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Quench plates, the man the myth the legend

Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by JTknives, Aug 4, 2019.

  1. JTknives

    JTknives Blade Heat Treating www.jarodtodd.com Knifemaker / Craftsman / Service Provider

    Jun 11, 2006
    So my quench plate set up is my big bottle neck now so I’m looking at adding at least 3 more sets. I would like a total of 6 sets of plates. The hurdle quickly becomes apparent when you start adding up cost for 6 sets. I’m also thinking of ditching the woodworkers vise and going Pneumatic.

    But the Issue at hand is the plates themselves. That much aluminum is a small fortune. This got me thinking why aluminum. I know it’s the “industry” standard so to speak, But why? The obvious answer is fast thermal conductivity but was it ever questioned? Or was it just assumed it was the best material because it’s whats used for heat sinks. But my question is how fast do you really need to be. We know from liquid quenching that faster is not always better. So I started digging into the numbers to see what’s going on.

    Let’s set the stage with the thermal conductivity of the standard. 6061 Aluminum has the thermal conductivity of 167W/(m·K). Now let’s look at something much cheeper like 1018 steel. 1018 has a thermal conductivity of 52W/(m·K). That’s quite a bit slower but does it matter.

    If we look at the steels we plate quench thy are designed to be air cooled in still air for the most part. I think the industry does s 3-5 bar nitrogen quench. But that is huge ovens with a ton of heat to remove. So let’s look at the thermal conductivity of air. Air actually has a better thermal conductivity then pure nitrogen. But it’s still between 0 to 100mW/(m·K), That’s milliwatts not watts. So at 1600°C air has a thermal conductivity of .1W/(m·K). That’s 1,670 times slower then aluminum and 520 times slower then steel. So it would seam that steel would be plenty fast enough to quench air gardening steels. But if you think about it another way your quenching a steel blade and it will only transfer heat up to a max rate of its self.

    But wait I know I can here people in the background yelling that steel holds is heat longer then aluminum so the quench plates will get hotter and take longer to cool off. That’s not necessarily true if you do the math. We will just average the math as it makes things simpler. So aluminum is half the weight of steel, It’s also half as ridged. Aluminum has a specific heat of .220Btu/(lb-°F). This means it takes .220 btu’s to increase 1lb of aluminum 1°f. Mild Steel on the other hand has a specific heat of .120Btu/(lb-°F). So as you can see aluminum looks much better as it seams like it can hold 2 the btu’s per deg then steel. But remember that’s per pound. So one pound of steel is half the size of the same weight of aluminum. So In fact thy hold the same btu’s per volume. So a 1”x4”x12” plate of steel and aluminum would infact be the same temp with x btu input from a blade. And about aluminum cooling off faster is also false. The plates will only cool as fast as the thermal conductivity of the air that’s around them.

    But this brings up another thing to think about when comparing steel to aluminum as quench plates. I had not thought about this till I started researching. Aluminum oxide is what aluminum creates on it’s outside skin. It’s not very thick but it’s still there. This could be the thing that tilts the scale one way or the other on weather one metal is better or not then the other. So I dug around and found a paper talking about thermal the conductivity of aluminum and aluminum oxide. I was shocked to see that aluminum oxide has a thermal conductivity of less then 35 w/mk. And as it gets hotter it gets less. At 250°C it’s sitting at under 20 w/mk. And if your aluminum purity goes down to 90% your sitting at just over 10 w/mk. This is crazy but makes sense if you think about it. One of the best refractory we use in forges is bubble alumina which is made out of aluminum oxide. It’s just a thin layer but it’s always there.

    So it looks like steel is not a bad option for quench plates and aluminum was chosen because it is assumed to be the better conductor. Sorry for the loooooog rambling but I figured I would document my thought process here.
    Natlek and DeadboxHero like this.
  2. Cushing H.

    Cushing H. Gold Member Gold Member

    Jun 3, 2019
    JT - just a couple things to add/correct - but I do not think they will change your conclusions.

    thermal conductivity of air (or nitrogen - difference is nil). You can not make that comparison just based on thermal conductivity. The mechanism responsible for removing energy from the hot blade sitting in air is convection (i.e. gas movement around the hot blade). Air near the blade will get hot, hot air rises and carries the heat away from the blade. Cold air comes in from the bottom to replace the hot air that has risen, that cold air then gets hot from the blade, etc, etc. (this is how hot water/steam radiators used to heat houses work). Convective mechanisms are MUCH more efficient that diffusion (which is what your thermal conductivity data indicates) mechanism. One can not be used to calculate the other - they are independent (there are some rather nasty look-up-tables to determine an air heat transfer coefficient for certain dimensions of metal and certain velocities of air flowing over the metal - all determined experimentally). that said - heat loss into a piece of steel versus air is likely still much more efficient than the air, unless you are using a really strong fan on the blade to up the convective loss.

    Aluminum by itself is an extremely chemically reactive metal (thats why aluminum fires are so potentially bad). The only reason we can work with it is because of that aluminum oxide layer you are talking about. Shave/sand/cut a piece of aluminum, and the newly exposed aluminum reacts to form that aluminum oxide layer. BUT, that aluminum oxide layer is extremely thin (i.e. on the order of molecules thin) - but it is enough to exclude more oxygen from getting through the layer. so once formed, that aluminum oxide layer is stable, and does not increase in thickness. It is so very thin, and stays that way, that it's effect on heat transfer is totally negligible given the bulk of aluminum below it.

