With all the interest The Forge has generated, I thought I'd take some time to lay out some of the things I've learned thus far about steel and the craft of making razors--and their duller cousins, knives. :) As time permits, I hope to make this an ongoing series, so let me know if there are topics you'd like to see covered.
Let me state at the outset that I'm still learning tons of stuff every day, so I'm far from an expert. As a relative beginner, I still remember a lot of the questions I had as I was starting out, so I'll endeavor to cover
the basics. I also want to point to guys like Mike Blue, Tim Zowada and Kevin Cashen, from whom I've learned just about everything I know at this point.
So enough intro stuff...
Heat-Treating Basics
At its most basic level, the heat-treatment of steel is a three-step process: Get it hot, get it cool, and relieve the stress. Before you can understand what the heat treating, or hardening, process does, you need to understand a few things about steel.
Let's keep it simple and consider plain carbon steel. Plain carbon steel is basically two elements--iron and carbon. There are some other trace elements in the mix, but we'll gloss over that for now. The ratio of iron to carbon is carefully controlled in the steel-making process. Too much carbon and the material becomes brittle. Too little and the material is soft. Most blademaking steels have between .6 percent and 1.5 percent carbon. There may be some exceptions, but that's a good general range to keep in mind.
As a sidenote, the four-number classification used for most steels will tell you how much carbon is in the blade. The last two digits of the sequence represent the carbon content. For example, "1095" designates a simple carbon steel (the "10" part) and .95 percent of carbon. A popular steel for big knives, 5160, has .60 percent carbon.
The carbon content of a steel is the biggest factor in determining how well a steel will harden. The simple carbon steel 1018, which is often known as mild steel, has only .18 percent carbon. It won't respond to a heat-treatment. This is the stuff you buy at Home Depot for welding projects. It works well for structural applications, but not blademaking.
A medium carbon steel, such as 1050, will harden to some extent. For blades, you generally need at least .60 percent carbon, and I'd suggest at least .80 percent for small knives and razors. (Swords are an example where 1060 might be better; a long sword needs to be flexible in addition to being hard, so less carbon can be an asset.)
For a new blademaker, I'd suggest 1080 steel. It makes an excellent blade and is pretty simple to heat-treat. We'll explain why later on...
Steel is a fascinating substance. More than many other materials, it undergoes radical internal changes as it is heated and cooled. When you get a piece of steel from the factory, it's usually pretty soft. Steel in the softened state is called "annealed" steel. In the annealing process, the carbon is allowed to precipitate out of the iron, so you end up with large swaths of pure iron and little pockets of iron mixed with carbon. The pure iron parts are very soft, so the steel is easy to cut, drill or grind, even using hand tools. Our 1080 steel is easily cut with a hacksaw in its annealed state, and you can drill it with regular black Home Depot drill bits.
As you heat the steel up, though, this structure begins to change. The carbon atoms start to intermingle more evenly with the iron atoms. When the temperature reaches a certain temperature, known as the "critical temperature," the carbon is fully dissolved back into the iron.
It's not a perfect analogy, but imagine a pot of water. Now pour a cup of salt into it. When the water is cold, the salt mostly settles to the bottom of the pot. The water and salt stay mostly separate. If you heat the pot to boiling, the salt will dissolve more readily into the water. It forms a solution.
Likewise, iron dissolves carbon as it is heated. When the maximum amount of carbon has been dissolved into the iron, the steel's crystal structure is known as "austenite." For 1080 steel, the austenizing temperature is a little under 1350 degrees Farenheit. (Different steel grades have different austenizing, or critical, temperatures. Thus there is no one-size-fits-all heat treatment.)
The next structural change occurs when the steel starts to cool. If the steel is cooled slowly, the carbon will
start to collect again, leaving patches of soft iron. In fact, this is how the annealing process works. The steel is heated above the critical temperature, then cooled very slowly in some kind of insulating material--25 degrees F per hour, in some cases. If allowed to cool at a natural rate in the open air, 1080 will be harder than it would be if annealed, but you could still drill and cut it fairly easily.
The real magic happens, though, if you cool the steel rapidly. For simple carbon steel, this is done by immersing the glowing blade in a bath of quenching oil. As the temperature plunges--more than 400 degrees F in less than 1 second--the carbon doesn't have time to escape from the iron. It is locked in place, and as the blade cools to 425 degrees, the austenite transforms into a jagged, needle-like structure known as "martensite." Marensite is very, very hard. If properly heated and quenched, the martensite in 1080 steel will reach 66 or 67 on the Rockwell C hardness scale. (By comparison, most razors are around 59-60 RC, and most knives are in the upper 50s.)
This sudden drop in temperature and radical change in structure creates tremendous stresses inside the steel. Did you ever pour boiling water into a chilled glass? The stress of the temperature change shatters the cup.
If you were to drop a newly hardened blade from waist high onto a concrete floor, it would probably break into several pieces. The internal stresses make the martensite very hard but also very brittle.
So the final step in heat treating is removing some of that stress to make the blade less fragile. This is known as tempering. The word "tempering" sometimes is misused to mean "hardening." In fact, tempering softens the blade slightly. To temper a blade, you heat the steel up just past the temperature where the martensite started to form as the blade cooled and then hold it there for several hours. For 1080, a good tempering formula might be three hours at 400 degrees. The blade will drop in hardness, maybe to 60 or 61 RC, and the internal stresses will relax.
Raising the tempering temperature results in softer and softer steel. A blade would become worthless in terms of edge holding at 600 or 700 degrees. The structure that results from a good temper is known as tempered martensite.
Tempered martensite--hard, but not too brittle. That's the target we're aiming at.
In my next post, I'll describe how I go about trying to achieve that goal. :)
Josh
