Kevin R. Cashen
Super Moderator
Get your cup of coffee, this will take a bit. 
I have been working on article and video ideas and wanted to see if this one could be of interest.
When I began learning about the fascinating effects of heat treatments on the properties of steel, I fell into the mindset that the three most important factors in a quality blade was – 1, heat treatment, 2. Heat treatment and, most of all, 3. Heat treatment. That was many decades ago and I now realize how wrong I was.
When I teach, I now emphasize what I call the quality blade triangle, in which the three critical things in creating a quality blade are- 1. Proper steel selection, 2. Proper geometry and 3. Proper heat treatment. None of these three can totally preplace the other as they are all completely interdependent, and if one gets out of synch with the others blade quality will suffer.
To illustrate that, I will share a story from a study I did for one of my regular corporate clients, that I consult for in another industry that deals with cutting implements. The client was faced with a decision of using blades from two different manufacturers and my job was to recommend which manufacturer would provide the blades that would hold up the best. After a large array of metallurgical evaluation and physical testing, I was faced with a lesser of two evils scenario. One manufacturer had their grinding and machining dialed right in for a very consistent geometry, but the heat treatment was one of the most appalling I had ever seen; the steel had been all but cooked alive with overheating. The other had nailed the heat treatment, fine grain, very homogeneous structures with excellent carbide distribution. But the edge geometry was all over the map, very inconsistent, with insufficient edge support yet a lot of resistance in the cut.
My recommended best solution was to get the good heat treaters to fix their grinding issues. But, short of that, I recommended the blade with the terrible heat treatment, because it had out-performed the other hands down. The folks with the terrible heat treatment understood the importance of geometry in cutting and overall edge stability and were able to adjust it to compensate for the failed heat treatment. The supplier with the great heat treatment didn’t understand that it was totally wasted on a blade that was ground to fail in use. That is how interdependent steel selection, heat treatment and geometry can be.
People often ask what is the best hardness for a given knife, and that is really a trick question, sort of like asking how long is a piece of string. The hardness of your edge will have to be tailored to the edge geometry, and the edge geometry will need to properly address the primary task of the blade. This is also were steel selection comes in. Each alloy will have a sweet spot in its maximum hardness (strength) for maximum toughness, with a steel like O-1, that spot will be up between 62-63 HRC, but for another steel it may be down around 59 HRC. If you find yourself lowering the HRC below this sweet spot in order to force the steel to work for your application you have missed the mark in steel selection. Examples of this would be O-1 or 52100 for a machete, or 5160 for a skinning knife. These are mismatches in steel for application can leave you forever short of the quality blade you are seeking.
There is no one geometry that works. Chopping cleaving blades require an entirely different edge shape than fine slicing blades. The physics of straight-line forceful penetration with wedging actions is entirely different than fine, draw cut, slicing. The edges will have to be designed with the strength, abrasion resistance and toughness the actions require. To this end, the heat treatment applied to the appropriate steel will need to address the geometry used. With the wrong geometry, the heat treatment will have to pick up the slack. And, if the wrong steel is selected, both the geometry and heat treatment may need to be compromised to deal with it. Picking the best in all three results in that knife we all hope to achieve.
One lingering myth that is every bit as pernicious as edge packing or quenching by the compass, is the idea of softening a blade for “sharpenability”. In fact, it is perhaps worse than the other myths because it causes us to make lesser blades rather than addressing the true problem; after all, you may still have a good blade after quenching an edge packed blade to true north, but not if you intentionally over-temper it. So much so that my radar always lights up red when I hear the term “sharpenability” when discussing heat treatment, which should be synonymous with “caveat emptor” when looking for a quality blade that will hold its edge.
While the hardness level of the blade will create a different feel when sharpening, as it resists the abrasion action, that same resistance is also what allows the blade to maintain that same edge in use. This is a logical follow through that should be self-evident, but seems to be lost with many folks.
It is a simple axiom that to cut (abrade) one material with another, the cutter/abrasive must have a higher strength than the material cut, the greater this discrepancy, the longer the action can be sustained. But there is another very important factor when dealing with that action- the surface area involved in the action itself.
The finer the knife edge, the easier it will penetrate a softer material, and the easier the cut, but such a fine cross section must have supporting strength or it will simply roll or abrade during the cut. Do the edges of your grinding belt cut faster than the middle? Of course they do, because of the reduced surface area of the cutting action.
For even better clarity let’s apply that to blade finish. You have two blades, both at 60HRC, but one is 2” wide while the other is only 1” wide. Even with simple aluminum oxide, which one will you be able to polish all the scratches out of quicker? Obviously, the narrower blade. Why? Because, at equal hardness, the less surface area, the less work. Now yes, if this was file work, different hardness levels would play a larger role because we are playing with differences of less than 2-3 points of hardness, but most abrasive particles are measured on something more like the Mohs scale, because they are orders of magnitude harder than steel can get.
