What makes a steel easy to sharpen?

[fsatsil]

Ive found that the softer the steel the easier it will be to grind but it's not always easier to sharpen. Really soft steel can be a challenge to get sharp because of how easily they form a burr and how difficult that burr can be to remove.

 

[Twindog]

All things equal, skill, hardness, heat treatment, etc what makes a steel thats easy to sharpen to a razor for instance super blue vs something more difficult like 440a? Same angles, geometry, abrasives, skill.

I think if you take out every variable other than the actual steel alloy, the steel that's going to be easiest to sharpen will be a simple carbon steel, especially if the "same abrasives" that you're using are relatively soft. If the "same abrasives" you're using and the "same skill" you're using are diamonds in the hands of someone with a lot of skill, the differences will diminish.

The hard carbides in steel that make it wear resistant will make it a little more difficult to sharpen, everything else equal.

But even if you make everything but the steel alloy the same, you will come up with different answers depending on what the same heat treat is for the steels, what the actual same sharpening skill level is, what the same hardness means to different steels, etc. For example, if your same hardness was very high versus very low, you will skew the sharpening results to different steels.

Even when you make everything the same, by making those decisions about what those levels of "sameness" will be for each of the many variables, you are predetermining the winner. Make steels the same in different ways, and you'll have a new winner.

 

[Charr]

The unfortunate truth of this question is that there are still a MASSIVE number of variables that will effect how easy a steel is to sharpen, even eliminating what the OP said.

The main distinct factors are going to be found in the chemical composition of the steel, but that is only in regard to the part that the steel itself plays in sharpening, which is about 50% of the process.
The other 50% of the process is all in the person who is sharpening, their skill, and what they consider "easy" and "sharp". For example, my cousin is not that good at sharpening to a fine edge, but he is very good at working his edges into a nice toothy finish that is still very sharp. For him, steels like S90V aren't really that hard to sharpen, but he has more issues with some of the more simple, low-alloy steels that don't take a toothy edge as readily, even though many other people consider those much easier to sharpen.
Not only that, but what you consider "sharp" is sometimes a little bit of a subjective thing as well. Some people don't think a knife is sharp unless it has a fine and keen edge that is polished and highly finished, but other people (myself included on some knives) only really need a working edge, because they know that is what is better for their particular cutting needs. That's also why you will hear people often have a very different opinion on how long it takes a blade to get dull, even if their method of sharpening is very similar. One person already thinks their edge is dull while the other is able to keep using it for quite a while longer.

Beyond that is also the inherent skill in sharpening, which is a hard thing to really acquire, but very valuable once you have it, because it will make sharpening almost any steel very easy when you have the right materials to get the job done.


But the steel itself will play, again, about 50% of the role in sharpening, if you're using the right materials to sharpen (I'll assume we are for this since that's a whole other issue by itself), and you already have the needed skills as well.

Grain size will play a role, in that a knife with larger and harder carbides will be more difficult to give a fine edge (i.e. S90V), while a knife with extremely small carbides and a very clean grain structure will take a polished edge much more readily, and will hold a fine cutting edge and smooth apex to that edge much longer in general (i.e. M390).
In addition to that, the wear resistance of the steel is largely determined by the carbide structure, and that is usually going to play a pretty big role in how difficult it is to sharpen as well. Higher hardness in the same steel will usually lead to higher wear resistance, but to varying degrees for different steels, and only given a proper procedure in the heat treat.
Extremely high wear resistance combined with a very fine grain structure can lead to a steel that is relatively easy to sharpen with the right tools and technique, but that still has extremely good edge retention and takes a very keen edge for a very long time. A good example of this is ZDP-189, when properly heat treated to high hardness, like is done by Rockstead in their knives. Though there is a fairly good amount of skill needed to sharpen their knives, if you have the right tools and have not just let your knife completely go dull like a novice, then touching up the edge or lightly sharpening it is very easy, and you will only have to do so VERY rarely, considering the insane level of edge retention their achieve on their steels.

That also addresses another very important part of sharpening, which is "What is your blade like before you sharpen it?"
Sometimes we forget this question, because it is a VERY important one to keep in mind when you're trying to sharpen your blades. If you have let your blade become a butter knife, or you have to re-profile the edge, then the difficulty of sharpening a high-carbide modern steel with high wear-resistance like M390, S110V, or S125V(God help you if you're trying to reprofile this one...) exponentially increases, but if you are bringing the sharpness back to a blade that has slightly lost it's edge, or it's "bite" then that task will be much easier, and won't really require much more effort than sharpening a steel of lower wear-resistance/grain refinement, such as 440C (again, assuming that you are going to be using the right tools and the right technique).


