Mike uses a technique for hardening O1 which I had never heard of before called autotempering or austempering to achieve a crystal structure in the metal known as bainite. According to Verhoeven, bainite was historically found to be tougher and more abrasion resistant than a more typical hardened carbon steel in its tempered martensite configuration.
The usual way to harden carbon steel is to heat it up until its crystal structure converts to the face centered cubic arrangement called austenite. Carbon is highly soluble in this structure, so the 0.95% carbon in O1 is fully dissolved in the iron matrix. Austenite is also paramagnetic rather than ferromagnetic, so the steel will lose its attraction to a magnet when it converts to austenite.
|Verhoeven, pp. 14|
If cooling is extremely quick, the carbon has no time to diffuse within the material very far before it is immobilized. The iron atoms attempt to reorganize themselves from the FCC crystal of austenite to the BCC crystal of ferrite. The carbon atoms are jammed into the crystal structure itself, distorting it from its desired BCC to a BCT (body centered tetragonal) crysal; essentially a highly strained version of ferrite. This form is martensite and has a needle like grain structure. Its vastly higher hardness and modulus come from the fact that it is already under a great deal of strain at the molecular level. This also makes it brittle. The usual way of heat treating carbon steel is to quench the austenite to transform it into martensite, then to raise the temperature again to temper the steel, or let some of the internal stresses dissipate and some of the martensite to transform into less hard and brittle (but tougher) forms through the formation of small carbides.
However if the austenite is cooled to an intermediate temperature first, allowing the solid to solid phase transition to happen there, then to room temp, another phase is possible to form; bainite. This material is similar to pearlite in that it is made up of a mixture of ferrite and cementite, but instead of being organized as laminated plates, it is more like interpenetrating aligned needles. From the micrographs in Verhoeven, the scale of the domains of ferrite and cementite is a little bigger in bainite. Depending on the intermediate temperature and the time it is held there, this technique can also produce fully pearlitic phases with non-eutectic compositions, or bainite with some martensite, or some retained austenite. Below is a phase diagram from Verhoeven showing the possible phase compositions by isothermal cooling:
|Verhoeven, pp. 35|
The variety of phases and structures that can be produced with iron and a little carbon are quite astounding. This subject has been studied deeply since it obviously has such great industrial significance, and has historically played a pivotal role in the technological development of human societies. I have not read the whole thing, but the book "Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel", by John D. Verhoeven is an excellent source which is available at no charge online.
Below is a picture of the two blades back from heat treat, with some light grinding done on the blade areas.
I continued to use my expanding rubber sanding wheel from Caswell, with 40 grit belts to begin with, but this time mounted on my buffer at home. This is a vintage three phase 208 tool, so I hooked it to a small VFD to turn the speed down to a more manageable level. I didn't realize that VFDs are famous for tripping GFIs which of course I have in my basement. After getting that straightened out, I realized I couldn't turn the thing down lower than 30Hz/1800rpm or else the expander wheel would not expand sufficiently to grip the sanding belt. It is a useable setup at 1800rpm though.
I now know that the 40 grit belts last about 5 minutes before the metal removal rate starts dropping off quickly. It took me a few hours of mostly wasted time using belts which felt gritty to the touch but in fact were not cutting the metal much to figure this out (duh!). These are just cheap aluminum oxide belts, and they are only 48cm long so it makes sense they give out faster than a quality 182+ cm long belt on a real grinder. Provided the belt is changed frequently, its not too bad to grind on this tool if you don't have a tremendous amount of metal to remove.
To cool the razors I positioned a 1 liter plastic bucket with water next to my grinder. When one razor got too hot to hold comfortably in my bare hands, I would put it in the water, remove the other from the water, and grind on it for a while. I had my fingers on the blade the whole time to alert me if the blade was getting too hot, since I didn't want to mess up the heat treating. Advice I received says that unless you see part of the steel turn blue you should be ok.
After I got close to an edge with the 40 grit, I went quickly (probably too quickly) through 80-120-240-320 grit belts.
Below is a picture of my vintage buffer/grinder I used for the both finish grinding and buffing.
Next I removed the expander wheel from the buffer and mounted an 8" sisal buff with black compound and worked this for a while. Buffing is done with the edge trailing, rather than leading as with the grinding. After the black on sisal treatment, I went through white on sewn muslin, and blue on loose muslin stages. Not completely smooth, but it looks pretty good. I think I should have spent more time in some of the intermediate sanding steps.
Though it looks cool, mirror finish is a bit of a nuisance in practice. I didn't spend enough time on mine to eliminate all grinding marks, but even at the level I am at any little scratch shows up extremely well. I rebuffed these a couple times after getting some scratches from stray abrasive slurry presumably pushed around by my fingers during honing.
I decided to leave the heat treated finish on the handle of razor #2, while on razor #1 I buffed the whole thing, then went back and did a satin finish on the handle using an extra fine 3M nonwoven abrasive pad.
