Steel D2

D2 steel is a die steel for cold forging with a higher content of additives than A2. In particular, the content of chromium and carbon are significantly high, which allows for the formation of large chromium carbides.

The austenitizing temperature of D2 (1010ºC) is slightly higher than that of A2 and is again gradually achieved; like in A2. The steel has high wear resistance due to the size of the carbides, but this reduces machinability and "grindability". Corrosion resistance is higher than that of tool steels (A2) due to the chromium content, however, it does not have the corrosion resistance of martensitic stainless steels because most of the chromium is contained in the carbides, considering the high carbon content available for bonding reactions and the low austenitizing temperature.

The carbides are coarse and can reach up to 50 µm in length, although the grain size according to ASTM is rated at no. 7.5. D2 is generally used in the industry for stamps, dies, and various types of knives. It is necessary that the residual austenite after quenching to room temperature is reduced by subcooling, known as "freezing". This provides a broad range for the annealing process of the steel after quenching and for the final martensite content, subsequently influencing hardness (300ºC - max. 64 HRC!). At a tempering temperature of up to 510ºC, the toughness increases to an acceptable level and hardness decreases to 58 to 60 HRC. High annealing temperatures transform residual austenite to martensite when cooling back to room temperature. The content of austenite is independent of carbide precipitation during annealing, which increases the MS value. Generally, multiple annealing can be used for the gradual transformation of newly formed martensite.

D2 can be austenitized to a temperature of 1120ºC with the intention to accelerate and increase the infiltration of alloying elements into the austenitic grain, which then reduces the MS and manifests as hardening hardness. This hardness is referred to as secondary hardness and is a reflection of secondary carbide precipitation and the transformation of residual austenite to martensite. High hardness is usually not beneficial mainly in terms of toughness.

Overall, D2 tool steel is widespread for its resistance due to the large content of coarse carbides and easily achievable hardness. Generally, it is a good steel for quality cutting blades with an average facet angle of the edge and special coarse surface finishes. Corrosion resistance is quite high for tool steel. This assessment does not apply to tests in a salt mist environment, also known as the salt test, where the corrosive action of chloride ions (Cl-) decreases the steel's resistance. However, this is a completely different requirement for corrosion resistance, which is considered, for example, when the steel is in contact with seawater, and it varies with each manufacturer.