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Sword tempering oven (propane fired)


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This is the tempering oven I'm going to build. I got into this craft wanting to make Damascus swords and now it's been 5 years and I'm just now working on my first mono-steel sword! :lol: I started gathering all these materials 2 years ago and im finally going to build it. Here is the materials I have so far. 

 

This is on the big side but I wanted to be sure to have room for just about any kind of straightening jig I may need.

 

In the metal box is ceramic fiber board and the cardboard box is kaowool. I have a Turkey fryer burner with a low pressure regulator (not good for a mini forge) to use as the heating "element" the electrical coils and everything will come at another time.

 

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Edited by Jeremy Blohm
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Overkill is almost enough, eh? ;)

 

The easiest way to make one is to get an old water heater shell or other big metal tank, cut one end off to make a door, then get a couple of long burners off a gas grill.  Plumb the burners in exactly like a gas grill, holes in the bottom of the tank and all.  Make a rack of some sort to hold the blade near the top, with vent holes in both ends of the tank and maybe a couple on top if you have trouble keeping an even heat.  What you're after here is just a big gas grill.  No insulation needed.  

 

If you want to go slightly more difficult, Jesus Hernandez's design (and the one I have, obtained from Dennis McAdams) uses a 1" black iron pipe running the length of the tank, one end capped, and one end connected to a Ward reducing T- type venturi burner.  (your turkey fryer plenum would work fine too).  There are 3/64" holes drilled every inch down both sides of the pipe to make a long thin grill burner.  

 

You're going to need an accurate thermocouple to tell your heat no matter what. Remember for swords you generally want a full spring temper.  That's going to be just above the blue-brittle range for your steel, anywhere from 575 to 800 F depending on alloy assuming simple or low-alloy blades.  

 

Now then: Your materials stockpile will make an excellent electric tempering oven, those do need insulation to work well.  

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This is what I came up with. I'm going to have to do the Jesus Hernandez's idea with the black iron pipe. I have some laying around somewhere.

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Nice I will have to get those. One question is would the burners be better above the blade so you dont get hot spots where the flame is hottest?

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With your insulated box, putting them off to the side would be fine.  The trouble with gas for tempering is keeping it cool enough, which is why mine is not insulated at all, and is in fact full of holes.  I'll get a pic soon.

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I thought about making a sort of burn chamber to try to keep it cooler. Where might I find a thermocouple locally?

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22 minutes ago, Jeremy Blohm said:

 

Other than the standard caveats about Harbor Freight tools, looks like it would work just fine.  The thermocouples they provide are replaceable, as those connectors are standard/universal.  I wouldn't expect the ones that come with it to last too long, but if you are only looking at tempering temps, you should be just fine.  

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Yeah it will only be used for temperimg. I need to come up with a better way to temper things. The wife is sick of openig the oven to see my blades or hammers in it. I'm trying to think of how to make a divider of sorts to block off sections of the oven so I dont have to heat the whole thing for a hammer.

 

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Rather than use gas, which as Alan says is relatively difficult to control because the temperatures needed are much lower than the flame temperature and the exhaust gases need to be able to get out, would you not be better off using Mineral Insulated Rod-type electrical elements and a PID controller? 

 

I'm pretty sure I could do it either way if I needed to, but I'd certainly be inclined to do it electrically because it's so much easier. The only reason I can think of to do it with gas would be because mains power is unavailable in the location. 

 

I would not try to partition the forge/oven for smaller items because the additional variables will be a pain to deal with. 

 

I have built electric HT ovens and gas HT "forges" for Austenitizing. I rather like the idea of using gas forges with a seriously reducing atmosphere for HT of Carbon steels (I'm pretty sure it can all-but-eliminate decarb), but it is not without its problems. The most serious of these is probably the massive levels of Carbon Monoxide produced: A perfectly tempered blade is good, but it tends to be better if the maker survives to do the other things that are needed to produce a finished article. I only ever use a gas HT forge outdoors. 

 

Search ebay for "6802 thermometer" and put the money saved towards a good thermocouple. The thermocouples supplied with the 6802s are often not much good for us at all. Sometimes you'll get a couple of PVC-insulated thermocouples which are only good to about boiling water temperatures. Sometimes you'll get glassfiber-insulated thermocouples that are good to about 400 degC/750 degF: useful for checking the temperature of a tempering setup. The ones pictured in the HF link look like the glassfiber ones.

 

I would recommend an Omega KHXL-14U-RSC-24 thermocouple assembly. https://www.omega.com/en-us/sensors-and-sensing-equipment/temperature/sensors/thermocouple-probes/khxl-nhxl/p/KHXL-14U-RSC-24?searchterm=KHXL-14U-RSC-24

 

It is not cheap, but it's not very much more expensive than the HF pyrometer in the HF link. Because it comes with a fitted cable and plug, there is no chance of wiring it wrong.

 

It is well worth making the time to talk to the technical sales guys at Omega and order by phone. They know their stuff and are helpful/patient, IME. If you need a bespoke thermocouple, they can supply and costs are very reasonable.

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37 minutes ago, Jeremy Blohm said:

so I dont have to heat the whole thing for a hammer.

 

You're missing the obvious solution:  Make more hammers to fill it up.  

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4 minutes ago, Jerrod Miller said:

Make more hammers to fill it up.  

 

I like your thinking!!!:lol:

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As usual, Tim has a better idea!  Scavenge the elements from a junked kitchen oven and install in your box. 

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This is a bit of a hijack, but this thread has made me question something.

 

An oven large enough to temper a sword will never have a perfectly even temperature.  Being within 10% would be pretty impressive.  As a controls systems guy, I'm not sure I would ever promise better than 5%, and I would use a lot of thermal mass to achieve that.  So I would think that we are talking roughly 25-50 degree F variance across the oven on a good day.

 

Assuming it is hotter at one end than the other, rather than hotter in the middle and cooler on the ends, which end of a sword should be tempered hotter?

Edited by Brian Dougherty
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2 hours ago, Brian Dougherty said:

 

Assuming it is hotter at one end than the other, rather than hotter in the middle and cooler on the ends, which end of a sword should be tempered hotter?

 

The tang end, assuming you stay out of the blue-brittle range.  If you break the tip off in battle you still have the rest of the blade. If it breaks at the guard the outlook is not favorable. ;)

And yeah, nobody is fighting with swords anymore.  But we'll just go with the assumption that they do.  

 

I am going to have to fire up my oven one of these days.  The pipe burner runs the entire length of the chamber for theoretically more even heat...

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25 minutes ago, Alan Longmire said:

The tang end, assuming you stay out of the blue-brittle range.  If you break the tip off in battle you still have the rest of the blade. If it breaks at the guard the outlook is not favorable. ;)

And yeah, nobody is fighting with swords anymore.  But we'll just go with the assumption that they do.  

 

At HEMA practice last Thursday my longsword broke at the tang/blade junction.  It certainly ended the fight.  I blame the sharp 90 degree transition (I will not be buying from that maker again, as they don't think that is anything wrong - I asked about buying a replacement blade).  

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I've was involved with a few foil and sabre breaks back in my fencing days.  It's kind of an "Oh crap" moment since a broken blade entering in through the armpit can be fatal.

 

I would imagine it is an even bigger startle in HEMA.

 

3 hours ago, Jerrod Miller said:

 

At HEMA practice last Thursday my longsword broke at the tang/blade junction.  It certainly ended the fight. 

 

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14 minutes ago, Brian Dougherty said:

foil and sabre breaks back in my fencing days

They made us wear kevlar plastrons ( sort of an armored half-vest worn under the white jacket for you non-fencers out there) for just that reason.  The club owned a bunch that rarely got washed.  Took a truly brave soul to put one on on a hot day... which is why I eventually bought a jacket with the whole right side made from thick kevlar.  

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It was un-expected, but we are a pretty under-control group.  Interestingly enough, in this context a tang break is the safest, as we may not have noticed if the last inch broke off, thus it would be pretty dangerous.  

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19 hours ago, Jerrod Miller said:

It was un-expected, but we are a pretty under-control group.  Interestingly enough, in this context a tang break is the safest, as we may not have noticed if the last inch broke off, thus it would be pretty dangerous.  

i hack at you sir for your own protection 

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