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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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  #1  
Old 09-27-2014, 09:54 PM
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The Hazards of Hardness Testing

This is the risk you take everytime you use a Rockwell hardness tester.

This was clay-backed 1080 after the quench but before the temper.


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Old 09-28-2014, 02:17 AM
jmccustomknives jmccustomknives is offline
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Idk, it kind of looks like there is a rough grain to the brake. That would indicate an improper heat treat. It might just be the pic though.
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Old 09-28-2014, 10:35 AM
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Never had an issue with mine , looks grainy to me as well......
also your test block looks like it has some serious hardness test dimples in it , may just be the picture


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Old 09-28-2014, 10:41 AM
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Yes, the test block has been used many times.

The break shows a finer grain texture in person. An identical blade HTd at the same time turned out fine with an as quenched hardness of 61RC.


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  #5  
Old 09-28-2014, 03:11 PM
WBE WBE is offline
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How and why are you applying the tester in the hollow grind, and why would you call 61 Rc as quenched good? Just asking.
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Old 09-28-2014, 03:43 PM
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Is there a reason to test the knife on the blade. I always test on the tang. I figure if I have a 60 Rockwell on the tang before temper I know the heat treat worked.
Steve


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  #7  
Old 09-28-2014, 04:15 PM
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I did a clay-backed HT on this blade with Satanite covering the spine and tang. They were not hardened. The hollow was the only part of the knife hardened, so that is where I test.

The Rams Rockford tester that I have is old and impossible to find parts for. My flat anvil is all I can use under the blade and penetrator. This may have contributed to the failure.

As for why 61 is good..., because I temper to 57-58. With 1080. 1084, and 1095, 61 is a very common as quenched hardness (1095 goes higher) as tested on my equipment. Because I almost always test on the hollow, I figure that there is a margin for error because the hollow on the underside is allowing a small degree of flex. On fully hardened blades out of the paragon and plate quenched and file steel quenched in brine, I get 62-64.

So WBE, you obviously have a difference of opinion. School me.


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Last edited by Andrew Garrett; 09-28-2014 at 04:59 PM.
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Old 09-28-2014, 05:55 PM
samuraistuart samuraistuart is offline
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I personally don't see how anyone could get reliable results testing their blades on a hollow grind. From what I understand, the surface to be tested must be perpendicular to the diamond penetrator, and at a decent polish (220 grit). A hollow grind would be a nightmare to try to get squared up.

With 1084, 61 as an as quenched hardness indicates you did something wrong. Each one of those steels should easily reach 65-66 HRC. A 61 RC value would be after a good tempering temp of 400F or so. You don't ever want to quench to your final hardness level. Always quench to get the maximum hardness possible, and then temper to the desired RC value. There is more to the steel than just how hard it is!

I have never heard of an RC test breaking a blade, either.
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Old 09-28-2014, 11:10 PM
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Thanks for the input Samurai.

Yes, I agree. Testing on the hollow..., not optimal. But..., I'm at a loss for options. I guess I could go back to the old file tests.

In any case, not satisfied with where this thread was going (making it look as if I'm a heat treating idiot), I took my broken blade back out to the shop when I got home from work and shed my uniform. I did about 15-20 more tests at different spots along the hardened edge. Some, where the hollow allowed space between the anvil and the blade, failed terribly--those readings were in the 30s or 40s. Some however, closer to the edge and those tested on a quickly ground and modified spare pipe anvil with a smaller diameter, measured as high as 64. I got that number 4 times.

Also at issue is my finish. As you see in the photo, it is a 60grit finish. This would soften the numbers too.

Additionally, I calibrated on my testing block several times and it too was inconsistent. These Rams Rockford testers use a spring rather than weights and temperature, humidity, etc. might alter results. I'm not sure, but I've never been totally happy with the device. When the numbers were consistent tonight, they were routinely about 1.5 points under test value (55.8) The standard variance is plus/minus 1.

Then, I went to the tattoo studio and retrieved my 12x loupe to closely examine the break and grain size. I was given a cheap coffee cup recently and decided to sacrifice it in the name of science. I broke of the handle and compared the textures--very similar. If baby powder is #1 on a scale from 1-10, and playground sand is #10, my grain looks to be about a 3 or 4. More scientifically, I estimated the break surface to be similar to a 320g grinding belt (not the finish they leave--the belt itself).

In conclusion:
I feel that my testing methods are flawed as my equipment is not adequate for testing edge-quenched hollow-ground blades, and my finish is not fine enough for reliable testing as evidenced by inconsistent results.
I feel that my HT is solid, but my testing is not.