MarkingI marked these guys with a 1064nm YAG laser we have at work. It was sold as a marking laser, but we mostly use it for cutting silicon. I did the graphics in Inkscape, a free Illustrator like program, then had to use some command line unix tools to convert the files into a form the silly laser software could deal with. The marks came out nicely.
Honing began on the rough side of my 220/1k Norton waterstone, with one layer of electrical tape over the spine to eliminate what was sure to be a lot of spine wear for the first honing. The stone seemed to cut the metal at a reasonable rate, and quickly showed up my embarrassingly wide and uneven bevels. Horrors! Well, I just decided to keep going and eventually got at least a little bevel all along the length of the blades. Then I went through 1k/4k/8k and a Shapton 12k.
Razor #1 was feeling sharp-ish, not super sharp, but I gave it some strops on webbing treated with CrOx, then a regular fabric and leather stropping. It was not as sharp as my kamisori right after it gets honed, but I shaved with it anyway. It was ok, but seemed rather dull and I had to press too hard to cut and as a result got a lot of irritation. I figured I would rehone it and try again, but after trying this (along with the initial honing of razor #2) both of my razors were pretty dull and clearly not ready to use. I had honed my kamisori at the same time and it came out fine, so it was definitely some issue with the two new blades, or at least my honing skills in relation to them.
It seemed to me that there could be one of two things going on:
1) I trashed the heat treat by getting the edge too hot during finishing
2) My honing was not up to snuff or complicated by some aspect of the new blades vs. the kamisori.
I posted a description of the issue on SRP, and got some helpful thoughts from people there. This, combined with an electronic consultation with Mike Blue convinced me that possibility 2 was much more likely. I had been figuring that a wide and uneven bevel was not such a big deal; it would make honing harder (due to the necessity of cutting more metal), but I would be far ahead of a wedge of the same size. Mike did warn me the bainite would be tough to hone, and others commented that a narrow bevel was a lot easier to deal with on the hones.
So I decided to go back to the grinder and narrow the bevels. This time I kept a plastic DMT plate next to the grinder and would give each side of the blade a few swipes after each cool down in the water to show me where I was with the bevels. Having the immediate feedback from the flat abrasion of the DMT was essential for gauging the bevel and I would certainly use this trick again for the initial finish grinding next time.
Using the 40 grit I got the bevels much more narrow and even, then went through 120-240-320, then buffing and honing. With all the extra grinding and honing I managed to put a bit of a smile in the #1 razor, but that shouldn't affect its functionality too much. This time, I had a much better feeling about the bevel setting on the plastic DMT plate than on the Norton 220/1k stone for some reason. After bevel setting, I went to the Norton 4k/8k, then a CrOx strop and regular stropping. I also tried putting on some paste furniture wax and buffing it off by hand to limit staining and rust on the carbon steel.
You can see the blade grind profile in the photo below.
One issue I ran into was that if I ground the bevel all the way down to an edge, the actual edge would get chips out of it due to the abrasive grains on the belt. So I had to sneak up on it and only get closer and closer to the bevels as I went up in grits. As a result I ended up with a not particularly straight edge. Probably this gets easier with practice!
Razor #1 now seemed relatively sharp; it was cutting armhair at a hover like the kamisori does when it is freshly honed. I'm positive its not as sharp as an experienced honer would get it, but how can I learn if not by doing? I've shaved with it for about a month and it has gone fairly well. Still very different than the kamisori as I mentioned above, but it seems to be a solid performer.
Razor #2 (the one with the more rounded toe and the unpolished handle) was a bit more tricky to get in shape. When I honed it the first time, it became quickly apparent that the line of the edge was not in plane with the spine, so the toe got ground more on one side and the heel on the other side. I this this is down to me leaving this razor with a meatier edge before heat treating, so I had more time to get into trouble with keeping the edge straight during finish grinding. If I had been checking it with the DMT plate while grinding it might have been ok, but as it was there was nothing to stop me from drifting off course. When I went back to try to improve the edges, I tried to correct this problem on the grinder but its still not fully rectified.
Still, I was able to eventually get the blade to what seems like a usable state, and I've been shaving with it for a couple weeks. The more rounded toe helps in getting my upper lip under my nose, and I like the slightly lighter weight and the unpolished handle.
Of course after regrinding I had to rebuff and remark as well.
Shaving is going fairly well. I'm doing a 2 pass shave every other day or so in the evening, which keeps me semi-presentable for work but doesn't take an unworkable amount of time from the day. I'm down to about 30 minutes for the shave now. I still have a long way to go on my skills at both shaving and honing, but the more I do it the better I am getting.
Overall, I think I might prefer a somewhat narrower and lighter razor, perhaps 19-20mm (6/8) rather than the 25mm I have on these two. Might have to order some new stock....