Are Non Clad Steels Stronger Laterally?

[PanChango]

I have a SB Endura and have been using it while doing a renovation for my house. I looked last night and the blade was bent enough that it would not close properly. It has been used fairly hard but I did not think I abused it enough to cause this. I know the 420J2 is soft and scratches, but I assume that it also didn't have the lateral strength either.

Will a non clad FFG VG-10 Endura hold up better than the clad Super Blue one? I really like the Endura as it is one of my favorite knives. I just don't want to get another FFG one have this happen to it again.

I will try and take a picture tonight when I get home?

[tvenuto]

Pics would be helpful, but keep in mind that a harder steel may still have failed, but just have fractured instead.

[PanChango]

That is true as well. I won't be able to take any until after work today.

[bearfacedkiller]

Cladding would in theory make it tougher, not stronger. I have always thought of San Mai as a modern equivalent to differential hardening like they used to do on their swords. A solid super blue blade should be stronger and a clad super blue blade should be tougher.

[tvenuto]

Well toughness (in the bulk sense) is the product of strength and ductility, so toughness is partially dependent on strength. What you say is correct, but could be misconstrued to mean that toughness and strength are independent of one another. See below for a decent high level example of the difference between an absolutely strong material and an absolutely ductile material. (Ignore the steel labels, this isn't meant to educate on which steel composition should have which property, this graph could just as accurately show 3 different heat treat states of the same composition).



Edit: So regarding your specific question and how it relates, let's say the solid superblue blade is the "strongest" example, and the clad blade is the "toughest" example. The strong steel does indeed resist permanent (plastic) deformation longer than the tough one, however, it does not deform very much before fracture (the end of the line). So solid superblue blade may indeed not have bent at all, as it can absorb more energy without doing so, but we can't really know what would have happened because we don't know exactly what magnitude of force caused it to bend, and how that force was applied.

[bearfacedkiller]

I was using them in the general sense of strength as resistance to deformation and toughness being resistance to breaking.

Thanks for adding that for some clarity. This confused me for a long time and I do not want to add to anyones elses confusion.

Here is an excerpt that helped explain it to me a while ago when I was trying to make sense of it.

From: http://zknives.com/knives/articles/knifesteelfaq.shtml" onclick="window.open(this.href);return false;

"Strength: The ability to take a load without permanently deforming. For many types of jobs, strength is extremely important. Any time something hard is being cut, or there's lateral stress put on the edge, strength becomes a critical factor. In steels, strength is directly correlated with hardness -- the harder the steel, the stronger it is. Note that with the Rockwell test used to measure hardness in a steel, it is the hardness of the steel matrix being measured, not the carbides. This, it's possible for a softer, weaker steel (measuring low on the Rockwell scale) to have more wear resistance than a harder steel. S60V, even at 56 Rc, still has more and harder carbides than ATS-34 at 60 Rc, and thus the S60V is more wear resistant, while the ATS-34 would be stronger.
Toughness : The ability to take an impact without damage, by which we mean, chipping, cracking, etc. Toughness is obviously important in jobs such as chopping, but it's also important any time the blade hits harder impurities in a material being cut (e.g., cardboard, which often has embedded impurities).
The knifemaker will be making a tradeoff of strength versus toughness. Generally speaking, within the hardness range that the steel performs well at, as hardness increases, strength also increases, but toughness decreases. This is not always strictly true, but as a rule of thumb is generally accurate. In addition, it is possible for different heat treat formulas to leave the steel at the same hardness, but with properties such as toughness, wear resistance, and stain resistance significantly differing." -zknives.com

[bdblue]

Pics would be helpful, but keep in mind that a harder steel may still have failed, but just have fractured instead.

The idea behind the clad steel is that the knife bends instead of breaking. You see it bend and stop what you are doing with it, but you still have the knife intact.

In knife steels there is the tradeoff between hardness (strength) and toughness (ductility). Increase one and you decrease the other. Different alloys might move the tradeoff point slightly up and down the scale.

There is another factor that might be involved with some geometries, and that is fractures that can initiate at notches, holes, etc. As hardness increases a knife becomes more sensitive to irregularities in the blade shape. As steel hardens its strength increases, in a perfect world, but the steel can become more sensitive to notches and thus a knife blade made out of the steel might fracture at a lower level of force than would bend a blade made out of softer steel. If you just test smooth bars the harder steel will have more strength than the softer steel, strength is what makes it hard. We test Rockwell hardness by putting a dent into the steel- the stronger steel resists the dent more than the weaker steel. As you test the smooth bars that softer steel will start to yield and will bend before it finally fractures, whereas the harder steel will bend less or almost none at all before it fractures.

An example of the bending of the softer steel would be a wire coathanger. You can bend one of those a lot and never get it to fracture until you bend it back and forth enough to work harden the steel. An example of a notch sensitive material would be a piece of glass. You can take a strip of glass and try to bend it, it actually resists breaking fairly well. Then scratch the surface with a glass cutter and the glass fractures under a lot lower force. Imagine this behavior in a knife blade with notches, holes, square corners where the guards fit, etc.

