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Damned If You Dewar, Damned If You Don't

Discussion in 'Other Tools' started by dancom, Jul 13, 2020.

  1. dancom

    dancom Dust Maker Best Shop Tool

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    Anyone have any insight on buying, owning, maintaining a dewar for liquid nitrogen?

    I am tempted to buy one new, but want to learn some details on where to refill, approx. cost per litre, how long the LN2 lasts on average. Does it have to be re-certified at any regular interval? I have a Linde/Messer and Air Liquide close to my work.

    Hoping that either a knifemaker or someone in livestock breeding can provide some info before I go and spend big on a dewar.

    Dan
     
  2. Griff

    Griff Active Member

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    Been a while since I looked into this Dan. But there was a service around the GTA that would rent Dewars and provide filling/refilling service to residential. At the time this seemed the most cost effective if/when I ever got to the stage to justify it, i.e batch making/ selling.

    Amazon.ca has some now though, they didn’t back when I was looking, here’s the type their selling



    I am on the fence. I think Cryogenic treatment is something worth doing. But I’m kinda (and I have said this before) leaning towards the Sandvick stainless steels that don’t need LN2, they just need a normal freezer. And the AEB-L I have, does a RC point or two higher justify the cost of the LN2 and Dewar?
     
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  3. FORGE

    FORGE Maker of the Year Best Knife

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    There is a good article today from Knife Steel Nerds about myths on heat treating , one topic was on cryo heat treating. In my opinion unless you really know what you are doing and always using the same steel are you really going to gain anything ?
    I don't think one point in hardness really justifies the expense.

    Cold Treatment Myths

    4. XXXX steel “needs” or “doesn’t need” cryo. I see frequently from knifemakers statement like, “I chose AEB-L because it doesn’t need cryo.” Or “Carbon steels don’t need cryo.” Or “Stainless steels need cryo.” This shows a fundamental misunderstanding of what cryo does or doesn’t do. I explained the mechanisms behind cryo in this article. It isn’t a light switch that either affects a steel or doesn’t. During quenching from high temperature, the steel hardens through the formation of a steel phase called martensite. The formation of martensite is measured with a “start” and “finish” temperature and the closer to the finish temperature you reach the more martensite forms and the closer to the maximum hardness you will achieve. Those formation temperatures are controlled by the steel composition and also the heat treatment, as higher hardening temperatures put more carbon and alloy “in solution” to affect the martensite formation. When the martensite finish temperature is below room temperature there will be some amount of “retained austenite” which isn’t transformed to martensite leading to lower hardness and other effects. Using cryo or other cold treatments means you get closer to martensite finish and retained austenite is reduced. Virtually any knife steel can be heat treated with or without cryo, depending on the hardening temperature that is selected. However, higher hardness levels can be achieved when using cold treatments, see the chart for AEB-L below, where the minimum temperature is labeled as “LN2” (cryo), “freezer” (a household freezer), or “room temperature” (no cold treatment):

    [​IMG]

    You can see that the hardness peaks around 62 Rc with no cold treatment, using a hardening temperature of 1925°F. Above that temperature the hardness drops because of excess retained austenite, usually amounts greater than 15-20%, which is undesirable. Using liquid ntirogen with the same hardening temperature gets about 63.5 Rc as-quenched. Yet higher hardening temperatures can be used in combination with liquid nitrogen up to a hardness of about 64 Rc.

    “Simple” carbon steels also have retained austenite and are affected by cold treatments. Here is a chart of retained austenite contents and hardness values achieved with and without cryo with low alloy steels:

    [​IMG]
     
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  4. dancom

    dancom Dust Maker Best Shop Tool

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    I think I'll stick with the stick it in the freezer route. I can't justify $600+ for a dewar and paying for a liquid that magically disappears in a month. Sounds to me kinda like beer. :D:beer:
     
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  5. FORGE

    FORGE Maker of the Year Best Knife

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    Dan you can always mix methanol and dry ice, if I remember correctly it get to about -130 Deg F
     
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  6. dancom

    dancom Dust Maker Best Shop Tool

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    The graph above is quite revealing. As I mainly use AEB-L and harden at 1060°C (1940°F), it indicates that my freezer step adds me over a point for free. Our freezer is around -18°C, so that's an extra 36°C or so below garage temperature. Now I am thinking that our brutally cold prairie winters can be an advantage. Bring on the -40...no wait, let's have at least another month of summer first.

