GB2153488A

GB2153488A - Nitrided steel piston ring

Info

Publication number: GB2153488A
Application number: GB08501651A
Authority: GB
Inventors: Takeo Kooroki
Original assignee: Nippon Piston Ring Co Ltd
Current assignee: Nippon Piston Ring Co Ltd
Priority date: 1984-01-23
Filing date: 1985-01-23
Publication date: 1985-08-21
Also published as: DE3501823C2; JPS60153456A; GB2153488B; DE3501823A1; GB8501651D0

Abstract

A wear-resistant, compression piston ring (2) for an internal combustion engine may be made of martensitic stainless steel (preferably one containing carbon and chromium). The ring has a nitrided outer layer with a hardness of Hv 950 to a depth of more than 50 mu m. <IMAGE>

Description

SPECIFICATION Steel piston ring This invention relates to a steel piston ring for an internal combustion engine, and in particular to an improved form of the compression piston ring, which in use, is positioned adjacent to the combustion chamber of the cylinder in which the associated piston is reciprocable. This piston ring is known as the first compression piston ring.

A piston ring of this type is greatly affected by combustion gas, because it is positioned immediately below the top land of the piston, which land is adjacent to the combustion chamber. This piston ring is pushed strongly against the cylinder wall by the gas pressure applied to the piston ring's inner wall, and produces a large frictional force that increases the wear on the piston ring. In such a compression piston ring, lubrication is rather difficult, and a wear phenomenon called "scuffing" is liable to take place.

In order to improve the efficiency of internal combustion engines, it is desirable to reduce this frictional force. Therefore, a recent tendency has been to decrease the thickness of this type of piston ring in the axial direction, thereby reducing the affect of the gas pressure applied to the piston ring.

If a conventional cast iron piston ring is reduced in thickness as described above, it loses much of its mechanical strength and stability. Consequently, thinner piston rings of this type have to be made of steel.

Unfortunately, steel piston rings suffer from the disadvantage that, since steel has no self-lubricating element (such as graphite in cast iron) they are susceptible to wear. In addition, at the operating temperatures of an internal combustion engine, steel piston rings lose tension because of repetitive fatigue.

In petrol engines using high lead petrol, steel piston rings are subject to significant wear. This is because lead compounds formed by combustion, such as lead halogenide and lead oxide, act as abrasive particles that accelerate the wear of the piston ring's sliding surface. Abrasive carbon particles are also unavoidably formed in diesel engines by combustion, and these abrasive particles accelerate wear on the sliding surfaces of such piston rings.

The aim of the invention is to provide a steel piston ring having nitrided layers formed in the surfaces of the steel piston ring, the nitrided layers being high in hardness, and excellent in wear resistance.

The present invention provides a piston ring for an internal combustion engine, the piston ring comprising an annular member consisting essentially of steel, the annular member having an outer layer consisting essentially of nitrided steel, the outer layer having a hardness of at least Hv 950 to a depth of more than 50 am.

Advantageously, the steel is a martensitic stainless steel containing at least from 0.25 to 1.20% by weight of carbon, and from 11 to 19% by weight of chromium.

Preferably, the outer layer has a hardness of less than Hv 1300, and a depth up to about 200 Am.

Conveniently, only the outer radial portion of the piston ring is nitrided.

The invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is part-sectional view of a steel piston ring constructed in accordance with the invention, the piston ring being positioned in the ring groove of a piston; Figure2 is a graphical representation of the hardness as a function of depth of various nitrided steel piston rings; Figure 3 is a graphical representation of the results of scuffing resistance tests; and Figure 4 is a graphical representation indicating results of a comparison of wear tests on various ring surface compositions.

Referring to the drawings, Figure 1 shows a compression piston ring 2 for an internal combustion engine.

The piston ring 2 is an annular member made essentially of martensitic stainless steel. The piston ring 2 is inserted in the uppermost ring groove of a piston 4 which is reciprocable within a cylinder 1. The ring 2 has a nitrided layer 3 which contacts the surface of the cylinder 1. Figure 1 shows the piston ring 2 having a nitrided layer 3 formed only in its outer wall. However, it would also be possible for the upper and lower surfaces (or all the surfaces) of the piston ring 2 to be nitrided, depending upon the conditions of use.

Compression piston rings of an internal combustion engine should have excellent gas-tightness, wear resistance, scuffing resistance and breakage resistance; and, in order to increase the output and speed of such an engine, such piston rings should be further improved.

The piston ring 2 of the invention is preferably made of martensitic stainless steel containing at least 0.25 to 1.20% by weight of carbon and 11 to 19% by weight of chromium. If the quantity of carbon is larger than that which is defined above, then the steel is so brittle that it is difficult to form a piston ring. Even if a piston ring can be formed with such large amounts of carbon, it is so low in impact resistance that it would break too easily in service. Accordingly, the quantity of carbon should preferably be within the above-described range. On the other hand, chromium is added to increase the heat resistance and strength of the base material and to produce carbides. The quantity of chromium should preferably be within the abovedescribed range, because of its effect on the brittleness of the base material which affects the machinability and breakage of the piston ring.

A piston ring 2 of such a martensitic stainless steel material may be used effectively to some extent.

However, it cannot be used in an internal combustion engine that is subjected to high loads. Accordingly, it is essential to subject the piston ring 2 to nitriding, and the nitriding remarkably improves the operating characteristics of the piston ring.

In other words, the formation of nitrided layers on the surfaces of the piston ring 2, and in particular on the sliding surface which is worn most, not only increases the surface hardness of the piston ring, but also makes the wear coefficient of the nitrided surfaces low. Moreover, the formation of nitrided layers on the outer surface of the piston ring 2, which is liable to be fatigued most, can improve the fatigue resistance of the piston ring.

