Hard versus Tough and why they don't mix.

Fred.Rowe

Fred.Rowe

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May 2, 2004
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As a knife maker I design a knife blade, its geometry, for "how" I want the edge to cut. How long the edge holds up is relative to its hardness and abrasion resistance; how much abuse and shock, the entire blade will absorb before failure is relative to its toughness. Tough steel can be very soft, like tie wire, tie wire is tough but not hard. Tough and hard are not really compatable. Its hard to have a tough edge that is hard or a hard edge that is tough. The best combination of the two is to have a hard edge tempered to the degree that it will support the geometry, while the body of the knife is "softer" and therefor tougher.

The Japanese were experts at combining hard and tough, but not in the same area of the blade; the edges of their katanas and wakizashi's were very hard, while the spines and body of the blades were relatively soft and tough. For them it was accomplished with the use of clay hardening. No factory blade that I know of has this feature; the entire blade is hardened and then the entire blade is tempered.


Edges are about trade offs you can't have it all, no matter what steel is used, what heat treatment is used and what the geometry of the blade is. Too hard and the edge chips, to soft and it bends. To thin and to hard and it chips, to thick and to soft and it fails. You decide on the job and make your selection from there.

A knife makers 2 cents.
 
So what about edge stabiliy vs. wear resistance? I've heard it thrown around that edge stability beats wear resistance.
 
singularity35 said:
So what about edge stabiliy vs. wear resistance? I've heard it thrown around that edge stability beats wear resistance.
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If your talking about deformation I think thats true. Plasticity is a big consideration in some blades. The blade can always be sharpened, but if it deforms its hopeless. Again, its a trade off.
A stable edge has much to do with its geometry. If I make a kitchen knife and go to chose a steel for the purpose my first consideration is how thin I can grind it and how hard it can be without chipping out. Slicing veggies does not take an edge that has long wear so mush as it does stability and hardness. "Tradeoffs"
If its a camp knife I am more interested in wear resistance than stability because the grind will be thicker and more obtuse and carrying a softer edge. "Tradeoff"

Ultimately it comes to the job being done, selecting a steel that can be ground and heat treated to a condition that is applicable to that work. They are all important depending on the application. Steel is a pretty amazing product. :)
 
I was talking about edge holding since what I read was mostly taking about high carbide volume steels and carbide tear out. The low alloy steels like AEB-L and 8Cr13Mov supposedly outcut the high carbide volume steels when taken to sub 10° bevel angles. Not really sure what to make of those "tests".
 
singularity35 said:
I was talking about edge holding since what I read was mostly taking about high carbide volume steels and carbide tear out. The low alloy steels like AEB-L and 8Cr13Mov supposedly outcut the high carbide volume steels when taken to sub 10° bevel angles. Not really sure what to make of those "tests".
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I've not read those test but I can see where that might be the case. After you reach .8 carbon in a steel the carbides move to the boundary of the matrix, the steel is not harder there are just more carbides to work with. I believe thats correct and there not as stable. Of course tungsten carbides as well as vanadium carbides are harder in themselves. As a maker I like using the more basic carbide steels, I find them easier to predict. I know how a 1095 or W2 edge is going to react to work stresses and abuse where the more exotic steels I am not as sure of myself. I believe many knife makers are of this mind.
Wear is less important to me than the edge being hard yet ductile, supported by a tough blade body and spine. I can always sharpen the knife. :)

This is the reason I don't use stainless of any kind; it feels like a brick in my hand compared to a differentialy hardened basic carbon steel blade. The Japanese sword makers were of the same mind.

Fred
 
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Great thread, thanks for starting it! Everything is a tradeoff, there's no magic or "best" steel/heat treatment. Period. I would also add, geometry is what cuts. If people would start with thinking about what kind of cutting they want, they could then get the geometry that suits it, then the steel/heat treat that supports it. Too often, we see someone wanting to try a steel with no thought as to the application or intended use.
 
sodak said:
Great thread, thanks for starting it! Everything is a tradeoff, there's no magic or "best" steel/heat treatment. Period. I would also add, geometry is what cuts. If people would start with thinking about what kind of cutting they want, they could then get the geometry that suits it, then the steel/heat treat that supports it. Too often, we see someone wanting to try a steel with no thought as to the application or intended use.
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:thumbup: Its the best approach if you want to end up with the most serviceable tool for the job.
 
sodak said:
Great thread, thanks for starting it! Everything is a tradeoff, there's no magic or "best" steel/heat treatment. Period. I would also add, geometry is what cuts. If people would start with thinking about what kind of cutting they want, they could then get the geometry that suits it, then the steel/heat treat that supports it. Too often, we see someone wanting to try a steel with no thought as to the application or intended use.
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So what about slicing abrasive materials? I'd say thin geometry but what kind of steel?
 
Making this more ungracefully complex :p, I would add a few more variables: Slicing/cutting style; wedge/non-wedge & stretch/compress coefficients; fracturing vectors. Box cutter plain & bi-metal is thin but obviously it needs better steel & ht. Grain and carbide type, size, density and distribution are also important.
 
