Production M390 - Expectation vs Reality?

So is an HRC of 51 2% harder than one of 50? See this is why I’m confused. Sure the bottom line is does the edge do what you want, but I’d like to understand the relationship between the HRC number and the actual hardness. And consequently what a difference between say 59 and 62 means in the case of m390.
The HRC is the acutal hardness.
Hardness is a component for edge performance.

You're wanting to know apply the HRC to actual edge performance in some type of calculation.
Larrin had made a regression formula from over a decade of CATRA test data to predict CATRA cut testing numbers using plug in values for edge angle, hardness and Carbide volume also with coefficients for carbide hardness.


Here is the formula for M390

-157+15.8*HRC-17.8*Inclusive edge angle+14.6*17.5% CrV7C3 carbides+26.2*2.5% MC type Carbides

So if we have
M390 at 50hrc
That would look like this
-157+15.8(50hrc)-17.8(30°inclusive)+14.6(17.5%CrV7c3)+26.2(2.5% VC)
= 420mm of predicted Total Cards Cut (TCC) in CATRA.
+1 HRC difference to 51hrc plugged into the same formula shows
=435mm TCC

For further explanation this is a 3 part series of articles here.
https://knifesteelnerds.com/2018/11/19/steel-edge-retention/
 
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Can we let this crap die already? Ferrum forge already made a video discrediting the importance of HRC, and that it's more a test point to see if "your" heat treating is going according to plan than an actual useful piece of data on how a steel will perform. Also the fact that edges are typically higher hardness than where you can Rockwell test a knife, you could have a 60 HRC pivot, and 61 HRC edge, or 58 pivot, 62 edge, and so on. Listen to ferrum forge's video, HRC is pointless as a specification for consumer purchases in the knife industry.
 
Nah, usually the edge is softer due to being overheated with poor practices grinding and sharpening used to hit volume.


Can we let this crap die already? Ferrum forge already made a video discrediting the importance of HRC, and that it's more a test point to see if "your" heat treating is going according to plan than an actual useful piece of data on how a steel will perform. Also the fact that edges are typically higher hardness than where you can Rockwell test a knife, you could have a 60 HRC pivot, and 61 HRC edge, or 58 pivot, 62 edge, and so on. Listen to ferrum forge's video, HRC is pointless as a specification for consumer purchases in the knife industry.
 
Can we let this crap die already? Ferrum forge already made a video discrediting the importance of HRC, and that it's more a test point to see if "your" heat treating is going according to plan than an actual useful piece of data on how a steel will perform. Also the fact that edges are typically higher hardness than where you can Rockwell test a knife, you could have a 60 HRC pivot, and 61 HRC edge, or 58 pivot, 62 edge, and so on. Listen to ferrum forge's video, HRC is pointless as a specification for consumer purchases in the knife industry.

That's not exactly accurate. He specifically addresses the issue of comparing pure HRC numbers from one steel to another. That is true and can be misleading. But comparing the HRC numbers within just the one steel is still telling. Using his 1095 example, at 65HRC that steel will exhibit certain characteristics but at 54HRC it will exhibit different characteristics. That's largely what this entire discussion has been about. M390 at certain hardness conditions and how it performs in those conditions. It is not saying M390 at 60HRC will be superior to S30V at 60HRC. It is about what hardness level for a specific steel brings out certain characteristics.

HRC does matter. It is not the end-all-be-all for sure and people do need to be careful and understand it. And even within a certain steel, it only tells part of the story. A higher hardness number doesn't necessarily mean better. Too hard and it's brittle, too soft and it's mushy. The key is knowing what properties will be enhanced at a certain hardness for a specific steel and understanding the compromises each of those inherently creates. But HRC does still matter and is important and can give the user good information.
 
Can we let this crap die already? Ferrum forge already made a video discrediting the importance of HRC, and that it's more a test point to see if "your" heat treating is going according to plan than an actual useful piece of data on how a steel will perform. Also the fact that edges are typically higher hardness than where you can Rockwell test a knife, you could have a 60 HRC pivot, and 61 HRC edge, or 58 pivot, 62 edge, and so on. Listen to ferrum forge's video, HRC is pointless as a specification for consumer purchases in the knife industry.

Think for a moment about why diamonds scratch glass.
 
