GB2153965A

GB2153965A - Nitrided steel crack resistant piston ring

Info

Publication number: GB2153965A
Application number: GB08501652A
Authority: GB
Inventors: Koji Umezawa; Hirotaka Teradaira
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: 1985-08-29
Also published as: JPS60155056A; GB2153965B; DE3501822A1; DE3501822C2; GB8501652D0

Abstract

A steel compression piston ring (8), having wear resistance as well as resistance to cracking, is formed with a nitrided steel outer surface (3). The inner radial surface (5) of the ring (8) has a hardness of about Hv 700 or less. The piston ring (8) is made by forming a piston ring structure, nitriding its outer surface, and then removing the nitrided layer (3) from the inner radial surface (5) of the nitrided structure. <IMAGE>

Description

SPECIFICATION Crack resistant piston ring and a method of manufacturing it This invention relates to a steel compression piston ring for an internal combustion engine.

Recently, it has been required to minimise the weight of an internal combustion engine as a means for economically using energy.

Therefore, efforts have been made to reduce the weight of each of the reciprocating components of the internal combustion engine to increase its efficiency.

There has been a strong interest in decreasing the weight of compression piston rings. In a conventional method of decreasing the weight of a compression piston ring, the width in the axial direction (B dimension) of the piston ring is decreased, whereby the dimensions of the piston are reduced, and the engine is made more compact. Using this concept, both the piston ring and the engine are reduced in weight.

On the other hand, if the width of a conventional cast iron compression piston ring is reduced, then its mechanical strength is decreased. This difficulty has been solved by using a steel compression piston ring.

However, in order to allow a steel compression piston ring to withstand the severe operating conditions of the newer engines, and to serve adequately as a piston ring, there are still severe problems to be solved.

One of the problems is the surface treament that is applied to the outer radial surface of the piston ring which slides on the cylinder wall. A steel compression piston ring that has not been subjected to surface treatment is impractical because its wear resistance is low.

In other words, a steel compression piston ring should be used only after having been subjected to surface treatment. For this reason, a variety of methods such as chromium plating, which is effective in increasing the wear resistance of a cast iron piston ring, have been tried. Moreover, it has been found that nitrided layers formed on all the surfaces of a steel compression piston ring remarkably improve the wear resistance thereof. In order to maintain the excellent wear resistance of a steel compression piston ring having nitrided layers for a long period of time, it is essential to form deep nitrided layers. However, the formation of deep nitrided layers makes a steel compression piston ring brittle. Accordingly, it may break during mounting on the piston, or it may crack during use.

As is well known in the art, a piston ring is engaged within a ring groove in the associated piston by opening the gap in the ring.

One of the conditions required for a piston ring is that the piston ring shall not break when the the gap is opened by a distance eight times the width (T dimension), in the radial direction, of the piston ring.

A steel compression piston ring having deep nitrided layers as described above suffers from the disadvantage that it tends to crack, the crack starting at the nitrided layer formed on the inner radial surface, which is greatly affected by stress when the gap is opened.

The aim of the invention is to provide a method of making a steel compression piston ring having excellent resistance to breakage due to the effects of deep nitrided layers. The invention also aims to provide an improved steel compression piston ring having reduced thickness, increased wear resistance, and a resistance to cracking or breakage during installation on a piston.

The present invention provides a method of making a steel compression piston ring comprising the steps of forming a piston ring structure from a base material consisting essentially of steel, the structure having an inner radial surface, an outer radial surface and upper and lower axial surfaces, forming a nitrided layer on at least the inner radial surface and the outer radial surface of the structure, and removing the nitrided layer from the inner radial surface of the structure to such an extent that the inner radial surface has a hardness of about Hv 700 or less.

Preferably. the nitrided layer is removed from the inner radial surface by grinding or machining.

Advantageously, after removing said nitrided layer from the inner radial surface. the inner radial surface has a hardness approximately equal to that of the base material.

Nitrided layers may be formed only on the inner radial surface and on the outer radial surface. Alternatively, nitrided layers may be formed on all the surfaces of the structure.

The invention also provides a steel compression piston ring having an inner radial surface, upper and lower surfaces and an outer cylinder-sealing surface, the outer cylinder-sealing surface having a nitrided layer thereon, the inner radial surface of the ring having a hardness of about Hv 700 or less.

Advantageously, the piston ring is made of steel base material, and has an outer layer of nitrided steel, the inner radial surface of the piston ring having a hardness approximately that of the base material. Preferably, the upper and lower sealing surfaces have nitrided layers thereon.

The invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a sectional view of a conventional steel compression ring; Figs. 2 and 3 are a plan view and a side view, respectively. showing a conventional way of supporting piston rings during nitriding; Fig. 4 is a sectional view of a steel compression piston ring made according to the nitriding method shown in Figs. 2 and 3; Figs. 5 and 6 are a plan view and a side view, respectively, showing another conventional way of supporting piston rings during nitriding; Fig. 7 is a sectional view of a steel compression piston ring which is made according to the nitriding method shown in Figs. 5 and 6; and Figs.8 and 9 are sectional views showing examples of a steel compression piston ring made in accordance with the invention.

