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Does overheating cause chipping/brittleness?
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Thread: Does overheating cause chipping/brittleness?

  1. #1

    Does overheating cause chipping/brittleness?

    There was a question in one of the blade forums about chipping and one response alluded to the possibility of overheating during factory grind. I've had a few blades that were prone to chipping, and always thought that they didn't get proper tempering to soften up the metal.


    The overheating and ruined temper are sometimes used synonymously, and I'm wondering if this is true. If you overheat, aren't you always going to soften up the metal? I.e. small amount of heat tempers the hardness, large amount of heat anneals, either way it would soften the blade and prevent chipping rather than cause it. Does anyone have any explanation for why overheating on belt grinder could lead to making the metal more brittle?

    Thank you.

  2. #2
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    I'm not an expert, but I'll explain it to you as I understand it. If steel is heated above the critical temperature / austenitization point (in the red hot range), then grain growth will occur. Larger grains reduce toughness and make chips more likely. This is not related to hardness. This type of temperature is very unlikely be reached on a belt grinder.

    After a blade is quenched from a point slightly above critical temperature, it is very brittle, and is tempered at temperatures which can be reached in a conventional oven (for carbon steel). This is to slightly change the crystal structure, making the knife sightly softer, but a lot tougher. If it is heated above this temperature during a later procedure such as grinding, then temper will be 'lost' and the knife will become softer than intended, approaching spring hardness depending on the temperature.

    Annealing is the process where the steel is heated above critical then left to cool very slowly. It is not simply a hot temper.

    So from my understanding, overheating (loss of temper) during belt grinding does not contribute to brittleness.

  3. #3
    I see.

    So if a blade is prone to chipping, would it be pretty safe to say that the cause is not from the belt grinder?

    I think this distinction matters in the sense that for production knives, grinding is done one blade at a time, whereas heat treatment is done in a large batch. So if one knife is chipping, then there might be a hundred other knives from that same batch that is also chipping?

  4. #4
    Overheating while grinding will actually soften the blade.

    Chipping and brittleness are signs of over-hardened or insufficiently tempered blade.


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  5. #5
    Quote Originally Posted by Marko Tsourkan View Post
    Overheating while grinding will actually soften the blade.

    Chipping and brittleness are signs of over-hardened or insufficiently tempered blade.
    Marko do you know if brittle behavior can also be a result of excessive grain growth?

  6. #6
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    I get chipping and crumbling on the edge from overheating on a grinder.

  7. #7
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    Don I know that large grain hardened untempered 52100 is more brittle than fine grain hardened untempered 52100. I would expect similar results after tempering.

  8. #8
    Quote Originally Posted by tk59 View Post
    I get chipping and crumbling on the edge from overheating on a grinder.
    I wonder if this is from temper embrittlement? I had thought that process was a result of holding at those temperatures for a long time though. I have no idea...

  9. #9
    Senior Member Benuser's Avatar
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    Never seen any brand new J-knife without some microchipping, until a new fresh edge was put on it. Looks like fatigued steel, perhaps from buffering?
    Seen a similar kind of chipping after poor leather + Cr2O3 stropping where too much pressure was involved.

  10. #10
    Quote Originally Posted by tk59 View Post
    I get chipping and crumbling on the edge from overheating on a grinder.
    Sometimes when you remove metal from the edge while grinding, it makes it look like tiny micro-serrations. Visor with magnifying lenses is great to spot those.

    Overheating the edge grinding (followed by change in color of the steel), at least in theory, would result in rolling, not chipping. Chipping is a result of impact fractures, caused by high hardness and low toughness. Fractures usually happens along metal grain boundaries, and it can be very small to moderately big, depending on steel and grain size. I had one instance when thinning White steel on Beston 500 would result in chips formation on the edge. It stopped when I re-tempered the blade.

    Quote Originally Posted by Benuser View Post
    Never seen any brand new J-knife without some microchipping, until a new fresh edge was put on it. Looks like fatigued steel, perhaps from buffering?
    Seen a similar kind of chipping after poor leather + Cr2O3 stropping where too much pressure was involved.
    When you put a new edge, you probably cut a less acute angle bevel - same principle as micro -bevels, thickening the edge. Chipping is a consequence of not-so-optimal heat treatment - too high quenching temp, or insufficient tempering, or both. Pressure on a thin area of over-hardened blade would increase chance of fracture, as you have observed stropping.


    "If there’s something worth doing, it’s worth overdoing.” - An US saying.

    If my KKF Inbox is full (or not), please contact me via Email: anvlts@gmail.com

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