Toughness?

[harrymole]

What does this mean concerning knife steels? Many would say, prying, impacting, edge retention, flexability or wear resistance. There even mabey others. This can be confusing seeing as everyone has a differant view of "toughness". Is there a specific definition for steel toughness? I see this term being used alot, so lets hear some experts expand on this. thanks.

 

[mete]

One of the basics is that some knives will be used for chopping such as a kukri and some 'survival' knives. Impact use requires toughness and the best steels are L-6, CPM 3V, 5160.

 

[harrymole]

Is toughness only for impacts during chopping? Does this include lateral use or twisting? I am just trying to get a thread started explaining this concept. I would like to know for myself also. thanks for the replies.

 

[ghost squire]

Toughness is the ability to absorb energy without failure, and is the combination of strength and ductility.

Strength is the ability to resist deformation, ductility is the plasticity exhibited by a material under tension loading. Plasticity is the ability to take deformation without breaking.

Those are the definitions from Metallurgical Theory and Practice by Dell K. Allen.
In practical terms, I think toughness could be measured by impact testing.

 

[ghost squire]

Ideally you see it would take a massive amount of force to deform a blade, but when it deformed it wouldn't break (break is defined as permanent deformation I believe, meaning no practical permanent deformation such as chips or rolls or dents), but merely temporarily deform.

 

[harrymole]

So, toughness is related to hard impacts? Is this in chopping only? So, if say Joe over here is looking for a tough knife that can chop, pry and take extreme use, you would look for strength for prying, toughness for impacts? Just to clarify.

Sorry, just re-read you description Ghost. You are saying that toughness is a combination of strength and ductility. So, toughness is a combination of terms not just one specific term. So, when looking at blade steel you want to look at the whole to get an idea of toughness?

 

[HoB]

Well to put this in more tangiable terms, toughness is the abilty of a material to absorb energy. In order to understand what that means we need to introduce two terms: yield strength and tensile strength. If you take a steel bar and bend it to a certain degree, it will snap back into its original position. The FORCE up to which this occurs is the yield strength. Meaning you can have two materials that behave very differently but have the same yield strength. One deforms very little because it is stronger, while the other one deforms (bends) by a large amount but still snaps back into its original position. This is called elastic deformation. If you stress it further than this point, the bar stays deformed. It has undergone plastic deformation. (For completeness sake, let's just mention that each of these stesses can be split up into shear, compression, tension and torsional stresses and strains).

Toughness is the ability to absorb energy, usually under a rapid impact. That means essentially that it is the measure of a material to yield (to undergo elastic deformation). But there is also a timing aspect. If the impact is so rapid that the material has no time to yield, it will shatter. So for high toughness a material must have both great yield strength and the ability to yield (rapid deformation in the elastic limit).

Ductility is the amount a bar can stretch during plastic deformation, before it breaks (tears). For large toughness, one usually prefers the material NOT to undergo plastic deformation, so usually ductility is not something that factors into toughness, but rather the ability to yield (which is essentially the "ductility", the amount a bar can stretch, during elastic deformation).

It is usually measure with one or several charpy impact tests.

Hope this helps. Please also keep in mind that this explanation is somewhat simplified.

 

[harrymole]

It does help HoB. So is this why stainless would not be the choice for large choopers? Carbon and tool steels both have greater toughness due to less chromium? Thanks.

 

[HoB]

Stainless steels have much lower toughness especially compared to simple carbon steels like 1060, 5260. A tool steel like A2 and L6 (is that even considered a tool steel? Mmmmh, probably) have a much higher toughness than stainless steels while D2 and M2 are at the upper end of the toughness range of stainless steels, but still in the same ballpark as stainless steels. So you can not really say that all toolsteels are tougher than stainless steels.

But don't forget about hardness. It is pretty pointless to compare on steel that has been hardened to 62 Rc and another that has been hardened to 56 Rc and say steel B is much tougher than steel A.

As to WHY stainless steels are less tough than carbon steels. I don't know for sure but it has clearly something to do with carbide volume, size, distribution, and type as well as with grain size. Chromium is a carbide former and it is the largest alloying constituent in stainless steels. Stands to reason, that the chromium carbides decrease toughness. Again tempering plays a large part aswell as it changes carbide size and distribution. More on this you can find here:
http://www.bladeforums.com/forums/showthread.php?t=396105

 

[harrymole]

Thanks Hob! It clarifies alot of the question I had. One more question that has nothing to do with this topic is how does 5160 steel compare to other steels as far as wear resistance, toughness and edge retention? I am looking at trading for an Ivan Campos knife that is made of 5160. Thanks.

 

[Gossman Knives]

S7 steel is tough. Nuff said.
Scott

 

[HoB]

I have no personal experience with 5160 though I am about to order a RD9. From the specs though 5160 has a small amount of Cr which should increase wear resistance a bit, but it should still be at the lower end of the range, edge retention also depends on the hardness. IIRC the 0.6% C are theoretically sufficient to quench to max hardness of steel, but usually more C helps with hardenability and 5160 seems to be run at lower Rc like 58 (give or take a couple of points). So edgeholding is probably not all that great in comparison to lets say ATS-34. Toughness on the other hand should be outstanding. Among the toughest steels out there. It probably won't beat S7 (which is a shock steel), but it was made for car springs, and can also be spring tempered. It is also used for swords (even though 1060 is probably more common) which should be an indication of its toughness. All in all it seems to me to be one of the steels that is ideally suited for a large chopper blade (wear resistance is of less importance cutting organic material, chipping and denting are much more common causes of blunting).

 

[Slaytanic]

**S7 steel is tough. Nuff said.
Scott**

Hi Scott, how does the edge holding compare to other carbon/tool steels?

Darrell.............

 

[smcfalls13]

Never had a knife with 5160, but I have several swords with the stuff. In my extremely inexpert opinion, it's nearly indestructible. Edge retention is moderate. Not good, but not bad.

L-6 and S-7 are supposed to be better in most respects, but I've not handled them.

 

[ghost squire]

Sorry, just re-read you description Ghost. You are saying that toughness is a combination of strength and ductility. So, toughness is a combination of terms not just one specific term. So, when looking at blade steel you want to look at the whole to get an idea of toughness?

Exactly. A very hard knife will be very strong, but won't generally exhibit good ductility. Thus this would be a poor choice for a large chopping style knife, or a machete where you would want the edge to roll rather then chip.

Copper is very ductile, but would be a poor choice for a machete or large chopper because it is TOO ductile.

You basically just want a balance depending on the jobs you expect your knife to do. Its a bit more complicated because one steel can't do all jobs.

 

[Keith Montgomery]

S-7 is about as tough a steel as you are going to find, but it is not noted for its edge holding ability.

A steel that is very tough and has excellent edge holding is CPM-3V.

 

[Gossman Knives]

Hi Scott, how does the edge holding compare to other carbon/tool steels?

Darrell.............

S7 holds an edge good enough but not as good as some of the other carbon steels. I got mine at about 57rc and feel it's pretty good. It's great for as a woodsman knife but wouldn't do well for cardboard or other fiberous materials. If I temper it to 59 (the highest it will go) then I sacrifice impact toughness. I would put it on the level with 5160 or very slightly under, as far as edge holding. I do know that it will stop a 41 magnum.
Scott

 

[Cliff Stamp]

S-7 is about as tough a steel as you are going to find, but it is not noted for its edge holding ability.

Edge holding ability can be strongly dependent on impact toughness for some work, large chopping blades don't benefit highly from wear resistance as they blunt mainly by deformation or fracture thus a S7 blade would have greater edge retention as an axe than a D2 blade, not less.

S7 is likely overkill for knives outside of the tacticals which are used for metal impacts and you are probably better offer with a steel with a more balanced perspective, but it is better to go extreme in that way than wear resistance which most of the non-forgers have been doing up until now.

-Cliff

 

[harrymole]

I saw the photos, Scott of the .41 magnum impact. VERY Impressive! You said S-7 is very similar to 5160 in edge holding. How does it compare to others teels so that I have some idea of its' edge holding ability? Thanks for all the great comments.

 

[Gossman Knives]

I saw the photos, Scott of the .41 magnum impact. VERY Impressive! You said S-7 is very similar to 5160 in edge holding. How does it compare to others teels so that I have some idea of its' edge holding ability? Thanks for all the great comments.

I'll put it this way, the best use for an S7 knife is as a camp knife or large chopper for batoning and chopping wood where high amounts of impact would be involved. Plus twisting and torquing of the blade. The edge will roll instead of chip and it's easy to resharpen in a minimal amount of time. It could even be done on a stone if necessary. If you want something for long term edge holding go with D2 or A2. One other thing about S7 with the edge, a convexed edge will stay sharper longer then a thin V-grind edge that will be more suspectable to rolling.
Scott