Head to Head Steel Comparison

[me2]

I'm thinking of making a couple of test blades, then doing identical tests until one fails. I'm leaning toward 154CM, A2, and S7, in 7" blades, 1/8" thick, 1-1.25" wide. Any suggestions on hardnesses or tests? I was thinking of having all of them the same hardness, but then started thinking about optimum hardness. S7 doesnt seem to go much above 58, and the others can go over 60.

 

[Cliff Stamp]

Generally you don't run steels at the same hardness as they are not desiged that way. 154CM and A2 can both reach 64/65 HRC with oil/cold and personally I would be interested in their performance with such a hardening in contrast to the typical 59/60 HRC commonly used. You could look at cutting cardboard on a push/slice, push/slice light metals (wire, cans) and harder dynamic cutting of wood, bones, thicker metals as well as general flex/impact tests and corrosion work. It would be interesting to see how 154CM at 64/65 HRC compares to S7 at 58/59 HRC and note the optimal edge angles and finishes for various tasks. Some of the work may be opposite as expected because S7 is going to be finer structured than 154CM and thus may better tolerate lower angles and thus have an advantage in push cutting.

-Cliff

 

[me2]

One of the tests I had in mind involved reducing the angle from 50 included to 20 included, 5 degree/side at a time. I may have to switch over to a more aggressive testing material, as the cans just dont seem to do anything other than take off the hair whittling edge. The blade shapes/sizes described were chosen for ease of manufacture. I figured it was more important for this test that they all be the same shape than some maximized shape that I could screw up the grinding on. I'd really like to throw an austempered blade just to see what would happen, but I've only found one place to heat treat that way, and a one shot deal may be too expensive.

Edited to add:
Cliff, I was looking through the A2 toughness/temperature charts from your site. Timkens chart shows a peak at 600 F and a hardness of ~57-58, and the other charts show a peak in both strength and toughness at 350-375 F and a hardness of ~64. A2's composition is fairly well defined, so what is Timken doing differently? Also, where are the other 2 charts taken from? They are 2 different tests, as Timken uses Izod and the others use torsional. Alvin often references ASM for their promotion of the torsional toughness due to the inability to get reproducable results from Charpy or Izod. Are the differences just from using 2 different tests, or is it due to differences in heat treatment or rolling direction related to how the impact samples are cut? For knives, torsional seems to make the most sense in terms of edge durability.

 

[Cliff Stamp]

One of the tests I had in mind involved reducing the angle from 50 included to 20 included, 5 degree/side at a time.

For test blades I would be thinking of just small shop knives, say 1-2" blades with a simple wharncliff pattern. You don't even need to pin the handles, just run a simple paring/slab construction.

Timkens chart shows a peak at 600 F and a hardness of ~57-58, and the other charts show a peak in both strength and toughness at 350-375 F and a hardness of ~64.

If you take the torsional responce and draw a smooth curve interpolating between the two peaks at 400 and 700 you get the Timken curve. What is problematic is that they are showing curves and not the actual points at which the toughness was determined so you don't know what is measured and what was just inferred. Ideally you always have the points which are repeated to give you some idea of confidence. What do you get if you repeat the same test again?

Also, where are the other 2 charts taken from?

ASM's "Tool Steels", I should be referencing that, I'll clean it up today.

... is it due to differences in heat treatment or rolling direction related to how the impact samples are cut?

This seems to logical to me because I have seen both charpy and izod show the embrittlement at 500 F and also fail to in steels which should have it, not all of them do. I assume in the ones that do the grain runs parallel with the impact vector and in the ones that don't they are perpendicular.

For knives, torsional seems to make the most sense in terms of edge durability.

Yes, they are torsionally loaded to failure in most cases, aside from just direct compression failure. Impacts on the spine are more like charpy/izod.

-Cliff

 

[me2]

Is it possible that some are using a higher purity sample to do the tests? I have seen in low alloy steels (10xx, 41xx, 43xx, and 5xxxx) that the peak will show itself in production grade steels, but not in higher purity experimental grade steels. Perhaps the plots are done using smaller batches that have lower impurity levels.

 

[Cliff Stamp]

Impuries are exactly one of the reasons for the embrittlement regions. They are not the only cause but they are known to make it worse.

