Rockwell hardness tests

[Gary W. Graley]

I have seen the testing of some blades but since to accurately measure it you have to test on a flat surface usually at the tang area, but my concern is that it doesn’t translate well out onto the edge of the blade and we all know that problems can come from not keeping the blade cool while grinding, some manufacturers flood the blade with coolant but not everyone.

If you try to get a measurement at the edge it would not be accurate unless you get the area angled to be flat and square to the testing bit which I haven’t seen anyone doing that

so what do you guys and gals think?

Are the testing of the tang area really representative of the actual blade’s hardness???

Thanks in advance
G2

 

[Eli Chaps]

To my knowledge, they will test the tang of one blade out of a batch at whatever state it is in post heat treat. If that tests good, they are all considered good and move through production. I would assume, but there's no way of knowing what everyone does, that early on, every blade was tested to ensure repeatability before dropping down to batch sampling.

I think someone during the "this M390 sucks" fiasco shimmed a blade for edge testing but don't remember for sure. I think your getting into thickness issues at that point. If the edge is thin enough that it just gives then you're not going to get an accurate result.

DeadboxHero and knarfeng will know much more.

I think in some ways, from the big boys, HRC is something we take on faith and consider as another piece of information with the performance telling the real story. I'm starting to fall deeper into the camp that thinks it doesn't matter all that much from most large makers and in most models as in general none of them run the high vanadium steels hard enough to get the most out of them anyway. I know Spyderco strives to go high on some models but overall I think you have to run 'em pretty danged hard to get the most benefit.

 

[Twindog]

You bring up an interesting question. I always assumed that a heat treat would end with the entire blade being of roughly equal hardness, except where the maker intentionally was creating a differential hardness.

Someone posted, maybe Deadbox, a series of hardness tests at the tang near the pivot, and the hardness was within 0.5 Rc in about four or so tries, if I remember correctly. But the edge is more difficult to test.

You could use a blade with a known hardness to do a scratch test. I had a folding blade that was custom heat treated, and the maker told me he tested it to 60 Rc. The blade didn't hold up in actual use -- cracked and dented easily. I used another blade from Benchmade that was 58-60 Rc. And I know Benchmade's hardness seems to be pretty consistent, although that's a fairly big range. But the results were the softer blade easily scratched the supposedly harder blade, but not vice versa. That test gave me confidence that the failed blade didn't achieve its supposed hardness, meaning I could infer that the whole heat treat was probably bad, as my experience led me to believe.

 

[Obsessed with Edges]

Some manufacturerers deliberately anneal the tang ends, after heat treat, to allow some blades to be bent or 'krinked' to fit into multi-blade traditional folders. Case does this with their blades (see below). Some might(?) also do it for tang-stamping's sake, although I think that can & should be done before heat-treat, ordinarily. On blades annealed for such purposes, a Rockwell hardness measurement in the tang portion could be misleadingly low.

This is a screen grab from a 'How it's Made' video (for pocket knives produced at Case) on YouTube. It shows the darkened, annealed portion of a folder's blade. Note how the annealed portion even extends some distance forward of the tang and ricasso. The video is embedded below the picture, with the tang-annealing segment in the 1:06 - 1:27 timeframe of the video.

 

[000Robert]

Good question, Gary. I can't afford an expensive hardness tester but I was thinking about getting some of those testing files to test blades with.

 

[DeadboxHero]

Just test on a flat, well finished area. Best practices will guarantee that the blade matches the test area (proper grinding, sharp abrasives, water-cooling)

 

[Gary W. Graley]

Best practices would be key for sure if everyone were to follow them. But not sure that everyone is committed to do that.

Test files would be handy to get some feedback on the actual edge
G2

 

[BITEME]

Some of the guys on YouTube, have rc values and their tests pretty much confirm that rc value is pretty consistent throughout the blade ,i.e. m390 at 62rc in their tests does cut a bit more cardboard than the same steel at a lower Rockwell say 58rc....another indicator of higher Rockwell values is burr removal during sharpening ,but each steel probably has its own characteristics for sure.
In my mind if it's too easy to sharpen I get a little worried esp. With some supersteels

 

[Danke42]

Are you making knives now or wanting to check knives you've bought?

