MXPA00000945A

MXPA00000945A - Surface heat treatment of piston rings

Info

Publication number: MXPA00000945A
Application number: MXPA/A/2000/000945A
Authority: MX
Inventors: Carrie Olivier; Staerck Jeanmarc; Maerky Christophe; Laine Olivier
Original assignee: Dana Corporation
Priority date: 1999-01-29
Filing date: 2000-01-27
Publication date: 2002-05-09

Abstract

A piston ring 32 for use in a piston of an internal combustion engine comprises a base material 38 of a first hardness formed intoa ring, and a secondary material of a second hardness formed by selective and superficial heat treatment of an outer portion of the base material. At least one outer surface 46 of the ring is selectively and superficially heat treated in one of a variety of ways to form an austenitic metal layer about 100 mu m thick on the surface of the ring. The ring is then rapidly cooled in an appropriate environment, resulting in the transformation of the austenitic compound into a martensitic compound 48 adjacent the base material at the point of heat treatment. Preferably, the martensitic metal has a hardness range of 800 to 1000 HV (Vickers Hardness scale) measured at a load of 100 grams.

Description

TERMOTRATAMIENTO OF THE SURFACE OF A PISTON RING The present invention relates to a piston ring, and more particularly, to a piston ring formed of a base material having an outer layer which is first selectively subjected to localized and superficial heat such that the outer layer is transformed to an austenitic form, and then a rapid cooling such that the austenitic form converts to a martensitic form.

BACKGROUND OF THE INVENTION Piston rings are well known.

They are usually received inside an annular groove arranged around an outer periphery of a piston. In turn, this piston is reciprocated within a cylinder of an internal combustion engine. Typically, the piston ring is discontinuous, and has two end portions. The end portions are spaced apart to expand the piston ring for insertion into a corresponding slot in the piston. Then the piston ring is compressed, bringing the end portions closer, to install the piston inside the cylinder.

A piston compresses fluids such as gases inside the cylinder. In an internal combustion engine, these fluids ignite and expand, forcing the piston away from the ignition point. The outer surface or contact face of a piston ring in an internal combustion engine is subject to high temperatures, corrosion, and frictional interaction with the cylinder walls.

To improve durability, wear resistance and rubbing, it is known to use a variety of wear surface materials bonded to an underlying substrate that forms the center of the piston ring. Examples include wear surface materials comprising ceramics, nickel-boron surface coatings and various nitride coatings such as chromium nitride coating. These wear materials have certain limitations. First, they are extremely fragile and subject to breakage and other unacceptable modes of failure. Second, there is often a problem in achieving sufficient coating thickness such that the coating does not wear out prematurely and exposes the base material, resulting in failure of the piston ring. Third, it is extremely difficult to machine a piston ring having a known wear material already bonded to the base material. Fourth, piston rings with known wear materials are often heavier, increasing the force of inertia and friction within the cylinder. Finally, known piston rings with wear materials attached to a base material are typically more expensive to manufacture.

Another way to improve the durability, resistance to wear and friction of the piston rings is to form the piston ring of a base material having a first hardness, and then subject the base material to a heat treatment, thereby the base material hardens. Typically, piston rings of this type are formed of powdered austenitic iron, and are then subjected to a heat treatment form to form martensitic iron in at least a portion of the piston ring. However, austenitic iron is difficult to work with, and forming piston rings from powdered austenitic iron can create undesirable manufacturing defects within the piston ring. Moreover, the entire piston ring is subjected to a heat treatment, which requires sufficient time and heat treatment to substantially affect the entire piston ring.

The invention The present invention is directed to a piston ring for use in a piston of an internal combustion engine. The ring includes a base material of a first hardness formed in a ring, and a secondary material of a second hardness formed by surface heat treatment and selective of an outer portion of such a base material. The heat treatment may be selectively and superficially limited to only the front and contact areas of the piston ring, or may involve the entire surface of the piston ring.

In a first embodiment, the ring is formed of cast iron, iron-aluminum alloys or steel alloys. At least a portion of the outer surface of the ring is surface and selectively heat treated in one of the varieties of shapes to form an austenitic metal layer around 25-200 μm, and more preferably 100 μm in thickness on a surface of the ring. The ring is then rapidly cooled in an appropriate medium resulting in the transformation of the austenitic compound into a fine martensitic compound adjacent to the base material at the point of heat treatment. Preferably, the martensitic metal has a hardness range of 800-1000 HV (Vickers hardness scale) measured at a load of 100 gr. In a second embodiment, the base material is a tempered and remelted alloy that includes coarse martensite. Heating a portion of the surface of the base material causes a transition to austenite, and the subsequent rapid cooling transforms the austenite into a fine martensite that is harder than the original base material.

