CPM 20cv flex toughness

[knife chop]

I had some experince with CPM20cv in the past and i have to say its a good tough steel but here's my question. there's charts on the web that 20cv has the same impact toughness with 440c. but how about flex toughness? Esee knives states that 440c from their testing will break at around 10-12 degrees if flexed. with the CPM process, will a 20cv suffer the same consequences when flexed? i would probably never to this to my knives but i'm just curious.

 

[gsx-rboy750]

I was doing flex testing on 3v (personal knofe not a SK) and concluded if I was flexing the knife even 2-7 degrees in real use I was not using the tool as efficient as possible. For instances I gather chaga. I pound the knife in a pry out. If I get a piece that it not coming out and I am starting to flex the .20 thick tool steel I should remove the knife and reinsert in a new area and try again.
I was flex testing a 3v knife at 63RC (over hardened) past 50' multiple times. For testing.
The thing you dont see is fracturing and internal micro cracking which can fail later when you need it.
For instance I broke my knife battoning very hard frozen maple about 1month after doing a bunch of testing. It believe it ultimatly was caused by stresses that remained from the testing.
My lesson was I would rather re adjust technique rather then stress the steel even if I dont "SEE" the effects right away.
It has tought me what I may not want to do to a tool I may need to count on later.
I understand the question.
I would recommend by a piece for 10-20$ have it hardened for 27$ and test that. I was fucking pissed when I broke my knife. Stainless WILL not flex well.

 

[chiral.grolim]

I had some experince with CPM20cv in the past and i have to say its a good tough steel but here's my question. there's charts on the web that 20cv has the same impact toughness with 440c. but how about flex toughness? Esee knives states that 440c from their testing will break at around 10-12 degrees if flexed. with the CPM process, will a 20cv suffer the same consequences when flexed? i would probably never to this to my knives but i'm just curious.

I would add that ESEE claims the following: http://www.eseeknives.com/warranty.htm

ESEE KNIVES ARE NOT THROWING KNIVES! They are hardened to a higher Rockwell than throwing knives and will most likely break if thrown, possibly harming the user. So, do yourself and your ESEE knife a favor and DO NOT throw it. Using any knife not meant to be thrown as a throwing knife is idiotic! We would rather idiots not buy our knives.

ESEE 1095 from Rowen is HT'd to 55-57Rc, soft for a knife blade, and it is their only products to which their "no questions asked" warranty applies. They assert that any knife as hard or harder than theirs will break when thrown, and anyone who thinks otherwise is an idiot... Wow. Then they state:

We do not warranty our 440C Stainless Steel knives from abuse.

Compare that to Entrek Knives which uses 440C exclusively: http://www.ennis-entrekusa.com/About Entrek USA.htm

If this Entrek USA knife fails for any reason it will be repaired or replaced at the option of Entrek USA at no cost to the original owner. To date we have replaced four knives in 12 years, all were damaged while being used for other than their designed purpose.

While I am interested in the ESEE Gibson and love the Izula, I generally avoid their products due to such lack of confidence on the part of the owners. *shrug*

To date I have not seen a broken S!K and only heard of one that broke in the skeletonized handles (per Ellie/Guy). The PM process should reduce/eliminate the carbide aggregation for which 440C is known and which contributes to low fracture toughness. As stated above, if you find yourself over-flexing the blade, it's probably best to stop and reset before risking microfractures regardless of how unlikely they are, because a flexing blade indicates inefficient prying - try again at another spot. Most S!K models are thick enough that such flexing would be minimal without use of a cheater-bar.

In any case, how tough is 20CV? I haven't tried it personally, but Guy's demonstrations of M390 and S90V (similar if not higher carbide loads) are telling:

(Skip to 11:00 is the video doesn't do so itself)

[video=youtube_share;JnAKtElk_PY]http://youtu.be/JnAKtElk_PY?t=10m59s[/video]

[video=youtube;gFzI9U64bjg]https://www.youtube.com/watch?v=gFzI9U64bjg[/video]

 

[B34NS]

Very well said Chiral.

Only thing I have to add is that in comparison witht he same models, the 4.1, I felt performed the same as the 3V. Only benefit to the 20CV is if you're in a wet environment and don't enjoy the cosmetic spotting of 3V. It has less impact resistance because of that feature, but not so much I'd put it on par with 440C. The flex breakage was about the same I'd feel comfortable with bending 3V, but didn't measure how many degrees, just that gut feeling of not wanting to push it too far, but sure in a pinch could do more.

 

[Silver Needle]

Chiral = Mr. Science :thumbup:

Seriously though, Chiral, what is your background?

 

[chiral.grolim]

My background is biomedical, not metallurgical, I was just relating what makers and manufacturers (who select alloying elements and HT-protocols based on metallurgy) have demonstrated. Phil Wilson is a maker who works closely with the steel suppliers and has produced thin/flexible blades out of high-carbide stainless steels: http://www.seamountknifeworks.com/about.htm
But flexing a knife as thick as the S!K models would take more than a normal amount of force. Per Charpy impact tests and what we know of how carbides affect fracture-toughness, there is good reason to be wary of high-carbide (e.g. 440C or 20CV) vs low-carbide (e.g. 420HC or 3V) steels, but "real-world" demonstrations by users and manufacturers help clarify the relevance of the data, and the CPM process was designed in part to limit the possibility of large inclusions (e.g. carbide aggregates) that can compromise fracture toughness (flex). Does that make CPM-20CV equivalent to INFI? No, but it seems pretty capable despite all that

 

[knife chop]

Chiral,

Sorry to bring up this old thread but what am I missing here? you mentioned that 440c is known for carbide aggregation which contributes to low fracture toughness and I've seen charts that implies 440c and CPM 20cv have the same toughness. how is it that if PM can reduce/eliminate carbide aggregation and these steels still have the same toughness?

