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| Metallography |
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| Finest abrasives. | ||
| Microbevels front and back. | ||
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| Copyright (c) 2002-12, Brent Beach |
Metallography
What is metallography?
Metallography is the study of the internal structure of metals and alloys, and of the relation of structure to composition and to physical, chemical and mechanical properties. The study of a particular specimen involves three steps: preparation, etching and microscopical examination. The step of most interest to sharpeners is the preparation step. Preparation typically progresses through four stages:
- mechanical cutting - which produces a rough and not very flat surface.
- filing or grinding - which flattens the surface but which still produces a rather rough finish.
- abrasion - which uses abrasive particles that are supported by a comparatively firm backing in which they are fixed rigidly and produces a very flat surface which is not yet polished.
- polishing - which used abrasive particles suspended in a nominally inert liquid and supported on an elastic backing, such as a cloth pad and produces a mirror surface. [Mirror surfaces reflect light with the incoming and outgoing angles the same. Rough surfaces scatter light. All surface combine a bit of both types of reflection. As we progress from rough to smooth the percentage changes from almost all scattered to almost none scattered.]
In fact these four stages differ in two ways: first, in the rate at which material is removed, and second, in the nature and depth of the the plastically deformed layer that is produced in the surface being formed. [An elastically deformed layer returns to its original shape after deformation, a plastically deformed layer is permanently changed. In our case, by the removal of filings as the abrasives scratch the surface. The scratched surface and the supporting metal to a depth depending on the technique used are plastically deformed during abrasion.]
This definition (and most of the material presented in this page) is based on Metallographic Polishing By Mechanical Methods by Leonard Samuels, third edition.
Why is metallography important?
What does metallography have to tell people who just want sharp tools?Cutting a sample of a metal to examine it and study its properties has the side effect of creating a deformed layers at the surface that are different from the underlying metal. Metallographers have worked out how to remove these shattered layers, gradually reducing the depth of the shattered layers. By doing this they are able to examine the metal itself, rather than the shattered layer. What they see are the properties of the metal, not the artifacts created during preparation.
Metallographers has studied the abrasion of metal in intensive detail, repeating those experiments in many labs over the years using all available inspection devices: all kinds of optical microscopes as well as scanning and transmission electron microscopes with magnifications up to 50,000 X.
How abrasive grits scratch steel
Metallography has determined that there are only three ways in which an abrasive grit scratches steel. Of those three modes, only one removes metal.
| A loose, tumbling particle. |
Much as if you rub sand between your hands, the particles can occasionally dig into the tool, but the particle quickly tumbles.
There are indentations, but almost no material is removed. The efficiency of the metal removal is almost 0%. |
| A fixed particle plowing a groove. |
Here the abrasive particle is fixed and the angle between the grit particle face and the tool surface in the direction of motion is small. Imagine a surfboard moving through water. Imagine a fork moving through peanut butter, the fork at a small angle to the surface.
A bulge forms in front, a ridge is left on either side, and a groove is left behind. There are grooves, but almost no material is removed, it is just rearranged. The efficiency of the metal removal is almost 0%. |
| A fixed particle cutting a chip. |
Here the abrasive particle is fixed and the angle between the grit particle face and the tool surface in the direction of motion is larger. [The critical angle at which plowing turns to cutting depends on the metal and its hardness.] While called a chip, the material removed usually looks like a curly shaving. Imagine a plane cutting a shaving.
A bulge forms in front, there is no ridge on either side, and a groove is left behind. The chip is formed from the entire content of the groove. The efficiency of the metal removal is almost 100%. |
How filings form
This drawing is from Samuels, figure 3.4, page 35.
The top shows plowing, the bottom cutting. The drawing shows that plowing removes almost no metal while cutting removes almost all the metal in the groove.
In order to remove any metal from the tool, the abrasive grit must make contact above the critical cutting angle. This drawing shows a critical angle between 60 and 120 degrees. This would be typical of softer metals.
This table shows the critical angle for steel at 3 different hardness points, using the Vickers hardness scale.
| Hardness | Critical Angle | Cutting points % |
|---|---|---|
| 150 | 80° | 40% |
| 255 | 90° | 25% |
| 855 | 10° | 95% |
Fortunately for people sharpening hardened tool steel, the critical angle is quite low - almost any angle will do so almost every grit particle in contact with the tool scratches. For soft steel, a smaller range of angles works! Soft steels tend to flow around the abrasive particle while hard steels cannot.
This diagram does not tell the whole story of what happens to the steel as it is scratched by the abrasive grit. It is concerned only with the removal of metal so shows only what happens on the surface. In fact, scratching changes the metal structure well below the surface. More on that later.
A cloth with embedded abrasive particles is a third case. The abrasive particle is not firmly held and does not roll. The grit can tilt, lowering cutting angle and reducing the depth of the scratch. It still removes metal but more slowly than the fixed abrasive case. This example is discussed below under polishing.
Scratch shape
The Vickers Hardness test uses a pyramid shaped diamond point under a fixed load to create an indentation in the material. The test uses the surface area of the resulting indentation, not the depth, as the hardness measure.
This diagram, Figure 3.12 from an earlier version of Samuel's book, shows what happens when a fixed load is applied to different shaped points. While the diagram shows 2-dimensional triangles, the diamond point is in face a pyramid.
The key idea is that for a given load the point stops going deeper when the surface area of the indentation reaches some fixed value. It is not the depth of the indentation, nor is it the volume of the indentation, it is the area of the indentation.
The first two shapes correspond to grits with different tip angles. A blunt grit produces shallower indents than a sharp grit.
Shapes 3 and 4 show what happens when the tip of a grit breaks off. For a small break, the grit still indents but the scratch is shallower. In the limit case, the surface area of the tip is large enough that there is no indentation.
Indenting solids is thus different from indenting liquids! With liquids, the weight of the volume of liquid displaced is equal to the weight of the object floating on the surface (thank you Archimedes). With solids, the surface area of the indent is related to the load.
Getting back to scratches, the shape of the grit determines the shape of the groove and thus the amount of metal removed during abrasion. Too many grits like drawing 4 and no metal is removed. Shape 2 removes the most metal. An even sharper point removes more metal, but may also fracture sooner resulting in shape 3 or 4 and reduced metal removal.
It is typical of abrasives that they start removing metal very quickly. During this break-in phase only the few tallest points are making contact and they all have shape 2. As these tallest grits fracture their points dull but other shorter grits make contact with the tool. With more contact points the load per point drops so the average scratch depth drops but we have more scratches. Over time all the grits making contact have shape 4 and metal removal stops.
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