Valve train component for an internal combustion engine, and method of making same
In order to provide a wear-resistant member which is excellent in wear resistance and pitting resistance, a valve train component is subjected to oxidation treatment to adjust its surface hardness to a preliminary value in a range between 550 and 800, followed by shot peening of at least a contact surface, to adjust the surface hardness to a value in a range between 800 and 1000. In a valve spring retainer, the contact surface is a seating surface against which a valve spring abuts. In a valve lifter, the contact surface is a sliding surface against which a cam lobe abuts. The components subject to the oxidation treatment are each made of a titanium alloy having an alloy composition including from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Ti and unavoidable impurities.
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The present invention claims priority under 35 USC 119 based on Japanese patent application No. 2006-091943, filed on Mar. 29, 2006. The entire disclosure of this priority document, including specification, claims and drawings, is incorporated by reference herein.
BACKGROUND OF THE INVENTION1. Technical Field of the Invention
The present invention relates to a wear-resistant titanium member, and to methods of making same. More particularly, the present invention relates to a wear-resistant valve train component of a cylinder head assembly in an internal combustion engine, where the valve train component is formed from an alloy containing titanium, and to methods of making same.
2. Background Art
In high-performance internal combustion engines of a type used mainly in automobiles and motorcycles, there has been a persistent demand for weight reduction in their internal component parts, in order to improve the limit rotation speed and decrease friction, thereby enhancing their efficiency from the viewpoints of performance improvement and reduction in environmental burden. Replacement of conventionally used steel parts with parts made of a titanium alloy material, excellent in specific strength, has a beneficial effect of weight reduction of some of these components.
However, titanium material has a high affinity for itself or another element, so that seizing occurs easily by sliding contact between adjacent titanium parts. Surface treatment is therefore required, for imparting such a titanium component with good sliding properties.
As the surface treatment of a titanium material, there is known a technology of coating TiN or CrN on the surface thereof, by an ion plating method. A nonmetallic surface is available by the use of this coating process, so that this technology can give wear resistance to the material while improving its anti-seizing property (refer to, for example, Japanese Laid-Open Patent Publication No. Hei 4-171206).
Another, different technology is also known in which shot peening is applied to the surface of a metal material, and a filler is subsequently added to the treated surface to give a compressive residual stress to the metal material, thereby heightening the pitting resistance of the material, and at the same time smoothing its external surface (refer to, for example, Japanese Patent Publication No. Hei 7-8803).
According to the technology as disclosed in Japanese Laid-Open Patent Publication No. Hei 4-171206, the coating formed by the ion plating process has a thickness of from about 3 to 5 μm. When the coating has a thickness exceeding this range, adhesion of it decreases. Such a coating from 3- to 5-μm thick is formed on the relatively soft surface of the titanium material, so that it cannot be applied to any site on the surface because deformation of the titanium material may occur at the sliding time under high surface pressure.
Composite plating with Ni is known as similar treatment for imparting a metal surface with wear-resistance. It ensures a sufficient plating thickness of 20 μm or greater, but owing to low adhesion strength on the interface between the titanium material and plating phase, exfoliation of the plating layer may occur after severe sliding. Thus, the composite plating with Ni can also be applied only to a limited site.
Another surface treatment different from the above-described coating, an oxidation treatment which is, at the same time, a diffusion treatment can be carried out. This oxidation treatment is advantageous in cost, because it basically requires only heating of the titanium material in the atmosphere. In addition, it is a diffusion treatment capable of providing good adhesion. The thickness of a hardened layer can be determined by selecting the treatment conditions.
In this oxidation treatment, however, the thickness of the hardened layer can be increased by raising the treatment temperature or treatment time, but it makes the surface layer brittle, and particularly it deteriorates pitting resistance.
According to the technology disclosed in Japanese Patent Publication No. Hei 7-8803, shot peening is effective for improving the pitting resistance of a metal material, but irregularities formed on the surface by this treatment must subsequently be smoothed out by the application of a filler to the treated surface, which decreases productivity.
SUMMARY OF THE INVENTIONWith the foregoing problems in view, the present invention has been made. An object of the present invention is to provide a wear-resistant titanium member which is excellent in both wear resistance and pitting resistance.
In order to attain the above-described object, a wear-resistant titanium member according to a first aspect of the present invention is characterized in that it is obtained by oxidation treatment, of at least an abutting surface thereof configured for contact with another member, to adjust a surface hardness Hmv (load: 0.1 kg) to 550 or greater but less than 800, followed by shot peening to adjust the surface hardness Hmv (load: 0.1 kg) to 800 or greater but not greater than 1000.
