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 Post subject: Induction Hardening / Impulse Hardening of kitchen knives?
PostPosted: Wed Aug 17, 2016 10:06 am 

Joined: Mon Mar 28, 2016 11:40 am
Posts: 95
Just curious - as a woodworker I see plenty of induction impulse hardened saw teeth - but don't recall ever seeing an impulse hardened kitchen knife edge.

https://en.m.wikipedia.org/wiki/Induction_hardening

It would seem you could do a very controlled modern honyaki with it. Or do current j-knife suppliers to cktg already use it - if perhaps for only uniform heat treatment instead of gas etc.

For example, you could take a basic 420j2 inexpensive blanked blade - at a mushy HRC 53 or so, and get the first 10mms or 15mms up to, what? 57? 58?

Make a $11 Chicago Cutlery Ashley Santoku into a pretty decent knife - for next to nothing (after the initial capital investment)...


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 Post subject: Re: Induction Hardening / Impulse Hardening of kitchen knive
PostPosted: Wed Aug 17, 2016 10:30 am 

Joined: Mon Mar 28, 2016 11:40 am
Posts: 95
Let's go a step further - suppose you applied saw-blade type pulse induction hardening along the continuous (not cut sawtooth) edge of a blade.

The blade edge hardness now varies cyclicly along the edge. Hard 'virtual teeth' - soft 'virtual gullies'.

Would the hard virtual teeth keep the soft virtual gullies from wearing and rollover? Would the soft virtual gullies support the hard virtual teeth and keep them from chipping out?

When sharpening, would you get an effect like sanding wood with distinct early and late grain? Humps and valleys? Or would it remain uniform, with the hard virtual teeth controlling?


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 Post subject: Re: Induction Hardening / Impulse Hardening of kitchen knive
PostPosted: Thu Aug 18, 2016 7:38 am 

Joined: Thu May 24, 2012 12:20 am
Posts: 4033
I am no expert, but a few thoughts...

I would imaging inductive heating would be quite uniform which is undesirable in a true honyaki. Honyakis are differentially hardened with a softer spine than edge. This is a design feature that promotes optimum edge performance with added toughness from the softer spine.

Just thinking out loud here, I suspect the reason saw blades might be induction heat treated is to accomplish differential heat treating where the teeth are hard but the body of the blade is softer and tougher... I don't know, just a thought.

There are monosteel knives that are not differentially hardened and induction heating would probably be fine for this application but I don't think it would be "better" in terms of the quality of the end product. Ultimately, either process involves heating the steel to an appropriate temp for an appropriate time, then quenching. I don't think it matters what your heat source is.

Improving upon inexpensive production knives would be pretty hard. 420j2 seems like a rather crappy knife steel really. It optimizes cost and stain resistance over edge taking and edge holding. Frankly, I suspect that knives in this steel are getting a good proportion of available performance out of the steel and you would be hard pressed to improve much on it.

On top of that, heat treating is a difficult process. You would need to skin any non metallic components off the knife not to mention softer metallic elements (brass bolsters, etc) because they would be ruined. Next you would probably want to anneal the steel. Lastly, you would heat treat which can be a very persnickety process. Then you would need to temper the knife. All the while, while heating the knife, you are risking bending or cracking the knife. Finally you would refinish (descale) and reassemble. Seems like a hell of an inefficient process to get a "cheap" nice knife. Though if you do not expect success and you have an over abundance of time, it could be a very fun and educational process.

Lastly, considering heat treatment serrations, I think this would amount to nothing useful. Best case scenario, the soft areas wear quicker and become real serrations. Worst case, the hard portions mask the soft portions keeping making the knife behave like a faster wearing knife off the higher hardness.

But that is just my instinct on the matter.


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 Post subject: Re: Induction Hardening / Impulse Hardening of kitchen knive
PostPosted: Thu Aug 18, 2016 7:58 am 
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Cedarhouse is right about honyaki blades: The differential heating is what makes it.

The most consistent form of heat treat that I know of right now is molten salt baths. It allows for the best temperature control and the most uniform application of heat to the blade. This, however, would not allow one to achieve a differentially hardened blade.

Induction hardening for knives is still in the infant stages right now as temperature control and even heat distribution are rather difficult unless you have a huge induction machine to do it with.

It is interesting though, and thank you for sharing this with us. I'm not shooting the idea down, but as I understand we aren't "quite there yet" to make this a viable alternative to current methods. I was always interested in playing with one of these though, I'd probably destroy a lot of things with one. :twisted:



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 Post subject: Re: Induction Hardening / Impulse Hardening of kitchen knive
PostPosted: Thu Aug 18, 2016 8:07 am 
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Just did a little snooping. Interestingly enough, you can differentially harden gears with induction. The coil is wrapped around the diameter of the gear and the thinner cross-sections of the teeth get heated faster, allowing only the teeth to be hardened. I imagine leaving the gear in there too long would heat the whole gear, but with many of these tool steels you only need to get the steel up to temp, and then water quench. No soaking at temp is usually required.

induction.jpg
induction.jpg [ 95.84 KiB | Viewed 151 times ]


I suppose if doing a water-quenched steel, you could possibly make a knife with a differential heat treat as long as the spine is substantially thicker than the rest of the blade. I don't think applying clay to the blade would help as induction does not apply heat from the outside and the clay would essentially be trapping more heat from leaving the blade, whereas normally in a gas forge or such the heat is coming from the outside and the clay is able to repel the heat from the beginning.

Okay, time for that second cup of coffee this morning. :)



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 Post subject: Re: Induction Hardening / Impulse Hardening of kitchen knive
PostPosted: Thu Aug 18, 2016 8:20 am 
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Just did a little more snooping, and lo and behold, a coil that would allow only the blade edge to pass through. Ha!



I wonder what factors are holding this technology from being applied to knives?

At this point I can only speculate as to either cost of equipment or running it. It might also be difficult to achieve uniform heat along a long blade surface, and also most stainless steels need to soak at temp for a bit, so this might present another difficulty. It's interesting though, but usually there's a reason these things haven't caught on yet. I'll try to do a little more snooping tonight, this should make for some good reading.



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 Post subject: Re: Induction Hardening / Impulse Hardening of kitchen knive
PostPosted: Thu Aug 18, 2016 9:53 am 

Joined: Mon Mar 28, 2016 11:40 am
Posts: 95
Thanks comments and insights, guys!

To add, Cedarhouse is right on his surmise on the bandsaw blades. The objective is hard teeth, and a flexible, tough backing.

I think I read the bandsaw blades are hardened by an induction machine that also includes a water quenching step immediately after 'lighting up' each tooth.

I also read that the depth of induction hardening can be controlled by varying the frequency of the coil energenation. The higher the frequency, the shallower the depth. Reminds me variable frequency AL TIG welding..

So, you might be able to make a 'virtual laminated' blade - hard on the front, soft in the back.

OK for single- bevel knives at least?


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 Post subject: Re: Induction Hardening / Impulse Hardening of kitchen knive
PostPosted: Mon Aug 22, 2016 10:36 am 

Joined: Wed Sep 04, 2013 9:16 am
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Location: Oxford,MA
I am pretty sure Don Nguyen posted a video awhile back with some induction heating, I think this method may be used with forging , where you are getting a blade to a ballpark temp so you can safely pound the steel without causing the structure damage.



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