 

[Fancier]

The inclusions are a defect, not a design feature. The manufacturer is not likely to publish test results from a sample that contained such a defect.

 

[me2]

CPM doesn't eliminate carbide aggregation, but does greatly reduce carbide segregation. The difference is the 20cv has more carbide than 440c, and it is better distributed. This means more consistency and slightly higher toughness potential. Also, flexibility has a great deal to do with geometry. To say a steel goes 15 degrees without thickness doesn't give enough information. I've made multiple 90 degree flexes with various steels, but the blades were less than 1/8" thick.

 

[me2]

The other thing to consider is how much effort did it take to get to 15 degrees? 10 lbs, not so good. 50, much better. This is even more controlled by geometry and up to a point is independent of steel type.

 

[chiral.grolim]

...what am I missing here? you mentioned that 440c is known for carbide aggregation which contributes to low fracture toughness and I've seen charts that implies 440c and CPM 20cv have the same toughness. how is it that if PM can reduce/eliminate carbide aggregation and these steels still have the same toughness?

This:

CPM doesn't eliminate carbide aggregation, but does greatly reduce carbide segregation. The difference is the 20cv has more carbide than 440c, and it is better distributed. This means more consistency and slightly higher toughness potential...

According to Crucible, the recommended HT/tempering of 440C results in only ~12% total carbide volume. In comparison, PM steels S30V and S35VN feature 14 & 14.5% carbide, CPM154 is 17.5% carbide, and S90V reaches full 20% carbide! However, because of the PM process to reduce aggregation, each of these steels can achieve similar toughness at 58 Rc by longitudinal charpy c-notch testing. High vanadium, high carbide steels with such toughness are impossible without PM due to carbide aggregation & segregation. CPM-20CV, falling between S35VN and S90V carbide-wise, can thus evince similar toughness. Crucible has also made a point of declaring transverse toughness comparisons to better show the advantage of PM manufacture, but it should be noted that the values thus achieved are much lower for all. Also, it is important to remember that these high-carbide steels were not developed for impact toughness, their high carbide volumes cannot compete with the likes of A2 and 3V that feature only ~5% total carbide volume. These steels all fall in the range of 25-30J at 58Rc (440C being the lowest). A2's toughness is 45-50J, and 3V is >50J even at 62Rc. The goal here is to enhance wear-resistance and corrosion-resistance along with machinability (word?) without excessive loss of toughness, using 440C as the benchmark.


BTW, I'm pretty sure me2 knows more about this than myself, so I am glad he posted.

 

[AmericanAsApple3.14]

Hope I'm asking this right... So why can steels like A2 and 3V still have such strong wear resistance/edge holding ability with such low carbide volume? Due to the impact toughness, they keep the ones they have longer?

 

[me2]

A2 and 3v are still quite wear resistant, just not as much so as the others, like 20cv. Carbide volume does not alone equal edge holding. Annealed 1095 has about 12% volume of carbides, but is nowhere near as wear resistant as hardened S30V, which has roughly 14%. A significant part of dulling is based on deformation of the edge, and for that 3v and A2 have no disadvantage, and perhaps even an edge, over stuff like 20cv.

 

[AmericanAsApple3.14]

A2 and 3v are still quite wear resistant, just not as much so as the others, like 20cv. Carbide volume does not alone equal edge holding. Annealed 1095 has about 12% volume of carbides, but is nowhere near as wear resistant as hardened S30V, which has roughly 14%. A significant part of dulling is based on deformation of the edge, and for that 3v and A2 have no disadvantage, and perhaps even an edge, over stuff like 20cv.

Which would help explain why people can bring 3V back to sharp after only a little effort on a strop? Because the edge was more along the lines of deformed, rather than actually removed? And also why at times 3V might seem to cut longer than 20cv (from what some people have claimed, not my personal experience)?

 

[chiral.grolim]

Which would help explain why people can bring 3V back to sharp after only a little effort on a strop? Because the edge was more along the lines of deformed, rather than actually removed? And also why at times 3V might seem to cut longer than 20cv (from what some people have claimed, not my personal experience)?

There are MUCH more important variables here than just steel type. Sharpening angle and technique (including burr-removal and final grit) are paramount. A edge that is too thin or presents a burr can easily fold over to leave a much thicker "apex diameter", but this same edge can also crack away and leave a sharp (narrow) apex diameter behind. If an apex folds over it will be dull, but a quick stropping can remove the fold and produce a convex microbevel that restores sharpness. To compare the 20CV and 3V, the blades need to have been sharpened exactly the same.

Assuming that, the higher carbide content of 20CV reduces fracture-toughness in that ultra-thin wire-edge such that it will more likely break off right away and (ideally) leave a more durable edge behind, but this edge will be of thicker (less acute) geometry that may not respond as well to stropping at too shallow of an angle. The user may want to resharpen to the desired angle (recommend 30 inclusive) and then burr removal at 40-inclusive.

In wood planar blades, A2 is highly regarded and 3V is supreme. The lower carbide content improves fracture toughness such that, at the same hardness (i.e. strength to resist initial deformation) as the 20CV, a wire-edge is less likely to crack away. If there is a wire-edge that folds, it can be successfully stropped away at a shallower angle and so seem to respond better to stropping in general (though the same sharpening angle and burr-removal is recommended).

So yes, deformed edge vs removed edge could certainly play into noticeable differences.
There may be some effect from the reduced abrasion resistance that lends to easy stropping in the lower carbide steel, but I am not convinced of that given that stropping usually involves sufficient abrasion per stroke to mask such a difference.

 

[Worldwatcher]

If someone wants to send me a 20cv survive ill gladly test it out and report my findings!