Patent Application
20130216856
The present invention concerns a mechanical component and a method for surface hardening at least one part of a surface of such a mechanical component.
Induction hardening is a heat treatment in which a metal component is heated to the ferrite/austenite transformation temperature or higher by induction heating and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness and brittleness of the surface of a metal component. Induction hardening may be used to selectively harden areas of a mechanical component without affecting the properties of the component as a whole.
U.S. Pat. No. 4,949,758 discloses a process for hardening the interior surface of a long (8-32 feet i.e. 244-975 cm), thin-walled (wall thickness ⅛ to ¼ inch, i.e. 3-6 mm), small inside diameter (1¼to 3¼ inch, i.e. 32 to 83 mm) tubular member. More particularly, it relates to a process involving progressive heating with an internally positioned, electromagnetic induction coil, followed by immediate quenching with a quench ring assembly, to develop a martensitic case on the inner surface of the tube. This method is used to obtain a surface having a hardness of at least 58 HRC and a substantially non-hardened core with a sharp demarcation between the hardened surface and the non-hardened case core.
When surface hardening a surface of a component it is however advantageous to obtain a hardness profile that does not exhibit a sharp demarcation between the hardened surface and the non-hardened core of the component. A smooth demarcation between the hardened surface and the non-hardened core, i.e. a transitional region in which the hardness decreases steadily with depth rather than abruptly minimizes or eliminates any stresses in the material in that region when the component is in use. Such a steadily decreasing hardness profile may be obtained by carburizing the surface of the component.
Carburizing is a heat treatment process in which iron or steel is heated in the presence of another material that liberates carbon as it decomposes. The surface or case will have higher carbon content than the original material. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the surface becomes hard, while the core remains soft (i.e. ductile) and tough.
An object of the invention is to provide an improved non-through hardened mechanical component.
This object is achieved by a mechanical component having a flat or non-flat surface, i.e. an interior or exterior surface, at least one part of which has been surface hardened by induction heating. The surface namely comprises a martensitic microstructure produced by induction hardening using an electromagnetic induction coil followed by quenching using a quenching device. A longitudinal or transverse cross section of the mechanical component through the surface exhibits a hardness surface at the surface and a hardness Hcore at the non-hardened core of the mechanical component (i.e. in the non-hardened base metal of the mechanical component. The hardness profile of the cross section (measured using the Vicker's Hardness Test or any other suitable method for example) exhibits a first region whose hardness is substantially equal to the hardness Hsurface at said surface, a third region whose hardness is substantially equal to the hardness Hcore at the non-hardened core of the mechanical component and a second region between said first and third regions. The hardness profile in the first region has an average hardness Y1, and the hardness profile in the third region has an average hardness Y3. If a line is drawn on the hardness profile in the second region between the points
where 0<k<2 and k is a real number, the hardness of the mechanical component in the second region determined along the line decreases by less than 50 HRC per mm.
At least one part of the surface of such a non-through hardened mechanical component will exhibit increased surface hardness, increased wear resistance and/or increased fatigue and tensile strength. Furthermore, the induction hardening heat treatment used to produce such a mechanical component is more energy efficient and cost effective than a carburizing heat treatment and it has a shorter cycling time and provides better distortion control than a carburizing heat treatment. Furthermore, properties, such as the hardness, microstructure and residual stress, of the at least one part of the surface may be tailored as desired for a particular application.
According to an embodiment of the invention, k is 1.
According to an embodiment of the invention k is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, or 1.9.
According to an embodiment of the invention the hardness of the mechanical component in the second region determined along said line decreases by less than 30 HRC, less than 25 HRC, less than 20 HRC, less than 15 HRC or less than 10 HRC per mm.
According to another embodiment of the invention the hardness Hsurface at the surface is between 55-75 HRC on the Rockwell scale, preferably between 58-63 HRC.
According to a further embodiment of the invention the hardness Hcore at the non-hardened core of the mechanical component is between 15-30 HRC.
According to another embodiment of the invention the first region extends from the surface to a depth of up to 6 mm below the surface preferably to a depth of 1-4 mm below the surface, i.e. the material of increased hardness may extend to a depth of about 0.5-6 mm below the surface, preferably 1-2 mm below the surface.
According to a further embodiment of the invention the mechanical component may be a steel bar, a cylinder, a rod, a piston, a shaft, or a beam.
According to an embodiment of the invention the mechanical component may for example be used in automotive, wind, marine, metal producing or other machine applications which require high wear resistance and/or increased fatigue and tensile strength.