The short version of this is that it is way to complicated to attribute to ANY one factor, even though we, as human beings, seem to have this ridiculous need to make everything simple and "black and white" when it comes to things that just aren't. There is never one cause for anything, and there is never really an easy answer when the question is in any way thorough. We just want it to be because it is easier that way and we can make ourselves feel better about not actually knowing what is going on.
If there is a need to frame your question with things like "forgetting/ignoring factors A, B, and C, what causes X?", then the question will not have one answer, almost guaranteed, because you cannot fully eliminate the extra variables in a question like that and scientifically determine only one cause, when all of the different variables are all constantly effecting the result you are questioning.

And that last bit isn't a dig at anyone or me being a jerk, and if it comes off like that I'm sorry. I'm a little tired of all of these questions that want a simple answer (at NO fault of the ones who ask them at ALL), and I'm damn glad it if Friday right now, so I might have gotten on my soap box a little bit there...

Also, I am omitting entirely the existence of Nitrogen-based steels in this comment/discussion, because I don't want to write EVEN MORE. So suffice it to say that Nitrogen-based steels would make this even more complicated than I already have here...yeah, this was the "short" version...

 

[Sunyata]

Very informative thread.

I use diamonds so everything is easy.

 

[kindasharp]

I find the easiest way to sharpen a knife is when someone is doing it for me. Sounds like you have all the information you need in the above posts.

 

[FCCBCT]

Ok I've looked at grain structure many times with an electron microscope, and a special light microscope to examine it. Essentially think of it as a crystalline structure and there happens to be various "types" of these crystalline forms. So the easiest way to see it is by taking a mirror polished flat surface etch with nitric acid and even without a microscope you might just be able to make out some shapes in your etched sample. Better still galvanising of steel you will see zinc crystals on the surface, not exactly the same but close enough.

"Carbides" is a term kind of short for elements in the alloy mix that combine with carbon to various degrees, for example chromium carbides, vanadium carbides etc etc. The more Chromium carbide there is the less Chromium (Cr) that is in what as known as solid solution to aid the alloy's resistance to corrosion (generally speaking).

Carbides are not the only the only way to make a steel hard. That's why you can have steel alloys with no carbon but the hardness is from Nitrides instead, i.e. elements combining with Nitrogen.

 

[FCCBCT]

What's a simple way to explain grain structure and carbides that won't blow my mind? I bought a material science book but its not specific or simple enough for me to understand.

Ok I've looked at grain structure many times with an electron microscope, and a special light microscope to examine it. Essentially think of it as a crystalline structure and there happens to be various "types" of these crystalline forms. So the easiest way to see it is by taking a mirror polished flat surface etch with nitric acid and even without a microscope you might just be able to make out some shapes in your etched sample. Better still galvanising of steel you will see zinc crystals on the surface, not exactly the same but close enough.

"Carbides" is a term kind of short for elements in the alloy mix that combine with carbon to various degrees, for example chromium carbides, vanadium carbides etc etc. The more Chromium carbide there is the less Chromium (Cr) that is in what as known as solid solution to aid the alloy's resistance to corrosion (generally speaking).

Carbides are not the only the only way to make a steel hard. That's why you can have steel alloys with no carbon but the hardness is from Nitrides instead, i.e. elements combining with Nitrogen.

 

[155440]

Thank you Charr for the very informative post.

 

[Charr]

Thank you Charr for the very informative post.

Glad I could help man!

I was about due for another one of my insanely long posts

 

[DeadboxHero]

Good stuff guys.

I'm not trying to beat a dead horse.

I'm fully aware that sharpening techniques, tools, and skill are a huge factor.

I am trying to isolate the metallurgy behind it.

For example, If one had the prowess to make a new knife steel at a foundry, how would they create a steel that sharpens fast and easy at a high hardness? I'm curious about understanding the metallurgy behind steels that sharpen quick like vg10, 14c28n, 52100, 1095.

I can read off the ingredients of each steel and state that "cobalt enhances the attributes" but,
I truly don't understand the why and the how behind it.

I sharpen knives daily, there is no denying how much faster these steels go from too dull to cut paper, to push cutting it.

Im able to comprehend the geometry, grinds, edges, sharpening tools and tactics, hardness, etc. But I want to know more about the metallurgical properties.

 

[Charr]

Good stuff guys.

I'm not trying to beat a dead horse.

I'm fully aware that sharpening techniques, tools, and skill are a huge factor.

I am trying to isolate the metallurgy behind it.

For example, If one had the prowess to make a new knife steel at a foundry, how would they create a steel that sharpens fast and easy at a high hardness? I'm curious about understanding the metallurgy behind steels that sharpen quick like vg10, 14c28n, 52100, 1095.

