Hardening of a zinc coated screw body

Abstract

A method for processing a screw body, including a screw body providing step in which a screw body made of a hardenable steel is provided, a primary coating step, in which the screw body is coated with a primary coating that includes zinc, wherein the primary coating step follows the screw body providing step, and a tip hardening step, in which a tip region of the screw body, which is coated with the primary coating, is selectively heated, at least in part and with the ad-mission of oxygen, to a temperature required for hardening, characterized in that the tip hardening step follows the primary coating step. A screw that can be obtained using this method is also provided.

Claims

1-12. (canceled)

13: A method for processing a screw body, the method comprising providing, in a screw body providing step, a screw body made of a hardenable steel; coating, in a primary coating step, the screw body with a primary coating including zinc, the primary coating step following the screw body providing step; and selectively heating, in a tip hardening step, a tip region of the screw body coated with the primary coating, at least in part and with the admission of oxygen, to a temperature required for hardening, the tip hardening step following the primary coating step.

14: The method as recited in claim 13 wherein no additional hardening step is performed on the screw body between the screw body providing step and the primary coating step.

15: The method as recited in claim 13 further comprising case hardening, in a case hardening step, the screw body, the case hardening step following the screw body providing step, the primary coating step following the case hardening step.

16: The method as recited in claim 15 wherein the case hardening step includes carburizing the screw body.

17: The method as recited in claim 13 wherein the screw body that is provided in the screw body providing step consists of a hardenable carbon steel with a carbon content in the range of from 0.1 wt. % to 0.5 wt. %.

18: The method as recited in claim 13 wherein the primary coating applied in the primary coating step has a thickness in the range from 20 ?m to 100 ?m.

19: The method as recited in claim 18 wherein the thickness is in the range from 30 ?m to 80 ?m.

20: The method as recited in claim 13 wherein in the primary coating step, the screw body is coated with the primary coating via hot dip galvanizing.

21: The method as recited in claim 13 wherein the primary coating step includes a spinning process of the screw body to remove excessive coating material.

22: The method as recited in claim 21 wherein the spinning process removes the excessive coating material from threaded regions of the screw body.

23: The method as recited in claim 13 wherein the tip hardening step includes inductively heating the tip region of the screw body.

24: The method as recited in claim 13 wherein further comprising, in a top coating step, coating the screw body with a top coating, the top coating step following the tip hardening step.

25: The method as recited in claim 13 wherein a ratio of a maximum outer thread diameter of a thread of the screw body to a pitch of the thread is in the range of from 1 to 2 at least in some regions of the thread.

26: The method as recited in claim 25 wherein the ratio is in the range from 1.2 to 1.6.

27: A screw comprising: a steel screw body; and a primary coating covering the screw body, the screw body having a tip region, the screw body having a higher surface hardness in the tip region as compared to regions of the screw body distant from the tip region, the primary coating including Zn phases and ZnFe phases, an average Fe content of the primary coating being higher at the tip region as compared to the regions of the screw body distant from the tip region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The invention is explained in greater detail below with reference to preferred exemplary embodiments, which are depicted schematically in the accompanying drawings. Individual features of the exemplary embodiments presented below can be implemented either individually or in any combination within the scope of the present invention.

[0049] FIG. 1 is a process chart of an exemplary method for processing a screw body.

[0050] FIG. 2 is a schematic longitudinally sectional view of an exemplary screw body that may be provided in the screw body providing step of the method of FIG. 1.

[0051] FIG. 3 is a schematic longitudinally sectional view of an exemplary coated screw body that may be obtained in the primary coating step (and before the tip hardening step) of the method of FIG. 1.

[0052] FIG. 4 is a schematic longitudinally sectional view of an exemplary coated screw body that may be obtained in the tip hardening step (and before the top coating step) of the method of FIG. 1.

DETAILED DESCRIPTION

[0053] A process chart of an exemplary method for processing a screw body is shown in FIG. 1.

[0054] The process sequence starts with a screw body providing step 1, in which a screw body made of a hardenable steel, in particular carbon steel (i.e. a steel with a carbon content that ranges from about 0.05% up to about 2.1% by weight), is provided. Preferably, the carbon content ranges from 0.1 wt. % to 0.5 wt. %. As shown, by way of example, in FIG. 2, the screw body includes a shank 21 and at least one generally helical screw thread 22 provided thereon. The screw body has a tip region 23, which is that region of the shank 21 that is intended to be first inserted into a substrate when the screw derived from the screw body is installed as intended. Optionally, the screw body might have a head 25, that is arranged on that end of the screw body that is opposite to the tip region 23, or the screw body might have another drive structure for imparting torque onto the shank 21. Again optionally, the screw body might have at least one cutting edge provided within the shank 21, in particular in the tip region 23.

[0055] The screw body might be obtained, for example, by performing forming operations, such as thread rolling and/or head upsetting, on a piece of wire. These operations could be considered to be part of the screw body providing step 1, or they can be considered to be part of a separate manufacturing step, which precedes the screw body providing step 1.

[0056] The screw body providing step is followed by an optional case hardening step 2. In the present embodiment, the case hardening step 2 is a carburizing step, which includes heating the screw body in an atmosphere containing carbon.

[0057] The screw body providing step 1 and, if applicable, the case hardening step 2, is followed by a primary coating step 3, in which the screw body is coated with a primary coating 30 that comprises zinc. In particular, the primary coating step 3 includes hot dip galvanizing to apply the primary coating 30 to the screw body (see FIG. 3). Preferably, the primary coating step 3 can also include a spinning process, in which the screw body is spun in order to remove an excess of the primary coating 30. The hot dip galvanizing is preferably carried out with the process parameters, in particular temperatures, defined in ISO 10684. The structure resulting from the primary coating step 3 is schematically shown in FIG. 3.

[0058] Note that the case hardening step 2 usually needs to be performed before the primary coating step 3, since the primary coating 30 usually would not withstand the high process temperature of a typical case hardening step 2.

[0059] The primary coating step 3 is followed by a tip hardening step 4. In the tip hardening step 4, the tip region 23 of the screw body is selectively hardened by heating, preferably by inductive heating, whereas the rest of the screw body is left generally unaltered. This step causes a local increase of oxidization of the zinc of the primary coating 30, but as explained above, this can be tolerated. As a consequence, the primary coating 30 has an oxidized first zone 31 (see FIG. 4), located in the tip region 23, where the percentage of zinc oxide in the primary coating 30 is higher as compared to a second zone 32 of the primary coating 30 that is located remote from the tip region 23, in particular rearwardly of the tip region 23. Alternatively or additionally, the average Fe content of the primary coating 30 (the primary coating 30 comprises Zn phases and ZnFe phases) is higher in the first zone 31, located in the tip region 23, as compared to the second zone 32 that is located remote from the tip region 23. In the tip region 32, the screw body has higher surface hardness as compared to regions of the screw body that are distant from the tip region, in particular rearwardly thereof. The resulting structure is shown in FIG. 4.

[0060] The structure shown in FIG. 4 can be considered to be a screw that is ready for use.

[0061] Optionally, the tip hardening step 4 might be followed by a cleaning step 5. In this cleaning step, loose zinc oxide (ZnO) and smoke residue can be removed from the screw body of the screw.

[0062] Optionally, the tip hardening step 4 and the optional cleaning step 5 might be followed by a top coating step 6, in which the screw body of the screw is additionally coated with a topcoat.