Carbide evolution for a quenched and tempered Dievar steel

Carbide precipitation, coarsening and hardness variations have been investigated for a quenched and tempered Dievar steel by SEM, TEM, STEM, HRTEM, XRD and Rockwell hardness testing. The carbide precipitation sequence has been ascertained as: M8C7 + bainitic/auto-tempered carbides -> M8C7 + M2C + M7C3 -> M8C7+ M7C3 +M23C6 -> M8C7 + M7C3 + M23C6 + M6C. Boundary carbides coarsen obviously faster than matrix ones. Hardness variations originating from the carbide evolution have been qualified. A fine-dispersion of M2C carbides leads to a high hardness tempered at 550 degrees C, whereas the hardness drops drastically for tempering at 600/650 degrees C predominately due to the dissolution of unstable M2C and the coarsening of carbides.