Oven tempering using a heatsink.

[Bm101]

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

 

[Woody2Shoes]

Were you using a fan oven? If so, I think what they try and regulate is the air temperature, rather than the temperature of lumps os material within the airspace.

 

[Sheffield Tony]

I wonder how much hysteresis the oven temperature control has. If the temperature cycles 200 - 250 - 300 - 250 - 200, a cake will not notice the difference between that and a constant 250, but the oxide colour probably reflects the peak temperature.

Wildly speculating here, you understand !

 

[Trevanion]

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. :-"

Ah, Sorry to hear about your cat going missing Ted... Here's some meatloaf I made yesterday to help keep your mind off it.

 

[sunnybob]

You need to calibrate your oven thermostat. Sand can not raise the temp. It can hold the heat for a very long time, so if you left the steel in there for longer than you should have the steel might have got hotter than your intended temp.

I have spent a lot of my working life with ovens, commercial and domestic, and its common to find a thermostat out by as much as a 100c. either way, although most are within +/- 5c by design.

 

[Eric The Viking]

I'm with Sunnybob on this. I have a cheap type-K thermocouple-based digital thermometer, which agreed with me that the oven was miles off. It under-reads by rather a lot (stuff wasn't being cooked properly). I estimate under by 10-15deg C. It's a Naff, er Neff. The fan oven is better, but the smaller oven that gets the most use has to be deliberately dialled "too high". The thermostat is right up in one back corner, where the it's hottest, which doesn't help.

SWMBO's oven thermometer is just a coil-driven needle (mechanical), and doesn't read at all reliably. But thermocouples should be consistent. For super accuracy one probe in melting ice or boiling water and t'other one measuring, IIRC. Mine lets you do that and came with two probes.

 

[ED65]

...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?

Maybe this is why:

I was roasting some potatoes and squash with the oven set to 350F, but the oven thermometer read 600.

:shock: :shock: :shock:

 

[ED65]

I wonder how much hysteresis the oven temperature control has. If the temperature cycles 200 - 250 - 300 - 250 - 200, a cake will not notice the difference between that and a constant 250, but the oxide colour probably reflects the peak temperature.

Wildly speculating here, you understand !

Yup, that's exactly how almost all ovens work. The heat goes on and off and the idea is that the oven averages out at the set temp. Or thereabouts. Approximately like. Okay okay roughly in the right ballpark careful not to put stuff near the back-left corner on the top shelf as that's a hot spot.

I have spent a lot of my working life with ovens, commercial and domestic, and its common to find a thermostat out by as much as a 100c. either way, although most are within +/- 5c by design.

I'm active in cooking circles as well as woodworking. From everything I've seen it would be very unusual indeed to find a domestic oven that was accurate. Even at the high end they can be pants.

User testing consistently shows that oven temp settings are off, with 30° or so being not at all uncommon, and sometimes way off (see above) with wild swings that make them a nightmare for home bakers. You wouldn't notice if you only cook roasts, casseroles and roasties, or ready meals and oven chips for that matter, which is why most people are unaware of the problem.

Not that rare to find ovens that are inconsistently off as well, okay-ish at lower temps but then suddenly decide they have an 11 setting when the dial is turned all the way up.

 

[MikeG.]

All of which makes the idea of a heat sink all the more useful. A heatsink holds temperature longer, be that up or down. In other words, a heatsink will tend towards the average temperature in its location, thus evening out the fluctuations. Someone wanting a constant temperature for tempering a piece of steel would be well advised to have both a thermometer and a heatsink, because even if the oven temperature fluctuates 50 degrees either side of his desired number, the heatsink will limit the range of temperatures that the steel experiences.

 

[sammy.se]

What about the dutch oven concept, i.e. a sealed unit within the oven, would that hold a more consistent temperature. Could that be adapted for tempering purposes? I know for bread baking the advantages are the even heat from the cast iron, plus the steam that is locked in.

Sent from my SM-G973F using Tapatalk

 

[sunnybob]

The oven thermostat will allow the element to heat untill the stat "reads" the temp its set to reach. it then cuts the power to the element, untill the temp reaches the lower set point.
The "blueprint" design difference is plus or minus 5c from the set temp.
On older mechanical thermostats (dials) the stat can be wrong from the word go, or can go out of spec as it ages. mechanical stats are very cheaply made and have very little quality control. All ovens are supposed to be calibrated when they are installed, by putting a thermometer in the middle of the central shelf. Nowadays with internet shopping and self build units thats out the window.
Old mechanical stats can be adjusted quite easily. On the most common types you pull the dial off and look down the hollow spindle. There is a tiny screw head showing. Use a screwdriver to adjust it.
DO NOT DO THIS UNLESS YOU HAVE A CHECKING TEMPERATURE PROBE. You could easily set it so high the oven will catch fire. :shock: (hammer)

If you have a digital stat, its more likely to be accurate. Most digital stats can be adjusted via the display unit but each manufacturer has its own system so the installation manual would be needed to find out how.
I had to attend a Tesco supermarket where the girl had managed to fool the digital temperature setting on the chicken rotisserie oven. at 260c it caught the chicken fat alight and quite literally blew the door off (cue Michael Caine) =D>

 

[MusicMan]

The colours seen on tempering are interference effects indicating the thickness of the oxide. The initial growth of the oxide at any temperature is initially fast then slows down (logarithmically) but keeps on growing. The colour charts are constructed for reasonably rapid heating for a short time, such as in a flame or forge, i.e. blacksmith guides. Tempering in a cooker will not produce the same thicknesses at different colours hence they are a poor guide. This is because the heat transfer rate is different in an oven to a forge or flame, and different again is the part is sitting in a sand heatsink. And if you leave it sitting in the oven for a long time, you will get the succession of tempering colours even though the temperature is not changing.