The invention according to a second aspect hereof is characterized in that in addition to the constitution of the first aspect, the shot peening is performed using media having a particle size of 0.03 mm or greater but not greater than 0.1 mm.
The invention according to a third aspect hereof is characterized in that, in addition to the constitution of the first or second aspect, an α-case layer is formed by the oxidation treatment, the α-case layer having a thickness between 5 μm and 20 μm.
The invention according to a fourth aspect hereof is characterized in that in addition to the constitution of any one of the first, second or third aspects, the shot peening is carried out with a coverage of from 100 to 500%.
The invention according to a fifth aspect hereof is characterized in that in addition to the constitution of any one of the first through fourth aspects, the member is made of a titanium alloy having, as an alloy composition, from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Ti and unavoidable impurities.
The invention according to a sixth aspect hereof is characterized in that in addition to the constitution of any one of the first through fifth aspects, the titanium member is a valve spring retainer made of titanium and having the abutting surface on which a valve spring abuts.
The invention according to a seventh aspect hereof is characterized in that in addition to the constitution of any one of the first through fifth aspects, the titanium member is a valve lifter, made of a material containing titanium, and having an abutting surface over which a cam lobe slides.
According to the first aspect of the invention, the wear-resistant titanium member is obtained by oxidation treatment, of at least an abutting surface thereof with another member, to adjust a surface hardness Hmv (load: 0.1 kg) to 550 or greater but less than 800, followed by shot peening to adjust the surface hardness Hmv (load: 0.1 kg) to 800 or greater but not greater than 1000. By the oxidation treatment which is low in cost and capable of providing good adhesion, the abutting surface with another member can have thereon a cured layer having a sufficient thickness, thereby having improved wear resistance. Moreover, owing to a residual stress given by the shot peening treatment, the material has improved pitting resistance. The wear-resistant titanium member can therefore have a good sliding property even under severe sliding conditions.
According to the second aspect of the invention, the shot peening is performed using media having a particle size of 0.03 mm or greater but not greater than 0.1 mm. This makes it possible to suppress appearance of irregularities on the treated surface as much as possible, thereby improving the wear resistance further.
According to the third aspect of the invention, an α-case layer having a thickness of 5 μm or greater but not greater than 20 μm is formed by the oxidation treatment.
This makes it possible to take full advantage of the effect of shot peening, thereby improving the wear resistance and pitting resistance while maintaining good balance therebetween.
According to the fourth aspect of the invention, shot peening is carried out with a coverage of from 100 to 500% so that pitting resistance can be improved by this shot peening and owing to a compressive residual stress given thereby, a fatigue strength can also be improved. Moreover, this treatment makes it possible to maintain wear resistance while providing the α-case layer with a sufficient thickness.
According to the fifth aspect of the invention, the wear-resistant titanium member is made of a titanium alloy having, as an alloy composition, from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Ti and unavoidable impurities. This makes it possible to improve the balance between the strength and processability and in addition, makes it possible to form a sufficiently thick α-case layer, which is formed by oxidation treatment, in a short time compared with another alloy and obtain the shot peening effects for improving wear resistance and pitting resistance easily.
According to the sixth aspect of the invention, the titanium member may be a valve spring retainer, made of an alloy including titanium, and having an abutting surface on which a valve spring abuts. The valve spring retainer having improved wear resistance and pitting resistance can therefore provide a good sliding property even under severe sliding conditions.
According to the seventh aspect of the invention, the titanium member is a valve lifter, made of an alloy containing titanium, and having an abutting surface over which a cam lobe slides. The valve lifter having improved wear resistance and pitting resistance can therefore provide a good sliding property even under severe sliding conditions.
For a more complete understanding of the present invention, the reader is referred to the following detailed description section, which should be read in conjunction with the accompanying drawings. Throughout the following detailed description and in the drawings, like numbers refer to like parts.
A method of producing a wear-resistant case hardened titanium member, and a wear-resistant titanium member which is a product of the described method will now be described relating to a selected illustrative embodiment of the present invention, and with reference to accompanying drawings. It should be understood that only structures considered necessary for clarifying the present invention are described herein. Other conventional structures, and those of ancillary and auxiliary components of the system, are assumed to be known and understood by those skilled in the art.
As illustrated in
The cylinder head 3 has an exhaust valve opening 7, which opens on an upper surface of the combustion chamber 6, and an exhaust port 8 leading to the exhaust valve opening 7. A stem 9a of an exhaust valve 9, provided for opening or closing the exhaust valve opening 7, is fitted slidably in a valve guide cylinder 10, disposed in the cylinder head 3.
