Minting is a perfect example of where art meets science and technical expertise.

Masterfully designed coin artworks, created by skilled artists, are engraved into steel using a computer-controlled engraving machine to produce a reduction die.

The reduction die is then placed in a hydraulic press, where through high pressure, ranging from 100 to 400 tons, the design is transferred onto a softened steel block. This process results in a master die that contains a negative impression of the coin’s design.

The master die is then subjected to a hardening treatment and subsequently used to produce several negative dies, or working dies, intended for the actual coining process.

Once pressed, the minting dies are shaped, hardened, and sometimes plated to enhance durability. Depending on the type of coin being produced, a single set of minting dies can be used to strike millions of coins.

The minting industry relies on the precision and longevity of its tools and dies to sustain high production volumes. During the coining process, the workpiece is subjected to intense pressure, inducing plastic deformation on its surface to imprint the negative image onto the coin. Typically, two metallic dies strike the coin simultaneously, one for each side of the workpiece, ensuring sharp, detailed impressions with every strike.

 

Figure 1: A tool steel coining die (courtesy of Staatliche Münzen Baden-Württemberg)

 

This process allows to produce hundreds of coins per minute, with high repeatability, even for designs having high relief or very fine features.

Dies used in the coining process are typically made from cold-work tool steel, which generally consists of high-carbon steel alloyed with elements such as chromium, molybdenum, tungsten, and vanadium. These elements provide an optimal balance of resistance to wear, chipping, and cracking.

Heat treatment is a critical process in enhancing the properties of the dies, ensuring they achieve a suitable combination of hardness and toughness.

Vacuum furnaces play a pivotal role in the heat treatment process, offering several advantages over more conventional methods. In this article, drawing on our 25 years of experience as an equipment provider for the coining industry, we’ll explain why!

 

The heat treatment of coining dies

Different grades of cold work tool steel are used in the minting industry.

The heat treatment of such grade typically consists in hardening the steel from a suitable temperature, quenching the parts at a rate sufficient for the martensitic transformation of the material, followed by ore or multiple tempering cycles to achieve the desired hardness and mechanical properties.

 

Figure 2: A typical heat treatment cycle for coining dies

 

In the following table, some of the most common commercial steel grades, used for the manufacturing of coining dies, are presented.

 

Commercial name	Chemical Composition (%)	Equivalent grade	Hardenability (1-10)
K455	C: 0,63, Cr: 1,10, Mn: 0,3, Si: 0,6, V: 0,18, W:2,00	1.2560 , 60WCrV7	6
K340	C: 1,10, Cr: 8,50, Mo: 2,10, V: 0,50, Mn: 0,4, Si: 0,9	Patented	8
ASP 2012	C: 0,6, Cr: 4,00, Mo: 2,00, V: 1,50, W: 2,10	1.3397	9
K890	C: 0,85, Si: 0,55 Cr: 4,35, Mo: 2,80, V: 2,10, W: 2,55 Co: 4,5	Patented	10
Caldie	C: 0,70, Cr: 5,00, Mo: 2,30, V: 0,50, Mn: 0,50	Patented	8
K110	C: 1,55, Cr: 11,30, Mo: 0,80, V: 0,95, Mn: 0,30	1.2379, X153CrMoV12	8
K460	C: 0,95, Cr: 0,55, Mn: 1,10, Si: 0,25, V: 0,10, W: 0,55	1.2510, 100MnCrW4	5

 

 

The hardenability value shown in the table is estimated based on the continuous cooling transformation curves for the specific alloy. As deducible from the hardenability values, while some steel grades can be effectively hardened by a relatively slow cooling, equivalent to air cooling, some others, such as K455 (60WCrV7) and K460 (100MnCrW4) are characterized by a significantly lower hardenability.

For those latter materials, the critical cooling rate (i.e. the cooling rate necessary to form 100% martensite upon quenching) can be as high as 23 °C/s and 31°C/s respectively, between 800°C and 500°C. Therefore, K455 (60WCrV7) and K460 (100MnCrW4) steel have been historically considered “oil quenching steel”; however, in the past years, vacuum furnaces equipped with integral high pressure gas quenching, proved to be effective in hardening dies manufactures using such steel grades.