I think of steel as having three qualities: resistance to wear, resistance to deformation and resistance to breaking (such as chipping). Geometry, steel composition, blade construction (such as lamination) and heat treat can affect all three.

For a kitchen knife, I like a thin, hard steel that is far more likely to chip than roll and that holds an edge for a long time and is easy to resharpen. (Aogami Super Blue at 64 HRc).

For a chopper, I want a balance between chipping and rolling, with a bias toward resistance to chipping. I like a medium hard (HRc 60), differentially treated blade with the balance set forward.

For an EDC, I like a thin, hard blade that resists rolling a bit more than chipping.

But I don't see everything as being a tradeoff. A super heat treat can make a steel perform extremely well, enhancing the blade's ability to resist chipping, deformation and wear resistance. Some steels respond especially well to heat-treat-tweaking, such as 52100.

Likewise, a super steel can greatly boost performance in one area without a dropoff in others. Powder steels are a good example. And steels like S90V can greatly improve wear resistance over, say S30V, with only a little loss in toughness.

There are so many variables -- and so few published and highly controlled tests -- that most of our conversations have to be a mix of information, misinformation and gut feel. But some of the new steels really stand out, such as CPM-M4.
 
Just throwing this out there. While it is true - in general - that softer steel is tougher, in the hardness range used for knives, there are often peaks in toughness at quite high hardness. Hitting the peak is desirable, if the steel has one.
 
There is probable a better word to be used than soft; a relative term. Isn't it true that in a steel thats in a normalized state its hardness rating is affected by the temperature at which the piece is tempered? I do believe the steel can be more brittle or less brittle depending on the temp. at which its tempered.
 
I'm not 100% clear on what you mean there Fred. Steel that's been normalized isn't normally tempered directly afterward, at least not for knives. Steel that's been normalized will typically be stronger than steel that's been annealed, but lower toughness is not a guarantee for the normalized steel.

That steel is more or less brittle depending on tempering temperature is 100% correct. However, the relationship between tempering temperature, hardness, and toughness is not a smooth line from high hardness/low toughness to low hardness/high toughness. That's all I'm getting at. Controlling toughness by tempering temperature is a workable substitute when steel choice (specifically carbon content) is limited.
 
me2 said:
I'm not 100% clear on what you mean there Fred. Steel that's been normalized isn't normally tempered directly afterward, at least not for knives. Steel that's been normalized will typically be stronger than steel that's been annealed, but lower toughness is not a guarantee for the normalized steel.

That steel is more or less brittle depending on tempering temperature is 100% correct. However, the relationship between tempering temperature, hardness, and toughness is not a smooth line from high hardness/low toughness to low hardness/high toughness. That's all I'm getting at. Controlling toughness by tempering temperature is a workable substitute when steel choice (specifically carbon content) is limited.
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I make the clamps for the Bubble Jig out of 01 and have tweaked the heat treat to get the most out of it. In the process I came upon charts that showed the effects of the different tempering temps used. Some temps on the chart showed the steel was more brittle than at other temps along the curve. It was not a smooth curve as expected, the lower the temp the more brittle the steel, it was at different places on the curve that the steel was less stable or more brittle. At 550 fh it showed the steel was more brittle than if tempered at either 500 or 600 fh. There were other places on the graph that indicated this as well. I had always been under the impression that the higher the tempering temp the less brittle the steel while this showed that it was relative to the selected temp along the curve. These clamps are deferentially hardened, with the fingers being quenched, leaving the loop end in a normalized condition. When I switched from 550 temper to 450 the breakage stopped both along the hardened arms as well as the loop end. My thinking was that the hardened area of the clamp as well as the loop end had become less brittle or prone to breakage. Sorry if my terminology is incorrect.
What is your understanding on this?
I used to break a percentage of the clamps of 01 until I started cycling at 1225 for two hours then ramping to 1600, cool, heating to 1525 and cool heating to 1500 and quench in med speed oil; tempering at 450. No breakage since.
I stayed clear of the 550 tempering temp because of the brittleness at this temp.

Fred
 
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I was mostly pointing out the lack of a smooth curve for those that didnt know. I figured you (Fred) had found that information.

Normalized O1 could have some martensite that would benefit from the tempering changes you mentioned. Out of curiosity, why use O1 instead of something like 1040 or 1055?
 
me2 said:
I was mostly pointing out the lack of a smooth curve for those that didnt know. I figured you (Fred) had found that information.

Normalized O1 could have some martensite that would benefit from the tempering changes you mentioned. Out of curiosity, why use O1 instead of something like 1040 or 1055?
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I wish I could; 5160 or the like. I can't find machine ground in the size I need. We can't order enough to have it custom cut. The 01 is ready to process when we get it. Do you have a source for 9/32 X 1/2 X 36 inch?

Fred
 
Unfortunately, no. I figured you're sources were better than mine. It just seemed expensive for the purpose, but cheaper than a custom ground 10,000 lb load.
 
singularity35 said:
So what about slicing abrasive materials? I'd say thin geometry but what kind of steel?
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In general, carbides can help here, assuming no shock or huge side loads on the thin blade. You can slice a long time with CPM 10V or S90V.
 
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High hardness and high toughness are not mutually exclusive - unless you limit your steel choices.
 
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