Nuwave cooktop

SinePari

SinePari

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Oct 24, 2013
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Grandma sprung for the BOGO (if you pay the S&H) on a Nuwave cooktop... I was experimenting cooking bacon the other day and the tolerances regarding heat transfer were accurate within 2 degrees of the displayed temp... It ranges from 100-575 degrees Fahrenheit... I'm thinking it might be the ticket for for tempering ferromagnetic blades as well as cooking bacon... Any thoughts on this possibility? After hitting the pan with the temp gauge about 20 times during the duration of cooking the pan stayed a consistently within 2 degrees tolerance, in spite of the ambient temp of the kitchen being 73 degrees... I'm unfamiliar when it comes to induction heating... Wouldn't it accomplish the same task as any other oven out ther in regards to tempering ferromagnetic blades?
 
The cooktop might be designed to have a circle of metal on top of it, not a knife blade shape. It might not heat evenly, or it might make the induction circuit unhappy. If there's no prohibition in the instructions about small or oddly shaped cookware. it might be worth an experiment with a scrap of mild steel to see if heats evenly end to end and side to side...and to see if any residual magnetism results in the scrap
 
Mahoney said:
The cooktop might be designed to have a circle of metal on top of it, not a knife blade shape. It might not heat evenly, or it might make the induction circuit unhappy. If there's no prohibition in the instructions about small or oddly shaped cookware. it might be worth an experiment with a scrap of mild steel to see if heats evenly end to end and side to side...and to see if any residual magnetism results in the scrap
Click to expand...

The only stipulation in the literature is the pan, vessel, or "cook-wear" must be ferromagnetic and not aluminum or glass... They demo the thing with 3/4 of a pan to show that the surface is cool to the touch, but cooks bacon in the pan... I'll give it a go and see what happens... hopefully it doesn't explode, because it cooks a mean pan of bacon.
 
Basic Theory of Operation:
1) The heating is by copper induction loops.
2) A feedback circuit is used to control those loops.
3) A ferromagnetic object must be placed over the TC ( probably dead center) to control the feedback circuit. It heats by the iron atoms being vibrated.
4) The ferromagnetic object must have large enough mass and high enough thermal conductivity to evenly distribute heat generated in the iron atoms. It does this by thickness, metal type, and most commonly by laminating a layer of copper in the material.
5) The object must be uniform in thickness to distribute the heat evenly.
6) The object must have its mass centered with and in close contact with the TC reading the temperature. (Ignore advertisement gimmicks like half pans)


From an engineering POV concerning HT of knives:
A knife blade has small mass, an uneven and varying thickness, tapers preventing even surface contact ( with the TC), and poor conduction from one area to another. While it will be heated by the induction loops, it is probably the worst object to heat on an induction circuit if you want the entire piece to be at the exact same temperature all the time. It is entirely likely that the blade will greatly overheat in the outer areas before the heat conducts to the spot over the TC. Uneven heating will be a virtual guarantee as I see it.
Additionally, I have no idea how rapidly vibrating atoms in the crystal structures will affect the conversions we want to happen in HT. I also doubt this has been studied concerning knives. From my take it could induce weak places in the metal ( basically stress risers).
 
So would testing a piece of knife shaped scrap be pointless? If I were to do so would there be any way of suggesting positive factors without putting the steel under a microscope? While the frying pan had consistent temps, it would be an assumption at best to say that it would heat something of knife profile evenly.... If the profile of a blade had even heat, during the duration of a 1 hr. window of time, testing it every 5-10 minutes, would there still be a chance of stress risers?

I'm not married to the idea being a success, and if the properties of consistent heat doesn't translate into proper HT, than I guess it's a dead end...
 
Induction heating and welding of steel is done widely in industry, so research about the suitability (or not) of induction heating for tempering is probably out there somewhere. That said...a Nuwave cooktop is about as similar to an industrial induction heater as a motorcycle is to a school bus. Doing the test won't be pointless, we can link the next guy who wants to temper on an inductive cooktop to this thread and your results...

If you test the piece of scrap and it heats evenly, then you can worry about the vibrations of the atoms. It's much more likely that the cooktop won't heat the knife shape very evenly, or might overheat thin areas like the edge
 
Mahoney said:
Induction heating and welding of steel is done widely in industry, so research about the suitability (or not) of induction heating for tempering is probably out there somewhere. That said...a Nuwave cooktop is about as similar to an industrial induction heater as a motorcycle is to a school bus. Doing the test won't be pointless, we can link the next guy who wants to temper on an inductive cooktop to this thread and your results...

If you test the piece of scrap and it heats evenly, then you can worry about the vibrations of the atoms. It's much more likely that the cooktop won't heat the knife shape very evenly, or might overheat thin areas like the edge
Click to expand...

Thanks Mahoney... I'll give her a shot and share the results, but I have no way of being able to truly identify issues on the atomic level, stress risers, or for that matter testing hardness...
 
Something to think about if the test does not work out...

It did occur to me the other day that perhaps putting the knife right on the cooktop might not be the only way. What about putting something like a cast iron dutch oven on the cooktop, lay a chunk of fire brick in it to get the knife blank up off the bottom, and treat it like a regular oven for tempering? The downsides, it will take longer to get the blade up to temperature, and you will be losing heat from the outside of the dutch oven, so your cooktop will be working hard and using lots of power to keep it hot. But accuracy within 2 degrees F is much better than most ovens can do.

As far as the vibration of the atoms...when something is hot, the atoms are vibrating. Doesn't matter really how it got hot, the vibrations are related to the temperature. But induction heating uses ginormous magnetic fields to induce local current flow and resultant heating, and how that magnetic field might interact with the desired crystalline structure in tempering is something you'd have to research. It looks to be possible though, a quick Google search brought up more than a few papers and the process is apparently used successfully in the automotive industry for tempering carbon steel components.
 
Placing the blade in a frying pan, or on a circle of 1/4" steel, might work better than a blade laying on the cooktop...but there will be a hot side and a cool side of the blade. HT and tempering are all about evenness of heat. Zones, random variable spots, etc. should all be avoided.
A Dutch oven would be a poor idea, as they are designed to be heated from all sides externally, and radiate that heat evenly inside due to the large thermal mass of the pot. Inductive cook tops are bottom only heat. (In a regular gas/electric kitchen oven, if you have variables in your oven heating, a Dutch oven with a fire brick platform for the blade would aid evenness of heat in tempering a blade. )

While induction is used in may industrial processes I seriously doubt they use a low cost tabletop cooker for the source. Their equipment has coils specifically shaped to heat a specific size object evenly. Good induction heating equipment with temperature control is quite expensive.

The simple answer here is to cook your soup on the Nuwave cook top and temper your blades in an oven. I get a kick out of people looking for elaborate answers to a simple question.
 
One of the knife makers [Mora ?] HTs knives with induction .They do and edge harden with one loop of copper tubing which heats the edge .It's then quenched , leaving a hard edge ,soft spine .There is a video of it.{haven't found it yet].
That of course is designed for one blade .Trying to do multiple sizes and shapes wouldn't work well. Time , frequency, shape of and closeness of the coils are significant .You can melt steel if you like as I did in my thesis back a few years !
 
SinePari wants to accurately temper the blades by induction. This is a far worse situation than hardening, which, as you say, is complex.
 
I think Sine Pari just want a damascus blade that looks like bacon !
 
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