9260 Spring Steel
Aug 27, 2009 18:41:17 GMT
Post by Deleted on Aug 27, 2009 18:41:17 GMT
To throw a comment into a topic where I am a complete neophyte (but which gives me a good excuse to break from studying my NATOPS):
I found [url
=http://zknives.com/knives/articles/knifesteelfaq.shtml]this web page[/url] to be very helpful when I first began investigating the whole steel blade business. Add that knowledge to what Mr. Salvati is providing about the tempering processes, which as I understand it can... draw forth one quality of a given steel at the expense of another, but can only work within the overall limits of that steel.
(Note on that site that some steels have various names and others are very close variations on a basic formulation. Others are newly named. Thus, for instance, 9260 does not appear on this page. I think I saw somewhere that 9260, 5260, and 5160 are all related to basic 1060, but I'm not sure. Perhaps Mr. Salvati can tell us.)
Finally, note that the site establishes and differentiates a number of terms related to a blade's resilience, including "strength," "toughness," and "edge-holding," all as separate qualities. While this is a vocabulary established within the context of that page and may not be accurate or shared throughout the industry, it goes a long way to demonstrating how many factors and stresses a smith must account for as he crafts a particular weapon.
With respect to your particular question, it might be a good idea to establish some basic definitions here as well: For our newbie purposes, through-hardened (TH) will mean that the blade is intended to have the same qualities of strength, toughness, hardness, etc. at all points on the surface and throughout the volume of the weapon. Differentially hardened shall mean that the blade is intended to have different qualities (within the overall limits of the steel, as mentioned before) in different regions of the sword, in a meaningful arrangement. Note that we use the word "intended" because, as Mr. Salvati says, just because you try to TH a blade doesn't mean you succeed in doing so. Any process can be screwed up.
If I were to guess, I'd say that the norm for most modern tools and weapons is TH, as aiming for a uniform result has to be simpler and less skill-intensive than aiming for a uniform distribution of results. In any case, numerous production, practical swords I have noticed are TH (including most of the Cheness line, for instance). The result of TH (if done correctly) is that the whole of the blade has one particular set of qualities.
For instance, a smith making a chef's knife might choose to bring out the steel's hardness potential, for a super-sharp, super-hard glass-like edge that breezes through tomatoes. The trade-off is that the steel's potential for strength and toughness are left behind. The resulting blade is brittle, and will chip, crack, or break if misused. (The extreme of this is Mr. Salvati's example of a blade that shatters like glass when dropped on a concrete floor.)
Alternately, a smith may try to bring out the steel's potential for strength and toughness, it's resistance to permanent damage like chips, cracks, breaks, and its ability to bend without taking a set. The trade-off is that the steel is now tempered to roll with the punches, and so its edge will roll or dent more easily as well. It won't hold its edge or resist rolling as well as a harder temper. An example is the Cheness 9260 TH blades which, if they come as advertised, are TH to optimize their forgiveness of mistakes, bad hits, bad targets, and so forth, but as a result may suffer cosmetic damage more easily and may have to be sharpened more often even after cutting something relatively soft like tatami mats.
(Again, all of this is subject to correction by Mr. Salvati and others. I'm speaking from my best academic learning to date, and nothing more. This becomes especially true in the following...)
What makes traditionally forged Katana swords so interesting to modern historians and collectors is that the Japanese smiths invented some unique methods which allowed them to improve the overall quality of the blade. (Among other things, folding over and over again was kind of like mixing. It promoted uniform distribution of the steel, in that local imperfections in the base stock were spread out until they effectively disappeared. In an era when smelting did not benefit from modern quality control technology, this method for ensuring the uniform quality of the blade steel would have been a remarkable improvement.) They also came up with methods for creating a more sophisticated blade, metallurgically. A more "advanced" blade, you might say. One of these is Differential Hardening. Instead of tempering the entire piece of steel uniformly, as with TH, the Japanese smiths would identify one region of the blade to temper with one set of qualities and another region that should display other qualities.
