Induction tempering of plain carbon steels

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Oct 19, 2016
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Hello everyone,

I recently got an induction stove for the kitchen and I simply could not resist putting some steel on it, just to see what happens...

I didnt really push it, for safety reasons, but I managed to get the most uniform straw color on a knife blank Ive ever seen.
All under two or three minutes.

This raised a question:

How long does a temper cycle need to be in order to be effective?

Can someone explain to me what happens when you temper freshly quenched blades made of simple hc steels?

Back in the day, thats all they had available and they did their tempering in the same coals they used to forge and quench.
Now, I have no idea how long, if at all, did they keep their work at the apropriate temperature. To my understanding, it was all over in a few minutes, at least per cycle.
If there are supposed to be more than one, I do not know.
When I do it, I heat to straw, which takes a couple of minutes and then either leave the blade to air cool or arrest further color change by quenching a little.
Either way, my blades rarely stay at those 200 or so celsius like a modern tempering oven does for you.

This method is historically proven to work and it has been working for me too. But given modern metalurgy has a thing or two to say about the traditional way of doing things, Im wondering whats the difference between the two approaches, traditional vs modern tempering?

Long story short, Im interested to know if using induction tempering is a viable option or is there something fundamentally wrong with my process of thinking here?

Because, to me, tempering in the forge and doing it on the stove is pretty much the same for the time it takes to accomplish both.
The only difference I can see is that with induction tempering, the heat comes from inside your piece, not from the outside like with regular way of heating. Or am I mistaken there too?

Anyways, Im interested to hear you opinions on this because it sure is convenient tempering like this :D

Thank you for your time and help, cheers.
 

Stacy E. Apelt - Bladesmith

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IF it is evenly heated throughout the entire blade, the change would likely only take a few minutes. Again, IF the blade was nearly instantly heated to an exact and even 400°F. it would likely take 5-10 minutes for the tempering ( just an educated guess). My worry would be how even the temperature was in all parts of the blade.

Most of the temper time in a HT oven is to insure that the temperature is exact and even throughout all the blade.
Martensite forms from austenite at literally the speed of sound. But the change from brittle martensite to tempered martensite is much slower.
 

HSC ///

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When you flash temper blades like hitachi high carbon simple steels (white), it is done in seconds. I do this all the time
 
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So basically, if my stove can heat the blade even, its fine?
Good to know :)
I wasnt planning to use large blades, only ones that can fit on the stove completely.
I would still temper larger things the usual way.
 

Stacy E. Apelt - Bladesmith

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Yes, I understand that tempering by color quickly has been done for a long time. Flash tempering and flame tempering tempers the thinner edge portion ( sand surface to some degree), but doesn't penetrate into the body of the blade. This is fine in a kitchen knife, but not as good in a camp chopper. Done properly and repeated a couple times it is a suitable method of tempering. It works best on very simple carbon steels which have low RA potential.

My answer was to the OP's concerns about his induction tempering method. It may be fine, I just was noting my concern about how even the tempering was.

Hopefully Larrin will chime in on this.
 
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doesn't penetrate into the body of the blade
Thats what I was getting at.

As I understand how induction heating works, its heating up the steel from the inside out.
If thats correct, then by the time the surface shows straw, the interior might be already way hotter than that. Or negligibly....
I have no idea. Thats my main concern.

I can easily do 20-30 ''flash'' tempering cycles in an hour without breaking a sweat with this thing, if thats what it takes.
As far as I can tell, Im getting uniform straw color without having to do anything besides pushing a button.

But Im worried that the nature of the heating process itself is giving me a softer blade than intended because of the way its heating it up.

Thats what I want to understand better.
 

Cushing H.

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A couple thoughts and a memory…

Tempering does not reach really high temperatures, but the surfaces of induction stoves is glass, so I might be concerned about getting a piece of steel so hot that you might end up cracking the glass in the stove top (when cooking food, the food in the pan helps keep the actual temperature of the pot/pan at a reasonable level) so … proceed with caution?

