EN8 vs EN19

EN8 and EN19 are the two most frequently specified forging steels in Indian engineering, and the choice between them is one of the most common material decisions a designer or buyer makes. It is also one of the most frequently made badly — usually by choosing on tensile strength figures alone, without considering section size.

The two steels contain almost the same amount of carbon, and carbon is what primarily determines how hard steel can become when quenched. So why do they behave so differently in practice?

When steel is hardened, it is heated until its structure becomes austenite and then cooled fast enough to transform into hard martensite before it can form softer structures such as ferrite and pearlite. The surface of a part cools quickly. The core of a thick part cools slowly, and in a plain carbon steel it cools too slowly to form martensite — so it ends up much softer than the surface.

Chromium and molybdenum slow down the formation of those softer structures. That gives the steel more time to transform to martensite, so even the slowly cooling core of a thick section can harden. This property — the depth to which a steel can be hardened — is called hardenability, and it is the whole difference between these two grades.

Molybdenum's second job

Molybdenum also reduces temper embrittlement, the loss of toughness some steels suffer when tempered in certain temperature ranges or cooled slowly through them. That helps EN19 keep useful toughness after tempering to high strength — one reason it is trusted for fatigue-critical and shock-loaded components.

This is the single most important point on this page. A steel's strength is not a fixed number — it depends on how thick the part is, because thicker parts cool more slowly during hardening. European standard EN 10083 publishes the achievable properties for each grade by diameter, and comparing C45 (the close counterpart of EN8) with 42CrMo4 (EN19) makes the difference very clear.

Two conclusions stand out. First, even in small sections 42CrMo4 reaches substantially higher strength than C45. Second — and more importantly for forgings — 42CrMo4 continues to deliver meaningful quenched and tempered strength in sections well beyond 100 mm, where plain carbon steel effectively stops responding to hardening at the core.

The hidden soft core

Here is the trap. A tensile test on a small coupon cut near the surface of a thick EN8 forging can show perfectly acceptable figures, while the centre of the part is significantly softer. The certificate looks correct, and the component fails early anyway. If a thick EN8 part must carry high load at its core, hardness should be surveyed across the section rather than taken from one convenient point.

A practical rule

• Up to roughly 40 mm, EN8 heat treats well and is usually the economical choice for moderate duty.
• Between roughly 40 and 60 mm, EN8 still serves for moderate loads; for demanding fatigue duty, EN19 becomes the safer choice.
• Above roughly 60 mm, where uniform properties through the section are needed, specify EN19.
• Above roughly 150 mm under severe load or shock, consider EN24, which hardens even larger sections.

Forgings are rarely used as-forged. The table shows the conditions in which each grade is most commonly supplied, with representative properties.

Property values vary with the exact composition of the heat, section size and heat treatment cycle, so these figures are for comparison rather than design. Hardness can be converted approximately to other scales with our hardness conversion calculator.

EN8

EN8 is most commonly supplied normalised, which refines grain and evens out the variation left by forging to give consistent hardness for machining. It can be hardened and tempered, but its low hardenability means the benefit is concentrated in thinner sections. Where wear resistance is needed, EN8 is very often induction hardened locally — raising a journal, a cam surface or a spline to around 55 HRC while the rest of the part stays tough and machinable.

EN19

EN19 is normally quenched in oil or polymer and tempered. The tempering temperature is chosen to land in the specified hardness band: lower tempering temperatures give higher strength and lower toughness, higher temperatures the reverse. EN19 also responds well to induction hardening and to nitriding, the latter producing a very hard, shallow case with little distortion on finished parts.

Our heat treatment guide and normalising versus annealing explain these cycles in more detail.

Machining

EN8 machines freely in the normalised condition, with predictable tool wear and good surface finish — one reason it remains so popular for general engineering parts. EN19 in the quenched and tempered condition at around 300 HB is noticeably harder on tooling, needs lower cutting speeds, and adds machining cost. Where extensive machining is needed, EN19 is sometimes rough machined in the annealed condition, then hardened and finish machined or ground.

Welding

Weldability is commonly estimated using the carbon equivalent, CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. A typical EN8 heat works out at roughly 0.5–0.6; a typical EN19 heat at around 0.8. The higher the value, the greater the risk of hard, crack-prone heat-affected zones.

• EN8 can be welded with preheat and controlled cooling, though it is not a naturally weld-friendly grade.
• EN19 is difficult to weld and needs careful preheat, controlled interpass temperature and post-weld heat treatment.
• On fatigue-loaded forgings, welding of either grade places a metallurgical discontinuity where the forging's advantage was meant to be. Design to avoid it. See can forgings be welded.

