Patent Application
20120312159
The present disclosure relates generally to a cavitation resistant covering, and more particularly, to a cylinder liner of an engine with a cavitation resistant covering.
Cavitation is material damage caused by the formation and collapse of bubbles, in a liquid. The cavities typically arise from rapid changes in pressure due to vibrations or turbulent flow. During cavitation (sometimes also called cavitation-erosion), the implosion of the bubbles formed in the liquid on the component surface erodes the surface. Cavitation is a source of concern in machine parts that are subject to vibratory forces while in contact with a liquid. Different materials offer different levels of resistance to cavitation. Cast iron is a material known to have relatively low resistance to cavitation. Examples of cast iron machine components that are susceptible to cavitation include, among others, housings of pumps and liners of engine cylinders.
A cylinder liner (referred to herein as a “liner”) is a removable cylindrical part fitted into an engine block of an internal combustion engine to form a cylinder. Typically, liners are made of steel or cast iron. Steels and cast irons are both iron alloys having primarily iron and carbon as the main alloying elements. Steels contain less than 2% (usually less than 1%) carbon, while cast irons typically contain more than 2% carbon. Pistons reciprocate within the cylinder to generate mechanical power. An inside surface of the liner, that serves as a sliding surface for the piston, bounds the combustion chamber of the cylinder. During operation of the engine, the liners get heated due to the combustion of fuel in the combustion chamber. To cool the liner, a liquid coolant (such as, water) is often circulated through a cooling jacket extending about a portion of the outer surface of the liner. Typically, the outer surface of the liner is in direct contact with the coolant circulating through the cooling jacket. It is known that the region of the liner in contact with the coolant experiences erosion from cavitation. To reduce cavitation-induced erosion, the outer surface of the liner may be coated, or treated, to increase its resistance to cavitation.
U.S. Pat. No. 7,617,805 (the '805 patent) discloses a method of heat treating the outer surface of the liner to provide a hardened layer of purely martensitic microstructure to inhibit cavitation-induced erosion. While the layer of purely martensitic microstructure of the '805 patent may provide some protection from cavitation induced erosion, the amount of protection provided may not be sufficient in some applications.
The present disclosure is directed to overcoming these or other limitations in existing technology.
In one aspect, a machine component is disclosed. The component includes a body made of cast iron. The body may include a surface configured to be subject to cavitation-induced erosion. The component may also include a hardened covering on the surface of the body. The covering may have a crystal structure including martensite and between about 5% to about 40% austenite.
In another aspect, a method of making a machine component that is configured to operate in communication with a liquid is disclosed. The method includes fabricating a body from cast iron. The body may include a surface that is configured to be subject to cavitation-induced erosion from the liquid. The method may also include forming a hardened covering on the surface of the body. The covering may have a crystal structure including martensite and between about 5% to about 40% austenite.
In yet another aspect, an engine is disclosed. The engine includes an engine block including one or more cylinder bores. The engine may also include a cylinder liner positioned in at least one of the one or more cylinder bores. The cylinder liner may include a hollow cylindrical sleeve with an inner surface and an outer surface extending from a first end to a second end along a longitudinal axis. The engine may also include a covering on the outer surface of the sleeve. The covering may be a surface layer of the outer surface where the crystal structure includes martensite and between about 5% to about 40% austenite.
During operation of the engine 10, combustion that occurs in the combustion chamber heats the liner 12. Engine block 14 may include a cooling jacket 18, which circulates a coolant (for example, water) along the outer surface 24, to cool the liner 12. Although
Liner 12 may be made of various iron alloys, including cast iron and steel. In some embodiments, liner 12 is an iron alloy containing greater than, or equal to, 50% of pearlite in its matrix. An iron alloy having greater than, or equal to, 50% of pearlite in its matrix is referred to herein as a pearlitic material. Pearlite is a two-phased, layered structure of alpha-ferrite and cementite. The pearlite may be present in the as-cast state of the iron alloy or may be produced by subsequent heat treatment. The pearlitic material may include several varieties of steel and cast iron. A pearlitic cast iron may include graphite in the form of flakes, compacted flakes, or nodular graphite depending on chemistry and cooling rate. Cast iron that contains flake graphite, compacted graphite, and nodular graphite are referred to as gray cast iron, compacted graphite iron (CGI), and ductile iron, respectively.
