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View Full Version : Does overheating cause chipping/brittleness?



xuz
05-24-2013, 04:23 AM
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.

Squilliam
05-24-2013, 06:35 AM
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.

xuz
05-24-2013, 07:10 AM
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?

Marko Tsourkan
05-24-2013, 10:19 AM
Overheating while grinding will actually soften the blade.

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

Don Nguyen
05-24-2013, 11:06 AM
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?

tk59
05-24-2013, 11:27 AM
I get chipping and crumbling on the edge from overheating on a grinder.

Squilliam
05-24-2013, 11:38 AM
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.

Don Nguyen
05-24-2013, 12:46 PM
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...

Benuser
05-24-2013, 02:32 PM
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.

Marko Tsourkan
05-24-2013, 03:11 PM
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.


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.

Zwiefel
05-24-2013, 03:21 PM
Very interesting thread guys...I feel like I understand my ignorance better now :)

Marko Tsourkan
05-24-2013, 03:30 PM
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

psfred
05-24-2013, 04:05 PM
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

Benuser
05-24-2013, 04:24 PM
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.

xuz
05-24-2013, 11:52 PM
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.

Benuser
05-25-2013, 08:31 PM
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.

DevinT
05-26-2013, 01:57 AM
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