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Oven tempering using a heatsink.

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sploo

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MusicMan":17ezf8er said:
Yes, colour indicates the thickness of the oxide, and nothing else. The well-known charts are compiled for a standard 'blacksmith' rate of heating, hence they are practically useful.

If you leave the metal at a fixed temperature for some time, the oxide will thicken and hence the colour will change even though the temperature has not.

If you want the maths, there is no such thing as the temperature that oxide starts to form, or rather, it is absolute zero. The rate of growth increases exponentially with temperature, and logarithmically with time at that temperature, assuming a coherent oxide that does not crack off. If it does, then the growth rate is more or less linear with time. Iron oxide is coherent when thin and incoherent when thick.
Thanks MM. That tallies with this (old, but very interesting) paper I found: https://nvlpubs.nist.gov/nistpubs/jres/ ... 63_A1b.pdf

EDIT: I find a lot of people are also incoherent when thick. Ahem.
 

MusicMan

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"Thanks MM. That tallies with this (old, but very interesting) paper I found: https://nvlpubs.nist.gov/nistpubs/jres/ ... 63_A1b.pdf"

Yes that paper seems pretty sound and NBS/NIST work was/is always excellent. (I was on the review panel for their Physics lab in the early 2000s). Understanding of the maths of chemical kinetics made a lot of progress in the 1950s and 60s, but that paper was sound for the time.
 

sunnybob

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I think you have it by now, but i will try one more time in laymans terms (cos I was naff at technical science)

The element heats to the temp set on the thermostat. (say 200c for the sake of argument).
IF The stat is correct, the power will be shut off to the element at 200c.
Because electricity is slow to heat and slow to cool, you get whats known as "overshoot" where the element will still heat for a very short time untill the element has absorbed all the electric.
Its possible for actual temp in the oven to reach 220c for a very short while untill the element reaches max temp and starts to cool. When the stat reads the low temp set point (usually 10% of the high setting), the element starts to heat again, and so on and so on.
All irrelevant if cooking food, 'cos the high and low temps even out every few minutes.

Air in the oven touching the outsides of the oven will cool fairly rapidly. A tub of sand will take much longer to cool because the heat in the middle of the tub is insulated by the dense sand around it.
This is the way the old economy 7 night storage heaters worked. Heat up very dense stone overnight and let them give off heat for many hours during the day.

Now we move onto the metal. Anyone who is hooked on Forged In Fire, will know that when working metal in a forge, timing is as critical as heat. Overheat it and it will go brittle and crack. Underheat it and it will be softer than a soft thing in the sun. Cool it too quickly and it will shatter.

Get it just the right heat FOR JUST THE RIGHT TIME, and sir, "it will cut"

=D> =D> =D> =D> =D> =D> (hammer)
Sorry Doug. :roll:
 

MusicMan

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sunnybob":e5132ar1 said:
Now we move onto the metal. Anyone who is hooked on Forged In Fire, will know that when working metal in a forge, timing is as critical as heat. Overheat it and it will go brittle and crack. Underheat it and it will be softer than a soft thing in the sun. Cool it too quickly and it will shatter.

Get it just the right heat FOR JUST THE RIGHT TIME, and sir, "it will cut"

=D> =D> =D> =D> =D> =D> (hammer)
Sorry Doug. :roll:
Your paragraph on the metal is too much of a simplification, especially the overheat statement, though it all depends on the particular alloy. But the last sentence is spot on!

Keith
 

Bm101

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I think the over simplification is on my behalf to be honest Keith. :wink:
Thanks all. I have a far better general understanding overall.
In one of those odd little moments of coincidence I came across one of those workshop practices books today. Number 1. Had completely forgotten I bought it (and a few others) and it was 'tucked away' by the Mrs in my desk.
The title?
Hardening, tempering and heat treatment'
:oops:

So. That's the Christmas Reading List sorted.
 

