You have a bar in your hand marked 42CrMo4. Somewhere else the same material is called 1.7225, and in another catalogue AISI 4140. All three are the same steel. This article explains how those codes are read, what each digit means, and how the systems translate into one another.
Why is there more than one system?
Steel designation systems grew up in different countries, at different times, with different logic. In America the AISI (American Iron and Steel Institute) and SAE (Society of Automotive Engineers) built a four- or five-digit system. In Europe the German DIN system spread first, and later the EN system was standardised for the whole of Europe.
All three are still in daily use. An old machine manual will use DIN, a customer drawing EN, a supplier catalogue AISI. So it pays to recognise all three.
The AISI / SAE system: four digits
An AISI-SAE code is usually four digits, and the logic could not be simpler:
The alloy groups you will meet most often:
| Group | What it means | Example | Typical use |
|---|---|---|---|
| 10xx | Plain carbon steel | 1020 · 1040 · 1095 | General engineering, shafts, pins, springs |
| 11xx | Resulphurised, free machining | 1140 | Automatic lathes, volume parts |
| 41xx | Chromium-molybdenum | 4140 | Shafts, gears, die holders |
| 43xx | Nickel-chromium-molybdenum | 4340 | Heavily loaded shafts, aerospace parts |
| 46xx | Nickel-molybdenum | 4620 | Case-hardening gears |
| 52xxx | High chromium | 52100 | Bearing rings and balls |
| 86xx | Nickel-chromium-molybdenum (low) | 8620 | Case-hardening gears and shafts |
| 92xx | Silicon-manganese | 9260 | Leaf and coil springs |
A few side by side – notice how the last two digits track the carbon:
| Code | %C | Other main elements | What it is good for |
|---|---|---|---|
| 1020 | 0.18-0.23 | Mn 0.30-0.60 | Low carbon: welds well, will not through-harden, suits case hardening |
| 1040 | 0.37-0.44 | Mn 0.60-0.90 | Medium carbon: can be quenched and hardened |
| 1095 | 0.90-1.03 | Mn 0.30-0.50 | High carbon: springs and cutting edges |
| 1140 | 0.37-0.44 | Mn 0.70-1.00 · S 0.08-0.13 | Same carbon as 1040 but with sulphur added: chips break, easy to machine |
| 4140 | 0.38-0.43 | Cr 0.80-1.10 · Mo 0.15-0.25 | The alloy lets it harden deeper |
| 4340 | 0.38-0.43 | Ni 1.65-2.00 · Cr 0.70-0.90 · Mo | Same carbon as 4140, but nickel adds toughness |
| 52100 | 0.98-1.10 | Cr 1.30-1.60 | Bearing steel – five digits, because carbon passes 1% |
| 8620 | 0.18-0.23 | Ni 0.40-0.70 · Cr 0.40-0.60 · Mo | The classic case-hardening steel |
| 9260 | 0.56-0.64 | Si 1.80-2.20 | Spring steel – silicon raises elasticity |
The European system 1: material number (1.xxxx)
Under EN 10027-2 every steel carries a material number. The leading 1. simply says “this is a steel”. The next two digits give the family:
| Number range | Steel family | Familiar example |
|---|---|---|
| 1.0xxx – 1.1xxx | Plain and special carbon steels | 1.0503 (C45) |
| 1.2xxx | Tool steels | 1.2344 (H13) · 1.2379 · 1.2714 |
| 1.3xxx | Special steels (bearing, high speed and so on) | 1.3505 (100Cr6) |
| 1.4xxx | Stainless and heat-resisting steels | 1.4301 (304) · 1.4404 (316L) |
| 1.5xxx – 1.8xxx | Structural, pressure vessel and engineering steels | 1.7225 (42CrMo4) |
This is the most useful shortcut on the shop floor: if it starts 1.2 it is a tool steel, if it starts 1.4 it is stainless. One glance at the number on the bar tells you what kind of material you are dealing with.