-Cliff

 

[me2]

So far I've been focusing on A2. A2 looks promising with the high hardness peak, and its toughness reputation. It seems like the knives you tested were near the bottom of the toughness trough with their hardnesses/tempering temperatures. Basically, what one chart calls a peak, the other shows as a minimum. I see what you mean though, since if you assume the peaks on the ASM chart are just opposite sides of a higher peak in the middle, you could get the chart from Timken's site. There is still the issue that we're comparing charts generated by 2 different tests, one of which is all but dismissed by ASM as being unreliable for tool steels. Maybe they really did get a data point at that peak, just by virtue of the different tests, or some processing characteristic we dont know about. The bottom line is I wont be able to tell how tough Rc 63-64 A2 is until I get my hands on some.

I sent an email about an estimate for A2 at 63-64, 153CM at 63-64, S7 at 57-58, and austempered O1, which hopefully can reach as high as 57-58. Based on the charts from Verhoevens book, they should be able to get to just below 60, as the carbon content would allow about 58, with the other elements just increasing it. I asked for an isothermal treatment at Ms + 25 degrees. In the book, the 180 degree bends were done at 58 Rc, and for some reason O1 comes to mind for that test. I may be wrong, the book is at work.

Regarding shape, I wanted to do a flex test, so 1-2 inch blades would be too short in my mind. I also wanted to do some chopping tests. The profile I've settled on is a machete grind, with the angles varying as noted above, on a slight drop point side profile. I'm not testing binding or cutting ability beyond edge holding, so this seems the simplest to do, with a simple jig for consistency.

 

[Cliff Stamp]

Yes, as I have noted I have not been impressed with A2 in general but that is more of a heat treatment issue than one of steel limitations it would seem, based on the materials data anyway. I would agree there are several possible reasons for the differences, but inherntly it seems problematic to me that the tests can be so similar but yield at times contradictory results while at other times being in agreement. You work should be interesting.

-Cliff

 

[Cobalt]

I agree, Cliff, HT is everything in that steel as well as most high end steels.

But the other issue is that Charpy testing while a guideline is not anywhere near accurate as a measure of a blades toughness. Think about this for a bit. A charpy notch test is done by creating a stress riser, on a rectangulat cross section piece of steel. The force of the impact against this to failure is the Charpy reading. Well, first off, you have to assume that the metal sample tested was stress relieved properly. You can get the exact same piece of steel with slightly different HT methods, and the same Rc, yet the Charpy will vary because of how the HT was achieved.

The other big factor is how the edge takes a toughnss reading. Impacting a flat piece of bar stock is fine, but doing the same to a very thin edge is quite different. A steel that is very tough when in rectangular cross section may crumble when ground down to an edge.

Charpy should be used as a baseline only and not to get a true measure of how it will act when ground out as a blade for a knife. The difference between CPM3v and S7 may not be much when testing rectangular samples, but when ground down to an edge, the difference could be dramatic, if both have equal HT quality.

my 2 cents.

 

[Cliff Stamp]

But the other issue is that Charpy testing while a guideline is not anywhere near accurate as a measure of a blades toughness. Think about this for a bit. A charpy notch test is done by creating a stress riser, on a rectangulat cross section piece of steel. The force of the impact against this to failure is the Charpy reading. Well, first off, you have to assume that the metal sample tested was stress relieved properly. You can get the exact same piece of steel with slightly different HT methods, and the same Rc, yet the Charpy will vary because of how the HT was achieved.

I do agree that the heat treatment is an influence, but in general we are talking about fairly serious materials research so we can assume a decent standard. You can also get unnotched charp/izod as well as the v/c notch. There is also torsional impact toughness as well as ductility, resilence, and then the actual area under the stress/stain graph. You need all of this to get a complete picture of durability, unfortunately from steel manufacturers you usually just get a few points of one test, usually chosen to show the strong points of the steel. You have to shop around and then see if you can't find some independent data to confirm/refute the pitch from the salesmen. A little skepticism is a healthy thing and not be so naive to believe that a knife maker or steel manufacturer isn't trying to actually sell you a product and may not be willing to be 100% open about the weak points of their products.

The other big factor is how the edge takes a toughnss reading. Impacting a flat piece of bar stock is fine, but doing the same to a very thin edge is quite different. A steel that is very tough when in rectangular cross section may crumble when ground down to an edge.

Bingo. This holds true for many other aspects as well. There are of course going to be dependances but they are not 1:1. Note for example that in many steels like W1, the impact test bars are actually thick enough so that they are effectively just case hardened and the core is actually pearlite. However the knife edge is going to be close to 100% martensite so it isn't going to behave exactly like basically the san mai test sample. You also have other issues like austenite stress transforming and carbides being larger than the edges.