The core of the weapon, the majority of its mass from the spine toward the edge, they tempered for resilience, for strength against breaking, cracking, or taking a set. Near the edge, though, and down to the edge itself, they changed the temper, bringing out the steel's potential for hardness, which allows it to retain the shape of its edge and not deform as it passes through a material softer than the metal. (Since no energy is used in deforming the edge, all the energy is used in the mechanical work of separating the target's bits from one another.) The idea is that the resulting sword is strong like a spring steel blade (ok, maybe that's an exaggeration, as steel of that era was not what it is today, but you get the idea) but could slice like a light-saber through countless leather-armored or wooden-armored targets without losing its edge.
Now, once again we have to mention that a given smith may or may not be capable of doing this right. But assuming he is, the DH process in effect creates two products in one, as if you had magically conjoined a kitchen knife's edge to a spring steel body. (You can see the point of union as the "hamon.") Now, that does not mean that a DH blade can be abused like a TH beater blade. The edge is still tempered for hardness at the price of brittleness, and it will chip or crack if it strikes something it is not meant to strike, in a way it was not meant to strike it. What DH means is that you get the benefits of a hardened edge for as long as you can keep that edge intact, and when you do screw up and chip or crack the edge, it will be only the edge that takes the damage. The sword as a whole will not break in twain and leave you standing on the battlefield with a big, expensive chef's knife. The edge cracks, but the blade as a whole survives the mistake, and the rest of the edge is still ready to fight. So the traditional Katana DH doth not a super-beater make. Rather, it is a combat compromise, a combination of qualities meant to give you the best tool for battle, for the duration of a battle. Again, assuming it is properly done at all. There's nothing to say a really dumb smith might not get it wrong (or even completely backwards, if he really tried)!
Assuming all of the above is at least reasonably correct--Mr. Salvati will I hope fix it if it ain't--the question of what to do with 9260 can be more meaningfully addressed. The answer, of course, is just what you've already been given: "You can do whatever you want!" But as Mr. Salvati pointed out, any given temper will only be able to work within the potential of the steel. A given steel has a limited potential for good qualities and an almost infinite potential for bad qualities. If you temper for strength or toughness, you will only be able to bring out the maximum strength or toughness of that particular steel, while easily creating a sword that can't hold any edge at all if you aren't careful. If you temper for hardness and edge, you will only be able to bring out the maximum hardness of which that steel is capable, while easily creating a sword which will shatter, if you aren't careful.
Cheness uses "9260 spring steel" for the advertised purpose of creating blades which are through-hardened for maximum toughness and strength--as required in a "beater sword" subject to practice cutting with frequent mistakes and the use of bad targets--while still taking and holding an edge to a reasonable degree. The exception is their Kaze Katana, which they differentially harden. Presumably the overall body of the sword is tempered for toughness as in their other weapons, but the edge of the Kaze is hardened for superior edge, resulting hopefully in a better cutting weapon--but less forgiving, as hardness comes always at the price of brittleness. The edge of the Kaze will be harder and may take a sharper edge than Cheness's other swords and hold it better, but must be protected from poor form and bad targets, as it will chip more easily.
"...than Cheness's other swords" is another critical point (not the last, but we're getting close, I promise). We must remember that everything Cheness does is within the limits of the 9260 steel. You're always up against, "How far can I take the good qualities without driving the bad qualities too far?" While 9260 may be a good steel for tough blades within this site's price range, it probably does not have the potential that someone like Howard Clark can draw from his choice steels, 1086 and, particularly, L-6, with which he is purported to work near miracles, if you can pay for 'em.
Now, the last point: we have not discussed blade geometry at all in this post. Given a particular steel at a particular hardness, you will introduce another spectrum of performance according to the shape you give it. It's not the subject of this thread, and I don't know enough to get into it, beyond simply noting a few things:
First, more acute edge angles will be more mechanically efficient in cutting, but will be less mechanically self-reinforcing, and thus more likely to degrade in one way or another. That is to say, acute angles are better slicers but more delicate, by and large. (There are always other considerations. Keep reading.)
