Metal treatment – Process of modifying or maintaining internal physical... – Carburizing or nitriding using externally supplied carbon or...
Reexamination Certificate
2000-12-01
2004-09-14
Ip, Sikyin (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Carburizing or nitriding using externally supplied carbon or...
C148S228000, C029S090700
Reexamination Certificate
active
06790294
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a surface treatment method for enhancing the performance of valve springs for internal combustion engines, clutch springs for transmissions of automobiles and the like, high-strength thin-sheet springs, and the like by projection of hard fine metal particles, and a high-performance spring produced by this surface treatment method.
BACKGROUND ART
The following are found to be prior art related to the present invention.
1. Japanese Patent Publication for Opposition No. 2-17607
“Surface Thermo-mechanical Heat Treatment Method for Metal Product”
This technique relates to a surface thermo-mechanical heat treatment method in which 40 to 200 &mgr;m shots having a hardness equal to or higher than that of a product to be treated are injected at a velocity of 100 m/sec or more so as to raise the temperature near the surface of the product up to an A3 transformation point or higher.
This method is a process in which a material to be processed is austenitized by heating the surface layer of the work due to collision and promptly cooled down so as to cause metallographic transformation of the work. This is different from the present invention in the technical principles and contents.
2. Japanese Laid-Open Patent Publication No. 9-279229, “Surface Treatment Method of Steel Work”
The disclosed technique mainly includes allowing a number of hard metal particles having a diameter of 20 to 100 &mgr;m to collide against the surface of a steel work at a velocity of 80 m/sec or more while controlling the temperature rise limit of the work surface to be equal to or higher than 150° C. but lower than a recovery/recrystallization temperature.
This disclosure does not mention nitriding, and hardly defines the properties of the metal particles, e.g. the specific gravity and hardness thereof. Although the collision velocity is defined as 80 m/sec or more, where the optimal velocity exists is unclear. Only the velocity of 180 m/sec is presented in an example and recognized to be effective. However, whether or not this velocity is optimal is unclear.
3. Japanese Laid-Open Patent Publication No. 10-118930,
“Shot Peening Method of Spring and Spring Product”
A 0.64% C—Si—Mn—Cr—Mo—V steel spring is nitrided, shot-peened with 0.5 to 1.0 mm dia. shots, and then peened with particles having a specific gravity of 12 to 16, a diameter of 0.05 to 0.2 mm, and a hardness of 1200 to 1600. As the result, a surface residual stress of &sgr;R=−1950 Mpa and a fatigue limit of 700±620 MPa at 5×10
7
times of repetition were obtained. This fatigue limit stress does not reach the level of the fatigue strength specified in the 8th embodiment of the present invention.
The objects and method of this disclosure are partly similar to those of the present invention, but different in that while this disclosure uses manufacturer-specific high-hardness and expensive cemented carbide particles having a size of 0.05 to 0.2 mm and a specific gravity of 12 to 16, the present invention uses more inexpensive and more available metal particles such as iron-based particles having a diameter of 0.01 to 0.08 mm. The resultant fatigue strength obtained in the present invention is superior to that obtained by this conventional method.
4. Japanese Patent Gazette No. 2613601 (Japanese Laid-Open Patent
Publication No. 1-83644), “High-strength Spring”
This disclosure describes a spring containing 0.6 to 0.7% of C, 1.2 to 1.6% of Si, 0.5 to 0.8% of Mn, 0.5 to 0.8% of Cr, a total of 0.05 to 0.2% of one, two or more types selected from V, Mo, Nb, and Ta, and iron and impurities as the remainder, having nonmetallic inclusions with a maximum size of 15 &mgr;m, a surface roughness Rmax of 15 &mgr;m or less, and a maximum compression residual stress near the surface of 85 to 110 kgf/mm
2
(833 to 1079 MPa). This patent describes that, if the maximum compression residual stress near the surface exceeds 110 kgf/mm
2
(=1079 MPa), the production of the spring becomes difficult and the surface roughness of the spring degrades, unintentionally decreasing the fatigue strength. One of the inventors of this patent together with other researchers report in detail the performance of a spring produced according to the technique disclosed in this patent in the ESF (European spring Federation) sponsored spring technology international conference held in Düsseldorf, Germany on Apr. 3, 1990 after the filing of the application of this patent. According to the related article, titled “A High strength spring for Automotive Engine” by M. Abe, K. Saito, N. Takamura, and H. Yamamoto, the maximum compression residual stress of a spring which corresponds to the invention of the patent (Gazette No. 2613601) is about 950 MPa in the surface layer and about 820 MPa on the outermost surface as is seen from
FIG. 9
of this article, and the surface roughness Rmax of the spring is 10.6 &mgr;m as is seen from Table 2 of this article. The fatigue limit of the spring is &tgr;m=588 MPa and &tgr;a=±(450 to 480) Mpa or so at 5×10
7
times of repetition as is seen from
FIG. 11
of this article, which does not correspond to the 9 th and 10th embodiments of the present invention.
According to the present invention, the surface roughness will not increase even if the maximum compression residual stress in the surface layer exceeds 1079 MPa. Moreover, the residual stress is maximum on the outermost surface or closely near the surface, effectively preventing fatigue breaking from the surface. Accordingly, it is possible to obtain a spring satisfying the fatigue limit represented by equation or expression (2) in the 10th embodiment of the present invention without nitriding.
5. Japanese Laid-Open Patent Publication No. 5-339763,
“Production Method of Coil Spring”
This disclosure describes that a spring product having a surface roughness Rmax of 5 &mgr;m or less and a fatigue strength of 60±57 kgf/mm
2
(588±559 MPa) at 5×10
7
times of stress repetition was obtained by descaling so as to suppress the surface roughness to a low level by shot peening, nitriding, and the following shot peening with 0.8 mm dia. cut wires. However, according to the data provided in an example of this disclosure obtained in the above manner, the fatigue strength does not satisfy expression (1) in the 8th embodiment of the present invention. Moreover, this document does not disclose projection of fine particles as disclosed in the present invention.
6. Japanese Laid-Open Patent Publication No. 7-214216,
“Production Method of High Strength Spring”
This disclosure describes that a steel wire spring may be electropolished, nitrided, and then subjected to two-stage shot peening using first particles having hardness of Hv 600 to 800 and diameter of 0.6 to 1.0 mm as first-stage shots and then particles having hardness of Hv 700 to 900 and diameter of about 0.05 to 0.2 mm as second-stage shots. However, no further analysis, examination nor consideration on the particle size of 0.05 to 0.2 mm are made. This disclosure reports that, in an example which uses steel balls having a grain diameter of 0.15 mm and a hardness of Hv 800 as the second-stage shots, the fatigue limit of the spring is 637 MPa as a mean stress and ±560 Mpa as an amplitude stress at 5×10
7
times of repetition. This does not satisfy expression (1) representing the spring fatigue limit in the 8th embodiment of the present invention. Moreover, the definition of the second-stage particle projection conditions is different from that of the present invention.
7. Japanese Laid-Open Patent Publication No. 5-177544,
“Production Method of Coil Spring”
This disclosure relates to a method including nitriding after spring formation and then shot peening the spring. In the shot peening, first-stage shot peening, low-temperature annealing, and second-stage shot peening using shots smaller than those used in the first-stage shot peening are sequentially performed. According to the detailed description of this disclosure, it is preferabl
Ishida Masaaki
Isono Yuji
Sasada Hironobu
Suzuki Hiroshi
Teratoko Keiichiro
Ip Sikyin
Suncall Corporation
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