An enquiry says "Material: EN8". Your stockist has C45 bright bar in the rack. The buyer's next drawing says "SAE 1045 or equivalent". Are these the same steel? Close, but not identical, and the gap matters when a drawing also calls for hardness, induction hardening or a test certificate.

What EN8 actually is

EN8 is an old British designation from BS 970:1955. Its later BS 970 name is 080M40. It is an unalloyed medium carbon steel, used for parts such as studs, bolts, axles, spindles and connecting rods.

The Smiths Metal Centres EN8 datasheet gives its composition as:

  • Carbon 0.36–0.44%
  • Silicon 0.10–0.40%
  • Manganese 0.60–1.00%
  • Phosphorus and sulphur 0.05% max each

In the "R" condition it lists tensile strength of 700–850 N/mm², 0.2% proof stress of 450 N/mm² min for sections up to 19 mm, and hardness of 201–255 HB. It can be flame or induction hardened.

What C45 is

C45 (material number 1.0503) is a European steel for quenching and tempering under EN 10083. The Saarstahl C45 sheet lists:

  • Carbon 0.42–0.50%
  • Manganese 0.50–0.80%
  • Phosphorus and sulphur under 0.045% each

Its listed equivalents include SAE 1045 and BS 080M46. In the quenched and tempered (+QT) condition, a section under 16 mm has tensile strength of 700–850 N/mm² and proof stress of 490 N/mm² min. In larger sections the minimum values drop. Between 40 and 100 mm the sheet gives 630–780 N/mm² tensile.

In India, the IS 1570 schedule for wrought steels uses a similar logic: the number after "C" shows carbon content, so C45 means roughly 0.45% carbon.

Why "equivalent" is not "identical"

Put the numbers side by side and the point is clear:

  • EN8 carbon range is 0.36–0.44%. C45 is 0.42–0.50%. They overlap only between 0.42 and 0.44.
  • The EN8 datasheet lists equivalents across a family, from C40 to C45 and SAE 1039 to 1045. That is a family, not a single match.
  • Mechanical properties depend on condition (as rolled, normalised, hardened and tempered) and section size, as much as on grade.
A grade name tells you the chemistry; only the condition and section size tell you the strength.

So C45 bar can often substitute for EN8, but a higher-carbon heat responds differently to induction hardening, and a drawing that asks for a minimum tensile in a 60 mm section needs checking against that section's values, not the small-bar values.

What to check before you quote or substitute

  • Grade and standard: Is it "EN8", "080M40", "C45 to EN 10083" or "IS 1570 C45"? Ask which standard governs if it is only a grade name.
  • Condition: Does the drawing say normalised, hardened and tempered, or as rolled? If it gives a hardness range, can the bar you buy meet it at that size?
  • Heat treatment notes: Induction hardening depth and surface hardness, and whether the customer expects you or the supplier to do it.
  • Certificate: Will the customer accept a stockist certificate, or ask for a mill test certificate traceable to the heat?
  • Substitution approval: For automotive and export customers, get written approval before switching grade, even if you consider it equivalent. For PPAP parts, a material change is a change that may need resubmission.

An illustrative example

For example, a drawing for a Ø32 mm pin says "Material EN8, hardness 201–255 HB". You plan to use C45 because it is in stock. The C45 carbon is likely to be higher, which helps hardness, but you still need the bar condition and a hardness check on the actual heat. A short email to the buyer, "We propose C45 to EN 10083 in place of EN8, hardness to be verified per drawing, please confirm", avoids a rejection later. (Illustrative scenario.)

How SpecReview helps

When you upload a drawing, SpecReview flags unclear material and heat-treatment notes as spec gaps to clarify before quoting, such as a grade with no condition or no standard. The quality plan it prepares then lists the material and hardness checks, so the certificate and test needs are known before the order is placed.