First, let us state that Aluminum reacts strongly with nitrogen in steel. There are other components in the steel that can react, such as chromium, but aluminum is the strongest reactor. If so, why is it common to Nitride die cast steel before the aluminum injection process? The reason is because aluminum nitrides form inside the steel structure, and expand. This puts the surface of nitrided steel under compression, which can be a good thing in the case of nitriding steels.

During the aluminum injection molding process, the same chemical reaction takes place, but with the aluminum and nitrogen coming from different places. An H13 tool steel who’s surface has been heavily nitrided will react with molten aluminum diffusing into the system to form the same types of aluminum nitride compounds. When this occurs, the same expansion occurs, but in an uncontrolled manner, leading not only to surface compression, but cracking as well.

Aluminum reacts with nitrogen compounds. The heat of formation for aluminum nitride is -76.47 kcal/mol while the heat of formation for aluminum carbide is -49.7 kcal/mol, both at 298.15 K. These numbers are only somewhat useful, as they depend upon many factors and assumptions including room temperature. But the aluminum nitrides lower delta G value means it is thermodynamically favored to form compared to aluminum carbide.

The reasons for this basic phenomenon gets us to the atomic level of where the effects of atomic sizes, bond structure, electronic valences, and crystalline shape all interact to result in a general reaction strength and mechanism that is indicated by the heat of formation value mentioned above.

The process of stopping the reaction of molten aluminum with an H13 surface is essentially the process of stopping the chemical reaction of aluminum with the H13 and the nitrogen added to its surface. It is a general principle in chemistry that pure materials tend to undergo chemical reactions much better than impure materials. Hence impurity, or positively stated, “alloy-complexity”, can lead to significant reductions in rates of these adverse chemical reactions. The reasons for this are numerous, but among them are steric interferences, where two chemicals cannot react because a third chemical physically blocks them from contact, and reaction spoiling effects, where a new crystal starts to form, but an impurity enters the structure and the shift in crystalline shape prevent further reaction.

It should be noted, before one errantly concludes to either not treated the H13, or simply carburize the H13 surface, that the aluminum will react very quickly with an H13 surface that has no nitrogen present and a purely carburized H13 is not as hard or stable. Solvenite’s combination of elements is what is essential for success.