The present disclosure relates generally to methods for treating a cast iron workpiece.
Cast iron materials may be used in applications where resistance to surface wear from friction is desirable. Untreated cast iron materials generally tend to corrode when exposed to the environments in which they are used. Some surface treatments, e.g., painting, tend to wear off quickly and/or may be deleterious to proper functioning of the cast iron materials.
In one example of the methods disclosed herein, a nanocrystallized surface layer at a finish surface of a cast iron workpiece is roller burnished. The roller burnishing reduces roughness of the nanocrystallized surface layer. In another example of the methods disclosed herein, a machined, finish surface of a cast iron workpiece is deformed by rubbing the machined, finish surface against a blunt tool to form a nanocrystallized surface layer at the machined, finish surface. The machined, finish surface is cooled simultaneously with the deforming to promote nanocrystallization of the machined, finish surface.
Features and advantages of examples of the present disclosure will become apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though perhaps not identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear.
Examples of the present disclosure advantageously provide methods for treating a cast iron workpiece. In the examples disclosed herein, a surface nanocrystallization process is performed to generate a nanocrystallized microstructure at a finish surface of the cast iron workpiece. This nanocrystallized microstructure contributes to faster or more energy efficient ferritic nitrocarburizing (FNC) treatments of the cast iron workpiece. In particular, the nanocrystallized microstructure accelerates/facilitates diffusion of nitrogen atoms and carbon atoms therethrough.
As used herein, the term “finish surface” refers to the surface of the cast iron workpiece that has been exposed to machining. Also as used herein, the term “nanocrystallized microstructure” refers to the finish surface after it has been exposed to nanocrystallization, i.e., the severe plastic deformation process that is very localized to the surface and near (i.e., up to a depth of a few tens of microns) the surface of the cast iron workpiece. The nanocrystallized microstructure has a refined microstructure which has smaller grains (e.g., from about 5 nm to ≦2000 nm) than the finish surface (having a grain size >2000 nm).
Examples of the method disclosed herein also involve cooling to enhance the surface nanocrystallization process, roller burnishing to reduce surface roughness resulting from the surface nanocrystallization process, or combinations of cooling and roller burnishing. As such, the methods according to examples of the present disclosure include treating the machined, finish surface during surface nanocrystallization and/or treating the nanocrystallized microstructure after surface nanocrystallization has been performed.
In the examples disclosed herein, the surface nanocrystallization may be accomplished, e.g., by deforming the machined, finish surface against a blunt tool. Nitrocarburizing may be accomplished using accelerated diffusion of nitrogen and carbon atoms through the nanocrystallized surface layer. Surface nanocrystallization and nitrocarburization form a substantially rust-free and high wear/fatigue resistant surface on the cast iron components/workpieces.
It is to be understood that in examples of the present disclosure, the deformation against the blunt tool (i.e., nanocrystallization) is severe, plastic deformation local to the location of contact between the blunt tool and the workpiece. The deformation occurs substantially without forming chips and without removing material in the process of deformation. Further, the local deformation of examples of the present disclosure is distinct from global deformation that would occur in wire drawing or sheet-metal rolling. Although the deformation of the present disclosure occurs in the vicinity of the blunt tool, a large surface of a workpiece may be nanocrystallized by systematically applying the blunt tool to the entire surface. In an example, a cylindrical surface may be nanocrystallized by rotating the cylinder while moving the blunt tool along the cylindrical axis. In the example, the blunt tool would take a spiral path over the entire surface of the cylinder. It is to be further understood that more than one pass may be made over the finish surface with the blunt tool. In an example, four passes are made over the finish surface with the blunt tool.
Conventional Ferritic NitroCarburizing (FNC) usually takes about 5 to 6 hours at about 570° C. to obtain a 10 micron thick hard layer on the surface of metallic parts (for example, brake rotors) for better wear, fatigue and corrosion resistance. In contrast, examples of the method of the present disclosure may advantageously reduce FNC time down to about 1 to 2 hours to achieve the same hard layer thickness and therefore considerably reduce the processing energy cost.
Referring first to
After the machining performed in step 106, one example of the method 100 then involves the steps shown along the path labeled 1, which include deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against a blunt tool (described further herein), thereby forming the nanocrystallized microstructure at the surface, as shown at reference numeral 108; roller burnishing the nanocrystallized microstructure, as shown at reference numeral 114; and nitrocarburizing the workpiece for a period of time ranging from about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., as shown at reference numeral 110.
