Common steel grades by AISI/SAE designation — type, typical application, hardness and machinability rating.
Data verified 2026-09-29 · based on n/a — the designation system is stable; compositions are revised periodically
Quick Answer
The AISI/SAE four-digit system encodes composition. The first digit is the major alloy type — 1xxx carbon, 4xxx chromium-molybdenum, 5xxx chromium, 8xxx nickel-chromium-molybdenum — and the last two digits approximate the carbon content in hundredths of a percent. So 4140 is a Cr-Mo steel with about 0.40% carbon.
Hardness and machinability are typical values for the usual condition of supply — annealed or normalised. Heat treatment changes both substantially. Machinability is on the AISI 1112 = 100% scale.
Reading the AISI/SAE Number
The four-digit designation is compositional, not arbitrary:
First digit — alloy type. 1 = carbon, 2 = nickel, 3 = nickel-chromium, 4 = molybdenum (usually with Cr), 5 = chromium, 6 = chromium-vanadium, 8 = nickel-chromium-molybdenum, 9 = nickel-chromium-molybdenum with higher Ni and Mo.
Second digit — approximate concentration of the major alloying element, or a sub-type indicator.
Last two digits — carbon content in hundredths of a percent. 4140 is 0.40% carbon; 8620 is 0.20% carbon.
The system breaks down for free-machining grades (11xx, 12xx), where the second digit signals the sulphur or lead addition rather than an alloy type, and for structural steels such as A36 and A572, which are specified by mechanical properties rather than composition.
Choosing Between the Families
Carbon steels are cheap and weldable, with strength limited to what the carbon content allows. Use them unless there is a reason not to.
Alloy steels buy hardenability and strength — the ability to harden through a thicker section — at the cost of machinability and price. 4140 is the default for shafts and gears that must be strong through their section.
Stainless steels buy corrosion resistance, with a large penalty in machinability and cost. Free-machining grades such as 303 and 416 recover some of the machinability by adding sulphur, at the cost of corrosion resistance and weldability.
Tool steels buy wear resistance and hot hardness. They machine in the annealed condition and are used hardened.
Frequently Asked Questions
What does 4140 mean in steel?
It is a chromium-molybdenum alloy steel with about 0.40% carbon. The first digit (4) indicates the Cr-Mo family, and the last two digits (40) give the carbon content in hundredths of a percent.
What is the difference between 1018 and 4140?
1018 is low-carbon plain steel — cheap, weldable, and not heat-treatable to useful hardness. 4140 is a chromium-molybdenum alloy steel at 0.40% carbon that hardens through thick sections to high strength. 4140 is roughly three times the price and about 30% harder to machine.
Which steel is easiest to machine?
12L14, at about 170% on the machinability scale, and 1215 at about 136%. Both are free-machining grades — the lead or sulphur additions break chips and act as a built-in lubricant. Neither is weldable.
What does the last two digits of a steel grade mean?
The approximate carbon content in hundredths of a percent: 1045 is 0.45% carbon, 4140 is 0.40%, 8620 is 0.20%. The convention holds for the 10xx, 11xx, 41xx, 43xx, 52xx and 86xx families.
Is A36 the same as 1018?
No, though both are low-carbon structural steels. A36 is specified by mechanical properties — a 36 ksi minimum yield — rather than by composition, so its chemistry varies between mills. 1018 is specified by composition. A36 is intended for structural work and is generally not used where a controlled chemistry is needed.
What is the hardest steel?
Tool steels reach the highest hardness: D2 and M2 harden to 60–62 HRC, and 440C stainless to about 60 HRC. Above roughly 45 HRC, machining is normally replaced by grinding.
Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.
#
Source
Type
Revision / method
[1]
AISI/SAE grade designations with typical properties
standard
n/a — designation system, with properties compiled from published data — source
Data Sources
Standard
Revision
What it covers on this page
SAE J403 / J404 — chemical compositions of SAE carbon and alloy steels
n/a — the designation system is stable; compositions are revised periodically
grade designations and their compositional meaning
Cross-checked against:
Designations and typical hardness values cross-checked against published steel reference charts
Machinability ratings cross-checked against the material data sheets on this site
Typical values for the usual condition of supply. Heat treatment, section size and mill practice all shift hardness and machinability — use the governing specification for design work.
Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.
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