WO2025015140A1

WO2025015140A1 - Fatigue resistance for shear knives

Info

Publication number: WO2025015140A1
Application number: PCT/US2024/037541
Authority: WO
Inventors: Albert-Jan DE BOER; Branislav PETROV; Menno Van Der Woude
Original assignee: Andritz Inc.
Priority date: 2023-07-12
Filing date: 2024-07-11
Publication date: 2025-01-16

Abstract

A method for fabricating a shearing tool for shearing high strength steel, the method includes: forming the shearing tool including a knife edge having a first surface, a second surface, and a cutting surface disposed between the first surface and the second surface; finishing the first surface and the second surface to have an isotropic surface finish substantially devoid of surface finish marks; cold forming a specified radius on the cutting surface; and deep rolling a portion of the first surface adjacent to the cutting surface and a portion of the second surface adjacent to the cutting surface to create a specified compressive stress in a volume of the knife edge corresponding to the portion of the first surface and the portion of the second surface to a specified depth within the knife edge.

Description

PATENT Attorney Docket No.057895-1443373-PAT-00324PCT FATIGUE RESISTANCE FOR SHEAR KNIVES BACKGROUND [0001] Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to being prior art by inclusion in this section. [0002] This application is directed to metal shearing knives or tools and their edges. Metal shearing knives or tools include shearing knives or any tool which separates metal. Metal shearing knives are made in a number of configurations, including but not limited to: straight, with one to four straight shearing edges; rotary, with one or two circular shearing edges; curved, with one to four curvilinear shearing edges; and helical, typically with one compound curved shearing edge. [0003] When shearing low to mid-strength metals, i.e., metals having less than approximately 500 MPa tensile strength, the edges of shear knives typically reach end-of-life when the edges become dull (worn) and are no longer able to shear the metal and provide the desired sheared edge quality. Abrasive wear of the knife edges typically produces undesirable burrs on the sheared edge of the metal. When the edges of a knife becomes too dull or worn to produce the desired quality of sheared edge, the shear knife edge may be resharpened by grinding or machining to remove knife material from one or both of the two surfaces that intersect and form the knife edge. Higher strength steels such as High Strength Low Alloy (HSLA) steel and Advanced High Strength Steels (AHSS) are commonly used to achieve lower weight, higher strength, and lower cost products, such as automobiles, pipe, tube, structures, and fabrications. These higher strength steels have tensile strengths well above 500 MPa. [0004] A problem associated with shearing higher tensile strength steels is the early failure of shear knife edges by fracturing due to fatigue rather than dulling by abrasive wear as in low to mid-strength metals. The fatigue failure of the shear knife edges is due to high contact stresses in the knife edges resulting from the higher forces required to shear the high strength steel. The higher the tensile strength and thickness of the metal being sheared, the higher the stress in the knife edges and the faster the edges fail. The higher stresses in the knife edges cause fatigue fractures to start earlier and to propagate faster until edge spalling occurs. Fatigue cracks develop and propagate on the knife edge until a piece of the knife edge breaks out of the knife, thereby producing an unacceptable condition (e.g., burrs) on the sheared edge of the material being sheared. [0005] FIG.1 illustrates a partial perspective view of a conventional knife edge 100 of a conventional metal shearing knife. The conventional knife edge 100 may have a ground surface finish with surface finish marks substantially parallel with the cutting surface. As shown in FIG. 1, the conventional knife edge 100 has a cutting surface 102 disposed between two surfaces 104, 106. The surfaces 104, 106 are portions of a first face/outside diameter 108 of the conventional metal shearing knife and a second face 110 of the conventional metal shearing knife. The conventional knife edge 100 has a ground surface finish with surface finish marks 112 substantially parallel with cutting surface 102. Failure of shear knife edges by fracturing due to fatigue when shearing high-strength steel can be exacerbated by the surface finish