Volume 8, Issue 9 (October 2011)
Impact Properties of Vacuum Heat Treated AISI D2 and 8 % Cr Cold Work Tool Steels
The impact properties of the traditional 12 % Cr AISI D2 steel and a family of new generation 8 % Cr cold work tool steels (average chemical composition fulfilling the following ranges: C = 1.00 ± 0.20; Cr = 8.00 ± 0.50; Mo = 1.75 ± 0.75; V = 1.10 ± 0.70; Nb = 0.50 ± 0.45; Si = 0.95 ± 0.10; Mn ≤ 0.50; P≤0.03; S ≤ 0.015) have been investigated. All steels have been vacuum heat treated simultaneously after austenitization at 1030°C, 1060°C, and 1120°C, followed by a cryogenic treatment and triple tempers at 520°C of 2 h each. The results showed for all steels that the higher the austenitizing temperature the higher the hardness and the lower the impact properties. For the same impact value, the 8 % Cr steels presented a 3–4 Hardness Rockwell C (HRC) points higher than the AISI D2 steel. The average impact values of the 8 % Cr steels were 33 %–45 % superior to the 12 % Cr steels, but only after austenitizing at 1030°C and 1060°C. At 1120°C, the impact values dropped dramatically for both 12 % Cr and 8 % Cr steels due to the higher hardness and grain growth differences between the investigated steels. The surfaces of the fractured impact tested samples were investigated via scanning electron microscopy + energy dispersive spectroscopy techniques. The fractographic analysis showed that at 1030°C the much coarser and higher volume fraction of primary chromium carbides found in the AISI D2 than in the 8 % Cr have contributed to its lower toughness. However, as the austenitizing temperature increased to 1060°C, and in particular at 1120°C, significant grain growth, higher hardness, and consequently, high strength of the matrix, took place and played the more important roles in dropping the toughness as shown by quasi-cleavage brittle fracture appearance. Austenitizing at 1060°C and tempering at 520°C showed the best compromise for toughness and hardness. Increasing the tempering temperature to 570°C and, in particular, to 620°C, remarkably increased the toughness of the 8 % Cr steels, when the % C is kept below 1 %. For the D2 and % C > 1.15 % the increase is negligible.