The Basics of Heat Treating, part I

 

This is an primer on the basics of heat treating, which is a rather fascinating science full of mystery. Rather than tell you what to do in a step-by step process, I find it's far more beneficial to give you a look into the steel itself and explain what's actually going on. This helps for testing, for altering, and for troubleshooting and gives you a feel for what is likely to happen in any given scenario. 

So, basics basics. Let's start with a fairly simple question.

What physical qualities make a high-performance blade?

Much of it has to do with geometry and the angles of the bevels and shape of the knife, but right now we're interested in the physical characteristics of the metal itself. 
 

  • A good blade will need to have the ability to be made exceptionally sharp. 
  • A good blade will need to have the ability to retain that sharpness after repeated use. This is generally known as "the ability to hold an edge".
  • A good blade will need to have the toughness to not break under stress. Preferably, to spring back to position rather than break or take a set (or bend).


We can disregard the first point for now, as it is directly related to the other two. Now, this is an important principle. In general, there are two physical qualities at opposite ends of a spectrum that steel can possess. 

The first is Hardness. In essence, hard steel is made up of molecules that are very resistant to moving without breaking bonds in relation to each other. This means it will break rather than bend, and chip rather than dent. A hard steel will hold an edge (not dull) very well, but it will be brittle. Hardness and brittleness are the positive and negative features of the same state. Think of a blade made of glass

The second quality is Toughness. In essence, the molecules are more fluid in relation to each other, and in stress they will modify position rather than break a bond. This means it will bend rather than break, and dent rather than chip. A tough blade will not break under stress, but it will dull very easily. Think of a blade made of copper

Now these two qualities are on a sliding scale. The tougher a blade is, the easier to dull it will be. The harder a blade is, the more brittle. Ideally, we want a blade that is somewhere in between. We want a blade that will hold a sharp edge through lots of use, and we want a blade to stay tough and strong through lots of impact and stress. 

These two qualities are of course not the be all end all, but they are a huge indicator of what exactly the goal is in heat treating. Remember, the technological progression of humanity is always about making it better; more efficient, prettier, easier and cheaper to attain. In handmade knives, easier and cheaper often goes out out the window because we are focused on making it the best. If you keep the principles outlined in this article in mind, then you begin to see why and how different materials for blades were used and replaced throughout history. But especially, these principles show you exactly where the end goal in heat treating is. 

Next article (probably next week), we'll look very in depth to the molecular structure and composition of steel.

P.S.,

Just a tidbit for you to chew on for next week: steel is an alloy that has many phases. We are used to the common phases of water: namely, Ice (solid), Water, (liquid), and Water Vapor (gas). You may have heard of a fourth phase: Plasma. Steel however, has sub-phases, which are all solids. Manipulating these phases and the transitions between them is the secret to a high-performance blade. And, just like water, they are manipulated by heating and cooling, and proper timing.