An Intro to Heat Treating, part III

 

Note: As I'm writing this I realize that the amount of information I originally wished to contain in this article was far too much, therefore this week's article is focusing on Normalizing alone

Now that you understand what goes on in the chemical structure of steel, we can focus on applying this knowledge. 

First, a couple disclaimers:

You may do everything to the letter I've outlined in this article and in the past few, and you won't be chopping boards and shaving hair. This is because different steels require different temperatures, and for truly excellent heat treating, you must start out with the temperatures and processes outlined in steel charts for the steel you're using, then further test and tweak to get it where you want it. 

So, let's get to the actual process. Heat treating is a general term for using various heating and cooling rates to change the physical properties of steel. Underneath this we can break simple heat treating down into three basic processes: Normalizing, Hardening, and Tempering. Now there are a few others, such as Cryo Tempering and Annealing, but we won't get into them until the last few articles. 

Normalizing

After forging, steel has a lot of stresses and a large grain size, which makes the steel quite brittle without being hard. Normalizing is done in several cycles, two or three after forging and another cycle after rough grinding.

If you remember last week's article, a steel bar will be formed of many grains, which are composed of steel atoms arranged in a particular pattern (BCC) along a certain plane. One grain will have a pattern oriented in a slightly different angle than the grain next to it.

Larger (and so, fewer) grains produces a weaker structure than smaller (and so, more numerous) grains. The goal of Normalizing is to minimize the size of the grains. 

As the steel is heated past critical temperature, new grains (FCC structure) will begin to grow along the borders of the already formed BCC grains. Soon past critical temperature, all the BCC grains have been converted to smaller and more numerous FCC grains.

A side note: If this temperature is held, and further, if the steel is worked say by a hammer, some grains break down and join their larger neighbors, and the overall grain structure is made larger. This is obviously not what we want. 

The process for normalizing thus far is as follows: the steel is heated up to critical temperature, often a little hotter to make sure of full grain conversion. Once you're sure it's evenly heated, remove the steel from the heat and let it air cool until it's completely black. 

So what does that do? As the steel cools below critical temperature, the grains once again begin to convert; BCC grains form and the junctions of the FCC grains. Again, these grains are slightly smaller than the previous ones. 

Heating and cooling like this is one cycle of normalizing. This process is repeated, usually 2-4 times, each time further refining and shrinking the grain size. 

Many knife makers will occasionally make a test blade, and after various chopping and cutting tests, and finally testing it to destruction where the blade breaks, will observe the grain structure. The steel will not break through the grains, rather, the weak points are the junctions in the grains, so a broken end of a steel bar will show you quite clearly the grain size, as in the cover photo at the top of this article. 

So there it is, the process for normalizing. To recap, normalizing is done by heating a bit above critical temperature then letting it air cool until completely black. This cycle is done three or four times. Many smiths prefer two or three cycles after forging, and another cycle after rough grinding. The effect of normalizing is to refine the grain structure and relieve stress, creating a blade with much higher overall strength. 


P.S. As mentioned at the beginning of this article, I originally intended to stuff all the different parts of heat treating into one article,  but that was far too much information. So, next week, we'll take a look at hardening. Before hardening, you have a "knife-shaped object". If it survives hardening, a real blade is truly born.