As is known in the art, a piston 26 reciprocates in the piston bore 16 of engine 10. As the piston 26 reciprocates, piston rings 36 (of piston 26) slide on the inner surface 22 of the liner 12. Due to the reciprocation of the piston 26, vibrations may be induced in the liner 12, and the inner surface 22 may be subjected to abrasive wear. To improve the wear resistance of the inner surface 22, a hardened shell, or case 40, is formed on the inner surface 22. Case 40 is a region of the inner surface 22 in which the matrix microstructure of the cast iron material is transformed to be substantially martensitic by, for example, heat treatment.
To form case 40, the inner surface 22 of the liner 12 is heated to a high temperature and then cooled rapidly (or quenched) to create a “case” of martensite on the surface. Any known surface heat treatment method may be used to heat treat the surface regions of the inner surface 22. For example, methods that employ direct application of a flame (such as, torch hardening) or methods such as induction heating or laser hardening may be applied to heat treat the inner surface 22. As is known in the art, when an iron alloy is heated to a temperature in the austenitic range and held at this temperature for a sufficient time, the crystal structure of the iron alloy changes to an austenite structure. When a cast iron alloy is held at this temperature, a portion of the carbon contained in the alloy dissolves and flows into the austenite. When the alloy is then quenched, the carbon atoms have insufficient time to diffuse out of the austenite, so that the iron-base matrix transforms to martensite. Transformation of austenite to martensite begins at the martensite start temperature. When the alloy cools further and reaches the martensite finish temperature, most of the austenite will have transformed into martensite. Thus, after quenching, a case 40 having a substantially martenisitic microstructure will be formed on inner surface 22. Typically, the residual amount of retained austenite in the substantially martensitic case 40 may be less than or equal to about 1%. Martensite is hard and wear resistant. Therefore, case 40 provides wear resistance to the inner surface 22. Case 40 may have a constant thickness, or different thicknesses, along the length of liner 12. In some embodiments, the thickness of case 40 at different regions may be selected to increase wear life while minimizing undesirable side effects.
During operation of engine 10, vibrations induced in the liner 12 (as a result of normal engine operation) result in the formation of vapor bubbles in the coolant. These bubbles may implode against an outer surface 24 of the liner 12. The implosion of these bubbles causes cavitation damage (or pitting) on the outer surface 24 of the liner 12 that is in contact with the coolant in coolant jacket 18. To protect the outer surface 24 from cavitation damage, a cavitation resistant covering 42 (hereinafter “covering 42”) may be applied to the outer surface 24. Covering 42 is a layer of material on outer surface 24 in which the crystal structure of the material is martensite with between about 5%-40% of austenite. The covering 42 may extend substantially along an entire length of the liner 12, or may only extend along selected portions of the length of the liner 12. In some embodiments, the covering 42 may cover the outer surface 24 of the liner 12 that is exposed to the coolant in coolant jacket 18. In some embodiments, covering 42 may extend circumferentially around liner 12 over substantially all portions of the liner 12 that forms a boundary wall of the coolant jacket 18. Although in general, the covering 42 may have a crystal structure of martensite with between about 5%-40% of austenite, in some embodiments, the amount of austenite may be between about 10%-30%. In some embodiments, the covering 42 may have a crystal structure of martensite with between about 20%-30% austenite.
