, a low-carbon steel with sulfur added specifically to improve machinability, and it's ideally suited to work on CNC or automatic machines, producing a clean surface finish, consistent chip formation, and long tool life over high-volume production runs. A leaded version, EN1A Leaded, adds a small amount of lead to improve machinability further still. Neither EN1A nor EN1A Leaded is suitable for welding.

EN8
General-Purpose Medium Carbon Steel

EN8's modern equivalent is 080M40, renamed from EN8 in 1970. The EN8 steel grade is an unalloyed medium carbon steel offering good tensile strength, excellent machinability and moderate wear resistance, which is why it turns up constantly across general engineering: spindles, axles, pins, shafts and a wide range of automotive components. EN8 is usually supplied untreated, though it can be surface-hardened by induction where extra wear resistance is needed on specific components, and it welds well in thinner sections, up to about 18mm, without preheating.

EN24
High-Strength Alloy Steel

EN24's modern equivalent is 817M40, and unlike EN1A and EN8, it isn't a plain carbon steel. It's an alloy steel containing nickel, chromium and molybdenum, which gives it significantly greater strength and toughness. It offers good ductility and shock resistance alongside a high resilience to wear, making it suitable for machined parts, gears and axles that face higher stress than EN8 can reliably handle. It's often supplied as EN24T, the "T" denoting a tempered, heat-treated condition, which is how BM Steel stocks it.

Choosing the Right Grade for Your Application

As a starting point: EN1A suits high-volume machined components where finish and machinability matter more than raw strength, especially on CNC or automatic lathes, and is available in BM Steel's bright round bar range. EN8 covers the vast majority of general engineering applications, shafts, pins, axles, and stressed components that need decent strength without the cost of an alloy steel, and is stocked in both metric and imperial sizes. EN24 is worth the extra cost specifically where a component faces higher stress or shock loading than EN8 can reliably take, gears and axles under real mechanical load being the classic case. Where a hot-rolled finish suits the job better than a bright, cold-drawn one, BM Steel's engineering black bar is stocked in these grades too.

Composition and heat treatment matter more here than in most structural specifying decisions, since these grades are chosen for how a specific component will be stressed, machined or hardened. Where a component's performance is critical, it's worth confirming the exact grade and condition with your supplier or engineer rather than assuming one EN grade is a drop-in substitute for another, even within the same broad family.

BM Steel's Engineering Steel Range

BM Steel's engineering steel range stocks EN1A, EN1A Leaded, EN3A, EN3B, EN8, EN32 and EN24T across bright round bar in both metric and imperial sizes. Whatever EN grade your drawing or spec calls for, our depot network can supply and cut it to the length you need.

FAQs about Mild Steel EN Grades

Is EN8 or EN9 the better choice?

It depends on what the component needs to do. EN9 (070M55) has a higher carbon content than EN8, roughly 0.50 to 0.60% compared to EN8's 0.36 to 0.44%, which gives it greater hardness and wear resistance once heat treated. That extra carbon comes at a cost, though: EN9 is harder to weld and generally needs more care in fabrication than EN8, which welds well in thinner sections without preheating. For general-purpose shafts, pins and stressed components, EN8 is usually the more practical choice. Where a component faces heavier wear, such as gears, cams or crankshafts, EN9's extra hardness can be worth the trade-off.

Which is harder: EN8 or EN24?

EN24. Where EN8 is a plain carbon steel, EN24 is an alloy steel containing nickel, chromium and molybdenum, and those alloying elements let it respond to heat treatment in a way plain carbon steel can't match. Once through-hardened and tempered, typically supplied as EN24T, it reaches a higher strength and hardness than EN8 can achieve even at its own upper limit, while still holding onto better toughness and shock resistance than a plain carbon steel of similar hardness. That's why EN24 is specified for components under real mechanical stress, gears and axles being the classic examples, where EN8 would be pushed past what it can reliably handle.

Is EN8 considered a mild steel?

Not technically, though it's often grouped with mild steel in everyday trade conversation. Mild steel is usually defined as low-carbon steel, under about 0.25% carbon, while EN8 sits at roughly 0.36 to 0.44%, which puts it in medium carbon steel territory. The distinction matters because that extra carbon is exactly what gives EN8 its greater strength compared with genuinely mild grades like EN1A or EN3B, along with reduced weldability. In practice, most suppliers and buyers use "mild steel" loosely to cover the whole family of general-purpose carbon and engineering steels, EN8 included, but if you're specifying by the strict definition, EN8 falls just outside it.

Which mild steel grade is the strongest?

Among the grades covered here, EN24T reaches the highest strength by a clear margin, once through-hardened and tempered it can significantly outperform EN8 or EN9 in both tensile strength and toughness, thanks to its nickel-chromium-molybdenum alloy content. It's worth noting this pushes EN24 outside the strict definition of "mild steel," but it's commonly sold and searched for alongside it, which is why it's worth knowing about if you're comparing options and strength is the priority. For components where a true low-carbon mild steel is required rather than a stronger alloy substitute, EN1A and EN3B are the more accurate fit, though neither is chosen for raw strength in the first place.

Posted by Craig Silvain
1st July 2026

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