marks parallel with the cutting surface. BRIEF DESCRIPTION OF THE DRAWINGS [0006] Aspects and features of the various embodiments will be more apparent by describing examples with reference to the accompanying drawings, in which: [0007] FIG.1 illustrates a partial perspective view of a conventional knife edge having a ground surface finish with surface finish marks substantially parallel with the cutting surface; [0008] FIG.2 illustrates a partial perspective view of a knife edge according to some aspects of the present disclosure; [0009] FIGS.3A and 3B are diagrams illustrating the cold forming process of the cutting surface of the knife edge according to some aspects of the present disclosure; [0010] FIG.4 is a table showing approximate maximum cutting surface radius for a thinnest steel to be sheared with a sheer knife according to some aspects of the present disclosure; [0011] FIG.5A is a diagram illustrating deep rolling of a knife edge according to some aspects of the present disclosure; [0012] FIG.5B is a diagram schematically illustrating an area of compressive stress of a knife edge resulting from deep rolling according to some aspects of the present disclosure; [0013] FIG.6 is a perspective view of a knife edge according to some aspects of the present disclosure; [0014] FIGS.7A-7C are diagrams illustrating front, side and perspective views of a shear knife having straight edges according to some aspects of the present disclosure; [0015] FIGS.8A-8C are diagrams illustrating front, side and perspective views of a shear knife having rotary edges according to some aspects of the present disclosure; [0016] FIGS.9A-9C are diagrams illustrating front, side and perspective views of a shear knife having curved edges according to some aspects of the present disclosure; [0017] FIGS.10A-10C are diagrams illustrating front, side and perspective views of a shear knife having a helical edge according to some aspects of the present disclosure; and [0018] FIG.11 is a flowchart illustrating a method for fabricating a fatigue resistance shear knife according to some aspects of the present disclosure. DETAILED DESCRIPTION [0019] While certain embodiments are described, these embodiments are presented by way of example only, and are not intended to limit the scope of protection. The apparatuses, methods, and systems described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions, and changes in the form of the example methods and systems described herein may be made without departing from the scope of protection. [0020] Aspects of the present disclosure can provide a shearing tool, alternately referred to herein as a shear knife, having a knife edge suitable for shearing high strength steels with reduced fatigue failure. A sheer knife according to the present disclosure may have a knife edge with a radiused cutting surface and faces having isotropic surface finishes. Portions of the faces may be deep rolled to reduce surface roughness and impart compressive stress to the knife edge. The radius can reduce stress concentration on the cutting surface during shearing of high strength steels, while reduced surface roughness can minimize nucleation of fatigue cracks on the surface of the knife edge and compressive stress produced from deep rolling and reduce the ability of fatigue cracks to propagate. [0021] FIG.2 illustrates a partial perspective view of a knife edge 200 according to some aspects of the present disclosure. Referring to FIG.2, the knife edge 200 may have a cutting surface 202 disposed between a first surface 204 of the shear knife and a second surface 206 of a shear knife. A plane of the first surface 204 and a plane of the second surface 206 may intersect at the cutting surface 202. The cutting surface 202 may be formed at a substantially 90 degree angle between the first surface 204 and the second surface 206 of the knife edge 200. The first surface 204 may be a portion of a first face/outside diameter 208 of the shear knife and the second surface 206 may be a second face 210 of the shear knife. The first and second surfaces 204, 206 may have non-directional (e.g., isotropic) surface finishes 212 produced by, for example, but not limited to, lapping polishing, burnishing, etc. [0022] The radiused cutting surface of the knife edge may be cold formed to produce the desired radius. Cold forming of the cutting surface may be performed using a tool designed to provide the required radius. FIGS. 3A and 3B are diagrams illustrating the cold forming process of the cutting surface of the knife