I can read off the ingredients of each steel and state that "cobalt enhances the attributes" but,
I truly don't understand the why and the how behind it.

I sharpen knives daily, there is no denying how much faster these steels go from too dull to cut paper, to push cutting it.

Im able to comprehend the geometry, grinds, edges, sharpening tools and tactics, hardness, etc. But I want to know more about the metallurgical properties.

I don't think you're beating a dead horse at all
I enjoy these conversations. My statement before was really just a rant on this type of question in general.

The main issue is that most of the chnages that can be made to the chemical mixture of the alloys tends to be a little bit of give and take.

If you wanted a blade that was supremely easy to sharpen in general, then all you would have to do is make a steel that is fairly low in terms of wear-resistance, even at higher hardness. Some carbon steels are like this, and in exchange for the loss in wear resistance, they achieve much higher toughness usually, and are easy to sharpen.

If you wanted to make a high-carbide steel easier to sharpen, then a good overall technique is the use of a powdered metalurgy method for production, which keeps the cystaline stucture of the steel very uniform, and keeps the carbides from forming into uneven groups, which they will tend to do otherwise because of ionic bonding that tends to happen in super-heated steel. This makes a steel that has very fine grain structure, very eaven carbide distribution, and makes it possible to have a much finer control over the properites of the steel, allowing much higher levels of alloy complexity.

In short, the first thing you want to do to make a steel that can perform very well and still be easy to sharpen in impliment a powdered metalurgy method of production.

Next, in terms of compostion, it really depends on what you want the steel to do. A steel that has high amounts of Vanadium in it will not generally be easy to sharpen for most people, becasue Vanadium forms very large and extremely hard carbides that are exceptionally good for making a toothy edge and holding it for a very long time, but they tend to give the steel a very high wear-resistance without having the super-fine grain structure that is needed to readily take an edge.
So, there are a few options, most of which we can see in many of the commonly used steels today:

For starters, you can use a small amount of several different trace metals, such as Molybdenum and Niobium, which can change the properites of the steel. For example, Niobium is used in S35VN to form carbides in place of some of the Vanadium carbides that were originally in S30V. This addition of Niobium into the steel had the immediate effect of making the steel tougher at almost all hardnesses, and the supplimental effects of increasing both grain refinement and easy of sharpening. It also had the added benefit of preventing a common issue in S30V where there would be micro-chipping on the edge, causing what seemed to be rapid dulling in use when cutting some media.
If you are stropping S35VN or running stones over it, it will feel much smooth and easier to work with in general than S30V will, at least in my experience, and that is largely because of this change in composition.
The added benefit of the Niobium is that it also replaced some of the carbides that were formed by Chromium, leaving more of it in the steel to act as a deterant for corrosion, and also increasing the edge retention (in theory, though only by a relatively small amount), since Niobium carbides are harder than those formed by Chromium.

Molybdenum aids in making a finer grain structure overall, but does not form carbides like Chromium, Vanadium, or Niobium, so it is not going to usually effect wear resistance in a positive way.
A good example of the use of this metal in alloys though is 154CM and CPM-154. Both of these steels have a relatively higher amount of Molybdenum in them, at 4% (double what is in CPM-S35VN), and that allows them to be easier to machine and polish, whish also translates to making them very easy to sharpen, while still maintaining higher hardness and wear-reistance than low-end steel like AUS-8 or 8cr13mov.

These two steels also highlight how important powedered metalurgy VS. traditional ingot steel can be when we talk about the properties of steel.
CPM-154 has notably higher wear resistance and is even easier to sharpen and especially takes a polish, satin, or other finishes rather easily. It is very good to use if you want to have a keen edge that has a polished and smooth apex to the edge.
Even though 154CM is also very easy to sharpen and takes a polish well, it will not finish quite as easily as CPM-154, will not have as high of wear resistance, and will not have as high ofg toughness either.

Another method is the use of nitrogen instead of carbon, like I mentioned briefly and FCCBCT also said.
Nitrides are smaller without having significantly lower hardness than most of their equivilant carbides, and form more readily under the right conditions. They also allow Chromium to be used almost exclusively for corrosion resistance, making more Nitrogen-based steels extremely corrosion resistant for the amount of Chromium that is in them.
the use of Nitrogen also helps in making the steel easier to machine, and easier to sharpen in that it also alllows for more alloying components to be added to the steel, producing an extremely fine grain structure, higher toughness in general, and easier polishing and machining, without losing wear resistance.
That's why high-carbide Nitrogen steels like Vanax are insanely good knife steels, but producing them is currently still very cost-intensive, so they are not widely used.

