JP3846811B2 - Piston for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a piston for an internal combustion engine, in particular, on the surface of the diesel engine piston, nitriding or relates piston subjected to soft nitriding.
[0002]
[Prior art]
In the past, an example of applying gas soft nitriding treatment to the required part of the outer shape of the piston body after finishing, which was made of ductile cast iron, was recently applied to a piston that required high heat resistance in a high-power diesel engine for vehicles. No. 3-87848 is known. This will be described with reference to FIG.
As shown in FIG. 6, a gas soft nitriding furnace having ammonia gas 22 introduced therein, 23 is a piston main body made of ductile cast iron after finishing, and 24, 25 and 26 are required for the outer shape forming surface of the piston main body 23. Location, that is, the surface of the outer periphery 23a of the piston main body 23, the surface of the piston ring groove 23b, the inner peripheral surface of the piston pin hole 23c, and the outer surface forming surfaces 28a, 28b, 28c, and 28d such as the inner surface of the combustion chamber 23d are heated. A high-frequency heating coil 27 is a mount for mounting the piston body.
When performing the nitrocarburizing process on the piston body 23, the piston body 23 is first placed on the frame of the nitrocarburizing furnace 21, and then the high-frequency heating coil 24 is placed near the outer periphery 23a of the piston body 23 and the piston ring groove 23b. The high frequency heating coil 25 is installed near the outer forming surface 28c of the piston pin hole 28c, and the high frequency heating coil 26 is installed near the outer forming surface 28d of the combustion chamber 23d.
Next, ammonia gas or the like 2 is introduced from the outside into the furnace 21, and the high-frequency heating coils 24, 25, and 26 are energized so that the outer periphery 23a of the piston body 23, the piston ring groove 23b, the piston pin hole 23c, and the combustion chamber 23d. Each of the outer shape forming surfaces 28a, 28b, 28c, 28d is heated to a predetermined temperature (about 550 ° C.) with a high frequency. For this reason, ammonia gas or the like is heated and decomposed into nitrogen gas and hydrogen gas, and the nitrogen gas is externally formed on the outer periphery 23a of the piston body 23, the piston ring groove 23b, the piston pin hole 23c, and the combustion chamber 23d. , 28c, and 28d, and gas soft nitriding is performed on the hatched portion in FIG.
As described above, when the outer periphery 23a of the piston body 23 made of ductile cast iron, the piston ring groove 23b, the piston pin hole 23c, and the outer shape forming surfaces 28a, 28b, 28c, and 28d of the combustion chamber 23d are subjected to gas soft nitriding treatment, The fatigue strength of these portions is about 1.5 times higher than when the gas soft nitriding treatment is not performed, and the seizure resistance and the wear resistance are further improved as compared with, for example, hard chrome plating.
For this reason, cracks due to the outer shape forming surfaces 28a, 28b, 28c, 28d of the piston main body 23 are prevented, and seizure resistance of the outer periphery 23a, the piston ring groove 23b, and the piston pin hole 23c, which are the peristaltic parts, is eliminated. And wear resistance are improved.
In addition, when the nitriding treatment is performed on the entire piston body 23, the piston body 23 must be heated. Therefore, there is a possibility that the piston body 23 may be deformed. Since it is heated, the piston body 23 is not deformed.
Further, since it is not necessary to heat the entire piston main body 23, it is not necessary to heat the entire gas soft nitriding furnace 21, energy saving can be achieved, and the gas soft nitriding process can be performed efficiently with a shortened processing time. be able to.
In the embodiment of the present invention, the case where the gas soft nitriding treatment of the piston body is performed by high-frequency heating has been described. However, various heating means are employed as well as high-frequency heating as long as the required portion can be heated. It goes without saying that various modifications can be made without departing from the scope of the present invention.
[0003]
[Problems to be solved by the invention]
However, the conventional technique has the following problems.
(1) In the case of Japanese Utility Model Publication No. 3-87848 of the prior art, the gas chamber is subjected to gas soft nitriding treatment, but the piston top surface portion is not subjected to gas soft nitriding treatment, There is a problem in that there is a risk of fatigue failure starting from a fine crack after a long period of operation.
In the piston of a high-power diesel engine, the combustion chamber and the piston top surface are exposed to high-temperature combustion gas during engine operation. For this reason, the temperature of the combustion chamber of the piston rises, and in particular, the temperature near the rim portion of the combustion chamber of the reentrant piston with improved combustion, and the vicinity of the exhaust valve operating at the piston top surface portion, is made of ductile cast iron. There is a problem that a piston that has not been subjected to soft nitriding treatment undergoes fatigue failure starting from a fine crack after a long period of operation.
