Blade Steel Rehardening

gunmike1 said:
Phil, that CPM S110V at full hardness sounds like wonderful stuff. I can't wait to see what you can do with CPM M4. At 62.5 RC it is really impressive stuff with great sharpness, very good edge retention, and excellent toughness for such a wear resistant steel. As expected corrosion resistance isn't a strength, but the trade off is worth it IMO due to the great performance. I would love to see what CPM M4 does at 64-65 RC.

Mike

I have a CPM M4 blade now at RC 66, tempered back from 67. In comparing it to one at 64 at this point I do not see much difference in edge holding when cutting 5/8 rope. Edge thickness behind the sharpening bevel is .008. Whittling and twisting out of the cut on fir I can see some very fine chips and also evidence of rolling. With the 64 blade and the same test I can see some rolling but no chipping. Looks like the best hardness is 64 since it gives some insurance against chipping. I don't have a feel for impact resistance (Charpy) yet. I do know that at these high hardness it is going to be very low. If for example a knife is dropped tip down or even on the edge on concrete for example --100% chance there will be some moderate damage.
A little insight on heat treating this steel also including CPM 10V and also CPM S110V. If you look at the data sheets you can see that 20 degrees difference in temp. can make one point hardness difference. The very best thermocouples and contollers can have have 3 percent error even when brand new ( I just changed all the thermo ocouples in my furnances). So, at 1000 degrees there can be 30 degrees error. For example--I have two brand new themocouples wired together so that the tips are exactly in the same place in the furnance and at 1000 degrees they read 40 degress different. Add some error in hardness testing and you can see that it is very tricky to sneak up on one pt hardness difference in the 62 to 65 range. I have enough experience with my hardness tester and Wilson calibrated test block that I feel confident quoting values to with in one point. More that most wanted to know but this is why the production guys are very conservative with hardness values and one guy's 61 might to another guy's 59. Phil
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Very interesting Phil. It really is amazing that large batches from most factories are as close to being right on as they are.
 
FlaMtnBkr said:
I'm not sure where to put this but figured a few that might check this thread might be interested.

I opened up my M4 Benchmade Rift the other day and noticed somehow the very tip was damaged and the steel was bent over. I don't remember doing anything that would have damaged it, but it was folded over instead of being chipped off. I took a steel to try and straighten it out and I'm not sure if it did or if it broke off but now the very tip is blunted a little. I'm talking very small though like 0.010" maybe less, so I'm sure it will sharpen out eventually.

I guess I figured if it was over 60rc that it would have chipped and I was hoping this was at least 61rc. But maybe it would roll even at a higher hardness?
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Is your M4 Rift stock? Some of the Benchmade M4 tested out to be 58 so that could be why. If you got it rehardened+regrind but left the tip really thin, then it could still roll even at high hardness. But stock shape, that blade tip is pretty beefy and shouldn't roll so easily at 60+ rc.
 
cotdt said:
Is your M4 Rift stock? Some of the Benchmade M4 tested out to be 58 so that could be why. If you got it rehardened+regrind but left the tip really thin, then it could still roll even at high hardness. But stock shape, that blade tip is pretty beefy and shouldn't roll so easily at 60+ rc.
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The edge has been thinned a little. I shot for 30 deg on the edge pro. So it might be slightly thinner but not much.
 
Just a Heads-Up Folks,

Coming soon to a Bladeforums thread near you, and just a mouse-click away...a Passaround of a COTDT rehardened and Knifenut1013 sharpened folding knife.

Chris
 
I'll be heat treating for two more months. Thanks to everyone to volunteered their knives, I've learned a lot.

Most of the feedback so far is "well the knife pretty much works as you said it would, especially with the thin edges, not much to say about it." Hopefully the RAT-1 passaround would shed some more detail on the effects of hardness.

The hardness is not the only difference actually, I also refine the grain structure. The heat cycling gets new grains to grow and I've broken steel bars to observe very fine, smooth, sparkly grains in the cross section. Under a 100x microscope, most stock heat treats get about ~200 grains per square inch (varies depending on steel), as they are heat treated from the annealed material, which is rather coarse with large spheroidal carbides clumped together. The microstructure before the heat treat has a huge impact on the final microstructure, so if you refine the microstructure before the final heat treat, you get something even finer.

