Pick up a piece of high-carbon tool steel. Pick up a piece of mild steel. They both looks alike to your eyes. Both are grey, metallic, and cold to the touch.
But if you were to do exact same thing to each, one would snap in half like glass, whereas the other bend into a curve with no problems at all. The secret language of metallurgy is written with carbon content and alloying elements that spell out if a part functions under load, or if it will fail catastrophically. It’s the difference between working prototype and a snapped prototype on the very first test run, and understanding that code makes all the differance.
Understanding Steel Grades
What does this all mean? Well, the SAE/AISI basically created their own numbering system that works like a simple filing system. The first set of numbers (the first two) indicate what family of steel it’s going to be. And the second set of numbers (last two) indicates how much carbon it have.
How do we know? Well, the last two digits are equal to one hundred times the percentage of carbon. So if the number ends with 45 that means it’s got about zero point four five percent carbon. This may sound like not much at all, but it’s what makes things like hardness or weld-ability different than usual. That little number make it all work.
So here’s the breakdown. The chart above show how the numbers correspond to physical properties such as yield strength and tensile strength. In other words, they take abstract science and turn it into practical engineering data you can use on shop floor.
Hardness is primarily driven by carbon. The higher the percentage of carbon, the harder the plain carbon steel will be, but it will also be less ductile or more brittle. Alloys with lower levels of carbon (ten; eighteen) are super forgiving. They can be welded easy and machine well. They also make great materials for shafts, brackets and general fabrication when ductility is more important then a razor-sharp cutting edge. Because they don’t have to be preheated much, if any, low carbon steels save energy and time.
Where low carbon steel fails, however, is if you desire a piece that holds an edge or resists wear. Then it’s time for the tradeoffs to play out. Hardness comes at the cost of machinability and weldability.
The in-between sits with medium carbon steels such as the ever popular ten-forty-five. This is great stuff that can be heat treated to high strengths ideal for axles and gears. However it is also less forgiving when welded. Many hobbyists fail to realize that pre-heating will help avoid cracking.
Then there are high carbon steels such as ten-ninety-five, used for springs and knives. These can be made extremely hard but are very hard to cut and almost impossible to weld unless special processes is used. The visual reference shows how this all progresses. It connects the amount of carbon to the application, so you don’t have to guess where it would work best.
Additions of nickel, molybdenum and chromium bring in alloy steels which have all kinds of crazy properties. They can be heat treated to very high yields (quenched and tempered) and still retain a bit of toughness; alloys such as forty-one-forty are used in both aerospace and automotive applications as a result.
Stainless steels has an altogether different rule set… It’s all about chromium forming a passive layer that repels corrosion. Ten point five percent chromium seems to be the minimum limit where steel starts acting stainless. Anything below that will rust. Anything above that is resistant. Within stainless steels there are differences too. Three-zero-four is typical for food service, whereas three-sixteen includes molybdenum to resist chlorides and is critical in a marine environment.
The last group would of be the tool steels. These are used for forming other metals. They are rated by letter instead of number, which shows how hard they are after hardening. Grades in the O series are oil-hardenable and found in most shops. The A series are the air-hardened steels with fewer distortions from heat treating. There are also hot work steels like H-13 that stand up to the repeated heating and cooling associated with die casting. Which tool steel you use will depend on what kills your tool first: thermal fatigue or wear resistance?
There’s no need to memorize all those numbers; choosing a steel grade boils down to prioritizing what matters most for your application: Does it have to be easy to weld? Low-carbon is the way to go. Must it resist rusting from salt water? Seek out stainless grades with molybdenum content. Should a part last tens of thousands of impressions before needing resurfacing? High-chromium tool steels will get you there.
These are just shorthand representations of physical characteristics involved. When you recognize the pattern, however, the steel grades cease to become a series of arbitrary codes and become clear. They shift from guesswork to engineered design. And that’s when a good idea transitions into a completed product that stands up to scrutiny.
