High-strength screw including an unhardening layer

Abstract

A high-strength screw (2) includes a threaded portion (7) having a thread (8). The screw (2) includes an inner core (16) as seen in cross-section of the screw (2), the core (16) having a first hardness. The screw (2) includes an outer surface layer (17) as seen in cross-section of the screw (2). The screw (2) includes an unhardening layer (18) forming the outer surface layer (17) in the threaded portion (7), the unhardening layer (18) having a second hardness being reduced compared to the first hardness of the core (16).

Claims

1. A high-strength screw, comprising: a longitudinal center axis; a threaded portion having a thread, the thread having a height H corresponding to a height of a sharp-cut imaginary profile section, the thread including a thread tip winding about the longitudinal center axis and having a diameter d and a thread base winding about the longitudinal center axis and having a diameter d3; an inner core as seen in cross-section of the screw, the core having a first hardness; an outer surface layer as seen in cross-section of the screw; and an unhardening layer forming the outer surface layer in the threaded portion, the unhardening layer having a second hardness being reduced compared to the first hardness of the core, the unhardening layer having a depth T1 existing along the thread tip winding about the longitudinal center axis and being measured in a radial direction perpendicular to the longitudinal center axis up to an imaginary axial connecting line between two positions of the thread base being adjacent and being offset by 360, the thread having one of the following pairs of values: height H: 0.4 mm to 0.7 mm; depth T1: 0.25 mm to 1.8 mm; or height H: 0.71 mm to 1.0 mm; depth T1: 0.25 mm to 2.7 mm; or height H: 1.01 mm to 1.5 mm; depth T1: 0.2 mm to 2.7 mm; or height H: 1.51 mm to 2.0 mm; depth T1: 0.2 mm to 2.7 mm; or height H: 2.01 mm to 2.5 mm; depth T1: 0.15 mm to 3.0 mm; or height H: 2.51 mm to 4.0 mm; depth T1: 0.15 mm to 4.0 mm, wherein the core has a first hardness of 400 HV to 650 HV, and the unhardeninq layer at its outside has a hardness of 300 HV to 380 HV.

2. The screw of claim 1, wherein the core is an inner portion of the screw as seen in cross-section up to a diameter of the core dk=d/4; and the first hardness of the core is the average of the hardness within the diameter of the core dk.

3. The screw of claim 1, wherein that the second hardness of the unhardening layer is the average of the hardness within the unhardening layer.

4. The screw of claim 1, wherein the depth T1 is between 0.1 mm and 3.0 mm.

5. The screw of claim 1, wherein the second hardness of the unhardening layer at its outside is reduced by at least 7% compared to the first hardness of the core.

6. The screw of claim 1, wherein the second hardness of the unhardening layer at its outside is reduced by between 10% and 50% compared to the first hardness of the core.

7. The screw of claim 1, wherein the core has a tensile strength of between 1300 N/mm.sup.2 and 2500 N/mm.sup.2; and the unhardening layer at its outside has a tensile strength of between 1000 N/mm.sup.2 and 1200 N/mm.sup.2.

8. The screw of claim 1, wherein the core has a tensile strength of between 1400 N/mm.sup.2 and 1850 N/mm.sup.2; and the unhardening layer at its outside has a tensile strength of between 1000 N/mm.sup.2 and 1200 N/mm.sup.2.

9. The screw of claim 1, further comprising: a head; a head bearing transition surface; and a threadless shank portion, the shank portion including a second unhardening layer forming the outer surface layer in the shank portion, the second unhardening layer having a third hardness being reduced compared to the first hardness of the core.

10. The screw of claim 9, wherein the shank portion has an axial length; and the second unhardening layer does not extend along the entire axial length of the shank portion.

11. The screw of claim 9, wherein the shank portion has an axial length; and the second unhardening layer extends along the entire axial length of the shank portion.

12. The screw of claim 1, wherein the screw is made of steel and includes a bainite structure.

13. A high-strength screw, comprising: a longitudinal center axis; a head; a threadless shank portion; a threaded portion having a thread; an inner core as seen in cross-section of the screw, the core having a first hardness of between 400 HV and 650 HV; an outer surface layer as seen in cross-section of the screw; and an unhardening layer forming the outer surface layer in the threaded portion, the unhardening layer having a second hardness that has been reduced compared to the first hardness of the core, the second hardness of the unhardening layer in its radial outmost portion being between 300 HV and 380 HV, wherein the core has a tensile strength of between 1400 N/mm.sup.2 and 1850 N/mm.sup.2; and the unhardening layer in its radial outmost portion has a tensile strength of between 1000 N/mm.sup.2 and 1200 N/mm.sup.2.

14. The screw of claim 13, wherein the thread includes a thread tip winding about the longitudinal center axis and having a diameter d; the core is an inner portion of the screw as seen in cross-section up to a diameter of the core dk=d/4; and the first hardness of the core is the average of the hardness within the diameter of the core dk.

