Heat Treatment of Coin Dies
One of the beneficial attributes of steel is that it can be heat treated in numerous ways to get a wide variety of properties for many different end applications. One of these applications are the dies used to strike coins. I’ve seen terms like “hardening,” “annealing,” and “quenching” in coin-related publications before, and after working with this subject for a year and a half, I’ve gotten to learn quite a bit about how the various heat treating processes work. I thought it would be helpful to share what goes on when heat treating a coin die, like the 1820 half eagle die shown below (I believe this is the ANA’s image).
To start out, steel is characterized as an iron-carbon alloy. Other alloying elements (e.g., manganese, chromium, nickel, molybdenum, etc.) are added to achieve a wide array of properties (e.g., strength, hardness, toughness, corrosion resistance, machineability, etc.). At room temperature, steel is generally made up of two phases with different amounts of carbon atoms interspersed amongst the iron atoms: ferrite (extremely low carbon content) and cementite (relatively high carbon content). At high temperatures, though, a third phase is formed: austenite. Austenite is not stable at room temperature, but once the steel is heated hot enough to completely transform to austenite (called “austenitizing”), it can be cooled in different ways to get a variety of different microstructures and properties.
Depending on how quickly or how slowly the steel is cooled will affect how much the carbon can migrate when the steel tries to return to the ferrite and cementite phases. Time is an important factor because the austenite phase is not stable once it cools below a certain point, so once a piece of steel is taken out of the furnace, the clock starts ticking.
The transformations can be plotted on a temperature versus time (logarithmic) plot to form a “TTT diagram” (“TTT” stands for “time, temperature, transformation”). It should be noted that each steel composition will have a different TTT diagram based on how the alloying elements affect how quickly the austenite reverts to ferrite and cementite. Below is a generic TTT diagram for a carbon steel that is roughly 0.77% carbon.
While this TTT diagram is relatively simple compared to diagrams for other steels, there are some things that require explanation. First, for this steel, the dashed line at 727°C represents the temperature above which austenite (Greek letter gamma γ) is stable (the “austenitizing temperature”). Below this line, austenite is no longer stable and begins to transform into ferrite (Greek letter alpha α) and cementite (Fe3C). Depending on how quickly the steel is cooled after dropping below 727°C will determine which microstructures (pearlite, bainite, or martensite) form.
Pearlite, bainite, and martensite have different mechanical properties. Pearlite has relatively low strength and low hardness, but is fairly tough. Martensite has high strength and high hardness, but has low toughness. Bainite’s properties are somewhere in between. For the sake of clarity, strength, hardness, and toughness are defined as follows:
Strength: ability to resist deformation when a force is applied.
Hardness: ability to resist friction/wear.
Toughness: ability to resist fracturing/breaking when a force is applied.
To get pearlite or bainite, the steel has to cross through the yellow shaded region. The left side of this region marks where the unstable austenite begins to transform into ferrite and cementite. The right side of this region marks where essentially all of the austenite has transformed into ferrite and cementite. The dashed line in between represents where 50% of the austenite has been transformed. Once the steel “passes” entirely through the yellow shaded region, the transformation process is essentially over and it can be cooled to room temperature without much effect on the microstructure.
If the austenite was transformed through the yellow shaded region above ~500°C, a pearlite microstructure will form. If the austenite was transformed below ~500°C, a bainite microstructure will form. For this steel, it would be easy to get pearlite to form by cooling it somewhat slowly. This could be accomplished by cooling along the green curve, which represents an annealing heat treatment where the steel is heated above 727°C and allowed to slowly cool with the furnace to room temperature.
To get martensite to form, the steel must be cooled rapidly enough to avoid crossing into the yellow shaded region. This could be accomplished by rapidly cooling along the red curve, which represents quenching (“hardening”), generally in a liquid medium like water or oil (though depending on the steel it could be gas quenched).
For this particular steel, you would need to cool it extremely quickly because at the “nose” of the yellow shaded region, the austenite will start to transform in less than 1 second around 550°C. Therefore, for the steel represented by this particular TTT diagram, it would be very difficult to completely transform all of the austenite to martensite. However, die steels used to make coin dies include other alloying elements that shift the yellow region further to the right and allow a little more time to get past the “nose.” This makes it possible to quench them and successfully achieve a martensite microstructure.
Circling back to how dies are heat treated, dies can be quenched (rapidly cooled) or annealed (slowly cooled) after heated above the austenitizing temperature. If the engraver wants to engrave a die, they would need to austenitize and then anneal the die to reduce the hardness and make it easier to work with. Once the engraver is finished with preparing the die, they would need to austenitize and then quench the die to develop the necessary strength and hardness. However, as mentioned previously, the martensite produced by quenching has low toughness, meaning it would be very susceptible to cracking and breaking apart (either due to striking coins or internal stresses). Since this is a very undesirable characteristic for a coin die, an additional heat treatment after quenching is needed: tempering.
For tempering, the steel is heated well below the austenitizing temperature, held at that temperature, and then cooled. In martensite, the iron and carbon atoms are arranged in a very strained manner (causing internal stresses), and tempering allows for the atoms to move into less-strained configurations. Tempering reduces the hardness and strength, but it increases the toughness. Generally the hotter the tempering temperature, the more negatively impacted strength and hardness become. Since coin dies need to be fairly strong and hard, they would probably be tempered at a lower temperature, with the understanding that their toughness may not be particularly great.
So, to sum it all up, a coin die would would generally have the following heat treatments:
- Annealing to soften the die prior to engraving.
- Quenching to harden the die after the engraving was completed.
- Tempering to improve toughness without sacrificing too much strength and hardness.
The effort that goes into heat treating a die was probably an important factor in the early days of the US Mint when it came to deciding to overdate an outdated die or just use the die as-is. If the die hadn’t been quenched yet, it would be fairly easy to overdate the die. However, once the die was quenched/hardened, it would be much more challenging to overdate the die. This is likely why there are very few early US coin dies that were overdated after being quenched/hardened and used to make coins.
Here are some links that cover modern heat treatment processes employed by the Philadelphia and Denver Mints, and the Royal Canadian Mint:
For a more thorough technical explanation of steel heat treatments, see the following ASM International publication: