Rockwell C scale

samuraistuart

samuraistuart

Joined
Dec 21, 2006
Messages
3,158
I just learned something I didn't know from Brad Stallsmith (Peter's heat treating). He was being interviewed by Blind Horse Knives for heat treating info, and the videos are on youtube. The Rockwell scale is not linear....it is graduated, or exponential. In other words, the difference between 60 and 61 on the Rockwell C scale is greater than the difference between 30 and 31. I learn somethin' new every day!
 
That's true, which is why I have to wonder my head when makers and manus list their knives as "56-58Rc" or "60-61Rc"... well, which is it? That could be a huge difference, and not knowing doesn't exactly inspire confidence in their procedures.

Brad is a great guy to talk to, and really knows his stuff :thumbup:
 
  • Like
Reactions: vba
Yep, prior to my knife making venture and speaking to Brad, I was unaware as well. He has told me a couple of times, that he can dial in whatever we as knife makers request for RC, to within a 1/2 point. For instance, I've got a batch of 52100 headed his way soon and whereas I was originally requesting say....59-60, I now request 59-59.5. According to him, this type of request is no problem at all. It's reassuring as a maker and the customer knows exactly what they're getting.
 
james terrio said:
That's true, which is why I have to wonder my head when makers and manus list their knives as "56-58Rc" or "60-61Rc"... well, which is it? That could be a huge difference, and not knowing doesn't exactly inspire confidence in their procedures.

Brad is a great guy to talk to, and really knows his stuff :thumbup:
Click to expand...

I always assumed the guys doing the range numbers were following a recipe without a tester and felt they should be in that range. With stepped tempering and three or more tempers as needed, I can get within Rc0.5 of my target, assuming the tester remains accurate between tests (so far so good).
 
The next questions should be...

What exactly IS the difference between 60 and 61HRC?
How does it translate to performance?
What is the relationship between geometry and HRC numbers?

When I read a maker's specs and they state "60HRC" I first have to wonder if they check. If so, where do they check and what equipment are they using? Then I wonder what sort of geometry they are using. I don't have an in-house tester, so when dialing in a heat treat I bring pieces to my guy and see where they are. If I stick to the temperatures and regularly calibrate my kiln... I should be in the ballpark. I sometimes give an "approx 58-59HRC" spec because that is what it is... an approximation. I focus more on geometry because in my experience, THAT is the meat and potatoes. I have seen great knives made out of 52100 at 60HRC and 58HRC. I have used some spectacular performing blades with "less than optimal" heat treat. I have also used some pretty awful knives with spot on HRC numbers from reputable heat treat service providers.

Don't get me wrong... I would kill for a Rockwell tester. Dialing in geometry AND hardness is the way to go... just don't get hung up on one or the other. How they perform as a team(in real-world use) is where it's at.
 
I agree with you 100%. For me, testing the steel is a verification that I have heat treated consistently. I find each steel has an optimum at a certain geometry and intended use. The number is meaningless in and of itself, but it assures me that something didn't go wrong with the heat treat.
 
Rick, maybe I'm totally wrong here, but it sounds like you're assuming we're not focused on hardness AND geometry. I can tell you with assurance, that would be an incorrect assumption....personally speaking. I've only worked with 52100 up to this point and I've spent the last year dialing in my geometry and hardness to find a happy medium for this particular steel. I've had Brad run my knives anywhere from 58 Rc to 61 Rc. After personal testing and customer feedback, I know that 59-59.5 Rc with the very thin grind geometry I aim for and a 25 degree inclusive bevel hits the sweet spot for my knives. I may be a year one maker, but I am determined to provide any potential customer, a knife that is of the highest quality AND performance.
 
I have a very accurite bench model hardness tester (at least according to my test
blocks). If I do a batch of blades and springs, plate quench, do the freeze, and the
draws, there can be 1/2 point or so between 2 different blades. I'd like to believe it
is more of an exact science or that some new steel will test exactly the same every time-
but I have'nt seen it yet. To be fair most blades in a batch will be closer than a 1/2 point
and often right on. I will say without a tester its pretty much " by guess" and "by golly".
I do test every blade and spring that I make.
Ken.
 
Last edited:
While it's true the scale is not linear, in the range used for most knives (55-61), it is very close. The change in one point of hardness represents roughly a 3% change in strength (2.8% in the mid 50's and 3.2% in the low 60's). There is also some repeatability inherent in the machines themselves, and generally a half point either way is quite good (60.5 + or - 0.5, or 60-61). Give or take a point (58-60) is more reasonable. Rockwell hardness is used as a minimally destructive (it does leave a dimple) way to quickly check for approximate strength quickly. I have seen the same steel have higher Rockwell numbers, but lower strength values and vice versa, depending on heat treatment. Keep in mind the strength levels are extremely high, 300,000 psi and up. Seemingly large changes on the order of 10,000 psi in strength are still only a couple of percent. Microstructure is the critical thing. Slow quenched 1095 without a temper and properly treated 1095 can both read 62-63 HRc, but I'd only want a knife out of the latter.
 
