Edge Quench and Tempering Spring Steel?

[wortbag]

Hi All,

A question out to the knife making hive mind.

We've just returned from Cambodia where to my surprise my other half agreed to join me on a knife making visit to a local smithy.The workshop is a normal, everyday smithy with locals dropping stuff off for work to be done etc, not just a tourist job. Details of the place are
https://www.backstreetacademy.com/siem-reap/535/medieval-knife-making
Not very medieval as grinding was all electric, as was the fan for the furnace, but hammers never really date! A really good experience that brings income to artisans that wouldn't be able reach outsiders without the help of local translators. Worth a look if you're lucky enough to visit Angkor Wat.

Knife blades were hammered and ground happily enough from leaf springs, even though it was 34C outside the workshop, but when it came to the heat treat and quench only the cutting edge was water quenched. He rolled the cutting edge into the water, then looked, and continued to lightly sprinkle water by hand along the edge. Probably never put the edge deeper in the water than 1cm I guess, the blade is approx 65mm deep and a curved cutting edge of approx 40cm, 6mm steel.

Advice required as to the need to temper this blade? Will the edge quench leave the spine soft anyway? Do I try and get this redone here? The interpreter was good enough for basic stuff but not quite up to translating the more detailed questions.

So over to you...

Cheers
John

 

[Stew]

Yes, an edge quench will leave the spine soft.

I might have missed you saying but if I have it right, you've purchased a blade and brought it back with you. I'm guessing no handle? If so, you could temper it and it'll do no harm to anything the original smith did but may be better for you.

He may have tempered it but you might not have realised.

 

[wortbag]

Thanks Stew,

no tempering process happened, whole thing was done in the workshop while we helped. You are correct, no handle was fitted while there, just wrapped in newspaper and back into the Tuk Tuk for the return journey. As the steel is unknown, I'm guessing for a general large slashing blade two hours as hot as our oven will go, 240 C, then slow cool down?

Cheers
John

 

[Snufkin]

Thanks Stew,

no tempering process happened, whole thing was done in the workshop while we helped. You are correct, no handle was fitted while there, just wrapped in newspaper and back into the Tuk Tuk for the return journey. As the steel is unknown, I'm guessing for a general large slashing blade two hours as hot as our oven will go, 240 C, then slow cool down?

Cheers
John

It's a long while since I've done any forging but if I remember correctly some of the lower carbon steels can be water quenched and require no further tempering but I know of a few folks who would temper them to stress relieve the blade. 240c may be too hot, I'd try 200-220c

 

[Stew]

I can't give you a definite answer as to temperature as your oven might lie and we don't know the steel.

I would start at 200, look for oxide colours and test the blade then go up in temperature until it works right for you.

An hour at a time per temperature.

 

[demographic]

Surely if its just quenched at the edge they can in effect temper the edge by allowing some heat back from the still hot spine?
Or even just leave the edge fully hard like some japanese blades are, although they use a refractive...err...dried...mud? On all the blade apart from the edge to keep heat in during the quench.

Maybe?

 

[Stew]

It's just a maybe though. It's all a big unknown.

Tempering it won't hurt.

 

[wortbag]

Thanks for your input fellas, 200 C for an hour it is, I'll get a picture up later if I can.

Cheers
John

 

[Dave Budd]

right. where to start?

the smith may have either 'slack quenched' or tempered as part of the hardening process.

It is common blacksmith practice to harden the edge of a tool and then use the residual heat in the body of it to draw the temper to the edge. It works really well with things like axes and cold chisels because they have a lot of mass and you can clean the metal with a grinder/sandpaper/stone between hardening and tempering (so you can see the oxide colours), but it is possible to use the way water boils and evaporates off the surface as a guide too (takes practice!).A heavy knife like a kukri is often done this way and there are loads of vids of smiths using a kettle or teapot to pour water over the blades rather than sticking the blade in a bucket.

Slack quenching is bringing the blade up to ALMOST hardening temperature and then quenching. That way not all of the carbon concverts to martensite and so doesn't become fully hard. It's not as tough as properly hardened and tempered steel, but is better than nothing at all. This method was the earliest heat treating technique that was knowingly and repeatedly used, by early I mean Roman and a bit after.

I doubt very much that the buy did the slack quenching and if he did I wouldn't trust it to not give a soft or a brittle edge. Ancient steel was shallow hardening and crude techniques work very well. Modern steels, especially most used in leaf springs, are deep hardening and complex alloys; so very unlikely to give good or repeatable results with techniques like this.


I suspect he either tempered when you didn't see or did it with the residual heat. If he didn't temper it at all, then definitely temper it yourself! stick it in the oven at 200C for an hour or so (once it has got to temperature: use an oven thermometer not the dial on the oven!).

 

[wortbag]

Thanks Dave for the reply, it's currently cooling down in the oven. I can only imagine residual heat as the only option, total process was two hours long and I'm not sure I noticed any reheating after quenching. Blade is large so your thoughts about large mass being used to temper may be spot on.
Cheers
John

 

[Nice65]

Thanks for your input fellas, 200 C for an hour it is, I'll get a picture up later if I can.

Cheers
John

That’d be great, I love a work in progress thread.

 

[Nice65]

Don’t know if anyone watched Our Guy in Japan, there’s a great scene where he helps Korehira Watan, one of Japan’s remaining traditional swordsmiths, forge a sword. Worth finding on All4 or wherever you can. Brilliant and inspiring stuff.