According to an embodiment of the invention, the mechanical component has a contact surface that allows a relative movement between the mechanical component and another component, e.g. a second mechanical component.
According to an embodiment of the invention, the contact surface comprises the at least one part that has been hardened.
According to an embodiment of the invention, the at least one part essentially corresponds to a contact surface.
According to a further embodiment of the invention the mechanical component comprises, or consists of a carbon or alloy steel with an equivalent carbon content of 0.40 to 1.10%, preferably a high carbon chromium steel. For example the mechanical component comprises/consists of 50CrMo4 steel having a composition in weight % 0.50 C, 0.25 Si, 0.70 Mn, 1.10 Cr, 0.20 P, 100Cr6 steel, or SAE 1070.
The present invention also concerns a method for surface hardening at least part of the interior or exterior surface of a mechanical component. The method comprises the steps of heating the at least one part of the surface with an electromagnetic induction coil to the ferrite/austenite transformation temperature or higher by induction heating, maintaining the at least one part of the surface at that temperature in order to allow for sufficient heat transport below the surface resulting in a sufficient austenitization of the at least one part, and quenching the at least one part of the surface in order to obtain a cross section of the mechanical component through the surface which exhibits a hardness Hsurface at the surface and a hardness Hcore at the non-hardened core of the mechanical component (i.e. in the non-hardened base metal of the mechanical component. The hardness profile of the cross section (measured using the Vicker's Hardness Test for example) exhibits a first region whose hardness is substantially equal to the hardness Hsurface at said surface, a third region whose hardness is substantially equal to the hardness Hcore at the non-hardened core of the mechanical component and a second region between said first and third regions. The hardness profile in the first region has an average hardness Y1, and the hardness profile in the third region has an average hardness Y3. If a line is drawn on the hardness profile in the second region between the points
where 0<k<2 and k is a real number, the hardness of the mechanical component in the second region determined along the line decreases by less than 50 HRC per mm.
In conventional induction hardening, in which a mechanical component is heated up quickly and/or heat is now allowed to spread through the component, instable in-homogeneous austenite is formed. In the method according to the present invention, heat is allowed to spread through the mechanical component for a period of 10 seconds, 20 seconds, 30 seconds, 40 seconds, 50 seconds or more so that stable, homogeneous austenite is formed. The expression “in order to allow for sufficient heat transport below the surface resulting in a sufficient austenitization of the at least one part,” is therefore intended to mean for a time period sufficient for stable, homogeneous austenite to form in the at least one part of the surface. By allowing homogeneous austenite to be formed, the formation of a transitional region, in which the hardness decreases steadily with depth below the surface rather than abruptly, is enabled/facilitated.
According to an embodiment of the method the hardness of the mechanical component in the second region determined along said line decreases by less than 30 HRC, less than 25 HRC, less than 20 HRC or less than 15 HRC per mm.
According to an embodiment of the method of the invention, k is 1. According to another embodiment of the invention k is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, or 1.9.
It should be noted that the expression “an induction coil” as used throughout this document with reference to the mechanical component and method according to the present invention is intended to mean one or more induction coils. A plurality of induction coils operating in the same or a different manner, for example at the same or different frequencies, may for example be used to simultaneously or consecutively heat a plurality of parts of an exterior surface and/or an interior surface of a mechanical component, or one or more parts of a plurality of the mechanical components. The induction coil(s) may be arranged to surround one or more parts of a mechanical component that is to be hardened or the entire mechanical component.
In an embodiment of the method the induction coil is removed from the mechanical component, and a quenching device, such as a quench spray or ring, is used to immediately quench the at least one part of the surface that has been heat treated.
In another embodiment of the method the mechanical component is removed from the induction coil, and a quenching device, such as a quench spray or ring, is used to immediately quench the at least one part of the surface that has been heat treated.
According to another embodiment of the method the first region extends from the surface to a depth of up to 6 mm below the surface preferably to a depth of 1-4 mm below the surface.
According to a further embodiment of the invention the mechanical component may be a steel bar, a cylinder, a rod, a piston, a shaft, or a beam.
The present invention will hereinafter be further explained by means of non-limiting examples with reference to the schematic appended figures where;
It should be noted that the drawings have not been drawn to scale and that the dimensions of certain features have been exaggerated for the sake of clarity.
An electromagnetic induction coil 14 is used to harden at least one part of the exterior 1510a, 10b of the shaft 10. A source of high frequency electricity (about 1 kHz to 400 kHz) is used to drive a large alternating current through the induction coil 14. The relationship between operating frequency and current penetration depth and therefore hardness depth is inversely proportional, i.e. the lower the frequency the greater the hardness depth.
