Finding the right balance of properties is the key to extending the life of the mold. Material properties are strongly linked to microstructural features, which, in turn, depend on the chemistry and processing of the material. Non-metallic inclusions and cleanliness of the alloy are of particular importance. The H13 compared to other alloys by itself does a good job with striking this balance of properties including: yield strength, thermal expansion, thermal conductivity, and elastic modulus. It is important that surface treatments used to extend die life enhance, but do not radically alter this balance.

One example of this is in heat checking, caused when hot metal coming in contact with the water-cooled die surface generates large thermal gradients. The surface of the die expands due to the temperature increase. At the same time, the colder zone below the surface resists movement. As a result, the hot working surface goes under compressive stress condition, which is then quickly reversed, subjecting the die to tensile stress. A network of fine cracks develops gradually on the working cavity surface of the die, deteriorating the surface quality of the cast product and eventually destroying the integrity of the die.

It is widely believed that to have a good die surface treatment, you must have the highest possible resistance to yielding in the temperature range between 1000 to 1200 oF. This is why there many companies trying to use superalloys and hard pure nitriding treatments to prevent heat checking. Many companies have also learned that despite the laboratory numbers these dies fail to live up to the theories in actual use.

To explain this let us think about the fact that H13 has a lower tempering temperature than many of the superalloys that have been tried, yet it retains its hardness in use even when the aluminum is injected at temperatures where the H13 temper should begin to fail. The reason for this is H13 has a much greater coefficient of thermal transfer than the superalloys. So even though the numbers say it will soften quickly, in the field, the die moves the heat away rapidly and holds its temper.

In the same manner, nitriding a surface puts the maximum yield strength into the surface, but the Solvenite process treats a die to have a higher yield strength, and a balance of elastic properties, This is one way that a unique treatment like Solvenite can outperform what the “lab numbers” say should not work.

Another way that Solvenite outlasts many of its competitors is its inherent resistance to diffusion of precipitating elements into the grain boundaries. Nitro-carburizing treatments can improve the resistance of H13 pins to washout (loss of base metal) but the secret to long term performance is a balanced approach. Thicker nitro-carburized layers provide better resistance to washout. However, they tend to crack more readily than thin coatings. Thick nitro-carburized coatings are desirable in “soldering intensive” applications; thin nitro-carburized coatings may be desirable in “thermal fatigue intensive” applications that can induce cracking. Since the formation of intermetallic layers along with mechanical interlocking are the primary reasons for soldering in heat resistant alloys, the diffusion blocking properties of Solvenite can significantly improve this as well.

Footnote: H13 is an excellent alloy, but making a long lasting mold starts with high quality H13 alloy and high quality machining to produce the mold. The higher quality of the material to start with, the more life the Solvenite process can add to it.