Carbon steel grades
5 carbon steel grades from 4 designation systems, each with a full data sheet. Sorted by system; the yield and tensile values are the first line of each grade's mechanical table.
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BS (superseded British)
| Grade | No. | Type | C % | Yield MPa | Tensile MPa | Equivalents |
|---|---|---|---|---|---|---|
| 080M40 | — | Unalloyed medium-carbon engineering steel (British standard) | 0.36 – 0.44 | 280 | 550–700 | c45, 1045, 42crmo4 |
ASTM / ASME
| Grade | No. | Type | C % | Yield MPa | Tensile MPa | Equivalents |
|---|---|---|---|---|---|---|
| SAE 1010 | 1.0301 | Low-carbon steel for cold forming, welding and case hardening | 0.08 – 0.13 | 180 | 325 | C10, S10C, 1008 |
AISI / SAE
EN (European)
| Grade | No. | Type | C % | Yield MPa | Tensile MPa | Equivalents |
|---|---|---|---|---|---|---|
| C45 | 1.0503 | Medium-carbon steel for quenching and tempering | 0.42 – 0.5 | 340 | ≥ 620 | 1045, S45C, 45 |
Carbon content is the whole story
Plain carbon steels are classified by how much carbon they carry, because carbon alone sets the hardness, the strength and the weldability. Low carbon below 0.25 % welds freely and forms well but cannot be hardened usefully. Medium carbon from 0.25 to 0.55 % — the C45, 1045, S45C family — hardens by quenching and is the workhorse of machined parts. High carbon above 0.55 % goes into springs, blades and wear surfaces and is difficult to weld.
The practical dividing line for a fabricator is 0.25 %: below it, weld without ceremony; above it, preheat and control the cooling or expect cracking in the heat-affected zone.
Why the same grade has four names
C45 (EN), 1045 (AISI/SAE), S45C (JIS) and 45# (GB) are the same idea expressed four ways, and the numbers are not coincidental: in each system the digits are the carbon content in hundredths of a per cent. 0.45 % carbon gives C45, 1045, S45C and 45#.
The differences are in the tolerances around that number and in what else is controlled. EN grades carry tighter manganese and residual limits than the older AISI grades, and the EN quenched-and-tempered condition is specified where AISI often leaves it to the buyer.
Delivery condition changes everything
A carbon steel bar is a different product depending on how it arrives. As-rolled is the cheapest and the least consistent. Normalised (+N) refines the grain and evens out the properties. Quenched and tempered (+QT) is where medium-carbon steel earns its keep — the same C45 can run from 600 MPa tensile as-rolled to over 900 MPa quenched and tempered.
Order the condition, not just the grade. A certificate for C45 tells you nothing about strength unless it also names the heat treatment.
faq.h
What is the carbon content of C45 steel?
About 0.42 to 0.50 %, which is what the number in the designation means — carbon in hundredths of a per cent. The same convention gives 1045 in AISI, S45C in JIS and 45# in GB.
Can carbon steel be welded?
Below about 0.25 % carbon, freely and without preheat. Above that the heat-affected zone hardens as it cools and can crack, so preheat, controlled interpass temperature and low-hydrogen consumables become necessary.
What is the difference between C45 and 1045?
They are near-equivalent medium-carbon grades with the same nominal carbon. EN C45 carries tighter limits on manganese and residual elements, and EN specifies the delivery condition more explicitly than AISI 1045 traditionally does.
Is mild steel the same as low-carbon steel?
In ordinary use, yes — mild steel is a trade term for low-carbon structural steel around 0.15 to 0.25 % carbon, typically S235 or S275 in EN terms. It is not a standard designation and should not appear on an order.
How hard can carbon steel get?
Attainable hardness follows carbon: about 55 HRC at 0.45 %, 60 HRC at 0.60 % and above. Hardenability — how deep that hardness reaches — depends on alloying, which is why thick sections need an alloy grade rather than more carbon.