The passage of current through the induction coil 14 generates a very intense and rapidly changing magnetic field, and the part of the exterior surface 10a, 10b to be heated is placed within this intense alternating magnetic field. Eddy currents are generated within that part of the exterior surface 10a, 10b and resistance leads to Joule heating of the metal in that part of the exterior surface 10a, 10b. The exterior surface 10a, 10b of the shaft 10 is heated to the ferrite/austenite transformation temperature or higher by induction heating and preferably maintained at that temperature for 10-40 seconds.
In order to select the correct power supply it is first necessary to calculate the surface area of the shaft to be heated. Once this has been established empirical calculations, experience and/or a technique such as finite element analysis may be used to calculate the required power density, heating time and generator operating frequency.
In the illustrated embodiment the induction coil 14 is then removed and a quenching device 16, such as a quench spray or ring is used to immediately quenching the at least one part of the exterior surface 10a, 10b that has been heat treated. The at least one part of the exterior surface 10a, 10b may for example be quenched to room temperature (20-25° C.) or to 0° C. or less. The quenching device 16 is arranged to provide a water-, oil- or polymer-based quench to the heated exterior surface layer 10a, 10b whereupon a martensitic structure which is harder than the base metal of the shaft 10 is formed. The microstructure of the remainder of the shaft 10 remains essentially unaffected by the heat treatment and its physical properties will be those of the bar from which it was machined.
It should be noted that that a plurality of surfaces of the mechanical component according to the present invention may be surface hardened. For example, at least part of an interior surface 12a, 12b of the mechanical component may also be subjected to the method according to the present invention. The interior surface 12a, 12b may for example be hardened to a depth 1-2 mm while the exterior surface 10a, 10b of the mechanical component 10 may be hardened to a depth of 4-6 mm, depending on the application in which the mechanical component 10 is to be used.
As an example, a 60-200kW power supply, a frequency of 20-60 kHz, preferably 10-30 kHz or 15-20 kHz a total heating time of 10-40 seconds and a quenching rate and time of 2001/min and quenching time of 40-70 s respectively may be used to obtain a mechanical component according to the present invention.
Furthermore, even though the shaft 10 in the illustrated embodiment has been shown in a horizontal position with the induction coil 14 and quenching device 16 being inserted horizontally, it should be noted that the shaft 10 may be oriented in any position. An induction coil 14 and quenching device 16 may for be moved vertically into place from the same or different ends of the roller 10. An induction coil 14 may for example be vertically lowered into place and a quenching device may be vertically raised as the induction coil 12 is withdrawn by raising it vertically.
The method according to the present invention results in the formation of a transition zone visible in both hardness and in microstructure. The heat treated part 18 of the exterior surface material 10a, 10b may namely have a hardness within the range of 55-75 HRC on the Rockwell, preferably 59-63 HRC. The material of increased hardness 18 may for example extend to a depth of up to 6 mm, preferably 1-4 mm below the exterior surface 10a, 10b measured radially downwards from the exterior surface 10a, 10b of the shaft 10 towards the interior surface of the shaft 12a, 12b respectively in the illustrated embodiment. Such a shaft 10 may be used for any application in which a part of the exterior surface 10a, 10b is subjected to increased wear, fatigue or tensile stress. Alternatively, the entire exterior surface 10a, 10b may be subjected to the method according to the present invention, depending on the application for which the shaft 10 is to be used.
The interior surface 12a, 12b of the shaft 10 may for example comprise a thread (not shown) arranged to mate with a corresponding thread of another component.
where 0<k<2, the hardness of the mechanical component in the second region determined along the line decreases by less than 50 HRC per mm.
It should be noted that if the interior surface 12a, 12b of the shaft 10 is subjected to a method according to the present invention it will also have a similarly shaped hardness profile although its hardness values may be selected to be different from the hardness values of the exterior surface 10a, 10b of the shaft 10. The depth of the first region 24 and second region 25 may be chosen depending on the application in which the mechanical component 10 is to be used.
The dashed line in
Further modifications of the invention within the scope of the claims would be apparent to a skilled person. For example, rather than moving an induction coil and/or quenching device into position relative to a stationary mechanical component, a mechanical component may be moved into position relative to a stationary induction coil and/or quenching device.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1000719 3 | Jul 2010 | SE | national |
This application is a National Stage application claiming the benefit of International Application Number PCT/SE2011/000094 filed on 27 May 2011, which claims the benefit of SE Application 1000719-3 Filed on 2 Jul. 2010.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE2011/000094 | 5/27/2011 | WO | 00 | 3/15/2013 |
