A method for production of a valve head portion of a hollow engine valve, in which the hollow engine valve has a hollow shaft portion, and the valve head portion comprising a material having heat resistance comparable to or better than heat resistance of a material for the hollow shaft portion, the valve head portion has a valve head portion hollow hole opening on a side welded to the hollow shaft portion, the valve head portion hollow hole is formed to have an increased diameter in a diameter-increased section of the valve head portion, and a maximum inner diameter of the valve head portion hollow hole is larger than a maximum outer diameter of the hollow shaft portion,
- the method comprising:
- a first step of producing a valve head portion semifinished product by working the material having the heat resistance comparable to or better than the heat resistance of the material for the hollow shaft portion to form the valve head portion semifinished product,
- in which the valve head portion semifinished product has a cylindrical barrel section and a diameter-increased section at one end of the barrel section, the diameter-increased section being integral with the barrel section,
- when the diameter-increased section is located below, a maximum outer diameter of the diameter-increased section is equal to a maximum outer diameter of the diameter-increased section of the valve head portion as a finished product, and
- the valve head portion semifinished product has a cylindrical hollow hole having an inner diameter equal to the maximum inner diameter of the valve head portion hollow hole of the finished product, the cylindrical hollow hole opening at an upper end, and having a lower end bottomed in the diameter-increased section,
- the first step including:
- using a cylindrical solid round bar as a material, the solid round bar comprising the material having the heat resistance comparable to or better than the heat resistance of the material for the hollow shaft portion, the solid round bar having a diameter larger than a minimum outer diameter of the valve head portion as the finished product, but smaller than a maximum outer diameter of the valve head portion as the finished product;
- while rendering a circular surface at one end of the solid round bar an upper surface, and a circular surface at another end of the solid round bar a lower surface, forming a cylindrical hollow hole opening at an upper end, and being bottomed at a lower end, an inner diameter of the cylindrical hollow hole being equal to the maximum inner diameter of the valve head portion hollow hole of the valve head portion as the finished product;
- and
- then increasing a diameter of a lower part of the solid round bar by forging to bring a maximum outer diameter of the lower part into conformity with the maximum outer diameter of the diameter-increased section of the valve head portion as the finished product,
- thereby producing the valve head portion semifinished product having the diameter-increased section whose maximum outer diameter coincides with the maximum outer diameter of the diameter-increased section of the valve head portion as the finished product, the valve head portion semifinished product having the cylindrical hollow hole having the inner diameter equal to the maximum inner diameter of the valve head portion hollow hole of the valve head portion as the finished product, the cylindrical hollow hole having the lower end bottomed;
- and
- a second step of gradually necking down an upper part of the diameter-increased section and the barrel section of the valve head portion semifinished product by cold forging in a plurality of stages, that is, gradually necking down the upper part of the diameter-increased section and the barrel section by use of dies for pressing the upper part of the diameter-increased section and the barrel section, inner diameters of the dies being decreased little by little as the stage advances, a number of the dies used being equal to a number of processes for necking,
- thereby obtaining the valve head portion as the finished product configured such that the maximum inner diameter of the valve head portion hollow hole in the diameter-increased section is maintained at the inner diameter of the cylindrical hollow hole, the inner diameter of the valve head portion hollow hole being decreased with increasing height and being equal to an inner diameter of a hollow hole of the hollow shaft portion at an upper end of the barrel section.