edge according to some aspects of the present disclosure. Referring to FIG.3A, a forming tool 350 configured with a portion having a specified radius 358 may be used to cold form a radius on the knife edge 200. During the cold forming process, the radius 358 of the cold forming tool 350 is pressed into the cutting surface 202 of the knife edge 200. The pressure used to press the radius 358 of the cold forming tool 350 into the cutting surface 202 of the knife edge 200 to form the radius on the cutting surface 202 may depend on the specified radius for the cutting surface 202 and the material from which the knife edge 200 is fabricated. [0023] As illustrated in FIG.3B, the cold forming tool 350 may be pressed into the cutting surface 202 of the knife edge 200 until edges 352a, 352b adjacent to the radius 358 of the cold forming tool 350 contact the first and second surfaces 204, 206 of the knife edge 200 thereby forming the corresponding radius on the cutting surface 202 determined by the radius 358 on the cold forming tool 350. The radius can reduce stress concentration on the cutting surface during shearing of high strength steels. Cold forming of the radius can also impart compressive stress to the knife edge 200. [0024] While FIGS.3A and 3B illustrate a schematic example of a forming tool 350 for cold forming a corresponding radius on a cutting surface 202 of a knife edge 200, it should be appreciated that actual cold forming tool configurations may be different and other cold forming processes may be used without departing from the scope of the present disclosure. [0025] The radius of the cutting surface of the knife edge may be different for different grades and/or thicknesses of steel to be sheared. FIG.4 is a table showing approximate maximum cutting surface radius for a thinnest steel to be sheared with a sheer knife according to some aspects of the present disclosure. As shown in FIG.4, the radius of the cutting surface of the knife edge may vary from a maximum radius of 0.3 mm to a minimum radius of 0.03 mm for shearing different grades/thicknesses of steel. Different cold forming tools with different radii may be used to form the different radii on the cutting surface of the knife edge. [0026] In order to impart compressive stress to the knife edge and provide a smooth surface finish, portions of the knife edge may be deep rolled. Deep rolling rather than other techniques such as shot peening may be utilized to impart compressive stress since shot peening, while providing compressive stress, can damage the surface finish needed to reduce fatigue cracking. Machining of the surface to restore the surface finish after shot peening can reduce the compressive stress. [0027] FIG.5A is a diagram illustrating deep rolling of a knife edge according to some aspects of the present disclosure. Referring to FIG.5A, a deep rolling tool 450, for example, but not limited to, a spherical deep rolling tool, may be utilized for deep rolling of a first deep rolled portion 404 of the first surface 204 of the knife edge 200 and a second deep rolled portion 406 of the second surface 206 of the knife edge 200. The diameter of the spherical deep rolling tool may control the depth of the compressive stress. The first deep rolled portion 404 of the first surface 204 may be adjacent to the radiused cutting surface 308 on the first surface 204 of the knife edge 200 and the second deep rolled portion 406 of the second surface 206 may be adjacent to the radiused cutting surface 308 on the second surface 206 of the knife edge 200. [0028] FIG.5B is a diagram schematically illustrating an area of compressive stress of a knife edge resulting from deep rolling according to some aspects of the present disclosure. As illustrated in FIG.5B, the first deep rolled portion 404 may extend a distance d1 adjacent to the radiused cutting surface 308 on the first surface 204 of the knife edge 200. Similarly, the second deep rolled portion 406 may extend the distance d2 adjacent to the radiused cutting surface 308 on the second surface 206 of the knife edge 200. The distances d1 and d2 may be in a range of 4-10 mm and may vary based on the geometry of the shear knife and the material to be sheared. The distances d1 on the first surface 204 and d2 on the second surface 206 may be different on the same knife edge 200. An area of compressive stress 410 extending in a range of 0.5μm – 0.5 mm into the knife edge 200 and corresponding to the first and second deep rolled portions 404, 406 may be formed in the knife edge 200. The deep rolling