Edge retention in chopping

[Michael Bulatowicz]

I spent some time thinking about the topic of edge retention in chopping while in the midst of chopping out the waste in a couple of Roy Underhill style bench hooks I made in oak this past weekend.

I realize that there is a perception that the topic of steel as it relates to edge retention has been beaten to death here and elsewhere. I disagree, particularly as it relates to edge retention in chopping. Various people have gone back and forth regarding hardness, brittleness, carbides of various compositions, toughness, sharpening to the finest possible edge in order to improve edge retention, abrasion resistance, and so on.

It occurs to me that there are a few important points that I have not seen raised on this or any other woodworking forum (perhaps I’ve simply missed those posts/threads).

In spite of my tendency to get overly verbose, I’ll try to keep the introduction here comparatively brief and details comparatively sparse; for the curious, we can add details in the discussion.

Point one: hardness as measured by an indenter (such as the Rockwell C hardness test) is a measure of the tendency of a material to resist “cold flow” and is not a measure of the hardness as one would measure on a basis of, for example, ability to resist being scratched (such as Mohs hardness). The Rockwell C number that one ends up measuring arises from a complicated interaction of the steel grain size; the size, distribution, concentration, and composition of carbides; steel crystalline structure distribution (how much martensite, austenite, etc. is in the steel); the alloy content, distribution, and structure; impurities; carbon content; and so on. It is, therefore, related to but not descriptive of the microstructure of the steel. It is in no way a measure of the brittleness of the material, though as has been hashed out many times it does correlate to brittleness. As pointed out here and elsewhere, one cannot, therefore, simply compare two steels of different composition or even the same composition but different processing details on the basis of a hardness spec alone.

Point two: metal fatigue. Bend a paper clip back and forth enough times and it’ll snap. If you haven't tried it before, it's a cheap and easy demonstration of metal fatigue. At no point are you applying a large enough load to break the paper clip all at once--you're not shearing through the steel or pulling it apart, but it breaks nonetheless due to the cyclic loading. The same basic idea appliees to a chisel edge in chopping; whether you’re using a hammer, mallet, or anything else to apply an impulse to the chisel edge (by way of hitting the handle, hopefully), you’re applying a cyclic load to the very thin cutting edge, which will eventually start to fracture and fail. Brittle failure, on the other hand, will happen upon a single application of a load that exceeds the fracture stress of the material (itself a complicated topic, but let’s leave that alone in the introduction). So, if you hit the chisel with a consistent blow any true brittle failure should happen very quickly—upon the first or second strike, if you’re consistent enough with applying the same force each time. An edge that, in straight-grained wood clear of knots, doesn't immediately chip but instead develops chipping only after many strikes has either been impacting relatively large imbedded hard particles in the wood (much larger than the typical imbedded silica one finds with many woods) or is instead degrading due to metal fatigue (often as exacerbated by abrasion). Fatigue failure starts with a small crack (or cracks) that develop and grow with repeated loading until the material breaks; without knowledge of the load history of a material and/or having a microscope to examine the fractured areas, fatigue failure is often easy to mistake for brittle failure after the fact, but is in truth a fundamentally different mechanism.

Many factors affect fatigue life. For maximum fatigue life, the list would include:
1. Fine grain structure—requires powdered metal and/or careful control of the mechanical deformation and temperature profile during processing.
2. Fine, well-distributed carbides that are well-bonded to the bulk steel (poorly bonded carbides worsen fatigue life even though they still increase hardness and abrasion resistance). So, impact forging (hammer or drop forging are examples) to shatter the carbides and mechanical deformation to distribute these broken-down carbides. Or, powdered metal that starts with inherently small carbides in the small powder particle size. Iron carbide exhibits the best bond strength to iron of any carbide of which I am aware. Stronger than chromium carbide or vanadium carbide, for example.
3. Minimum impurities. During processing and heat treatment, impurities tend to gather at grain boundaries, weakening the grain-to-grain bonds. Impurities in general and sulfur (sulphur) in particular tend to make steels brittle and fragile by weakening the grain-to-grain bonds.
4. Properly oriented grain structure (think “long grain” along the chisel’s long axis, with the chisel’s bevel being like a miter on the end of a board—a rather poor metaphor, but applicable nonetheless). The advantages here should be obvious to any woodworker. This is achieved by mechanical deformation, for example by hammering or pressing the steel at temperatures too low for full recrystallization.
5. Hard steel—this includes hardening by mechanical means such as burnishing (for example, stropping on a plain strop). All else being equal, a harder steel will have a longer fatigue life under identical cyclic loading.
6. High carbon content. All else being equal, a steel with more carbon will have a longer fatigue life (as long as the carbon content remains low enough to actually still call it steel). Edges that have experienced decarburization will fail much faster, for example (as already pointed out by a number of people).
7. Very smooth steel—keeping the scratches as small as possible at the sharp edge will maximize the fatigue life.
8. Oriented scratches: side-to-side sharpening can be expected to result in an edge that fails earlier due to fatigue compared to forward-backward sharpening because of the direction of the applied cyclic loading.