And the reason that steels like VG-10 and 1095 sharpen very easily in general is because they have much lower wear-resistance than something like S35VN or CPM-154, and don't have many carbides that form in very large numbers that increase the abbrasion-resistance of the steel.

 

[DeadboxHero]

Wow, thank you. This post is bursting with knowledge. It take true understanding to explain complicated things in simple ways. Your the man Charr

 

[link2derek]

I have always been taught that steels with more chromium and less carbon content are generally softer and easier to sharpen. Of course, they are also easier to dull from use. More carbon and less chromium means harder to sharpen, but it also means it will hold an edge better. Kind of a trade-off.

 

[DeadboxHero]

It more complicated the carbon vs chromium.

For example, I have a 5" carbon untilty knife and 10" stainless chef knife. Both with similar grinds, both dull with rounded tips, both sharpened free hand on a DMT extra coarse diamond stone.

The carbon blade removed material faster and left a sharper edge with less burr removing time the stainless however needed much more time to sharpen and remove the burr. It also holds its edge less.

Both were left finished on a extra coarse grit and stropped one deniem laid flat on the stone then pulled through soft wood to remove any stragglers.

The stainless takes more attention to detail and time and does not hold it edge.


Corrosion resistance is the only advantage for low end stainless.

 

[DeadboxHero]

Vanadium carbides are large and hard, So carbides have an independent hardness then the whole blade?

Also what fills the gaps in steels with low carbide volume?

 

[The Aflac Duck]

I'm

Vanadium carbides are large and hard, So carbides have an independent hardness then the whole blade?

Also what fills the gaps in steels with low carbide volume?

Iron and carbon are what makes steel. Iron being the most of it.

 

[BluntCut MetalWorks]

Answer to Op question can be generalize into optimization of physics for 2 functional variables. Shaper (abrasive + applied forces & interactions) and Subject(cutlery material in this case).

Shaper - optimal shaper should able to remove subject material at smallest unit with lowest or no energy transfer - from shaper to subject's base. e.g a material displacer/transport would remove a top layer of subject material without transfer any thermal/kinetic/etc... to the rest of the subject.

Subject - Optimal attributes for shaping: smallest fracturing unit - highly coherent, non interlocking; high strength; lowest variance in hardness; smallest range of elasticity & zero ductility; etc...

Now in sharpening world, it's a matter of matching best shaper (abrasive type+size; interaction type 2 or 3 bodies; velocity and angle (force vector)). In steel, choose one that most uniform in structure and hardness and fracturing unit much smaller than abrading/shaper unit. A monograin (i.e. no grain nor subgrain in micro structure) pure martensite matrix (no carbide of any kind) at highest hardness possible. Yep, a brittle steel but that is a concern if the goal is to shape a smallest apex radius using light sabre abrader.

Now, perhaps you can see why simple low alloy steels usually very easy to achieve high sharpness, especially when hardness > 58rc. Combination of fine grain + fine carbide + no RA matrix yield a best small size fracturing unit. Grain size factor in sharpening is really about dealing with grain boundaries, porosity + stress + carbide aggregation heavily influence the fracturing unit. So instead of smallest fracturing martensite unit ~6-9nm, fracturing involves GB could be in 100 or 1000s of nm. Ok, if we have a light sabre type of abrader then sure subject's hardness consistency doesn't matter much. But in practical world, even the best abrader (100% sharp CBN on high speed wet grinder), carbides are much harder than matrix so there will be impacts that dislodge or crack the carbide to matrix interfaces.

brain-to-words translator is broken

 

[fsatsil]

Molybdenum aids in making a finer grain structure overall, but does not form carbides like Chromium, Vanadium, or Niobium, so it is not going to usually effect wear resistance in a positive way.

I could be wrong but I always though molybdenum was a carbide former?

 

[fsatsil]

Vanadium carbides are large and hard, So carbides have an independent hardness then the whole blade?

Also what fills the gaps in steels with low carbide volume?

Vanadium carbides are not always large but yes carbides have their own hardness separate from the overall hardness of the steel. In order of softest to hardest it would be chromium carbide, molybdenum carbide, tungsten carbide, vanadium carbide, and niobium carbide. Iron makes up most of the composition of steel and is what "fills the gaps".

 

[Charr]

I could be wrong but I always though molybdenum was a carbide former?

Sorry misspoke on that one. Molybdenum does form carbides, yes, and they are fairly small in comparison to most other ones generally, and harder than Chromium carbides as you stated as well. They aid aid in refinement by allowing the steel to form a cutting edge out of smaller carbides instead of the larger ones, and allowing more of the steel to be evenly distributed with carbide formations.