# How do you temper swords usually?



## inferno (Sep 11, 2019)

So today i ordered some 5mm, 500 long (20 inches) carbon steel that i will make a "home utility sword" out of.
Because 500mm is the longest i can fit in my home oven when tempering.

Most real swords are a lot longer than this though.

Now i'd like to know how (i guess _where_ is the keyword here) swords are usually tempered. most of them seems to be hardened in gas/coal forges. but its seems kinda hard to temper them in these. or?

So how is this supposedly done? generally.


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## Matus (Sep 11, 2019)

... assuming that this is a real question ...

How do you plan to HT the steel? Do you have a forge? What kind of experience and tools do you have?


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## inferno (Sep 11, 2019)

Matus said:


> ... assuming that this is a real question ...
> 
> How do you plan to HT the steel? Do you have a forge? What kind of experience and tools do you have?



I have a gas forge that i do my kitchen blades in. it can take almost any length, its modular. and i just turn up the gas and move faster.

I have seen many videos of japanese smiths quenching blades in water. I have never seen any of them temper anything though.

So now i wonder how this is usually done with lowtech stuff. they just put it back in the coal for a minute??

i mean this must be highly trivial if you have a long paragon or evenheat, but i dont (yet).


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## Matus (Sep 11, 2019)

Japanese guys indeed temper mostly in the coal forge (Carter shows that in one of his videos where he makes a kurouchi nakiri)


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## milkbaby (Sep 11, 2019)

If you can't run the forge low enough to temper in, you can draw the temper with a handheld torch down the middle of the blade. Clean it up totally then draw the color from the center of the blade towards the edge until you have about the color temper you want. You can also do this on a single edged blade drawing the temper from the spine to the edge. I would practice some before ruining a big piece that you put a lot of work into tho.


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## Qapla' (Sep 11, 2019)

What would a "home utility sword" be? A machete?


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## milkbaby (Sep 11, 2019)

Here's a good video for the forge HT and temper of a sword (starting at 39:18)


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## milkbaby (Sep 11, 2019)

Here's a video showing the handheld torch method of drawing the temper (starting at 10:32)


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## Kippington (Sep 12, 2019)

Milkbaby got it right. Tempering long pieces of steel are often done by watching the tempering colours on the surface. You need a freshly ground surface to see it happening.

Alternatively you can use the water drop test, which is to sprinkle water on the hot blade and to judge the temp by the way the water behaves on the surface. This works best when you have a large heat source such as a sword makers forge, or something that can hold the bulk of the blade. A torch won't cut it here.



Matus said:


> Japanese guys indeed temper mostly in the coal forge (Carter shows that in one of his videos where he makes a kurouchi nakiri)


I believe he uses coke for forging and charcoal for heat-treating. Many of the Japanese smiths believe coal to be too 'impure', and think of the steel as a sponge willing to soak it all up. I've never seen a video/picture of them using coal.


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## Matus (Sep 12, 2019)

Yeah, with coal I indeed meant charcoal.


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## bryan03 (Sep 12, 2019)

be friend with a baker, they have big oven


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## Kippington (Sep 12, 2019)

milkbaby said:


> Here's a good video for the forge HT and temper of a sword (starting at 39:18)



Some interesting notes on this video:
- He doesn't check the temperature of the temper at all (Ai-tori at 41:15), just blind heating with experience as the indicator. It would be cool to find out how accurate he was.
- When applying clay for the hamon (37:08), I've noticed that many of the swordsmiths will add clay lines all the way down to the cutting edge. These act as periodicly spaced shock absorbers down the length of the blade. Stresses that are created during the harsh water quench get absorbed into these softer zones before the blade can build up enough force to crack. This is something I haven't seen yet on kitchen knives (after all, they're shorter and less likely to crack), but I'd like to know if it ever gets done.


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## Kippington (Sep 12, 2019)

Matus said:


> Yeah, with coal I indeed meant charcoal.


Yeah its understandable to mix them up (charcoal/coal/coke), but once you've used all three of them you'll spot the differences from a mile away. Each of them have very distinctive pros and cons over the others.


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## Beau Nidle (Sep 12, 2019)

The only sword I've actually finished so far was tempered with a torch, watching the colours as I ran it down the middle as someone else has already mentioned. I've also seen it done in oil, if you can get a container large enough to fill with oil and then heat it to your tempering temperature, put the sword in there for 1-2 hours just like you would a larger oven.


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## inferno (Sep 12, 2019)

Thanks guys for your replies. It really makes sense to simply just judge the colors.

