In the alloy designation 4130, what does the first digit indicate?

• A. Basic alloying element
• B. Carbon content
• C. Heat treatment
• D. Manufacturing process

Answer: (A)

In SAE/AISI steel designations, the first digit shows the broad class of steel by its main alloying category. A leading digit of four identifies alloy steel, meaning elements beyond carbon are part of the composition. That’s why the first digit is read as indicating the basic alloying element category. The rest of the digits specify which alloying elements are present in the middle positions and the carbon content in the last two digits (for 4130, about 0.30% carbon). So the first digit tells you it’s an alloy steel, not plain carbon steel, and not the heat treatment or manufacturing process.