Method for manufacturing valve umbrella portion of hollow engine valve, press device of valve umbrella portion of hollow engine valve, and hollow engine valve

A method for manufacturing a valve umbrella portion of a hollow engine valve having a valve umbrella portion hollow which opens at a side to be welded to a hollow shaft portion or to a shaft end sealing material, the valve umbrella portion hollow being formed with an expanded diameter within an expanded-diameter section of the valve umbrella portion, a maximum inner diameter of the valve umbrella portion hollow being larger than a maximum outer diameter of the hollow shaft portion,

• the method for manufacturing, comprising:
• a first step of producing a valve umbrella portion semifinished product from a round solid bar as a raw material;
• and
• a second step of converting the valve umbrella portion semifinished product into the valve umbrella portion in finished form by warm forging,
• wherein the first step is adapted to obtain the valve umbrella portion semifinished product which
• has an expanded-diameter section integral with a body of a cylindrical shape at an end of the body, a maximum outer diameter of the expanded-diameter section being equal to a maximum outer diameter of the expanded-diameter section of the valve umbrella portion as a finished product when an expanded-diameter section side of the valve umbrella portion semifinished product is placed below, and
• has a cylindrical hollow having an inner diameter equal to the maximum inner diameter of the valve umbrella portion hollow of the finished product, the cylindrical hollow being open at an upper end thereof and being bottomed within the expanded-diameter section at a lower end thereof, and
• the second step is adapted to subject the valve umbrella portion semifinished product to forging at a temperature within a range of room temperature to 870° C. to draw an upper part of the expanded-diameter section and the body gradually in a plurality of stages
• in such a manner as to gradually perform drawing by use of dies for pressing the upper part of the expanded-diameter section and the body of the valve umbrella portion semifinished product, a number of the dies being equal to a number of drawing processes, and inner diameters of the dies decreasing little by little as the stages proceed, while holding an entire space itself including works, the dies, and punches at a constant temperature,
• thereby obtaining the valve umbrella portion as the finished product in which the maximum inner diameter of the valve umbrella portion hollow within the expanded-diameter section is held at the inner diameter of the cylindrical hollow, and the inner diameter of the valve umbrella portion hollow becomes smaller more upwardly.