Toughness - Expansion on Charpy C-Notch Values

C

cm_bushman

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Oct 22, 2012
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Okay guys, for everyone who always asks about toughness, I was doing some searching today and found this thread from 2003 that details some research this user did on Charpy numbers of a number of tool steels. As soon as I saw it, I was amazed it wasn't made a sticky back then, and so instead of doing the whole resurrection thing, I figured I would add some numbers from some other threads I'd happened across and beef the list up a little. I hope this helps someone!

Note: All of these numbers come from this forum, from various threads I've found. I can't verify these since I have no access to a Charpy machine at present.

Tool Steels:

Steel-----Hardness-----Charpy C Ft. Lbs.

CPM-15V......@Rc60..........10
CPM-10V......@Rc60..........20
CPM-3V.......@Rc58..........85
CPM-3V.......@Rc60..........60
CPM-3V.......@Rc62..........40
CPM-4V.......@Rc60..........50
CPM-4V.......@Rc62..........36
CPM-M4.......@Rc65.5.......20
CPM-M4.......@Rc63.5.......28
CPM-M4.......@Rc62.........32
CPM-M48......@Rc64.........16
CPM-T15......@Rc65.........20
M2..............@Rc62.........20
D2..............@Rc60.........20
A2..............@Rc60.........40
S7..............@Rc58........120
S7..............@Rc57........125
L6..............@Rc60.........40
O1..............@Rc60........30
H13.............@Rc47.......125
A11.............@Rc61........20
Z-Wear PM.......@Rc60......65
Vanadis 4.......@Rc60........50

Stainless:

Steel-------Hardness-----Charpy C Ft. Lbs.
CPM-S90V......@Rc56..........20
CPM-S90V.....@Rc58...........19
CPM-S60V.....@Rc56...........16
CPM-S30V.....@Rc58...........28
CPM-S35VN....@58Rc...........32
CPM-154......@60Rc............30
154CM..........@Rc58...........28
440C............@Rc56...........26
440C............@Rc58...........16
420HC...........@Rc58...........24
M390............@Rc60...........22


Note: I've cross-referenced these with Simply Tool Steel and Crucible to try and bring the numbers in line, doing this has cleared up a few inconsistencies I had noticed here on the forum. It also allowed me to add a few more Stainless grades. Now, when someone asks about 3V's toughness, for example, instead of statements like "it's about triple D2", you can prescribe a number to the statement to get both verification and a more exact idea.

This is still not a very comprehensive list. If anyone has more numbers or a good reference to expand this list further, please post here or let me know and I will add the figures.
 
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So does this mean you can bump up the hardness on 3v to say Rc63 for even more edge retention or would it really fall off the table after 62. I know it's supposed to be tough, but I'm just curious. Also would the opposite be true for 10 or 15v. Drop the Rc value a point and get a noticeable increase in toughness but still have awesome edge retention. I know you always give up something in order to gain something. And often pushing steel to something it's not is easily resolved by picking a different steel. Again my questions are just out of curiosity. BTW thanks for pulling this together it's informative.
 
Do you know the sample size or dimensions of the pieces used for testing?

Erasteel has some of the sample size dimensions as reference in some of their datasheets. Might be worth looking into.
 
I can't remember where I read it, but I think (not know) it was in an ASM handbook. Anyway, the source cautioned comparing impact test values for steels from different sources, especially for steels we're interested in here (relatively low toughness, very sensitive to notch effects, etc.). The tests still work, but from lab to lab they said there was a lot of scatter in the data. I also want to clarify "relatively low" toughness. Knife steels generally have fairly low toughness when compared to other steels or compared to the same steels heat treated for different purposes. As an example, A-992 can have a toughness of 300 ft-lbs using a V notch test at just above freezing (40 deg F). Nothing in knifedom even comes close to that. When researching impact testing, one is bound to run across a full range of data, so it helps to keep relative terms like "tough" in perspective. Of course, most any knife steel will cut pieces of A-992, but this thread is about toughness.

Also, I have been thinking lately that, except for knives whose purpose involves direct impact, impact toughness isn't really that critical. A certain minimum is needed, but that minimum is fairly low, like around 20 ft-lbs. Large choppers, throwing knives, and knives intended for batoning will have the most direct need for high impact toughness. Thoughts?
 
Cyo said:
This thread inspired me to learn about the Charpy test. I found this interesting youtube video showing how it's done. Now I have a better sense as to what this actually measures.

http://www.youtube.com/watch?v=tpGhqQvftAo
Click to expand...

What the crap?! His machine has a crank!! Back in my day we had to haul that hammer up by hand. (No I'm not joking, every test we had to haul that thing back to the top). He also scares me on that first test run. It looks for all the world like that hammer is going to take him out.
 
Please help me understand. I though "toughness" was about resistance to breaking from impact - as distinct from "strength: being resistance to deformation from force.

Would he ability to resist being "cut" by another tool reflect "toughness" or "strength" - or both?
 