*shrugs* maybe I just got lucky with my 2 M390/20CV blades. I dont know the tested rockwell of either but they both perform great. Probably my favorite steel of my 8 different choices.
 
I don't get into If is right or not , but answering a question made about hardness here, an australian site that calculate percentage difference between 2 hardness numbers:
http://knifegrinders.com.au/Manuals/Rockwell_Hardness_Difference_Comparator.html


Thank you this answered the question I was asking. So while not apparently linear there is not a big gain in hardness as the numbers go up, so not logarithmic either.

Thanks also for the answers to question I had not thought of yet.
 
Thank you this answered the question I was asking. So while not apparently linear there is not a big gain in hardness as the numbers go up, so not logarithmic either.

Thanks also for the answers to question I had not thought of yet.

It’s a tricky thing to quantify in use. Larrin has described a potential 5%-10% increase in CATRA edge retention tests per point of hrc. In human use, with lateral force introduced, there is also the matter of increased hardness helping edge stability.
 
Here's a graph of the entire range of the web tool. I'm sure this 0-100 range is well beyond the range of HRC testing. This graph shows the % difference calculated for each HRC # and the next HRC # (ex: 20 to 21, 50 to 51).

yZ0rzmHl.jpg

For the curious, this curve looks nearly identical to a graph of the equation y = 1/(100-HRC #). We don't know if it's correct or just someone's guess to calculate the % difference in hardness between HRC numbers.

EDIT: I have no idea if part of this curve used for blade hardness (~50-70 HRC) accurately shows % differences in blade hardness. This is just a graphic representation of the function that web site is using to calculate % differences.

Edit 2: I got a question about what this means. It means that for for that web site they're using this curve to calculate the % difference between HRC numbers. If you look at the range for most blade steels it's a less aggressive curve than you see toward the right side of this graph. I also got asked what the graph means. It's means that the difference in Y increases as you get closer to 100, at which point it's 100%.

Here's another graph showing the range we're likely to see for knife blades.
MlBcSsCl.jpg
 
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https://drive.google.com/file/d/11ZT0D-d__u5Fo5-mKj3QvsrGF4n3OG4L/view

CATRA results.

These guys crowd funded the knives to be tested. Cost was $586 to have CATRA do all the testing on one run.
CATRA sharpened the knives with there CATRA Sharp machine, CBN wheels, convex edge, coarse finish, unknown grit

CATRA did Vickers hardness and Rockwell hardness averages

CATRA measured the angles with a laser gonimeter, unknown behind the edge thickness

Hardness*

58.5rc Lionsteel M390
*62rc Kershaw M390
61rc Spyderco S30v
60rc Spyderco M390

Intial cutting performance, first 3 cycles*

Lionsteel 100.5mm
*Kershaw M390 119.7mm
Spyderco S30v 109.8mm
Spyderco M390 104.5mm

Total Cards Cut after full 60 cycle test*

Lionsteel 626mm
Kershaw 707.7mm
Spyderco S30v 641.9mm
*Spyderco M390 784.7mm

Edge angle*

Lionsteel 31.5° inclusive, (18° Right, 13.5° Left) {from reading Gonimeter image}
Kershaw 32° inclusive, 17° Right, 15° Left
*Spyderco S30v 30° inclusive 13° Right, 17° Left
Spyderco M390 32° inclusive 16° right, 16° left. (most symetric)


Spyderco M390 had the most cards cut after the full 60cycles

Kershaw M390 was the hardest and had the highest intial cutting performance but did not have the highest after 60cycles.

Unknown if it's because of how Kershaw processed and heat treated, or if higher Rockwell does worse or if it's how it was sharpened. It's usually the sharpening that needs to be ruled out first.


Deburring methods are unknown at this time, the CATRASHARP is a dual spiral wheel motorized pull through sharpener with CBN coated wheels and is advertised as not needed any deburring because of the "Gothic Arch" (convex) edge Geometry it creates from the spirals. Unknown if true.
Look like a slight recurve was created using this sharpener and the bevels are uneven.

The spyderco in black is mislabeled as M390 but you can see it says S30v on the blade.


Appears that the lionsteel at 58.5 rc and 31.5° included angle did the worse amongst the other 3 knives including the harder and slightly lower angle S30v.



I have no involvement or affiliation with this test group.
They definitely take action. They organized and funded testing for the public to see, I enjoyed reading it.
 
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