Making a Custom Knife: Carbon Steel

A gentleman of my long acquaintance asked me to fabricate a knife for the woman in his life. Since I only do blade work for myself or close acquaintances this posed an interesting problem for me. There can be a certain intimacy in making a blade for a person as what their needs are becomes important in the overall design and can be answered only by rather personal questions. I am not in the habit of asking other women personal questions but accrued obligations are the sine qua non of my life so I accepted.  Having said that I'll walk through the process to get it clear in my own thoughts. For working with steel there is one diagram that should be engraved in the mind. The Iron-Carbon Phase Diagram.

I'm only showing it out to the 2 percent carbon point as this is the blade maker's area of interest. From the diagram you can see the eutectoid point that occurs at 0.76 wt.% carbon at around 730 Degrees C. Whether a steel is chosen to the left, right or at the eutectoid point is up to the maker. For much of my work I use steel that has a carbon content at or greater than the eutectoid point. The phase diagram goes directly to the heat treatment required for a given plain carbon steel.

There is much discussion about heat treatment, what is meant by it, what exactly are you trying to achieve, and so on. In it's simplest form it's heating to the Austenite point then cooling of the steel to achieve specific properties. Subsequent heating and cooling may be used to modify those properties.

When the blade is heated to the appropriate temperature all of the steel is in the gamma phase or Austenite. During quenching it is the blade maker's goal to achieve the Martensitic phase at the edge which gives high hardness. To determine the appropriate temperature for heating to the Austenite Temperature the following graph devised from the phase diagram is useful. If I choose a 1 wt.% carbon steel I'll want to Austenitize the steel to the heat treatment line (dashed line) so I would heat it to 850 Degrees C (1560 Degrees F)

The next thing to do to the steel is quench it from the Austenitizing temperature. As I said we want to introduce a Martensitic phase to the steel so it must be quenched according to the ITT diagram shown in the next graph. To go from Austenite to Martensite the cooling rate must be such that we avoid the "knee" of the graph and reach the start of the Martensite formation (Ms) line via the dashed red line. A water or oil quench is usually sufficient.

A tempering of the steel is required to reduce the potential brittleness of the blade. The next graph shows hardness as a function of carbon content and final hardness after tempering at three different temperatures.

So now I have the basics to choose a steel. I've chosen a 1084 which will provide high hardness for a keen edge and I can subsequently temper it quite easily. It is also easy to use in knife making while providing for a quality finished piece. Here are some links to steel suppliers for blade making. Alpha Knife Supply  and Jantz Supply. In later posts I'll show the steps of making this knife.