may be performed at a pressure in a range of 150-400 bars and may produce a compressive stress of approximately 2500 megapascals (MPa). Compressive stress produced from deep rolling can reduce the ability of fatigue cracks to propagate. In some instances, the area of compressive stress may be determined by testing hardness since hardness has a close relationship to mechanical properties such as strength, ductility, and fatigue resistance. The testing methods may measure the subsurface microhardness gradually. [0029] While FIG.5B is a two-dimensional illustration, it should be appreciated that the radiused cutting surface 308 and the first and second deep rolled portions 404, 406 extending a width direction of the knife edge 200, and that the illustrated area of compressive stress 410 occupies a volume corresponding to the width of the knife edge 200. Further, the area of compressive stress 410 shown is representative for purposes of illustration and the actual area/volume of the knife edge in which compressive stress is created can be different. In some cases, the compressive stress is at a maximum at approximately 0.1 mm below the deep rolled surface and then decreases with increasing depth. [0030] Deep rolling of portions of the first and second surfaces 204, 206 may result in an average surface roughness (Ra) less than 0.4 Ra on the first deep rolled portion 404 and the second deep rolled portion 406. The reduced surface roughness can minimize nucleation of fatigue cracks on the surface of the knife edge. [0031] FIG.6 is a perspective view of a knife edge 200 according to some aspects of the present disclosure. The perspective view of FIG. 6 illustrates the radiused cutting surface 408, the deep rolled portions 404, 406, adjacent to the radiused cutting surface 408 on the first and second surfaces 204, 206, respectively, and the area of compressive stress 410 extending into the knife edge 200 and corresponding to the first and second deep rolled portions 404, 406. Fatigue resistance of a knife edge 200 according to the present disclosure may extend to a depth of approximately 1 mm below the surfaces of the deep rolled portions 404, 406 of the knife edge 200 compared to conventional knife edges where fatigue resistance extends only to a depth of approximately 1 μm. [0032] One of ordinary skill in the art will appreciate that embodiments of the fatigue resistance shear knife may include various sheer knife configurations, for example, but not limited to, sheer knives having straight edges, sheer knives having rotary edges, sheer knives having curved edges, sheer knives having helical edges, etc., without departing from the scope of protection. Sheer knives according to the present disclosure may have a hardness in a range of 50-65 on the Rockwell hardness scale. [0033] FIGS.7A-7C are diagrams illustrating front, side and perspective views of a shear knife having straight edges according to some aspects of the present disclosure. As shown in FIGS. 7A-7C, the sheer knife 70 with straight edges may include a body 72 including four edges. At least one edge, for example, edge 74, may include a cutting surface disposed between a first face 76 and a second face 78. In the embodiment illustrated in FIG.7A-7C , the first face 76 may contact the metal being sheared. [0034] FIGS.8A-8C are diagrams illustrating front, side and perspective views of a shear knife having rotary edges according to some aspects of the present disclosure. As shown in FIGS.8A- 8C, the shear knife having rotary edges 80 may include a body 82 including two edges 84a, 84b. At least one edge may include a cutting surface disposed between an outside diameter 89 and a first face 86. In the embodiment illustrated in FIG.8A-8C, the outside diameter 89 may contact the metal being sheared. [0035] FIGS.9A-9C are diagrams illustrating front, side and perspective views of a shear knife having curved edges according to some aspects of the present disclosure. As shown in FIGS. 9A-9C, the shear knife having curved edges 90 may include a body 92 including four edges. At least one edge, for example, edge 94, may include a cutting surface disposed between a first face 96 and a second face 98. In the embodiment illustrated in FIG.9A-9C, the second face 98 may contact the metal being sheared. [0036] FIGS.10A-10C are diagrams illustrating front, side and perspective views of a shear knife having a helical edge according to some aspects of the present disclosure. As shown in FIGS.10A-10C, the shear knife having a helical edge 1000 may include a body 