The covering 42 may be formed on outer surface 24 in any manner. In some embodiments, the material on the surface layer of the outer surface 24 may be transformed (for example, by surface heat treatment) to form the covering 42. In other embodiments, a layer of material separate from the material of the liner 12 may be attached to the liner 12 to form the covering 42. In some embodiments, liner 12 may be a two layer liner formed by, for example, a process such as centrifugal casting. It is also contemplated that in some embodiments, in place of a separate covering 42, the covering 42 may be made of a material of the liner 12 (entire thickness of the liner is made of the covering material). In embodiments where the surface layer of outer surface 24 is transformed to form covering 42, a surface heat treatment, or a surface hardening, process may be applied to the outer surface 24 of the liner 12 to form the covering 42. Any known surface hardening process, such as, laser hardening, flame hardening, induction hardening, etc. may be applied to the outer surface 24 to selectively harden the surface layer of the outer surface 24 and form covering 42. In some embodiments, the same or a similar surface heat treatment process that is used to create case 40 may be applied to the outer surface 24 to form covering 42. The heat treatment process used to form covering 42 will be configured to produce a microstructure that is martensite with between about 5%-40% of austenite.
To form covering 42, the outer surface 24 is heated to a temperature in the austenitic range (from about 800° C. to about 1100° C.) and quenched. As explained with reference to the formation of case 40, when the outer surface 24 of an iron-based liner is heated to a temperature in the austenitic range, the crystal structure of the alloy in the outer surface 24 changes to an austenite structure. And, during quenching, this austenitic microstructure is transformed to martensitic. However, if the amount of carbon in the austenite is high, the amount of residual austenite in the microstructure of the cooled alloy will be relatively large. The heat treatment process used to form covering 42 is tailored to produce between about 5-40% of retained austenite in the covering 42 after quenching. Since the heat treatment parameters that may be varied to control the amount of retained austenite after quenching are known in the art, they not extensively discussed herein. In some embodiments, the amount of retained austenite in covering 42 after quenching may be increased by increasing the temperature to which the outer surface 24 is heated during heat treatment and/or by increasing the soak time at this temperature.
Although any surface heat treatment process may be used to form covering 42, in some embodiments, an induction heat treatment process may be used to transform a layer of material on the outer surface 24 to covering 42. During this process, an induction coil scans the outer surface 24 of the liner 12 and applies an alternating magnetic field on the outer surface 24. This alternating magnetic field induces a current flow that heats the outer surface 24 by Joule heating. As is known in the art, by varying parameters of the scanning (such as, frequency, power level, scan speed, etc.), the depth of covering 42 may be varied. While a thick covering 42 may seem desirable from a cavitation life point of view, it may have undesirable side effects. For instance, increasing the thickness of the covering 42 may require increasing the thickness of the liner 12. Increasing the thickness of the liner 12 may undesirably increase the weight of the liner 12. Further, a thicker covering 42 may induce higher residual stresses on liner 12. Therefore, the thickness of the covering 42 is selected to achieve a beneficial increase in cavitation resistance while minimizing undesirable side effects.
In some embodiments, covering 42 may have a constant thickness on all areas of liner 12, while in other embodiments, the thickness of covering 42 in different regions may be different. Covering 42 of different thicknesses may be obtained by varying the parameters of the hardening process at different regions. For instance, in embodiments where an induction hardening process is used to form covering 42, a thicker covering 42 may be formed in selected regions by decreasing the frequency of the alternating magnetic field applied to this region, increasing the power level of the magnetic field applied to this region, and/or decreasing the scan speed of the induction coil in this region. Although
The disclosed machine component may be applied in any application where it is desired to increase the resistance of the component to cavitation-induced damage. A cavitation resistant covering is formed on a surface of the component that operates in communication with a liquid, and may therefore be subjected to cavitation-induced erosion. This cavitation resistant covering includes between about 5%-40% of retained austenite therein. The cavitation resistant covering may be formed by any method. In some embodiments, a layer of material on the surface of the component may be transformed to form the cavitation resistant covering by a heat treatment process. An exemplary method of forming a cavitation resistant covering 42 on the outer surface 24 of a cylinder liner is described below.
Although the inventive aspects of the current disclosure are described using a cylinder liner, in general, a covering 42 including martensite with between about 5%-40% of retained austenite may be used to increase the cavitation resistance of any cast iron component having a pearlitic microstructure. It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed machine component. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed machine component. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.
