BS 970 Chromium Molybdenum Tubing
BS 970 was the British standard for wrought steels for many decades, and its grade names still appear on older drawings. For chromium-molybdenum steel those names need translating into a current specification before tube can be ordered.
BS 970 and Its EN Successors
BS 970 is withdrawn for new designs in favour of the harmonised EN standard series:
- BS 970 708M40 → superseded by EN 10083 42CrMo4 / 1.7225 (closer to AISI 4140)
- BS 970 708M30 → approximated by EN 10083 30CrMo4 / 1.7228 (closer to AISI 4130 in carbon)
The AISI 4130 equivalent in Europe is 25CrMo4 (1.7218). The exact BS 970 grade on an old drawing should be identified before assuming it maps directly to 4130.
| BS 970 Grade | Approximate EN Equivalent | AISI Approximate |
|---|---|---|
| 708M40 | 42CrMo4 / 1.7225 | ~4140 |
| 708M30 | 30CrMo4 / 1.7228 | Between 4130 and 4140 |
| 25CrMo4 (EN) | 1.7218 | ~4130 |
| Note | Substitution requires engineering comparison of actual limits |
Reading a BS 970 Grade
BS 970 grades take the form of three digits, a letter and two digits, such as 708M40. The first three digits identify the steel type, with the 700 series covering chromium-molybdenum steels. The letter shows how the steel is supplied: A for analysis, M for mechanical properties and H for hardenability. The last two digits give the carbon content in hundredths of a percent, so 40 means about 0.40 percent.
BS 970 was primarily a bar standard and has been superseded by European standards. A drawing that quotes a 700-series grade with around 0.25 to 0.30 percent carbon is describing a steel of the 4130 and 25CrMo4 type; one ending in 40 is describing the higher-carbon 4140 type.
To order tube against such a drawing, agree a modern equivalent with whoever is responsible for the design, then specify the current grade and standard. See EN 25CrMo4 tubing and 4130 equivalent material.
Frequently Asked Questions
Can I use 4130 where a BS 970 drawing specifies 708M40?
No — 708M40 corresponds more closely to 4140 (higher carbon). Substituting 4130 for a 708M40 specification requires a formal engineering comparison and approval, as the strength levels achievable in equivalent conditions differ significantly.
What do the last two digits of a BS 970 grade mean?
The nominal carbon content in hundredths of a percent.
Is BS 970 still current?
It has been largely superseded by European standards, though its grade names remain on legacy drawings.
Which BS 970 series covers chromium-molybdenum steels?
The 700 series, for example 708 and 709 grades.
Was BS 970 a tube standard?
No. It was primarily a standard for bar and forging steels.
Enquire About BS 970 / EN Equivalent Tubing
Provide the BS 970 grade, the application, and whether EN equivalent material is acceptable for a procurement discussion.
- BS 970 grade
- Product form
- Dimensions
- Application
- Substitution approval status