| 摘要 |
PROBLEM TO BE SOLVED: To provide a mold which is excellent in heat impact resistance, heat fatigue resistance, wear resistance and impact resistance, and long in service life by forming a nitrogen diffusion layer of specified hardness using a bright nitrogen diffusion method on the surface of the die. SOLUTION: A nitrogen diffusion layer is 800-1200 in Vickers hardness (load: 100 gf). A radical nitrided layer 3C of 30-300μm in thickness is formed on a sculpture surface 3B of a cavity. An upper mold and a lower mold 3 which are degreased and cleaned are placed on a cathode in a vacuum furnace, and the furnace is evacuated, and heated at 400-580 deg.C in the atmosphere of a mixture gas consisting of 5-99% hydrogen and 1-95% ammonium, the DC voltage of 300-500 V is applied between the cathode and an anode in the vacuum furnace, and the gas is ionized through the glow discharge to diffuse nitrogen on the mold surface. The nitrided layer is hard, thick and high in toughness, and the hardness of the nitrided layer is continuously reduced, and thus, the nitrided layer is excellent in close tightness to a base steel layer. The nitrided layer is sufficiently wear resistant, and free from the brittleness, and excellent in cracking resistance and heat impact resistance.
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