It can get a little complicated, but generally I'd say it's both, though it depends on how you're cutting if impact toughness is an issue. Take gray cast iron for instance. Very low toughness due to the shape of the graphite particles in the iron matrix. Also a fairly low strength. By all accounts, it machines like butter. That low toughness makes chips that break off and get out of the way. High speed machining of gray cast iron looks like a shower of sparkly chips. Strength has a lot to do with it as well. To make a cut, you have to start a crack right at the edge of the cutting tool, the propogate it. High strength material is resistant to starting cracks, but once there, they might propogate easier, depending on material. Perhaps this video will shed some light.

http://www.youtube.com/watch?v=mRuSYQ5Npek

All very tough materials. There would be a lot more cracking if the toughness were lower, though there is enough as it is.
 
Thomas Linton said:
Please help me understand. I though "toughness" was about resistance to breaking from impact - as distinct from "strength: being resistance to deformation from force.

Would he ability to resist being "cut" by another tool reflect "toughness" or "strength" - or both?
Click to expand...

The simplest way to think of the difference between strength and toughness is to think about the load rate. Silly putty is very weak and stretchy, but if the load rate is high enough and the load itself is enough, it will crack. Steel is similar. A low load rate and you have a tensile test. A high load rate (like impact) and you have an impact test. They start to blend when you have intermediate load rates. That same bar that broke cleanly on the Charpy video would likely stretch a fair amount if loaded slowly.
 
Any chance of having 52100 and 1095 added? Being as they are pretty common when it comes to woods knives (Swamp Rat and Scrapyard for 52100, and ESEE and Becker for 1095)
 
I was surprised at how low the toughness scores were for 440C. It is often hardened to RC 57-59.
 
Thomas Linton said:
http://www.barrelsandscrewsby4deng.com/pdf/Crucible CPM 10V.PDF

"typical values"

A2 60 38
CPM M4 64 32
10V 60 26
D2 59 21.5

http://www.crucible.com/eselector/prodbyapp/highspeed/cpm4hch.html

CPM REX 65.5 20
63.5 28
Click to expand...



Thanks for some additional numbers TL! The only one that puzzles me is the 10V; I thought the 10V result I had found was sound, since it nearly matched my (separate) A11 numbers. Who knows, however, as has been said, this test is inconsistent based on very small factors, and I could be rather far off the mark.


Me2,

In regards to impact toughness, I agree that the main parameter that this test would predict is at what force your chopper or machete would deform permanently or break. This is very important in that regard, since no one wants a broken blade or possible injury. Besides that, (and to a much lesser extent since it is a much less direct predictor, and the forces involved are of different levels of magnitude) it could be used in high-Rc steels when you are looking to reduce chipping and improve edge stability at very low angles (note: toughness is one aspect I think can affect this, among many, and I do not think it is the most important).
 
For small blades that see little or no impact, like pocket knives and other such folders, I tend to think impact toughness has little to do with chipping. This is a recent line of thought, so I've not fully gotten through it. There are other kinds of toughness besides impact that I think have a bigger role, but they are also related to grit finish. Also, ductility and strength are key elements in chipping when the load rate it low and when the impact energies are low. Basically, any blade will chip if you push hard enough and the blade itself is hard enough to hold an edge, which is the case since we're discussing the higher hardness steels. Also, it seems to me that many users will report chipping when feeling any roughness to the edge, whether its actually chipping or just rolling or other deformation. To me it doesn't make much difference, since both have to be repaired/sharpened, but it seems the case to me. While compiling these numbers, it's important to remember that heat treatment has a huge influence on toughness. I can make you some annealed 1095 with toughness not much better than hardened D2 if you need it.
 
Many thanks to cm_bushman for compiling these data. Great effort and very interesting.

But I worry about investing too much faith in these numbers because of the lack of sourcing or the credibility of the sources. And I worry even more about comparing steels without referencing the heat treatment, even though the hardness numbers are available.

I've had CPM-M4 hardened to 64 Rc shatter into three pieces on me from just hand pressure. Other CPM-M4, even harder, has proved remarkably tough.

Can you really compare 52100 steel elaborately and skillfully heat treated by someone like Ed Fowler to another maker that does a quick and dirty heat treat? Does heat treating by Bos really add no extra value to the steel? Is Dozier D2 really no better than D2 treated by a novice smith? I doubt it.

I have several blades in S30V and they don't all behave alike.

I suspect that if we gave multiple blades of equal geometry and in the same steel to multiple smiths, they would all test out differently; and the spread in many cases would overwhelm the differences intrinsic to many steels.
 
TD, I for one think you have the right view of it. In matters of performance, we should buy based on the maker and his record of success, not the ingredients used.
 
Thomas Linton said:
TD, I for one think you have the right view of it. In matters of performance, we should buy based on the maker and his record of success, not the ingredients used.
Click to expand...

I agree 100 percent on this. Its why when I go shopping for a custom knife I find the design I like then I ask the maker what steels he likes the most, because I feel that if a maker knows 01 inside and out he will make a better knife out of 01 than he would 3V.
 
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