Second, practical sharpness, as we observe it, is a combination of the actual refinement of the edge (the point to which it tapers) and the overall angle of the various bevels (the rate at which it tapers). You may have seen one of those electron microscopy scans of a needle tip which reveals it to be much more blunt than it appears to the human eye. How close you can get to a perfect edge at a given angle is dependent on the metal's properties at a particulate level and is the purview of an expert metallurgist, but suffice to say... it varies. The ideal would be a material that you could convince to taper down to a single atom at the tip--if you could make such a thing that wasn't impossibly delicate. Having selected a metal and hardness which gets you as close to this as is practically possible, you must select an angle that gives it the best balance of mechanical efficiency and mechanical reinforcement, with angles as wide as 40 degrees being used for maximum resistance to degredation, and as thin as a few degrees being used for maximum slicing power. Straight bevels are the easiest to apply and are most common. Concave bevels (where the angle is very acute at the edge and the surfaces curve away from one another toward the back of the blade) are used to achieve the finest taper in razors and similar tools, and are most delicate. Convex bevels are, as I understand it, the shape of choice for traditional Katanas. A convex bevel (the "appleseed" cross-section) starts out with a wide angle at the tip which quickly curves to parallel.
***
Aside:
If you have a metal and hardness at the edge that can taper to a very refined point, then this seems to me a fairly clever solution. It provides as much mass as possible near the edge, to improve reinforcement at the micro level, while quickly and smoothly reducing the wedge angle to improve mechanical advantage at the macro level. Most soft, organic materials have a somewhat elastic behavior. Human skin and leather (which is made from skin) are excellent examples. Slice either with a razor, and you will observe that the elastic action in the surrounding surface pulls the slit open somewhat. One needs only the refined edge to make the cut in the material; one does not need an acute bevel following that edge to force the material apart. (This is as opposed to solid, compressed materials like a tree, where the weight and structure of the tree does nothing but press in on your blade at all levels.) Once the wide-angled edge has opened the target, the rest of the Katana follows, quickly introducing a more acute wedge for parting flesh at the macro level where it becomes advantageous. When the blade encounters something that it must splinter or crush rather than properly cut (such as a bone), it has the mass at the edge and the refinement of edge to apply force in a strong but focused way, much as a policeman's window-breaker shatters a window.
This profile, if correctly fashioned, could easily be deceptive. It would not feel sharp in the way that a razor feels sharp, due to the wide initial angle, but the refinement of the edge (the degree of taper, not the rate of taper) would permit it to easily open a fingertip with an errant stroke. Once the flesh is open, the rest of the blade can fall in and continue to spread the cut material, using that portion of the blade where the rate of taper is more efficient.
***
So, given all of this as I presently understand it, how would I proceed in selection of a blade? At this time, I have no training in swordsmanship. I would begin with a wooden sword (a shinai or waster, depending on your tradition) and a good teacher, to learn the fundamentals of good cutting and fencing technique. I would then get myself a sub-$300 beater, probably a through-hardened production sword from a maker highly rated on this and other forums and on Mr. Southren's websites. The Cheness TH 9260 blades are one possibility. I would use this to cut stuff, accepting that it may take cosmetic damage and it may require regular sharpening, until I had developed the skill to deliver clean cuts with proper technique even under unusual or unideal circumstances, such as when surprised or off balance. Then I would upgrade to a differentially hardened blade of higher quality which I would treat with greater respect but would continue to use in practice, further refining the efficiency and precision of my technique, accepting the occasional mistake which might result in a chip or crack.
When I reached the point where my skills merited such a weapon and could maximize its advantages, I would secure multi-thousand-dollar weapon (probably something along the lines of Howard Clark's L-6 blades, as I am a fan of fusing modern learning with time-tested wisdom) as my ultimate practical sword, with which I would continue to master the art of sword-play and fantasize about being a Samurai.