1002 including an edge 1004 having a cutting surface disposed between a first face 1006 and a second face 1008. In the embodiment illustrated in FIG.10A-10C, the first face 1006 contacts the metal being sheared. [0037] It will be appreciated that these configuration as well as other variations of the disclosed configurations may be used without departing from the scope of the present disclosure. [0038] FIG.11 is a flowchart illustrating a method 1100 for fabricating a fatigue resistance shear knife according to some aspects of the present disclosure. Referring to FIG.11, at block 1110, a basic configuration of the shear knife may be fabricated. The shear knife may be fabricated from steel having have a hardness in a range of 50-65 on the Rockwell hardness scale and may have straight edges, rotary edges, curved edges, helical edges, etc., as illustrated in FIGS.7A through 10C. [0039] At block 1120, an isotropic surface finish may be formed on the surfaces of the knife edge. Non-directional (e.g., isotropic) surface finishes may be formed on first and second surfaces adjacent to the knife edge by, for example, but not limited to, lapping polishing, burnishing, etc. [0040] At block 1130, a radiused cutting surface of a knife edge may be formed. The radiused cutting surface of the knife edge may be cold formed to produce the desired radius. The radius may be a radius in a range of 0.03 mm to 0.3 mm. During the cold forming process, a cold forming tool having a specified radius is pressed into the cutting surface of the knife edge. The pressure used to press the radius of the cold forming tool into the cutting surface of the knife edge to form the radius on the cutting surface may depend on the specified radius for the cutting surface and the material from which the knife edge is fabricated. [0041] At block 1140, portions of the surfaces of the knife edge may be deep rolled. A spherical deep rolling tool may be utilized for deep rolling of a first deep rolled portion of a first surface of the knife edge adjacent to the cutting surface and a second deep rolled portion of a second surface of the knife edge adjacent to the cutting surface. The diameter of the spherical tool may control the depth of the compressive stress. [0042] The first deep rolled portion may extend a specified distance adjacent to the radiused cutting surface on the first surface of the knife edge and the second deep rolled portion may extend a specified distance adjacent to the radiused cutting surface on the second surface of the knife edge 200. The specified distances may be in a range of 4-10 mm and may vary based on the geometry of the shear knife and the material to be sheared. An area of compressive stress extending in a range of 0.5μm – 0.5mm into the knife edge and corresponding to the first and second deep rolled portions may be formed in the knife edge. The deep rolling may be performed at a pressure in a range of 150-400 bars and may produce a compressive stress of approximately 2500 megapascals (MPa). [0043] The specific operations illustrated in FIG. 11 provide a particular method for fabricating a fatigue resistance shear knife according to an embodiment of the present disclosure. Other sequences of operations may also be performed according to alternative embodiments. For example, alternative embodiments of the present disclosure may perform the operations outlined above in a different order. Moreover, the individual operations illustrated in FIG. 11 may include multiple sub-operations that may be performed in various sequences as appropriate to the individual operation. Furthermore, additional operations may be added or removed depending on the particular applications. [0044] Embodiments of the present disclosure provide a shear knife fabricated from steel having a hardness in a range of 50-65 on the Rockwell hardness scale with a knife edge having a radiused cutting surface and deep rolled portions having reduced surface roughness adjacent to the radiused cutting surface with compressive stress induced below the surface of the knife edge resulting from the deep rolling process. These embodiments provide shear knives having increased resistance to fatigue failure as compared to conventional shear knives. [0045] The examples and embodiments described herein are for illustrative purposes only. Various modifications or changes in light thereof will be apparent to persons skilled in the art. These are to be included within the spirit and purview of this application, and the scope of the appended claims, which follow.

