Metal treatment – Stock – Carburized or nitrided
Reexamination Certificate
1998-08-10
2001-05-01
Sheehan, John (Department: 1742)
Metal treatment
Stock
Carburized or nitrided
C148S325000, C148S333000, C420S034000, C277S434000, C277S443000
Reexamination Certificate
active
06224687
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a piston ring material and a piston ring used in an internal combustion engine.
PRIOR ART
In recent years, the internal combustion engine has been provided with various improvements such as low fuel consumption design, high performance design, light weight design, exhaust gas-purifying design, and etc.
Among them, a piston ring, which is a sliding portion in the internal combustion engine, is strongly demanded for improvements in characteristics such as fatigue characteristic, wear resistance, and scuffing resistance because of thickness-thinning design as the engine has light weight design and high rotation design. Thus, a cast iron piston ring conventionally used is being replaced with a steel piston ring which is excellent in strength and fatigue characteristic.
At present, materials used for the steel piston ring mainly include material based on Si—Cr steel equivalent to JIS SWOSC-V, or martensite stainless steel containing 12-21% of Cr (hereinafter called “high-Cr martensite stainless steel”).
Typically, a piston ring made of Si—Cr steel is Cr-plated on its surface for use. However, the Cr-plating layer formed on the piston ring surface has insufficient wear resistance required for use.
In addition, when it is applied to an internal combustion engine with high load, there arises a problem that ring matrix is exposed due to peeling of a plating area. Consequently, scuffing can immediately occur on the inner wall of the cylinder.
In addition, Cr-plating treatment involves various problems associated with waste liquid generated after the treatment and such as adverse effects on environment, or increase of waste liquid treatment cost in recent years.
On the other hand, many of the piston rings made of high Cr martensite stainless steel are treated for nitriding on its surface for use.
The nitride layer not only has higher wear resistance when compared with the Cr-plating layer, but also has no problem of peeling of the treated layer because the nitriding is treatment utilizing diffusion, so that it has very excellent characteristics for the piston ring.
In addition, since the nitriding is inexpensive in its treatment cost, and causes less impact on environment, it is a more advantageous treatment than the Cr-plating treatment.
Furthermore, even if it is necessary to provide he Cr-plating treatment depending on its application, high r martensite stainless steel can be also used as a piston ring material for Cr-plating because the material itself has higher heat resistance, wear resistance and corrosion resistance than Si—Cr steel.
As described above, the conventional piston ring has been used for a high load internal combustion engine relatively placing emphasis on performance, but its use is not limited to such high load internal combustion engine, and being gradually expanded for range in the background of demands for the recent internal combustion engine such as low fuel consumption design, high performance design, light weight design, exhaust gas-purifying design and etc.
SUMMARY OF THE INVENTION
Typically, in manufacturing the high Cr martensite stainless steel as the piston ring material, the material formed in a flat wire or a deformed wire stock is once heated to 900-1100° C., quenched and hardened, and then tempered at a relatively high temperature.
After the above heat treatment, the high Cr content martensite stainless steel stock would be formed into a predetermined ring shape. However, its hardness should be conditioned to as relatively low as 35-45 HRC when compared with the post-heat treatment hardness of 45-55 HRC of Si—Cr steel to increase the bending workability (curling property) in forming into the ring shape.
Higher heat treatment hardness is desirable if emphasis is essentially placed on wear resistance, scuffing resistance, and fatigue resistance as a piston ring.
However, the high Cr content martensite stainless steel containing much residual carbide has a problem that its heat treatment hardness should be conditioned to a lower level although its properties is more or less deteriorated because it may be broken during bending work if it has a high heat treatment hardness.
To reduce such problem, there is known a low alloy piston ring proposed in JP-A-59-166653 and JP-A-63-140066.
Such approach lowers Cr content to a low level of 2.0 to 9.0%. Although it can improve breakage resistance property, the scuffing resistance is significantly lowered. Thus, the composition proposed by the above has a problem in the properties as a piston ring, and it is the current status that it is not widely put in practical use.
In addition, since the high Cr martensite stainless steel has higher work hardening than the Si—Cr steel, work ratio cannot be increased until it is finished as a flat wire or deformed wire stock.
It calls for a large number of annealing stages during drawing or rolling process, so that there is a problem that cost for manufacturing wire stock becomes expensive.
Then, in view of the above, the present invention is intended to provide a piston ring material and a piston ring for which the manufacturing cost can be reduced by improving the warm or cold drawability and rolling workability, and the breakage during forming a ring can be reduced without deteriorating characteristics required for a piston ring.
First, the inventor thoroughly examined a wire-shaped piston ring stock after hot rolling for factors affecting its warm and cold drawability and rolling workability.
Furthermore, the inventor thoroughly studied bending workability after heat treatment of the stock, and scuffing resistance and wear, most important as piston ring, resistance for factors affecting its properties.
As the result, the inventor found that the properties largely depend on morphology of carbide existing in the structure of piston ring material, and found optimal morphology of carbide which provided very good workability even during manufacturing and after heat treatment without deteriorating the required wire stock properties as a piston ring.
In addition, the inventor found that conditioning of C and Cr contents as elements contained in the piston ring material is effective in further attaining the various properties described above, and in attaining the optimal carbide state, and reached the preset invention.
That is, the piston ring material according to the present invention essentially consists, by weight of 0.2 to 1.2% C, 5.0 to 25.0% Cr and the balance Fe and incidental impurities, M
7
C
3
type carbide content existing in the structure being 4.0% or less in terms of area percent to attain excellent scuffing resistance and workability. Preferably, C is 0.2 to 0.7%, and Cr is not less than 5.0% but less than 12.0%.
With the piston ring material according to the present invention, it is possible to attain excellent scuffing resistance and workability even for a low Cr content martensite piston ring material which contains Cr of not less than 5.0% but less than 12.0%.
In addition, the piston ring material according to the present invention can attain very good workability even after heat treatment without deteriorating required properties as a piston ring by limiting Cr (wt. %)/C (wt. %) to 12-45. Preferably, Cr (wt. %)/C (wt. %) is 15-45, more preferably 18-30.
In addition to above, the piston ring material according to the present invention can further improve the properties required for the piston ring material by conditioning, by weight, Si to not more than 0.25%, Mn to not more than 0.30%, or one or more of Mo, W, V and Nb to not more than 2.5% in total. It is also possible to condition Cu to not more than 4.0%, Ni to not more than 2.0%, or Al to not more than 1.5%.
With the piston ring material with excellent bending workability according to the present invention, even when a wire stock to be bent is bend along a longer side of its small cross-section, breakage is difficult to occur regardless of hardness conditioning after its heat treatment. That is, the present invention can provide a piston ring with
Hitachi Metals Ltd.
Oltmans Andrew L.
Sheehan John
Sughrue Mion Zinn Macpeak & Seas, PLLC
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