The present invention relates to an improvement in the conventional method of making a steel compression ring. In such a method. a piston ring 1 (see Fig. 2) is formed from base material consisting essentially of steel. A partial cross-section of such a structure is shown in Fig. 1, where the piston ring 1 has an inner radial surface 5, an outer radial surface 4, an upper axial surface 6, and a lower axial surface 7.

In order to improve the wear resistance of a piston ring, it is necessary to form a nitrided layer in its outer radial surface which slides along the cylinder wall; and, if required, to form nitrided layers in its upper and lower axial surfaces which contact the piston ring groove.

Figs. 2 and 3 show a conventional way of supporting a number of piston rings 1 during a nitriding process. In such a process, a nitrided layer is formed in the base metal of each steel piston ring by a nitriding method such as salt bath nitriding, gas nitriding, soft nitriding or ion nitriding.

When it is required to form a nitrided layer 3 only on the outer radial surface 4, nitriding is carried out with a plurality of steel piston rings 1 set on a jig 2 (see Figs. 2 and 3) and fixed with a fixing jig (not shown). Accordingly, nitrided layers 3 are formed both in the outer radial surface 4 and the inner radial surface 5 (see Fig. 4).

On the other hand, if it is required to form nitrided layers 3 in the outer radial surface 4 and the upper and lower axial surfaces 6 and 7, a plurality of steel piston rings 1 may be set on a jig 2 in such a manner that they are spaced apart from one another (see Figs. 5 and 6). When such a configuration is subjected to nitriding, nitrided layers 3 are formed on the inner and outer radial surfaces 5 and 4, and on the upper and lower axial surfaces 6 and 7 (that is to say on all of the surfaces) of the piston rings 1.

Steel compression piston rings made in this way are conventional ones having nitrided layers on their inner radial surfaces. As mentioned above, such a conventional steel compression piston ring suffers from the disadvantage that, if the nitrided layer is formed deep into the ring, the ring may break during fitting or during use.

Figs. 8 and 9 show steel compression piston rings 8 made in accordance with the invention, in which case the nitrided layer 3 is removed from the inner radial surface 5. preferably by machining using a grinding machine or a lathe, until the surface hardness is not more than about Hv 700.

The steel compression piston ring 8 shown in Fig. 8 is obtained by removing all the nitrided layer 3 from the inner radial surface 5 of a piston ring of the type shown in Fig. 4.

until the surface hardness of the inner surface reaches the hardness of the base metal The steel compression piston ring 8 shown in Fig. 9 is obtained by removing about twothirds of the nitrided layer 3 from the inner radial surface 5 of a piston ring of the type shown in Fig 7. until the surface hardness reaches Hv 650 If the nitrided layer formed on the inner radial surface of a piston ring is removed until the surface hardness reaches Hv 700 or less.

then the ring will not crack. even if a large stress is applied to its inner wall in mounting it on the piston. that is to say its resistance against breakage is remarkably improved It will be apparent from the above description, that the deep nitrided layers formed on the outer radial surface (and optionally on the upper and lower axial surfaces) of piston rings constructed in accordance with the invention are high in wear resistance. The nitrided layer formed on the inner radial surface of such a piston ring is removed at least partially, so that its surface hardness is Hv 700 or less.

Accordingly. even if such a steel compression piston ring is expanded to mount it on the associated piston. it is not cracked by the stress which acts on the inner surface of the ring, that is to say its resistance against breakage is considerably improved. Thus, this invention makes it possible to put into practical use steel compression piston rings having deep nitrated layers.

Claims

1. A steel compression piston ring having an inner radial surface, upper and lower surfaces and an outer cylinder-sealing surface, the outer cylinder-sealing surface having a nitrided layer thereon, the inner radial surface of the ring having a hardness of about Hv 700 or less.

2. A steel compression piston ring as claimed in claim 1, wherein the piston ring is made of steel base material, and has an outer layer of nitrided steel, the inner radial surface of the piston ring having a hardness approximately that of the base material.

3. A steel compression piston ring as claimed in claim 1 or claim 2, wherein the upper and lower sealing surfaces have nitrided layers thereon.

4. A steel compression piston ring substan tially as hereinbefore described with reference to, and as illustrated by, Fig. 8 or Fig. 9 of the accompanying drawings.

5. A method of making a steel compression piston ring comprising the steps of forming a piston ring structure from a base material consisting essentially of steel, the structure having an inner radial surface, an outer radial surface and upper and lower axial surfaces, forming a nitrided layer on at least the inner radial surface and the outer radial surface of the structure, and removing the nitrided layer from the inner radial surface of the structure to such an extent that the inner radial surface has a hardness of about Hv 700 or less.

6. A method as claimed in claim 5, wherein said nitrided layer is removed by grinding said layer from the inner radial surface.

7. A method as claimed in claim 5, wherein said nitrided layer is removed by machining.

8. A method as claimed in any one of claims 5 to 7, wherein, after removing said nitrided layer from the inner radial surface, the inner radial surface has a hardness approximately equal to that of the base material.

9. A method as claimed in any one of claims 5 to 8, wherein nitrided layers are formed only on the inner radial surface and on the outer radial surface.

1 0. A method as claimed in any one of claims 5 to 8, wherein nitrided layers are formed on all the entire surfaces of the structure.

11. A method of making a steel compression piston ring substantially as hereinbefore described with reference to the accompanying drawings.