The AISI Numbering System (often referred to as the AISI-SAE system) as a standardized four-digit numerical classification developed by the American Iron and Steel Institute and the Society of Automotive Engineers to categorize steel based on its chemical composition. This system is the global language of metallurgy, ensuring that engineers can specify materials with mathematical certainty.
An AISI number typically consists of four digits (XXXX), where each position serves a specific diagnostic function:
The First Digit (Major Alloying Element): This digit identifies the primary alloying class. For example, 1 represents Carbon Steel, 2 is Nickel, 4 is Molybdenum, and 5 is Chromium.
The Second Digit (Secondary Alloy Modification): This indicates the percentage of the primary alloy or a specific modification within the series. In the 10XX series, the zero indicates a "Plain Carbon" steel with no significant alloying elements.
The Last Two Digits (Carbon Content): These are critical for heat treatment. They represent the carbon concentration in hundredths of a percent. For instance, AISI 1045 steel contains approximately 0.45% carbon.
Why It Matters in Engineering?
The AISI number allows a mechanical engineer to instantly predict the material’s properties. A 10XX steel is highly weldable and ductile, whereas a 41XX (Chromoly) steel offers a superior strength-to-weight ratio for structural applications. For stainless steels, the system shifts to a three-digit series (e.g., 300 series for Austenitic and 400 series for Martensitic/Ferritic), where the first digit denotes the metallurgical structure.
Understanding these codes is vital for quality control in the tube and pipe industry, as it dictates the welding parameters, tensile strength, and corrosion resistance of the final product.