JPH0625596B2 - Highly durable piston ring manufacturing method - Google Patents

Description

The present invention relates to a method of manufacturing a highly durable piston ring.

Hardening treatment by hard Cr plating is generally used as a surface hardening treatment for piston rings for internal combustion engines. However, with this hardening treatment alone, diesel engines have been developed with the recent trend toward higher output and higher exhaust gas regulations. In such engines, troubles such as shortening the engine life due to ring wear have occurred.

As a countermeasure against these problems, various hardening treatments have been studied, and a piston ring whose surface is hardened by spraying a Mo-based material, a Fe-Cr-C-based material or the like for high-lead gasoline is partially used. However, although this one has no problem with its own wear, it has a problem with compatibility due to the aggressiveness to the mating material caused by thermal spraying properties and the mating material being cast iron material and poor wear resistance, and it is a mating material. The wear of the piston ring is large and the life of the engine is insufficient, and it is necessary to cure the piston ring in harmony with the relative wear.

The present invention extends the engine life by extending the life of the piston ring by making the cylinder wear the current level.

Generally, as a method to extend the life of the piston ring,
There are a method of improving the material of the ring, a method of further improving hardening wear resistance, a method of increasing the thickness of the existing hardening layer, and the like.

However, since the cylinder material is limited to cast iron or the like in the method, it is difficult to adjust the compatibility, and a method of increasing the thickness of the hardened layer itself is used to improve durability.
Hardening treatments that are currently in practical use include treatments such as hard Cr plating or nitriding treatment. These processing rings are satisfactory in both performance and durability in normal engines, but are not satisfactory in durability in diesel engines or engines using high-lead gasoline that have advanced exhaust gas countermeasures, and inevitably have a hardened layer. Increased thickness is required. However, each of these treatments has a problem that the thickness cannot be arbitrarily set due to the requirement only from durability.

For example, hard Cr plating having low toughness cannot follow the deformation of the base material, and a vertical crack is generated in the Cr layer. Since the crack penetrates to the base material, it acts as a notch and reduces the fatigue limit of the base material. Moreover, the cracks grow larger as the plating thickness increases, resulting in greater stress concentration. As described above, the thick Cr plating deteriorates the mechanical properties of a component, such as a piston ring, which is repeatedly stressed.

The following characteristics of electroplating are also important problems in using thick plating.

The change in the surface shape becomes large, which causes a problem in the forming process as a piston ring.

Inconsistencies in plating thickness increase, making it difficult to complete processing.

Surface roughness increases sharply and machining cost increases.

From the above points, the upper limit plating thickness is 0.25 mm or less on the safe side, although it depends on the strength of the base metal and the usage conditions.

On the other hand, in the nitriding treatment, the nitriding rate decreases with the nitriding thickness,
Further, nitriding of thin materials such as piston rings causes a large deterioration in toughness and breaks due to the expansion stress when assembled to the piston, so there is a limit to the nitriding thickness, and 20% or less of the thickness of the base material is Practical, eg 1 for side rails with a thickness of 0.6 mm
The thickness is preferably 20 μm or less.

As mentioned above, in the single layer, the upper limit of each thickness is regulated,
It is impossible to improve the durability and achieve the hope only by increasing the thickness.

As described above, the hard Cr plating has the property of lowering the fatigue limit. On the other hand, it is widely known that nitriding treatment causes residual stress due to diffusion of N and improves fatigue strength. This effect is not lost even when the compound layer having a high N concentration forming the outermost surface portion of the present invention (described later) is removed. Therefore, by plating after nitriding treatment, hard Cr having a high fatigue limit is obtained.
The plating ring can be manufactured.

Therefore, by combining the advantages of both, the nitriding layer contributes to the wear resistance effect after abrasion of the hard Cr plating and contributes to the improvement of durability. Therefore, the hard Cr plating on the nitriding surface improves the mechanical properties. And it is expected that the wear resistance will be improved, making it an ideal long-life piston ring.

The present invention has been made in view of the above points, and an object thereof is to provide a method for manufacturing a highly durable piston ring.

The method for producing a highly durable piston ring according to the present invention is a method in which a piston ring made of martensitic steel is used in a mixed gas of NH ₃ gas and N ₂ gas or in an NH ₃ gas atmosphere in an amount of 500 to 6
After the whole surface is gas-nitrided by holding at 00 ° C, at least the compound layer having a high N concentration forming at least the outermost surface of the nitrided layer on the outer peripheral surface is removed, and the compound layer having a high N concentration exists. A non-nitrided layer with a thickness of 30 μm or more and 20% or less of the piston ring thickness is formed, and a hard Cr plating of 50 μm is made on the nitride layer from which this compound layer is removed by a fluorination bath.
It is characterized in that it is applied in a range of from m to 250 μm.

