Selective tempering... is it really make knife tougher?

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shqxk

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Mar 26, 2012
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Im talking about full blade quenched, regular 400F tempering and extra tempering by torching the spine while edge sit in the water.


Always wonder if this method really has it benefit and will the softer yet more springier spine make the knife less bending resistance?

Have heard numbers of respectable knife makers who obviously has great metallurgy knowledge still doing it. To name a few, Fisk, Foster, DesRosier, Kramer and many more...
 
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Sure it has a benefit. Edge stays hard and resistant to rolling and the blade it less likely to break under impact or pressure.

About bending resistance: Wouldn't you rather be able to bend your blade back after bending it 90 degrees and it staying somewhat bent than having it break in two?
 
It is totally a blade survival thing. I think it is most prominent among the ABS guys and those who learned from them.

Does it have a benefit - yes, but not as far as cutting and edge life. A proper full temper will suffice on all but extreme abuse blades, or those designed to survive the ABS bend test.

It makes the spine much less likely to start a crack in severe batoning or a blade bend test. It makes the upper bevel more ductile and less likely to snap in half. The edge may chip or crack, but the blade will survive.
 
Really two schools of thought on the subject... Katana vs. European swords. Katanas were differentially heat treated, with a soft spine and a hard edge. European swords were through tempered.
Both were used in battle, and broke in battle. Which is worse a bent sword? Or a broken sword? Both are un-useable.

I personally think that I would rather have a through tempered blade, at the appropriate hardness for the task it was designed for.
I do however really love the aesthetic of the hamon, all whispy and beautiful, as well as the stark line of an edge quench.
But as far as function, I think through temper wins out.
 
A.McPherson said:
Really two schools of thought on the subject... Katana vs. European swords. Katanas were differentially heat treated, with a soft spine and a hard edge. European swords were through tempered.
Both were used in battle, and broke in battle. Which is worse a bent sword? Or a broken sword? Both are un-useable.
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They should have a saying, reductio ad katana, which stipulates that all edged tool discussion eventually leads to a comparison of European swords and Katanas. :D

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SirSpice said:
They should have a saying, reductio ad katana, which stipulates that all edged tool discussion eventually leads to a comparison of European swords and Katanas. :D
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Lol!! Very true! They are the extremes of knife craft though, and I think they are good argument points...
 
Ken H> said:
Well, a broke sword is sorta hard to put back together, but a bent sword can be straighten and used again.
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Sure. But which is better? A sword that bends 50% of the time it meets a resisting object or a sword that breaks 1% of the time? I obviously don't know how often either situation happens but am just trying to illustrate that in my opinion a fully tempered martensite blade, while having a higher risk of breaking, will have a far lower frequency of occurrence than a partial martensite, partial pearlite blade will have bending.

Also this really isn't pertinent to shqxk's OP as that edge quenched katana is vastly different from a spine tempered blade - both in structure (condition?) and hardness. Superior in every physical sense I would say. He's talking about a blade that is tempered martensite through and through, with a 60 RC edge and say a 50 RC spine. It will be slightly more brittle than the katana spine but with higher yield. It's not all that different than comparing an actual spring to a piece of cold rolled 1018 in the shape of a spring.

I really hope (and have requested) the various differential hardness methods/results are a topic of one of Larrin's articles soon. I think he could clear a lot of air in what the benefits are, just how beneficial, and exactly what is going on when we use those different methods.
 
You do raise a good point there Kuraki!

And I do have to admit that I was talking about differentially hardened blades not about differentially TEMPERED blades...

In the second case, I just don’t have enough knowledge to have an opinion really.
 
JTknives said:
We did learn that katanas don’t fare so well on knife or death lol
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Well, regardless of the quality of that blade, the way it wrapped around the ice block is exactly what I anticipated would happen. We also saw a western sword become a much shorter western sword at the ice block. When the katana bends around it, it's acting how it's designed to act. When the western sword broke, it's because it wasn't heat treated properly. When a properly heat treated western sword faced the ice block, it neither bent nor broke.
 
Since this can of worms has been opened:

There is a big difference between "Traditional Blades" and "Modern Blades". We make modern blades, even when we are copying traditional styles.

About 70% of the reason is the steel. Traditional blades were from simple carbon steel and of much lower carbon content than what we use. Alloying was virtually zero in most cases, and what alloying there was came by the way of impurities from the ore sources. No smith had any idea exactly what the steel had in it before the 20th century.

Another 20% is modern metallurgical techniques that allows our better steels to be pushed to limits far beyond the past. We have testing, laboratories, engineers, etc. to determine things they just guessed at. We can harden a blade and temper to Rc 63 and know that is what we have.

