Let's Explore The Evidence And How To Avoid It Happening
It is common knowledge among knife enthusiasts that factory knives often have subpar edges. Their sharpness and keenness are often lacking and do not maintain their edge well. As a result, many knife owners opt to sharpen their new knives right away. Many even suggest that a factory knife needs to be sharpened several times before it can be accurately assessed.
Worth noting is that factory knives may have uneven edges due to being ground with either belts or wheels and quite often by hand. I have observed factory knives with up to a ten-degree variation in bevels from one side of the edge to the other and a seven-degree difference from the heel to the tip. However, this is not the topic I wish to discuss today. Today's topic of concern is whether grinding the apex or edge of a knife causes any damage to the steel remaining and the apex itself. It's a fact that grinding steel produces friction, which in turn generates heat. If the heat exceeds the original tempering temperature of the hardened steel, it can lead to over-tempering, resulting in the steel becoming softer than before. This is something that needs to be avoided. Furthermore, if the heat intensifies, the edge will re-harden, turning into untempered martensite once again.
When it comes to grinding a knife blade, a moderate amount of heat is generally acceptable in thicker areas. The steel volume of the blade acts as a heat sink, allowing the heat to be distributed throughout. However, it's a different story regarding the microscopically thin edge. A properly sharpened kitchen knife has an edge width of half a micron or less (for comparison, a micron is about forty-millionths of an inch). These widths are so small that they can only be seen clearly with a scanning electron microscope. It's in this tiny, critical area where the heat can potentially cause the most damage. When making knives, they are heated to high temperatures, quenched quickly, or cooled down to room temperature, and then tempered to reduce internal stresses. Typically, the tempering temperature is around 400°F (200°C ). If a knife being sharpened is heated to a temperature lower than that, it's unlikely to cause damage. However, the temperature can easily exceed that limit in the sub-micron area of the edge's apex within a second or less, resulting in a compromised "heat-affected zone". In this zone, edge stability and edge retention will be adversely affected. This HAZ may be only a couple of thousandths of an inch back from the apex, but if it is there, your edge is degraded. Your knife won't hold an edge as well as it SHOULD. So is it best to avoid the situation altogether?
There are ways to mitigate creating this heat-affected zone (HAZ). Let's go over a few:
1) Only Hand Sharpen Knives-- This isn't a viable option for professional knife makers and sharpeners. Still, it could be the answer for the home sharpener who only wants to keep personal knives keen and ready.
2) Grind wet-- Add some water or a Kool Mist to your setup. It's messy, but it saves edges! Some sharpening machines come ready for wet grinding as well.
3) Grind Slowly-- Slow down the speed of your machine. Does your machine have only one speed? Then don't grind knives on it. If you're ruining your or customers' knives, or even your own, why are you doing that?
4) Use Structured Abrasives-- Structured abrasive belts such as 3M Trizact generate far less heat than other types of belts. 5) Dip The Blade-- Dipping the blade in cold water after each pass on your grinder will help prevent the build-up of excessive heat. 6) Hand Sharpen After Belt Grinding-- I do this for every knife I sharpen. They ALL get a hand sharpening after a touch-up on the belt sander. At Keith Nix Knives, we use all the other precautions when belt sharpening, thinning edges, or repairing chips, cracks, or broken tips. Hand sharpening that final apex assures we deliver the finest edges possible with no chance for a HAZ to ruin your blades. SUMMARY: It is common for knife edges to overheat during power sharpening operations, as the small volume of steel at the blade's apex makes it prone to this issue. To minimize the chance of overheating, it is advisable to grind wet, grind slowly, use structured abrasives, and dip the blade in water. Hand sharpening ONLY is an effective way to eliminate this issue. Hand sharpening a small micro-bevel after grinding can also help. Exercising great care while sharpening a fine knife is essential, as it can be ruined in mere seconds if not appropriately handled.