How to Identify Carbon Alloy Steels

Written by timothea xi
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How to Identify Carbon Alloy Steels
(brushed steel background texture image by JoLin from

Carbon steels are among the most widely used steel types, comprising more than 85 per cent of all steel produced and shipped in the U.S.

The introduction of carbon to the alloying medium can render significant impact on the final product, with hardness and tensile strength enhanced with greater carbon content and ductility and weldability reduced with lower carbon content.

Plain carbon steel is different from low alloy and high alloy steels in that it has less than 1 per cent of carbon in a mixture of mostly iron and trace elements.

Skill level:


  1. 1

    Identify according to the AISI definition:

    Determine whether the steel in question meets the criteria of the American Iron and Steel Institute for what constitutes carbon steel: 1) no minimum content is specified for any alloying elements, such as vanadium, tungsten, titanium, molybdenum, nickel, cobalt or chromium, 2) the specified minimum for copper does not exceed 0.40 per cent and 3) the maximum content does not exceed 1.65 per cent for manganese or 0.60 per cent for both silicon and copper.

  2. 2

    Identify according to carbon composition:

    Find out how much carbon is in your carbon alloy steel. Up to 2 per cent total of any given carbon steel can be composed of alloying elements. Alloys with carbon levels of up to 0.30 per cent and manganese levels up to 1.5 per cent, such as flat rolled products and steel structural plates, fall into the low carbon category. Medium carbon steels range from 0.30 to 0.60 per cent carbon and up to 1.65 per cent manganese and are used in axles, gears, crankshafts, rails, railway wheels and quenched processes. High carbon steels contain from 0.60 to 1 per cent carbon and up to 0.90 per cent manganese and are used in springs and wires, while ultra-high carbon steel contains between 1.25 to 2.0 per cent carbon and is experimentally formed to produce ultrafine granular microstructures.

  3. 3

    Identify according to deoxidation practices:

    Determine whether the carbon alloy steel was rimmed, capped, killed or semi-killed. These terms refer to the deoxidation processes during steel forging, in which the amount of oxygen still remaining in the product prior to or during casting affects the formed steel's characteristics. Rimmed carbon steel involves minimal deoxidation and results in a steel in which the outer rim contains relatively higher concentrations of carbon, sulphur and phosphorus than the inner core. Capped carbon steel is similar to rimmed steel except that the process is mechanically or chemically controlled to produce a rise in the mould, which then comes into contact with a metal cap. Killed carbon steel is strongly deoxidized with the addition of aluminium and silicon, resulting in a uniform product, whereas semi-killed carbon steels exhibit features in between those of killed and rimmed carbon steels.

  4. 4

    Identify according to SAE/AISI classification:

    Follow the Society of Automotive Engineers and AISI's coding system for identifying steels of various types. The first number in the code refers to the basic type of steel, which in this case would be 1, for carbon steel. The first two numbers together refer to the series within the basic alloy group, depending on the primary alloying elements in that series. For carbon steel, "10xx" would refer to plain carbon steel, whereas "11xx" signifies resulphurized carbon steel. The last two or three numbers designate the permitted range of carbon in the steel in hundredths of a per cent.

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