15. A high-strength screw, comprising: a longitudinal center axis; a head; a threadless shank portion; a threaded portion having a thread, the thread including a thread tip winding about the longitudinal center axis and having a diameter d; an inner core as seen in cross-section of the screw, the core being an inner portion of the screw as seen in cross-section up to a diameter of the core dk=d/4, the core having a first hardness being defined as the average of the hardness within the diameter of the core dk, the first hardness being between 400 HV and 650 HV, and the core having a tensile strength of between 1400 N/mm.sup.2 and 1850 N/mm.sup.2; an outer surface layer as seen in cross-section of the screw; and an unhardening layer, the unhardening layer forming the outer surface layer in the threaded portion, the unhardening layer in its radial outmost portion having a second hardness being reduced compared to the first hardness of the core, the second hardness of the unhardening layer in its radial outmost portion being between 300 HV and 380 HV, and the unhardening layer in its radial outmost portion having a tensile strength of between 1000 N/mm.sup.2 and 1200 N/mm.sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, the invention is further explained and described with respect to preferred exemplary embodiments illustrated in the drawings.

(2) FIG. 1 illustrates a (partial) longitudinal section through a screw joint including a new high-strength screw.

(3) FIG. 2 illustrates a longitudinal section through a part of the screw.

(4) FIG. 3 illustrates the course of the hardness over the cross-section of the screw in the threaded portion.

(5) FIG. 4 illustrates the course of the hardness over the cross-section of the screw in the shank portion.

(6) FIG. 5 illustrates exemplary values of the unhardening layer associated with different screw sizes in a table.

DETAILED DESCRIPTION

(7) FIG. 1 illustrates an exemplary embodiment of a screw joint 1 including a new high-strength screw 2 as well as a first component 3 and a second component 4. The screw 2 includes a head 5, a threadless shank portion 6 and a threaded portion 7 having a thread 8 being designed as an outer thread. The first component 3 includes a bore 9 and the second component 4 includes a bore 10. The bore 10 includes an inner thread 11 corresponding to the outer thread of the screw 2. The clamping force required for attaining the secure screw joint 1 is realized by the engagement of the threads 8, 11 and the head bearing surface 12 of the head 5 of the screw 2.

(8) The screw 2 is a high-strength screw 2 having a tensile force of at least 800 N/mm.sup.2, especially an ultra-high-strength screw having a tensile strength of at least 1400 N/mm.sup.2. The screw 2 includes a bainite structure that has especially been produced by austempering and that extends substantially over the entire cross-section of the screw 2.

(9) FIG. 2 illustrates a cross-section through the high-strength screw 2 having a longitudinal center axis 13. The thread 8 furthermore includes a thread tip 14 winding about the longitudinal axis 13 in a usual way and a thread base 15 also winding about the longitudinal axis 13. The thread 8 may be a metric thread 8 or a different thread 8.

(10) The screw 2 includes a core 16 extending in the region of the longitudinal center axis 13 and the radial portion between the lines 19 and 20 surrounding it.

(11) A surface layer 17 is located in the portion of the screw 2 being distant to the core 16 in a radial direction. According to the invention, the surface layer 17 is designed as an unhardening layer 18 having a reduced hardness compared to the core 16 of the screw 2. This reduced hardness has been produced subsequently. The radial outer limit of the unhardening layer 18 is formed by the outer surface of the screw 2. The radial inner limit of the unhardening layer 18 is fluent. However, it is symbolized by a line 21.

(12) In the present example, the unhardening layer 18 does not extend along the threaded portion 7, but also along the shank portion 6. In the shank portion 6, the depth T2 of the unhardening layer 18 is located between the line 21 and the outer surface of the screw 2.

(13) In the threaded portion 7, the unhardening layer 18 also extends up to the outer surface of the screw 2. The illustrated depth T1 however only designates a part of the depth of the unhardening layer 18. The depth T1 exists along the thread tip 14 winding about the longitudinal center axis 13, and it is measured in the radial direction perpendicular to the longitudinal center axis 13 to the imaginary axial connecting line 22 between two positions of the thread base 15 being adjacent and being offset by 360.

(14) FIG. 3 illustrates the course of the hardness over the cross-section of the screw 2 in the threaded portion 7. The variables contained in FIG. 3 have the following meaning:

(15) d=outer diameter of the thread

(16) dk=diameter of the core of the screw

(17) HK=hardness of the core of the screw

(18) HR=hardness of the surface layer (unhardening layer)

(19) H=hardness difference in the core of the screw

(20) T1=(part of the) depth of the surface layer in the threaded portion

(21) FIG. 4 illustrates the course of the hardness over the cross-section of the screw in the shank portion 6. The variables in addition to the ones used in FIG. 3 have the following meaning:

(22) ds=shank diameter of the screw

(23) T2=depth of the surface layer in the shank portion

(24) FIG. 5 illustrates exemplary values of the unhardening layer 18 in case of different sizes of the screw 2 in a table. The variables being additionally used in FIG. 5 have the following meaning:

(25) P=pitch of the thread

(26) H=height of the sharp-cut imaginary profile triangle

(27) Many variations and modifications may be made to the preferred embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention, as defined by the following claims.