Woody2Shoes

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sunnybob":13eha3lo said:
MikeG.":13eha3lo said:
Woody2Shoes":13eha3lo said:
.........Yebbut I think we've agreed that it can probably increase the average temperature of a piece of steel inside it, relative to an identical piece alongside but outside it - due to its thermal inertia..........
No, I don't think so. The average temperature of both would be essentially the same, assuming they both started at the same temperature. The range of temperatures would be much different, but the average the same.
As mike (and I) say, anything inside the oven that is inert (like sand) CAN NOT increase the temp. It can only hold the temp for longer than the surrounding air.
Length of time at any one temperature can affect the metal colour.
I think you're missing my use of the word 'average' - that's important. The typical temperature regulation in an electric oven/furnace is a bit like the graphs on here:

https://www.eurotherm.com/en/temperatur ... made-easy/

My point is that the heat input to the "sytem" is continuously fluctuating. A lump of stuff - closely thermally coupled with the workpiece - which has a high thermal inertia will smooth out these fluctuations in a way that helps to increase the average temperature of the workpiece over time (versus the equivalent setup without the added thermal mass) - partly because it stops it cooling in the 'undertemperature' parts of the cycles. Obviously, any effect depends on the relative time constants involved and will be less noticeable where the oven is well thermally insulated.

Cheers, W2S
 

MikeG.

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Woody2Shoes":1f7aolv0 said:
.......My point is that the heat input to the "sytem" is continuously fluctuating. A lump of stuff - closely thermally coupled with the workpiece - which has a high thermal inertia will smooth out these fluctuations in a way that helps to increase the average temperature of the workpiece over time (versus the equivalent setup without the added thermal mass) - partly because it stops it cooling in the 'undertemperature' parts of the cycles........
Yeahbut.......the heatsink will also keep the workpiece cooler than the oven when the oven goes above the set temperature, which is why I contend that the average temperature will be the same.
 

MikeG.

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Here is what I think is happening, with the dashed line being the average temperature in the oven, the blue being the air temperature in the oven, and the red being the heatsink temperature:

 

CHJ

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When heat treating small critical components in Industrial grade ovens we treated it as normal practice to place a larger thermal mass in the oven and let the oven temperature stabilise before introducing the critical component in close contact with the thermal mass.

I would certainly do this with a domestic oven if I was aiming for a temperature in the tempering region for a cutting blade.
And due to the lack of internal thermocouples for most instances putting in a test piece to watch for the colour cast achieved before submitting the work piece.
 

Droogs

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From what I've learned from horologists is that the use of a heat-sink usualy brass fillings rather than sand is not to raise the temp but to provide a more regulated and even temp across the whole piece being tempered to enable a much more consistent thickness of oxide layer thus providing a uniform colour as the light refracts on the surface once polished. Admittedly this is usually for when bluing hands etc but I'm sure the principle works for plane irons as well. Very well explained process here:

https://www.youtube.com/watch?v=NhjiIPohUywtemp

So the bluing on the OPs iron is due to length of time at a particular temp giving a thicker layer and not how fast it is heat up or cooled down
 

TheTiddles

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Much discussion here, mostly correct.

Using “heat sinks” (a term engineers avoid using unless they’re electrical as they often don’t get that there’s no such thing as a “heat sink” like there is a current sink) does not magically make things hotter (or colder, the point of realisation at which I usually get brought in). In the same way as there is no such person that can’t lose weight by eating less... you just can’t cheat at thermodynamics.

One thing people are noticing is that measuring temperature is actually quite hard, many think they are measuring the temperature of something without realising they are actually only measuring the temperature of the thermometer, thermocouple etc... which may be very different to what they actually want to know the temperature of.

Aidan
 

Max Power

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In another thread concerning tempering tool steel in an oven I gave out a bit of misinformation in error concerning the use of a sand heatsink and that it would raise the Temps in a domestic oven to above the normal range. I was gently enlightened that a heat sink can't raise the Temps. I have no reason to disagree and I am always happy to learn but it does puzzle me.
The original thread is here : making-a-brass-infill-plane-hattori-hanzo-dp-t120331.html
Rather than clutter that excellent build thread up I thought to raise the question here.
Hattori shows a wheat coloured temper on his O1 steel that seems to indicate an oven temperature of around 200 celcius. Ok no worries there. Most ovens in the uk go to 250.


But if the heatsink (sand in a turkey baking foil tray) doesn't actually raise the Temps, then when I did this 4mm thick iron for my first plane, how did achieve this blue colour that grades somewhere between 300 and 330 celcius?


