Update: Michael Drinkwine sent me another report from Global where they reported factory sharpened and watertone sharpened knives. I added it to the “CATRA Testing” section of the article.

Update 2: Roman Landes gave me some microhardness measurements of an edge after being ground with 220 grit belts. I added them to the “Effect on Knife Edges” section.

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Bad Edges on Factory Knives

It is commonly reported that a new knife needs to be sharpened a few times before its performance can be evaluated, reportedly because the grinders that put the final edge on the knife will overheat the edge. Is this a real effect?

Grinding of edges to sharpen them is relatively common for knife makers and companies. So if that technique does lead to lesser performance it would be a concern. Among knife users it is somewhat less common though the Work Sharp gets some use as well as Harbor Freight 1×30 grinders:

Abrasion of Steel

When grinding with a belt or a wheel hard media abrades away steel as fine particles:

This image from [1]

The steel often sparks as it is removed from the steel:

You might think that the steel has to be reaching extremely high temperatures to be “burning up” like that as it leaves the steel. However, it is really only the particles that leave the steel that are reaching high temperatures. The formation of iron oxide (rust) is an exothermic reaction (it generates heat). Normally that reaction is slow but with small particles they have a very high surface area and the reaction occurs immediately in air. If using very fine iron particles they will react in a similar manner when exposed to air even without grinding:

Heating of Steel

Instead it is the heat generated by friction in the grinding process that can lead to overheated edges. The amount of heat generated depends on the geometry of the knife/part, grinding media, force against the sharpening media, the speed of operation, etc. At relatively low temperatures the first thing that happens to a knife edge is that it is overtempered. Typically a knife is heated to high temperature, quenched, and then tempered to the desired hardness. A typical tempering temperature for a knife steel is 400°F or so, resulting in the desired hardness such as in the range of 58-62 Rc. Here is a tempering curve for a simple high carbon steel, 1095:

This image from [2]

You can see that as the temperature goes up the hardness goes down. After tempering, if the steel is heated to some temperature below the tempering temperature, the hardness will likely not be affected unless it is held there for very long periods of time. However, if the tempering temperature is exceeded then softening will continue and it will travel down the hardness curve above. Here is an image of a high speed steel tap which is darker at the edge indicating overtempering:

This image from [3]

If the edge is greatly overheated the steel can become so hot that it can actually reach temperatures sufficient to reharden the steel, usually 1400°F or above, depending on the steel in question. In this case the steel will be untempered and brittle. Below is the same high speed steel tap which was overheated to the degree that it was rehardened at the surface, which can be seen by the bright color at the edge:

[3]

Microhardness measurements of that tap edge showed the effect of overheating:

[3]

Away from the overheating in the dark region the hardness is at the original value of 65 Rc, it then transitions to the 58-50 Rc region where it was overtempered. Then at the surface where it was rehardened it is 62 Rc, this value is lower than the original 65 Rc because it was improperly hardened.

Effect on Knife Edges

Grinding of edges can indeed heat up the edge. Knife edges have an extremely small volume of material so they are quite easy to overheat. A very sharp edge can have a diameter under a micron. You can read more about how small edges are and what controls sharpness in this article. Because of the very small volume at the apex, it heats up much more quickly than the rest of the knife. Holding the knife with bare hands or dunking in water between passes is not likely to be sufficient since the heating occurs so quickly.

This image from [4]

Roman Landes gave me some very interesting measurements of 8660 steel (German 1.2795) which is a 0.6% carbon steel with about 0.8Mn-0.5Cr-0.2Mo-0.6Ni. There is also a result from “1065 Cr mod” which has 0.65% carbon along with 0.7Mn-0.25Cr. The knives or scissors were sharpened by hand on a grinder with 3M 220 grit Trizact belts. Interestingly, there is also a comparison between bainite and martensite microstructures. Typically bainite is less affected by tempering than martensite. However, in both cases a hardness drop was measured at the edge. Up to a 5 Rc difference was measured, and of course even microhardness can only get so close to the apex of the edge.

