ht'ing with a temp controlled kiln

[mcraigl]

I tried to harden a cablemascus blade in my wife's kiln last weekend. I set the kiln for 1500 since my Machinery's said that was a good temp for the various 10xx steels (figuring I'd lose a few degrees going from the kiln to the quench). First go 'round I left the blade in the kiln for 6 minutes then quenched. File cut it real easy, not hardened. So I thought maybe it takes longer for the blade to come up to the ambient temp of the kiln. Second go around I left it in for 30 minutes and quenched. A little harder, but definately not what I expect from cable. So in desperation, I cleaned up the blade and fired up the gas forge and heated a little past where the magnet would't stick then quenched. It got hard this time, but still not as hard as I'd like. So I have a few questions.

First of all, am I expecting too much from a cable blade to get hard enough for a file to skate?
Second, do you think the triple quenching this blade got ruined it? Grain structure wise?
Third, How long do you wait for a blade to come up to temp in a kiln if you use one? Would it be better to put the blade in at a lower temp and let it come up to temp with the kiln?
Fourth, how do you keep the scale down if using a kiln?

I had thought this would be the cat's meow since there's a computer controller on the kiln and I can set whatever temp I want, but I'm not so sure now. I do have some O1 blades that will need heat treated soon, and was hoping to use it for them. I have a little bit worried about the amount of scale that was produced on the cable and usually O1 scales more on me that cable. I thought about putting some hardwood charcoal in when I put the steel in, but didn't do it. (wife was worried it'd pollute her glass/bead kiln).

Any help y'all could suggest would be most appreciated.
ML

[Ed Caffrey]

The first suspect in my mind is the cable itself. You don't mention if this was new cable, or salvaged. I used to build a lot of cable blades...and no longer do, simply because of the situations like you experienced. The key to cable blades is this: The reason you see a pattern in a finished cable blades is because of the decarb that occurs between the individual wires when welded. Most cable that folks use tends to have smaller diameter individual wires, thus they decarb excessively. That is likely ONE of the causes of your blade not hardening properly. The very best cutting blades I ever produced from cable came from a drag line cable that was overall 3" in diameter, with individual wires that were just over 1/8" in diameter. Because of the individual wire diameter, there was minimal decarb, and it just produced a much better blade as far as cutting, than anything I ever produced with smaller wired cable.

Next, generally you want this type of material to go to 1525-1550F. If you only went to 1500, its very likely that going from the kiln to the quench you lost critical heat. The time you allowed it to soak would not have mattered, if you only went to 1500, and it looses heat between the kiln and quench, you were likely a hundred and fifity degrees or more below critical temp when you hit the quench.
I usually recommend that plain carbon steels NOT be soaked for any duration prior to quenching. Bring them up to temp, then get them into the quench. Since plain carbon steels do not contain any level of alloys, there is no need for a soak, which is intended to get all those alloys into "solution". Most cable is simple steel.

All that being said, my suspicion is that the cable used in the blade you mentioned had very small diameter wires to begin with, which decarbed considerably during forging/welding, and then when you tried to harden it, the temp was not high enough.

Finally, the scale issue. That was a product of the steel being exposed to the heat for too long of a duration, causing the scale...hence my suggestion for getting the steel up to temp with NO SOAK..that soaking is what causes the scale.

[SVanderkolff]

When I use my electric kiln to heat treat carbon steel I always do so from room temp. Take the steel to 1550 and then quench in olive oil. Although I tend to get a lot of scale on the blade the vast majority of it comes off ithe quench leaving an almost clean blade. The times I have had excessive scale I have either not set the kiln temp correctly and been to cool or I have fumbled with the kiln to quench and let the blade cool to much. I have no experience with cable damascus but I tend to use a fair bit of carbon damascus. Hope that helps.
Steve

[mcraigl]

Thanks a ton for the replies. I have made a few other blades in the past from this same material, and it is salvaged cable from a skyline yarder logging system. About 1 1/4" dia. with "fairly" coarse individual wires. In the past I think I was overall pretty happy with the heat treat just eyeballing it from the gas forge etc. However in the intervening year or so I've been playing with O1 and maybe my expectations have changed as the O1 tends to HT real hard from quench. So I definately am taking Ed's comments to heart here. I'm kind of "over" spending the hours on recycled cable. I have thought about buying new cable of known alloy and doing a few more simple ones just 'cause people absolutely love them. Next time if I use the kiln I go a little hotter. And I think I will try putting a piece of charcoal in with the blade.

Now for the a clarification of Steve's comments, since I've got 5 blades from O1 that are supposed to be Christmas presents that need to be HT'd in the next week or two. Are you saying you put the blade into the kiln when you start the kiln? Before it comes to temp? Then once the kiln pyrometer or whatever reads your target temp, you assume the blade is at that temp also and go straight to quench? My kiln takes a good hour or so to go from the 55'ish of my shop to 1500, and that's why I had let it preheat to the target temp, then put the blade in. I wasn't sure how long it takes the blade to go from ambient to target temp inside the 1500 degree kiln.

Do you have some sort of fixture in the kiln that holds the blade edge up? Or do you just lay it in there on the kiln shelf? I've got some scrap peices of SS that I've been thinking of building a simple fixture, but thought I'd see if I can make the overall system work first. For this go-round I just used a couple of little ceramic thingy's to prop the blade between to hold it edge up.


Also a follow up for both of you, with the O1 that I'm using right now, should it get a soak, or should it go straight to quench once it reaches target temp?

Again, thanks a ton for the replies. I'm kind of excited about these O1 blades I'm doing right now, they're my first attempt at integrals, and also I've just got my wood lathe up and running and am excited about the handles for these. If I can make up my mind on material selection for the handles anyway.

ML

[SVanderkolff]

I would be very interested t hear other folks answers but for myself, I always put my blades into the oven when it is cold. That way I can make sure it is standing up correctly and not likely to cause a warp . It also makes sure that by the time the kiln reaches the right temp the blades are heated completely through as well, that way I don;t need a soak time either. My kiln is about an hour to 1500 , so yes the blades are in there for an hour and then straight from the kiln to the oil.
I hope that helps.
Steve

[mcraigl]

Steve,
Thanks a ton for that. I will go ahead and make some SS fixtures to hold the blades and try these O1 blades from room temp.
ML

[WBE]

In the 01 Ht, you need a soak at 1200/1250? for maybe 20/30 minutes, then ramp up to 1475? minimum, to 1500? and soak at that temp for 20/30 minutes. Then quench. It is best to have decarb protection. I use PBC powder, or the liquid version. Both are available from Brownell's, and work very well, leaving a very clean blade surface.

[mcraigl]

I had been considering getting some of the pbc and trying it. I was unaware that there was a liquid version. Interesting.

[WBE]

I find the liquid to be much easier to apply. It is water based, and painted on. The powder has to be applied on a blade heated to 600/700?. Both are removed with hot water, although the liquid type usually comes off in the quench with any remaining coating easily chipped off with your fingers.