***
Notes:
I used the words "harden" and "temper" interchangeably in this post, which is not technically correct. I'm pretty sure these are separate steps in the overall process with which we are here concerned, of producing the final qualities we desire in our given piece of steel.
Everything above is subject to correction by more knowledgeable folk, such as professional smiths/metallurgists like Mr. Salvati. I look forward to reading their responses, to see how close I am and to learn a little more, as always!
A question for such experts: In the region of the hamon, is the harder metal extant throughout the thickness of the blade, or does it form as a surface layer around a core of the softer metal?
"Why did my 9260 take a set? I thought it could bend 90 degrees!" Note that the ability of a piece of metal to bend without setting is not really measured in angle but in radius of curvature. Radius of curvature is the radius of a circle whose circumference matches the bend you're talking about. It's a combination of the angle of the bend across the length of steel being bent. In visualizing this, an absurd example may be helpful: it is easier to imagine a seven-mile-long piece of wire bending to 90 degrees across its length and then returning to straight than it is to picture a seven inch knife blade doing the same. Why does this matter? Because when you make a bad hit on an object, the forces involved will attempt to bend your sword around that object, as if you had laid it over the sharp edge of an anvil and struck it with a hammer. You are inducing not a smooth bend along the whole length of the sword but rather a sharp bend along maybe an inch of metal. It may not be many degrees, but applying that bend in such a short length gives you a small radius of curvature and is the equivalent of twisting the whole length of the sword into a pretzel. So what of the tests? If you take two swords of equal length, clamp them at the hilt and tip, and bend, the you have a valid test. The one that bends further without taking a set is going to be, by and large, the one that will do better in use without taking a set. Note also that while one may take a set more easily, it may require more force to bend, which might be just as good. (In this test, you are testing not only their respective metals, but also their geometry, mass, etc, which affect both their resistance to bending and their resistance to setting under a bend.) Do TH and DH affect this? My guess would be (and this is a guess) that one could not make a direct connection, because the ability of a blade to resist bending or bending damage would depend on the material, shape, and treatment of the overall blade, regardless of how the edge was or was not differently treated.
Finally: Good grief. Good thing this site has a liberal per-post character limit...
I found [url
=http://zknives.com/knives/articles/knifesteelfaq.shtml]this web page[/url] to be very helpful when I first began investigating the whole steel blade business. Add that knowledge to what Mr. Salvati is providing about the tempering processes, which as I understand it can... draw forth one quality of a given steel at the expense of another, but can only work within the overall limits of that steel.
(Note on that site that some steels have various names and others are very close variations on a basic formulation. Others are newly named. Thus, for instance, 9260 does not appear on this page. I think I saw somewhere that 9260, 5260, and 5160 are all related to basic 1060, but I'm not sure. Perhaps Mr. Salvati can tell us.)
Finally, note that the site establishes and differentiates a number of terms related to a blade's resilience, including "strength," "toughness," and "edge-holding," all as separate qualities. While this is a vocabulary established within the context of that page and may not be accurate or shared throughout the industry, it goes a long way to demonstrating how many factors and stresses a smith must account for as he crafts a particular weapon.
With respect to your particular question, it might be a good idea to establish some basic definitions here as well: For our newbie purposes, through-hardened (TH) will mean that the blade is intended to have the same qualities of strength, toughness, hardness, etc. at all points on the surface and throughout the volume of the weapon. Differentially hardened shall mean that the blade is intended to have different qualities (within the overall limits of the steel, as mentioned before) in different regions of the sword, in a meaningful arrangement. Note that we use the word "intended" because, as Mr. Salvati says, just because you try to TH a blade doesn't mean you succeed in doing so. Any process can be screwed up.
If I were to guess, I'd say that the norm for most modern tools and weapons is TH, as aiming for a uniform result has to be simpler and less skill-intensive than aiming for a uniform distribution of results. In any case, numerous production, practical swords I have noticed are TH (including most of the Cheness line, for instance). The result of TH (if done correctly) is that the whole of the blade has one particular set of qualities.
