Coolant-containing hollow poppet valve and process for producing the same

This invention relates to a hollow poppet valve charged with a coolant and having sufficient heat resistance, wear resistance, and strength in accord with required use conditions of the valve, and to a method of manufacturing such hollow poppet valve. More particularly, the invention relates to a hollow poppet valve having a cap that is integrally welded to the fillet portion that is integrally formed at one end of the cylindrical stem portion of the poppet valve, and to a method of manufacturing such hollow poppet valve.

A typical prior art poppet is disclosed in the patent document cited below. As shown in FIG. 8, this prior art hollow poppet valve has a thick edge (hereinafter referred to as thick portion) at the upper open end of the fillet region provided at the upper end of a cylindrical stem portion (G region) of the hollow poppet valve. A cap welding portion A is provided on the inside of the thick part and away from the seat portion 150 (of E region). It is anticipated that the seat portion 150 retains a higher Vickers hardness in the range from 300 to 349HV than regions A, B, C, and D, whose Vickers hardnesses are lowered by welding heat. Thus, the seat portion 150 is anticipated to have sufficient wear resistance against repeated abutment with a cylinder to maintain the seat portion 150 in intimate contact with the cylinder during closing/opening operation of the valve.

As discussed above, the prior art valve has been regarded capable of retaining sufficient fatigue strength and creep strength when the seat portion 150 and other portions of the valve including the fillet region 140 (region F) are hardened as high as possible (in the range from 350 to 399 HV). CITED PATENT DOCUMENT 1: International Publication WO 00/No. 47876.

However, it has been found from the measurements of strengths of actual hollow poppet valves conducted by the inventors of the present invention that those portions of the poppet valves exposed to high temperatures, e.g. fillet region 140, are liable to deterioration in fatigue strength and creep strength, and loose their initial strengths given by hardening, as shown in FIGS. 9 and 10.

FIG. 9 shows measured fatigue strengths (ordinate) of cold-worked austenitic-stainless-steel materials for hollow poppet valves after they were hardened to three different levels of hardness (151, 316, and 380 HV in Vickers hardness) and subjected to 10 million times of fatigue tests under different temperatures (abscissa). FIG. 10 shows creep strengths (ordinate) of the same materials as shown in FIG. 9, which were also cold-worked to three different levels of hardness and tested at 600° C. The abscissa represents value of Larson Mirror parameter.

It is seen in FIG. 9 that the larger the hardness of a valve material is, the higher its fatigue strength, and that the fatigue strength decreases with a relatively moderate rate in the temperature range between the room temperature (25° C.) and 400° C. irrespective of the hardness of the sample. Above 400° C., however, the fatigue strength of the material hardened to 380 HV decreases with an extremely large rate as compared with other materials hardened to 151 HV and 316 HV.

That is, fatigue strength of the material having hardness of 380 HV quickly decreases when used at or above 400° C., and, when used at 600° C., decreases to a level comparable to that of the material having hardness of 151 HV, which level is far below the fatigue strength of the material having hardness of 316 HV.

On the other hand, it is seen in FIG. 9 that if the valve material hardened to 380 HV is used at 600° C., it looses creep strength greatly in time as compared with the materials hardened to 151 and 316 HV. A reason for this great loss in fatigue and creep strengths of the material which was initially hardened to 380 HV and used at 600° C. may be due to recrystallization of the texture of the material caused by hardening to 380 HV.

Thus, in the event that the valve is excessively hardened and the seat portion 150 undergoes repetitive abutment against the seat metal of a cylinder during its closing/opening operations at a temperature above 500° C., the cold-worked head portion, i.e. the fillet region 14, of the valve is subjected to repetitive strong tensile stresses and fails to retain required fatigue strength and creep strength, thereby resulting in problematic ruptures for example.

On the other hand, the stem portion of the poppet valve or the cylindrical shaft portion that corresponds to region G extending downward from the lower end of the head portion as shown in FIG. 8, is in sliding contact with the valve guide section of the cylinder and repeatedly subjected to tensile stresses in the same manner as the head portion. As a consequence, if it is cold worked to the same hardness as the head portion, it suffers the same problem that it cannot retain necessary fatigue strength and wear resistance.

Further, the seat portion 15 that comes into contact with the seat metal of a cylinder needs to be cold worked harder than the head portion in order to allow the seat portion to come into intimate contact with the cylinder as much as possible, otherwise it cannot sustain wear resistance necessary to secure the intimate contact.

In view of the prior art problems discussed above, this invention seeks to provide a cold-worked, heat-treated hollow poppet valve charged with a coolant, and a method of manufacturing such poppet valve, such that the valve has a fillet region (or head portion) having sufficient fatigue strength and creep strength to stand repetitive use at high temperatures.

The invention further seeks to provide a hollow poppet valve charged with a coolant and having a stem portion (or shaft portion) having predetermined fatigue strength and wear resistance and a seat portion having predetermined wear resistance. The invention also seeks to provide a method of manufacturing such hollow poppet valve.

To this end, there is provided a hollow poppet valve charged with a coolant as claimed in claim 1, having different portions of different hardness which are formed by steps of:

cold pressing a material into a cup-shaped object having a thick edge at the open end of the cup-shaped object;

intermediate annealing to soften the cup-shaped object;

cold drawing to form a head portion having a flared opening at the open end of the cup-shaped object, a cylindrical hollow shaft portion integral with the head portion, and

a tapered seat portion around the outer periphery of the thick edge, whereby the head portion is hardened to Vickers hardness in the range from about 250 HV to 350 HV inclusive.