In the number '4130' designating chromium molybdenum steel, what does the first digit indicate?

In the context of the alloy designation '4130' for chromium molybdenum steel, the first digit indicates the basic alloying element, which in this case is chromium. The American Iron and Steel Institute (AISI) and Society of Automotive Engineers (SAE) use a four-digit numbering system to classify different steel alloys. The first digit identifies the primary alloying element present in the steel.

In '4130', the '4' specifically designates that the primary alloying element is chromium, while the '1' indicates a specific series or type of steel with carbon content, in this case, around 0.30%. This system allows those working with materials to quickly identify the steel's properties and potential applications based on its composition.