There’s no single, universal steel. The fact that steel is an alloy, a mix of different elemental minerals, practically guarantees that there is more than one way to make the stuff. As a result we have steel in a variety of compositions, each of which can withstand a certain amount of abuse and perform well under specific conditions. We assign grades to steel to make these differences clearer. Generally, the more carbon a steel fastener contains, the more it can be hardened, and the addition of other metals may provide other desirable qualities as well.
Let’s examine a few of the steel grades commonly assigned to bolts. Grade 4.8 steel is your generic hardware-store stuff — not particularly strong, but affordable and useful for general applications. Grade 12.9, 10.9, and 8.8 is considerably harder than Grade 4.8, so bolts featuring this grade of steel tend to see use in automobile construction and other industrial manufacturing. For components that face unusual stresses, such as car suspension systems, manufacturers turn to the even harder Grade 12.9 steel. When strength is the absolute top priority and flexibility doesn’t matter, you can go for the hardest possible steel, known as alloy steel. And as we discussed in a previous article, you can mix chromium into your steel to get stainless steel, though technically it’s not a grade.