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Identical Second Temper Has A Lot Of Blue

Discussion in 'Heat Treating' started by RussGen, Jan 27, 2021.

  1. RussGen

    RussGen New Member

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    I'm really not sure what went wrong but my ht was successful and my first temper at 400 produced an even straw color. The blanks were sanded and washed after ht. Second temper at 400 was a mix of bronze and blue with blue on most edges. I added a large chunk of metal to curb the temperature swings. This was a toaster oven but a high end one. Should have used my Evenheat but it takes so long to cool down. Question is should I ht and temper again or is this just an optical illusion? Temperings were identical! Any advice would be greatly appreciated.
     
  2. Griff

    Griff Active Member

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    What steel bud?

    I’m just wondering what the swing in temperature is with a toaster oven.

    Below is what I do to get the kiln temperature down fast...it’s sucking heat out btw, not blowing air into the kiln, I didn’t want to possibly damage the coils.

    The kiln was set at 1475F for O1 and that was about ten minute mark for down to 800F or so.

    Also I’ve found that the kiln doesn’t suddenly jump up higher when I close the door so this is very effective at sucking residual heat out of the bricks:beer::beer:

     
  3. RussGen

    RussGen New Member

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    Great info. That's actually better information than I was could have expected. The real problem was I couldn't use my oven because it was too hot. Thanks
     
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  4. RussGen

    RussGen New Member

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    Hey Griff, it was 1084 and the wide short one 80crv2
     
  5. dancom

    dancom Dust Maker

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    By the colour charts I've seen, blue would indicate temperature over 250°C. Sounds like the toaster oven might have a sticky thermostat. I have seen this before with a Breville unit I had some time ago. I smelled a hotter than usual smell. I do have an oven thermometer that I keep in there , but I was too late. The thermometer needle was pinned. Bye-bye toaster oven.

    Personally, I would HT again. It might just be me, but it would always be in the back of my mind.

    Dan
     
  6. Griff

    Griff Active Member

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    Here’s a good reference pic I came across

    [​IMG]
     
  7. Joelsund

    Joelsund Active Member

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    I agree with Dan on redoing it. There’s no other good explanation for it going blue other than it hitting around that 500F+ mark. While waiting for my HT oven to cool down I typically leave my knives in LN2 or in the freezer overnight, even with low carbon. Never had a crack.
     
  8. RussGen

    RussGen New Member

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    Lots of good advise, thanks. Got the fan already for next time. Two things though; where to get the LN2 and do I need to normalize again before a second HT?
     
  9. Joelsund

    Joelsund Active Member

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    I've Gotten my LN2 from Praxair on McPhillips. Their other locations don't have it. You'll need a dewar though. Im not sure the cost to benefit is justifiable on carbon steel though. It does give a reduction in retained austenite but not as drastic as it does for some stainless. If you do pick up a dewar, just ask them to fill half. They end up filling the whole thing anyway lol. its about $8/litre + some hazardous material handling fees. I believe ive paid $60 for 5 litres, then they fill my tank the full 10L.

    I would tend to think you should be fine skipping normalizing. The grains should still be small from the first time. People claim doing multiple quenches actually reduces the grain size further. I havent found any studies to convincingly prove that though. Im also not sure what the temper in between would do.

    https://www.researchgate.net/figure...ize-with-tempering-temperature_fig1_282318918
    This seems to showt higher tempering temp makes grain size smaller.
     
    Last edited: Jan 29, 2021
  10. Nieman Knives

    Nieman Knives New Member

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    I started using a toaster oven as well, mid to high range price wise. I had similar results with colours often getting blue when aiming for straw and so I bought a thermometer for the oven and found the temps were all over the place. I still use it for making kydex sheaths but now only use my kitchen oven for tempering. I found my kitchen oven very consistent but needed to be set about 10• cooler than my target temp.
     

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