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Response To:
Thanks, Michael! ()

Michael Bulatowicz
TL:DR I think, without specific experimental data to back up this assertion, that microstructure (at and below what you can see with an optical microscope) is more important than macroscopic properties in terms of what you seem to be looking for in extreme edge taking/holding and corner robustness.

As I mentioned, I am not a metallurgist (nor do I have specific references to provide off the top of my head); the following is an argument cobbled together from what I do know and that I hope might provide some insight. I don't have any definitive answers, as my knowledge is limited to the academic on much of this (I haven't experimentally investigated most of this myself), but I hope this is helpful in getting a feel for what's going on with different alloys. I'll also be speaking in generalities, which I hope doesn't add too much confusion.

For extreme edge taking/holding and corner retention, I'd argue that microstructure is more important than macroscopic properties. Macroscopic properties are important, too, but don't tell the whole story (as you and many others have observed). While the macroscopic properties such as hardness, yield strength, and toughness can provide some insight into the microstructure of the steel, such insight is rather limited. For example, large carbides, and the presence of cementite as opposed to pearlite or solution carbon, for example, can significantly impact harness measurements by impeding plastic deformation of the material caused by the indenter--indenter geometry can play a significant role there, too.

So, on to microstructure and how I think it might be relevant to edge/corner taking/holding.

As you're aware, steel as it cools tends to organize into a multitude of crystals (grains). Each of these crystals will be randomly oriented with respect to the others and will exhibit "principal crystalline planes" with different properties of stiffness, strength, and so on along each direction. The randomized orientation means that the bulk material exhibits the average of these along any direction, and so has relatively uniform properties.

These crystals will not be perfect; one type of crystalline defect is a dislocation (where the atoms in the lattice don't quite line up in an orderly fashion). For a 2D visualization, think of a honeycomb; it's an orderly lattice with a hexagonal structure. A dislocation would consist of either a missing cell (with the adjacent cells kind of shoving their way in toward the empty spot) or an extra cell (shoving the adjacent cells away). Why does this matter? These dislocations are weak points in the crystal that allow motion of the atoms under some kind of load (i.e. deformation of the steel). There will be many dislocations at the grain boundaries--where one grain meets another--and so even in a pure material the grain boundaries tend to be the points at which the material is most easily deformed/broken. Dislocation motion is a major source of "yield" of the material (the stress level at which the material will flow instead of just stretching or compressing).

As an aside, the difference between the yield stress and the proportional limit is that the yield stress is defined as the point at which, when the stress is removed, the material has permanently deformed by 0.2%. The proportional limit is less well defined--for some materials, it simply corresponds to 0% permanent (plastic) deformation upon removal of the stress, but to how many digits of precision? For others, it's the point at which the spring rate goes nonlinear (but, again, to what level of precision)? The elastic modulus (stress divided by strain) is reported as one number, but it'll exhibit nonlinearity on some level even for a pure, ideal crystal. The elastic modulus is all about stretching or compressing the atomic bonds, which are themselves inherently nonlinear, but that's another very-involved topic.

Worth noting is that these crystalline defects can move at energy levels (temperatures) below the recrystallization temperature or even the annealing temperature. This is one mechanism that leads to "creep" motion of the material, but can also be exploited for improving the microstructure. Heating below the recrystallization point allows the worst of these to settle out, but if the material is allowed to sit too long at a high temperature the impurities can be pushed to the grain boundaries and make the material macroscopically weaker instead of stronger.

This brings me to another type of defect. A second type of defect is an inclusion in the crystal--for example, a sulfur atom. This can be thought of like a more extreme version of a dislocation, and tendency toward forming a crystalline lattice will tend to apply pressure to force the inclusion out. These inclusions, then, tend to diffuse to the grain boundaries where they will be rejected by adjacent crystals. Meanwhile, they don't bond well to these adjacent crystals. So, impurities (a term I am using to indicate materials not part of the intended alloy and that don't bond well with the crystalline matrix) tend to gather at grain boundaries. This is true of other potential alloyants as well; some tend to gather at grain boundaries or form their own structures such as carbides. These processes happen faster at higher temperatures (below the temperature at which the alloyants are dissolved into the overall material).