I use an ames portable hardness tester and give a range for the hardness value of a knife. I do this for a couple reasons. First, according to ames, the tester is supposed to be accurate to within 1 point, so if I gave a single number and represented it as THE hardness, I feel like that could be kind of misleading. The other reason is that the hardness will be slightly different at different points on the knife, even though the entire blade is theoretically supposed to be of a uniform structure, and thus hardness (unless of course it's differentially heat treated). So what I do is take three tests at three points on the blade (usually at the ricasso, a third of the way down, and half the way down if the flat goes that far) and give the lowest and highest number. Because I know very little about math and science, I am not sure if this is a better or "truer" representation of the hardness than just doing one test and saying its HRC X, +/- 1, but it seems better to me.

From what I understand, the value on the HRC scale is equal to 100 - (penetration depth / .002mm); in other words, for each .002mm the indenter penetrates into the specimen, you subtract 1 from 100. So if it goes .002" deep (after test load) you'd have HRC99, if .004" deep, then 98, etc. with 100 being infinite hardness. And the reason the scale is described as nonlinear is that for each .002mm the cone-shaped indenter goes, the force is spread over a surface area that increased at a rate equal to the square of the cone's radius at that point, in other words, very quickly. So this means that going .002mm into an object when the radius is very small, e.g., from 100HRC to 99HRC is not nearly the same as going .002mm when the radius is way bigger, say from 40 to 39.

That said, assuming the the HRC values increase geometrically or exponentially or whatever, wouldn't the range that we as knifemakers are working with, say, 57 to 62, or even 55 to 65, represent just a small section of that curve, and thus look pretty close to linear? In other words, maybe the difference between 58 and 59 is not so huge, or at least thinking about values in that range in a linear-ish way is not too far off to be considered totally useless/completely inacurate?

Thanks for letting me think out loud.

-Mike
 
me2 said:
While it's true the scale is not linear, in the range used for most knives (55-61), it is very close. The change in one point of hardness represents roughly a 3% change in strength (2.8% in the mid 50's and 3.2% in the low 60's). There is also some repeatability inherent in the machines themselves, and generally a half point either way is quite good (60.5 + or - 0.5, or 60-61). Give or take a point (58-60) is more reasonable. Rockwell hardness is used as a minimally destructive (it does leave a dimple) way to quickly check for approximate strength quickly. I have seen the same steel have higher Rockwell numbers, but lower strength values and vice versa, depending on heat treatment. Keep in mind the strength levels are extremely high, 300,000 psi and up. Seemingly large changes on the order of 10,000 psi in strength are still only a couple of percent. Microstructure is the critical thing. Slow quenched 1095 without a temper and properly treated 1095 can both read 62-63 HRc, but I'd only want a knife out of the latter.
Click to expand...

I started writing my post before you posted this and so I didn't see it till after I posted; it really clears up a lot of questions I had. Thank you!
 
I do a series of 5 tests now and average them to come up with my number. There was an interesting demonstration at the Southern Alberta Hammer In where the variance in Rc decreased with each temper cycle. Three tempers resulted in the tests being within Rc0.5 of each other. With two tempers the variance was Rc1.0, and one temper showed more variation. I don't know if this would change real world performance, but it can't hurt either. I used to do three tests, but it was suggested that 5 was more accurate, so that is what I have been doing since.
 
Are Rc numbers the be-all/end all? Of course not.

Rick Marchand said:
What is the relationship between geometry and HRC numbers?
Click to expand...

As you know, it's complicated, and varies from knife to knife and perhaps moreso between different steels. Obviously, I don't run big choppers with the same edge thickness and hardness as super-thin slicers made of the same alloy. That would be dumb.

I can control geometry within a thousandth of an inch fairly easily. That's my job as a grinder/sparkmaker.

But I'm just not set up to control my entire HT process and test it to anywhere near that degree of accuracy... and very few small shops are.

Peters' is set up for that, and they Rockwell-test every one of my blades at least twice to be sure. That lends a great deal of peace-of-mind to me and my clients.