    But as I said, I doubt these two points would change your thinking in any way. Good idea! (industries keep doing the same thing over and over, because "it was always done that way" - even if there is a better or equal way sitting out there for the taking). I would get a single set of those steel plates, and try it.
    Storm W, Nick Dunham and Ken H> like this.
  3. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    pure silver quench plate if your really want quality and speed.
    Copper would be a poor mans option.
  4. weo

    weo KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Sep 21, 2014
    I like how your brain works, JT (not that my opinion matters).
    Probably the best way to convince those that read this forum is to do an experiment, quenching some blanks using both aluminum and steel and then comparing the steels after quenching.
    Nick Dunham and Ken H> like this.
  5. JTknives

    JTknives Blade Heat Treating www.jarodtodd.com Knifemaker / Craftsman / Service Provider

    Jun 11, 2006
    That’s the plan. Do some hardness tests from both plates and see what we get.
  6. kc custom

    kc custom KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Apr 20, 2005
    I used steel plates for years, and kept them in the freezer same as my aluminum ones. Never saw any big difference, more like none
    at all on the hardness tester.
    Ken H> and razor-edge-knives like this.
  7. Nick Dunham

    Nick Dunham

    Feb 15, 2018
    In @DeadboxHero's video with Phil Wilson, it sure looks like Phil is using steel plates:

  8. seanj

    seanj Gold Member Gold Member

    Mar 1, 2010
    If you increase the surface area on the outside of your plates by milling slots you will effectively increase the heat transfer. That's why heat sinks on CPU's always look the way they do. I don't have the math to know how much of a difference it would make, but it might be worth a try.
  9. Cushing H.

    Cushing H. Gold Member Gold Member

    Jun 3, 2019
    good idea - but may not make a huge difference (vs. cost of doing it) relative to the heat capacity of the plates themselves????? something else for JT to consider doing if he finds non-equivalance with aluminum when using flat plates I guess .......?????
    seanj likes this.
  10. JTknives

    JTknives Blade Heat Treating www.jarodtodd.com Knifemaker / Craftsman / Service Provider

    Jun 11, 2006
    Cooling the plates is not the issue. The issue is wanting to batch knives up in the oven and have them all have a set of plates to go in right away.
  11. Larrin

    Larrin Gold Member Knifemaker / Craftsman / Service Provider Gold Member

    Jan 17, 2004
    You’re unlikely to see a hardness difference with air hardening steels.
    bjansen, P.Brewster, Ken H> and 3 others like this.
  12. seanj

    seanj Gold Member Gold Member

    Mar 1, 2010
    Based on other replies by people much more knowledgeable than me on this subject, I would agree. It doesn't appear there would be enough of a difference to make it worth while.
  13. Cushing H.

    Cushing H. Gold Member Gold Member

    Jun 3, 2019
    Seanj - my apologies - your thought really IS a good idea. I really meant nothing more than it would have to be tried to see what the amount of difference would be. My fault for having it come across as something different
    seanj likes this.
  14. seanj

    seanj Gold Member Gold Member

    Mar 1, 2010
    No problem. Thanks though. I think I misinterpreted...lack of sleep does that to me. It would be an interesting experiment.
  15. HSC ///

    HSC ///

    Nov 7, 2012
    Also I’m pretty certain the density of aluminum is 1/3 that of steel.
    So alum is 1/3 the weight for the same size plates. May not affect your calculations Or your conclusions

  16. JTknives

    JTknives Blade Heat Treating www.jarodtodd.com Knifemaker / Craftsman / Service Provider

    Jun 11, 2006
    You are correct. Some how I thought it was about half. Which means steel can hold 1/3rd energy per 1° temp increase.
    HSC /// likes this.
  17. Ken H>

    Ken H>

    Dec 31, 2011
    I think quench plates are more for preventing warp than actual fast cooling. Sandvik says as long as the temp falls below 1100F in less than 2 minutes you're good. I've never tried just air hardening due to concern of warps. Unless you can find the steel in scrap prices not sure steel is going to be that much less than aluminum in cost. Have you compared prices?
  18. Nathan the Machinist

    Nathan the Machinist KnifeMaker / Machinist / Evil Genius Moderator Knifemaker / Craftsman / Service Provider

    Feb 13, 2007
    Steel quench places work as well as aluminum.

    You can see this by the time it takes to cool a blade enough to handle it. Try it. They work the same.

    The limiting factors in the effectiveness of quench plates tends to be how well they contact your work (air gap is a great insulator) and if they're already hot. If you can't handle your blade with bare hands after one minute something needs work. Quench rate is very important with some steels so you need to get this right.

    Heat transfer out of your work is a function of the quality of the contact and the difference in temperature between your work and the quench plate. This temperature difference at the point of contact is helped by the improved thermal conductivity of aluminum, but harmed by its reduced heat capacity.

    Pound for pound, aluminum is probably a little better. Volume for volume steel is probably a little better. The difference doesn't matter if your plates are dinged up, warped or filthy.

    The real advantage of aluminum is you can machine it flat without it warping and it doesn't rust when you dip it in water. It is also more flexible so it's possible the surface may conform a little better to your work giving you better contact.

    None of this matters if your plates aren't flat. As-extruded aluminum might be .015" out across 6". That won't work well. if it isn't flat you need to deck it flat in a mill or buy ground plate.
    Mecha, Shawn Hatcher and seanj like this.
  19. daizee

    daizee KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Dec 30, 2009
    What if your blades aren't flat? :confused:
    How do you guys handle tapered tang blades with plate-quenching?
    Taper after hardening? (ugh)
  20. JTknives

    JTknives Blade Heat Treating www.jarodtodd.com Knifemaker / Craftsman / Service Provider

    Jun 11, 2006
    Yeah grind after heat treating. 99% of the blades we get are not ground before heat treating. It seams to be the direction a lot of people are going. I would also say you get a much better plate quench on blades that are not ground.
    Russ Andrews likes this.

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