|An older sword revived.
||[Jul. 15th, 2011|12:40 am]
I dragged out an old project, left half-fogotten in a pile of similar projects. 5160, about 30" long, sized to fit my equipment in fact, single edged, tapered blade. The original stock was only about 12" long, the rest of the length was drawn out during forging, via a rigged up rolling mill. |
More behind the cut
It had some heavy surface rust, but since one side was only partially descaled, and there was still a lot of meat on the blade that was going to have to come off eventually, it wasn't a major problem. There was a slight twist to the blade I wanted to do something about, and a minor bend. I couldn't remember if I had fully annealed or just normalized the blade, so I normalized it again, taking advantage of the heat to take out the twist and straighten it all back up.
The 36 grit coated belts I had used for Titanium, but had clogged up quickly, were just sitting the the belt rack, their orange-ish coating making them stand out against the muted browns, reds, and grays. I had just reinstalled the resurfaced wheel on the big 5hp grinder, and it needed a workout. I put the one on the other, and went to work.
First I just concentrated on getting to a good, even surface: all the forge scale removed, an even taper from back to edge, from guard to tip. Then I worked on the profile, giving a slight arc from handle to tip, and adjusting the edge geometry until it looked just right. I switched up to some new 80 grit belts and removed the scratches from the 36 grit by working it at an angle to the longitudinal lines of the coarser belt. A final grind at A100 Norax structured abrasive (about 200 grit) and it was time for heat treatment.
3560 is best treated by a 7 minute soak at 1525 followed by a warmed oil quench, then a tempering of around 400F for an hour. This is a sword, so I'm tempering a bit more on the tougher side, plus I expect to have to adjust the blade, best done if I completely distemper a portion of the back of the blade, which will also serve to make the blade tougher overall.
It was dark out tonight by the time I finished grinding, probably about 10pm. I rigged up a thermocouple to the old heat-treat oven, used some firebrick to extend the furnace out onto it's lowered door. It took some adjusting to get the cranky old regulator and blower in sync, but in a few minutes I heard the fierce roar of the two burners going strong. The furnace just wasn't deep enough -- it was quickly clear that I'd never get the whole sword to an even heat. I quickly moved the propane feed to the gas forge, and did the juggling act of trying to keep a blade at an even temperature when the thin parts want to be too hot and the thicker are taking forever to come to temperature.
After a couple of minutes, I got the rhythm down -- holding it by the handle with a pair of tongs, move it all the way in slowly, so the thinner part of the blade has time to come to temperature, then send it out the passthrough in the back, and move it back and forth in a constant motion, faster when the tip is in the forge, slower when the base is there, but never stopping, keeping the whole thing at just the right temperature.
And I have to admit I'm guessing a little here. I got a good look at the sword in the oven with the thermocouple, and used that recent memory plus my knowledge of metallurgy and my experiences in hardening to get the temperature where I believed it should be. The lights were out, and I was watching the steel change states -- just slightly darker clouds moving around on the surface of the blade, showing where different crystalline forms of steel were forming and changing. For seven long minutes I ketp it right at the critical temperature, slowly chasing all the clouds away until the blade was, in the darkness, a bright shining streak of steel.
In the darkness, then, I plunged the glowing blade edge first into a quench tank of oil. I had been expecting a flare of oil burning off, but instead all I got was smoke. After a minute, I pulled it out and let it air cool down to room temp in the vice. Then, and only then did I take a good look at it.
The edge is almost dead straight. That's the good news. On the back side, however, there's a fairly pronounced "S" curve. Looks like I'll have to do some adjusting. But, also on the bright side -- no cracking, no dramatic warps, no sabering. Off to tempering before I do anything stupid. Again.
The timer just went off, letting me know that it's been in a 400F oven (the aforementioned limit to my equipment) for an hour. I'm going to let it cool down, then tomorrow hopefully straighten things out without breaking anything. Yes, I could still lose this sword if a crack develops.
I hope not, however, and I think this one may be finished soon.