The stem 9a protruding from the valve guide cylinder 10 has a valve spring retainer 12 fixed at the end portion thereof via a halved cotter 11 (valve keeper). A coiled valve spring 14, surrounding the stem 9a, is disposed in a compressed state between the valve spring retainer 12 and a spring seat member 13, supported by the cylinder head 3. The exhaust valve 9 is biased in a valve closing direction by a spring force exerted by this valve spring 14.
The cylinder head 3 also includes an inverted, cylindrically cup-shaped valve lifter 15, which fits over and surrounds an upper portion of the stem 9a, an upper portion of the valve spring 14 and the valve spring retainer 12. The upper end of the stem 9a is abutted concentrically against an inner surface of a blocking end portion of the valve lifter 15, via an inner shim 25 disposed therebetween. This valve lifter 15 is fitted slidably in a cylindrical guide hole 16 formed in the cylinder head 3.
A valve-operating cam lobe 18, mounted on a cam shaft 17, is slidably abutted against the outer surface of the blocking end portion of the valve lifter 15. With the rotation of the cam shaft 17, the valve-operating cam lobe 18 pushes the stem 9a downwardly, via sliding contact with the valve lifter 15, acting against the spring force of the valve spring 14, whereby the exhaust valve 9 opens and starts its operation.
As illustrated in
The valve spring retainer 12 has a stem-fixing tapered hole 20 disposed to axially penetrate therethrough, and the halved cotter 11 fits in the tapered hole 20, in such a manner that it is sandwiched between the stem 9a to be inserted in the tapered hole 20 and the valve spring retainer 12.
As illustrated in
The valve spring retainer 12 and valve lifter 14 which are wear-resistant titanium members are made of a titanium alloy formed by cold forging. The titanium alloy is including from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Ti and unavoidable impurities.
These valve spring retainer 12 and valve lifter 15 are subjected to oxidation treatment all over the surfaces by heating in the atmosphere. The valve spring retainer 12 and valve lifter 15 thus subjected to oxidation treatment each has a surface hardness Hmv (load: 0.1 kg) of 550 or greater but less than 800.
In addition, the valve spring retainer 12 and valve lifter 15 are each subjected to shot peening of at least an abutting surface which provides the sliding portions thereof for making repeated contact with another member during engine operation. The sliding position of the valve spring retainer 12 subjected to shot peening is a sliding surface of the seating surface 19 against which the valve spring 14 is abutted, while the sliding position of the valve lifter 15 subjected to shot peening is the sliding surface 23 with which the valve operating cam lobe 18 is brought into sliding contact.
By the oxidation treatment of these valve spring retainer 12 and valve lifter 15, an α-case layer having a thickness of 5 μm or greater but not greater than 20 μm is formed over their surfaces and by the shot peening of the seating surface 19 and sliding surface 23, their surface hardness Hmv (load: 0.1 kg) is heightened to 800 or greater but not greater than 1000.
The valve spring retainer 12 and valve lifter 15 used for an intake valve (not illustrated) for opening or closing an intake valve opening (not illustrated) also have a similar constitution.
The valve spring retainer 12 and valve lifter 15 are case hardened according to the present invention by being surface treated in the manner described below. Description will be made with the valve spring retainer 12 as an example, with the understanding that the valve lifter 15 is case hardened by a substantially similar process.
Oxidation treatment is given to the surface of the valve spring retainer 12 formed by cool forging of a titanium alloy including from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Ti and unavoidable impurities. Oxidation treatment is carried out by placing the valve spring retainer 12 in an oxygen-containing atmosphere, for example, in an atmospheric furnace and heating at from about 600 to 800° C. for several minutes to several hours.
As illustrated in
Shot peening treatment is then given to the seating surface 19 of the valve spring retainer 12 having a surface hardness Hmv (load: 0.1 kg) adjusted to 550 or greater but less than 800 by the above-described oxidation treatment. The shot peening treatment is carried out, for example, by projecting media made of fine particles toward the seating surface 19 of the valve spring retainer 12 by using an air nozzle type shot peening apparatus.
Arc height (shot peening intensity) and coverage (peening ratio of media) are determined by controlling the conditions of shot peening treatment such as projection pressure, projection distance, kind of media (shape, material quality), nozzle speed, work rotation speed and the number of passes. The media including fine particles to be used for this shot peening treatment have a particle size of preferably 0.03 mm or greater but not greater than 0.1 mm in consideration of the irregularities of the surface treated with them.
