SAE specification numbers

By the 1920's the Society of Automotive Engineers had developed a system for identifying the most commonly used steel alloys by number. Generally the number had four digits, but five and even six were possible.

The first digit indicates the principal alloying element:

Carbon steels 1
Nickel steels 2
Nickel-chromium steels 3
Molybdenum steels 4
Chromium steels 5
Chromium-vanadium steels 6
Tungsten steels 7
Silico-manganese steels 9

The second digit indicates the approximate percentage of the predominant alloying element. If the percentage is greater than 9%, this second digit  becomes two digits. The figure given was nominal. For example, the digit in the specification number might be "2," but the actual percentage in the alloy 1.5%. Actual composition and tolerances were given on the society's specification sheets for each alloy.

The last two digits indicate the carbon content in hundreths of a percent. If the carbon content exceeds 1%, instead of two digits there will be three.

Some examples:

2340 nickel steel, about 3% nickel, about 0.40% carbon
71360 tungsten steel, about 13% tungsten, about 0.60% carbon
3245 nickel-chromium steel, about 2% nickel, about 0.45% carbon
52100 chromium steel, about 2% chromium, about 1% carbon
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