PURPOSE: To obtain a steel die for extrusion molding of aluminum having a bearing surface subjected to a nitriding treatment with a uniform hardness distribution.
CONSTITUTION: A steel die blank formed with narrow slits is formed of a tool steel. The steel die blank packed with a polyvinyl chloride(PVC) resin in the slits is housed into a hermetic vessel and is kept at a high temp. of ≥400°C to thermally decompose the PVC. After the inside wall surfaces of the slits are cleaned by the gases formed by the thermal decomposition, the atmosphere in the hermetic vessel AS substd. with the nitriding atmosphere and the blank is subjected to the nitriding treatment. As a result, the halogen and hologen hydride formed by the thermal decomposition of the PVC are filled in the slits and the bearing surface is cleaned. The nitriding reaction is thus made uniform over the entire area of the bearing surface.
KAWAI TOSHIO
TAKAZAWA REIKO
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