The media are shot out from a nozzle 42 of the shot peening apparatus which has been inclined downward at about 45 degree while rotating the valve spring retainers 12 together with the supporting rod 41 and are then projected to the seating surface 19 of the uppermost valve spring retainer 12.
Completion of the shot peening to the uppermost valve spring retainer 12 is followed by the relative movement of the supporting rod 41 and nozzle 42 and then shot peening of the valve spring retainers 12 is carried out successively from the upper one to the lower one.
By the shot peening treatment of the valve spring retainer 12 in the above-described manner, the oxide scale layer 31, which is a scale on the treated surface, is removed. The projection of fine particles increases the hardness of the α-case layer 32, thereby increasing its surface hardness Hmv (load: 0.1 kg) to 800 or greater but not greater than 1000. At the same time, it imparts the retainer with a compressive residual stress and thereby heightens the fatigue strength, whereby the retainer has improved pitting resistance.
When the media are projected to the seating surface 19 of the valve spring retainer 12 from the oblique direction as described above, they are also projected to a corner 19a between the seating surface 19 and a wall portion adjacent to the seating surface 19, which greatly enhances the fatigue strength at the corner 19a.
Shot peening treatment after the oxidation treatment imparts the α-case layer 32 with a residual stress, which results in great improvement in pitting resistance. Deterioration in the pitting resistance can therefore be suppressed even if the thickness becomes greater. After shot peening treatment, even if the α-case layer 32 has a thickness of 20 μm, pitting resistance is substantially equal to that when the α-case layer 32 has a thickness of 5 μm or less. Moreover, with an increase in the surface hardness brought by the shot peening treatment, the wear resistance also improves greatly.
When the α-case layer 32 is thicker than 20 μm, even shot peening treatment cannot improve the pitting resistance sufficiently because of the breaking tendency of the grain boundary of recrystallized crystal grains. The thickness of the α-case layer 32 is therefore preferably 5 μm or greater but not greater than 20 μm in consideration of the wear resistance and pitting resistance.
Surface treatment given to the valve lifter 15 is substantially similar to that employed for the valve spring retainer 12. In the case of the valve lifter 15, the shot peening treatment after the oxidation treatment also heightens the surface hardness Hmv (load: 0.1 kg) on the sliding surface 23 to 800 or greater but not greater than 1000 and as a result, it has improved wear resistance and pitting resistance.
The titanium alloy employed here has an alloy composition including from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Ti and unavoidable impurities. The alloy composition within the above-described range makes it possible to improve the balance between the strength and processability, makes it possible to form a sufficiently thick α-case layer by the oxidation treatment in a short time compared with another alloy, and facilitates obtaining the effects of shot peening for improving wear resistance and pitting resistance.
As described above, in the valve spring retainer 12 and valve lifter 15 made of titanium according to this Embodiment, the seating surface 19 against which the valve spring 14 abuts and the sliding surface 23 against which the valve operating cam lobe 18 abuts are adjusted to have a surface hardness Hmv (load: 0.1 kg) of 550 or greater but less than 800 by the oxidation treatment and then adjusted to have a surface hardness Hmv (load: 0.1 kg) of 800 or greater but not greater than 1000 by the shot peening treatment. This makes it possible to obtain a cured layer having a sufficient thickness and including the α-case layer 32 and oxygen diffusion layer 33 by the oxidation treatment which is low in cost and exhibits good adhesion, thereby improving wear resistance. At the same time, pitting resistance can be improved owing to a residual stress given by the shot peening treatment. As a result, the valve spring retainer 12 and valve lifter 15 made of titanium and having a good sliding property even under severe sliding conditions can be obtained.
Shot peening with media having a particle size of 0.03 mm or greater but not greater than 0.1 mm makes it possible to suppress the formation of irregularities on the treated surface as much as possible, thereby improving the wear resistance further.
Moreover, since the α-case layer 32 having a thickness of 5 μm or greater but not greater than 20 μm is formed by the oxidation treatment, it is possible to improve wear resistance and pitting resistance in a good balance by making the best use of the effects of shot peening.
The shot peening is carried out with a coverage of from 100 to 500%. This makes it possible to improve the pitting resistance and the fatigue strength attributable to the compressive residual stress also brought by shot peening, and at the same time, maintain the wear resistance while providing the α-case layer 32 with a sufficient thickness.