Adhesion is a problem in electroplating the nitriding surface. Especially when high adhesion such as piston ring is required, the nitriding method, plating pretreatment, and plating conditions are important factors for piston ring manufacturing. It becomes a big requirement.

As the nitriding method, there are a soft nitriding method for diffusing N, C or N, C, O and a nitriding method for diffusing only N. However, N, C or N, C is excessively applied to the surface of electroplating. , O on the surface of which a non-metal layer is formed by combining with O cannot obtain a strong adhesion. The salt bath method, which is usually used as the soft nitriding method, has a harmful surface oxide layer (black layer) and porous layer thicker than other methods, and the layer thickness is proportional to the processing time. There are many disadvantages above.

In this case, the NH ₃ gas nitriding method of the present invention, in which only N is diffused, is preferable as the plating base surface.

Also in the NH ₃ gas nitriding method, N generated in 2NH ₃ → 2N + 3N ₂ diffuses and infiltrates from the surface, so the N concentration on the outermost surface is very high and is very thin compared to the salt bath method, but NH ₃
Since the compound layer is also formed in the gas nitriding method, when the hard Cr plating is applied to the surface, the compound layer having a high N concentration forming the outermost surface portion must be removed before plating.

The main state of the nitride layer is ζ-Fe ₂ N, ε-Fe from the outermost surface.
_2-3 N, γ'-Fe ₄ N, N solid solution layer in α-Fe are formed and distributed. Among these, the compound that impairs the adhesion of plating is a compound layer having a high N concentration forming the outermost surface portion, that is, ζ-Fe ₂ N
And ε-Fe _2-3 N, and by subjecting the surface from which this layer has been removed to the hard Cr plating treatment, a piston ring with good adhesion of the hard Cr plating can be obtained.

When performing the nitriding treatment, it is preferable to select a material that minimizes the generation of this compound layer, and also considering the function as a piston ring, the material for the piston ring is generally a martensite-based material. Steel (SUS44
0B, SUS420J2, etc. and their equivalent materials) are preferably used. Martensitic steel is excellent in heat resistance and wear resistance, and also has high electric conductivity (Cr plating current is about 50 A / dm ^2, which is 20 times higher than that of other plating). Further, the Cr-plated surface needs to be electrochemically active because of its adhesiveness, and the SUS material requires a high-level passivation film removal technology, and the martensite-based is more advantageous than the austenite-based. is there. Also,
In an austenite system, the contained Ni hinders thick nitriding.

Next, a nitriding method and processing conditions for making the compound layer as thin as possible are selected. The salt bath method is because it is difficult to control the nitrogen potential in the nitriding atmosphere.
The longer the treatment time is, the thicker the compound layer formed in the outermost layer is, which is unsuitable for the production of piston rings.

On the other hand, the NH ₃ gas nitriding method, which is the nitriding method of the present invention, adjusts the decomposition rate of NH ₃ gas according to the treatment temperature and the flow rate to minimize the formation of the compound layer, and the decomposition rate of NH ₃ is based on the catalyst of iron. Therefore, the decomposition rate automatically decreases as the nitriding progresses, and the nitrogen potential automatically decreases, so that there is an advantage that the thickness of the compound layer does not increase due to a slight difference in processing time. On the high temperature side, the treatment temperature is set to 600 ° C. or lower in order to prevent a decrease in the internal hardness of the base material (SUS420
Internal hardness Hv350 after treatment with J2, SUS440B, etc.
Above), since the high-hardness nitrided layer (nitriding rate decreases but hardness increases as the temperature decreases) may peel off on the low temperature side, the temperature is set to 500 ° C or higher, but 560 to 580 ° C is optimal.

Salt bath nitriding also uses iron as a catalyst to generate N in the generator, but because of its high nitrogen potential, SUS materials also have the advantage of requiring no special pretreatment such as gas nitriding and having a relatively high nitriding rate. However, when trying to thin the compound layer for this purpose, the opposite effect is obtained.

When the unnecessary compound layer generated by the nitriding method is ground and removed by lapping or the like, the compound layer generated by the NH ₃ gas nitriding method is 10 μm or less, and the strain amount due to the nitriding treatment, the expansion amount due to the nitriding, and the surface roughness. There is no problem because the correction machining allowance such as the increment of can be sufficiently removed with the minimum grinding allowance.

