Editor’s Note: This is an excerpt from Murray Carter’s new book, Bladesmithing with Murray Carter. Click here to learn more from this renowned knifemaker.
The first and most logical approach is to select steel by doing some research on cutlery grade steel. Topics to especially pay attention to are the chemical composition of different steels and the effects of each chemical; isothermal transformation graphs for different steels; the effects and importance of thermal cycles, including annealing, quenching and tempering; the availability and cost of the steels and, perhaps most importantly, how easy the steels are to work by hand.
The above paragraph only took a few lines to write and a few minutes to compose, and yet many metallurgists will spend their whole lives researching those points. Pursue it to your heart’s content, and to that end I have included a thorough bibliography at the back of this book. However, to get you focused back on the task at hand, suffice it to say that there are some specifi cs you will need to concentrate on when selecting steel. These are:
• The amount of carbon in the steel
• Forging temperature range
• Annealing temperature, quenching temperature and tempering temperature
• Overall workability
Let’s examine each of these in a little more detail.
Carbon is added to iron (Fe) to make steel. Carbon is the element that enables steel to harden when it is quenched at the proper temperature. Steel is unique in this aspect; all other metals soften when subjected to the same thermal cycle. The amount of carbon is very important. Too little carbon will fail to make steel harden when quenched, and too much carbon turns cutlery steel into cast steel. Generally speaking, less than 0.5 percent carbon is considered low carbon steel, and not suitable for blades. More than 1.6 percent carbon is considered extremely high carbon steel, and is very tricky to make into a blade. More than two percent usually equates to cast steel. Hence, most blades in the world have a carbon content between 0.5 percent and 1.5 percent.
In this range, all other factors being equal, the more carbon, the harder the blade gets when quenched. The harder the blade, the finer an edge and the longer it will stay sharp.
Forging temperature range is the temperature at which you can “work” or manipulate the steel. Most steel can be worked between a bright red heat and orange/yellow heat (approx .700~900 degrees Celsius, 1290-1650 Fahrenheit) and it will yield under the blow of the hammer, bend or twist etc.
Below this range the steel will cease to yield to manipulation and can be damaged by subjecting it to stress. Likewise, steel can be irreversibly damaged from working it at too high a temperature.
Annealing, quenching and tempering are the three phases of heat treating steel. These temperatures are very critical figures to commit to memory for the steel you are working. The heart and soul of a blade is the heat treat, as the final quality of the blade will be determined by how successfully these three operations are accomplished. Knowing the proper temperatures, and knowing what they look and feel like, is a critical skill for the bladesmith.
Availability determines whether or not you will be able to try forging that ‘super-steel’ you have been reading about. If you cannot locate or buy the steel in question, the pursuit becomes meaningless. Cost is another factor. Even if you locate the steel of your dreams, it may cost too much to have it shipped to your location. You want to know if you can acquire the steel for a reasonable cost and if it will be available in the foreseeable future.
Overall workability considers how the steel in question compares to other steels. Is it easy to manipulate under the hammer when you are forging it? How does it heat treat? Is it prone to warping, bending or cracking? Is it easy to straighten after heat treating? How does it take a final polish?
These questions are easier to answer once you have experience with a few of the common steels.