[tvenuto]

The idea behind the clad steel is that the knife bends instead of breaking. You see it bend and stop what you are doing with it, but you still have the knife intact.

Although I'm not sure it's relevant to the question at hand, I think the idea behind this particular clad steel was increased corrosion resistance by the lower percentage of non-stainless steel exposed to the elements.

[bearfacedkiller]

I assumed it was a combination of the two. However, clad steels seem to be extremely popular in Japanese kitchen knives and I do not see lateral toughness being a major concern in kitchen knives so maybe it is more to do with corrosion resistance. Except for the Damascus and vg10 clad knives which seem to be purely aesthetic.

[Holland]

I would love to see a photo of it

[PayneTrain]

I gotta wonder, is actually it the blade that bent or the pivot? If it is on the blade, whereabouts?

The thing that always puzzled me with clad steels like Super Blue/420Jx is the fact that while the cladding may be softer, the Super Blue inside is still hard and (relatively) brittle and if the blade sees a load enough to break the Super Blue but not the 420, wouldn't the Super Blue still break? Like laminated glass, the outer film may not break but the glass inside still shatters, and is just held together by the laminate. That's why I've always seen cladding as more for corrosion protection than any kind of mechanical advantage over solid harder steel. Is this not the case? I really don't know how the two steels interact in there, or how they put them together at all so I could be wrong but that's my impression.

[Jazz]

We've gone through this whole thread before. The 420 is real soft and bends easy. Just bend it back. I don't like it either, but that's what they use. If I had the time, I'd find a link to the thread. Try bending it back and see if it works. Mine was a tad off and it bent real easy.

[bdblue]

The thing that always puzzled me with clad steels like Super Blue/420Jx is the fact that while the cladding may be softer, the Super Blue inside is still hard and (relatively) brittle and if the blade sees a load enough to break the Super Blue but not the 420, wouldn't the Super Blue still break?

This will not happen with a little bit of bending but could happen in an extreme case. When you bend a solid object, the bending stresses vary linearly from 0 at the center to maximum at the outside surface. The Super Blue laminated in the center is relatively thin and would only see a small amount of bending stress because it is all close to the center. The outside layers are softer and at the point when they start to yield, the hard inside layer is seeing less stress and is at only a fraction of its higher strength. Now if you keep bending the blade farther the outside layers continue yielding. The inside layer has not yielded or fractured yet so it continues increasing bending stress and at some point it actually would reach its fracture point. I suspect that it would take a lot of bending to achieve this but theoretically it is possible.

[Cliff Stamp]

Will a non clad FFG VG-10 Endura hold up better than the clad Super Blue one?

The clad blade will take a set at a lower force, however it will be easier to break the VG-10 blade in half. In general the question is complicated because you end up trying to make a decision on how much strength do you want to give up for ductility. However in regards to the clad blades they are very weak, you can bend them just by cutting things hard so they are light use only typically.

[Cliff Stamp]

In knife steels there is the tradeoff between hardness (strength) and toughness (ductility).

Generally most discussions on toughness are talking about fracture toughness, specifically under impact and/or resistance to micro-chipping. These are related to, but not the same as ductility and very different than measuring toughness form the area under the stress/strain curve.

[PanChango]

Sorry for the delay in getting back to the thread, yesterday turned in to a 14 hour work day that I was not expecting. Thanks for the replies. Sorry I have missed the previous discussion. It is the blade that is bent not the pivot, I probably tried to do too much with it and should have used something stronger like my military. It was a good knife, but I don't think I will be able to bend it back enough to keep a consistent edge for sharpening.

Here are a bad cell phone pics of it.

[Bodog]

I bent my PSF27 mule team blade at a more severe angle than that and was able to bend it back adequately enough to still sharpen pretty much equally on both sides. Grab a hammer and two pieces of wood and a straight edge. If it's so bad right now that you won't likely use it again then what will it hurt if you try to bend it back and it breaks? Either way you're out a knife but one way you may be able to make it useful again. Just a little love, a little bit of patience, and some firm but gentle prying and banging should do the trick.

[tvenuto]

Just a little love, a little bit of patience, and some firm but gentle prying and banging should do the trick.

Sounds like good relationship advice as well.

Pan, that's pretty crazy, I certainly wouldn't have expected that severe of a bend. I guess I also missed the last discussion as I was also suspecting some sort of pivot issue, but that's definitely the blade. Hope you're able to get it back to at least somewhat serviceable shape.

[PanChango]

I will give it a shot to see if I can get it back to usable. I really didn't know how to straighten one, but the between the 2x4s is a good idea.

Thanks everyone.

[Jason B.]

As someone that sharpens a lot of Japanese blades this is very common. Its part of the knives design to bend and be bent back, for larger blades they sell wooden sticks made just for the task of bending blades straight. The idea is to encase the hard brittle metal with a softer metal so you do not break or damage the hard core steel.