After machining at step 106, another example of the method 100 then involves the steps shown along the path labeled 2, which include simultaneously cooling and deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against the blunt tool (described further herein), thereby forming the nanocrystallized microstructure at the surface, as shown at reference numerals 108 and 112; roller burnishing the nanocrystallized microstructure, as shown at reference numeral 114; and nitrocarburizing the workpiece for a period of time ranging from about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., as shown at reference numeral 110.
After machining at step 106, still another example of the method 100 then involves the steps shown along the path labeled 2′, which include simultaneously cooling and deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against the blunt tool (described further herein), thereby forming the nanocrystallized microstructure at the surface, as shown at reference numerals 108 and 112; and nitrocarburizing the workpiece for a period of time ranging from about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., as shown at reference numeral 110. This example of the method ends after the nitrocarburizing step.
Referring now to
After machining at step 106, an example of the method 100′ then involves the steps shown along the path labeled 3, which include deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against a blunt tool (described further herein), thereby forming a nanocrystallized microstructure at the surface, as shown at reference numeral 108; roller burnishing the nanocrystallized microstructure, as shown at reference numeral 114; and nitrocarburizing the workpiece for a period of time ranging from about 5 hours to about 10 hours at a temperature ranging from about 370° C. to about 450° C., as shown at reference numeral 110′.
After machining at step 106, another example of the method 100′ then involves the steps shown along the path labeled 4, which include simultaneously cooling and deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against the blunt tool (described further herein), thereby forming the nanocrystallized microstructure at the surface, as shown at reference numerals 108 and 112; roller burnishing the nanocrystallized microstructure, as shown at reference numeral 114; and nitrocarburizing the workpiece for a period of time ranging from about 5 hours to about 10 hours at a temperature ranging from about 370° C. to about 450° C., as shown at reference numeral 110′.
After machining at step 106, still another example of the method 100′ then involves the steps shown along the path labeled 4′, which include simultaneously cooling and deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against the blunt tool (described further herein), thereby forming the nanocrystallized microstructure at the surface, as shown at reference numerals 108 and 112; and nitrocarburizing the workpiece for a period of time ranging from about 5 hours to about 10 hours at a temperature ranging from about 370° C. to about 450° C., as shown at reference numeral 110′. This example of the method ends after the nitrocarburizing step.
In each of the examples above of the present methods, the FNC renders the nanocrystallized microstructure into i) a friction surface (described further below at reference numerals 46, 46′), or ii) a corrosion-resistant surface (e.g., reference numerals 86, 86′ in
This formation of the nanocrystallized microstructure prior to FNC allows a higher diffusion rate of nitrogen and carbon into the cast iron workpiece, which leads to a considerably more efficient FNC process.
Without being bound to any theory, it is believed that at least the following three aspects are improved with methods of the present disclosure: 1. at conventional FNC temperatures (e.g., method 100), the FNC processing time may be reduced down to about 1 hour to 2 hours (from the conventional 5 to 6 hours); 2. alternatively (e.g., method 100′), FNC may be performed at a low temperature at which conventional FNC cannot thermodynamically create a hard nitride layer. This low temperature treatment may lead to a better dimensional stability, thereby eliminating the need for a stress relief step (i.e., step 104) in some instances; and 3. the surface nanocrystallized microstructure may itself contribute to better wear and fatigue performance of the workpiece. In addition, it is believed that the addition of cooling during deformation may enhance the effectiveness of nanocrystallization by slowing down or suppressing dynamic recrystallization, where deformed grain structures grow and become undesirably coarse. Still further, it is believed that a surface burnishing step may be performed to reduce surface roughness that may result from the nanocrystallization. Decreasing the surface roughness may improve the performance of the cast iron workpiece.
In the examples disclosed herein, machining 106 may be accomplished by, for example, turning, milling, sand blasting, grit blasting, grinding, and combinations thereof.