    Gotta love Larrin's work.
     
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  7. Griff

    Griff Active Member

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    Awesome stuff *Thumb up*
     
  8. Scott Kozub

    Scott Kozub New Member

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    Hey Dan.

    I found a 25l Dewar of Kijiji about a year ago that paid about $130 for. Some guy got it from a medical building he was renovating. They pop up fairly frequently. I'm told if you go the used route you run the risk of the vacuum being broken but I got lucky. My reasoning for getting the dewar was that I can only get dry ice at praxair during the week and its a pain. I was heat treating knives in batches of 20 or so. With the LN on hand, it's very convenient to be able to heat treat knives when ever I want, like during the week after work or custom orders.

    My Dewar has a mouth that's about 2.5" so I can easily do chef's knives. I had it filled back in march and did about 10 knives or so. Then my steel order was trapped at the boarder and I haven't been able to get it due to the shutdown so my LN has been sitting as I have no steel or belts left. I filled it up on April 8th and checked last weekend and still had about 3-4" left in the bottom so that's pretty good for over 3 months. Most guys get about 3 months but it depends on how much you use it. It cost me $208 to fill the full 25l and they won't do partial fills. I would think a 10l with a wide mouth would be perfect but a couple hundred every few months is no big deal. The knife doesn't need to be submerged in LN. As long as there is still a bit in the bottom, the air will be cold enough.

    One big disadvantage is not being able to clamp a knife between quench plates when chilling to fight the warp. I do a lot of AEB-L and have gotten used to straightening with a carbide peening hammer I made so no big deal there. You always could dump the LN into a cooler if you wanted. The LN could be cheaper than dry ice depending on if you batch knives or not. If we lived in the states and could get dry ice at the grocery store I would have stuck with that.
     
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  9. Scott Kozub

    Scott Kozub New Member

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    in regards to hardness. I have noticed no additional hardness with the LN on AEB-l over dry ice. I tend to get about 62 HRC as quenched at 1985. Then a first temper at 200 gets me up to 64.5 ish. I then temper back accordingly. Dry ice or LN may get an additional point over a freezer but it isn't much. It still warps like crazy too. Multiple knives hanging off a wire in LN can warp as well as the outside cools faster.
     
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  10. dancom

    dancom Dust Maker Best Shop Tool

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    Thanks Scott. Lots of good info. Exactly what I was looking for!
     
  11. Griff

    Griff Active Member

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    There are Dewars with 4”+ inside diameter necks, that you could pass through a clamped knife, but the pricing on those Dewars new is astronomical, some are $4K. You’d have to be a Bob Kramer to justify it :D
     
  12. Scott Kozub

    Scott Kozub New Member

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    Ya and the bigger the opening the faster the loss of LN. That's where most of it goes so the smallest size you can get away with the best.

    I do however suggest getting something big enough to fit a large piece of broccoli for fun. One advantage we didn't discuss is the ability to make great ice cream once you have LN. I could probably eat enough ice cream to justify the $200 fill.

    Seriously though, having LN on hand can be useful for other things like freezing tight fitting shafts. Or you can be like this guy that nearly takes his face off!

     
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  13. Griff

    Griff Active Member

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    That’s hilarious and pretty freaking cool haha
     
  14. John Noon

    John Noon Well-Known Member

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    Something I have not seen discussed is the use of a small freezer box. You plumb it with a coil for the liquid to pass through before venting into the box then atmosphere.

    There are controllers, switches and temperature probes all readily available for this type of project
     

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