An Intro to Heat Treating, Pt II

 

Last week we ran through not what we do in heat treating, or how, but why. The why is always imperative. It's hard to go on a journey if you have no idea where you're going. 

For a quick refresher, there are two basic qualities of steel on opposite ends of a sliding scale. One the one side is hardness, which equates to brittleness. Think of it like glass. On the opposite end of the scale, is toughness, which equates to softness. Think of it like copper. A very hard knife will hold an edge but will break easily, a tough knife will keep strong and  intact but will dull easily. A good knife will be somewhere in between the two. 

We can put our blade in between those two qualities by varying heating and cooling at different rates. This is called heat treating. 

That brings us to today's subject, the composition and structure of steel.

If last week was why, and next week is how, today will be more or less whathappens in heat treating. 

This guide will focus only on simple carbon steels, and not more complicated tool or stainless steels. If you wish to learn about those, a solid foundation of knowing carbon steel is again imperative. 

We start with the question, what exactly is steel? Steel can be defined as an iron based alloy containing carbon, and often a few other elements. Alloy, for the record, means a metal containing two or more elements. 

For knife making purposes, we are concerned with two types of steel: Mild steel, and Carbon steel. Carbon steel generally contains .3%-1% carbon by weight, and mild steel is generally anywhere below that, for our intents and purposes. Carbon steel is more specifically defined by the ability to harden. So, only carbon steel (also known as high carbon steel) can be used for blades. 

As you've probably guessed by now, steel requires a certain amount of carbon in order to be manipulated properly. So with that in mind let's get down to steel structure. 

Steel Structure

Steel, or iron specifically, generally takes on a specific atomic structure. Think of it as a lattice of cubes: one iron atom at every corner of the cube, and another atom in the center. Other atoms such as carbon are floating in between. This is called Body Centered Cubic, or BCC. 

Now, something extremely important to keep in mind is that this structure forms over a three-dimensional plane. In a given chunk of steel, you will have some areas where the lattice structure is oriented one way, and some where the lattice is oriented a slightly different way. These chunks, or grains, defined by specific orientation in which one atomic cube is connected to the same plane as the one across the grain, can be of varying sizes, from coarse sand size to almost powder. The border between two grains will be a weak point. A bar of steel with smaller and more numerous grains will require much more force to break than a bar with larger and fewer grains. We will get further into this in future articles, especially on normalizing, but this information is enough for now. 

BCC (Body Centered Cubic) is the standard formation for steel at room temperature. But, as soon as you heat it to roughly 1600 degrees Fahrenheit (varying from one alloy to another), grains of different atomic structure begin to grow and form at the borders of the BCC grains.

These new formations are again cubic, but instead of one atom at every corner and one in the center (BCC), they are comprised of one atom at every corner, and one atom at the center of every face. This is called Face Centered Cubic (FCC). So, while a BCC cube will have 9 iron atoms, an FCC cube will have 14. 

Like I mentioned in the previous article, this transformation is a phase transformation. Turning liquid water into a solid (ice) is one phase transition, turning liquid water into water vapor is another. Now while these two examples are liquid-solid, and liquid-vapor, the BCC to FCC transition is a solid-solid phase change. They each have specific temperatures at which they change, based mostly upon composition. 

The temperature where the steel is transformed from BCC to FCC is known as "Critical Temperature" among knife makers. An interesting thing to note is that as soon as this phase change happens, the steel loses its magnetism. So as a bladesmith, you can tell when you've reached the right temperature by touching the steel with a magnet. If it sticks, you're still BCC. If it doesn't, you've reached Critical and the steel has transformed to FCC. 

Make sense?

Now, the following gets more complicated the more one studies, but for a basic understanding, I am keeping it simple to suit our needs for basic heat treating. If you heat from room temperature to critical, you transform from BCC to FCC. If you let the steel cool slowly from Critical down to room temperature, it will transform from FCC back to BCC. The key word there is "slowly". If you can cool the steel quickly enough, and go from critical to a certain lower temperature fast enough, it's not merely changing to BCC. 

So what does happen? Let's go back to the structure. 

Remember, BCC are smaller compact cubes with carbon atoms floating in between one cube and another. FCC are larger and more spacious. So when the steel is converted to FCC, the carbon atoms slip inside the cubes. If the steel cools quickly enough, the FCC switches back to BCC, transforming the cubes, compacting them, and becoming smaller. 

And the carbon?

The carbon atoms are stuck inside boxes too small for them. This creates a lot of stress, and grains with large amounts of carbon in them while other grains have comparatively little. This creates a very rigid, hard structure across the steel. The atoms are rigid in relation to each other: it is difficult to move them, but when they do, they break bonds rather than deform

If you recall the previous article, this should seem familiar. Hard, but brittle. 

This pressure and stress may be removed by heating to a significantly lower temperature than critical, roughly 350-450 degrees Fahrenheit. This can be thought of as opening up the BCC cubes just a bit to relieve "pressure". The hotter the temperature, the softer the steel. 

To recap: 

Steel at room temperature is generally in Body Centered Cubic formation. Heated past critical temperature, BCC converts to FCC, or Face Centered Cubic. A fast enough cooling will revert formation quickly enough to "compress" and put pressure on the carbon atoms by trapping them in too small of cubes. This converts the steel to a very hard but brittle structure. This can be relieved to a varying degree by heating again, although to a significantly cooler temperature than critical. 

Next week, we'll be looking at exactly how this all applies and how we, as knifemakers and bladesmiths, or really anyone who works with steel, can use this knowledge to make the most physically capable blades and tools possible, through very simple means.

Remember, knowing the why and the what is the secret to knowing the how