-Cliff

 

[me2]

"The other big factor is how the edge takes a toughnss reading. Impacting a flat piece of bar stock is fine, but doing the same to a very thin edge is quite different. A steel that is very tough when in rectangular cross section may crumble when ground down to an edge."

This is precisely the reason to do this test. The impact values are only a starting point. Also, any steel will crumble when ground down to an edge, you just have to push hard enough for the cross section. The edge angle variances are intended to determine the steel with the lowest functional edge angle for a given application. Also, if you assume that there were no thermal gradients or other oddities in the heat treatement due to changing cross section, the edge steel should be the same as the body steel, in terms of material properties, strength, ductility, etc. Of course a half inch square bar is stronger than a wire of the same material, and that is where the differences will be seen between the rectangular cross section and the thin edge. Of course the assumption of no thermal gradient causing different properties is a big one, but its one we seem willing to take, since most hardness tests arent done right on the edge. Short of cracks or warpage, the edge is generally assumed to react the same way the main body does to the same treatment. This is not a universal assumption in industry, but for knives seems sufficient. Does anyone know of experiments where an edge is sectioned and the very edge given microhardness tests?

 

[Cliff Stamp]

Also, if you assume that there were no thermal gradients or other oddities in the heat treatement due to changing cross section, the edge steel should be the same as the body steel, in terms of material properties, strength, ductility, etc.

Not all steels are through hardened depending on the thickness, this is usually more of an issue for really thick knives though, 1/4"+ . There are also issues with carbides tearing out of the edge and austenite stress changing to martensite.

-Cliff

 

[Cobalt]

This is precisely the reason to do this test. The impact values are only a starting point. Also, any steel will crumble when ground down to an edge, you just have to push hard enough for the cross section. The edge angle variances are intended to determine the steel with the lowest functional edge angle for a given application.

Unfortunately we do not get this kind of info from Mfg's of steel. We can do impromptu tests ourselves or the knifemaker can, if so inclined.

It would be nice to have a multi sample test of each steel with varying edge grind angles, but as Cliff pointed out, they are not about to show any weakeness in their products, usually.

 

[Cliff Stamp]

There is a lot of data available on most tool steels, however it is in reference books. The online PDF files are really sparce in comparison and trying to use them to get an idea of steels is like opening a book and reading 1% of it at random. It is of course better than just looking at the title/cover but not a whole lot more. Guys like me2 willing to actually do comparative test samples are what the industry needs a lot more of. A lot more information with direct comparisons to other steels and a lot less hype of everything being tested in isolation.

-Cliff

 

[me2]

Here are the tests I'm thinking of: 2x4 chopping w/ polished edge, 1/2" rope cutting w/ a fine India edge, coat hanger wire cutting w/ a claw hammer and wood cutting block. It probably wont cut all the way through w/o driving into the wood, but it will tell me what I want to know. Finally after all the cutting is done, I'll try wood tip break outs, from 1/2" or 1" penetration, I'll decide after trying a few. Finally, the lateral flex locked in a vice. I'd like to try each blade at the hardnesses listed above and at the conventional hardness, if appropriate, but dont have the resources now. Maybe later.

Ever since I had my 2nd metallurgy course I've wondered why so many blades are tempered in temp ranges that were shown to be detrimental to the toughness and possibly other properties. I've noticed a trend in metals and materials in general that there is usually a peak somewhere in the treatment for whatever property you're looking for. I wasnt sure where it was for steels until seeing some of Alvins posts on rec.knives. Whatever steel comes out on top for the different uses will be what I start using for that type of knife. Each of the ones listed are fairly readily available, O1 being the easiest to find in a baffling array of sizes.

If I could just remember half the stuff I've read, I'd be way ahead. The first book I read on knifemaking, David Boye's "Step by Step Knifemaking" says a lot of what Alvin has been saying and I just forgot. He recommends a draw of 62-63 HRc for L6, 62 for A2, and in general runs his blades higher than most do. Of course this is a fairly old book, and he may not do anything the same now, but its interesting to note that for many of the steels in his index he recommends a temper lower than the trough in toughness seen in many tool and low alloy blade steels. The ones specifically I remember are L6, F2, and A2. F2 sounds very promising. Does anyone know where to get any in usable sizes?

I havent heard back from the heat treating company I wrote. I'm hoping the total cost for the experiments wont be over $200, but I dont know. On line buying of tool steels from some of the popular sites here certainly does its part to keep the cost down.

 

[Cliff Stamp]

Here are the tests I'm thinking of: 2x4 chopping w/ polished edge, 1/2" rope cutting w/ a fine India edge, coat hanger wire cutting w/ a claw hammer and wood cutting block. It probably wont cut all the way through w/o driving into the wood, but it will tell me what I want to know. Finally after all the cutting is done, I'll try wood tip break outs, from 1/2" or 1" penetration, I'll decide after trying a few. Finally, the lateral flex locked in a vice. I'd like to try each blade at the hardnesses listed above and at the conventional hardness, if appropriate, but dont have the resources now.

Monetary concerns are always a limitation, time being the other, you need an apprentice to do the grunt work while you make the knives. As a suggestion, push cutting the rope and slicing would be informative to see if you support Landes position on carbide effects on edge tear outs as typically you can't go fine enough on the edge and still do 2x4 cutting for this to be critical. This isn't going to be long runs as typically the sharpness end points are quite high, can't push cut newsprint perpendicular to the grain for example.

Ever since I had my 2nd metallurgy course I've wondered why so many blades are tempered in temp ranges that were shown to be detrimental to the toughness and possibly other properties.

Because it is promoted by makers, the arguement is self-supporting as there is not a lot of independent verification and even if it is done not a lot of people are willing to stand up and say everyone else is wrong, that tends to make you unpopular. There is the common perception that 60 HRC is the "right" hardness, one of the main knife myths.

F2 sounds very promising. Does anyone know where to get any in usable sizes?

I have done a few searches and asked a few people, according to Alvin, quoting ASM, the finishing steels have been replaced by the HSS's. Thom has had better luck with steel sources than me, you might want to ask him.

-Cliff

 

[Phil Wilson]

Cliff, I have never been able to get 154CM to a hardness much over about 62 as quenched. This is with a high temp soak, oil quench and cryo cycle. With a temper at 400 the end result is 60/61. With the high temp temper maybe one could get a little more. I would be interested in anyone out there who has a recipe that would end up with a final 64 RC. I could learn something from this. Also IMHO this steel at 63/64 would be way to brittle to be any use as a knife blade. PHIL

 

[Cliff Stamp]

Phil, Crucible lists in their steel selection book with oil/cold you can get as quenched hardness of 63 and with a 212F temper it is raised to 65 HRC, likely due to carbide precipitation. This falls to 62 HRC to a temper at 400F, so interpolating between those two should get 62-65 HRC. Based on the PDF file the soak temperature is 1950F. Yes you are not getting a lot of stress relief at the low tempers, I would be curious as to the impact data that low.

-Cliff

 

[Phil Wilson]

Cliff, I have the same reference you are looking at, the "orange book". yes I see now the reference to RC 65. This is at a temper of 212 as you mentioned. I have never tried to duplicate this beacuse a temper that low is not going to do much for stress relief I do not think. Would it make a useable knife blade? My experience is that RC61 on this steel is pushing it for toughness, impact resistance. It also shows a finished hardness of 62 with a 400 temper, oil quench and cryo. This must be lab conditons, I have never been able to get much over 61 with this recipe. The new stuff, CPM154 heat treats much nicer than 154CM. Easy to get 63 as quenched and then with a cryo and 400-500 temper get 61 like clock work.. Also very nice to finish and work with... More on this one later.. PHIL

 

[Cliff Stamp]

In general most references like that tend to be ideal results plus likely seeing the effect of favorable rounding as well. That low a temper does not actually do much stress relief on the martensite and mainly just forces the precipitation of transition carbide. Even for the really low alloy steels generally higher tempers are recommended, about 325F being a minimum.

I have not seen any data on 154CM at that temper though there is lots of D2, which has maximum wear resistance at 325 F, which generates 62-64 HRC with oil/cold. The impact toughness is about 75% of maximum. W1 has a similar responce but the temperatures are shifted lower due to lack of alloy, I would assume 154CM would be closer to D2 obviously.

I will be interested in the work with CPM-154CM. I would assume with less alloy segregation it would be easier to get required solute levels and thus a more even and consistent hardening responce. Have you ever worked with RWL34? Have you tried the ZDP-189 yet? Any luck getting decent size stock in AEB-L or 12C27/13C26?

-Cliff