For instance, a smith making a chef's knife might choose to bring out the steel's hardness potential, for a super-sharp, super-hard glass-like edge that breezes through tomatoes. The trade-off is that the steel's potential for strength and toughness are left behind. The resulting blade is brittle, and will chip, crack, or break if misused. (The extreme of this is Mr. Salvati's example of a blade that shatters like glass when dropped on a concrete floor.)
Alternately, a smith may try to bring out the steel's potential for strength and toughness, it's resistance to permanent damage like chips, cracks, breaks, and its ability to bend without taking a set. The trade-off is that the steel is now tempered to roll with the punches, and so its edge will roll or dent more easily as well. It won't hold its edge or resist rolling as well as a harder temper. An example is the Cheness 9260 TH blades which, if they come as advertised, are TH to optimize their forgiveness of mistakes, bad hits, bad targets, and so forth, but as a result may suffer cosmetic damage more easily and may have to be sharpened more often even after cutting something relatively soft like tatami mats.
(Again, all of this is subject to correction by Mr. Salvati and others. I'm speaking from my best academic learning to date, and nothing more. This becomes especially true in the following...)
What makes traditionally forged Katana swords so interesting to modern historians and collectors is that the Japanese smiths invented some unique methods which allowed them to improve the overall quality of the blade. (Among other things, folding over and over again was kind of like mixing. It promoted uniform distribution of the steel, in that local imperfections in the base stock were spread out until they effectively disappeared. In an era when smelting did not benefit from modern quality control technology, this method for ensuring the uniform quality of the blade steel would have been a remarkable improvement.) They also came up with methods for creating a more sophisticated blade, metallurgically. A more "advanced" blade, you might say. One of these is Differential Hardening. Instead of tempering the entire piece of steel uniformly, as with TH, the Japanese smiths would identify one region of the blade to temper with one set of qualities and another region that should display other qualities.
The core of the weapon, the majority of its mass from the spine toward the edge, they tempered for resilience, for strength against breaking, cracking, or taking a set. Near the edge, though, and down to the edge itself, they changed the temper, bringing out the steel's potential for hardness, which allows it to retain the shape of its edge and not deform as it passes through a material softer than the metal. (Since no energy is used in deforming the edge, all the energy is used in the mechanical work of separating the target's bits from one another.) The idea is that the resulting sword is strong like a spring steel blade (ok, maybe that's an exaggeration, as steel of that era was not what it is today, but you get the idea) but could slice like a light-saber through countless leather-armored or wooden-armored targets without losing its edge.
Now, once again we have to mention that a given smith may or may not be capable of doing this right. But assuming he is, the DH process in effect creates two products in one, as if you had magically conjoined a kitchen knife's edge to a spring steel body. (You can see the point of union as the "hamon.") Now, that does not mean that a DH blade can be abused like a TH beater blade. The edge is still tempered for hardness at the price of brittleness, and it will chip or crack if it strikes something it is not meant to strike, in a way it was not meant to strike it. What DH means is that you get the benefits of a hardened edge for as long as you can keep that edge intact, and when you do screw up and chip or crack the edge, it will be only the edge that takes the damage. The sword as a whole will not break in twain and leave you standing on the battlefield with a big, expensive chef's knife. The edge cracks, but the blade as a whole survives the mistake, and the rest of the edge is still ready to fight. So the traditional Katana DH doth not a super-beater make. Rather, it is a combat compromise, a combination of qualities meant to give you the best tool for battle, for the duration of a battle. Again, assuming it is properly done at all. There's nothing to say a really dumb smith might not get it wrong (or even completely backwards, if he really tried)!
Assuming all of the above is at least reasonably correct--Mr. Salvati will I hope fix it if it ain't--the question of what to do with 9260 can be more meaningfully addressed. The answer, of course, is just what you've already been given: "You can do whatever you want!" But as Mr. Salvati pointed out, any given temper will only be able to work within the potential of the steel. A given steel has a limited potential for good qualities and an almost infinite potential for bad qualities. If you temper for strength or toughness, you will only be able to bring out the maximum strength or toughness of that particular steel, while easily creating a sword that can't hold any edge at all if you aren't careful. If you temper for hardness and edge, you will only be able to bring out the maximum hardness of which that steel is capable, while easily creating a sword which will shatter, if you aren't careful.