It is because of this that almost every case of poor “sharpenability” that I have seen was a matter of poor geometry, and not a heat treatment issue at all. Even if it was, we have diamond hones- and nobody can heat treat steel that well! My favorite kitchen knife, that I made for myself, approaches 64 HRC at the edge, but it doesn’t take any longer to sharpen than a 59 HRC camp knife. This is because the edge geometry is a very fine one that matches the heat treatment and is designed for its purpose.
I can walk through a knife show and spot knives that have poor “sharpenability”, without even picking them up. They will have a very wide, and highly visible edge line cut on them by the stone as it tediously wore away the unnecessary material of an overly thick grind. The irony of this is often that the edge is left so thick because the maker is trying to compensate for lower Rockwell.
An unavoidable fact is that the edge has to have strength to hold up, it can get it through hardness or, it can get it through cross section. Most knifemakers who worry about “sharpenability” may be decent heat treaters but they may need more contemplation on geometry, and simply fixing their grinds would solve the problem. The Catch-22 is that if you soften your knives for “sharpenability”, it will probably necessitate a thicker geometry to make up for the loss of strength. For leaving the edges at the same thickness, while sapping their strength with greater tempering temperatures, would most likely reveal the flaw in this approach when they quickly dulled or rolled in use.
There are certain factory knives that I get requested to sharpen more than others, most have a really good heat treatment but are impossible to sharpen because of the atrocious high-angle edges that are machine cut onto them. I will always ask the owner if they would like me to sharpen the knife so that they will need to bring it back to me, or would they like me to sharpen it so they can do it themselves the next time. If they want to bring it back, I will try to match that horrible angle on the stone. But, if they allow me to, I would prefer to step over to the grinder and reset the geometry so they can sharpen it, and on better knives the HRC is high enough that the finer edge will work even better. I charge the same because, it doesn’t take much longer to entirely rework the grind, than to try to sharpen that cold chisel masquerading as a knife.
In conclusion, I hope the main take way of this screed is that, in making a quality blade, one simply cannot dismiss the equally critical contributions of steel selection, heat treatment and geometry. And the need to adjust them so that they complement each other is the essence of the art of “cut”.
I have been working on article and video ideas and wanted to see if this one could be of interest.When I began learning about the fascinating effects of heat treatments on the properties of steel, I fell into the mindset that the three most important factors in a quality blade was – 1, heat treatment, 2. Heat treatment and, most of all, 3. Heat treatment. That was many decades ago and I now realize how wrong I was.
When I teach, I now emphasize what I call the quality blade triangle, in which the three critical things in creating a quality blade are- 1. Proper steel selection, 2. Proper geometry and 3. Proper heat treatment. None of these three can totally preplace the other as they are all completely interdependent, and if one gets out of synch with the others blade quality will suffer.
To illustrate that, I will share a story from a study I did for one of my regular corporate clients, that I consult for in another industry that deals with cutting implements. The client was faced with a decision of using blades from two different manufacturers and my job was to recommend which manufacturer would provide the blades that would hold up the best. After a large array of metallurgical evaluation and physical testing, I was faced with a lesser of two evils scenario. One manufacturer had their grinding and machining dialed right in for a very consistent geometry, but the heat treatment was one of the most appalling I had ever seen; the steel had been all but cooked alive with overheating. The other had nailed the heat treatment, fine grain, very homogeneous structures with excellent carbide distribution. But the edge geometry was all over the map, very inconsistent, with insufficient edge support yet a lot of resistance in the cut.
My recommended best solution was to get the good heat treaters to fix their grinding issues. But, short of that, I recommended the blade with the terrible heat treatment, because it had out-performed the other hands down. The folks with the terrible heat treatment understood the importance of geometry in cutting and overall edge stability and were able to adjust it to compensate for the failed heat treatment. The supplier with the great heat treatment didn’t understand that it was totally wasted on a blade that was ground to fail in use. That is how interdependent steel selection, heat treatment and geometry can be.
People often ask what is the best hardness for a given knife, and that is really a trick question, sort of like asking how long is a piece of string. The hardness of your edge will have to be tailored to the edge geometry, and the edge geometry will need to properly address the primary task of the blade. This is also were steel selection comes in. Each alloy will have a sweet spot in its maximum hardness (strength) for maximum toughness, with a steel like O-1, that spot will be up between 62-63 HRC, but for another steel it may be down around 59 HRC. If you find yourself lowering the HRC below this sweet spot in order to force the steel to work for your application you have missed the mark in steel selection. Examples of this would be O-1 or 52100 for a machete, or 5160 for a skinning knife. These are mismatches in steel for application can leave you forever short of the quality blade you are seeking.
