Rockwell C scale

[Willie71]

Chuck, I found similar performance improvements by going harder with 15n20. If used at rc58/59, it's no better than 1084 or 1075. Going to Rc61-63 (the highest I've tested without chipping so far, higher is probably still ok, 62 is definitely fine in the kitchen or in a skinner) and the wear resistance goes way up. The nickel toughens the edge, and it really resists chipping. If you use it like a simple 10xx steel, you leave performance on the table.

Bluntcut, at the hammer in I was at a while ago, it was discussed that the number after temper for testing purposes is less important than the out of quench test. If you have less than Rc66 (for many steels) you have either not brought enough carbon into solution or the structure is messed up, or inversely if heated too high, you will bring too much carbon into solution resulting in retained austentite or excess carbon in carbides, depending on alloying. I did this recently with W2, and there was about a 10f window that gave me rc68. 10f up or down and the results were less consistent, and 20f out, Rc values dropped to 64 or lower. Starting at the most consistent structure will obviously give you the best blade once tempered down to the intended range.

 

[Rick Marchand]

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.

This is worth quoting again. I wasn't implying that folks weren't paying attention to geometry or that Rockwell numbers aren't that helpful. I was trying to illustrate that ALL of these factors(chemical composition, thermal treatment, structure, hardness, geometry, ergonomics, etc...) are what make a knife perform well. There is plenty of wiggle room and anyone who thinks they've found "THE" way to do it just hasn't been humbled yet.

That said... if anyone has a spare Rc tester kickin' round, lemme know.

 

[james terrio]

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

Right on; it's hugely important to eliminate as many variables as possible when trying to track down why's and wherefores. :thumbup:

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.

I could not agree more. I have learned a great deal from Brad, and trust him with all my blades. He has the know-how and the equipment to ensure consistent, verifiable, tested results every time.

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.

Absolutely right. Probably the single most important thing I have learned over the last few years (thanks to folks like Chuck, Aldo, Stacy, and others) is to balance alloy selection, geometry and heat-treatment to suit the purpose of a given knife. Along with making the thing comfortable to use, once you have a grip on that, the rest is just icing on the cake.

 

[stezann]

AEB L is an air hardening steel, if the difference in hrc is due to retained austenite of course the blade won't perform well...the higher hrc means only you reached full martensite.
if you get the lower hrc by tempering i won't bet for an awful performance

 

[Darrin Sanders]

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.

The steel that I've found to give the most consistent readings is S35VN. The readings vary by a needle width or less. Other steels usually vary by approx. 1/2 point like you said.

 

[Darrin Sanders]

AEB L is an air hardening steel, if the difference in hrc is due to retained austenite of course the blade won't perform well...the higher hrc means only you reached full martensite.
if you get the lower hrc by tempering i won't bet for an awful performance

With Brad's equipment and experience I'm sure that his Martensite percentages are maxxed out no matter what the final hardness is.

 

[BluntCut MetalWorks]

Finally got my tester setup. Here is some reading - pertaining to my thread about aldo W2; applicable to what RC # really mean.

Aldo latest W2, Don's W2 (generously sent to me). 2 knives from each source, ht with almost identical ht params - aust 1450F evenheat KO; 3 minutes soak; tempered ~385F (estimated because of toaster oven).

Std blocks calibrated tester.

Aldo W2 - 63rc (triple reading per knife). Very sharp & tough but low wear resistant. Knives in my W2 thread around 66-62rc corresponded to range of temper temps. All exhibits wear against cardboard similar to blade with carbon around 0.8 to 0.7%.

Don's W2 - 61.5rc (triple reading per knife). Very sharp + tough + excellent wear resistant (almost match my 52100).

Willie - Your (2nd mentioned) about hammer in & how carbon in solution + other metallurgy are well received & appreciated. I agree with your assessment. Rationally we all are trying to zero-ing some balanced ht params to achieve near optimal blade per intended use. Unfortunately guessing is part of the game.

Again to me, RC is just a reference # for steel type + ht params. Rc# in itself is not a performance indicator.

