Method and apparatus for improving the distribution of compressive stress

Abstract

A surface treatment element having a textured surface with a plurality of deforming features is pressed against the surface of an article to induce deep and shallow layers of compressive residual stress to form a continuous layer of compressive residual stress extending from the surface to a depth beneath the surface. A method whereby a surface treatment apparatus is pressed into the surface of the apparatus to cause Hertzian loading thereby inducing a deep, high magnitude compressive residual stress below the surface. The deforming features of the surface treatment element cause the lateral displacement of material on the surface of the article thereby cold working the surface and providing a more shallow layer of compressive residual stress at the surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:

FIG. 1 is a plot illustrating a typical pattern of depth versus magnitude of compressive residual stress for shot peening and burnishing showing the effect of lateral displacement of surface material on the magnitude of compression for each treatment;

FIG. 2 is a schematic illustration of a preferred embodiment of the surface treatment element that is a subject of the present invention showing a surface treatment element where the deforming features are dimples;

FIG. 3 is a schematic illustration of another preferred embodiment of the surface treatment element that is a subject of the present invention showing a surface treatment element where the deforming features are hemispherical nodules;

FIG. 4 is a cross sectional illustration showing a method of using a preferred embodiment of the surface treatment element of the present invention;

FIG. 5 is a cross sectional illustration showing a method of using another preferred embodiment of the surface treatment element of the present invention;

FIG. 6 is a plot illustrating a typical pattern of depth versus magnitude of compressive residual stress distribution induced in the surface of an article by the surface treatment element and method of the present invention; and

FIG. 7 is an illustrative example of an article having a metallic portion with a residual stress distribution induced along the surface by the apparatus and method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention addresses the limitations of currently available surface treatment methods by providing an apparatus and method for producing an article having at least one metallic portion with a continuous, deep layer of high magnitude compressive residual stress that extends from the surface to a depth beneath the surface of at least one of the portion(s).

Referring to FIG. 2, the apparatus for improving the distribution of compressive stress is a surface treatment element 100 for use in conjunction with hydrostatic, ball-type burnishing tools, wheel or roller burnishing tools, indenting tools and impact/pinch peening tools. By way of example, the surface treatment element 100 shown in FIGS. 2 and 3 is a spherical element for use in conjunction with a conventional hydrostatic, ball-type burnishing tool. Alternatively, the surface treatment element may have a variety of forms including cylindrical, hemispherical, conical, or disk-shaped depending on the particular embodiment of the surface treatment apparatus in which it is employed.

The surface treatment element 100 has at least one contact face 102. The contact face 102 has a plurality of small deforming features 104 located thereon. The deforming features 104 may comprise hemispherical nodules, irregularly shaped nodules, dimples, ridges, crevices and combinations thereof. For an examplanary illustration, the preferred embodiment shown in FIG. 2, the deforming features 104 are in the form of semi-circular dimples. In another examplanary illustration the preferred embodiment shown in FIG. 3 the deforming features 104 are in the form of hemispherical nodules. The deforming features 104 may be of uniform size and geometry, comprise a distribution of sizes and geometries, or have random sizes and geometries. In one preferred embodiment, the dimensions of the deforming features 104 are about 3.2 mm (0.125 inches) in diameter to about 0.10 mm (0.004 inches) in diameter. The deforming features 104 may be arranged in a regular pattern or randomly positioned over the curved contact face 102. In another preferred embodiment, the size and/or spacing of the deforming features 104 is sufficient to produce local contact stresses at the deforming feature sufficient to indent the deforming features into and along the surface of the article being treated thereby causing plastic deformation and lateral flow of the surface.

In another preferred embodiment, the geometry, size, and/or spacing of the deforming features 104 are designed to produce a specified amount of plastic deformation, cold work, and/or magnitude of compressive residual stress for a given material forming the article or a portion of the article being treated. It should understood that one skilled in the art can, such as by the use of test specimens, can select the proper combination of geometry, size and spacing of the deforming features 104 to arrive at the proper combination to design the deforming feature 104 for generating the desired amount of plastic deformation, cold work, and/or magnitude of compressive residual stress.

Referring now to FIGS. 4 and 5, the surface treatment element 100, in connection with a standard surface treatment tool, such as a burnishing or pinch peening tool (not shown), is brought into contact with the surface 108 of a portion 106 of an article 110 to be treated. The portion 106 of the surface 108 of the article 110 is metallic. The surface treatment element 100 is pressed into the surface 108 of the article 110 by the application of a normal force 118. The magnitude of the normal force 118 is sufficient to cause Hertzian loading of the surface treatment element 100 on the surface 108 of the article 110 thereby introducing deep, high magnitude compressive residual stresses beneath the surface 108 of the portion 106.

