What actually dulls an edge?

BJE · May 15, 2007

It is a little hard to explain, but I have been wondering how a material that is softer than the blade steel, when cut, can dull the edge? I understand why hard materials dull it, even if they aren't as hard, because they can roll the edge, but how does something softer than the steel make a blade dull? I have always thought that minerals and other impurities, which are harder than the steel, are found in the material and thats what dulls it. If this is true, then in theory, if you cut a pure material that was softer than the blade steel and contained nothing harder, then you could cut forever without dulling the blade? It might be a little confusing, but I am curious as to the answer, so if anyone has any thoughts on this then please share them with me.

Knife_Geek · May 15, 2007

Its probably somethign along the lines of how a soft rubber tire can over the years wear down the much harder asphalt roadway.

yablanowitz · May 15, 2007

Cutting pure, soft materials will take a really long time to dull a good, hard steel with a moderate edge angle. When you get down below ten degrees included, even marshmallows can roll the edge if you are not careful. Usually it is some small degree of lateral force that begins an edge rolling, i.e. not keeping the cutting edge perfectly perpendicular to the material being cut. Once the thin edge is bent a tiny bit out of line by that lateral force, it will get bent more by each successive cut, ending up as a rolled edge. As far as the edge simply wearing away, water can erode steel over time and even our water here is a little softer than steel. Very high pressure or velocity will erode steel much faster (such as a water jet cutter).

When I saw the thread title, I was going to post a one word answer. What dulls a knife edge? ME

samhain73 · May 15, 2007

The reason a knife dulls other than blunting is oxidation. Just strop it to remove.

FoxholeAtheist · May 16, 2007

IIRC, there are four methods of dulling...

1. Abrasion - Wearing away the steel, as in cutting cardboard
2. Corrosion - Rusting away the edge
3. Rolling - Bending the edge to the side
4. Chipping - Breaking pieces out of the edge

Different steels resist these different dulling methods with various degrees of success, and the choice of the proper steel for the job is often recognizing what kind of dulling method you'll be chiefly encountering.

Broos · May 16, 2007

Think in terms of that very small area of steel at the edge that makes the cut. Then think in terms of putting your muscle into a tough cut, maybe applying 50-100 lbs of force on that tiny itsy bitsy area of steel. The pressure that is exerted on that very small area of steel is VERY high, and I am pretty sure it would account for some dulling (rolling, chipping, etc.).

A scientist will make a lot of assumptions in trying to theorize how a material is going to react. For example, scientists do not really understand or agree on what IS explained by the results of a charpy test. Theoretical analysis of simple tensile or compressive strengths assumes that the test piece of steel (or whatever) is perfectly homogeneous (which no test piece is, really).

There are whole fields of engineering understanding that are understood by purely empirical data, and very hazy or shaky theory (heat transfer, airfoil performance to name a couple). In other words, some of our most complicated machines we use, and some of the highest technology we deploy, may be based on very little theory and was developed using the old tried and true method, trial and error, trying to learn how to make it better from each failure. After enough tests are done, the scientist simply best fits his data to an equation.

I don't know how I went off topic like that. I guess I'm trying to say that it is at times fruitless and quite possibly wasted effort to attempt to imagine or theorize the mechanics of why a knife edge performs the way it does. I've seen a lot of folks here get all tied up in trying to understand the micro mechanics of edge behavior, when it seems often to me to be nothing but conjecture. I sometimes wonder why, especially when it is better and a heck of a lot easier to get real good understanding of how the knife will perform cutting a specific material by just cutting it a lot. Don't mean to diss this too much though, it is natural to try to understand, and I'm guilty of it myself.

dantzk8 · May 16, 2007

FoxholeAtheist said:
IIRC, there are four methods of dulling...

1. Abrasion - Wearing away the steel, as in cutting cardboard
2. Corrosion - Rusting away the edge
3. Rolling - Bending the edge to the side
4. Chipping - Breaking pieces out of the edge

Different steels resist these different dulling methods with various degrees of success, and the choice of the proper steel for the job is often recognizing what kind of dulling method you'll be chiefly encountering.

Great post. You have said a lot in few words.

dantzk.

siguy · May 16, 2007

if you use a knife often, such as using an EDC to cut boxes every day at work etc, it will mostly be abrasion wearing away at the edge.

if you don't use a knife often, such as a hunting knife sitting in a closet over the off season, the major dulling factor will be corrosion.

if you use the knife wrecklessly or on materials that the edge angles/steel is not suited to your problems will be rolling and chipping mostly.

for me personal use, i worry mostly about abrasion, because i cut alot of things and cutting all of them can wear the steel away from the edge.

for you, it may be a different factor to worry about. if you have alot of rolling/chipping problems you may need a thicker edge bevel.

j22knife · May 16, 2007

BJE said:
It is a little hard to explain, but I have been wondering how a material that is softer than the blade steel, when cut, can dull the edge? I understand why hard materials dull it, even if they aren't as hard, because they can roll the edge, but how does something softer than the steel make a blade dull? I have always thought that minerals and other impurities, which are harder than the steel, are found in the material and thats what dulls it. If this is true, then in theory, if you cut a pure material that was softer than the blade steel and contained nothing harder, then you could cut forever without dulling the blade? It might be a little confusing, but I am curious as to the answer, so if anyone has any thoughts on this then please share them with me.

The key is to undertand that "hard" and "soft" are relative terms. When you rub a hard and soft material together (knife and a block of wood) - the hard material will wear the soft material more than the soft material will wear the hard material.

But each will wear the other to some degree.