Bluntcut, at the hammer in I was at a while ago, it was discussed that the number after temper for testing purposes is less important than the out of quench test. If you have less than Rc66 (for many steels) you have either not brought enough carbon into solution or the structure is messed up, or inversely if heated too high, you will bring too much carbon into solution resulting in retained austentite or excess carbon in carbides, depending on alloying. I did this recently with W2, and there was about a 10f window that gave me rc68. 10f up or down and the results were less consistent, and 20f out, Rc values dropped to 64 or lower. Starting at the most consistent structure will obviously give you the best blade once tempered down to the intended range.

 

[stezann]

Bluntcut, congrats for the new reader (i wish i could soon afford one myself). :thumbup::thumbup::thumbup:
Did you consider taking reading as quenched vs after tempering? I think it should be more indicative about what's happened in the ht, and sure will remove any incoerency from the tempering into a toaster oven.
Both readings (after & before) will give the complete picture we can get from an hardness testing device.

 

[BluntCut MetalWorks]

Sure, later on I will do reading along the whole range AQ; after cryo; after 1 temper; so on. Perhaps to catch RA to mart conversion(if any); carbide precip along the way. RC# is a handy ref # - probably useful as interpolating performance correlation rather than coefficient or extrapolation.

Bluntcut, congrats for the new reader (i wish i could soon afford one myself). :thumbup::thumbup::thumbup:
Did you consider taking reading as quenched vs after tempering? I think it should be more indicative about what's happened in the ht, and sure will remove any incoerency from the tempering into a toaster oven.
Both readings (after & before) will give the complete picture we can get from an hardness testing device.

 

[Willie71]

Bluntcut, my question is the Rc values with such low tempering temperature. Knowing the Rc value out of quench will help sort out what is happening in the steel. The composition of Aldo's W2 is almost identical to Don's. It's puzzling why there is such a performance difference between the two for you. I'm quite curious.

 

[BluntCut MetalWorks]

OK, this where I am guessing + speculating however try to stay within RC topic.

Ignoring the minor factor of carbide in W2 RC reading. ~0.6%C go into the mart matrix. And Aldo W2 at 63RC (off the tang) after 385F tempered, perhaps some FeC precip (hypothetically - 0.1%C). OK, then alloyed + precip carbides should add to (0.33%C + 0.1%C). The matrix(ignoring grain size & carbide size/distribution) at this hardness has sufficient strength to resist & hold onto carbide, thus expect to give good wear resistant.

Further question whether out of 0.33%c minus alloyed, how much stuck in spheriodized form (large & bad performance)? If there are spheriodize carbide, the edge won't support dry shave keenness. Well, these w2 are super sharp and durable in hanging-hair-tests territory. Deduced to no bad large/coarse spheriodize carbide. Similarly deduction apply to whether draw that probably no lumpy cementite.

From perf test, Don's W2 exhibits wear resistant near Aldo's 52100 (0.94%C). While I can't ht (probably due to my technique or simply lame at ht) Aldo W2 blade to 25% of Don's W2 wear resistant against cardboard. Aldo W2 63rc is less than 25% perf compare to Don's W2 at 61.5rc. Clearly RC # mostly reflects somewhat what the harden state of steel matrix, not much more.

Begging the question, whether high RC at quench indicate good ht? 'Maybe' is a reasonable, however a 'No' in my doubtful/skeptical perspective (per a pinch of ht experience thus far, hahah perhaps my empty can of knowledge makes loud noise ).

Bluntcut, my question is the Rc values with such low tempering temperature. Knowing the Rc value out of quench will help sort out what is happening in the steel. The composition of Aldo's W2 is almost identical to Don's. It's puzzling why there is such a performance difference between the two for you. I'm quite curious.

 

[samuraistuart]

I appreciate the input from you guys. This is great stuff...for a steel nerd. Totally weird how there is such a performance difference in Bluntcut's testing of Don W2 vs Aldo W2 (get in the ring!). Don's W2 is C: .95 / Mn: .22 / Cr .15 / Si: 23 / Mo: .013 / V: .19 / Ni: .08 / Cu : .14 Aldo's W2 is C: .934 / Mn: .233 / Cr: .077 / V: .166 Numbers are very similar.

 

[Willie71]

OK, this where I am guessing + speculating however try to stay within RC topic.

Ignoring the minor factor of carbide in W2 RC reading. ~0.6%C go into the mart matrix. And Aldo W2 at 63RC (off the tang) after 385F tempered, perhaps some FeC precip (hypothetically - 0.1%C). OK, then alloyed + precip carbides should add to (0.33%C + 0.1%C). The matrix(ignoring grain size & carbide size/distribution) at this hardness has sufficient strength to resist & hold onto carbide, thus expect to give good wear resistant.

