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Grade Comparison

708M40 vs 709M40 — What's the Difference?

It's one of the questions we're asked most. 708M40 and 709M40 are both BS 970 designations for EN19 chromium-molybdenum steel — and they are identical in every element but one. The single difference is molybdenum: 708M40 is 0.15–0.25%, 709M40 is 0.25–0.35%. Here's what that actually means in practice, and when to specify each.

708M40 Data Sheet 709M40 Data Sheet Get a Quote
Side by Side

Comparison Table.

Property708M40 (EN19)709M40 (EN19)
Carbon (C)0.36–0.44%0.36–0.44%
Silicon (Si)0.10–0.35%0.10–0.35%
Manganese (Mn)0.70–1.00%0.70–1.00%
Chromium (Cr)0.90–1.20%0.90–1.20%
Molybdenum (Mo)0.15–0.25%0.25–0.35% — the only difference
P max / S max0.035 / 0.0400.035 / 0.040
Tensile, H&T “T” condition850–1000 N/mm²850–1000 N/mm²
Hardness, “T” condition248–302 HB248–302 HB
Through-hardening sectionTo ~65mmDeeper — higher Mo aids large sections
WeldabilityLimited — preheat & PWHTLimited — preheat & PWHT
European equivalent42CrMo4 / 1.722542CrMo4 / 1.7225
AISI/SAE equivalent4140 / 41424140 / 4142
Old EN numberEN19EN19
Bar-end colour codeYellow / WhiteYellow / White
3.1 cert availableYesYes
What the Molybdenum Does

One Element, One Practical Effect.

Molybdenum is the element that controls hardenability — how deeply a steel will harden right through its cross-section during quenching, rather than just at the surface. 709M40's higher molybdenum (0.25–0.35% vs 0.15–0.25%) shifts the hardenability curve slightly, so a large bar quenched in 709M40 reaches full hardness deeper toward its core than the same bar in 708M40.

In plain terms: on a 50mm shaft, you would struggle to tell 708M40 and 709M40 apart — both through-harden comfortably. On a 90mm+ section that needs full-section properties, 709M40's extra molybdenum gives a measurable edge. For everything else, they behave the same and meet the same EN19 mechanical bands.

This is why 709M40 is often treated as the slightly higher-specification, modern-preferred EN19 number, while 708M40 remains entirely standard for the vast majority of shafts, gears, studs and machined components. Both are EN19 / 42CrMo4 / AISI 4140; both quench-harden and temper to the same strength.

When to Choose Each

Making the Right Call.

Choose 708M40 when:
  • General high-tensile shafts, gears, studs and components
  • Section sizes up to ~65mm where full through-hardening is easy
  • The drawing calls out 708M40 specifically
  • You simply need EN19 / 42CrMo4 / 4140 from stock
  • Cost and availability are the priority (it's the more common number)
Choose 709M40 when:
  • Larger sections (roughly 90mm+) needing full-section hardness
  • More demanding heat treatment or deeper hardenability is specified
  • The drawing calls out 709M40 specifically
  • You want the marginally higher-spec, modern-preferred EN19 number
  • Heavy-duty crankshafts, large gears and big drivetrain parts

If your drawing doesn't specify which, either grade is acceptable for most work — tell us the application and section size and we'll supply the right one. For the step up to a nickel-bearing alloy, see EN19 vs EN24T; for the step down to plain carbon, see EN8 vs EN19.

Both Grades in Stock.

708M40 and 709M40 EN19 bright and black bar from stock at Sherwood Park. Cut to length from one piece. 3.1 certs available on both.

708M40 Data Sheet 709M40 Data Sheet ► Get a Quote ►