However, since the plating base surface is electrochemically insufficient in activity, it is necessary to improve the hard Cr plating conditions in order to obtain stronger adhesion. Higher adhesion can be obtained with the fluorinated bath type than with the standard hard Cr plating bath type chromic acid-sulfuric acid based Sargent bath. This is because the catalyst of the fluorination bath, F ⁻ , has higher activity than SO ₄ ^2−, which is the catalyst of the Sargent bath.

It is widely known that the fluorination bath can be plated with a wide current density, has a good covering power, and that it is easy to perform Cr plating on Cr. The problem in applying the solution is, for example, that the plating surface is instantaneously passivated when the plating is interrupted, and the passivation film is completely removed in a standard bath (surgery bath) that uses only sulfuric acid as a catalyst, and advanced plating is performed. Technology (re-plating method, etching → change polarity to plating, 2.5V to 1
It is widely known that if the plating is not performed by increasing the voltage by 0.5 V every minute to a specified current), poor adhesion will result. Meanwhile, the fluoride bath it is possible to re-plating continues without requiring special processing be interrupted plating, this is of the bath F ^-
It depends on the removal effect of the passivation film by the ions. The ring material of the present proposal is a martensitic steel material and has a property that it is easily passivated, and is similar to the Cr plating on Cr in the above example. A fluorinated bath is required as a stable Cr plating bath for martensitic steel + nitrided surface.

As described above, the hard Cr plating treatment method as the manufacturing method according to the present invention is premised on the condition of the fluorination bath.

The following baths are suitable as the fluorination bath.

^{_{CrO 3 200~300 g / Cr 3+ 0.5~5}} g / H 2 SO 4 1~3 g / fluorides 1 to 5 g / Temperature 45 to 60 ° C. fluoride is large, more than 30 species, but in general use What is mentioned is N ₂ SiF ₆ or K ₂ SiF ₆ .

The present invention is a plating bath in which F ⁻ ions are present.

It is more preferable to use the following replating method.

1) Etching (reverse voltage) 30 A / dm ² x 1 min 2) Plating (positive voltage) 1.9 V x 5 min Reduction cleaning treatment Plating 2.5 V (decomposition voltage), 1 V stepping up by 1 V every minute, and specified current (45 to 50 A) / Dm ² ).

The thickness of the nitriding layer increases as the nitriding time increases, and it may be set by the life of the nitriding layer. If the hardness of the top and bottom surfaces of the top ring is Hv 700 or more and the thickness is 10 μm or more, a normal diesel It is possible to meet the engine life, but considering the hardness of the base material and the fluctuation of usage conditions, etc., 30
The safety side is above μm. In addition to fatigue strength, the upper limit is 20% or less of the wall thickness of the ring because the toughness deteriorates with the nitriding thickness and breaks during assembly to the piston. Hard C
The thickness of the r plating is 50 μm or more, and preferably 250 μm or less in consideration of the forming process after the Cr plating, the fatigue limit and the like.

FIG. 1 shows an embodiment of a piston ring manufactured according to the present invention.

The piston ring 1 of the present example has a gas nitriding layer on the entire surface, but the gas nitriding layer on the surfaces other than the inner peripheral surface has removed the compound layer having a high N concentration forming the outermost surface portion. That is, 2 is a piston ring base material, 3 is a gas nitriding layer of Hv 700 or more formed on the entire surface of the piston ring, 4 is a hard Cr plating layer of Hv 850 or more formed on the gas nitriding layer 3 of the sliding surface, 5 Is a compound layer having a high N concentration, which remains on the inner surface of the gas nitride layer 3 without being removed.

In this example, since the compound layer having a high N concentration on the sliding surface and the upper and lower surfaces was removed, the hard and brittle compound layer fell off and acted as abrasion powder to cause the opposite wear,
It does not accelerate the wear of itself and cause abnormal wear or scratches on the cylinder inner peripheral surface or the ring surface.

An example of the manufacturing method of the present invention is shown below.

1) Rough finishing process (from wire to ring shape process) 2) Nitriding process (NH ₃ gas nitriding) 3) Compound layer with high N concentration forming the outermost surface removal grinding process (preparation of plating base) 4) Degreasing (organic) 5) Surface activation treatment (blasting at an air pressure of 5 kg / cm ² using an aqueous solution containing 10 to 30% (volume ratio) of 280 to 350 mesh alumina abrasive grains) 6) Hard Cr plating (fluorine) 7) Completion processing (peripheral lap, upper and lower surface polishing (compression ring), adjustment of abutment) The adhesion test of hard Cr plating will be described below.