The part where the temperature near the exhaust valve is activated on the piston top surface is subjected to soft nitriding to increase the high temperature fatigue strength.
That is, in cast iron pistons operating under high temperature conditions, the material strength of the rim portion of the combustion chamber is substantially determined by the high temperature fatigue strength and the strength of the iron-based oxide film generated during operation. Breakage of the rim portion of the cast iron piston piston running under high temperature conditions is a fatigue fracture originating from the fine cracks of the iron-based oxide film that occurs rim surface. An iron-based oxide film is generally brittle compared to an iron base material, and its strength is lower than that of an iron base material. The strength of the iron-based oxide film decreases with increasing thickness. Further, the thickness increases with the engine operating time or the piston rising.
[0004]
(2) Piston slap also becomes larger due to higher output of diesel engine, and nitriding treatment is applied to the skirt portion of the piston to improve seizure resistance and wear resistance, but nitriding treatment is applied to the piston skirt portion. As a result, there is a problem that the shape is easily deformed. A shape that does not easily deform during nitriding and a nitriding method that does not easily deform are needed.
[0005]
The present invention focuses on conventional problems as described above, the piston for an internal combustion engine, nitriding or surface of the piston, treated soft-nitrided and nitrided or shape is hardly deformed during the nitrocarburizing treatment shape, and, in the deformed hardly nitriding method, to avoid fatigue fracture originating microcracks in long-term operation of the combustion chamber rim, high-temperature oxidation resistance, high temperature strength, wear resistance superior piston The purpose is to provide.
[0006]
[Means for Solving the Problems]
To solve the above object, the first invention of the internal combustion engine piston of the present invention, the cast iron piston or iron piston for an internal combustion engine,
The thickness of the skirt portion of the piston is provided on the entire circumference, and the ratio of the thickness t1 at the center of the skirt to the length L from the center of the piston pin hole to the lower end of the piston, and the thickness t2 of the lower end of the skirt with respect to the length L The ratio product is in the range of 0.0125 to 0.2500, and at least the skirt portion surface of the piston surface is subjected to nitriding treatment or soft nitriding treatment.
A nitriding-treated piston is obtained in which the rigidity of the skirt portion of the piston and the dimensional change before and after the nitriding treatment of the skirt portion by nitriding treatment are small.
In addition, by performing nitriding in this way, fatigue failure starting from fine cracks can be prevented by operating the combustion chamber rim of the piston for a long time, and the skirt portion of the piston can be prevented from burning due to high output. It is possible to improve the wear resistance of the stickiness, wear resistance, and top ring groove and piston pin hole.
[0008]
Oite to the first aspect of the present invention, nitriding of the surface of the piston or, nitrocarburizing treatment is an ion nitriding.
Such ion nitriding is an electrical nitriding method in which anions generated in plasma are vapor-deposited on a cast iron piston surface or an iron-based piston surface . The nitriding potential is high, the processing time is short compared to the gas nitriding method, and a deep nitride layer can be obtained. In addition, there is no need for post-processing to suppress the formation of a surface embrittlement layer. Further, partial nitriding is easy, distortion is small, and the processed surface can be cleaned.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the first embodiment will be described in detail with reference to FIGS.
The first embodiment or nitride on the surface of the cast iron piston example that a soft nitriding treatment, and examples of ion nitriding.
First, the structure of the cast iron piston 1 whose surface is nitrided or soft nitrided on the left side of FIG. 1 will be described. The left side of FIG. 1 shows a half sectional view of the cast iron piston 1. The material is ferritic annealed spheroidal graphite cast iron, which is subjected to nitriding or soft nitriding after machining. A soft nitriding treatment 2 is applied to the entire surface of the cast iron piston 1 as indicated by a one-dot chain line in the figure. That is, the surface of the piston top portion 3, the combustion chamber 4, the rim portion 4a, the top ring groove 5, the second ring groove 6, the oil ring groove 7, the skirt 8, and the piston pin hole 9 is processed.
[0010]
Next, the operation will be described. By applying the soft nitriding process 2 to the entire surface of the cast iron piston 1, distortion occurs. This distortion is large in the skirt 8 portion of the cast iron piston 1. The deformation of strain when soft nitriding is performed will be described with reference to the right side of FIG.