Effects of finer grain? Not sure, it does improve performance in various toughness tests. With the higher hardness of the rehardening the toughness still goes down overall. I don't think microstructure has much to do with edge retention though, in my use the most important things are getting rid of retained austenite and higher hardness. Both minimize rolling deformation. Rockwell hardness is measured by a diamond indenter which is resistance to rolling type deformation.

The micrographs in research papers clearly show that initial edge wear is simply rolling, only when the edge width surpasses ~12 microns ("semi-sharp") does abrasive wear (cutting abrasive material) and adhesive wear (steel rubbing off) come into play. Adhesive wear doesn't depend much on hardness just on the steel, whereas abrasive wear does depend on hardness but also on the steel. But if you have a super sharp edge (0.4 microns), the initial wear is rolling so that's what you want to combat.
 
Well, I just had a RAT-1 rehardened by cotdt, and I decided to give it with the 2x4 test for edge holding. What I discovered was not the edge holding abilities of the harder blade, but that the construction of the RAT-1 is simply not appropriate for heavy chopping duty. It is in vitually all other respects an outstanding knife.

Here's the video. Be warned, I am no James Cameron when it comes to camerawork.

[youtube]WAEensOF5mc[/youtube]
 
Sir Cantaloupe,

How was the edge retention on the broken RAT 1 folder after your chopping test? The point of the test was to evaluate the rehardened blade, so tell us the result, please. We know how the stop pin failed, but would like to know about the blade steel, too.

As an aside, I did not know that folding knives were designed to do chopping, but am ready to learn something new. What are some folders that excel at this sort of duty? Thanks.

Bill
 
knifenut1013 said:
Chopping with a folder is never a good idea.
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I certainly learned that lesson. As for Bill's very relevant question, well, I haven't come to a conclusion. I haven't had a chance to test the performace of the rehardened blade, so I can't really make any statements. I will say that after that aborted chopping test I had a fair amount of micro chipping in the edge, nothing that 20 minutes sharpening didn't fix. Keep in mind, normal use for this knife is cutting cardboard, zip-ties, and occasional use as a gardening tool. So I'm not so concerned with chipping, more with edge retention. I intend to put it through it's paces and report back soon.
 
I find it funny that so many people are doubting the OPs knowledge and ability to reharden the blades, this is not magic, nor is it only big companies that can pull it off, its the opposite, everything is known, its just that the big companies are trying to save costs by underhardening the steels.

I like what I have read at least.

His method is simple, and its the only right method, aim for max hardness (or choose another alloy), do everything as quick as possible, quench as as aggressive as possible without the blade cracking, no wait periods, keep the process going, cryo at the right time, quenching after tempering, its all the way it "should" be done. To get the perfect heat treat you can only do one or a few blades at a time, even in the same kiln the temps will vary too much from place to place so you cannot get an optimal HT all over, this is why mass produced blades suck, you need to work fast and you can only do a few at a time to be able to get them all good.

People actually doubt it works this way?

When austenizing stainless at 100 degrees more you burn like twice the power, this is why everything is so soft from the factory. They try to save money.

I would not hesistate to send out my knives to this guy, even if he´s drunk it would be better than factory.

With experience comes wisdom, the wisdom to read between the lines or TTT charts in this case, the guy probably knows exactly what will happen if he alters the mill recommended HT. And so does a lot of people, almost no steels are made for knives and all the suggested HTs are for non knife objects. Stamps, tool holders, dies or whatever, but not knives.

cotdt and Phil Wilson, I wish you all good luck, you 2 seem to be some of people with the most knowledge in this business, and work good stable equipment, or maybe some of the few that tries to do this as good as possible, no cutting corners.

I´d rather have AUS6 or 420hc made by you than S30V by Benchmade. :foot:

It was a good thread. And now its back.
 
What cheapish steel do you think performs well at 63-65 rc for light duty cutting, like rope?

I am interested in sending a hardened knife to Ankerson to see how it performs on his cutting tests.
 
Doughnut said:
What cheapish steel do you think performs well at 63-65 rc for light duty cutting, like rope?
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M2. 1095 if you can get someone to harden to that level.

cotdt hasn't visited BF in over 4 months, and has not posted in over 5.
 
Yeh, I noticed he has an AIM account listed. Maybe that is a good bet.

I also noticed that he mentioned he would only be doing this for a specific amount of time, so he might not be going it anymore.
 
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