"Ballpark" hardness values based on datasheets, guesswork and several other factors (austenizing temp, color charts, homebuilt forges, quenchant speed, calibrated eyeballs, toaster-oven tempering cycles etc - every one of which is very likely to be off by who-knows-how-much) is simply not good enough for me. There's a certain amount of wiggle-room between optimum and half-vast, but I prefer optimum. All that stuff usually works out fairly well, but it *could* add up to a total failure, in a worst-case scenario.

JT got no time for that. That's why they invented Rockwell testers in the first place... as a major step of quality control to see if everything probably went right, or might be horribly wrong.
 
Last edited:
  • Like
Reactions: vba
me2 said:
While it's true the scale is not linear, in the range used for most knives (55-61), it is very close. The change in one point of hardness represents roughly a 3% change in strength (2.8% in the mid 50's and 3.2% in the low 60's). There is also some repeatability inherent in the machines themselves, and generally a half point either way is quite good (60.5 + or - 0.5, or 60-61). Give or take a point (58-60) is more reasonable. Rockwell hardness is used as a minimally destructive (it does leave a dimple) way to quickly check for approximate strength quickly. I have seen the same steel have higher Rockwell numbers, but lower strength values and vice versa, depending on heat treatment. Keep in mind the strength levels are extremely high, 300,000 psi and up. Seemingly large changes on the order of 10,000 psi in strength are still only a couple of percent. Microstructure is the critical thing. Slow quenched 1095 without a temper and properly treated 1095 can both read 62-63 HRc, but I'd only want a knife out of the latter.
Click to expand...

+1...
 
me2 beat me to it. While alinear, it is not exponential. Within our range of use, the difference is more or less linear. For anyone but a metallurgical lab with very precise testing equipment, the difference between Rc 58-59-60-61-62 can be considered as even steps.
 
bladsmth said:
me2 beat me to it. While alinear, it is not exponential. Within our range of use, the difference is more or less linear. For anyone but a metallurgical lab with very precise testing equipment, the difference between Rc 58-59-60-61-62 can be considered as even steps.
Click to expand...

That was what I gathered from Brad. Exponential isn't the right term, and I knew that when I typed it (so why did you type it dummy?). Alinear....that is more accurate. The difference between 60 and 61 isn't as big a difference between 30 and 31, not enough to notice in real life, probably. I like what Roman Landes said, "Geometry cuts, heat treat determines how long." Kinda goes with our rockwell discussion.

I searched the "interweb" trying to find a quantitative graph, or info, so we can nail down exactly how parabolic, or alinear, the curve is. The question has been asked a few times on forums before, "Is the C scale linear or alinear, and if alinear, by how much?" I would love to quantify that difference. Oh wait....what's this?.......rockwell_c_conv.jpg The image isn't super detailed, but you can make out that, at least against the vickers scale, the difference between 30 and 31 is less than 60 and 61.....just barely tho.
 
Last edited:
CATRA Testing shows the difference in HRC makes:

Elmax:
HRC 60.5 - 761.7 TCC
HRC 62.0 - 930.7 TCC

TCC = Total cards cut

I like looking at CATRA testing data because the variables of geometry, cutting media and cutting style have been eliminated.

An example in my personal testing is AEB-L. At HRC 59 the performance is mediocre. Raise the hardness 3 points and the steel is amazing.

I can't say enough good about Brad Stallsmith at Peter's Heat Treating. His work is excellent. Ask him for specific a hardness. He will deliver. We recently had a billet of CPM M4 heat treater X could not get above HRC 52. Heat treater X told the knifemaker the steel was 420HC. We had the steel tested and it was CPM M4. We sent the steel to Brad and he hit HRC 62 like the knifemaker requested. We've had multiple experiences with Brad over the last few years where he always delivers what says. He is the only heat treater we now recommend.

I do not believe there is one optimum hardness for a given steel. If a knife will not be used for chopping, why heat treat for optimum toughness? Ask your customers how they will use the knife and then select a hardness that balances edge retention and toughness. This is an advantage custom knifemakers have over factory knives. Leverage this when talking to your customers and never give up the advantage.

Chuck
 
  • Like
Reactions: vba
IMHO - beside geometry...

RC hardness # is a useful point of reference. However this measurement of indentation depth can't delineate what & how variables at involved. Variables such as: element distribution, grain size, carbide size, micro structure, phase transformation state. Rephrase 'Me2' above - a good 62RC beats bad 62Rc :D So bottom line, the knife performance (for all indented/target tasks) is all that matter. Now, what does PetersHT/Brad 62.00RC (even if at this precision) mean or just a game barrier to ht entry?

I can't find Lagrangian's hardness post here on BF, hence About Hardness

btw - my hardness tester package should arrives today. Hehehe, I need some reference points after all.
 
You must log in or register to reply here.
Back
Top