Heat Treating Simplified

This is an article to explain heat treating to a person who is not really interested in steel structure terms such as Martensite, Austensite and Pearlite. They just want it explained in everyday language.

So here goes. When we talk about “heat treating” steel we are talking about the process of making a piece of steel as hard or soft as needed to perform a specific job. The following is the series of events in order.

Heat treating can vary bladesmith to bladesmith and steel type to steel type. For example, one bladesmith may have a different process for heat treating a high carbon steel for a carving blade and yet another for heat treating a filet knife. The bladesmith can equate to a chef, they all have thier “special recipes” and methods to create thier individual product.

For our purpose, critical temperature is when the molecules are moving around without burning up or destroying each other.

Annealing
Annealing is heating steel to a critical temperature (sometimes 2000º) and letting it cool down very slowly to room temperature in an oven, forge or insulating mediums such as woodstove ashes, vermiculite, or ceramic insulation.

Why do we do this?
This is simply making steel as soft as possible. When we start to form our blade, whether it is with a file, sandpaper or grinder, it is more easily formed when it is in a softer state. For example, M2 steel is a high speed alloy steel that is used for planer and moulding blades. Very hard stuff. It is sharpened and cut with hard ceramic stone. To try and file it in its hardened state is like trying to push a needle into glass. When properly annealed, you can file it and sand it like a piece of copper.
Picture in your mind a bundle of rubber bands that you are grasping in both hands. We are taking the bundle of super stretched rubber bands and putting it in a totally relaxed state so that we are able to easily change its form and remove rubber bands from the bundle.

Normalizing
Normalizing is heating steel to a critical temperature, holding it there for a few minutes, and then taking it out of the forge to cool in still air.

Why do we do this?
We do this to help the steel adjust to it’s new form after forging, straightening or grinding. When we change the form of the steel we create stresses that are uneven in the steel.

To use the rubber band analogy, when you try to stretch them apart, some bands are very tight and some are still relaxed. When we normalize we are trying to get all of the steel in the same state of tension.

Hardening
Hardening is heating steel to a critical temperature where all of the molecules of the steel are at their best arrangement and stopping them abruptly using a coolant or quench.

Why do we do this?
This is making steel as hard as possible. A cutting edge is best when we can form the hardest, finest, smoothest edge possible.

Back to the rubber band bundle. With the bundles of rubber bands relaxed you can put them across your leg and they don’t make much of a impression. Stretch them as far as you can and push them into your leg and they make a good impression.

Tempering
Tempering is removing hardness from steel with heat to give it more toughness and flexibility.

Why do we do this?
This process helps the blade become more stable. If left in its hardened state the blade would be as hard and brittle as glass. The blade will snap like a potato chip when we try to carve with it. When we temper it we make it relax to a more usable state.

Rubber bands again. When we stretch the rubber bands to their limits and push them against our leg some of them break. But if we can remove some of the tension, and still make a dent in our leg without breaking them we’ve achieved a perfect hardness.

Heat treating can vary bladesmith to bladesmith and steel type to steel type. For example, one bladesmith may have a different process for heat treating a high carbon steel for a carving blade and yet another for heat treating a filet knife. The bladesmith can equate to a chef, they all have thier “special recipes” and methods to create thier individual product.

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