BS 4360 43A/50B, BS 970 080M40 …. Issued by BSI. 1 grades and 1 standards are documented so far; each grade links to a full data sheet and a graded cross-reference to the other systems.
British steel designations come from two families that work quite differently. BS 4360 covered weldable structural steels with names like 43A, 43C, 50B and 50D — the number is tensile strength in tons per square inch and the letter is the impact class. BS 970 covered engineering steels with six-character codes like 080M40 and 709M40.
Both were replaced by EN standards — BS 4360 by EN 10025, BS 970 by EN 10083 — and both still appear on drawings and in maintenance documentation across the UK, the Gulf and the Commonwealth.
Reading a BS 970 code
The six characters are structured. The first three digits give the steel family: 000–199 carbon and carbon-manganese, 200–240 free-cutting, 300–499 stainless, 500–999 alloy. The letter is the specification basis — M for mechanical properties, A for chemical analysis, H for hardenability. The last two digits are carbon content in hundredths of a per cent.
So 080M40 is a carbon-manganese steel specified on mechanical properties with 0.40 % carbon — the old designation for what is now C40E or AISI 1040. 709M40 is an alloy steel with 0.40 % carbon, equivalent to 42CrMo4 and AISI 4140.
As with DIN, the safest practice is to read the old code for what the designer wanted — strength, toughness, machinability — and quote the EN grade that delivers it, noting the substitution on the paperwork.
faq.h
What does 080M40 mean?
A BS 970 engineering steel: the first three digits place it in the carbon-manganese family, M means it is specified on mechanical properties, and 40 is the carbon content of 0.40 %. It corresponds to EN C40E and AISI 1040.
What is the EN equivalent of BS 4360 50D?
S355J2. The 50 was tensile strength in tons per square inch and D was the impact class, corresponding to testing at −20 °C.
What is 709M40 steel?
The BS 970 designation for a chromium-molybdenum engineering steel with 0.40 % carbon — equivalent to EN 42CrMo4 (1.7225) and AISI 4140.
Are BS steel standards still valid?
BS 4360 and BS 970 were withdrawn and replaced by EN 10025 and EN 10083. The designations remain in use on legacy drawings and maintenance records, and should be read as pointers to the EN grade rather than ordered against directly.
What does the M mean in a BS 970 code?
The steel is specified on mechanical properties. A means it is specified on chemical analysis and H on hardenability — the same nominal steel can appear with any of the three letters.