Understanding the Carbon Steel Grade System

Engineering handbooks point out that the classification of steels comes under the authority of the Society of Automotive Engineers (S.A.E.) or American Iron and Steel Institute (A.I.S.I.) for its coding system. The system of coding was originated in the early 1940’s and it was done with the expressed intention of giving engineers, designers, draftsmen, or heat treaters specific information on the types and grades of steel available. The system is very effective for steels using the A.I.S.I. and S.A.E. standards.

The S.A.E. or A.I.S.I. classifications contains 4 to 5 numeric characters with additional alpha characters added to designate special characteristics of the steel. Here is the coding system definition. (Examples: 1018, 12L14, 4140, 41L40 )

The very first number depicts a general category grouping of steels. That is:

1indicatescarbon steel:  1XXXX

4indicatesmolybdenum steel:  4XXXX

5indicateschromium steel:  5XXXX

6indicateschrome vanadium steel:  6XXXX

8indicatesnickel chromium molybdenum, steel:   8XXXX

9indicatessilicon manganese steel:  6XXXX

The second number gives indication if there are elements present that effect attributes of the steel. The last two digits (three digits for a few grades) represents the actual nominal percentage of carbon content present.

Take for instance 1018 steel. The zero in the 10XX indicates there are no major secondary elements present such as sulfur. Sulfur in steel increases machinability, but all free machining agents, such as sulfur, lead, calcium, etc., are in essence dirt, or elements taken directly from the earth. Such free machining elements improve the machinability but do not homogenize in the steel making process and can cause pockets, stringers or other faults that can affect some applications.

The last two characters represent the carbon content of the steel. For instance, 1018 is a basic carbon steel, with no added alloying elements, and contains .18 % of carbon. The actual standards will show a carbon range of .15/.20%. During the steel making process the carbon and alloy contents cannot be controlled to a specific percentage and thus the percentage shown is stated as the nominal.

Now let’s us examine what takes place if you see an 11XX number. The first ‘1’ indicates it to is also a simple carbon steel, the second ‘1’ indicates the basic analysis has been modified. In steel bearing the ‘11’ designation examining the chemistry tells us sulfur has been added to improve machining. These steels are typically used in automatic screw machine operations, or often when threading needs to be done. (thus any grade carrying ‘11’ refers to re-sulfurized steels. Re-sulfurized means the sulfur was added to the crucible at the end of the heat, prior to pouring into an ingot, which prevents it from being burning away. Familiar grades such as 1113, 1117, 1141 are re-sulfurized grades.)

There are some gaps in the numbering system such as 2XXX, 3XXX and 7XXX. At one time there were steels in some of these categories but were not popular enough to continue in manufacturing and were retired.

Often you will see an alpha character has been added in between the code groups, such as: 11L17, 11L41, 12L14 or a 50B40. The ‘L’ designates a lead addition. The lead like sulfur improves machinability and if combined with an already re-sulfurized grade, improves it that much more. The letter B stands for Boron which is on occasion added to low carbon steels to aid in increasing the hardness of the steel.

Steels with the letter ‘H’, which simply means ‘hardenability’, added to the end of the grade designation, signifies the steel is guaranteed to be capable of hardening to a specific depth.

We already briefly looked at the 1XXX grade classification and explained 2XXX, 3XXX and 7XXX are not being used, now here are the remaining classifications of steel categories.

4XXX Series are Chromium Molybdenum Steel.


  • 41XX Chromium Molybdenum (4150)
  • 43XX Chromium Nickel Molybdenum (4340)
  • 48XX Nickel Molybdenum (4815)

5XXX Series are Chromium Steel Low Chromium.


  • 51XX High Chromium (52100)
  • 6XXX Series are Chromium Vanadium Steel
  • 8XXX Series are Nickel-Chromium-Molybdenum Steel
  • 9XXX Series are Silicon Manganese Steel

The material presented in this article is intended for general educational information only. It should not be used for a specific application without careful analysis and study of the intended use. Anyone using this information or relying on it assumes all risk and any liability arising from their applications and use.

COPYRIGHT © April 2007, by Advisor In Metals

The author, Bill Bryson, Advisor In Metals has had numerous years and extensive experience in the heat treating of tool steels. He has conducted over 250 seminars to leading companies in the U.S. to train their tool makers and engineers on proper steel selection and heat treatment practices. He is also the author of the book called “HEAT TREATMENT, SELECTION AND APPLICATION OF TOOL STEELS” published by Hanser-Gardner Publications.

If you want practical information on the heat treatment process in understandable everyday language, inquiry to: Advisor In Metals or via e-mail at thegateway@metrocast.net. Information about the book or seminars is available on line at: Advisor In Metals

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