In the movie Conan the Barbarian, we hear a great deal about “the riddle of steel.” We are never told exactly what that riddle is, but in modern times, it might be: What’s the difference between 4150 and 1020 steel? If you’ve been around a machine shop, you’ve probably heard the AISI/SAE numbers, but if you didn’t know what they mean, [Jason Lonon] can help. The video below covers what the grade numbers mean in detail.

The four digits are actually two separate two-digit numbers. Sometimes, there will be five digits, in which case it is a two-digit number followed by a three-digit number. The first two digits tell you the actual type of steel. For example, 10 is ordinary steel, while 41 is chromium molybdenum steel. The last two or three digits indicate how much carbon is in the steel. If that number is, say, 40, then the steel contains approximately 0.40% carbon.

A common example of a five-digit code is 52100 steel. That’s ball bearing steel, and it has 1% carbon. You’ll notice that of the first two digits, the first digit changes when the main alloying element changes. That is, 2000-series steel uses nickel while 7000-series uses tungsten.

Tool steel has a different system, with a letter indicating the type of steel and a number indicating its alloy properties. Tool steel can be quenched in oil, air, or water. It can also be hot or cold drawn, and the letters will tell you how the steel was made. As you might expect, each type has different properties, which you may care about in your application.

For example, type W — water-hardened — isn’t used much today because it warps and cracks more often than steel produced with oil quenching.

If you want a list of steel grades, Wikipedia is your friend. You’ll see there are a variety of letters you can throw in to indicate hardness, and things like boron or lead added to the alloy, but these aren’t very common. Stainless steel also has a coding system that the video doesn’t cover, but you can find more information on the Wikipedia page.

If you want to work with steel, you’ll need heat. Next time you use tungsten steel, marvel at the fact that the Earth’s crust has about 1.25 parts per million of the rare element. Yet the world produces more than 100,000 tonnes of it a year.