Referring now to
The tool 80, 80′ applies a deforming force to the machined, finish surface of the workpiece. In an example, the blunt tool 80, 80′ may be advanced by rotating a leadscrew that controls the advancement of the blunt tool 80, 80′ into a rotating finish surface of the workpiece by about 0.03 mm beyond first contact between the rotating workpiece and the blunt tool 80, 80′. It is to be understood that advancing the blunt tool 80, 80′ by about 0.03 mm does not necessarily create penetration of 0.03 mm in the workpiece because of elastic deformation of the workpiece, the pellet 82, and the holding fixture of the blunt tool 80, 80′. Further, the pellet 82 is not sharp and does not cut the finish surface. Blunt tool 80 reorganizes the crystal structure of the finish surface substantially without removing material therefrom. It is to be understood that the deformation of the finish surface may not be visible to the naked eye. However, a change in the reflective properties of the finish surface may be observable to the naked eye.
In an example, the tool 80, 80′ may cause pellet 82 to vibrate relative to the workpiece (as indicated by the double sided arrow V shown in phantom in
In examples of the present disclosure, cooling may be performed simultaneously with the deformation of the workpiece surface (reference numerals 108 and 112 in
Examples of the liquid coolant may also have phase changing material particles suspended therein to further enhance the cooling effect. An example of the phase changing material particles includes micro-encapsulated particles, such as sodium sulfate (Na2SO4.10H2O) particles or lauric acid particles. The volume fraction of added phase changing material particles may range from about 1% to about 30%.
The coolant may have a temperature ranging from about −100° C. to about 25° C. In one example, the temperature of the coolant is about 0° C.
Rapid cooling may be performed via any suitable method. In an example, the coolant is in the form of a cooling gas jet that is directed toward the surface where deformation is taking place.
In the example shown in
In any of the examples involving cooling, the cooling rate may vary depending, at least in part, on the workpiece and tooling setup, the material of the workpiece (e.g., its thermal conductivity), the feeding rate (i.e., how fast the workpiece is deformed), and the cooling conditions.
It is to be understood that the vortex tube 14 (or other cooling device) may be separate from the blunt tool 80 (as shown in
The deformation or deformation and cooling of the machined, finish surface promotes the nanocrystallization of that surface, resulting in the formation of the nanocrystallized microstructure 70. It is to be understood that the nanocrystallized microstructure 70 (see
In some examples of the method disclosed herein, it may be desirable to surface burnish 114 the nanocrystallized microstructure 70. An example of the method utilizing surface burnishing 114 will now be described in reference to
Referring now to
As shown in
In an example, roller burnishing reduces the roughness of the nanocrystallized microstructure 70 from Ra=1 μm to 5 μm to Ra <0.1 μm. It is to be understood that the reduction in surface roughness may depend, at least in part, on the material of the workpiece and the roller burnishing conditions that are used.
The roller 90 may be any commercially available cylindrical or spherical roller available from ECOROLL AG/ECOROLL Corp. In an example, the radius of the roller 90 ranges from about 1 mm to about 200 mm. In another example, the radius of the roller ranges from about 5 mm to about 10 mm. The roller 90 generally has a smooth surface, which is polished and has a surface roughness, Rz, less than 1 μm.
Roller burnishing has a relatively short cycle time. In an example, the cycle time ranges from about 10 seconds to about 120 seconds.
In the examples disclosed herein, after nanocrystallization (with or without active cooling), or after nanocrystallization (with or without active cooling) and surface burnishing, the method(s) involve nitrocarburizing (reference numerals 110 and 110′).
Examples of the methods of the present disclosure are relatively simple to execute and can be applied to many workpieces (one example of which is a component with axial symmetry that can be rotated during metal work, e.g., components having a disc shape or round bar shape).
An example of a cast iron workpiece is a rotational member of a vehicle brake. A brake 10 (shown in
A vehicle brake 10 may be a disc brake 20, drum brake 50, and combinations thereof.
Referring now to
As shown in
It is to be understood that a disc brake 20 may be combined with a drum brake 50. As shown in
The rotational member 12, 12′, 12″ includes the friction surface 46, 46′ that is engaged by a friction material of the brake pad 36 or the brake shoe 62. As a brake is engaged to retard a vehicle, mechanical wear and heat may cause small amounts of both the friction material and the rotational member 12, 12′, 12″ to wear away. It may be possible to reduce the rate of wear of the rotational member 12, 12′, 12″ or the friction material by reducing the coefficient of friction between the two, but a lower coefficient of friction may make the brake 10 less effective at retarding the vehicle.