Cheness uses "9260 spring steel" for the advertised purpose of creating blades which are through-hardened for maximum toughness and strength--as required in a "beater sword" subject to practice cutting with frequent mistakes and the use of bad targets--while still taking and holding an edge to a reasonable degree. The exception is their Kaze Katana, which they differentially harden. Presumably the overall body of the sword is tempered for toughness as in their other weapons, but the edge of the Kaze is hardened for superior edge, resulting hopefully in a better cutting weapon--but less forgiving, as hardness comes always at the price of brittleness. The edge of the Kaze will be harder and may take a sharper edge than Cheness's other swords and hold it better, but must be protected from poor form and bad targets, as it will chip more easily.
"...than Cheness's other swords" is another critical point (not the last, but we're getting close, I promise). We must remember that everything Cheness does is within the limits of the 9260 steel. You're always up against, "How far can I take the good qualities without driving the bad qualities too far?" While 9260 may be a good steel for tough blades within this site's price range, it probably does not have the potential that someone like Howard Clark can draw from his choice steels, 1086 and, particularly, L-6, with which he is purported to work near miracles, if you can pay for 'em.
Now, the last point: we have not discussed blade geometry at all in this post. Given a particular steel at a particular hardness, you will introduce another spectrum of performance according to the shape you give it. It's not the subject of this thread, and I don't know enough to get into it, beyond simply noting a few things:
First, more acute edge angles will be more mechanically efficient in cutting, but will be less mechanically self-reinforcing, and thus more likely to degrade in one way or another. That is to say, acute angles are better slicers but more delicate, by and large. (There are always other considerations. Keep reading.)
Second, practical sharpness, as we observe it, is a combination of the actual refinement of the edge (the point to which it tapers) and the overall angle of the various bevels (the rate at which it tapers). You may have seen one of those electron microscopy scans of a needle tip which reveals it to be much more blunt than it appears to the human eye. How close you can get to a perfect edge at a given angle is dependent on the metal's properties at a particulate level and is the purview of an expert metallurgist, but suffice to say... it varies. The ideal would be a material that you could convince to taper down to a single atom at the tip--if you could make such a thing that wasn't impossibly delicate. Having selected a metal and hardness which gets you as close to this as is practically possible, you must select an angle that gives it the best balance of mechanical efficiency and mechanical reinforcement, with angles as wide as 40 degrees being used for maximum resistance to degredation, and as thin as a few degrees being used for maximum slicing power. Straight bevels are the easiest to apply and are most common. Concave bevels (where the angle is very acute at the edge and the surfaces curve away from one another toward the back of the blade) are used to achieve the finest taper in razors and similar tools, and are most delicate. Convex bevels are, as I understand it, the shape of choice for traditional Katanas. A convex bevel (the "appleseed" cross-section) starts out with a wide angle at the tip which quickly curves to parallel.
***
Aside:
If you have a metal and hardness at the edge that can taper to a very refined point, then this seems to me a fairly clever solution. It provides as much mass as possible near the edge, to improve reinforcement at the micro level, while quickly and smoothly reducing the wedge angle to improve mechanical advantage at the macro level. Most soft, organic materials have a somewhat elastic behavior. Human skin and leather (which is made from skin) are excellent examples. Slice either with a razor, and you will observe that the elastic action in the surrounding surface pulls the slit open somewhat. One needs only the refined edge to make the cut in the material; one does not need an acute bevel following that edge to force the material apart. (This is as opposed to solid, compressed materials like a tree, where the weight and structure of the tree does nothing but press in on your blade at all levels.) Once the wide-angled edge has opened the target, the rest of the Katana follows, quickly introducing a more acute wedge for parting flesh at the macro level where it becomes advantageous. When the blade encounters something that it must splinter or crush rather than properly cut (such as a bone), it has the mass at the edge and the refinement of edge to apply force in a strong but focused way, much as a policeman's window-breaker shatters a window.