This gathering of impurities at grain boundaries and the defects associated with random orientation of adjacent grains tends to make large-grained materials relatively weak and ductile (and more-susceptible to corrosion embrittlement, gas-phase embrittlement, and so on).

Worth noting is that after heating to solution temperature (normalizing the material by dissolving all the constituents into the overall matrix) and the quench/temper process (which sets the intended crystalline structure) one can still manipulate the grain size. The typical way to do this involves plastic deformation and heat. Plastic deformation stretches and deforms the grains, providing stored elastic energy (stress) in the material. Adding sufficient heat provides enough energy for nucleation of new un-stressed grains within or at the boundaries of the old. As long as the heat is applied for a short enough time (or if the plastic deformation is continued during the high-temperature period) the new grains will be smaller than the old. This can only be taken so far because the initial rate of grain growth (when the grains are very small) can be quite high. Alloyants can impede grain growth, making this easier, and/or can form/grow carbides during this process. Further, the motion of the crystalline lattice again tends to push impurities toward the grain boundaries, potentially weakening the material.

Meanwhile, smaller grains also means that the bulk material has a higher grain-surface to grain-boundary ratio. The time for diffusion of impurities to the grain boundaries goes as grain size squared (and is a function of temperature). So, a fine-grained material has more area to spread out impurities, but those impurities reach the grain boundaries faster during processing. It becomes a question of impurity content, among other things, as to how far one would ideally push the grain fineness. Poorly-bonded impurities in the grain boundaries (which can actually diffuse into the material from gases during processing--impurity atoms can find their way through the atom-scale interstices between grains) tend to lead to weak and brittle macroscopic behavior--the material becomes much easier to break along the grain boundaries while the grains themselves might avoid most of the damage.

As many have observed, edge taking has much to do with the fineness of the grain structure, which is itself a function of material composition and processing. Finer grains tend to have fewer defects, and those defects can reach the boundaries faster (dislocations diffuse through the grains, too, as the crystalline lattice settles in to become more "perfect" at elevated temperatures).

Composition, of course, is also important. It's my understanding that carbon not only bonds well with iron, it tends to contribute little to crystalline defects--the iron and carbon atoms are of similar size. Tungsten and vanadium, on the other hand, are significantly larger atoms that don't bond as well with iron and therefore impede crystal growth as well as encouraging dislocations and being able to bond with the carbon to form carbides. Impeding (slowing) crystal growth is good for achievable fineness, but the increase in dislocations and the potential for carbide formation can impede development of other desirable properties.

So, as you and so many others have observed, it's a balancing game of trying to optimize the properties for each material and application.

What does this have to do with extreme-edge taking, retention and corner robustness? A well-sharpened edge with sharp corners is, if I am not misunderstanding the mechanisms, going to be relying on the microstrucural properties of the material to avoid deformation, chipping, and erosion. Relevant questions include: what size are the grains, how perfect are the crystals, how pure are the grain boundaries, what is the composition of the crystals, and how many/what size and distribution of carbides do you have? The corners, in particular, seem likely to be susceptible to failure at the grain boundaries (which would show up as micro-chipping) because the grains are relatively unsupported there.

I'd argue that a powdered metal blade failing by way of micro-chipping (as I believe was mentioned elsewhere in this thread) makes sense in the microstructural context--it may be more likely to fail at the grain boundaries rather than within a given grain. It's my understanding that the constituents are melted together into solution, then that solution is "atomized" and cooled, forming a powder. The powder is then compressed and sintered into a billet at "full density" (whatever that means--it seems highly unlikely that the bulk material matches the density of any one grain to arbitrary precision) and processed from there. In the microstructural/grain boundary picture, this seems likely to result in fine and robust individual grains (it sounds to me like most individual particles of the powder are likely to be single-crystal), which would in turn suggest grain boundaries as contributing the most to material failure. Do I know any of this for certain? No.

Another way of summarizing this is that my understanding is that there are various competing mechanisms at the microscopic level that all contribute to the edge-taking/holding and corner retention, and it becomes a game of "some of this, a little of that, but not too much of any of these."

Experimentation is key.

The forging process has the potential to optimize much of the available tradeoff space, and I don't think it should come as a surprise that (as many have observed) the experimentally-determined optimal steel compositions and processes that have been developed over a very long time period by a great number of intelligent people are the favorites of many and are difficult to improve upon.

I hope this helps.


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