A piston ring in which the hardness of the nitrided layer is Hv 950 or less, and whose depth in section is 50 ijm, is not sufficiently high in wear resistance and scuffing resistance. Such a piston ring cannot be used in the newest internal combustion engines under severe operating conditions. Accordingly, the depth in section of the nitrided layers whose hardness is at least Hv 950 should be more than 50 Fm.

A feature of the invention resides in the fact that the hardness of the nitrided layers is greatly increased.

This is accomplished by forming the nitrided layers in the above-described martensitic stainless steel material.

A nitrided layer is formed excessively deep in an ordinary low allow steel, while it is formed shallow in a high alloy steel. According to the invention, nitrided layers are formed using a stainless steel which essentially contains carbon and chromium. Moreover, since the steel material is of martensitic, the hardness of the steel itself is high, and therefore the upper limit of the nitrided layer's hardness may be of the order of Hv 1300. Furthermore, the upper limit of the depth in section of the nitrided layer may be of the order of 200 ,LLm when the function of the piston ring is taken into account.

Examples of methods of forming nitrided layers are: a salt bath nitriding method, in which a piston ring is treated in a salt bath; a gas nitriding method, in which a piston ring is treated in a nitriding atmosphere; and an ion nitriding method, in which electric discharge is utilised.

Figure 2 indicates hardness at various depths in section of compression piston rings which are made of tempered martensitic stainless steel containing at least 0.65% by weight of carbon and 12% by weight of chromium, and which are nitrided. The graph of Figure 2 shows the depths in section of the rings and average hardnesses (average values of hardnesses in a radial direction and those in a vertical direction in Hv, Vickers hardness). In Figure 2, curve A is for a piston ring which was subjected to salt bath nitriding for three hours, curve B is for a piston ring which was subjected to salt bath nitriding for five hours, curve C is for a piston ring which was subjected to gas nitriding for five hours, curve D is for a piston ring which was subjected to ion nitriding for eight hours, and curve E is for a piston ring which was subjected to gas nitriding for ten hours.

As was described above, it is rather difficult to supply sufficient lubricant to a compression piston ring, and therefore abnormal wear (called "scuffing") is liable to take place on such a piston ring. Therefore, a scuffing resistance test was performed with respect to the material used for the piston rings of the above-described curves B and C. The test results are as follows: The test conditions were as listed below: Speed: 300 r.p.m.

Load: The start load was 30 kg, and the load was increased by 10 kg every five minutes until scuffing took place.

Lubrication: Lubricant was applied to the rotating part before the test, and no lubrication was made during the test.

Mating material: FC25 (rotating part) Test pieces: The piston rings corresponding to those used for the curves B and C.

Under the above-described conditions, the scuffing resistance test was carried out. In the test, the surface pressure was increased, and the surface pressure at which the scuffing phenomenon occurred was referred to as "a scuffing limit surface pressure".

As is apparent from Figures 2 and 3, the hardness of the nitrided layer decreases as the depth from the surface increases. However, this is not because of differences between the variety of nitriding methods employed. It has been found that hardness and depth in section are essential factors for scuffing resistance.

As shown in Figure 3, the piston ring corresponding to the curve B is low in scuffing resistance and cannot be used. On the other hand, the piston ring according to the invention (that is to say the piston ring corresponding to the curve C) is high in scuffing resistance. It can, therefore, function efficiently as a compression piston ring, and it results in a greatly decreased amour-L of initial wear (scuffing) of the inner wall of the cylinder.

Figure 4 indicates the results of a test which was performed under the following conditions: In Figure 4, the bar graph designated (1) illustrates the amount of wear in a steel piston ring which was subjected to the nitriding method corresponding to the curve B in Figure 2; the bar graph (2) illustrates the amount of wear in a steel piston ring which is subjected to the nitriding method corresponding to the curve C in Figure 2; and the bar graph (3) illustrates the amount of wear in a steel piston ring which was plated with 0.2 millimetres of hard chromium in a conventional manner. These piston rings were used as first compression piston rings, and thereafter the amount of wear in the outer wall of each ring was measured.

The operating conditions were as listed below: Engine type: 4-stroke piston engine Bore X stroke: 83 mm X85 mm Speed: 5600 r.p.m. (full load) Lubricant: SAE 30 Fuel: High lead petrol The cylinder used was of cast iron (corresponding to FC 20).

As is apparent from Figure 4, the amount of wear of the nitrided piston ring (2) (that is to say the piston ring constructed according to the invention) was less than about 1/3 of that of the conventional chromium-plated piston ring (3), and about 1/2 of that of the nitrided piston ring (1). Thus, the nitrided piston ring of the invention is the highest in wear resistance.

Claims

1. A piston ring for an internal combustion engine, the piston ring comprising an annular member consisting essentially of steel, the annular member having an outer layer consisting essentially of nitrided steel, the outer layer having a hardness of at least Hv 950 to a depth of more than 50 Fm.

2. A piston ring as claimed in claim 1, wherein the steel is a martensitic stainless steel.

3. A piston ring as claimed in claim 2, wherein the martensitic stainless steel contains at least from 0.25 to 1.20% by weight of carbon, and from 11 to 19% by weight of chromium.

4. A piston ring as claimed in any one of claims 1 to 3, wherein the outer layer has a hardness of less than about Hv 1300.

5. A piston ring as claimed in any one of claims 1 to 4, wherein the outer layer has a depth of up to about 200 lim.

6. A piston ring as claimed in any one of claims 1 to 5, wherein only the outer radial portion of the piston ring is nitrided.

7. A compression piston ring substantially as hereinbefore described with reference to, and as illustrated by, Figure 1 of the accompanying drawings.