By using a titanium alloy having an alloy composition including from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Ti and unavoidable impurities, the balance between the strength and processability can be improved and at the same time, a sufficiently thick α-case layer 32, which is formed by the oxidation treatment, can be formed in a short time compared with another alloy, whereby effects of shot peening for improving the wear resistance and pitting resistance are available easily.
The present invention is not limited to the above-described embodiment, but can be changed or modified as needed.
In this Embodiment, a valve spring retainer and valve lifter were described as examples of the wear-resistant titanium member, but the invention is not limited to these members but can be applied to various products including engine parts such as crank shaft and rocker shaft.
In the valve lifter according to this Embodiment, shot peening is given to the sliding surface with a cam, but shot peening may be given to not only this sliding surface but also another sliding surface with a guide hole of a cylinder head.
The titanium member of the present invention is not limited to that having the above-described composition, but Ti-6Al-4V alloy, Ti-3Al-2.5V alloy or pure titanium of JIS 2 grade may be employed instead.
EXAMPLES (Test 1)Tests were made on the influence of the conditions of oxidation treatment and presence or absence of shot peening treatment on the wear resistance and pitting resistance of a valve spring retainer. As test specimens, valve spring retainers were made by cool forging of a rod made of a Ti-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm. Oxidation treatment was then given to them under varied treatment conditions as follows: 500° C.×5 hours, 600° C.×5 hours, 700° C.×5 hours, 800° C.×5 hours and 900° C×5 hours. Valve spring retainers to be disposed on the abutting surface with a valve spring were subjected to shot peening further. Shot peening was performed with “#400 High Speed Steel” as media under the following conditions: distance of 200 mm, projection pressure of 0.5 MPa and 300% of coverage.
Surface hardness was determined by measuring it at ten points under a load of 100 g by a Micro Vickers hardness tester after removal of irregularities from the surface by buffing with alumina polishing particles of 0.3 μm in size and then averaging the measured values at eight points except the maximum and minimum values. Wear resistance and pitting resistance were evaluated based on the results of a motoring durability test made on a valve spring having, in the vicinity of the end portion thereof, a burr. The results of motoring durability test are shown in Table 1.
In table 1, “C” means appearance of severe wear or pitting, “B” means appearance of wear or pitting, and “A” means exhibition of a good sliding property without appearance of wear or pitting. These results have revealed that the valve spring retainers having both wear resistance and pitting resistance are those subjected to oxidation treatment under the conditions of from 600° C.×5 hours to 800° C.×5 hours, followed by shot peening treatment. These valve spring retainers had a surface hardness Hmv (load: 0.1 kg) of 800 or greater but not greater than 1000.
The valve spring retainer subjected to oxidation treatment under the condition of 900° C.×5 hours, on the other hand, was not evaluated because its surface was markedly coarsened owing to a large amount of oxide scales generated after the oxidation treatment.
(Test 2)A test on the wear resistance and pitting resistance of a valve spring retainer was conducted while changing the diameter of media to be used for shot peening treatment. Test specimens were, similar to Example 1, valve spring retainers manufactured by cool forging of a rod made of a Ti-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm. The resulting test specimens were subjected to oxidation treatment under the conditions of 700° C.×5 hours, followed by shot peening while changing the diameter of a media. As in Test 1, evaluation was made based on the results of the motoring durability test. Conditions are similar to those employed in Example 1 except that the diameter of the media was changed. The results of the motoring durability test are shown in Table 2.
It has been found that the test specimens have good wear resistance and pitting resistance when media having a particle size of 0.03 mm or greater but not greater than 0.1 mm are used, while use of media having a particle size exceeding 0.3 mm do not lead to good results because it gives a damage to the cured layer including an α-case layer and oxygen diffusion layer formed by the oxidation treatment.
(Test 3)Tests on the wear resistance and pitting resistance of a valve spring retainer were made while changing the thickness of the α-case layer. The thickness of the α-case layer varies, depending on the conditions of oxidation treatment. Test specimens were, similar to Example 1, valve spring retainers manufactured by cool forging of a rod made of a Ti-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm. The resulting test specimens were subjected to oxidation treatment under various conditions, followed by shot peening under similar conditions to those employed in Test 1. After the motoring durability test of the thus-prepared valve spring retainers as in Test 1, each retainer was cut, buried in a polishing resin and polished. Then, the thickness of the α-case layer on the surface was observed. The α-case layer is a portion of a titanium structure changed to an cc phase and becoming a hard layer by the oxygen penetrated from the surface and diffused in the member and it looks white on a microscope owing to etching. No change in structure is found even inside of the α-case layer but a so-called oxygen diffusion layer, that is, a layer having an increased hardness by the diffusion of oxygen exists. The results of the motoring durability test are shown in Table 3.