As the surface treatment element 100 is pressed into the surface 108 of the portion 106, the deforming features 104 located on the surface treatment element 100 cause the material on the surface 108 of the portion 106 to be laterally displaced and plastically deformed. The deforming features 104 create a greater contact surface than a smooth surface treatment element of similar dimension thus causing greater displacement and plastic deformation at the surface. The displacement results in deformation on the surface 108 of the portion 106 similar to that caused by shot peening. However, the excessive deformation and cold work associated with shot peening is avoided due to the elimination of the random impacting required to achieve full coverage of the surface by shot peening.

The plastic deformation at the surface 108 causes the introduction of a shallow layer of high magnitude residual compressive stress in the treated portion 106 similar to that from shot peening but with lower cold working as the amount of material displacement is controlled and the redundant impact of shot is eliminated. The surface treatment element 100 is then rolled, as in the case of a hydrostatic ball, wheel, or roller tool, across the surface 108 of the portion 106 to introduce both deep and shallow compressive residual stresses in the predefined area. Alternatively, as in the case of a pinch peening, impact peening, or indenting operation, the surface treatment element 100 is repositioned relative to the surface 108 of the portion 106 being treated followed by application of force to indent the surface, and the process repeated.

The resulting treated metallic portion(s) 106 has an improved residual stress distribution with a continuous layer of compressive residual stress 112 extending from the surface 108 to a depth beneath the surface of each treated portion(s) 106 that is greater than that of conventional shot peening. A typical residual stress distribution for the surface treatment element is shown in FIG. 6. As illustrated the distribution has both highly compressive surface residual stresses and due to the lateral displacement of surface material and deep, high magnitude subsurface residual stresses due to Hertzian loading. For an illustrative example, as illustrated in FIG. 7, a residual stress distribution of a article having a metallic portion formed from a highly work hardening austenitic stainless nickel base alloy, Alloy 22, is shown having been treated by conventional smooth tool burnishing and by the method and apparatus of the subject invention. For both samples, the forces and tool dimensions were identical, differing only in the texture of the surface of the surface treatment element. For purposes of this illustration, the smooth tool burnishing represents a conventional burnishing element having a smooth surface. The tool of the subject invention comprises the surface treatment element as described above. As illustrated, a layer of high compressive residual stress extends from the surface to a depth beneath the surface of the portion treated with the textured ball that is greater than that produced by the conventional smooth tool burnishing. As shown, the surface layer of low compression has been eliminated by the apparatus and method of the subject invention.

It should now be understood to one skilled in the art that the surface treatment apparatus and method of the subject invention leaves the surface of the treated portion(s) roughened. This roughened surface results in improving the adhesion of any coatings subsequently applied to the treated surface.

In another preferred embodiment of the invention, following treatment using the apparatus and method of the present invention, the surface of the treated portions is further burnished using a smooth ball, such as a conventional burnishing apparatus, to remove any dimples, ridges, etc., thereby causing further lateral displacement of the surface and even higher surface compression.

It should now be apparent to one skilled in the art that the subject method and apparatus may be used to improve the fatigue and stress corrosion cracking performance of a variety of metallic articles or metallic portions of articles and systems, such as articles used in the aircraft, automotive, rail, shipping, nuclear and petrochemical industries. This includes, but is not limited to, aircraft, naval, steam and ground-based turbines including turbine blades, disks, shafts, aircraft structural parts, aircraft landing gear and parts, metallic weldments, piping and parts used in nuclear, fossil fuel, steam, chemical, and gas plants, distribution piping for gases and fluids, automotive parts such as gears, springs, shafts, connecting rods, and bearings, ship hulls, propellers, impellers, and shafts, rail transport parts and tracks, and various other parts and structures too numerous to be mentioned herein.

It should also now be apparent to those skilled in the art that the described invention has many advantages, including the ability to produce deep, high magnitude compressive residual stress accompanied by higher magnitude surface compression than is otherwise achievable using smooth surface treatment elements, particularly in work hardening materials or surfaces previously cold worked. This eliminates the need for multiple surface treatments or other operations in order to produce the desired residual stress distribution and thereby reduces manufacturing costs and time.

Another advantage of the present invention is the ability to produce high magnitude surface compression with minimal cold work for improved thermal stability of compressive residual stresses.

Another advantage of the present invention is the ability to introduce beneficial residual compressive stresses in the part while simultaneously improving the mechanical bonding characteristics of the surface of an article so as to improve the adhesion of platings, paints, and other coatings.