(1) Adhesion test method As shown in FIG. 2, for the adhesion test, a slit 6 having a width B = 2 mm is formed from the inner peripheral side to the outer peripheral side of the piston ring 1 from the outer peripheral surface to the position A = 3 mm. . And the third
As shown in the figure, fix one side of the piston ring 1 and expand the other side from the inside to the outside at a constant speed until it breaks.
The fractured state of the plating layer 4 is observed and the adhesion is evaluated.

The evaluation is good when the hard Cr plating 4 completely remains as shown in FIG. 4, and slightly good when the hard Cr plating 4 partially remains as shown in FIG. 5, as shown in FIG. Hard C
The case where the r-plating 4 does not remain is regarded as defective.

(2) Test material Piston ring material: SUS420J2 Piston ring size: 83φmm × 2.0mm (B) × 3.5mm (T) Surface hardening treatment is shown in Table 1. Test Nos. 1 and 6 are examples without nitriding.

The nitriding treatment conditions are shown in Table 2.

Hard Cr plating conditions are shown in Table 3.

The manufacturing process is as follows: roughing process → nitriding process → removal grinding process of compound layer having a high N concentration forming the outermost surface → degreasing → surface activation process → hard Cr plating → finishing process. Degreasing is performed by steam cleaning with a fluorine-based organic solvent, and surface activation is performed by blasting at a pneumatic pressure of 5 kg / cm ² using an aqueous solution containing 30% by volume of 280 mesh alumina abrasive grains.

(3) Results As shown in Table 1, the results obtained by performing the 10-piece adhesion test for each of the test Nos. 1 to 6 in Table 1 above show that test No. 5 of the present invention shows the most stable adhesion. You can see that you can get it.

As described above, according to the present invention, the following effects are obtained,
It is an extremely excellent method for producing a highly durable piston ring.

Hard Cr plating layer (thickness 50 μm) on gas nitriding layer (thickness 30 μm to 20% or less of piston ring thickness)
As described above, since it has a thickness of 250 μm or less), high durability can be obtained.

Since a martensitic steel is used as the material, the function as a piston ring can be sufficiently satisfied, and a hard nitrided layer can be obtained by nitriding.

By gas nitriding, it is possible to form a thick effective nitriding layer that is hard and has excellent wear resistance.

As described above, nitriding and soft nitriding are different, and nitriding diffuses and permeates nitrogen into the object to be treated, whereas soft nitriding simultaneously diffuses and permeates nitrogen and carbon into the object to be treated. Alternatively, the treatment atmosphere is different, and the obtained physical properties are also different. Compared with gas nitriding, salt bath nitriding causes a thick compound layer having a high N concentration, which is harmful to adhesion, to be formed thick, and an effective nitriding layer cannot be thickened. On the other hand, in gas nitriding, a compound layer having a high N concentration, which is harmful to adhesion, is thinly formed, an effective nitriding layer can be thickly formed, and wear resistance is excellent as compared with an effective nitriding layer by soft nitriding. .

A piston ring that is exposed to harsh conditions requires a high degree of adhesion between the nitride layer and the hard Cr plating, but depending on the removal of the compound layer having a high N concentration and the plating conditions of the fluorination bath, the piston ring It is possible to secure sufficiently satisfactory adhesion even under severe conditions where the ring is exposed.

During the grinding and removing work of the compound layer, the grinding and removing work is difficult because the compound layer is hard, but by selecting the martensitic steel material and the gas nitriding treatment, the grinding and removing work is performed because the compound layer does not become thick. Easy and easy to manufacture.

[Brief description of drawings]

FIG. 1 is a sectional partial view showing an embodiment of a piston ring manufactured according to the present invention, FIG. 2 is a partial plan view of a piston ring provided with slits for a contact test, and FIG. 3 is an explanatory view of the contact test. FIGS. 4 to 6 are views showing the contact evaluation of the piston ring after the adhesion test are good, slightly good, and poor. 1 ... Piston ring, 2 ... Base material, 3 ... Gas nitriding layer, 4 ... Hard Cr plating layer, 5 ... Compound layer with high N concentration

Claims (1)

[Claims] [Claim 1] After the mixed gas or NH ₃ gas atmosphere the entire surface is held at 500 to 600 ° C. in the NH ₃ gas and N ₂ gas piston rings made of martensitic steel and the nitriding gas, at least the outer peripheral surface Of the nitride layer, the compound layer having a high N concentration forming at least the outermost surface is removed, and the nitride layer having no compound layer having a high N concentration is removed by 30 μm.
m or more and 20% or less of the piston ring thickness, and hard chromium plating is applied to the nitride layer from which this compound layer is removed by a fluoride bath in a thickness of 50 μm or more and 250 μm or less, high durability Piston ring manufacturing method.