As a result, when the α-case layer had a thickness of 2 μm, the resulting member was inferior in wear resistance itself, while when the α-case layer had a thickness of 24 μm, pitting appeared because the boundary of recrystallized crystal grains broke easily. Test results were good when the α-case layer had a thickness of 5 μm or greater but not greater than 20 μm.
(Test 4)Next, tests were made on the influence of the conditions of oxidation treatment and presence or absence of shot peening treatment on the wear resistance and pitting resistance of a valve lifter. Specimens used for the test were valve lifters manufactured by forging and mechanical processing of a rod made of a Ti-1Fe-0.30 (wt. %) alloy and having a diameter of 30 mm. The specimens were subjected to oxidation treatment under varied treatment conditions as follows: 500° C.×5 hours, 600° C.×5 hours, 700° C.×5 hours, and 800° C.×5 hours. The results of Test 1 showed that oxidation treatment under the condition of 900° C.×5 hours was not worthy of evaluation so that oxidation treatment under this condition was omitted. Valve lifters subjected to shot peening further at the sliding surface thereof with a cam were also prepared. The measuring method of surface hardness and shot peening conditions were similar to those employed in Test 1.
The wear resistance and pitting resistance of the valve lifters were evaluated in accordance with a motoring durability test using a cam shaft having a cam width reduced by 25%. The results of the motoring durability test are shown in Table 4.
In table 4, “C” means appearance of severe wear or pitting, “B” means appearance of wear or pitting, and “A” means exhibition of a good sliding property without appearance of wear or pitting. These results have revealed that the valve lifters having both wear resistance and pitting resistance are those subjected to oxidation treatment under the conditions of from 600° C.×5 hours to 800° C.×5 hours, followed by shot peening treatment. These valve lifters had a surface hardness Hmv (load: 0.1 kg) of 800 or greater but not greater than 1000. Thus, the valve lifters showed similar results to those of the valve spring retainers.
(Test 5)Tests on the wear resistance and pitting resistance of the valve lifters were made while changing the diameter of media to be used for shot peening treatment. Test specimens were, similar to Example 4, valve lifters manufactured by forging and mechanical processing of a rod made of a Ti-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm. The resulting test specimens were subjected to oxidation treatment under the conditions of 700° C.×5 hours, followed by shot peening while changing the diameter of media. As in Test 4, evaluation was made based on the results of a motoring durability test. Conditions are similar to those employed in Example 4 except that the diameter of the media is changed. The results of the motoring durability test are shown in Table 5.
It has been found that the test specimens shows good wear resistance and pitting resistance when media having a particle size of 0.03 mm or greater but not greater than 0.1 mm are used, while use of media having a particle size exceeding 0.3 mm do not lead to good results because it gives a damage to the cured layer including an α-case layer and oxygen diffusion layer formed by the oxidation treatment. Thus, the results of the valve lifters were similar to those of the valve spring retainers.
(Test 6)Tests on the wear resistance and pitting resistance of valve lifters were made while changing the thickness of the α-case layer by changing the conditions of oxidation treatment. Test specimens were, similar to Example 1, valve spring retainers manufactured by cool forging of a rod made of a Ti-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm. The resulting valve spring retainers were subjected to oxidation treatment under various conditions, followed by shot peening under similar conditions to those employed in Test 1. After the motoring durability test of the thus-prepared valve spring retainers as in Test 1, each retainer was cut, buried in a polishing resin and polished. Then, the thickness of the α-case layer of it was observed. The results of the motoring durability test are shown in Table 6.
As a result, when the α-case layer had a thickness of 2 μm, the resulting member was inferior in wear resistance itself, while when the α-case layer had a thickness of 24 μm, pitting appeared because the boundary of recrystallized crystal grains broke easily. Test results were good when the α-case layer had a thickness of 5 μm or greater but not greater than 20 μm. Thus, the evaluation results of the valve lifters were also similar to those of the valve spring retainers.
(Test 7)Tests were made while changing the alloy composition of the valve spring retainers or valve lifters. Those having an alloy composition including from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O, and the balance of Ti and unavoidable impurities were employed. It has been found that when the Fe and O contents falls within the above-described ranges, respectively, tensile strength increases almost in proportion to their amounts. Within the above-described ranges, however, a difference in fatigue strength is not so large as that in tensile strength, meaning that this composition range is advantageous for mass production control. It has also been found that the advantage of the present invention can be utilized more within this composition range because an alloy having the above-described composition range enables formation of a thicker α-case layer compared with another alloy.