The European system 2: material name (C45, 42CrMo4, X5CrNi18-10)
The EN 10027-1 naming system splits into three cases.
a) Non-alloy steels – start with C
C45 – the letter C says non-alloy, and 45 is one hundred times the carbon content, so 0.45% carbon. The AISI equivalent is 1045.
b) Low-alloy steels – start with a number
Here the code begins with one hundred times the carbon, then the symbols of the alloying elements, then numbers giving how much of each. Those numbers are not the percentage directly; each group of elements has its own dividing factor:
| Factor | Elements |
|---|---|
| 4 | Cr, Ni, Mn, Si, Co, W |
| 10 | Al, Be, Cu, Mo, Nb, Pb, Ta, Ti, V, Zr |
| 100 | Ce, N, P, S |
| 1000 | B |
Take 42CrMo4 apart:
- 42 – one hundred times the carbon, so 0.42% C
- Cr and Mo – the main alloying elements are chromium and molybdenum
- 4 – the amount of the first element, chromium. Chromium has a factor of 4, so 4 ÷ 4 = 1.0% chromium
Compare that with the composition of AISI 4140: 0.38-0.43% C and 0.80-1.10% Cr. It matches – because 42CrMo4 = 1.7225 = AISI 4140 is one and the same steel.
c) High-alloy steels – start with X
If any alloying element exceeds 5%, the code gets an X in front and no factors are used; the numbers are straight percentages.
X5CrNi18-10 means 0.05% carbon, 18% chromium, 10% nickel – the familiar 304 grade stainless (1.4301).
X40CrMoV5-1 means 0.40% carbon, 5% chromium, 1% molybdenum – the hot-work tool steel H13 (1.2344).
The three systems side by side
| EN name | Material no. | AISI / ASTM | What it is used for |
|---|---|---|---|
| C45 | 1.0503 | 1045 | General machine parts, shafts |
| 42CrMo4 | 1.7225 | 4140 | Loaded shafts, gears, die plates |
| 100Cr6 | 1.3505 | 52100 | Bearing components |
| X40CrMoV5-1 | 1.2344 | H13 | Hot-work dies, injection cores |
| X37CrMoV5-1 | 1.2343 | H11 | Hot-work dies, extrusion |
| 55NiCrMoV7 | 1.2714 | L6 | Forging dies |
| X5CrNi18-10 | 1.4301 | 304 | General purpose stainless |
Traps to watch for
- A code is not a guarantee. What is stamped on the bar and what is inside it are not always the same. For critical work ask for a material certificate (3.1 inspection document).
- “Equivalent” does not mean identical. 42CrMo4 and 4140 are very close, but the tolerance bands may not overlap exactly. On precision work, compare the composition ranges in both standards.
- The same code can arrive in different delivery conditions. 42CrMo4 is sold annealed (soft) or quenched and tempered (+QT). Their hardness and machinability differ considerably.
- Old DIN names are still around. Ck45 is now C45; St37 has become S235JR. Older books will still use the old names.
Frequently asked questions
What does 1.2344 mean?
It starts 1.2, so it is a tool steel. Its name is X40CrMoV5-1 and its AISI equivalent is H13. It is a hot-work tool steel with 0.40% carbon, 5% chromium and 1% molybdenum, used for hot forging, injection moulding and die-casting tools. Type that code into the Steel Grade Cross-Reference tool to see its JIS and AFNOR equivalents, composition and heat treatment data as well.
I do not know what my material is – how can I find out?
The certain method is spectrometer analysis. For a rough idea there are three approaches: a hardness reading hints at the carbon level; the shape of the spark on a grinding wheel changes with carbon content; and if a magnet does not stick, austenitic stainless is likely. None of these replaces a certificate.
Why does one of two steels with the same carbon harden deeper?
Carbon decides how hard it can get; the alloying elements decide how deep that hardness reaches. 1040 and 4140 carry almost the same carbon, but the chromium and molybdenum in 4140 let the core of a thick part harden too.
What do codes with an S in them mean?
They indicate added sulphur. Sulphur breaks the chip and makes machining easier, at the cost of weldability and transverse toughness. The AISI 11xx series and EN grades such as 11SMn30 belong to this group.