Edge Retention of Belt Sharpened Knives

A study was conducted in 2016 [5] where they compared a knife sharpened on the Work Sharp belt sharpener with traditional sharpening on stones. To test the edges they used an Anago sharpness tester which I have not previously discussed. In that test, the entire knife is drawn through a grid-like material so that sharpness values are found over the entire length of the edge:

The Anago measures sharpness on a 10 point scale where 10 would be an infinitely sharp edge (unattainable). They wore the edges of the knives by making cuts into bamboo and measured the sharpness after 30, 60, and 90 cuts. The Work Sharp led to better sharpness (9) than the stone sharpening that they performed (8.5), but dulled much more rapidly:

The belt sharpened knife was only 6.9 after only 30 cuts on the bamboo; the stone sharpened knife was still a 7 even after 90 cuts. After 60 cuts the belt sharpened knife was only a 6.5 as measured with the Anago while the stone sharpened knife was still a 7.4.

CATRA Testing

Roger Hamby of CATRA also tells me that in their testing of edge retention that over 75% of the knives they test suffer to some degree of edge softening due to the sharpening process. He reports that nearly all manufactured knives have the problem, while hand sharpened knives generally do not. I have written extensively about the CATRA test in the past, such as in these articles: Part 1 and Part 2.

The knife company GLOBAL compared their factory sharpened knives to “whetstone” sharpened knives and found a big difference [6], the information has since been removed from the website but I have hosted it here: GLOBAL CATRA testing

Image from [6]

How to Avoid Overheating of Edges

The easiest way to avoid overheated edges is to sharpen by hand, of course. However, that is not always possible. There are water cooled methods, such as the Tormek sharpener or adding a water misting system to a belt grinder:

Using water cooling prevents steel from heating up during the grinding and polishing process and therefore may prevent overheating of edges and loss of edge stability and edge retention.

Different Steels

To some extent steel choice can help. Certain steels are designed to be tempered at high temperatures. High speed steels, in particular, are designed for operating at high temperatures without softening. They do this through additions of elements like tungsten and molybdenum that form very tiny carbides at temperatures in the 900-1100F range that increase hardness:



This image from [2]

Non-high speed steels with sufficient molybdenum or tungsten additions can also be tempered in that high temperature range. Particularly tool steels like Vanadis 4 Extra, 4V, 3V, and others. Even some stainless steels like 154CM can be tempered in that range. However, high temperature tempering also leads to a reduction in corrosion resistance so it is usually not recommended if corrosion resistance is a priority. Using steels with a high temperature temper can help prevent softening if the edge remains below the tempering temperature of 1000°F or so. If that temperature is still exceeded then it won’t matter, of course. Measuring whether or not the edge has actually exceeded any given temperature is very difficult given that it is the very fine edge of the knife that we are interested in.

Summary and Conclusions

Knife edges are easily overheated during powered grinding due to the small volume of the edge and the friction buildup from grinding. This softens the edge and worsens edge retention. The use of steels capable of high temperatuhttps://www.youtube.com/watch?v=D4aFy3tRpI4re tempering treatments help to some extent since they can withstand more heat. Hand sharpening or water cooled sharpening are the best methods for preventing overheating of edges.

[1] Klocke, Fritz, Olaf Dambon, and Barbara Behrens. “Analysis of defect mechanisms in polishing of tool steels.” Production Engineering 5, no. 5 (2011): 475-483.

[2] Roberts, George Adam, Richard Kennedy, and George Krauss. Tool steels. ASM international, 1998.

[3] https://customer.cartech.com/assets/documents/datasheets/Bulletin_104.pdf

[4] http://www.hroarr.com/wp-content/uploads/2014/04/sharpness.pdf

[5] Mulder, Joshua, and Jonathan B. Scott. “The measurement of knife sharpness and the impact of sharpening technique on edge durability.” (2016): 1-7.

[6] https://web.archive.org/web/20160118100853/http://global-knife.com/catra/

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