(Test 8)Tests were made on the influence of the conditions of oxidation treatment and presence or absence of shot peening treatment on the wear resistance and pitting resistance by using a Fabry wear tester. Fabry wear test specimens were prepared by mechanical processing of a rod made of a Ti-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm. The specimens were subjected to oxidation treatment under varied treatment conditions as follows: 500° C.×5 hours, 600° C.×5 hours, 700° C.×5 hours, 800° C.×5 hours, and 900° C.×5 hours. Specimens subjected to shot peening treatment further were also prepared. In the wear test, wear resistance was evaluated when the test specimen was used on the block side, while pitting resistance was evaluated when the test specimen was used on the pin side. A partner material employed here was an SCM carburized material having a surface hardness Hmv (load: 0.1 kg) of about 750. The measuring method of the surface hardness and shot peening conditions employed for the test specimens were similar to those employed in Test 1. The test results are shown in Table 7.
In table 7, “C” means appearance of severe wear or pitting, “B” means appearance of wear or pitting, and “A” means exhibition of a good sliding property without appearance of wear or pitting. These results have revealed that the test specimens having both wear resistance and pitting resistance are those subjected to oxidation treatment under the conditions of from 600° C.×5 hours to 800° C.×5 hours, followed by shot peening treatment. These test specimens have a surface hardness Hmv (load: 0.1 kg) of 800 or greater but not greater than 1000. The test specimen subjected to oxidation treatment under the condition of 900° C.×5 hours, on the other hand, was not evaluated because its surface was markedly coarsened owing to a large amount of oxide scales generated after the oxidation treatment.
(Test 9)Tests on the wear resistance and pitting resistance of a valve lifter were conducted while changing the diameter of media to be used for shot peening treatment. As in Test 8, Fabry wear test specimens were prepared by mechanical processing of a rod made of a Ti-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm. The resulting test specimens was subjected to oxidation treatment under the conditions of 700° C.×5 hours, followed by shot peening while changing the diameter of media. Influence of it was evaluated by the wear test. Conditions are similar to those employed in Example 1 except that the diameter of the media is changed. The test results are shown in Table 8.
It has been found that the test specimens have good wear resistance and pitting resistance when media having a particle size of 0.03 mm or greater but not greater than 0.1 mm are used, while use of media having a particle size exceeding 0.3 mm do not lead to good results because it gives a damage to the cured layer itself formed by the oxidation treatment.
(Test 10)Tests on the wear resistance and pitting resistance were made using a Fabry wear tester while changing the thickness of the α-case layer by changing the conditions of oxidation treatment. Fabry wear test specimens were made in a similar manner to that employed in Test 8 by the mechanical processing of a rod made of a Ti-1Fe-0.30 (wt. %) alloy and having a diameter of 10 mm. The resulting test specimens were subjected to oxidation treatment under various conditions, followed by shot peening treatment under similar conditions to those employed in Test 8. After the wear test of the test specimens, they were each cut, buried in a polishing resin and polished. Then, the thickness of the α-case layer on the surface was observed. The test results are shown in Table 9.
As a result, when the α-case layer had a thickness of 2 μm, the specimen was inferior in wear resistance itself, while when the α-case layer had a thickness of 24 μm, pitting appeared because the boundary of recrystallized crystal grains broke easily. Test results were good when the α-case layer had a thickness of 5 μm or greater but not greater than 20 μm.
(Test 11)A test was made using Ti-6Al-4V alloy, Ti-3Al-2.5V alloy and pure titanium of JIS 2 grade under similar conditions to those employed in Example 8 except that oxidation treatment was conducted under the conditions of 700° C.×5 hours. As a result, any of these titanium materials showed good wear resistance and pitting resistance when subjected to shot peening. They had a surface hardness Hmv (load: 0.1 kg) of 910, 884, and 818, respectively, after shot peening.
(Test 12)Shot peening was given, under similar conditions to those employed in Example 4, to a rocker shaft made of a Ti-6Al-4V alloy and used after oxidation treatment and a bench durability test on the resulting rocker shaft was conducted. The oxidation treatment is ordinarily performed at 600° C. for 6 hours. The rocker shaft of the present invention showed durability ten times as much as that of the conventional one in the ultimate durability test, showing that the invention has a great effect on a practical part.
Although the present invention has been described herein with respect to a number of specific illustrative embodiments, the foregoing description is intended to illustrate, rather than to limit the invention. Those skilled in the art will realize that many modifications of the illustrative embodiment could be made which would be operable. All such modifications, which are within the scope of the claims, are intended to be within the scope and spirit of the present invention.
