BS 970 — Wrought steels for mechanical and allied engineering purposes
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Scope
The British engineering steel standard: carbon, carbon-manganese, alloy and stainless wrought steels supplied as bar, billet, forging stock and rod. Part 1 sets the six-character designation system (three digits for the steel group, a letter for the requirement type, two digits for nominal carbon), the delivery conditions R to W by tensile band, and the limiting ruling section principle by which mechanical properties are guaranteed. It replaced the numbered En series of BS 970:1955, which is still the name most engineers use.
Grades covered by BS 970
| Grade | No. | Type | C % | Yield MPa | Tensile MPa | Nearest equivalents |
|---|---|---|---|---|---|---|
| 080M40 | — | Unalloyed medium-carbon engineering steel (British standard) | 0.36 – 0.44 | 280 | 550–700 | c45, 1045, 42crmo4 |
Revision history and superseded standards
| Year | Change |
|---|---|
| 1955 | En-number series — En1 to En363 — defined for the first time |
| 1970 | Six-character designation introduced: 080M40 replaced En8 |
| 1983 | Restructured into six parts |
| 1991 | Part 1 current edition; Parts 2 and 3 withdrawn in favour of EN standards |
Supersedes: BS 970:1955 (the En series: En8, En19, En24, En31…); BS 970:1970 / :1983 Parts 1–6.
Where to buy the standard
The full text is copyrighted by BSI (British Standards Institution) and is not reproduced here. Purchase the current edition from BSI Group or your national standards body. The values on this site are the normative limits as published; the standard's own tables, notes and test methods govern.
Related standards
Six characters that say what the bar is
BS 970 replaced the wartime En series — En8, En19, En24, En31 — with a six-character code, and the code is readable once the pattern is known. 080M40: the first three digits place the steel in a group (080 is carbon-manganese with about 1 % manganese), the letter states what the bar is supplied to, and the last two digits are nominal carbon in hundredths of a per cent.
The letter is the part that gets missed. M means supplied to mechanical property requirements in a stated condition; A means supplied to chemical analysis only, with no guaranteed strength; H means supplied to a hardenability band. 080M40 and 080A40 are the same steel and different purchases.
Limiting ruling section, and the EN that is not EN
BS 970 does not guarantee a property for a grade; it guarantees a property for a grade in a stated maximum diameter. Conditions R, S, T, U, V and W each name a tensile band, and each band is valid only up to its limiting ruling section — because the core of a thick bar cannot be cooled fast enough to reach it.
A separate warning belongs on every page that mentions this standard: the EN in En8 stands for Emergency Number, from the British wartime specification series. It has no connection to EN 10025, EN 10083 or any CEN document, and the coincidence has caused a great many mis-specified orders.