This profile, if correctly fashioned, could easily be deceptive. It would not feel sharp in the way that a razor feels sharp, due to the wide initial angle, but the refinement of the edge (the degree of taper, not the rate of taper) would permit it to easily open a fingertip with an errant stroke. Once the flesh is open, the rest of the blade can fall in and continue to spread the cut material, using that portion of the blade where the rate of taper is more efficient.
***
So, given all of this as I presently understand it, how would I proceed in selection of a blade? At this time, I have no training in swordsmanship. I would begin with a wooden sword (a shinai or waster, depending on your tradition) and a good teacher, to learn the fundamentals of good cutting and fencing technique. I would then get myself a sub-$300 beater, probably a through-hardened production sword from a maker highly rated on this and other forums and on Mr. Southren's websites. The Cheness TH 9260 blades are one possibility. I would use this to cut stuff, accepting that it may take cosmetic damage and it may require regular sharpening, until I had developed the skill to deliver clean cuts with proper technique even under unusual or unideal circumstances, such as when surprised or off balance. Then I would upgrade to a differentially hardened blade of higher quality which I would treat with greater respect but would continue to use in practice, further refining the efficiency and precision of my technique, accepting the occasional mistake which might result in a chip or crack.
When I reached the point where my skills merited such a weapon and could maximize its advantages, I would secure multi-thousand-dollar weapon (probably something along the lines of Howard Clark's L-6 blades, as I am a fan of fusing modern learning with time-tested wisdom) as my ultimate practical sword, with which I would continue to master the art of sword-play and fantasize about being a Samurai.
***
Notes:
I used the words "harden" and "temper" interchangeably in this post, which is not technically correct. I'm pretty sure these are separate steps in the overall process with which we are here concerned, of producing the final qualities we desire in our given piece of steel.
Everything above is subject to correction by more knowledgeable folk, such as professional smiths/metallurgists like Mr. Salvati. I look forward to reading their responses, to see how close I am and to learn a little more, as always!
A question for such experts: In the region of the hamon, is the harder metal extant throughout the thickness of the blade, or does it form as a surface layer around a core of the softer metal?
"Why did my 9260 take a set? I thought it could bend 90 degrees!" Note that the ability of a piece of metal to bend without setting is not really measured in angle but in radius of curvature. Radius of curvature is the radius of a circle whose circumference matches the bend you're talking about. It's a combination of the angle of the bend across the length of steel being bent. In visualizing this, an absurd example may be helpful: it is easier to imagine a seven-mile-long piece of wire bending to 90 degrees across its length and then returning to straight than it is to picture a seven inch knife blade doing the same. Why does this matter? Because when you make a bad hit on an object, the forces involved will attempt to bend your sword around that object, as if you had laid it over the sharp edge of an anvil and struck it with a hammer. You are inducing not a smooth bend along the whole length of the sword but rather a sharp bend along maybe an inch of metal. It may not be many degrees, but applying that bend in such a short length gives you a small radius of curvature and is the equivalent of twisting the whole length of the sword into a pretzel. So what of the tests? If you take two swords of equal length, clamp them at the hilt and tip, and bend, the you have a valid test. The one that bends further without taking a set is going to be, by and large, the one that will do better in use without taking a set. Note also that while one may take a set more easily, it may require more force to bend, which might be just as good. (In this test, you are testing not only their respective metals, but also their geometry, mass, etc, which affect both their resistance to bending and their resistance to setting under a bend.) Do TH and DH affect this? My guess would be (and this is a guess) that one could not make a direct connection, because the ability of a blade to resist bending or bending damage would depend on the material, shape, and treatment of the overall blade, regardless of how the edge was or was not differently treated.
Finally: Good grief. Good thing this site has a liberal per-post character limit...