|knives, more heat treatment, and brushed finishes
||[Dec. 5th, 2004|05:13 pm]
Spent quite a bit of time this past weekend (my weekend is Friday & Saturday), got a lot done.
The rough forged knives were all successfully profiled, beveled, and smoothed, using a 36 grit belt to do the profiling and beveling, and A100 (about 200 grit) for the smoothing. While I forged out 10, one seems to have run away, probably lurking under some equipment in the workshop somewhere. Final grinding before heat treat was A30, about 1000 grit. I used a new Trizact "gator" A100 belt from 3M -- it's a structured abrasive, but instead of the pyramids, it has a different pattern, diagonal lozenges about 2mm wide by half a centimeter long. It seems to leave a better scratch pattern than the older style, and is supposedly waterproof and less likely to split. So far, can't argue -- I cool my blades in a quench bucket while grinding, and it was always a pain to have to dry off the knife completely when using the old style -- and if you didn't, even a drop was enough to mess up a belt -- the abrasive would turn into a paste, and clog up the belt.
Structured abrasives (Trizact from 3M, Norax from Norton Abrasives) are definitely the way to go. You can go straight from a 36 grit ceramic (non-structured) to the 100 micron structured abrasives with no problem, and without having to spend too much time cleaning up the 36 grit grind marks. I usually go for a mirror polish at 36 ceramic, structured A100, structured A36, then A16 and A6, and if I really want it to shine buff with pink rouge on a soft cotton buff.
These knives, however, are going to be a brushed finish. After some experimentation, I decided to go straight from the 220 grit to heat treat, then clean and finish up with a medium Scotchbrite belt.
But first I had to heat treat. Deciding to do things the "right way," I carefully assembled a package of the blades and a thermocouple in a stainless steel foil envelope, with a sacrificial carbon source (old oily rag) in to keep a reducing atmosphere. Into a stone cold heat treat oven, heat to 1,800 - 1,875 degrees F for 20 minutes. Or, as the case turned out, burn out your thermocouple somehow about halfway through and break out the spare and use that to determine the envelope temperature, then add in an extra 20 minutes to ensure thorough heat soaking and so get full hardness. A file test showed that the blades were at least harder than the file -- it skated right off of all of the blades without leaving a mark.
A quick air-quench later, and off to the cooler, where the quench is finished with liquid nitrogen. I put a couple of supports on the bottom of the cooler to keep the blade off the liquid nitrogen, poured in a half-inch, and gave it half an hour to get the blades down to cryogenic levels, adding in lN2 as needed. Once I was sure the blades wouldn't be shocked into pieces, I submerged them in the lN2 and let them sit for an hour.
Done with the cryo-quench, off to the heat treat oven. D2, the steel I used for these, has 2 maximum toughness/hardness peaks, the best one at 450 deg, about RC59-60, so that's what I tempered these at (2 tempering cycles, one hour each.)
Once the blades cooled down for the 2nd time to room temperature, I tried a couple of different Scotchbrite belts, decided that while the coarse worked faster, the medium definitely had a better look to it. Somehow got some grease in the belt, though -- I'm going to try to get that out without damaging the belt somehow.
Having determined the finish, I did a round of sharpening. This is the preliminary sharpening -- while it gives a good, sharp edge, there are enough operations between now and the final product that I'll need to do one last sharpening before sending out the blades. I decided, since I had a few liters of liquid nitrogen around, to try something I've seen done with dry ice -- chill down the blade before grinding the edge in on the belt grinder. The theory is that with the blade at a chilly -321 °F, to exceed the tempering temperature of 450°F requires some 771 degree temperature rise. In practice, it seems to work -- using an A6 (2000+ grit) belt on slow speed usually requires a bit more care than I exercised, yet didn't distemper any blades, even on the tips (the hardest part to avoid distempering.) Still, at $3/liter, I'll be using dry ice next time, it's a lot cheaper.
So, the blades are all sharpened, but need a cleanup, the brushed finish put on, and then I should be done.