steel D2
STEEL D2 is a die steel for cold forging with a higher additive content than A2. Especially the chromium and carbon content are high, which allows the formation of large chromium carbides. Austenitizing temperature D2 (1010ºC) is slightly higher than A2 and is again reached gradually; as in A2. Steel has a high wear resistance due to the size of the carbides, but the machinability and "grindability" decreases with it. The corrosion resistance is higher than that of tool steels (A2) with regard to chromium content, but it does not have the corrosion resistance of martensitic stainless steels due to the fact that most of the chromium is contained in the carbides, given the high content of carbon available for bonding reactions and the low austenitization temperature. The carbides are coarse and can reach up to 50 µm in length, although the ASTM grain size is rated No. 7.5. D2 is generally used in industry for punches, dies and various types of knives. It is necessary that the residual austenite after quenching to room temperature is reduced by subcooling, so-called "freezing".This provides a wide area for the annealing process of the steel after hardening and for the final martensite content and thus the subsequent hardness (300ºC - max. 64 HRC!). At tempering temperatures up to 510ºC, toughness increases to an acceptable limit and hardness decreases to 58 to 60 HRC. The high annealing temperature transforms the residual austenite into martensite when cooled to room temperature again. Austenite content is independent of carbide precipitation during annealing, which increases theMS value. In general, multiple annealing can be used to gradually transform the newly transformed martensite. D2 can be austenitized up to 1120ºC with the intention of accelerating and increasing the content of alloys penetrating the austenitic grain, which then reduces theMS and manifests itself as hardening hardness. This hardness is referred to as secondary hardness and is a reflection of secondary carbide precipitation and transformation of residual austenite to martensite. High hardness is not usually beneficial (www - Crucible, Timken, Böhler) especially in terms of toughness.