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Last edited by Andrew Garrett; 09-28-2014 at 11:14 PM.
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  #10  
Old 09-29-2014, 05:55 AM
WBE WBE is offline
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No one is calling you an idiot, and your conclusions are probably dead on. Typical Rc testers must have the test piece at 90? to the penetrator, and have full support underneath the piece being tested. If these requirements are not met, you have wasted your time, because you will not get an accurate reading. A good comparison of grain size is to match against a broken "quality" file, such as an older Nicholson, Simmonds, or other trusted brand.
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Old 09-29-2014, 08:12 AM
Kevin R. Cashen Kevin R. Cashen is offline
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I think this is a clear case of one of those internet communication shortcomings. Andrew, I think the format of your initial post put some folks into ?debunk? or at least heavy analytical mode. What you are showing is not a hazard of Rockwell testing in the experience of myself or anybody I know who has ever done the test, but rather a freak incident. If your opening sentence had been ?Is this the risk you take everytime you use a Rockwell hardness tester?? the input would probably have been different, but the phrasing makes is sound like you are stating a fact as if to warn people away from a very standard and ubiquitous test. The topic shift to heat treatment I think was to help understand what happened before the wrong conclusions are drawn about a test as useful as Rockwell.

With a certain tone established folks probably viscerally went immediately to gathering evidence to support a case to defend the Rockwell test, which would be nothing personal but just an attempt to set the record straight. WBE (I call him Wick), had some pretty good advice with breaking a quality file, rather than ceramics, powders or sands, grain size does not transfer from different materials that well. But what I can say is that if you can see the individual grains, they are way too large, even under 12X magnification. ASTM grain size 8 or 9 are pretty good but they need at least 100X to resolve enough to measure to the ASTM scale (well, you would also need the proper mounting, reagents etc?)

You also have a valid point about the tester and test. The handheld units can be good if they are in great shape and reading dead on, but they are much more prone to damage and drift if they are not in excellent condition. Your observation on the blade finish is spot on, I prefer to take any area to be HRC tested to at least 400X as the coarser scratches will cause the problems you pointed out. But then I can?t fault the folks for pointing out what are facts regarding the issues with taking an HRC test on an edge bevel, but then you mentioned that there were no other options in test surfaces; I would probably just go with a file test and forgo an HRC test I can?t trust.

Now blast me if anybody wants to, it wouldn?t be the first time, but that is why I decided to give some input on the misunderstanding, I have been there enough times myself that I know the way they happen.
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Old 09-29-2014, 11:01 AM
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Thanks for the clarity gentlemen. When I posted this thread it was just to show the photo of an event that I'd never experienced. Then, it seemed as if it was becoming an indictment of my HT procedures. I wasn't prepared for that, but I do genuinely appreciate the input--there are lessons there.

My tester did not come with much in the way of instruction where set up is concerned. It only indicates on a metal instruction plate how to obtain a reading. I was very ignorant about items being level, but it make sense (deflection). I was also ignorant as to finish grade, but now I know. Thank you.

Since I make so many blades that have no flat hardened surfaces, I will go back to a file test--maybe I'll even get a set of testing files to try and be a bit scientific about it.

As for grain growth, yea, I HT carbon blades in an atmospheric forge and I can see how an occasional over-heat could happen. I assume that a standard normalization or two will undo that damage, or will I need to do a full annealing?

Thanks again for the help guys, and forgive my defensive response.

Cheers.


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Old 09-29-2014, 11:19 AM
Brad Johnson Brad Johnson is offline
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Andy,
Any blade in the as quenched state is going to be brittle. Try testing post temper as it will pull some of the hardness out and it will not be so brittle.
Think about it, the tester is wanting to force the ball at the end of the ram via pressure alone a pre-determined distance into the steel. I have never heard of using a tester pre-temper. If you want to test for hardness as quenched use a file.
Just my 2 cents,


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Old 09-29-2014, 11:36 AM
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Brad, It is my understanding that testing post-quench AND post-temper is the norm. I'm sure someone will come along and confirm that or tell me that I'm all wet in a minute or two.


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Old 09-29-2014, 11:42 AM
samuraistuart samuraistuart is offline
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Brad, not trying to derail Mr Garrett's thread here....and at the risk of sounding like I am critiquing you (I wish we were face to face...I'm really a nice guy!!!!!). Testing the RC value post quench pre temper is the only way that i know of to get a good idea if you are reaching max RC out of the quench. A file test isn't going to tell you jack. It will tell you if it is harder than the file or not...that is about it. The post quench RC value is very important. It should be as high as you can get it. Testing the RC values after tempering is great to know if you are hitting what you are aiming for in reducing the overall hardness....but if you aren't reaching max RC out of the quench....you aren't realizing the steel's full potential. Leaving meat on the plate....so to speak. And I think the VERY experienced (not me) will say that the tests individually (RC, file skate, brass rod flex, temper colors, etc etc etc) do very little in giving you the overall picture of what has happened with the structure. Put them all together and you get a much better picture. Just my 2 pesos.
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