Sunday, March 27, 2005

Blade steels

There are a lot of different alloy steels out there, many of them useful for blades(an 'alloy' basically being steel with other elements added). One thing to remember is that while you can make a blade out of anything, some work better than others, and some are better for some uses than others.

A word on alloys. Steel itself is an alloy made by adding carbon to iron. Additional alloys used are chromium, nickel, sulphur, molybdenum, and a whole lot of others. They add different qualities. For instance, adding molybdenum allows a steel to harden with a slower quench- oil instead of water- which lessens the thermal shock of quenching and thus reduces the chance of cracking or warping. Stainless and stain-resistant steels generally add chromium and nickel. Problem is, you can't add an element in and it only adds that quality to the mix; it reacts with everything else. This is one reason why the truly 'stainless' steels often don't hold an edge very well, you add enough chromium and nickel to the mix to make it stainless, it tends to reduce the wear-resistance, which means it dulls faster. Sulphur can make a steel easier to machine to shape, but if present in too-high amounts it weakens the piece.

Big blades
Swords, big dirks, bowie knives, etc. I've wound up using 5160 spring steel for these. This is the same stuff you find in leaf and coil springs in your car's suspension. It generally won't hold an edge as well as a higher carbon tool steel, but it's also tougher. In a blade that's made to hold up in heavy cutting and chopping, that's a very good thing. And in smaller blades, if it's shaped and heat-treated right, it'll hold an edge quite well. In carbon content, this steel is just about identical to they've found in swords that have been tested.

Smaller blades
I've mostly used W2 and O1 for these. W2 is commonly used in files, and O1 is an oil-hardening tool steel used in industry for all kinds of things. Both of these make fine blades. Old files have been ground or forged into everything from fighting knives to garden and kitchen knives, and O1, usually starting with either round or rectangular bar stock, the same. Fairly easy to work, simple to heat-treat, and can hold an edge very damn well. One thing to watch on files is many of the cheap ones were made with a high-sulphur content steel, easier to machine but not as strong. These do an interesting thing if you forge them; they literally crumble when you hammer them. I meant it when I said too much sulphur is bad(one old trick for punching a hole in heavy iron was to put a lump of sulphur on the spot, then heat the iron to red and hold it there a few minutes, the iron in that spot will be weakened and easier to punch through).
Addition: I forgot to bring up one other steel that's very good for small & medium size knives: 52100B. Sounds impressive, doesn't it? It's a high-quality bearing steel, used for many ball bearings, roller bearings and bearing races(the track the bearings run in). I haven't looked in a while, it may be possible now to find it in bar stock, but I've always had to start with a bearing and forge it out. This stuff can be forged about the same as O1, and heat-treats about the same; about 1500F temperature, and a warm oil quench, and uses about the same tempering temperatures. Odd thing about this stuff: I usually give the other stuff two quenches, meaning bring up to heat, quench, let cool to ambient temp, repeat, followed by three tempering heats. 52100 seems to work best with three quenches, with 24 hours between them. I first read of this oddity in an article by a maker named Ed Fowler, so I gave it a try, and it does seem to give a better blade. Don't ask me why. I say for small & medium blades, but one of my first with this stuff involved turning a 2" ball bearing into a dirk blade about 13" long that goes through wood like an axe. The friend carrying it says it's also very good for slicing through PVC pipe.

For those I use mild steel, closest I can get to wrought iron, as the body. Cut the proper shape from some strap, wrap the middle around a mandrel to form the eye, then weld the halves together. Leave the end open, shape a piece of medium or high-carbon stock to fit in and a little bit sticking out, and weld that in. Grind to shape and harden & temper the bit(every wonder where the term 'bit of the axe' came from?), fit a haft and you're in business. 5160 is very good here, also; hard enough to hold an edge, tough enough not to chip or crack.

Stainless steels
I haven't messed with them much. Some are a flat bitch to forge; all of them require some special steps in heat-treatment, including higher temperatures, a controlled atmosphere and/or a sub-zero quench to properly harden them; and some of them just don't hold an edge well. A lot of knifemakers, maybe most, who work with them don't do the heat treatment themselves, they send the finished blades off to someone with the equipment to do it right, and I like to do it all myself if I can.

440c stainless, for example. You can forge it to shape, but after the oil quench, you have to do a sub-zero quench for it to completely harden. Best would be with liquid nitrogen, but you can do it with dry ice and acetone. After all that, you have a blade that won't rust, but won't hold an edge very well either.

There are some stainless/stain-resistant steels that do make good blades: ATS-34, D2, 154CM for instance. But since they require the extra steps to treat, I don't mess with them much. The exception is D2. They make planer blades with it, and as long as you don't overheat it you can grind one to shape and not have to heat-treat it.

That's a quick and dirty rundown of the stuff I've used. One nice thing about all of them, they're easy to get for good prices. 5160 can be had either as new bar stock, or you can straighten & flatten a section of coil spring, or just use a section of leaf spring. O1 and all the other tool steels can be bought as bar stock from a number of industrial supply houses(Enco, for instance) for good prices. Considering what our ancestors had to do or pay to get decent steel, the price of the stuff is rediculously cheap. Which is good, because when you screw up a piece and have to start over, you haven't ruined stock that you can't replace.

Some things really have gotten better over the years.


og said...

Dude, you need to mail me, I got some serious questions about heat treating knife steel. I'm gonna try to make some knives, and I'm gonna need all the help I can get

Kevin said...

Do you do any commission work? I sometimes have access to some REALLY BIG ball bearings, & I think a nice dagger made from one would be very cool.