It is indeed not true that a heat sink (or any passive object not itself emitting energy) could possibly increase the temperature of an oven. Heat can only flow spontaneously from hot to cold, Though shalt not violate the Second Law of Thermodynamics. Actually, you can't.

So what to do? You have found empirically a heat treatment that suits your needs. Stick with it. That is how tempering procedures were devised (with a bit more measurement involved).

Keith

 

[Woody2Shoes]

.....
It is indeed not true that a heat sink (or any passive object not itself emitting energy) could possibly increase the temperature of an oven. Heat can only flow spontaneously from hot to cold, Though shalt not violate the Second Law of Thermodynamics. Actually, you can't.
....

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. Also that the indicated temperature of the oven is very much an indicative estimate of the target temperature and nothing more - largely because of the (rather primitive) way that ovens regulate their temperature.

You can have just as much fun trying to measure the temperature inside a fridge - which I had to do recently. There's a stratification effect, plus the effect of air changes, plus the thermal inertia of the solid/liquid contents, plus a similarly crude temperature regulation mechanism.

Cheers, W2S

 

[MikeG.]

.........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.

 

[Bm101]

Thanks for all the replies so far Gents. This is turning out to be an informative thread. I don't have the right type of mind to at least easily understand some of the concepts being proffered but it's definitely aiding this bear of little brain understand some of the principal concepts. Both of tempering process and how ovens work.
Essentially in my case at least Musicman's point that empirically it works is the most pertinent for me but it's fascinating reading the replies.
Many thanks as always.

 

[sunnybob]

.........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.

 

[Bm101]

I'm a bit baffled here Bob.
For the sake of argument, let's suggest we have a premium temperature controllable kiln for example. Its mustard because Nasa made it to work properly. You put a lump of steel in and turn the dial to the precise temperature you want.
When that steel reaches that temperature how does it ever increase past that from length of exposure?
Not saying you're wrong (obviously) but I don't understand how the colouration can change if the same temp is meticulously maintained at a constant.
I pick a set of cold keys up and hold them. They warm up but don't keep warming. Is it because the temperatures are high enough to alter the physical properties of the steel in the oven? I can't picture it tbh.
:|

 

[ED65]

When that steel reaches that temperature how does it ever increase past that from length of exposure?

It doesn't but the oxide skin thickness increases over time. Tempering colours are traditionally achieved quickly and then there'd be a second quench to halt heating.

The colours seen on tempering are interference effects indicating the thickness of the oxide. The initial growth of the oxide at any temperature is initially fast then slows down (logarithmically) but keeps on growing. The colour charts are constructed for reasonably rapid heating for a short time, such as in a flame or forge, i.e. blacksmith guides. Tempering in a cooker will not produce the same thicknesses at different colours hence they are a poor guide.

 

[sploo]

I'm a bit baffled here Bob.
For the sake of argument, let's suggest we have a premium temperature controllable kiln for example. Its mustard because Nasa made it to work properly. You put a lump of steel in and turn the dial to the precise temperature you want.
When that steel reaches that temperature how does it ever increase past that from length of exposure?
Not saying you're wrong (obviously) but I don't understand how the colouration can change if the same temp is meticulously maintained at a constant.
I pick a set of cold keys up and hold them. They warm up but don't keep warming. Is it because the temperatures are high enough to alter the physical properties of the steel in the oven? I can't picture it tbh.
:|

I'm assuming that MusicMan's earlier reply is pertinent; at some temperature (I don't know what) an oxide layer will start to form on the surface of the metal. If kept above some temperature (again, I don't know what) the oxide layer will continue to grow, slowly changing the colour.

However, you could instead rapidly heat the metal up to a specific temperature (a shown in the diagram in your first post) in order to quickly achieve a target colour, then immediately let the metal cool.

I assume that if a metal item is in a "blanket" of sand it'll initially heat up slightly slower, but as the oven element cycles on and off it'll maintain a much more constant temperature due to the insulating layer of sand. I suppose if the oxidation only happens above a certain temperature then metal sitting open on an oven shelf may stop oxidising, as it rapidly drops below the oxidation temperature when the oven element is off (whereas metal in the sand "blanket" will lose heat more slowly, thus staying above the oxidation temperature). Thus two identical metal parts (one open, one in sand) in the same oven may oxidise at different rates.

All complete speculation on my part; having no experience (but a passing interest) in this.

 

[MusicMan]

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.