Production M390 - Expectation vs Reality?

Discussion in 'General Knife Discussion' started by Cosmodragoon, Jun 11, 2019.

  1. DeadboxHero

    DeadboxHero Triple B Handmade, Custom Knives Knifemaker / Craftsman / Service Provider

    Mar 22, 2014
    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.
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  2. Banter 247

    Banter 247

    Feb 22, 2019
    I’m on IG regularly. Banter___247 (three underscores) Feel free to reach out!

    Also, @DeadboxHero — thank you for the replies in here.
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  3. cabraljr


    Aug 27, 2010
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  4. David Richardson

    David Richardson Gold Member Gold Member

    Nov 30, 2018
    We may have an answer. This shows increasing hardness difference as you move up the scale. 50 to 52 is 8.2%. 60 to 62 is 10.3%. 70 to 72 is 13.8%.
    Last edited: Jul 23, 2019
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  5. `br4dz-

    `br4dz- Gold Member Gold Member

    Jan 18, 2016
    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.
  6. Larrin

    Larrin Gold Member Knifemaker / Craftsman / Service Provider Gold Member

    Jan 17, 2004
    Not usually. Unless it is a differentially hardened knife, but this discussion is primarily about production folders.
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  7. DeadboxHero

    DeadboxHero Triple B Handmade, Custom Knives Knifemaker / Craftsman / Service Provider

    Mar 22, 2014
    Nah, usually the edge is softer due to being overheated with poor practices grinding and sharpening used to hit volume.

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  8. Eli Chaps

    Eli Chaps Gold Member Gold Member Basic Member

    Apr 20, 2018
    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.
  9. Banter 247

    Banter 247

    Feb 22, 2019
    Think for a moment about why diamonds scratch glass.
  10. Kaibab270

    Kaibab270 Gold Member Gold Member

    Jul 20, 2019
    *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.
  11. Applecider3

    Applecider3 Gold Member Gold Member

    Feb 4, 2019

    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.
  12. Banter 247

    Banter 247

    Feb 22, 2019
    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.
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  13. David Richardson

    David Richardson Gold Member Gold Member

    Nov 30, 2018
    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).

    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.
    Last edited: Jul 26, 2019
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  14. Larrin

    Larrin Gold Member Knifemaker / Craftsman / Service Provider Gold Member

    Jan 17, 2004
  15. DeadboxHero

    DeadboxHero Triple B Handmade, Custom Knives Knifemaker / Craftsman / Service Provider

    Mar 22, 2014

    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


    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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