METHOD FOR PRODUCING A RETORT FOR A NITRIDING FURNACE AND RETORT

Abstract

A method for producing a retort for a nitriding furnace, in which metallic workpieces are heat-treated in a pre-determined atmosphere, includes pickling at least the surfaces of the retort, which are configured to come into contact with the pre-determined atmosphere while the nitriding furnace is operating, by using a pickling agent. The pickled surfaces may then be electropolished and passivated. A retort may be produced according to this method and the retort may be used in a nitriding furnace.

Claims

1. A method for producing a retort for a nitriding furnace, in which metallic workpieces are heat-treated in a pre-determined atmosphere, wherein the retort is produced from a stainless steel, comprising: pickling at least the surfaces of the retort, which are configured to come into contact with the pre-determined atmosphere while the nitriding furnace is operating, by using a pickling agent.

2. The method according to claim 1, wherein the pickling agent comprises nitric acid and hydrofluoric acid.

3. The method according to claim 1, further comprising, after the pickling, polishing the surfaces by using an electrolyte in a galvanic bath.

4. The method according to claim 3, wherein the electrolyte comprises phosphoric acid and sulfuric acid.

5. The method according to claim 3, further comprising passivating the polished surfaces with nitric acid.

6. The method according to claim 1, wherein the retort is cylindrical shaped, and the retort is made from metal sheet cut-outs, which are welded together.

7. A retort for a nitriding furnace, in which metallic workpieces are heat-treated in a pre-determined atmosphere, wherein the retort is made from a stainless steel, and at least the surfaces of the retort, which are configured to come into contact with a pre-determined atmosphere while the nitriding furnace is operating, have been pickled by using a pickling agent.

8. The retort according to claim 7, wherein the retort is made from metal sheet cut-outs, which are welded together.

9. The retort according to claim 7, wherein the retort is cylindrical shaped.

10. The retort according to claim 7, wherein the retort concentrically encompasses a guide cylinder configured to hold the workpieces during the heat treatment.

11. The retort according to claim 7, wherein the pickled surfaces of the retort have been electropolished.

12. The retort according to claim 11, wherein the pickled and electropolished surfaces of the retort have been passivated with nitric acid.

13. A nitriding furnace, in which metallic workpieces are heat-treated in a pre-determined atmosphere, comprising the retort according to claim 7.

14. The retort according to claim 12, wherein: the retort is made from metal sheet cut-outs, which are welded together; the retort is cylindrical shaped; and the retort concentrically encompasses a guide cylinder configured to hold the workpieces during the heat treatment.

15. The method according to claim 5, wherein the pickling agent comprises nitric acid and hydrofluoric acid.

16. The method according to claim 15, wherein the electrolyte comprises phosphoric acid and sulfuric acid.

17. The method according to claim 16, wherein the retort is cylindrical shaped and the retort is made from metal sheet cut-outs, which are welded together.

18. The method according to claim 17, wherein inner surfaces of the retort are pickled, electropolished and passivated.

19. The method according to claim 18, wherein the pre-determined atmosphere contains ammonia.

20. The method according to claim 1, wherein the pre-determined atmosphere contains ammonia.

Description

[0019] In the following, the invention will be described in more detail on the basis of a preferred embodiment.

[0020] A retort is made for an evacuable nitriding furnace. First, metal sheet cut-outs are made from rolled stainless steel sheet, an essentially cylindrically shaped retort is formed and the metal sheet cut-outs are then welded together. The retort comprises a known guide cylinder made from stainless steel. The guide cylinder serves to accommodate or store the workpieces during the nitriding process.

[0021] The inner surfaces of the retort and the guide cylinder are pickled. For this, a mixture of nitric acid and hydrofluoric acid is used. The surfaces treated by using the pickling agent then exhibit only a low catalytic activity. The build-up of a high nitriding potential during the nitriding in the retort is thereby possible with a low ammonia quantity.

[0022] After the pickling, the pickled surfaces are electropolished in a galvanic bath composed of phosphoric acid and sulfuric acid. Thereafter, the pickled surfaces are passivated with nitric acid. The electropolishing leads to a reduction of the surface roughness, i.e., in total to a smaller overall surface. In this way the adsorption of ammonia on the surface as a pre-stage to the dissociation is reduced. Also advantageous is the higher resistance to corrosion. The service life of the retort thereby significantly extends.