1018, 4140, 304, 316 … with UNS numbers. Issued by SAE International. 18 grades and 5 standards are documented so far; each grade links to a full data sheet and a graded cross-reference to the other systems.
ASTM A240 — Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications
ASTM A276 — Standard Specification for Stainless Steel Bars and Shapes
ASTM A29 — Standard Specification for General Requirements for Steel Bars, Carbon and Alloy, Hot-Wrought
ASTM A295 — Standard Specification for High-Carbon Anti-Friction Bearing Steel
ASTM A564 — Standard Specification for Hot-Rolled and Cold-Finished Age-Hardening Stainless Steel Bars and Shapes
The AISI/SAE system for carbon and alloy steels is the most readable of all the designation schemes. The last two digits are the carbon content in hundredths of a per cent; the first two identify the alloy family. 1045 is plain carbon steel with 0.45 % C. 4140 is a chromium-molybdenum steel with 0.40 % C. 8620 is a nickel-chromium-molybdenum steel with 0.20 % C.
The family prefixes worth knowing: 10xx plain carbon, 11xx resulphurised free-cutting, 41xx chromium-molybdenum, 43xx nickel-chromium-molybdenum, 51xx chromium, 86xx triple-alloy, 92xx silicon-manganese spring steels.
The stainless numbering is a different scheme entirely
AISI stainless numbers do not follow the same logic. The 300 series is austenitic chromium-nickel, the 400 series is ferritic or martensitic chromium, and the 200 series substitutes manganese and nitrogen for part of the nickel. Within a series the numbers are serial, not descriptive: 304, 316 and 321 carry no arithmetic meaning.
Most of these designations are now formally superseded by UNS numbers, but the trade uses 304 and 316 daily and will continue to. The UNS equivalents — S30400, S31600 — are what appears on a modern certificate.
Where AISI and EN diverge
AISI 4140 and EN 42CrMo4 are called equivalent and are close, but the EN grade carries tighter limits on phosphorus, sulphur and residual elements, and EN specifies the delivery condition where AISI traditionally leaves it to the buyer.
For a machined part with a hardness requirement, that difference rarely matters. For a part that will be welded, or where fatigue life is critical, the residual limits are exactly what matters, and the grades should not be swapped without checking the certificate.
faq.h
What does 1045 mean in AISI?
The first two digits identify the family — 10 is plain carbon steel — and the last two give the carbon content in hundredths of a per cent, so 0.45 % carbon. The same logic gives 4140 as a chromium-molybdenum steel with 0.40 % carbon.
What is the EN equivalent of AISI 4140?
42CrMo4 (1.7225) is the standard near-equivalent. The EN grade has tighter residual-element limits and a specified delivery condition, so the two are interchangeable for many machined parts but not automatically for welded or fatigue-critical ones.
Why do AISI stainless numbers not follow the same rule?
Because the stainless series was numbered separately: 300 for austenitic chromium-nickel, 400 for ferritic and martensitic chromium, 200 for manganese-substituted austenitic. Within each series the numbers are serial rather than descriptive.
Is AISI still an active standard?
The American Iron and Steel Institute stopped maintaining the designations; SAE and UNS carry them forward. The names remain in universal trade use, and a modern certificate typically shows both the familiar number and the UNS designation.
What does the 11xx series mean?
Resulphurised free-cutting steel. Extra sulphur forms manganese sulphide inclusions that break the chip, which makes the steel far easier to machine at the cost of reduced ductility and poor weldability.