The right side of FIG. 1 shows the height of the cast iron piston 1 on the vertical axis, and corresponds to each part of the left piston 1 of FIG. The horizontal axis represents the piston outer diameter dimension mm on the thrust side perpendicular to the piston pin hole 9 of the cast iron piston 1. Profile A in the figure, that is, a, b, c, d shows the shape after the completion of machining and before soft nitriding, and profile B, ie, e, f, g, h, the shape after soft nitriding. Is shown. Deformation from profile A to profile B by soft nitriding. Although there is no distortion deformation due to the soft nitriding process at the lower ends d and h of the skirt 8, the distortion amount ΔD is maximized in the vicinity of the center of the piston pin hole 9 at c to g and b to f. This amount of distortion is determined by the rigidity of the lower surface of the skirt 15. The relationship between the stiffness and the strain amount will be described with reference to FIG.
FIG. 2 shows the skirt deformation ratio on the vertical axis and the ratio (ΔD / D) with the amount of distortion ΔD with respect to the outer diameter D of the piston. The relationship between the ratio (t1 / L) of the wall thickness t1 and the ratio (t2 / L) of the wall thickness t2 of the lower end of the skirt to the length L (t1 / L × t2 / L) is shown. A value obtained from various experimental results. If the rigidity is large, the strain amount is small, and if the rigidity is small, the strain amount is large. When the value of t1 / L × t2 / L is 0.0125 or less, the distortion amount of ΔD / D increases rapidly.
FIG. 3 shows that the product of the ratio of the wall thickness t1 at the center of the skirt to the length L from the piston pin hole center Q to the piston lower end and the ratio of the wall thickness t2 at the lower end of the skirt to the length L is 0.0125-0. It indicates that the range is 2500 (hatching range). The vertical axis shows the skirt aspect ratio and the thickness t2 (t2 / L) of the skirt bottom with respect to the length L. The horizontal axis shows the skirt aspect ratio and the thickness t1 (t1 / L) of the skirt bottom with respect to the length L. Show. The lower limit value E indicates t1 / L × t2 / L ≧ 0.0125, and the upper limit value F indicates t1 / L × t2 / L ≦ 0.2500. The lower limit E of 0.0125 is the allowable limit value as seen from the amount of strain caused by the soft nitriding treatment, and the upper limit F of 0.2500 is the allowable limit value as seen from the weight increase of the piston.
Next, the effect of soft nitriding with spheroidal graphite cast iron whose material is made of ferritic annealing will be described with reference to FIG. In FIG. 4, the vertical axis represents the oxide film thickness, and the horizontal axis represents the engine durability operating time. The oxide film thickness on the vertical axis is a characteristic that represents the deterioration of the piston. The larger the thickness, the larger the deterioration. From past experience, the allowable limit is based on 70 μm. It is said. The measurement location is the rim of the combustion chamber where the piston is easily cracked. The solid line G in the figure is the deterioration characteristic without the soft nitriding treatment, and reaches the allowable limit at the end of the durable operation time 950h at the point m. Also, the dotted line H in the figure is a deterioration characteristic with soft nitriding treatment, and is in a safe range with a margin with a durable operation time of 1000 h. When the presence / absence of soft nitriding treatment is compared with the endurance operation time of 500 h, the oxide film thickness is reduced from j point to k point (thick dotted arrow) and reduced to about 1/5.
[0011]
Next, ion nitriding will be described. Ion nitriding is performed in a low pressure gas atmosphere of 1 to 10 mmHg using a furnace body as an anode + and an object to be processed as a cathode, and applying a direct current voltage of 350 to 1000 V to generate glow discharge to perform nitriding. The nitriding gas is ionized by the glow discharge, accelerated by the electric field, and collides with the object to be processed. This energy is converted into thermal energy to heat the object to be processed, and causes cathode sputtering to knock out atoms and electrons such as iron (Fe, C, O) from the surface. The ejected iron atoms combine with atomic nitrogen in the glow plasma to form iron nitride FeN, which is deposited on the surface of the workpiece. Nitrogen N released in the decomposition process into the lower nitride diffuses into the steel. The iron atoms that have released nitrogen return to the glow plasma to form FeN again, and repeat the above-described reaction to promote nitriding.
Such ion nitriding is characterized in that the processing time is shorter than that of the conventional gas nitriding method, distortion is small, and the processed surface can be cleaned. Further, there is no need for post-processing to suppress the formation of the surface embrittlement layer.
[0012]
Next, a second embodiment will be described with reference to FIG. The second embodiment is an example in which nitriding or soft nitriding is applied to the piston top surface. FIG. 5 shows a half cross-sectional view of the cast iron piston 1. The same parts as those in the first embodiment are denoted by the same reference numerals, and description thereof will be omitted below.