Claims

WHAT IS CLAIMED IS: 1. A method for fabricating a shearing tool for shearing high strength steel, the method comprising: forming the shearing tool comprising a knife edge having a first surface, a second surface, and a cutting surface disposed between the first surface and the second surface; finishing the first surface and the second surface to have an isotropic surface finish substantially devoid of surface finish marks; cold forming a specified radius on the cutting surface; and deep rolling a portion of the first surface adjacent to the cutting surface and a portion of the second surface adjacent to the cutting surface to create a specified compressive stress in a volume of the knife edge corresponding to the portion of the first surface and the portion of the second surface to a specified depth within the knife edge.

2. The method of claim 1, wherein a plane of the first surface and a plane of the second surface intersect at the cutting surface.

3. The method of claim 1, wherein the specified radius is formed by utilizing a tool configured with the specified radius applied to the cutting surface with a pressure sufficient to conform the cutting surface to the specified radius.

4. The method of claim 1, wherein the specified radius is a radius in a range of 0.03 mm to 0.3 mm.

5. The method of claim 4, wherein the radius is determined based on a grade or thicknesses of steel to be sheared.

6. The method of claim 1, wherein the deep rolling forms a specified surface finish on the portion of the first surface and the portion of the second surface.

7. The method of claim 6, wherein the specified surface finish has an average roughness less than 0.4 Ra.

8. The method of claim 1, wherein the portion of the first surface extends 4-10 mm on the first surface from the cutting surface and the portion of the second surface extends 4-10 mm on the second surface from the cutting surface.

9. The method of claim 1, wherein the compressive stress extends to a depth of 0.5μm – 0.5mm into the knife edge.

10. The method of claim 1, wherein the shearing tool is formed as one of a sheer knife having straight edges, a sheer knife having rotary edges, a sheer knife having curved edges, or a sheer knife having helical edges.

11. The method of claim 1, wherein the shearing tool is formed from steel having a hardness in a range of 50-65 on the Rockwell hardness scale.

12. A shearing tool for shearing high strength steel, the shearing tool comprising: a knife edge having a first surface and a second surface; and a cutting surface disposed between the first surface and the second surface, the cutting surface having a specified radius, wherein the first surface and the second surface have a specified surface finish, and wherein a compressive stress is formed in a portion of the knife edge corresponding to a portion of the first surface adjacent to the cutting surface and a portion of the second surface adjacent to the cutting surface and extending a specified depth into the knife edge, wherein the compressive stress and the specified surface finish are created by deep rolling the portion of the first surface adjacent to the cutting surface and the portion of the second surface adjacent to the cutting surface.

13. The shearing tool of claim 12, wherein a plane of the first surface and a plane of the second surface intersect at the cutting surface.

14. The shearing tool of claim 12, wherein the specified radius is a radius in a range of 0.03 mm to 0.3 mm.

15. The shearing tool of claim 14, wherein the radius is determined based on a grade or thicknesses of steel to be sheared.

16. The shearing tool of claim 12, wherein the specified surface finish has an average roughness less than 0.4 Ra.

17. The shearing tool of claim 12, wherein the portion of the first surface extends 4-10 mm on the first surface from the cutting surface and the portion of the second surface extends 4-10 mm on the second surface from the cutting surface.

18. The shearing tool of claim 12, wherein the compressive stress extends to a depth of 0.5μm – 0.5mm into the knife edge.

19. The shearing tool of claim 12, further comprising steel having a hardness in a range of 50-65 on the Rockwell hardness scale.

20. The shearing tool of claim 12, wherein the shearing tool is one of a sheer knife having straight edges, a sheer knife having rotary edges, a sheer knife having curved edges, or a sheer knife having helical edges.

21. A method for processing a shearing tool comprising a knife edge having a first surface, a second surface, and a cutting surface disposed between the first surface and the second surface, the method comprising: deep rolling a portion of the first surface adjacent to the cutting surface and a portion of the second surface adjacent to the cutting surface to create a specified compressive stress in a volume of the knife edge corresponding to the portion of the first surface and the portion of the second surface to a specified depth within the knife edge.

22. The method of claim 21, wherein the compressive stress extends to a depth of 0.5μm – 0.5 mm into the knife edge.