Various methods can be used to heat the surface of the piston ring. In one method, electrons emitted by a cathode are accelerated and concentrated on an outer surface of the piston ring while in a high vacuum medium. As the electrons interact with the surface of the ring, energy is transferred to the surface of the ring, thereby heating the surface of the ring. In this way, the electron beam heats the surface of the ring beyond the temperature of austenite formation.

In another method, the surface of the ring is heated by focusing a laser on the surface of the ring such that the temperature of the surface of the ring exceeds the temperature of austenitic iron formation. In other methods, the surface of the ring is heated by an induction furnace or by exposure to a plasma torch.

In all cases, the base material comprising the piston ring is relatively ductile and malleable, selected from the group comprising cast iron, iron-aluminum alloys or steel alloys. The base material must be capable of transition to both an austenitic form and a martensitic form having the desired end characteristics described below. In a first embodiment, the base material has not been hardened or pre-treated in any way, and should not include any austenite. In another embodiment, the base material has been tempered and tempered in such a way as to include some martensite. In both modalities, the base material is relatively easier to initially form and configure. The heat treatment according to the present invention occurs only superficially in the piston ring, in such a way that not all the ring is subjected to the heat treatment. In fact, because all methods described for surface heating can be selectively oriented, portions of the surface of the piston ring can be selectively hardened. Thus, the heat treatment can be limited to only the face or contact areas of the piston ring. As a result, the original ductility of the base material is sufficiently maintained to allow the formation of the piston ring with the groove of a piston.

The heat treatment of the surface also occurs for a relatively short period of time, compared to the time required to heat treat an entire piston ring. By limiting the scope of the heat treatment, the time required to heat treat the piston ring is reduced, resulting in manufacturing efficiencies. The manufacturing time is further reduced without adversely affecting the quality of the piston ring. Finally, because heat treatment occurs only at a limited depth in the piston ring, only those faces that require hardening need to undergo treatment, thereby maintaining the ductility and elasticity of the base material.

Drawings The features and inventive aspects of the present invention will become more apparent upon reading the following detailed description, claims and drawings, of which the following is a brief description.

Figure 1 is a view of a piston for use in an internal combustion engine that includes several piston rings of the present invention.

Figure 2 is a sectional view of a piston ring taken along lines 2-2 of Figure 1, showing an untreated piston ring prior to surface hardening according to the present invention.

Figure 3 is a sectional section of a piston ring, taken along lines 2-2 of Figure 1 showing the inventive piston ring after a secondary layer has been selectively formed on the exterior of the material of base selectively treating the surface of the base material with heat.

Figure 4 is a partial section taken along lines 2-2 of Figure 1, showing the inventive piston ring after a secondary layer has been selectively formed on the outer surfaces of the piston ring by treatment with heat of the entire surface of the base material.

Detailed description A piston assembly 20 is illustrated in Figure 1 which includes a piston 22 movable reciprocally along a longitudinal axis A-A within a bore 24 of a cylinder 26 of an internal combustion engine. The piston 22 includes a plurality of annular grooves 28 around a peripheral outer surface 36 extending circumferentially. A ring-shaped annular piston ring or sealing member 32 is mounted within each of the slots 28 to provide sealing engagement between the peripheral outer surface 36 and a corresponding cylinder wall 34.

A first section of an untreated piston ring 32 is illustrated in Figure 2. The piston ring 32 is shown for simplicity in Figure 2 as having a square section but it should be understood that the present invention is effective regardless of the geometry of the piston ring. piston ring and sectional shape.

The piston ring 32 starts with a metal base material 38 of a first hardness. Preferably, the base material 38 is any of an iron-aluminum alloy, steel alloy or simply cast iron, but can be formed of any metal having desirable characteristics. In particular, the base metal must be capable of transition to both austenite and martensite. Alternatively, the base material 38 may be formed of a tempered and remelted alloy including coarse martensite. In such a case, the heat treatment of the surface transforms the coarse martensite into austenite, and a rapid cooling transforms the austenite into fine martensite. In both cases, the base material 38 is relatively ductile and easy to machine in the required shape of the piston ring 32, either by embossing, die-cutting or casting. The ductility (ie, non-rigidity) characteristic of the base material 38 is necessary to allow the piston ring 32 to conform tightly against the peripheral outer surface 36 of the piston 22.