I'm genuinely intrigued.
With this iron actually, I fluked. In use it holds an edge at least as long as as any I have bought. Longer I think. Obviously I'm not applying DW. Type testing conditions! Just seems to work.
Mind you if you ever eaten an animal you killed yourself it's maybe the same delusion as that lol. Fish you catch yourself always tastes better.
Next doors cat too if you can get away with it. :-"

What do we think people?
Cheers
Chris
Very interesting and useful thread, I have no knowledge at all on the subject so handy to refer to.
 
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Jelly

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My point is that the heat input to the "sytem" is continuously fluctuating. A lump of stuff - closely thermally coupled with the workpiece - which has a high thermal inertia will smooth out these fluctuations in a way that helps to increase the average temperature of the workpiece over time (versus the equivalent setup without the added thermal mass)
[Note: On reflection, explaining my logic clearly here would be much better done with some diagrams, and/or a system of equations... But that's apparently not something my brain is willing to do for me at1am on a Tuesday morning. Sorry!]

So I'd agree that:
In no case could the thermal mass equilibrate to a temperature greater than the average temperature of the hottest element within the system (i.e. the heating element) because that would violate thermodynamic principles.

But, there's an additional factor to consider... Does some property of the thermal mass alter the modes of thermal transfer relative to the oven's design intent?

Ovens are designed for heat transfer via convection (be that natural or forced) to predominate, so the average temperature of the air as a heat transfer medium will act as a limit on the temperature an item in the oven can reach, exactly how you describe.


However many of the cheaper, naffer electric ovens I've used in my years as a connoseur of inexpensive rented houses, had elements which would get far hotter than the desired air temperature when on, then just switch off for a bit.

When I say far hotter I can be confident of this because the elements were visibly glowing, and incandescence generally begins to occur at no less than 525°C, that incandescence also means Radiative heat transfer is beginning to become relevant.

So... If the heating element is at a much higher temperature than the air, and you insert a thermal mass which has a substantially higher absorptivity than air it could equilibrate with the air temperature, and then continue to absorb energy via radiative thermal processes and reach a surface temperature greater than the air around it.

That hot (relative to the air) surface of the thermal mass would then begin to equilibrate with it's surroundings, and if it's a thermally conductive material, conduction would predominate so most of the thermal energy absorbed would be transferred into the interior of it, until it began to reach internal thermal equilibrium, meaning convective heat transfer to the air around it would start to predominate.

At which point the geometry of the thermal mass becomes important, because it is possible to have geometries in which the heat rate for radiative heat transfer into the thermal mass becomes equal to the heat rate of convective losses from the thermal mass, at an object temperature greater than the surrounding air temperature.

Only once the air temperature began to rise above it's setpoint as a result of that convective transfer from the thermal mass to the air, would the oven element turn off.


So in principle there's a plausible mechanism by which a thermal mass with the right geometry and material properties could reach a temperature higher than the average air temperature of the oven; provided the oven in question had a sufficiently crude control architecture to make the radiative heat transfer mechanism viable.
 

Eric The Viking

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The oven I was complaining about (above) blew up in the week before lockdown. The failure took out the main logic board, making it uneconomic to repair. I paniced, and rang round locally to source a replacement. Ended up back at our usual white goods supplier, Horders, who I can't praise enough for quality of service, (definitely going the extra mile so we weren't left without an oven for lockdown). So what follows is by no means criticism, and I wholeheartedly recommend them both for prices AND service...

... Our new "Bosched" Neff oven is horrid, in comparison to the proper Neff one it replaced. I have measured it with a type K thermocouple (all I have that will cope with the temperatures. When the oven is set to 200 C, it initially overswings by about 30 C, with a hysteresis range of about 50 C overall. It settles to an average temp of about +10 to +20 C of target. I have "re-calibrated" the dial with tape and a Sharpie.

Control was electronic in the last two Neff ovens (i.e. using actual thermocouples and measuring voltages), but is now down to oil-filled tubes and springy contacts. The new one's engineering is a throwback to 1950s design, and it is incapable of measuring the actual oven temperature, hysteresis notwithstanding.

This is one of many cost reducing (I guess) changes that have been made to the design since Bosch took Neff over, most of which are, in my view, simply cheapening it.

I am furious I got conned by the brand name (pretty much as expensive as its predecessor, too). I won't buy another, and I can see why people like Aga ovens. You may not be able to control the oven temp easily, but at least you get what it tells you!

E.
 
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