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.

Steel Grades Chart — Carbon, Alloy, Stainless and Tool Steel

Sources and scope for this page: SAE J403 / J404 — chemical compositions of SAE carbon and alloy steels (the designation system is stable; compositions are revised periodically), covering grade designations and their compositional meaning. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. Full sources, values and limits on this page.

Page last changed 2026-10-19 · 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.

Find a grade

Common Steel Grades

Grade^[1]	Type^[1]	Typical use^[1]	Hardness^[1]	Machinability^[1]
1018 #	Carbon steel	Low carbon, general purpose	126 HB	78 %
1020 #	Carbon steel	Low carbon, case hardenable	126 HB	72 %
1045 #	Carbon steel	Medium carbon, heat treatable	190 HB	57 %
1095 #	Carbon steel	High carbon, spring and blade	230 HB	45 %
1112 #	Carbon steel	Free-machining reference grade	170 HB	100 %
1141 #	Carbon steel	Free-machining medium carbon	210 HB	72 %
1215 #	Carbon steel	Free-machining, resulphurised	160 HB	136 %
12L14 #	Carbon steel	Leaded free-machining	130 HB	170 %
4130 #	Alloy steel	Cr-Mo, aircraft tubing	200 HB	70 %
4140 #	Alloy steel	Cr-Mo, shafts and gears	215 HB	56 %
4340 #	Alloy steel	Ni-Cr-Mo, high strength	240 HB	50 %
4620 #	Alloy steel	Ni-Mo, carburising	180 HB	66 %
52100 #	Alloy steel	Bearing steel, high carbon	210 HB	40 %
6150 #	Alloy steel	Cr-V, spring steel	220 HB	52 %
8620 #	Alloy steel	Ni-Cr-Mo, carburising	180 HB	66 %
9310 #	Alloy steel	Ni-Cr-Mo, high hardenability	220 HB	58 %
A36 #	Structural	Structural carbon steel	120 HB	72 %
A572 Gr50 #	Structural	High-strength low-alloy	150 HB	65 %
303 #	Stainless	Austenitic, free-machining	180 HB	78 %
304 #	Stainless	Austenitic, general purpose	185 HB	45 %
316 #	Stainless	Austenitic, Mo-bearing	190 HB	36 %
321 #	Stainless	Austenitic, Ti-stabilised	185 HB	36 %
410 #	Stainless	Martensitic	160 HB	55 %
416 #	Stainless	Martensitic, free-machining	180 HB	90 %
420 #	Stainless	Martensitic, cutlery	200 HB	45 %
440C #	Stainless	Martensitic, high carbon	230 HB	40 %
17-4 PH #	Stainless	Precipitation hardening	330 HB	48 %
O1 #	Tool steel	Oil hardening, cold work	200 HB	42 %
A2 #	Tool steel	Air hardening, cold work	220 HB	42 %
D2 #	Tool steel	High chromium, cold work	230 HB	38 %
H13 #	Tool steel	Hot work	210 HB	45 %
M2 #	Tool steel	Molybdenum high speed	230 HB	39 %
S7 #	Tool steel	Shock resisting	200 HB	45 %

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.

Value Sources

References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

#	Source	Type	Revision / method
[1]	AISI/SAE grade designations with typical properties	reference	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

Verification scope

No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.

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. Use the source notes and recorded verification scope above to assess each value. Calculated values follow the stated derivations; compilation values may lack a confirmed original document or product condition. 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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Plain text

Engineering Reference. "Steel Grades Chart." https://refmanual.com/charts/steel-grades/ (accessed 2026-11-09). Data from SAE J403 / J404 — chemical compositions of SAE carbon and alloy steels, n/a — the designation system is stable; compositions are revised periodically.

BibTeX

@misc{steel-grades,
  title        = {Steel Grades Chart},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/steel-grades/}},
  note         = {Accessed: 2026-11-09},
  source       = {SAE J403 / J404 — chemical compositions of SAE carbon and alloy steels, n/a — the designation system is stable; compositions are revised periodically}
}

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TY  - ELEC
TI  - Steel Grades Chart
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/steel-grades/
PY  - 2026
DA  - 2026-11-09
N1  - Source: SAE J403 / J404 — chemical compositions of SAE carbon and alloy steels, n/a — the designation system is stable; compositions are revised periodically
ER  -

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