 

[Eli Chaps]

Just test on a flat, well finished area. Best practices will guarantee that the blade matches the test area (proper grinding, sharp abrasives, water-cooling)

Think that's what Gary is getting at Shawn. Acme Knife Co. can say they tested their XYZ steel at 61HRC, taken on the tang. But then they go dry grind the bevel in and now who knows what hardness the actual edge is?

For small, custom oriented makers like yourself, no worries, but for the large mass production outfits, this is something that some folks wonder about.

 

[000Robert]

Think that's what Gary is getting at Shawn. Acme Knife Co. can say they tested their XYZ steel at 61HRC, taken on the tang. But then they go dry grind the bevel in and now who knows what hardness the actual edge is?

For small, custom oriented makers like yourself, no worries, but for the large mass production outfits, this is something that some folks wonder about.

I wonder how far back from the edge would have lost its heat treat by the time you feel the thicker part of the blade getting warm? That is a question that I have wondered about, but I have no way of recording it. Maybe someone that has a highspeed thermal imaging camera capable of slow-motion, could see it? I don't know much about photography either.

 

[Gary W. Graley]

Are you making knives now or wanting to check knives you've bought?

knives that I bought but putting the question out there on how to go about it

G2

 

[DeadboxHero]

The problem is even if the anvil had a tilt feature which it doesn't, the Rockwell tester is sensitive to flatness and surface finish.

If the tang, pivot area is super flat and well finished and the blade has a rough grind with a very subtle convex than that will give different readings but not due to poor grinding practice, but due simply to different levels of flatness and surface finish.


The Rockwell tester measures depth with each Rockwell point equal to 2um in depth.

For perspective a red blood cell has an average of 8um in diameter.

So folks aren't kidding when they say the test piece needs to be flat, to a certain level of surface finish and cleaned well before testing.

I've seen folks such as newer makers think there is variation in the blade hardness due to inconsistency in hardness measurement without ruling out surface RA and flatness. Just shows why the details are important.

Rockwell hardness testing is a simple test and it's quick and easy but there are details involved for best results.

So there is not a good way to test near the edge using Rockwell hardness testing.

Think that's what Gary is getting at Shawn. Acme Knife Co. can say they tested their XYZ steel at 61HRC, taken on the tang. But then they go dry grind the bevel in and now who knows what hardness the actual edge is?

For small, custom oriented makers like yourself, no worries, but for the large mass production outfits, this is something that some folks wonder about.

 

[Eli Chaps]

The problem is even if the anvil had a tilt feature which it doesn't, the Rockwell tester is sensitive to flatness and surface finish.

If the tang, pivot area is super flat and well finished and the blade has a rough grind with a very subtle convex than that will give different readings but not due to poor grinding practice, due simply to different finish and flatness.

The Rockwell tester measures depth with each Rockwell point is equal to 2um in depth.

For perspective a red blood cell has an average of 8um in diameter.

So folks aren't kidding when they say the test piece needs to be flat, to a certain level of surface finish and cleaned well before testing.

I've seen folks such as newer makers think there is variation in the blade hardness due to inconsistency in hardness measurement without ruling out surface RA and flatness. Just shows why the details are important.

Rockwell hardness testing is a simple test and it's quick and easy but there are details involved for best results.

So there is not a good way to test near the edge using Rockwell hardness testing.

I agree Shawn. That's why I think that coming from the big houses, we all just need to take the HRC as just another piece of information. Trying to judge it as a reflection of the edge itself is probably only so helpful. But one thing I think it can tell us is, if the tang is say 58HRC, then we know the edge isn't likely to be any harder.

You're of course spot on about the RA too. In my work, we have to make sure the RA is in a specific range prior to Rockwell testing.

 

[Gary W. Graley]

A friend where I use to work...(I'm retired now ) had a small folder in M390, it measured low when it should have been about 60, then our machinist tried to do a rockwell test on the side of the blade, which read really low but we discovered as you neatly detailed that you can ONLY really do an accurate rockwell test on a flat well finished surface and nothing that has an angle to it so we threw those values out. I ended up sending the knife back to the dealer and he exchanged it with another folder.

