METHOD FOR PRODUCING A SCREW, AND SCREW

Abstract

The invention relates to a method for producing a screw, having the following steps: (a) rolling a screw wire made of low-alloy carbon steel to produce screw (10) having a thread; (b) heating the entire screw (10) to an austenitizing temperature under a carbon atmosphere and/or nitrogen atmosphere and maintaining the temperature; (c) quenching the entire screw (10) to a bainitizing temperature and maintaining the bainitizing temperature until the screw has a bainitic structure over its cross-section. The invention is characterized in that the screw (10) is subsequently hardened locally at its tip (22), by the tip (22) being heated to an austenitizing temperature and the screw (10) being subsequently quenched to a temperature below the martensite starting temperature (MS).

Claims

1. A method for producing a screw, comprising the following steps: rolling a screw wire made of low-alloy carbon steel to produce a screw having a thread; heating the entire screw to an austenitizing temperature under a carbon atmosphere and/or nitrogen atmosphere and maintaining the temperature; quenching the entire screw to a bainitizing temperature and maintaining the bainitizing temperature until the screw has a bainitic structure over a cross-section thereof, wherein the screw is subsequently hardened locally at a tip thereof, the tip being heated to an austenitizing temperature and the screw being subsequently quenched to a temperature below a martensite starting temperature Ms.

2. A method according to claim 1, wherein the carbon atmosphere has a higher carbon content than the screw and/or that the nitrogen atmosphere has a higher nitrogen content than the screw.

3. A method according to claim 1, wherein the screw is kept at an austenitizing temperature in the carbon atmosphere until an edge zone of the screw has a carbon content which is at least 0.2% higher than a carbon content of a core thereof.

4. A method according to claim 1, wherein the screw is tempered.

5. A screw comprising a thread-bearing shaft and a tip, the shaft having a substantially bainitic structure in a core thereof, wherein the tip substantially consists of a hardened martensitic structure, at least in an edge zone thereof.

6. A screw according to claim 5, wherein in the edge zone, the screw has a structure with a higher carbon content than that of a core thereof.

7. A screw according to claim 5, characterized in that in its core, the shaft has a substantially tempered bainitic structure, and in its edge zone, the shaft has a tempered structure with a higher carbon content than that of the core, with the tip having a tempered hardened martensitic structure, at least in its edge zone.

8. A screw according to claim 5, wherein the screw is produced using a method comprising the steps of rolling a screw wire made of low-alloy carbon steel to produce a screw having a thread; heating the entire screw to an austenitizing temperature under a carbon atmosphere and/or nitrogen atmosphere and maintaining the temperature; quenching the entire screw to a bainitizing temperature and maintaining the bainitizing temperature until the screw has a bainitic structure over a cross-section thereof, wherein the screw is subsequently hardened locally at a tip thereof, the tip being heated to an austenitizing temperature and the screw being subsequently quenched to a temperature below a martensite starting temperature Ms.

9. A screw according to claim 8, wherein the carbon atmosphere has a higher carbon content than the screw and/or that the nitrogen atmosphere has a higher nitrogen content than the screw.

10. A screw according to claim 8, wherein the screw is kept at an austenitizing temperature in the carbon atmosphere until an edge zone of the screw has a carbon content which is at least 0.2% higher than a carbon content of a core thereof.

11. A screw according to claim 8, wherein the screw is tempered.

Description

[0025] In the drawings,

[0026] FIG. 1 is a schematic sectional view of a low-alloy carbon steel screw manufactured by rolling the screw thread onto the shaft;

[0027] FIG. 2 is a schematic sectional view of the screw after case-hardening bainitizing;

[0028] FIG. 3 is a schematic sectional view of the screw after local case hardening of the tip;

[0029] FIG. 4 is a schematic temperature-time diagram of the method according to the invention for producing the screw and case hardening of the tip.

[0030] FIG. 1 is a schematic sectional view of a rolled screw 10 made of conventional screw steel after a process step. The screw has a shaft 20 comprising the screw head and a free screw end which is referred to here as the tip 22 and is located opposite the head in the axial direction. The screw according to the invention is manufactured by forming, in a process step, the screw from a screw wire of low-alloy carbon steel having an alloy content of less than 3% of alloying elements. Manufacture of the screw in particular involves rolling the thread onto the screw. Preferably, 23MnB4 is used as the material for the screw wire. This kind of steel can be processed well in a rolling process.

[0031] FIG. 2 is a schematic sectional view of the screw 10.

[0032] To achieve the state illustrated in FIG. 2, the screw 10 was heated to an austenitizing temperature in a carbon atmosphere having a higher carbon content than the screw 10 itself and exposed to this carbon atmosphere until a carbon content was reached in the edge zone 12 of the shaft and in the edge zone 18 of the tip of the screw 10 which is at least 0.2% higher than that in the core of the screw, and which in particular is between 0.6% and 1.5%. In the present view, edge zone 12 and edge zone 18 are only schematically illustrated and may vary in depth. As an alternative or in addition to carburizing, nitriding can also take place analogously.

[0033] After reaching the desired carbon saturation in edge zone 12 and edge zone 18, the screw 10 is quenched, in particular in a molten salt bath, to a bainitizing temperature, which bainitizing temperature is above the martensite starting temperature Ms. The screw is kept at bainitizing temperature until its shaft substantially has a bainitic structure 14 over its cross-sectional area. The screw 10, according to FIG. 2, has a higher carbon content in edge layer 12 and edge layer 18 than the core. A substantially bainitic structure is defined as at least 80% of the cross-sectional area having a bainitic structure. Other structures may also be present in some cases.

[0034] FIG. 3 is a schematic sectional view of a screw 10 according to the invention, in which the tip 22 of the screw has been heated again locally to an austenitizing temperature and then cooled down to a temperature below the martensite starting temperature Ms to form martensite, so that a hardened martensitic microstructure 16 with a carbon content of about 1% is present in the tip 22, in particular in the edge layer 18. This results in the creation of an ultra-hard tip.

[0035] FIG. 4 is a schematic temperature-time diagram of the manufacturing method according to the invention. The screw is first heated to an austenitizing temperature that is higher than the A.sub.3 temperature. If the wire material does not have a sufficient carbon content of between 0.6% and 1.5%, such heating can be performed in a carbon enriching atmosphere. The carbon content of the atmosphere has a higher carbon concentration than the wire material, so that carbon will diffuse from the carbon atmosphere into the edge zone of the screw during heating.

[0036] The screw is then quenched to a bainitizing temperature. The bainitizing temperature is the temperature at which the wire material is in the bainite phase field of its time-temperature diagram. The quenching time is selected to prevent both ferrite and pearlite formation during the quenching process. The screw is held at the bainitizing temperature until substantial portions of the cross-section of the screw exhibit a bainite structure. The screw is then cooled down to room temperature.

[0037] After the screw manufactured in this way has been cooled down to room temperature RT, its tip is locally reheated to an austenitizing temperature and then quenched again to below the martensite starting temperature Ms so that a martensitic structure is formed at least in the edge zone of the tip.