With reference to both Figures 1 and 2, the cross section of the piston ring 32 includes an upper surface 40, a lower surface 42, a radially inner surface 44 and a radially outer surface 46. Upon assembly on the piston 22, the radially inner surface 44 is positioned adjacent the radial peripheral outer surface 36 of the piston 22, while the radially outer surface 46 is positioned adjacent the cylinder wall 34. During operation, the radially outer surface 46 of the piston ring 32 is subjected to substantial thermal and friction stress. As a result, it is desirable to harden a thin layer of the material comprising the radially outer surface 46 of the piston ring 32.

As seen in Figure 3, instead of applying a heat treatment to the entire body of the piston ring 32, piston ring 32 is instead selectively and superficially treated. The piston ring is selectively heated only until a predetermined portion of the outer surface of the piston ring is subjected to a heat treatment. And the heat treatment process is designed to affect only a small thickness of the base material, thereby having only a surface effect. As seen in Figure 3, only the radially outer surface 46 is treated, since it includes the portion of the piston ring 32 that is in contact with the cylinder wall 34 (see Figure 1) during the reciprocation of the piston. The process of heat treating the radially outer surface 46 involves applying energy, represented by the arrows 41, directly to the radially outer surface 46, raising the temperature of the radially outer surface 46 above the temperature at which austenite is formed. The surface is then rapidly cooled in an appropriate medium to form a martensite layer 48 on the surface 46. The martensite has the hardness characteristics required to withstand friction and wear while functioning as a seal in an internal combustion engine. Preferably, the thickness ti of the layer 48 is between 25 and 200 μm thick, and more preferably it is approximately 100 μm thick. Because a piston ring is only about 3 mm wide, heat treatment affects less than ten percent of the thickness of the base material to form layer 48.

As noted above, the piston ring 32 may include any of a metal base material 38 of a first hardness, or may include a quenched and tempered alloy that includes a coarse matrix of martensite. When the base material is not treated, any of several methods may be used, so long as the resulting temperature is sufficient to form austenite on the surface of the piston ring. Upon reaching a temperature sufficient to form austenite, the piston ring is placed in a medium that rapidly cools the piston ring, which substantially transforms all of the austenite into martensite. However, a transition zone is formed at the interface between the martensite layer and the base material. The transition zone of base material to martensite may include some austenite, although not in appreciable thickness. Also, the transition of base material into martensite is usually abrupt and may include some austenite when cast iron is used as the base material. In a method for heating the surface layer, an electron beam is concentrated and accelerated on the surface of the piston ring. As the electrons interact with the surface of the piston ring and decelerate, they deposit large amounts of energy in the surface layer of the piston ring, thereby heating the surface layer. In another method, a laser is oriented and concentrated on the surface of the piston ring. Again, the interaction of the laser with the surface of the piston ring heats the surface layer of the ring, thereby heating the piston ring superficially. In yet another method, the piston ring is placed in an induction furnace which includes an electric current on the surface of the piston ring, heating the surface. In addition, a plasma torch can be used to superficially heat the surface of the piston ring.

Using either method, a surface of the piston ring 32 is subjected to a high temperature and then rapidly cooled. For example, acceptable results have been obtained by subjecting the radially outer surface of 280 piston rings to an electron beam for 134 seconds., where the electron beam measures 0.4 mm by 21.2 mm and moves at a speed of 2.2 centimeters per second, and is from a source that has an energy of 350 watts-s / cm2. Acceptable results have also been obtained using a laser that has an energy of 20.5 Joules and a cross section of 5 mm by 1.5 mm that moves at 6 meters per second. In both cases, the surface temperature of the surface rings reaches approximately 800 ° C, and then it is allowed to cool to room temperature. However, improved cooling methods can be used to increase the conductive and convex cooling of the piston ring after heat treatment. As a result of selective and surface heating, followed by rapid cooling, the surface of the piston is hardened to a hardness range of 800 to 100 HV, with a load of 100 grams applied to measure the hardness of a 100 μm thick layer .