Further question whether out of 0.33%c minus alloyed, how much stuck in spheriodized form (large & bad performance)? If there are spheriodize carbide, the edge won't support dry shave keenness. Well, these w2 are super sharp and durable in hanging-hair-tests territory. Deduced to no bad large/coarse spheriodize carbide. Similarly deduction apply to whether draw that probably no lumpy cementite.

From perf test, Don's W2 exhibits wear resistant near Aldo's 52100 (0.94%C). While I can't ht (probably due to my technique or simply lame at ht) Aldo W2 blade to 25% of Don's W2 wear resistant against cardboard. Aldo W2 63rc is less than 25% perf compare to Don's W2 at 61.5rc. Clearly RC # mostly reflects somewhat what the harden state of steel matrix, not much more.

Begging the question, whether high RC at quench indicate good ht? 'Maybe' is a reasonable, however a 'No' in my doubtful/skeptical perspective (per a pinch of ht experience thus far, hahah perhaps my empty can of knowledge makes loud noise ).

My thoughts are that you are overthinking this. Don is getting Rc63/64 at 425f temper, and I got very similar, about Rc0.5 down from him. For you to be at Rc63 or 61.5 with a 385f temper, means you did not get full hardness out of the steel to start with, or your tempering oven is out by at least 50f. If you only got Rc63/65 out of quench, then there is a lot of performance left in the steel that is not tapped into. This is where the Rc tester shows its value, letting you know if you got optimum hardness from your quench, then temper back to the toughness you want based on real world experience. Without accounting for the quench hardness, the number becomes meaningless, as you don't have optimum performance to start with. The matrix is set up by normalizing, then grain refinement. In the final austentizing cycle, you just want to get 0.8- 0.85% carbon into solution, and not mess with much else requiring temp and time control. The lower probable numbers mean you likely got less than the ideal amount of carbon into solution, which would be no better than 1070 for example, or you got too much in, resulting in retained austentite. Again, performance decrease. You could get the retained austentite to convert with subzero, but then you also probably have the more fragile form of the martensite with microfractures, again compromising performance. This doesn't even account for the carbide distribution/size etc.

 

[BluntCut MetalWorks]

You probably right on my overthinking and possibility of masked underlying problem of insufficient carbon in solution which reflected in low AQ hardness. Let's look at this more simply.

Take 2 W2 knives with same profile and ht the same, except sourced from different place (hence diff composition). Says same heat treated: normalize 1600F, grain refine 1550 & 1500, hardened 1450F. I expect their wear resistant within -+ 50% of each other. In my case, one knife still cut newsprint after 500 linear feet of cardboard, however the other failed after 50 linear feet of cutting. I heat treated Don's W2 with his recommended way & my super quench way vs Aldo W2 BF recommended way & my superquench way. Same big performance disparity.

RC reading can be affected carbide size & density, microstructure, latch type and may even grain size. Steels with secondary hardening hump clearly demonstrated overly increase of hardness reading due to carbide precip exceeded the loss of matrix hardness. Steel response to temp & time depends on composition+state+etc. As for precise temper temperature control, I used to snap-temper in toaster and regular temper temp in kiln - after it cool down. But found that +-30F variation doesn't negatively affect my experiment, which to say I look for large performance delta (50%+) so I became indifferent about 1 or 2 RC delta(by guessing before my grizzly rc tester).

As for carbon in solution excess of 0.6% doesn't mean RA. Higher lattice stress configuration could limiting phase transformation from aust to mart, so cryo or subzero to reduce energy state and perhaps temporary dimensional contraction enabling the RA to mart conversion. Excess carbon in solution mostly carbide forming long before Mf.