Claims
1. A wear-resistant valve train component member formed from a material comprising titanium, said valve train component member being a product of a process including steps of:
- case-hardening at least one surface of said member, designated as an abutting surface configured for abutting contact with another member, by oxidation treatment to adjust a surface hardness Hmv (load: 0.1 kg) of said surface to an oxidized value in a range between 550 and 800 Hmv;
- followed by shot peening said abutting surface to adjust the surface hardness Hmv (load: 0.1 kg) of said surface to a final value in a range between 800 and 1000 Hmv.
2. A wear-resistant valve train component according to claim 1, wherein the shot peening is performed using media having a particle size in a range from about 0.03 mm to about 0.1 mm.
3. A wear-resistant valve train component according to claim 1, wherein an α-case layer, having a thickness in a range from about 5 μm to about 20 μm, is formed by the oxidation treatment.
4. A wear-resistant valve train component according to claim 1, wherein the shot peening is carried out with a coverage of from 100 to 500%.
5. A wear-resistant valve train component member according to claim 1, wherein the member is made of a titanium alloy having, as an alloy composition, from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Ti and unavoidable impurities.
6. A wear-resistant valve train component member according to claim 1, wherein the member is a valve spring retainer having an abutting surface on which a valve spring abuts.
7. A wear-resistant valve train component member according to claim 1, wherein the member is a valve lifter having an abutting surface on which a cam lobe slides.
8. A wear-resistant valve train component member formed from a material comprising titanium, said valve train component member being a product of a process including steps of:
- case-hardening at least one surface of said member, designated as an abutting surface configured for abutting contact with another member, by oxidation treatment to adjust a surface hardness Hmv (load: 0.1 kg) of said surface to an oxidized value in a range between 550 and 800 Hmv;
- followed by shot peening said abutting surface to adjust the surface hardness Hmv (load: 0.1 kg) of said surface to a final value in a range between 800 and 1000 Hmv;
- wherein the shot peening is performed using media having a particle size in a range from about 0.03 mm to about 0.1 mm;
- and wherein an α-case layer, having a thickness in a range from about 5 μm to about 20 μm, is formed by the oxidation treatment.
9. A wear-resistant valve train component according to claim 8, wherein the shot peening is carried out with a coverage of from 100 to 500%.
10. A wear-resistant valve train component member according to claim 9, wherein the member is made of a titanium alloy having, as an alloy composition, from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Ti and unavoidable impurities.
11. A wear-resistant valve train component member according to claim 8, wherein the member is either a valve spring retainer or a valve lifter.
12. A method of case-hardening a valve train component member formed from a material comprising titanium, said method including the steps of:
- case-hardening at least one surface of said member, designated as an abutting surface configured for abutting contact with another member, by oxidation treatment to adjust a surface hardness Hmv (load: 0.1 kg) of said surface to an oxidized value in a range between 550 and 800 Hmv; an subsequently, shot peening said abutting surface to adjust the surface hardness Hmv (load: 0.1 kg) of said surface to a final value in a range between 800 and 1000 Hmv.
13. A method of case-hardening a valve train component member according to claim 12, wherein the shot peening is performed using media having a particle size in a range from about 0.03 mm to about 0.1 mm.
14. A method of case-hardening a valve train component member according to claim 12, wherein an α-case layer, having a thickness in a range from about 5 μm to about 20 μm, is formed by the oxidation treatment.
15. A method of case-hardening a valve train component member according to claim 12, wherein the shot peening is carried out with a coverage of from 100 to 500%.
16. A method of case-hardening a valve train component member according to claim 12, wherein the member is made of a titanium alloy having, as an alloy composition, from 0.5 to 1.5 wt. % of Fe, from 0.2 to 0.5 wt. % of O and the balance of Ti and unavoidable impurities.
17. A method of case-hardening a valve train component member according to claim 12, wherein the member is a valve spring retainer having an abutting surface on which a valve spring abuts.
18. A method of case-hardening a valve train component member according to claim 12, wherein the member is a valve lifter having an abutting surface on which a cam lobe slides.
Type: Application
Filed: Mar 14, 2007
Publication Date: Oct 4, 2007
Applicant: Honda Motor Co., Ltd. (Tokyo)
Inventors: Hiroyuki Horimura (Saitama), Kosuki Doi (Saitama)
Application Number: 11/724,100
International Classification: C23C 8/06 (20060101);