In cast iron, corrosion is mainly the formation of iron oxides. Iron oxides are porous, fragile and easy to scale off. Further, corrosion on a friction surface may be non-uniform, thereby deleteriously affecting the brake performance and useful life. Thus, corrosion may lead to undesirably rapid wear of the friction surface 46, 46′ and the corresponding friction material.
Ferritic nitrocarburization produces a friction surface 46, 46′ that resists corrosion and wear. In examples of the present disclosure, ferritic nitrocarburization is used to render the nanocrystallized microstructure 70 (which may have been exposed to surface burnishing) into a compound layer 70′ on the rotational member 12, 12′, 12″ of the workpiece (e.g., brake 10). In an example, rotational member 12, 12′, 12″ has a compound layer 70′ disposed at the friction surface 46, 46′ or the corrosion-resistant surface 86, 86′. The compound layer 70′ may have an exposed surface in contact with an atmosphere, for example, air. An example of the compound layer 70′ resulting from nitrocarburization is shown in
As depicted in
In an example, a ferritically nitrocarburized rotational member 12, 12′, 12″ having a friction surface 46, 46′ formed by methods of the present disclosure exhibits a friction material wear of less than 0.4 mm per 1000 stops at about 350° C. An experiment using the test procedure in Surface Vehicle Recommended Practice J2707, Issued February 2005 by SAE International may be conducted. An Akebono NS265 Non Asbestos Organic (NAO) friction material may be used in the experiment.
The workpiece/rotational member 12, 12′, 12″ may be made from cast iron. The friction surface 46, 46′ may exhibit a hardness of between about 56 HRC and about 64 HRC. Hardness is directly related to wear resistance.
In the examples of the methods 100, 100′ disclosed herein, it is to be understood that nitrocarburizing 110, 110′ includes a gas nitrocarburizing process, a plasma nitrocarburizing process, or a salt bath nitrocarburizing process. The salt bath nitrocarburizing process may include immersing at least the friction surface 46, 46′ of the rotational member 12, 12′, 12″ into a nitrocarburizing salt bath, and then immersing at least the friction surface 46, 46′ of the rotational member 12, 12′, 12″ into an oxidizing salt bath.
It is to be understood that the rotational member 12, 12′, 12″ may include a brake disc 39, a brake drum 56, or a combination thereof.
Overall, examples of the present methods 100, 100′ may improve corrosion resistance similarly to FNC methods performed without first forming nanocrystallized surface layer 70. In summary, examples of the method of the present disclosure may reduce FNC cycle time by a factor of about 5 to 10 (e.g., reduced from about 5 to 6 hours to about 1 to 2 hours at 570° C.). Alternately, examples may enable low temperature FNC (reduced from 570° C. to about 400° C.-450° C.) to reduce part distortion. Examples of the present disclosure further produce workpieces with improved wear/fatigue resistance and corrosion resistance. Increased productivity is achievable compared to other surface nanocrystallization processes. For example, nanocrystallization by shot peening may require about 36 seconds per square centimeter. In sharp contrast, examples of the method disclosed herein may take about 2 seconds per square centimeter. Some examples of the method 100, 100′ also produce a desirably smooth surface.
Numerical data have been presented herein in a range format. It is to be understood that this range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a time period ranging from about 5 hours to about 10 hours should be interpreted to include not only the explicitly recited limits of about 5 hours to about 10 hours, but also to include individual amounts such as 5.5 hours, 7 hours, 8.25 hours, etc., and sub-ranges such as 8 hours to 9 hours, etc. Furthermore, when “about” is utilized to describe a value, this is meant to encompass minor variations (up to +/−10%) from the stated value.
In describing and claiming the examples disclosed herein, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.
Additionally, reference throughout the specification to “one example”, “another example”, “an example”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the example is included in at least one example described herein, and may or may not be present in other examples. In addition, it is to be understood that the described elements for any example may be combined in any suitable manner in the various examples unless the context clearly dictates otherwise.
While several examples have been described in detail, it will be apparent to those skilled in the art that the disclosed examples may be modified. Therefore, the foregoing description is to be considered non-limiting.
Number | Date | Country | Kind |
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PCT/CN2012/085510 | Nov 2012 | CN | national |
This application is a continuation-in-part of International Application Serial No. PCT/CN2012/085510, filed Nov. 29, 2012.