Another advantage of the present invention is that the apparatus could be used at load levels much lower than what would be needed for Hertian loading of the full surface treatment element. In this way by pressing the deforming features into the surface of the article would produce the same results as shot peening (if the deforming features are the same size) but without the high cold work or the “laps and fold” type of damage caused by multiple impacts of shot peening. In addition, the method and apparatus of the present invention would eliminate scattered shot typically occurring with shot peening and which is not permitted or acceptable where shot left over could damage the mechanism or cause corrosion problems. Such systems include, but are not limited to for nuclear systems, jet engines, and high performance motors. Further, the present invention eliminates surface chemical contamination as often happens with ferrite residue on the surface of articles caused by shot peening by forming the surface treatment element from a hardened version of the article alloy or a harder similar alloy. The present apparatus can be used for many articles that previously must be dismantled, removed from another device and the like in order to be shot peened. It should now be apparent to one skilled in the art that the apparatus of the present invention can be of a size permitting many such articles to be treated without dismantling or removal from its location.

Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. Therefore, the scope of the appended claims should not be limited to the description of the preferred embodiments contained herein.

Claims

1. A surface treatment element for introducing deep and shallow compressive residual stresses in an article, the element comprising: a surface for contacting and applying a Hertzian load to a surface of the article, the surface having a plurality of deforming features effective for causing the plastic deformation and lateral displacement of material on the surface of the article.

2. The surface treatment element of claim 1 wherein the surface treatment element is in the form of a sphere, spheroid, ellipsoid, hemisphere, cylinder, cone or disk.

3. The surface treatment element of claim 1 wherein the surface treatment element is selected from the list consisting of pinch peening elements, impact peening elements, indenting elements, and burnishing elements.

4. The surface treatment element of claim 1 wherein the deforming features are uniformly spaced.

5. The surface treatment element of claim 1 wherein the deforming features are randomly spaced.

6. The surface treatment element of claim 1 wherein the deforming features are bumps, nodules, ridges, dimples, craters, crevices and/or combinations thereof.

7. The surface treatment element of claim 1 wherein the deforming features are less than about 0.125 inches in diameter.

8. A method of improving the surface compression of an article having at least one metallic portion comprising the acts of: contacting the surface of the metallic portion with a surface treatment element having a contact surface with a plurality of deforming features disposed thereon; andapplying pressure to the surface treatment element of sufficient magnitude to cause Hertzian loading thereby inducing deep compressive residual stresses beneath the surface of the metallic portion and causing the material at the surface of the metallic portion to be plastically deformed by the deforming features of the contact surface so as to induce shallow compressive residual stresses.

9. The method of claim 8 further comprising the act or rolling the surface treatment element over the surface of the metallic portion.

10. The method of claim 8 wherein the surface treatment element is selected from the list consisting of pinch peening elements, impact peening elements, indenting elements, and burnishing elements.

11. The method of claim 8 wherein the deforming features are less than about 0.125 inches in diameter.

12. The method of claim 8 further comprising the step of burnishing the surface of the metallic portion with a smooth burnishing element to eliminate surface roughness and cause additional lateral displacement and higher magnitude surface compression in the surface of the metallic portion.

13. A method of improving the surface compression of a portion of an article comprising the acts of: applying a Hertzian load to at least one metallic portion of the surface of the article to introduce a deep compressive residual stress; andplastically deforming discrete portions of the at least one metallic portion to introduce shallow compressive residual stress.

14. The method of claim 13 wherein the acts of applying a Hertzian load and plastically deforming are performed in a single operation.

15. The method of claim 13 wherein the acts of applying a Hertzian load and plastically deforming are performed using a surface treatment element having a surface for contacting and applying a Hertzian load to the surface of the metallic portion to introduce a deep compressive stress, the curved surface having a plurality of deforming features to cause the plastic deformation and lateral displacement of material on the surface of the metallic portion to introduce a shallow layer of compressive stress.

16. An article having at least one metallic portion with an improved residual compressive stress distribution comprising: a surface plastically deformed at discrete locations;a continuous residual compressive stress distribution extending from the surface to a depth beneath the surface of the metallic portion which is a depth greater than that obtainable by shot peening.

17. The article of claim 16 wherein the article is selected from the group comprising aircraft and aircraft engine parts, power generating parts, automotive and automotive engine parts, gas and fluid distribution parts, steam generator parts, and nuclear components and weldments.

18. An article with an improved residual compressive stress distribution comprising: a metallic portion having a surface, the surface having a surface layer of compressive stress caused by plastic deformation at discrete locations along the surface, the compressive residual stress extending beneath the surface as a result of Hertzian loading.