Thermal cycling

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Natlek

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Jun 9, 2015
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We need to protect steel when we do thermal cycling after forging steel .There are different way to protect steel while doing the process......
I know that tray are used for HT Stainless steel , they fill the tray with gas and the steel is protected.
What if we use that tray filled with fine AO sand ? There should be no air inside ?
More energy would be consumed but if it works ?

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There should be no need to go to those lengths. Carbon steel will be fine after thermal cycling. It isn't being held at high temperatures for long times like stainless steel.
Because the heat transfer rate will be very slow, packing the blade in sand/AO would require very long cycles, and the heating/cooling rates may be undesirable.
The decarb layer is not as deep as many fear it will be. Just grind it off in post-HT finishing. If you do the bevels post-HT, the mater is moot.
 
Stacy E. Apelt - Bladesmith said:
There should be no need to go to those lengths. Carbon steel will be fine after thermal cycling. It isn't being held at high temperatures for long times like stainless steel.
Because the heat transfer rate will be very slow, packing the blade in sand/AO would require very long cycles, and the heating/cooling rates may be undesirable.
The decarb layer is not as deep as many fear it will be. Just grind it off in post-HT finishing. If you do the bevels post-HT, the mater is moot.
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I'm not sure I understand what you mean by that.............heat transfer rate will be very slow
If oven is stabilized in set temperature .............pull the try out , insert knives and back in oven ? It can be done very fast so no much heat loss. Sand would heat knives very fast , and due to the mass of sand there should be very little temperature loss ?

Let say that we follow this DET protocol from Larrin for 52100 steel........
Oven is on 1700 F , we pull try out insert knives and let them there for 20 minutes. Then we pull try out , take out the knives one by one to air cool .While doing this the temperature will drop in the furnace and in the tray quite ? Anyway we wait if necessary for temperature to drop to next desired temperature and continue with cycling ?
Decarb layer is there Stacy............30 minutes on 1700 Fahrenheit on open air ??
If this works, if sand completely protect carbon steel from decarb................can be used for stainless steel ??
I can't try it now, I borrowed my oven to one friend who makes series of small parts on special machines / I don't know what they are called / that need to be hardened...But I will weld tray this evening , I have lot of 316L stainless which will work for this .
1700°F for 20 minutes, air cool

1460° for 30 minutes, air cool

1460° for 30 minutes, cool at 670°F/h to 1260°F, air cool

Austenitize at 1500-1525°F for 15 minutes followed by a fast oil quench
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I cant see sand in the granular form really blocking oxygen, if the sand mixture had a low melting point of like 1000f I could see that working, however then you have to break off the sand/glass mixture. foil is probably just easier and better. but hey give it a shot, it may work great and change the game
 
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Skiller1nc said:
if the sand mixture had a low melting point of like 1000f I could see that working,
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Then you'd have basically the same thing as a salt bath.
Natlek said:
There should be no air inside ?
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Won't there will still be air inside? Although by volume, there's less oxygen in the oven than without the sand, but you don't have an air-tight seal and there's no vacuum, so the steel will still be exposed to oxygen, no?
 
A granular material has a much lower contact surface area, and will transfer heat slower. It will also take a much longer time to bring to an even temperature throughout the mass. Granular materials act as insulators. That is part of why a sand bath has to be fluidized by moving air/gas or high vibration to make it work. It is also why a small sand bath doesn't work well.

For effortless tempering or other thermal processes, salt baths are the quickest and most accurate. The temperature change is almost instantaneous. The negative is that since it take a good while for a change in temperature of a salt bath you need multiple baths to do things like quench and temper, marquench, ausquench, etc. This can be somewhat made easier by using the HT oven for lower temperature tasks and the salt bath for ones where decarb will be an issue.
 
Skiller1nc said:
I cant see sand in the granular form really blocking oxygen, if the sand mixture had a low melting point of like 1000f I could see that working, however then you have to break off the sand/glass mixture. foil is probably just easier and better. but hey give it a shot, it may work great and change the game
Click to expand...
I m talking about aluminium oxide sand ..........it can handle temperature behind melting point of steel :)
 
weo said:
Then you'd have basically the same thing as a salt bath.

Won't there will still be air inside? Although by volume, there's less oxygen in the oven than without the sand, but you don't have an air-tight seal and there's no vacuum, so the steel will still be exposed to oxygen, no?
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How much air can be in this ? I don t think that there would be air which can affect steel
This is AO sand / I forget micron size/ for my Sand oven ................
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Stacy E. Apelt - Bladesmith said:
A granular material has a much lower contact surface area, and will transfer heat slower. It will also take a much longer time to bring to an even temperature throughout the mass. Granular materials act as insulators. That is part of why a sand bath has to be fluidized by moving air/gas or high vibration to make it work. It is also why a small sand bath doesn't work well.

For effortless tempering or other thermal processes, salt baths are the quickest and most accurate. The temperature change is almost instantaneous. The negative is that since it take a good while for a change in temperature of a salt bath you need multiple baths to do things like quench and temper, marquench, ausquench, etc. This can be somewhat made easier by using the HT oven for lower temperature tasks and the salt bath for ones where decarb will be an issue.
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It is extremely fine sand Stacy , it would act like solid so I don t think that will transfer heat slow....
Did you forget ? JT has hardened steel in sand pot , in cold sand just fluidized......if that is slow heat transfer ...
Only thing which might be impractical is time to bring to an even temperature throughout the mass .
BW , the only reason why I haven't finished My sand pot oven yet is that it's impractical for me for now. Too much energy and time spent for one or two knives at time .
 
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Natlek said:
but also they are very light and when heated they want to go UP
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Sure, but all that will do is create a 'swirling effect', no? When the hotter oxygen molecules go up, they will push the ones already up there down, no? And/or if the oven isn't air tight, then any vacuum created on the bottom by the molecules going 'up' as you say will merely be replaced with cooler oxygen molecules entering the bottom of your kiln.
I'd say this might work only if you were notice that removing the blade from the sand is difficult due to any vacuum created by, as you say, the molecules going "up".
If your phenomenon was correct, wouldn't we all suffocate in hot weather?
 
Based on the density value I find for AO (~4g/cc) and the bulk density of AO blast media (120 lb/ft^3=2g/cc), I think your sand is about 50% air.

FWIW, if you could get all the energy of a 2kW heater into your sand, it would take you around six minutes to heat one kilogram of AO sand (heat capacity 880 J/(kg °C) by 800°C.
 
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Treat AO sand as a membrane variable in Fick's Law.

Prior put in blade: there is no O2 partial pressure gradient between hot air and static hot sand, thus no O2 diffusion.

Buried blade (Depth/thickness) in sand: O2 in sand go into decarb, now there is pp gradient which O2 will flow from hot air into hot sand at rate allows by sand membrane. Of course this rate much less than just air (membrane of zero thickness) => with sufficient thickness/depth and low sand volume (narrow and tall tray), maybe decarb be small enough to ignore/live-with.

Fluidized sand = affectively making membrane less permeability(less porous due to added kinetic/motion). Heading toward salt bath configuration, where membrane is fluid and very thick, affective/practicality vacuum.
 
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