A2 vs M2 vs 440v (???)

OK folks, given the same blade configurations and geometery, which of these is preferable for a sport utility / tactical folder?

(Assuming blade coatings of some sort, I'm not interested in stain resistance.)

It looks like A2 is a much simpler tool steel than M2 but is the high-speed (Molybdenum?) attribute of M2 really that important in a folding knife? I've heard people prise the 440v steel but some folks have said it's too brittle.

What's the story?

Jon
 

Cliff Stamp

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I don't see any advantage of M2 over A2 in regards to knife steels as its primary advantage is of no use (it retains high abrasion resistance at extreme temperatures). Comments by knifemakers in regards to ELU advantages are welcomed. As for A2 over 440V it depends on if you want high toughness or abrasion resistance. Both factor into edge holding depending on what you cut. High toughness (charpy value) means the edge will resist rolling, impacting and chipping strongly and can be taken to a much lower included angle. High abrasion resistance simply means it takes longer for the edge to wear away.

Something to think about :

A2 : 60 RC : Charpy 21 : Wear resistance : 2-3

CPM-440V : 56 RC : Charpy 18 : Wear resistance 15/25
CPM-440V : 58 RC : Charpy 13 : Wear resistance 20/30
CPM-440V : 59 RC : Charpy 12 : Wear resistance ?
CPM-440V : 60.5 RC : Charpy 11 : Wear resistance ?

These are straight from the CPM sheets :

www.physics.mun.ca:80/~sstamp/knives/index.html

I assume that 3-4 and 20/30 are ranges.

You can see that A2 would have a much tougher edge (and a much higer yeild strength) and that 440V would have much better wear resistance. Deciding between the two means that you have to figure out how much of each property you need. For example if the extra toughness of A2 is of no use then obviously you get 440V. However if 440V chips out during regular use its high abrasion resistance is of no value.

Note in regards to CPM-440V, while people have commented on its weakness before (broken tips mainly) Sal Glesser has stated that is as as tough as any other steel they have used

www.bladeforums.com/ubb/Forum3/HTML/000247.html

For me, that means that the gloves are off when my new Military comes in.

Another aspect to consider is the kind of edge that each takes. According to MPS, A2 scores very high in this respect. It is better than Boye's 440C, which is saying something.

-Cliff


[This message has been edited by Cliff Stamp (edited 19 May 1999).]
 

Cliff,

Wow, thanks for that response!

How do those Wear Resistance values compare to something like ATS-34 and do you know how it is measured?

I think I remember reading that it (ATS-34)has a 13 to 14 Charpy rating at 59-60RC. One might guess that the difference is more academic than anything.

Jon
 

Cliff Stamp

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Jon, 440C has a charpy rating of 26 and a wear resistance 3/4 at a RC of 56/57. ATS-34 is known to be significantly lower in toughness so your value makes sense. Its abrasion resistance is slightly higher than 440C but not by much.

The subject of material specifics is one I am interested in as it cuts directly through the hype. Most knife companies claim extensive testing which has lead to thier choice of knife steel but very few will release anything other than a RC rating. It would be really nice to see all companies make the stats on their materials public - the elastic constants and the critical points, the charpy value, the abrasion resistance and adhesion resistance, etc.

The problem with this is that it puts a limit on what you can claim - which it obviously a disadvantage to most. For example Chris Reeve has stated that his Project would be a better blade in BG-42 than in A2 and the only reason he does not do this is that it would more expensive to make. This makes no sense to me, I would have assumed the opposite and would rather a Sebenza in A2 than Bg-42. If I am wrong, that's cool, just show me the numbers. I have asked with no reply.

Mission is fairly direct in this respect and have compared their steel and Ti knives quoting several numbers like tensile strength. This is a very, very nice practice and one I would like to see continue but it obviously will not unless pressure is put on by the consumers.

-Cliff


[This message has been edited by Cliff Stamp (edited 19 May 1999).]
 

Cliff Stamp

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There are three basic elastic constants. The Stretch Modulus, the Shear Modulus, and the Bulk Modulus. These are all measured in force per area.

A few examples are (units are in 10^10 N/m^2)

Annealed steel : 20 : 8 : 14
cast iron : 12 : 4.6 : 9.0
copper : 10 : 3.9 : 11
aluminum : 7.0 : 2.5 : 7.5

The numbers are Stretch, Shear and Bulk modulus, respectively. You will note that generally as one increases the others do as well. These moduli represent the resistance a material has to being stretched, sheared (deformed) and compressed.

What is really important though is what the breaking points are. For example the breaking point of the stretching is called the tensile strength. If you exceed this then the material will suffer extreme failure (come apart). Something to consider, tensile strengths (10^8 N/m^2) :

annealed steel : 5
Mission's Ti : 15.5
Mission's A2 : 21.3

A higher stretching modulus usually implies a higher tensile strength.

In regards to knives all these elastic constants are important. They represent the ability of a steel to resist pulls and pushes applied in various directions. Usually when you are cutting, prying or whatever, you are applying a combination of stresses. This includes what is being applied to the edge. Its only in the lab where you can focus on one particular aspect. It would be very interesting to see these constants for all the various steels and the associated deformation limits. The charpy value for example is very dependent on the bulk modulus.

In regards to edge retention in particular, these elastic constants are very important as when a knife gets blunted its usually because you passed an elastic limit and it rolled, impacted or chipped out.

Note, I am not a materials engr. and would welcome the insight of one in this regard.

-Cliff


[This message has been edited by Cliff Stamp (edited 19 May 1999).]
 

Cliff,

OK, I'm missing something I think.

A2 at 60RC has the same wear resistance as 440C at 56/57RC?

That would easily put below ATS-34 at ~60RC, right?

Jon
 

Cliff Stamp

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Jon, yes. I just rechecked the numbers from CPM and that is what they list. It is important to note that high wear resistance does not mean better edge holding, it is only one factor. A2 has a much higher ability to hold its edge due to the increased toughness over ATS-34. Wear resistance is only the dominant factor is you are doing very precise cutting like machines cutting paper in a factory. For general utility the elastic breaking points are much more important.

-Cliff

[This message has been edited by Cliff Stamp (edited 20 May 1999).]
 
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Thanks Cliff. Your dissertation helped a lot, I think. I had forgotten what little I did manage to learn during my engineering studies at the university. One thing that is still throwing me a little (because I have forgotten it; have to use it to retain it I guess) is how the moduli you list pertain to the differentation between rolling the edge or chipping the edge. If it has a low, let's say, shear modulus. It you were to, say, cut into a staple while cutting up a cardboard box, how would the edge deform. Would it roll, or would it chip? Or does that depend on the heat treat? Or is it something else? Elastic modulus? Or something like that? Wish I still had my structures books here at the office.
 

Cliff Stamp

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Outlaw_Dogboy, the moduli basically allow you to calculate how much a material will stretch, deform and compress but that is not really that useful from a knife perspective. As you commented what is really important is what happens at the breaking point.

For example the stretch modulus has three associated breaking statistics. The first is the elastic limit - this is the stress that causes a perm. distortion. The second is the tensile strength - this is the stress that causes the material to break. The third is the elongation at rupture - this is the amount of distortion when you pass the rupture point.

For reference (these are all Aluminum):

<table>
<tr><th align="left">Annealed</th> <th>6.9 </th> <th>1.2 </th> <th>4.7 </th> <th>49
</th></tr>
<tr><th align="left">Cold Rolled</th> <th>6.9 </th> <th>10.6 </th> <th>11.2 </th> <th>5
</th></tr>
<tr><th align="left">Typical Alloy</th> <th>7.1 </th> <th>15 </th> <th>20 </th>
<th>3
</th></tr>
<tr><th align="left">High-Performace Alloy </th> <th>7.2 </th> <th>50 </th> <th>56</th>
<th>11 </th></tr>
</table>

These are : the stretch modulus (in 10^10 N/m^2), the elastic limit (in 10^7 N/m^2), the tensile strength (in 10^7 N/m^2) and finally the elongation at the rupture point ( in percent).

You will note that while the moduli all remain pretty much the same the breaking points do not. This is why it is very important for them to be know as well.

Fact is that any knifemaker trying to cut through all the hype has to simple quote these numbers for his steels. If Busse wants to prove that INFI is so much better than A2 then simply list these numbers.

The short answer to your question would be it will indent if you pass the elastic limit and it will chip if you pass the breaking point. Some materials have these quite close together and some do not.

-Cliff


[This message has been edited by Cliff Stamp (edited 21 May 1999).]
 

Cliff Stamp

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Note that while you can measure the moduli easily, the limits are obviously destructive tests.

-Cliff

[This message has been edited by Cliff Stamp (edited 21 May 1999).]
 
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Guys: I have been following this post, and, I am working on a response, but, it's not ready. The question being posed is a complicated one, and, unfortunately, presenting the material properties doesn't give the complete answer. I'll try and get a response up here this weekend, OK?
RJ Martin
 
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