Dixcel - HD type | The Art of Heat Treated Brake RotorsHD Type In pursuit of higher stability The heat treatment stabilizes the bonds between the elements in the structure of the cast iron. This prevents disc distortion and strain which is common immediately after installing new discs. The treatment also optimizes the surface hardness, which improves brake response. Higher performance can be achieved by using the HD discs along with the Z type or R series pads. We recommend this disc for advanced drivers