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I am going to be heat treating my next knife and had a question concerning baffle pipes. Has anyone ever tried filling the pipe with some kind of material ( like vermiculite) to help control temperature fluctuations? Or is this just crazy talk? Thanks.

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Thermal mass is what you need and is the absolute opposite to an insulator like vermiculite. A very thick pipe would help but temp fluctuations should be managed mostly at the source. 

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Thanks for the reply Joël. I never really thought of vermiculite as an insulator but I guess it is. I was thinking it would act as thermal mass, once it got hot, that you could slide the blade in and out of as you thermal cycled.

 

What would be the minimum wall thickness recommended?

 

BTW been meaning to ask you what kind of whisky?

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A quarter inch should be fine. 

About the whisky, I like pretty much any kind :lol:

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I've been using 1/8" wall pipe for several years.  It has lost a lot to scale on the outside, but should be good for another several years.  This is in a coal forge.  With gas you should have a more even heat to start with.  And you don't want anything in the pipe except a small lump of charcoal pushed to the far end.  That way you can see decalescence and avoid scale at the same time.

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Do you close off the back end of the pipe?

 

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Yes, that is a requirement.  Otherwise you just get a chimney effect and extra oxidation.

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Well I got the knife heat treated and for the first time I did a sacrificial blade before the real knife to break it and check the grain. I didn't end up using a muffle pipe. I did end up using this heat treat forge that I threw together a while ago with the thought it would be good for longer pieces once I get to that level. I was really curious to see how it would work. It's 10 inch diameter duct pipe lined with one inch of Kaowool ( or the equivalent - don't remember the brand I bought) and two layers of satinite. Burner is a tiny 1/2 inch side arm. IMG_1267.jpg

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I had it running at a sputter (pressure didn't even register on the gauge - it sounded like a steam locomotive). Once I had it set, it kept temperature pretty well between 1475 and 1625 depending on the setting. Just touching the gauge dial up or down could change the temperature. The one down side is it's real difficult to see the sides of the knife so couldn't really watch for decalescence. I basically waited until the knife was the color of the inside of the forge (based on thermocouple temperature) waited 3 more minutes then pulled it out, either for thermocycling or the quench.

 

Now the real question. The steel I'm using it 80CrV2. Thermocycled three times 1575F, 1525F, 1475F letting cool to black in still air between temperatures. I then heated to 1500 and quenched in a high flash point vacuum pump mineral oil (no idea on the speed). I think the grain looks pretty good, but I would really appreciate more learned opinions. Thanks for feed back.

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Yep, that's what you're after, grain-wise.  Good job!  And you might be able to drop a lump of charcoal into that furnace to provide a reducing atmosphere if you start having issues with scaling and decarb.  I am told 80CrV2 is more prone to decarb than 1084 or 5160, but I've never been offered a reason that should be.  

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What would scale and decarb look like when heat treating? All the times I’ve heat treated the only time I get something on the blade is after the quench there is a flaky black coating on the blade. I just figured it was the quenchant.

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Yeah, the flaky black stuff is burnt oil.  Scale shows up in oxidizing atmospheres, and is basically magnetite formed from the iron in the steel combining with oxygen, and appears as a hard crusty gray deposit that leaves a pitted surface.

Decarb doesn't show visually, but can be noticed if the outer few thousandths of an inch are softer than they should be.  This happens in mildly oxidizing atmospheres when there is just enough oxygen to scavenge carbon from the surface of the steel. 

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