As shown in FIG. 5, the material is ferritic annealed spheroidal graphite cast iron, which is subjected to nitriding or soft nitriding after completion of machining. As indicated by a one-dot chain line 2 in the figure, at least the piston top surface portion 3 and the combustion chamber 4 are subjected to a nitriding or soft nitriding partial treatment 10. That is, nitriding or soft nitriding partial treatment 10 is also applied to the rim portion 4 a between the piston top surface portion 3 and the combustion chamber 4. The top ring groove 5, the second ring groove 6, the oil ring groove 6, the skirt 8, and the piston pin hole 9 are subjected to nitriding treatment for preventing nitriding or soft nitriding.
By performing nitriding on the piston top surface portion and the combustion chamber of the piston in this way, fatigue failure starting from a fine crack can be prevented during long-time operation of the combustion chamber rim portion. Further, since there is no change in dimension and shape due to the nitriding treatment of the skirt portion of the piston, it can be accurately processed to a predetermined dimension.
[0013]
【The invention's effect】
As described above, according to the present invention, the ratio of the wall thickness t1 of the center part of the skirt to the length L from the piston pin hole center Q to the piston end,
The product of the ratio of the length L to the wall thickness t2 of the lower end of the skirt is in the range of 0.0125 to 0.2500,
In addition, since the surface of the piston has been subjected to nitriding or soft nitriding treatment, a piston having a small dimensional change between the rigidity of the piston skirt and the nitriding treatment before and after nitriding of the skirt can be obtained. .
In addition, by performing nitriding in this way, it is possible to prevent fatigue failure starting from microcracks due to long-time operation of the combustion chamber rim portion of the piston, and the piston skirt portion due to high output can be prevented. The seizure resistance, the wear resistance, and the wear resistance of the top ring groove and the piston pin hole can be improved.
Further, by subjecting the piston top surface portion and the combustion chamber of the piston to nitriding or soft nitriding, it was possible to prevent fatigue failure starting from fine cracks during long-time operation of the combustion chamber rim. Further, since there is no deformation of the dimensional shape due to the nitriding treatment of the skirt portion of the piston, it can be accurately processed to a predetermined dimension.
Also, in cast iron pistons, ion nitriding is applied to the piston surface as nitriding or soft nitriding treatment, so that the processing time is shorter than conventional gas nitriding methods, deformation due to distortion is less, and the processed surface is clean. And the need for post-processing is eliminated in order to suppress the formation of a surface embrittlement layer.
[Brief description of the drawings]
FIG. 1 is a half sectional view showing a cast iron piston according to a first embodiment of the present invention and a diagram showing a change in piston outer diameter before and after soft nitriding.
FIG. 2 shows the ratio of the amount of distortion to the outer diameter of the piston on the vertical axis, the ratio of the wall thickness of the center of the skirt to the length from the piston pin hole center to the lower end of the piston on the horizontal axis, and the lower end of the skirt with respect to the length. It is a figure which shows the relationship with the product of ratio with wall thickness.
FIG. 3 shows the relationship between the ratio of the thickness of the skirt lower end to the length of the vertical axis and the ratio of the thickness of the skirt central portion to the length from the piston pin hole center to the piston lower end of the horizontal axis. FIG.
FIG. 4 is a graph showing the relationship between the oxide film thickness on the vertical axis and the engine durability operating time on the horizontal axis.
FIG. 5 is a half sectional view showing a cast iron piston according to a second embodiment of the present invention.
FIG. 6 is a diagram for explaining nitriding treatment of a cast iron piston according to the prior art.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Cast iron piston, 3 ... Piston top part, 4 ... Combustion chamber, 8 ... Skirt, 9 ... Piston pin hole.

Claims (2)

In cast iron pistons or iron-based pistons for internal combustion engines,
The thickness of the skirt portion of the piston is provided on the entire circumference, and the ratio of the thickness t1 at the center of the skirt to the length L from the center of the piston pin hole to the lower end of the piston, and the thickness t2 at the lower end of the skirt with respect to the length L A piston for an internal combustion engine, wherein the product of the ratio is in a range of 0.0125 to 0.2500, and at least the skirt portion surface of the piston surface is subjected to nitriding treatment or soft nitriding treatment. Nitriding or the piston surface, nitrocarburizing treatment, the piston for an internal combustion engine according to claim 1 is an ion nitriding.

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