Now, the start of that issue was the folder would dull...quickly just cutting a little bit of cardboard. It was very perplexing, so we went the route of trying to test it. The 'new' folder worked far better and did not dull like the other one had, so between the two it seem readily apparent that one was not head treated as well as the other.

And boy aren't we so lucky to have youtube, makes it amazing to see how some makers make/grind their knives, but as with anything too much of it can be confusing or lead to confusion. Everything with a grain of salt is still a good adage.

I appreciate all of your comments and thoughts!
G2

 

[DeadboxHero]

It can be a complex problem, sometimes the lower hardness (sub 59rc) at the edge means not only lower strength but also makes it very difficult to remove the burr at the edge (depending also on HT not just HRC)

What happens is that the burr is very stubborn and gets straightened out rather than broken off and removed. A wispy wire edge that feels sharp until the first several uses and than it instafolds over making the edge seem to go dull very fast.

What is also frustrating is that if deburring too aggressively one can remove the burr better but the edge underneath gets crushed and comes up smooth. So it's back and forth for mediocre results.

When I was a sharpener I hated it since we would get tons of crummy knives to sharpen, my only desire to be a sharpener at the time was to sharpen all the cool knives but that's not how it works

Issues like this were a huge catalyst for me to start making my own knives and chase those details.

My opinion,
Some of the folks that make these production knives that don't actually use these knives nor do they think deeply about how the microstructure works, just chase numbers like lower HRC means more toughness = more performance like some kind of power point sales chart.

At face value, the lower hardness means it's tougher but toughness is not the same as edge stability and bottoming out the strength due to fear and lack of understanding is not the best route especially for a higher end steel designed for cutting performance on a small blade in my opinion.

Conversely, I think we should respect that there is a lot of money that is invested in doing a 2000-5000 Pcs run of blades. Folks get cold feet when investing $25,000 in a run of knives, there can be many unknowns behind the scenes that most folks on the other side of the fence will never know or understand without doing it themselves so it's not fair to judge.
I feel making in small batches one by one by hand and making at larger scale both have unique challenges and one cannot do what the other does.

I like doing small batch because that is simply the only way possible to reach closer to the physical limit of what is possible for performance at consequence to high cost and low quantity.

A friend where I use to work...(I'm retired now ) had a small folder in M390, it measured low when it should have been about 60, then our machinist tried to do a rockwell test on the side of the blade, which read really low but we discovered as you neatly detailed that you can ONLY really do an accurate rockwell test on a flat well finished surface and nothing that has an angle to it so we threw those values out. I ended up sending the knife back to the dealer and he exchanged it with another folder.

Now, the start of that issue was the folder would dull...quickly just cutting a little bit of cardboard. It was very perplexing, so we went the route of trying to test it. The 'new' folder worked far better and did not dull like the other one had, so between the two it seem readily apparent that one was not head treated as well as the other.

And boy aren't we so lucky to have youtube, makes it amazing to see how some makers make/grind their knives, but as with anything too much of it can be confusing or lead to confusion. Everything with a grain of salt is still a good adage.

I appreciate all of your comments and thoughts!
G2

 

[knarfeng]

Some manufacturerers deliberately anneal the tang ends, after heat treat, to allow some blades to be bent or 'krinked' to fit into multi-blade traditional folders. Case does this with their blades (see below). Some might(?) also do it for tang-stamping's sake, although I think that can & should be done before heat-treat, ordinarily. On blades annealed for such purposes, a Rockwell hardness measurement in the tang portion could be misleadingly low.

That's a good point for traditional patterns. Aside from the stamping, blades which are crinked also require the tang to be annealed. I found that to be true for both Case and Camillus .

I've had had good luck measuring the hardness on the tang, although you need to make certain that there is enough room for a proper measurement. You don't want to take a measurement out next to the edge of the tang. I've had a couple of knives for which I couldn't get a measurement because there was not enough exposed tang area and I did not want to disassemble them. (The chief metallurgist in the engineering lab at work was a friend and used to let me use the Rockwell tester. He's since retired, so I'm now cut off from that resource.)