My thoughts are that you are overthinking this. Don is getting Rc63/64 at 425f temper, and I got very similar, about Rc0.5 down from him. For you to be at Rc63 or 61.5 with a 385f temper, means you did not get full hardness out of the steel to start with, or your tempering oven is out by at least 50f. If you only got Rc63/65 out of quench, then there is a lot of performance left in the steel that is not tapped into. This is where the Rc tester shows its value, letting you know if you got optimum hardness from your quench, then temper back to the toughness you want based on real world experience. Without accounting for the quench hardness, the number becomes meaningless, as you don't have optimum performance to start with. The matrix is set up by normalizing, then grain refinement. In the final austentizing cycle, you just want to get 0.8- 0.85% carbon into solution, and not mess with much else requiring temp and time control. The lower probable numbers mean you likely got less than the ideal amount of carbon into solution, which would be no better than 1070 for example, or you got too much in, resulting in retained austentite. Again, performance decrease. You could get the retained austentite to convert with subzero, but then you also probably have the more fragile form of the martensite with microfractures, again compromising performance. This doesn't even account for the carbide distribution/size etc.

 

[BluntCut MetalWorks]

I don't have much experience & skills in metallurgy to lean on, so just triangulate whatever I can. Aldo 52100 & 1095 knives about 62.5-63RC, with pretty identical ht as W2, perform above & about Don's W2. Hitachi White#2 at 64rc best them all on shave+whittle+cardboard tests. Even Aldo 1084 can easily pass 100 linear ft cardboard cutting test. Aldo D2 also performed well in shave & cardboard but didn't meet my standard in whittle dry knotty hardwoods.

I appreciate the input from you guys. This is great stuff...for a steel nerd. Totally weird how there is such a performance difference in Bluntcut's testing of Don W2 vs Aldo W2 (get in the ring!). Don's W2 is C: .95 / Mn: .22 / Cr .15 / Si: 23 / Mo: .013 / V: .19 / Ni: .08 / Cu : .14 Aldo's W2 is C: .934 / Mn: .233 / Cr: .077 / V: .166 Numbers are very similar.

 

[stezann]

Bluntcut, are you sure about the hardening temperatures? Do you rely on calibrated termocouples/PID for the HT, or by sensation/magnet etc..?
I think, when we name steels, we need to be clear on what we do and how, in order not to let people to get wrong impressions.
Thanks

Stefano

 

[Don Hanson III]

It is critical to get the highest hardness at quench along with the finest grain. This will give best results in testing. So Blunt you need to test as quenched hardness.

Not sure what's up with the differences in the two W2 batches. But the lower Rc #s at such low temper temp are way off from what I get. Could it be the snap temper in the toaster oven? I don't do that...

Blunt, I'll get around to running some tests on that piece of W2 you sent me soon.

 

[BluntCut MetalWorks]

Heat treated with Evenheat KO 18 w/ ramp master, also oven temperatures confirmed/calibrated with external TC.

Bluntcut, are you sure about the hardening temperatures? Do you rely on calibrated termocouples/PID for the HT, or by sensation/magnet etc..?
I think, when we name steels, we need to be clear on what we do and how, in order not to let people to get wrong impressions.
Thanks

Stefano

Yes/right & thanks, I will take AQ hardness next time. Grain of knives from these mentioned W2 knives look finer than 1500 grit sand paper. Microscrope view from break off chunk (tip or tang 0.5") or sometime blade snap in half. For experiment sake, knives made from your W2, one missed a tip(now shortened), other lost half tang .

For more precise RC reading, for the next few times(i.e. special case, not always because of time consuming), I will let knives remain in LN2 dewar and wait for the kiln to cool down 100* below tempering temperature.

It is critical to get the highest hardness at quench along with the finest grain. This will give best results in testing. So Blunt you need to test as quenched hardness.

Not sure what's up with the differences in the two W2 batches. But the lower Rc #s at such low temper temp are way off from what I get. Could it be the snap temper in the toaster oven? I don't do that...

Blunt, I'll get around to running some tests on that piece of W2 you sent me soon.

 

[me2]

Have you repeated the cutting tests? I have compared 2 knives and had them trade places in terms of edge holding, then had them switch back on a third try. Even when the knives didn't trade places, one or the other would change considerably from one trial to the next.

 

[BluntCut MetalWorks]

Done around 6 times or so but only twice looked&confirmed at apex thickness width using a usb microscope, on top of pass/fail newsprint slicing. We are talking about 10x delta - yeah easy to spot a grand canyon on its rim. As you said about good rc vs bad rc, well could be that I don't know how to properly ht Aldo W2, thus produced bad 63rc blades.

Have you repeated the cutting tests? I have compared 2 knives and had them trade places in terms of edge holding, then had them switch back on a third try. Even when the knives didn't trade places, one or the other would change considerably from one trial to the next.