These points serve to help explain the observations of a number of people regarding the edge retention of various steels in chopping. If fatigue failure is a dominant or at least major edge failure mechanism in chopping the following should exhibit maximum edge life, when done “right” for edge retention:
1. Impact-forged high carbon steel (including but not necessarily limited to hand-forged high-carbon steel)
2. Powdered metal compositions

Okay, so it appears I’ve once again failed to overcome my tendency towards verbosity. Or, have I? There’s lots more where that came from.

Questions? Disagreements? Insults for having dared to re-open such a topic? I welcome them all.

Best regards,
Michael Bulatowicz

[James Pallas]

Edge retention is only one part of the tale. Technique is the bigger part. Knowledge of what you are chopping, the angle you are chopping, how hard the blows are the angle of the wedge the width of the edge. Edge retention can be controlled a good bit by user knowledge. The longevity of how long the edge lasts can differ greatly between users. If I wish to I can destroy a good edge rather quickly no mater the metal.

[Jim Koepke]

Edge retention is only one part of the tale. Technique is the bigger part. Knowledge of what you are chopping, the angle you are chopping, how hard the blows are the angle of the wedge the width of the edge. Edge retention can be controlled a good bit by user knowledge. The longevity of how long the edge lasts can differ greatly between users. If I wish to I can destroy a good edge rather quickly no mater the metal.

What he said. My attention span wanes when so much type is on the screen without a few line breaks in the text. My eyelids get heavy after a while.

As James said, "the angle of the wedge" will have a lot to do with edge retention.

My paring chisels (~15º bevels) are not used for chopping.

My firmer chisels (~30º bevels) are seldom used for paring.

jtk

[Thomas Wilson]

Michael, thanks for the short course in metallurgy. There is a lot of good information. It is unfortunate that tool advertising gives so little scientific data about chisel steel.

You stop short of comparing specific steel types for chisels. Which ones do you think provide the best compromise on steel properties specifically for chopping?

Thanks,

TW

[Andrew Hughes]

I’ve noticed my chisels I use for chopping out wood between pins and tails are not chipping on the edge.
My scientific analysis is that my hammer blows are straight and true.
The harder steels don’t like any prying action.

[Jim Koepke]

I’ve noticed my chisels I use for chopping out wood between pins and tails are not chipping on the edge.
My scientific analysis is that my hammer blows are straight and true.
The harder steels don’t like any prying action.

Is there metallurgical testing indicating compression force from straight on blows being less stress to the edge than the stress from torsional stress caused by prying action?

If my memory is working, most of the posts on SMC about catastrophic failures of chisels have been related to prying.

jtk

[Michael Bulatowicz]

James and Jim,

Agreed. Lower the bevel too much and, in chopping, you invite other “immediate” failure mechanisms such as edge crumbling (large and/or poorly bonded carbides), brittle failure if the steel is brittle, rolling if it’s ductile. Continue to raise it and metal fatigue is eventually no longer a factor unless you’re pounding very hard and/or using a narrow chisel; metal fatigue is very much dependent on the stress. If you’ll forgive the math, stress is itself dependent on force applied times the cosine of the bevel angle over the sine squared of the bevel angle, divided by width of the edge (assuming a straight-on mallet blow).

Technique also figures in to fatigue life because it can greatly affect the magnitude and direction of the repeated load.

There are many ways to make an edge fail; my goal was to help provide the “why” to observations from others regarding edge retention in chopping for various steels, in a way that I hadn’t yet seen discussed.

For just one example, Derek Cohen’s test of various chisel steels in chopping jarrah. For another, Brian Holcombe’s observations of edge life in his mortise chisels. Another; Warren Mickley’s vintage hand-forged chisels. The list goes on.

Best regards,
Michael

[Michael Bulatowicz]

Hi Thomas,

I am sure that the lack of information is deliberate, though it is probably no more nefarious than trying to protect one’s recipes from competitors.