The selective and surface heat treatment according to the present invention allows the treated surface to harden sufficiently to withstand the stress within an engine and to prevent premature wear and failure of the piston ring. The inventive method allows the use of a ductile, non-rigid and easy-to-handle base material 36 such that the piston ring can maintain a tight fit and seal. However, the heat treatment is selective and superficially applied to the base material to increase the resistance of the piston ring to high temperatures, corrosion, and friction interaction with the cylinder walls 34, and if necessary with the surfaces that they are coupled from the piston slot 28. Thus, the heat treatment can be limited only to the face or contact areas of the piston ring, thus retaining the original ductility of the base material enough to allow the formability of the piston ring 32 against the piston 22.

Additionally, the time required to deal with selective heat and superficially piston rings is significantly less than the time required to heat treat a complete piston ring. As a result of the present invention, the time required to manufacture a heat treated piston ring is greatly reduced without adversely affecting the quality of the piston ring thus produced.

The preferred embodiments of the present invention have been described. A person with ordinary skill in the art will realize, however, that some modifications could pertain to the teachings of this invention. Therefore, the following claims should be studied to determine the true scope and content of the invention.

Claims

1. - Piston ring comprising: a base material forming a ring, the base material is selectively transformable between at least two material phases, the second material phase is harder than the first material phase; a first thickness of the base material is superficially heated and then cooled, the material phase of the first thickness is transformed through at least one of the heating and cooling.

2. - Piston ring according to claim 1, wherein the first material phase is a coarse martensite matrix, and the second material phase is a fine martensitic matrix.

3. Piston ring according to claim 1, wherein the first material phase is an austenitic material, and the second phase is a martensitic material.

4. Piston ring according to claim 3, wherein the martensitic material is coarse or fine.

5. - Piston ring according to claim 3, wherein the base material is selected from a group consisting of alloys of steel, aluminum-iron alloys and cast iron.

6. - Piston ring according to claim 5, wherein the base material has been hardened and tempered.

7. Piston ring according to claim 1, wherein the base material is selectively heat treated in only a portion of the outer surface thereof.

8. - Piston ring according to claim 7, wherein the portion includes a radially outer surface of the piston ring for contacting an inner wall of a cylinder bore.

9. Piston ring according to claim 1, wherein the first thickness has a thickness between 25 and 200 μm.

10. - Piston ring according to claim 9, wherein the first thickness has a thickness of approximately 100 μm.

11. Piston ring according to claim 10, wherein the second phase of hardness has a hardness range of 800 to 1000 HV.

12. - Piston ring according to claim 11, wherein the heat is applied by one of a group of methods consisting of induction heating, electric flow, laser or plasma torch.

13. - Piston ring according to claim 1, wherein an austenitic transition zone is formed between the base material and the first thickness.

14. Piston ring comprising: a base material that forms a ring, the base material is selectively transformable between three material phases, the first material phase is harder than the second material phase and the third phase of material is material is harder than the first and second material phases; a first thickness of the base material surface heated and then cooled, the material phase of the first thickness is transformed through at least one of the heating and cooling.

15. - Piston ring according to claim 14, wherein the first phase of hardness is a thick martensite, the second hardness phase is austenite, and the third hardness phase is fine martensite.

16. - Piston ring according to claim 15, wherein the first thickness has a thickness between 25-200 μm.

17. Piston ring according to claim 16, wherein the first thickness has a thickness of about 100μm.

18. - Piston ring according to claim 17, wherein the second hardness phase has a hardness range of 800 to 1000 HV.

19. - Piston ring according to claim 18, wherein the heat is applied by one of a group of methods consisting of induction heating, electron flow, laser or plasma torch.

20. Piston ring according to claim 19, wherein the austenitic transition zone is formed between the base material and the first thickness.

21. Method for treating with selective and superficial heat a piston ring, comprising: manufacturing the piston ring of a base material, the base material is selectively transformable between at least two phases of material, the second phase of material is harder than the first material phase; surface heating a first thickness of the base material and then cooling the first thickness, the material phase of the first transformed thickness through at least one of the heating and cooling.

22. Method according to claim 20, wherein the first material phase is a thick martensitic matrix, and the second material phase is a fine martensitic matrix.

23. Method according to claim 20, wherein the first material phase is an austenitic material, and the second phase is a martensitic material.

24. Method according to claim 23, wherein the martensitic material is coarse or fine.

25. Method according to claim 20, wherein the first thickness has a thickness between 25 and 200 μm.

26. Method according to claim 25, wherein the first thickness has a thickness of approximately 100μm.

27. Method according to claim 20, wherein the heating step is applied by one of a group of methods consisting of induction heating, electron flow, laser or plasma torch.

28. Method according to claim 20, wherein the third phase of hardness has a hardness range of 800 to 1000 HV.