Over cooked

[dtalbot]

Hi All,
I've just cleaned up the blade I heat treated in the Rayburn and think I may have over done the heat treatment. Some parts of the surface (and more pronounced on one side than the other) have a granular appearance, is this a bad sign?
Never mind it it is it's all good practice!
Thanks
David

[Trond]

I am not sure what you mean, but if you had some scale that jumped off in the ht, it is a good sign, try with a file on the edge if it does not take any off the edge, just slide over it, it is good, I think ;)

[dtalbot]

I am not sure what you mean, but if you had some scale that jumped off in the ht, it is a good sign, try with a file on the edge if it does not take any off the edge, just slide over it, it is good, I think ;)
Yep,
I did that and the file slid of plus the edge of the HTed blade plus I colud scrape some steel of the edge of some EN42 with it. I tempered it at 200C for 45 min twice over letting it cool naturally to room temp in between.
I've now cleaned up the tempered blade and given it a bit of a polish, looking closley at the polished surface I can see what looks to be a pattern of small adjoining cells (say about 0.5mm in diameter) like looking down on touching soap bubbles on water. Not having done any metal work to speak of it looks a bit strange for steel to me!
David

[dtalbot]

I am not sure what you mean, but if you had some scale that jumped off in the ht, it is a good sign, try with a file on the edge if it does not take any off the edge, just slide over it, it is good, I think ;)
Hopefully the attached picture of the thing I'm talking about will help (taken at x60 magnification on an Intel QX3 computer microscope). I overestimated the 'grain' size on my last post I guess its more like 1/4mm.
Cheers
David

[Colin KC]

Yah Dave, I suspect you've cooked it a bit. Best thing to do is break the blade (do some chopping to test the edge, sharpen at various angles to see what each does, try various grits to see how it reacts under each, then stick it inna vice 'n' (wearing protective stuff) give it a bend, then another, then a whack (should do the job) let us know what the grain structure looks like)

How's that sound? No point in just plain destroying it, test the **** out of it first;)

[dtalbot]

Yah Dave, I suspect you've cooked it a bit. Best thing to do is break the blade (do some chopping to test the edge, sharpen at various angles to see what each does, try various grits to see how it reacts under each, then stick it inna vice 'n' (wearing protective stuff) give it a bend, then another, then a whack (should do the job) let us know what the grain structure looks like)

How's that sound? No point in just plain destroying it, test the **** out of it first;)
Thought so,
It's just in some isolated patches so I guess the rayburn fire isn't an even enough heat source, time to come up with a plan B!
Cheers
David

[C_Claycomb]

I guess that you have already had a go at grinding those marks out. Simply cleaning it up with 400 grit wouldn't have been enough to be sure you were through surface erosion caused by the quench.

It would be good to have a pic that showed the whole blade, to give a sense of scale. I have had marks on a number of my blades which looked a bit like that, but they were quite big. I always thought that it was to do with oxidation, over heating, uneven heating, or something like that. The other day though I was talking to our company materials engineer and he suggested it was cavitation damage caused during the quench. He told me that the industrial quench tanks that we used to have had to be agitated to prevent this happening. I must admit that the damage to my blades always looked like the edges of bubbles and I had thought it was the oil before I was told all about over cooking etc.

If you do decide to test to destruction, and you haven't anything better to do with them, would you like to send me the bits? The materials engineer said he would help me with sectioning up a blade to look at the grain under the microscope. They have the equipment to diamond saw up the blade, polish and measure grain against a graticule in the scope. At the moment I don't have a blade that I really want to break that much. ;)

Chris

[C_Claycomb]

Oh, just thought. It could be that you are right about the uneven heat, that could cause the oil to boil more in some areas than others...

[dtalbot]

I guess that you have already had a go at grinding those marks out. Simply cleaning it up with 400 grit wouldn't have been enough to be sure you were through surface erosion caused by the quench.

It would be good to have a pic that showed the whole blade, to give a sense of scale. I have had marks on a number of my blades which looked a bit like that, but they were quite big. I always thought that it was to do with oxidation, over heating, uneven heating, or something like that. The other day though I was talking to our company materials engineer and he suggested it was cavitation damage caused during the quench. He told me that the industrial quench tanks that we used to have had to be agitated to prevent this happening. I must admit that the damage to my blades always looked like the edges of bubbles and I had thought it was the oil before I was told all about over cooking etc.

If you do decide to test to destruction, and you haven't anything better to do with them, would you like to send me the bits? The materials engineer said he would help me with sectioning up a blade to look at the grain under the microscope. They have the equipment to diamond saw up the blade, polish and measure grain against a graticule in the scope. At the moment I don't have a blade that I really want to break that much. ;)

Chris

Nope,
This was a fairly light cleanup done by hand using medium and fine wet and dry followed by a quick polish with my cheapo wollies imitation Dremmel in about 15 minutes, I'll hit it a bit harder to take the surface down a touch and report back.
As to testing I think this will be a general practice knife for everything from sharpening to handle making, as my first go I'll be happy to make somthing the actually cuts. Learning the finer points of metalurgy along the way is a big bonus!
David

[Trond]

David, i do not agree that it nesseseraly are overcooked. when you heat the blade for hardening, there is allways (unless you use hardening foile with a papersheath, or heat the blade in charcoal) going to be scales on some part of the surfase on the blade, due to the air around the blade, when you cool the blade, those scales wil jump off, and leave those marks on the surface.

[dtalbot]

David, i do not agree that it nesseseraly are overcooked. when you heat the blade for hardening, there is allways (unless you use hardening foile with a papersheath, or heat the blade in charcoal) going to be scales on some part of the surfase on the blade, due to the air around the blade, when you cool the blade, those scales wil jump off, and leave those marks on the surface.
Makes sense,
If I remember right the pattern is strongest where the scale fell away in the quench. I'll do some more and report back. Sorry if these are daft questions, I'm learning more and finding even more I don't know with every step of the way!
Regards
David

[Trond]

Makes sense,
If I remember right the pattern is strongest where the scale fell away in the quench. I'll do some more and report back. Sorry if these are daft questions, I'm learning more and finding even more I don't know with every step of the way!
Regards
David
Dont worry my friend they are not daft :)

[Kevin]

If it has been overcooked and the grain has grown to large there are a couple of ways to check. Snapping it and looking at the greain structure is one way..should be smooth like satin.

If you sharpen it, an over cooked blade is difficult to put a good edge on and the edge feels 'toothy' and rough

[dtalbot]

David, i do not agree that it nesseseraly are overcooked. when you heat the blade for hardening, there is allways (unless you use hardening foile with a papersheath, or heat the blade in charcoal) going to be scales on some part of the surfase on the blade, due to the air around the blade, when you cool the blade, those scales wil jump off, and leave those marks on the surface.
A bit more study and I think you are spot on! I was looking at a piece of EN42 strip that hasn't had anything done to it yet and the surface (still black as EN42 arrives) has very similar marks, I presume from whenever it was last heated. Just my inexperience causing me to worry :)
David

[Colin KC]

Yay:D

Give it that final rub & STILL test the **** out of it, but with a bit more respect;)

[dtalbot]

Yay:D

Give it that final rub & STILL test the **** out of it, but with a bit more respect;)
Certainly will test it good and proper, though probably not to destruction as a bit of time working through the wet and try paper grades has left me with a surface that look as good as my Sabatier kitchen knives!
Just got to wait for my spyderco sharpening set to arrive now :)
David