I’d like to emphasize that I was only filling in a part of the story that I hadn’t seen mentioned; fatigue is dominant only in certain circumstances. Specifically, with bevel angles around 30 degrees (depending on the steel), when the chisels are used with good technique and no prying, in wood that’s not too abrasive (again, “too abrasive” depends on the steel). Raise the bevel angle enough, and fatigue goes away (“enough” depends on technique, steel, chisel design, and wood).

Based on what I’ve seen from real world testing done by others (my own hands-on experience with scientific metal fatigue testing was not with chisels, and my own chisel use has a very limited variety) for edge life in chopping with good technique and a bevel angle around 30 degrees I’d recommend:

1. Forged high purity high-carbon steel. Modern options are pretty much limited to Japanese tools. White #1 or blue #1 steel, if done very skillfully, should provide an extremely long fatigue life, to the point that even at a 30 degree bevel angle fatigue is probably not going to be the failure mechanism you encounter. Vintage hand-forged western chisels have the potential to be just as good, but QC was sorely lacking by comparison, so it’s more of a crapshoot. I haven’t seen modern western forged options. ETA: Scratch that last; I believe Barr chisels are forged.

2. PM-V11. Of the powdered metals, this seems to have hit the best mix of fatigue life properties along with good abrasion resistance and yield strength while also taking an excellent edge on common sharpening media.

Again, though, metal fatigue is only one part of the story.

Best regards,
Michael

[Michael Bulatowicz]

More mechanical engineering calculations than metallurgical testing.

It’s specifically the bending component of stress rather than the compressive component that is of concern. As you’re no doubt well aware, when the chisel penetrates the wood, force straight along the handle will result in the edge trying to bisect the bevel angle. So, a 30 degree bevel tries to “dive” 15 degrees toward the flat back. The back gets in the way (once the chisel is a little way in or if the back is registered against something) and acts to prevent this diving motion. This applies a bending moment on the edge. The edge can break off under repeated applications of this bending load even if no individual load is large enough to cause breakage or rolling of the edge.

Kind of like how you can break off a burr by bending it back and forth, though attempts at a direct comparison get into the topic of low-cycle versus high-cycle fatigue, a whole other can of worms.

Prying is indeed likely to generate a much higher bending moment than chopping using straight on blows, and therefore is much more likely to cause catastrophic failure, or even fatigue failure farther up the chisel for egregious use such as repeatedly getting the chisel stuck and prying back and forth until the chisel comes out.

Best regards,
Michael

[Michael Bulatowicz]

Also, given a very skilled blacksmith, white #1 should be ahead of blue #1 for fatigue life, but the other way around for abrasion resistance (which can be quite important depending on the wood you’re chopping).

[Brian Holcombe]

The factor that will affect this most is technique. An assured hand and utilization of the technique of riding the bevel keeps the edge alive.

Gripping the blade and chopping kills the edge in a hurry, scraping kills the edge, twisting kills the edge and prying kills the edge. A new or moderately experienced user is likely doing all of these things and not even aware of it.

My White #1 chisels outlast my western chisels but the most important factor remains the user.

[Michael Bulatowicz]

No argument here. Technique is indeed a major and potentially dominant factor in fatigue loading, which is itself only part of the story.

That said, for a particular user (especially one such as myself bumbling forward, if I may steal a phrase from your signature, without the benefit of a master woodworker’s oversight) additional insight into why an edge is failing and what might help to combat this failure can be helpful to both choose tools and to improve technique. It certainly is so for me.

Many factors in edge failure (wear, etc.) have been hashed out previously on this and other forums, but not, to my knowledge, metal fatigue; hence, this thread.

Best regards,
Michael

[Thomas Wilson]

Hi Thomas,

I am sure that the lack of information is deliberate, though it is probably no more nefarious than trying to protect one’s recipes from competitors.

...

l

In my ideal world, there would a woodworking class on tool design and metallurgy just as there are classes on cutting dovetails. I would get to test tool steel in a lab and see electron micrographs of edges I have sharpened and stuff like that. I’m a dreamer.

TW

[Warren Mickley]

Vintage hand-forged western chisels have the potential to be just as good, but QC was sorely lacking by comparison, so it’s more of a crapshoot.

Best regards ,
Michael

Where does this information come from?

[Michael Bulatowicz]

Hi Warren,

The information was extrapolated from information contained in the introduction of Tool Steels, 5^th Edition by George Adam Roberts et al.

Perhaps my choice of “QC was sorely lacking by comparison” was hasty and/or an exaggeration, but I do maintain that modern metallurgy and materials testing capability has a much better understanding (and control, hence my statement) of carbon content and carbide composition, both of which are important to fatigue life (which is itself only part of a large story on edge retention).

Best regards,
Michael