A memory: growing up I had a very good (older) friend, an electrical engineer, who worked at a company called “radio frequency inc”. They produced (often custom) manufacturing equipment for inductively heating metals to , among other things, harden and anneal those metals (usually on a continuous basis) (I worked for the company for a couple summers). The guts of the equipment were quite big (12 - 18” diameter) heavy gauge induction coils where the metal was drawn axially through the center of the coil.

So it clearly can be done … but I guess careful control would be key.

(I actually just came into possession of an induction cooktop. The pain is that it requires specialized cookware. The cool thing is that the heating process is almost as responsive as a gas burner. Pretty cool technology.
 
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Thats what I was getting at.

As I understand how induction heating works, its heating up the steel from the inside out.
If thats correct, then by the time the surface shows straw, the interior might be already way hotter than that. Or negligibly....
I have no idea. Thats my main concern.

I can easily do 20-30 ''flash'' tempering cycles in an hour without breaking a sweat with this thing, if thats what it takes.
As far as I can tell, Im getting uniform straw color without having to do anything besides pushing a button.

But Im worried that the nature of the heating process itself is giving me a softer blade than intended because of the way its heating it up.

Thats what I want to understand better.
Google .......................induction tempering process
I found some very interesting things :)
 
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proceed with caution
Yeah, that was my main concern, but it went well this time. If I ever decide to try this again, which I very well might, Ill put something insulative below, to keep the glass cool :)
Google .......................induction tempering process
I found some very interesting things :)
It didnt even occur to me to google this lol
I trust you guys more than plain ole google :D

The first article I read stated the main benefit as time saving, cool.
I was really worried that such speed is giving little time to the steel to change its structure.
Guess not...
Id still like to give it at least a couple of cycles, since it cant hurt anything if I dont overheat...

My main problem would be what I mentioned above:
Since the steel is being heated from the inside, would that result in a softer core by the time straw color appears on the surface?
Or not at all, because the whole process is so short?
 

Stacy E. Apelt - Bladesmith

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Induction heating of metals is done by putting the metal in a radio frequency zone. This agitates the molecules/atoms and creates internal friction, and thus heat. Depending on the equipment, this agitation happens from a few thousand times a second to a couple million times a second. Heat is created REALLY fast.

My main point is that commercial induction treatments of steel (annealing, hardening, tempering) is not done on a home range top. The equipment can cost up to a million bucks in some cases. It is very controlled and has hade a lot of testing to reach the desired result.
Things like the teeth on a bandsaw blade are hardened by impulse/induction. Continuous annealing if wire and such is common.
 
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Yeah, that was my main concern, but it went well this time. If I ever decide to try this again, which I very well might, Ill put something insulative below, to keep the glass cool :)

It didnt even occur to me to google this lol
I trust you guys more than plain ole google :D

The first article I read stated the main benefit as time saving, cool.
I was really worried that such speed is giving little time to the steel to change its structure.
Guess not...
Id still like to give it at least a couple of cycles, since it cant hurt anything if I dont overheat...

My main problem would be what I mentioned above:
Since the steel is being heated from the inside, would that result in a softer core by the time straw color appears on the surface?
Or not at all, because the whole process is so short?
Get some temperature measurement device.Insert thermocouple/ one which need contact with steel to measure / between two piece of steel and watch what is happening.......That was what i done when i finished my tempering oven .......So i was able to read temperature ON steel and temperature on PID from TC in oven ...
You need something like this .They come with two thermocouple , one for high temperature and one for low temperature ,around 250 Celsius ...
ydUFG4G.jpg
 

Stacy E. Apelt - Bladesmith

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That won't work while the steel is in the RF zone. The RF would also heat the thermocouple.
 
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I might do that in the future. In fact, I almost certainly will.
But not at this time :D
I was just messing around with this newly found toy and I was hoping somebody else already tried something along these lines.
 

Larrin

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Induction tempering is usually done at significantly higher temperatures for short periods of time. At short times there are big differences in the degree of tempering with relatively small time and temperature differences. In other words the difference between 30 seconds and 1 minute is much greater than the difference between 2 hrs and 2 hrs plus 30 seconds. So I would be concerned about repeatability without a thermocouple.
 

