高クロム高コバルト系熱間ダイス鋼について

抄録

The fundamental characteristics of 12Cr-7W-0.5V-5Co (DSE) and 12Cr-7W-0.5V-10Co (DSF) type steels for hot work dies have been investigated.The results obtained are as follows:
(1) Transformation temperature on heating, specific gravity, specific heat and coefficient of thermal expansion are measured, and particularly it is noticeable that the coefficients of thermal expansion show small values.
(2) Secondary hardening occurs in the temperature range from 500 to 550°C, and that resistance to tempering softening becomes larger with increasing austenitizing temperature. The temperature range suitable for hardening is from 1050 to 1100°C, so the austenitizing temperature of 1050°C only is used for all tests.
(3) The nose of isothermal transformation curve situates at the temperature of 750°C and 15min for DSE, and 750°C and 25min for DSF.Ms temperature is 205°C for DSE and 175°C for DSF.
(4) At the testing temperature of 600°C, hardness, tensile strength and impact strength are Hv 225, 93 kg/mm₂ and 8 kg m/cm² for DSE, and Hv 270, 107 kg /mm² and 5 kg m/cm² for DSF.
(5) The dimensional change of a round bar at the tempering temperature of 600°C is +0.045% for DSE and + 0.040% for DSF in the direction of diameter, and 0% for DSE and -0.010% for DSF in the direction of length.
(6) The tempering at 600°C gives the best impact strength and wear resistance at room temperature.
(7) When both steels are isothermally tempered for 1000 hr at 600, 650 and 700°C, hardness decreases linearly and impact strength slightly with increasing tempering time at each temperature.
And in the process of these temperings, the following sequence of changes in carbides takes place: M₆C+M₇C₃→M₆C+M₇C₃+M₂₃C₆→M₆C+M₂₃C₆→₆C+M₂₃C₆+Fe₂W→M₂₃C₆+Fe₂W.
(8) Resistance to heat checking is better than that of SKD5 (JIS) steel of 10W-3Cr-0.5V type.
(9) Resistance to oxidation at high temperatures is very excellent as compared with SKD 5 steel.