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Thread: Does overheating cause chipping/brittleness?

  1. #11

    Zwiefel's Avatar
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    Very interesting thread guys...I feel like I understand my ignorance better now
    Remember: You're a unique individual...just like everybody else.

  2. #12
    Quote Originally Posted by Squilliam View Post
    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.
    That's correct. If the austenitizing temperature is too high, even after proper tempering, the blade will still have low impact toughness (resistance to chipping) because of the large grain.

    You can see it with a lo10-20 loupe magnifier pretty well if you look at the grain.

    M


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  3. #13
    There is another phenomenon that results from overheating while grinding that has nothing at all to do with changing the temper or hardness of the steel.

    When grinding an edged tool, be it a knife or a chisel, if you are grinding only at a sharp edge, the friction will heat the edge much more than the steel further back in the bevel. This results in the thin edge expanding more than the base metal, and can easily result in microfractures along the edge. This effect can be greatly magnified by dipping the blade or tool in water occasionally while grinding as I've seen suggested, as this cools the edge faster than the bulk of the metal.

    Hand grinding is unlikely to generate enough heat to cause trouble, but a belt sander or grinding wheel can easily, especially if the abrasive is incorrect for the job or dulled (the horrible gray wheels that always come installed on pedestal grinders are a perfect example -- far too hard to stay "sharp" resulting in excessive pressure to cut rubbing on dull grit, lots of heat and little work).

    The harder the material, the worse the problem as it is inevitably more brittle than softer material. With the tendency for very hard Japanese knives to chip on hard stones due to brittle edges or micro-fracture with power tools, it's best to sharpen them on soft stones with very light pressure. I'm sure the same advice would be good for western style knives made of very hard materials as well, even with less acute edge angles.

    The best tool for power grinding is, of course, a wheel with flood coolant so that no significant temperature changes occur. If you don't have coolant, by all means use very light pressure and quick passes.

    Peter

  4. #14
    Senior Member Benuser's Avatar
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    Quote Originally Posted by Marko Tsourkan View Post
    When you put a new edge, you probably cut a less acute angle bevel - same principle as micro -bevels, thickening the edge.
    Sorry Marko, cannot confirm. Once the original edge removed, and the microchipping gone, I may put so to say any edge on it.

  5. #15
    Thanks for really nice responses, especially Marko. That was exactly the kind of information I needed.

    There was a suggestion that you should grind away a good portion of the edge with "ruined temper" on a $300 knife!!!
    I suggested that problem is probably not with the overheating issues, and should be dealt in another way.

    I personally love hearing about metallurgy stuff.

  6. #16
    Senior Member Benuser's Avatar
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    Quote Originally Posted by psfred View Post
    There is another phenomenon that results from overheating while grinding that has nothing at all to do with changing the temper or hardness of the steel.

    When grinding an edged tool, be it a knife or a chisel, if you are grinding only at a sharp edge, the friction will heat the edge much more than the steel further back in the bevel. This results in the thin edge expanding more than the base metal, and can easily result in microfractures along the edge. This effect can be greatly magnified by dipping the blade or tool in water occasionally while grinding as I've seen suggested, as this cools the edge faster than the bulk of the metal.

    Hand grinding is unlikely to generate enough heat to cause trouble, but a belt sander or grinding wheel can easily, especially if the abrasive is incorrect for the job or dulled (the horrible gray wheels that always come installed on pedestal grinders are a perfect example -- far too hard to stay "sharp" resulting in excessive pressure to cut rubbing on dull grit, lots of heat and little work).

    The harder the material, the worse the problem as it is inevitably more brittle than softer material. With the tendency for very hard Japanese knives to chip on hard stones due to brittle edges or micro-fracture with power tools, it's best to sharpen them on soft stones with very light pressure. I'm sure the same advice would be good for western style knives made of very hard materials as well, even with less acute edge angles.

    The best tool for power grinding is, of course, a wheel with flood coolant so that no significant temperature changes occur. If you don't have coolant, by all means use very light pressure and quick passes.

    Peter
    A very interesting explanation, which matches with my experience. A very local overheating also, causing small fractures.

  7. #17
    This is a common problem in industry. Over heating can cause micro cracking on the edges of knives and tooling. The metallurgists at Crucible steel have been telling me this for years. They first learned about this when some of their customers had tool failure, which, when they looked at it under magnification, found the micro cracking. The customers were blaming the problem on the steel, of course. This can happen even with flood coolant when the abrasives get some what dull.

    This does not happen every time. There are a lot of cases where it does act like a higher tempering of localized areas, like at the tip or at the heel.

    Love and respect

    Hoss

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