Mecha

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I know someone who uses an induction heater while forging and one of his favorite things about it is that it does NOT overheat the thinnest part of the blade edge. The heat is very even throughout the entire billet regardless of thickness.

This isn't for heat treatment, however, just for forging.

OP I'd say to test the hell out of it of the blades to see how it's working.
 

john april

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make a knife using it, then video a brass rod test showing if the edge flexes back to its original position.
 
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Why not put a steel plate in the oven, warm it up and let the oven hold the temp.

I trust modern temp control way way more than any eyeball.
 
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Hello everyone,

I recently got an induction stove for the kitchen and I simply could not resist putting some steel on it, just to see what happens...

I didnt really push it, for safety reasons, but I managed to get the most uniform straw color on a knife blank Ive ever seen.
All under two or three minutes.

This raised a question:

How long does a temper cycle need to be in order to be effective?

Can someone explain to me what happens when you temper freshly quenched blades made of simple hc steels?

Back in the day, thats all they had available and they did their tempering in the same coals they used to forge and quench.
Now, I have no idea how long, if at all, did they keep their work at the apropriate temperature. To my understanding, it was all over in a few minutes, at least per cycle.
If there are supposed to be more than one, I do not know.
When I do it, I heat to straw, which takes a couple of minutes and then either leave the blade to air cool or arrest further color change by quenching a little.
Either way, my blades rarely stay at those 200 or so celsius like a modern tempering oven does for you.

This method is historically proven to work and it has been working for me too. But given modern metalurgy has a thing or two to say about the traditional way of doing things, Im wondering whats the difference between the two approaches, traditional vs modern tempering?

Long story short, Im interested to know if using induction tempering is a viable option or is there something fundamentally wrong with my process of thinking here?

Because, to me, tempering in the forge and doing it on the stove is pretty much the same for the time it takes to accomplish both.
The only difference I can see is that with induction tempering, the heat comes from inside your piece, not from the outside like with regular way of heating. Or am I mistaken there too?

Anyways, Im interested to hear you opinions on this because it sure is convenient tempering like this :D

Thank you for your time and help, cheers.

Look into traditional Japanese tempering, Yaki Modoshi, it is the process after Yaki Ire and it is all fast tempering techniques. Usually done right after quench, and the water droplet test is done to test the temp. You drop water onto the blade and whent he water dances and starts to bubble you know it's been tempered.
This is usually done with blades that have been clay quenched during Yaki Ire process.
Play around and do some tests, see if the temper is as solid and strong as a long temper, check the cross section after break, see how much force it can take in a bend before it takes a set etc.
Is it more brittle, larger grain?
If you can do Yaki Modoshi correctly, you should produce a good knife, better than a long temper? you find out.
Just know, I switched from Yaki Modoshi to 2 hour oven temper cycle for a reason.
 
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I would think that it would be hard to control and like Larrin said getting it to repeat would be very hard. Tempering involves both time and temperature so if I understand things right for short tempers rather than the 1 to 2 hour first temper would need to be done at a higher temperature than the normal longer temper. It's commonly used to preheat and heat treat parts while welding but thermocouples are attached to the work and temperature control is tight and a record is made of the entire cycle.

Since there is an oven right underneath that would do a good job I'm going to assume that you want to do it just because it's fun and kinda cool. That's a totally valid reason. Lol. With a timer and Tempstick crayons you can probably work out a procedure. They aren't that expensive. Get ones lower than your planned temperature and just make a bunch of stripes on your piece including ones that go over and you should be able to watch the temperature come up. I don't know how fast it works but you might use a magnet to grab it. Since your tempera are going to be short it might be best to quench the piece as soon as it hits target temperature so you don't have to factor in air movement and temperature into the dwell time. Remember that you are going to be hotter than if use use more time in the oven. Reading through Larrin Larrin s article on tempering would be a good idea and maybe he could give you some idea where to start for temperature. I suppose you could also see if a laser thermometer would work. You would need some sort of procedure for one though since things like distance can make a difference.
 
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