Professionally over tempered?

C_Claycomb

C_Claycomb

Joined
Dec 11, 2000
Messages
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I just got a couple of O-1 blades back from heat treating by a professional knife maker. I was hoping to use them as a bench mark and target for my own heat treating efforts. The maker described the process used as follows, a 15 minute soak in an electric oven (no inert gas) at 1454degF followed by an edge quench in oil, cryo and triple temper. No other thermal cycles.

Having ground the edges, sharpened them and done some tests I am pretty sure that they are much too soft, at about 56Rc. I called the maker to check what he thought he had hardened them to and he was positive that they would be 59Rc, according to his process. I felt acutely embarrassed to suggest that they might be soft and the maker shrugged off the results I mentioned. I have since done more testing and am even more sure of my results.

Could different sources of O-1 make so much difference to the final hardness, so that while his came out hard, and mine did not?

My natural inclination is to think the blades were over-tempered. I am curious, for a long soak with O-1, aiming for a 58-60Rc hardness, what temperature should be used for tempering and what color would you expect? I had expected it would be in the browns but the blades came back a fairly dark blue.

If the process could be off target, is it worth letting the maker know?

I am going to have to re-treat the blades, even if it is only with a propane torch and hollow fire brick as per my normal procedure. I don’t think it could hurt anything, at least, I can’t make them worse. As I understand it, since it has been soaked thoroughly once, it should be fairly easy to get enough carbon back into solution, even without further soaking. Coming from soaked steel, would it be better to cycle the steel, or just go for a single heat and quench?

Chris
 
C_Claycomb said:
I just got a couple of O-1 blades back from heat treating by a professional knife maker. I was hoping to use them as a bench mark and target for my own heat treating efforts. The maker described the process used as follows, a 15 minute soak in an electric oven (no inert gas) at 1454degF followed by an edge quench in oil, cryo and triple temper. No other thermal cycles.

Having ground the edges, sharpened them and done some tests I am pretty sure that they are much too soft, at about 56Rc. I called the maker to check what he thought he had hardened them to and he was positive that they would be 59Rc, according to his process. I felt acutely embarrassed to suggest that they might be soft and the maker shrugged off the results I mentioned. I have since done more testing and am even more sure of my results.

Could different sources of O-1 make so much difference to the final hardness, so that while his came out hard, and mine did not?

My natural inclination is to think the blades were over-tempered. I am curious, for a long soak with O-1, aiming for a 58-60Rc hardness, what temperature should be used for tempering and what color would you expect? I had expected it would be in the browns but the blades came back a fairly dark blue.

If the process could be off target, is it worth letting the maker know?

I am going to have to re-treat the blades, even if it is only with a propane torch and hollow fire brick as per my normal procedure. I don’t think it could hurt anything, at least, I can’t make them worse. As I understand it, since it has been soaked thoroughly once, it should be fairly easy to get enough carbon back into solution, even without further soaking. Coming from soaked steel, would it be better to cycle the steel, or just go for a single heat and quench?

Chris
Click to expand...
dark blue is not accurate for a temperature reading but I dare say the steel is too soft at that color from a temper heat. it should be around a straw,,
but too,,,, you'd have had colors from the quench he did unless he reground it for you..
then tempered it
 
you'd have had colors from the quench he did unless he reground it for you..
then tempered it
Click to expand...

Good point. I didn't think he re-ground anything, but then there wasn't any black gunk left on the blades either.

The reason I was wondering about the straw vs blue is that several people have written that when they soak O-1 they have to up their tempering temperature to get back down to high 50s Rc. I just don't know how much that increase in temperint temperature would normally be, 100deg, 200:confused:
 
Temper color is not an exact science since it depends on alloy, surface condition [clean , oil etc] and temperature.The only way is to do a hardness check on a decarb free surface.
 
The blades were flat ground from 5/32” stock with cutting edges left at about 0.030” for treating, when they returned I ground them to an edge using a power file and hand held water stones. Following the rough grind I attempted to sharpen one blade and test its cutting ability.

Tests include cutting ¾” manila rope, news paper, a seasoned ash stave, and brass rod edge flex. The performance was overall rather disappointing, it took quite some doing to get a remotely sharp edge on the blade and it never reached that scary, “grabby” level. To begin with it would not shave or slice paper, but a little more work got it doing both, but not with any authority. On a ceramic rod I could feel a very distinct burr being pushed back and forth. Rope cutting was not even as good as my better fire brick treated blades (which have been cycled but not soaked), it just didn’t hold all that well and only managed about 30 cuts before dragging and needing more effort. On the brass rod it was possible to see how far I had run the knife along the rod because the edge reflected light differently before and after a single pass along the rod.

All the above made me highly suspicious and I tried a 55Rc test file on the edge, it didn’t just mark the surface, but bit in enough that the scratch that took 5 minutes to remove with an 800 grit waterstone. When testing my own heat treating I have found that the files will mark steel about 1 point in hardness higher than they are rated, but not much more.

The maker reckonned that there would only have been a couple of thou of decarb, and that it should be cleared with a couple of sharpenings. Is that overly optimistic?

He said that it should be expected that a 55Rc file would mark the edge, it was only if it really bit deep that it counted. Not being entirely convinced I took the blade to work where I used their hardness tester to test a spot on the Ricasso right next to the edge/plunge. I didn't grind this surface, but it was sanded clean. Three readings all came back at 56Rc. Unfortunately I can’t rig the machine to test the ground bevel, but I can easily sand the ricasso down further and do another test tomorrow.

None of the above, alone, would make me sure the blades are soft, but taken together they all seem to be pointing the same way.

Still not sure what to do about it though.

Chris
 
mete said:
Temper color is not an exact science since it depends on alloy, surface condition [clean , oil etc] and temperature.The only way is to do a hardness check on a decarb free surface.
Click to expand...

Dan Gray said:
dark blue is not accurate for a temperature reading but I dare say the steel is too soft at that color from a temper heat. it should be around a straw,,
but too,,,, you'd have had colors from the quench he did unless he reground it for you..
then tempered it
Click to expand...
just to add to what I said and clarify if you do use Color

straw is not accurate for a temperature reading either :)
I mean to say, between the two there could be a heap of difference in a Rockwell:)
 
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