Proper way to perform a "file test"

JW Knives

JW Knives

Joined
Feb 22, 2011
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566
I HT my own blades and have been getting very good results.

I often see to check that your HT has worked that you can do a file test by running a file over the HT'd blade and seeing if it skips or bites.

How much pressure do you put on the file?
Do you run it over a flat or an edge?

It seems to me that if you press hard enough you can get the file to grab at something (maybe it's just the post HT scaling)

Thanks
 
Run it over the edge. You do need to ensure you're running it on the steel and not the forge scale, many beginners think there's a problem with HT because they get the file to bite on the scale. I usually test on the actual edge (having cleaned/filed the scale off first), running the file parallel with the proposed bevel, because having left about 1mm thickness at the edge for HT, you know that any file marks will disappear when you finally do put the edge on it.
 
One of the guys on this forum once posted that he takes a hacksaw to his edge to check for hardness. I've never heard anyone else say that and still don't know what to think. I don't think he was kidding, since he posted that comment in the for sale forum. As for me, I don't put a lot of pressure on the file, just enough to see if it skids or bites.

Dave
 
To get the most accurate reading, you need samples of known hardness to compare with. Also, it helps if the file is good quality and sharp. Pressure and the amount of surface area in contact also have an effect. If you can't tell by feel alone, lay a sheet of white paper down, and see if particles of steel are coming off.
 
Can't you get hardness test files, a set with different degrees of hardness to compare to the blade?
 
The problem is that files don't work as well once they get a little dull. A dull file will skate on a Rockwell 50.
 
Heres a set of pre hardened files i use to gage "bite" on the stuff i heat treat myself. Found mine on craigslist 20 bucks.
 

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File testing is only slightly better than worthless, most people do not grind below their decarb before testing
files will skate off of scale
files will skate off of pearlite
files will bite into the decarb layer of a full hardened blade making you believe your heat treatment failed

I wasted close to a week troubleshooting someone's heat treat process because he was using hardness testing files, and kept coming up soft. He swore he was not in the decarb layer so he finally took his test coupons to a machine shop that had a rockwell tester, the readings were completely different from what the files gave him, but still soft, I had him surface grind 10 thousandths, his readings went up on the tester, I had him grind 10 thousandths at a time until he got the same reading 3 times and his heat treat had been dead on the whole time, he just had .045" thick decarb layer.

-Page
 
Wow, .045" decarb. Was that .045" total or .045" each side? How long was he soaking? Or what do you think caused such excessive decarb.?
 
SO... there is no proper way to file test...SO...point drops, antler hacking, brass rod testing, clothes drier steel ALOT thicker than cans and garage door are the alternatives and there funner!! To sunshadow, sounds like your guy had a bad quench.
 

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the guy tried a couple of times, and had no atmosphere control in his furnace.

It wasn't a problem with his quench, it was his testing method. When he finally got through the excessive decarb he was right on the money. He didn't believe he had decarb, the steel disagreed with him.

I use lump charcoal a little thrown in the sealed kiln a minute before putting the blades in to create a reducing atmosphere in the kiln which has reduced my decarb to 5-25 microns (based on my measurements of cross sectioned samples) your mileage may vary.

-Page
 
Any test is only "slightly better than worthless" or completely worthless, if you don't do it right and understand it's drawbacks.
 
A file test is a pass or fail test to see if your blade hardened at all, IMO. It takes experience and experimentation to know what it should feel like on a hardened blade vs. a soft blade, and as others have said, the condition of the file and the workpiece itself can skew the test to a wide margin of error.
Not too dissimilar from gauging wind speed with a wet finger. Using actual test files is more accurate but still not a number I would put much faith in.
 
i tried a file on some of the other blades i had laying around that were of similar steels...it didnt leave me feeling very confident, it bit a little depending on how hard i pressed or the angle i used, i couldnt really tell squat as a beginner so im not sold on the idea...
 
Good input. I didn't even know that there were hardness testing files! I am confident that my knives are hard and they do perform well, I've just yet to find a resource for getting a legit rockwell test done...
 
JW, you can buy the testing files individually for $18.00 each. They come in 5 point increments. If you know the hardness range you want, you can buy two, much like headspace gauges for guns. A go, and a no go, but the set is nice to have on hand. They will not replace or equall a Rockwell tester, but will give a ball park idea of what you have. I had a couple of blades I tested with them, sent them off to be Rced, and was only 3 points off the Rc tester. I think, not bad for a file test. They will wear out, but you can replace them easily enough. http://store.flexbar.com/shop/pc/Individual-Hardness-Testing-Files-Black-26p5384.htm
 
We dont have a RC tester so we try and make sure the heat treat is spot on..We heat treat all our knives in a muffle, in a gas forge. Temperature is controlled manually with a pyrometer..I was supprised but it will hold within 1*-2* degrees for as long as you want..Soaking 1095 @ 1475* for 5-7 minutes is no problem at all.We also now use commercial fst quench oil.
I will try a file just for peace of mind but its a brand new nicholson thats used only for that purpose..
Axes and hawks are heat treated more old school.Then again heat treating a axe and a knife are totally different things....The thing is we have made some many dang axes that heat treating one is like taking a breath..Its just second nature..
 
Just in case heres our set up..The pyrometer and probe were about $40 I think...Then the muffle is homemade..Not as nice as a $1200 oven but its controlled, even temp...Thats what matters..
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Scratch hardness and Rockwell hardness are two different things. You can't determine Rockwell hardness with a scratch test or vice versa. However, the scratch test isn't completely worthless, pass/fail or anything like that. One test is for the resistance to scratching and the other to penetration or indentation . The indentation test is the most widely accepted and used for steels in big industry, but is not the end all of end alls either... and may not be practical for your average knifemaker.

Hardness in itself is somewhat hard to define singularly and is multi-faceted. So we are into another sort of gray area.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Hardness.htm

Quote:
"The lack of a fundamental definition indicates that hardness is not be a basic property of a material, but rather a composite one with contributions from the yield strength, work hardening, true tensile strength, modulus, and others factors."

Since the Mohs scale of scratch hardness wasn't designed for metals, that's why I recommend using some samples of steel with known "hardness’s" (and composition) to compare to, with the file test. Because of the limitations of the scratch hardness test for steel, it should be used in conjunction with some other type of test/tests to get a better idea. I think that chopping into a brass or copper rod at different angles will help, but there again you need something to compare to.

In the final analysis, chopping on wood, slicing different softer materials and just field testing the blade for it's intended purpose probably makes the most sense for us humble knifemakers.
 
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One difference between hardness testing steel with files and the Mohs scratch hardness test, is that with the Mohs you are testing one material or mineral against another. With the file test you are testing steel against steel, so it should apply much better....

The file test "scale" applies better to what we are doing than the Mohs.
 
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