The final 10% is that the traditional blades were much softer than our blades. They were hardened to whatever it came out to be and tempered to ranges between the upper 40's and lower 50's. We would consider any blade a HT failure in those ranges.

What happens when you smack a Rc 50 sword against a tree and what happens to the same sword at Rc 60 is very different. That does not mean the old sword was "better", just softer and more malleable. The edge life was often for one use and then it required resharpening. After a year or so of use, many knives and swords were "worn out". We all remember Grandpa's pocket knife where the blades were barely over 1/4" wide from repeated sharpening. With proper HT and tempering, wecan make weapons that would far surpass the "Traditional Blades". About 99% of the mythological properties of those blaes was pure hype and myth. Swords don't cut anvils in half in the real world.

Final comment is that we don't fight with swords and Bowie knives anymore. The FIF Knife or Deathe show showed how much our blades can be made to take, and how poorly a traditional blade may perform.
 
As a matter of interest, Ben Lilly, who used his knife to kill bears stated he would rather have a knife break than bend. I think his reasoning was a knife that would not bend but break would stand more abuse before breaking than one that would bend. In other words, a knife that would bend would bend before one that would break would break.
 
Why does this have to be a can of worms? Traditional, historical blades are irrelevant. There are real, physical differences between a differentially hardened blade, and a differentially tempered one. That's why I'm holding hope Larrin will address it because it will be with a much greater degree of accuracy than I'm able.

Even the 80 year old Barong that was used (well) on the show is a contemporary blade.
 
My understanding:

If you start with a tough steel, nail your geometry and temper back the thicker section (spine) of the blade, you will have a FAR more resilient knife than you would if you left the spine unhardened. If you leave the spine with a spring temper, and the geometry of the knife is designed to its task, it will be essentially impossible to seriously damage the blade through intended use.

Nearly any blade, no matter how thin or hard, is virtually immune to damage from forces applied in plane (in other words from edge to spine or vice versa). It is side loads that are most likely to cause blade damage. From a strength perspective, you adjust spine thickness and geometry based on what sort of side loads/twisting/etc. that the blade is likely to encounter.

In addition to thickening the spine, you can also use a differential temper. This temper will allow the thicker sections of the spine, which are more prone to cracking under deflection, to withstand a greater degree of deflection prior to cracking. Essentially as thickness increases, minimum bend radius increases for a given heat treat. Increasing the temper temperature of a steel of a given thickness will generally decrease (improve) the minimum bend radius before cracks form.

Take steel that has not been heat treated and compare it to steel that has. All geometry and alloying being the same, steel that has not been hardened will reach its elastic limit prior to hardened and tempered steel. Steel that has been insufficiently tempered will essentially fracture prior to reaching its elastic limit (snap before it takes a permanent bend). There are a few youtube videos that I can't look up right now, but I think are from 'Real Engineering' that visually depict the relationship between hardness, elastic, and plastic deformation in a given steel.

So basically if you temper the spine back on a knife with appropriate geometry (think full flat or convex grinds, but tall saber grinds and the like also work), you allow the knife to undergo a deeper bend before they start cracking. It also may, depending on the as tempered hardness of the knife you are comparing it to, allow you to bend the knife more prior to the knife taking a permanent bend (undergoing plastic deformation).

What it won't do for you is change the amount of deflection or bend that a given bending moment will create. That is, as far as I am aware, only a function of geometry (thickness) and alloy.

Disclaimer: I am not a metallurgist, so maybe someone who is smarter than I on this score can add info or correct (Larin, you around?).
 
I have differential tempering vs differential hardening on my list of article ideas. I don’t know when I will get to it. I’m not sure what literature is out there on properties of materials with similar property differentials. I can certainly write about the different phases present and their properties and how they might interact with each other.
 
I’ve read a paper somewhere in the internet regarding the transition zone from hard to soft blade. Can’t find that paper anymore, but I think they where mentioning that that transition zone would be problematic, regarding toughness/durability. As I said, can’t find that paper anymore. Maybe I’ve drunk too much, sorry!
 
I spring temper the spines on my hard use machetes made from 52100. They are seriously tough and I’ve never actually managed to break one. Even bending them in a vice, they will take a set after 120 degrees instead of snapping.

Now I’m not sure exactly how much of a difference the spring tempered spine makes, but for an extra cost, I think the edge in performance is worth it— even if that extra performance is mostly theoretical and not truly empirically proven. At the very least, it makes the blade less likely to crack from the use of hard batons.
 
Huntsman Knife Co. said:
At the very least, it makes the blade less likely to crack from the use of hard batons.
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Unless, you start looking into tempered martensite embrittlement (TME). Theoretically, if you tempered out the spine, you would have a transition area that would be more brittle than your edge. I have no idea how that plays out in real life but it is metallurgical fact.
 
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