Other than achieving a sufficiently high quenching rate, the equipment and quenching media selected for the heat treatment of cold work tool steel, as the ones presented in the table above, should be able to satisfy the following criteria:

• Guarantee an optimal temperature uniformity during heating and cooling, to avoid microstructural inhomogeneity and excessive residual stresses.
• Avoid surface contamination of the material, to avoid undesirable effects such as decarburization and, consequently, loss of mechanical properties.
• Being fully programmable, to allow for a precise execution of complex heat treatment programs that include multiple preheating steps and different heating and cooling rates

Modern vacuum furnaces can meet all the aforementioned requirements and are therefore inherently well-suited for processing cold work tool steel; they can guarantee temperature uniformity better than +/5°C, by operating in a vacuum or partial pressure of inert gas (i.e. pressure below atmospheric) they eliminate the risk of surface contamination and, finally, they can perform complex heat treatment programs with minimum deviation from the setpoints and reaction times in the order of seconds.

 

Figure 3: A TAV vacuum furnace dedicated to quenching and tempering of coining dies

 

In vacuum furnaces with integral high pressure gas quenching, the cooling effect is generated through circulation of high pressure inert gas, inside the furnace, through a water cooled heat exchanger. Such a system provides several benefits over the more traditional oil quenching process, including the elimination of oil residuals on the parts, a more uniform heat transfer and a significant reduction in the safety and environmental impact.

 

Figure 4: Schematic representation of the gas quenching process in a vacuum furnace

 

In the past, the possibility of using high pressure of gas up to 20 bar, highly increase the cooling rates achievable through gas quenching; at the same time, other technological advancements such as gas recuperation systems, made economically viable to use helium as a quenching gas compared to the more traditional nitrogen. The lower molecular mass and higher thermal conductivity of helium, in fact, further increase the heat transfer coefficient during high pressure gas quenching, allowing to achieve cooling rates comparable to those of oil quenching.

Later on, further developments such as the improvement of the heat exchanger materials and manufacturing technologies, other than optimization of the gas flow velocity and distribution through intensive R&D aided by computational fluid dynamics simulations, allowed to further increase the quenching rates achievable at a given pressure. This made it possible to effectively quench oil hardening cold work steel grade effectively, even at lower gas pressure and using nitrogen gas, leading to a reduction in the operational costs required for the process.

 

Figure 5: A batch of coining dies ready to be heat treated

 

More recently, the development and growing popularity of multi-chamber vacuum furnaces have opened up new possibilities for the heat treatment of tool steel.

In its simplest form, a multi-chamber vacuum furnace consists of a dual-chamber design.

In a single-chamber vacuum furnace, the entire heat treatment process takes place with the load inside the hot zone. In contrast, dual-chamber vacuum furnaces feature a separate cold chamber dedicated to quenching. Despite their increased complexity, these furnaces offer several advantages.

First, the graphite-insulated hot chamber is never exposed to ambient air during loading or unloading. This allows the hot chamber to remain preheated at the treatment temperature, whereas in single-chamber furnaces, the hot zone must always be loaded and unloaded at room temperature to prevent oxidation damage to the graphite insulation. As a result, dual-chamber vacuum furnaces enable faster heating cycles and lower energy consumption, as a significant amount of energy is required to reheat the hot zone in single-chamber designs.

Additionally, since quenching takes place in a separate chamber, the hot zone insulation in dual-chamber vacuum furnaces can be enhanced by increasing the thickness of the graphite board without compromising cooling performance.

 

Figure 6: TAV DC4, Dual Chamber vacuum furnace

 

Furthermore, the presence of a dedicated cold chamber, designed to remain at room temperature, enables significantly faster quenching rates compared to single-chamber designs. The heat transfer coefficient can increase by 50% to 100%, depending on quenching pressure and gas type. This improvement enhances the furnace’s ability to achieve the desired metallurgical properties, supports higher load capacities, and allows for effective heat treatment even at lower quenching pressures.

Conclusion

The evolution of vacuum furnaces has revolutionized the heat treatment of coin dies, making them an indispensable tool in the minting industry.

With advantages such as precise temperature control, preservation of surface properties, and fast, uniform quenching rates through gas quenching, vacuum furnaces have surpassed conventional methods in both efficiency and quality.

The introduction of multi-chamber designs further enhances performance, leading to significant cost savings and sustainability improvements. 
As the minting industry continues to demand higher precision and durability, vacuum furnace technology will remain at the forefront, ensuring the production of superior coining dies for years to come.