Is there any reason why one want to use the torch in the middle of the blade instead of the spine? does it get too weak otherwise?



Qapla' said:


> What would a "home utility sword" be? A machete?


not really. I'm not sure yet actually. I'll have to figure it out as i go along. But i'm fairly certain it will look similarish to some japanese sword in the end. But i have no intention of producing it like a japanese sword. too much work.


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## Beau Nidle (Sep 12, 2019)

I think most people have assumed that a sword would be double edged, hence telling you to temper along the middle. If it’s a single edged sword like a Katana (I guess your size is more of a wakizashi) then yes, run the torch along the spine and watch the colours towards the edge.


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## John N (Sep 13, 2019)

The ones I have done have been tempered in a tube of hot marquenching oil.


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## inferno (Sep 13, 2019)

i did some experiments today at work with a small torch. I wanted to see if it could take a piece of steel to non magnetic. it was a smaller 2kW handheld type (sivert powerjet). and my findings were that this torch would suck ass for any actual HT-ing. no ****._ almost_ impossible to bring any steel to non magnetic in open air.

so then i polished that steel with a soft spongey type disc on the angle grinder and tried to pretend this was infact a sword blade that i was tempering with the torch.
and it worked extremely well and was easy to control. i could get all the tempering colors.

However. this raised some questions. now if i get, lets say, a golden brown color at the edge in like 30 seconds. and a regular tempering cycle is 2x1h or 2x2h to get this color.
I must be giving up something here right?? or?

I always thought this was time vs temperature kinda process. where the time component was actually important somehow.

Isn't it so?

let say i take 80crv2 and flame temper it to the equivalent color of 200C.
Or do 2x2h at 200C, will i get the same resulting toughness and hardness?? I kinda doubt that. But i dont know.

is the flame tempering instantaneous? but the oven tempering not? why do the steel manufacturers suggest 2x1h and not a 30 seconds tempering?


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## inferno (Sep 13, 2019)

the more you learn the more confusing it gets.


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## bryan03 (Sep 14, 2019)

the color you get in tempering is a surface oxidation, it will depend on the steel composition , the way it is degreased and the heating speed. do not focus on that.


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## Kippington (Sep 14, 2019)

inferno said:


> I always thought this was time vs temperature kinda process. where the time component was actually important somehow.
> 
> is the flame tempering instantaneous? but the oven tempering not? why do the steel manufacturers suggest 2x1h and not a 30 seconds tempering?


What you are referring to is the kinetics side of heat treating. You are absolutely correct in your assumptions above, but for tempering simple carbon steels the only thing we need to consider is how the carbon atoms come out of solution. Two important factors come into play here: Carbon atoms are very small and move through the iron lattice surprisingly quickly (given enough temperature), and importantly they don't have to travel far (we're talking sub-micron distances) to find a nucleation point or existing precipitate to latch on to.

The reason it's suggested to hold the steel at temperature for some time during tempering is because the result has been accurately measured using a tested and repeatable formula. If you shorten the duration (at the same temp) it can lead to different results, but there's no doubt that the steel will still have been tempered to at least a lesser degree. It's possible to make up for this with a higher temper at a shorter time-span, but this would obviously be moving into uncharted territory until you decide to test and record the results yourself.

The second tempering cycle is often suggested to temper any fresh martensite that was formed during the first temper cycle, which will sometimes be created out of any retained austenite that was oversaturated with carbon during the quench. The first temper will allow this carbon to move out of the RA and will in many cases allow a martensitic transformation to occur upon cooling.



inferno said:


> Is there any reason why one want to use the torch in the middle of the blade instead of the spine? does it get too weak otherwise?


To increase accuracy you should aim the heat-source at the thicker part of the blade and allow the heat to migrate through the steel towards the thinner edge. Keep in mind that on many swords the middle of the blade _*is*_ the spine, which makes your question a little strange.
Overheating the spine is not to much of a problem, but overheating the edge is a different kettle of fish entirely.


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## Kippington (Sep 14, 2019)

bryan03 said:


> the color you get in tempering is a surface oxidation, it will depend on the steel composition , the way it is degreased and the heating speed. do not focus on that.


If you're going to dismiss a traditional method, you should at least suggest something better... keeping the original post in mind.
If your answer is to buy a bigger oven, you're completely missing the point of this thread.


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## bryan03 (Sep 14, 2019)

Kippington said:


> If your answer is to buy a bigger oven, you're completely missing the point of this thread.





bryan03 said:


> be friend with a baker, they have big oven


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