EN19 costs more than EN8 as raw material, is slower and more expensive to machine, and usually requires a quench and temper cycle rather than simple normalising. Together those differences can add materially to part cost.

That premium is money well spent when the part is thick, heavily loaded or fatigue-critical — the failure it prevents costs far more than the steel. It is money wasted on a light bracket, a lightly loaded pin or a 30 mm shaft at modest torque, where EN8 performs the job just as well. We regularly recommend customers step back from EN19 to EN8 after reviewing the actual load case, and step up from EN8 to EN19 when a section size makes EN8's apparent strength an illusion.

The principles become clearer applied to real decisions. The four examples below are illustrative engineering cases, not specific customer parts, and each shows the reasoning rather than just the answer.

Example 1 — a 35 mm clevis pin on a farm implement

The pin carries shear load through a bush and wears where it turns. The section is small, the load is moderate, and the failure that actually retires the part is wear at the bearing surface rather than fracture. EN8, induction hardened on the bearing zone, is the sensible choice: the surface resists wear at around 55 HRC, the core stays tough enough to bend rather than snap under a shock overload, and the part costs less and machines faster than it would in EN19.

Example 2 — an 85 mm axle shaft under reversing torque

Here the load is heavy, it reverses, and fatigue governs. At 85 mm the part sits in the 40–100 mm band of EN 10083, where quenched and tempered C45 reaches about 630–780 MPa while 42CrMo4 reaches about 900–1,100 MPa — and, just as importantly, 42CrMo4 develops those properties at the core. EN8 would leave a relatively soft centre under the most demanding load case. EN19, quenched and tempered to roughly 280–320 HB, with induction hardened bearing seats and splines, is the right specification.

Example 3 — a heavy diesel crankshaft with 140 mm webs

Sections this thick fall in the 100–160 mm band, where plain carbon steel is effectively beyond useful through-hardening and even 42CrMo4 is down to roughly 800–950 MPa. EN19 is the minimum, and for a highly rated engine or shock loading EN24 deserves serious consideration because its nickel keeps toughness and hardenability in larger sections. See EN19 versus EN24.

Example 4 — a lightly loaded 20 mm bracket welded to a frame

The loads are low, the section is thin, and the part must be welded in assembly. EN19 would add material cost, machining cost and a difficult welding procedure for no functional gain. EN8 in the normalised condition, welded with appropriate preheat, is the practical answer.

EN8 and EN19 are two steps on a wider ladder. If neither fits your part cleanly, one of their neighbours probably will.

These are the errors we see most often on drawings and purchase orders that call for EN8 or EN19.

1. 01Naming the grade without a condition. "EN19" alone does not say whether the part should be normalised, annealed or quenched and tempered. State the condition and a hardness or strength band.
2. 02Specifying strength that the section cannot reach. A high tensile requirement on a thick EN8 part may be met at the surface and missed at the core.
3. 03Measuring hardness at one convenient point. On thick parts, define where hardness is measured, or require a survey across the section.
4. 04Upgrading to EN19 for wear alone. If the problem is surface wear, induction hardening EN8 is often cheaper and just as effective.
5. 05Forgetting weldability. Specifying EN19 for a part that must be welded in assembly creates a difficult welding procedure that was never needed.
6. 06Accepting a certificate without verification. Grade substitution between these two steels is common precisely because they look identical. Spectro verification catches it.

Our forging drawing checklist covers the other items a complete forging specification needs.

Work through these questions in order.

1. 01What is the governing failure mode? Light static load favours EN8. Heavy fatigue or shock loading favours EN19.
2. 02What is the largest section that must carry load? Above roughly 60 mm needing uniform properties, choose EN19.
3. 03Does only a surface need to resist wear? If yes, consider EN8 with induction hardening rather than EN19 throughout.
4. 04Will the part be welded? If yes, EN8 is easier; if EN19 is required, plan preheat and post-weld heat treatment — or redesign to avoid welding.
5. 05How much machining is involved? Heavy machining favours EN8, or EN19 machined in the annealed condition before hardening.
6. 06Is cost the deciding factor on a lightly loaded part? Choose EN8.

Drawings from different countries name these steels differently. The designations below are close equivalents rather than identical specifications, since composition ranges differ slightly between standards.

Use our steel grade equivalents lookup to search other designations. Whatever the paperwork says, the only reliable confirmation of a grade is spectro analysis of the material itself, which we carry out on every incoming heat lot.