There is no one geometry that works. Chopping cleaving blades require an entirely different edge shape than fine slicing blades. The physics of straight-line forceful penetration with wedging actions is entirely different than fine, draw cut, slicing. The edges will have to be designed with the strength, abrasion resistance and toughness the actions require. To this end, the heat treatment applied to the appropriate steel will need to address the geometry used. With the wrong geometry, the heat treatment will have to pick up the slack. And, if the wrong steel is selected, both the geometry and heat treatment may need to be compromised to deal with it. Picking the best in all three results in that knife we all hope to achieve.
One lingering myth that is every bit as pernicious as edge packing or quenching by the compass, is the idea of softening a blade for “sharpenability”. In fact, it is perhaps worse than the other myths because it causes us to make lesser blades rather than addressing the true problem; after all, you may still have a good blade after quenching an edge packed blade to true north, but not if you intentionally over-temper it. So much so that my radar always lights up red when I hear the term “sharpenability” when discussing heat treatment, which should be synonymous with “caveat emptor” when looking for a quality blade that will hold its edge.
While the hardness level of the blade will create a different feel when sharpening, as it resists the abrasion action, that same resistance is also what allows the blade to maintain that same edge in use. This is a logical follow through that should be self-evident, but seems to be lost with many folks.
It is a simple axiom that to cut (abrade) one material with another, the cutter/abrasive must have a higher strength than the material cut, the greater this discrepancy, the longer the action can be sustained. But there is another very important factor when dealing with that action- the surface area involved in the action itself.
The finer the knife edge, the easier it will penetrate a softer material, and the easier the cut, but such a fine cross section must have supporting strength or it will simply roll or abrade during the cut. Do the edges of your grinding belt cut faster than the middle? Of course they do, because of the reduced surface area of the cutting action.
For even better clarity let’s apply that to blade finish. You have two blades, both at 60HRC, but one is 2” wide while the other is only 1” wide. Even with simple aluminum oxide, which one will you be able to polish all the scratches out of quicker? Obviously, the narrower blade. Why? Because, at equal hardness, the less surface area, the less work. Now yes, if this was file work, different hardness levels would play a larger role because we are playing with differences of less than 2-3 points of hardness, but most abrasive particles are measured on something more like the Mohs scale, because they are orders of magnitude harder than steel can get.
It is because of this that almost every case of poor “sharpenability” that I have seen was a matter of poor geometry, and not a heat treatment issue at all. Even if it was, we have diamond hones- and nobody can heat treat steel that well! My favorite kitchen knife, that I made for myself, approaches 64 HRC at the edge, but it doesn’t take any longer to sharpen than a 59 HRC camp knife. This is because the edge geometry is a very fine one that matches the heat treatment and is designed for its purpose.
I can walk through a knife show and spot knives that have poor “sharpenability”, without even picking them up. They will have a very wide, and highly visible edge line cut on them by the stone as it tediously wore away the unnecessary material of an overly thick grind. The irony of this is often that the edge is left so thick because the maker is trying to compensate for lower Rockwell.
An unavoidable fact is that the edge has to have strength to hold up, it can get it through hardness or, it can get it through cross section. Most knifemakers who worry about “sharpenability” may be decent heat treaters but they may need more contemplation on geometry, and simply fixing their grinds would solve the problem. The Catch-22 is that if you soften your knives for “sharpenability”, it will probably necessitate a thicker geometry to make up for the loss of strength. For leaving the edges at the same thickness, while sapping their strength with greater tempering temperatures, would most likely reveal the flaw in this approach when they quickly dulled or rolled in use.
There are certain factory knives that I get requested to sharpen more than others, most have a really good heat treatment but are impossible to sharpen because of the atrocious high-angle edges that are machine cut onto them. I will always ask the owner if they would like me to sharpen the knife so that they will need to bring it back to me, or would they like me to sharpen it so they can do it themselves the next time. If they want to bring it back, I will try to match that horrible angle on the stone. But, if they allow me to, I would prefer to step over to the grinder and reset the geometry so they can sharpen it, and on better knives the HRC is high enough that the finer edge will work even better. I charge the same because, it doesn’t take much longer to entirely rework the grind, than to try to sharpen that cold chisel masquerading as a knife.
In conclusion, I hope the main take way of this screed is that, in making a quality blade, one simply cannot dismiss the equally critical contributions of steel selection, heat treatment and geometry. And the need to adjust them so that they complement each other is the essence of the art of “cut”.
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