Short answer: A BS 970 code such as 817M40 describes the steel in six characters. The first three digits give the steel type or alloy family, the letter says how it is specified (A = analysis, M = mechanical properties, H = hardenability, S = stainless), and the last two digits are the carbon content in hundredths of a percent. So 817M40 (EN24) is a nickel-chromium-molybdenum steel with 0.40% carbon, supplied to mechanical properties. A letter after the code, as in 817M40T, is the heat-treated strength condition.
In this guide: How to read a code · The first three digits · The letter · Condition letters P to Z · EN to BS 970 table · BS 970 today · FAQ
How to read a BS 970 code
Take 817M40T, the full name for EN24T:
| Part | Example | What it means |
|---|---|---|
| First three digits | 817 | Steel type or alloy family: here, nickel-chromium-molybdenum |
| Letter | M | Supplied to specified mechanical properties |
| Last two digits | 40 | Nominal carbon: 0.40% |
| Condition (optional) | T | Hardened and tempered to 850–1000 MPa tensile |
These six-character codes replaced the old EN numbers (EN1 to EN362) in the 1970s. Indian buyers still use both, which is why drawings say "EN24" while mill certificates may say "817M40".
The first three digits: steel type
| Digits | Steel type | What the digits tell you |
|---|---|---|
| 000–199 | Carbon and carbon-manganese steels | Manganese content × 100 (080M40 = 0.80% Mn) |
| 200–240 | Free-cutting steels | Sulphur content × 100 (230M07 = about 0.30% S) |
| 250 | Silicon-manganese spring and valve steels | Family number |
| 300–499 | Stainless and heat-resisting steels | Family number (e.g. 304, 316, 410, 431) |
| 500–999 | Alloy steels | Family number for the alloy combination |
For alloy steels the number identifies a family rather than a measurement. A few families you will meet often:
- 6xx: nickel-chromium steels, such as 655M13 (EN36) and 637M17 (EN352).
- 7xx: chromium-molybdenum and chromium-vanadium steels, such as 709M40 (EN19) and 735A51 (EN47).
- 8xx: nickel-chromium-molybdenum steels, such as 817M40 (EN24), 815M17 (EN353), 805M20 (EN362 / SAE 8620) and 832M13 (EN36C).
- 9xx: other alloy steels, such as the nitriding steel 905M39 (EN41B).
The letter: how the steel is specified
- A (analysis): supplied to a chemical composition. Example: 735A51 (EN47), 535A99 (EN31).
- M (mechanical properties): supplied to guaranteed strength and other mechanical properties. Example: 817M40 (EN24), 080M40 (EN8).
- H (hardenability): supplied to a hardenability (Jominy) range, mainly for case-hardening and through-hardening steels where heat-treatment response matters.
- S (stainless): marks a stainless steel. Example: 304S15 (EN58E), 431S29 (EN57). In stainless codes the last two digits are a serial number, not the carbon content.
Condition letters P to Z: heat-treated strength
A letter added after the code gives the tensile strength range after hardening and tempering. This is where names like EN24T, EN19T and EN24V come from.
| Condition | Tensile strength, MPa |
|---|---|
| P | 550–700 |
| Q | 620–770 |
| R | 700–850 |
| S | 770–930 |
| T | 850–1000 |
| U | 930–1080 |
| V | 1000–1150 |
| W | 1080–1240 |
| X | 1150–1300 |
| Y | 1240–1400 |
| Z | 1540 min. |
Each condition is only achievable up to a certain bar size (the "limiting ruling section"), because thick bars do not harden as fully as thin ones. See our EN24 vs EN24T guide and hardness guide for what this means in practice.
EN number to BS 970 code table
Common grades used in India, with the six-character code that replaced each EN number. Grades in bold are ones we stock in Mumbai.
| EN number | BS 970 code | Type |
|---|---|---|
| EN1A | 230M07 | Free-cutting steel |
| EN3A | 070M20 | Low-carbon steel |
| EN8 | 080M40 | Medium-carbon steel |
| EN9 | 070M55 | Medium-high carbon steel |
| EN16 | 605M36 | Mn-Mo steel |
| EN18 | 530M40 | Chromium steel |
| EN19 | 709M40 | Cr-Mo steel |
| EN24 | 817M40 | Ni-Cr-Mo steel |
| EN26 | 826M40 | Ni-Cr-Mo steel |
| EN30B | 835M30 | Ni-Cr-Mo air-hardening steel |
| EN31 | 535A99 | Bearing steel |
| EN36 / EN36A / EN36B | 655M13 | Ni-Cr case-hardening |
| EN36C | 832M13 | Ni-Cr-Mo case-hardening |
| EN39B | 835M15 | Ni-Cr-Mo case-hardening |
| EN40B | 722M24 | Cr-Mo nitriding steel |
| EN41B | 905M39 | Cr-Al-Mo nitriding steel |
| EN45 | 250A53 | Si-Mn spring steel |
| EN47 | 735A51 / 735A50 | Cr-V spring steel |
| EN352 | 637M17 | Ni-Cr case-hardening |
| EN353 | 815M17 | Ni-Cr-Mo case-hardening |
| EN354 | 820M17 | Ni-Cr-Mo case-hardening |
| SAE 8620 / EN362 | 805M20 | Ni-Cr-Mo case-hardening (≈ SAE 8620) |
| EN56A | 410S21 | Stainless (martensitic) |
| EN57 | 431S29 | Stainless (martensitic) |
| EN58E | 304S15 | Stainless (austenitic) |
| EN58J | 316S16 | Stainless (austenitic) |
For European (DIN), AISI/SAE, JIS and Indian equivalents of these grades, see our equivalent grades chart.
BS 970 today
BS 970 has largely been replaced by European standards: EN 10083 for quenched-and-tempered steels such as 34CrNiMo6 (EN24), EN 10084 for case-hardening steels such as 20NiCrMo2-2 (SAE 8620), and EN 10089 for spring steels such as 51CrV4 (EN47). The old names are still the everyday language of the Indian steel trade, so a good supplier should recognise all three: the EN number, the BS 970 code and the European name.
Two things to watch for:
- "EN" means two different things. EN24 is an old BS 970 number; EN 10083 is a modern European standard. They are unrelated naming systems that happen to share two letters.
- One EN number can map to more than one code. For example, EN36A and EN36B both became 655M13, while EN36C became 832M13 because it contains molybdenum (see our EN36 guide). Check the full code on the drawing.
Stock in Mumbai
Sai International stocks EN24 / 817M40, EN36C / 832M13, EN353 / 815M17, SAE 8620 / 805M20 and EN47 / 735A51 as round bar at Darukhana, Mumbai, with EN47 also in square and flat. Minimum order is 100 kg, and every dispatch comes with a mill test certificate. Use our round bar weight calculator to work out weights before you order.
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BS 970 codes: frequently asked questions
What does 817M40 mean?
817M40 is the BS 970 code for EN24. 817 identifies the nickel-chromium-molybdenum alloy family, M means the steel is supplied to specified mechanical properties, and 40 is the nominal carbon content, 0.40%.
What do the letters A, M, H and S mean in BS 970 codes?
A means the steel is supplied to a chemical analysis, M to mechanical properties, H to a hardenability requirement, and S marks a stainless steel. For example, 735A51 (EN47) is supplied to analysis, while 817M40 (EN24) is supplied to mechanical properties.
What does the T in EN24T or 817M40T mean?
The letter after the code is the heat-treated condition. T means hardened and tempered to a tensile strength of 850–1000 MPa. Other letters run from P (550–700 MPa) up to Z (1540 MPa minimum).
What is the BS 970 code for EN36C, EN353 and EN47?
EN36C is 832M13, EN353 is 815M17 and EN47 is 735A51 (also listed as 735A50). Plain EN36, EN36A and EN36B are 655M13.
Is BS 970 still a current standard?
BS 970 has largely been replaced by European standards such as EN 10083 (quenched and tempered steels) and EN 10084 (case-hardening steels). Its EN numbers and six-character codes are still widely used on Indian drawings and purchase orders.
Are 735A50 and 735A51 different steels?
Both are BS 970 codes listed for EN47 chromium-vanadium spring steel, with a nominal carbon of 0.50% and 0.51% respectively. In the Indian market they are generally treated as the same grade. If your drawing names one specifically, check the carbon range on the mill test certificate.