Method for treatng a nitrided/nitrocarburised part

Abstract

In a method of treating a nitrided/nitrocarburized workpiece, at least a portion of the workpiece is subjected to a first step in which at least one laser beam is moved in at least one pass over the portion, until the surface layer of the portion is transformed in part or in full, and until the distribution of the nitrogen concentration in the diffusion zone is modified. In a second step at least one laser beam is moved in at least one pass over said portion so as to enable the nitrogen concentration in the underlying diffusion layer to be reduced.

Claims

1. A method of treating a nitrided or nitrocarburized workpiece, comprising: subjecting at least a portion of the workpiece to a first step in which at least one laser beam is moved in at least one pass over said at least a portion, until a surface layer of the at least a portion is transformed such that a distribution of nitrogen concentration in an underlying diffusion zone is modified; and subjecting the at least a portion to a second step in which at least one laser beam is moved in at least one pass over said at least a portion, thereby reducing the nitrogen concentration in the underlying diffusion zone.

2. The method according to claim 1, wherein, for the first step, a plurality of passes are made with an inter-line space between the passes that lies in the range 0.01 mm to 0.05 mm.

3. The method according to claim 2, wherein the inter-line space is 0.02 mm.

4. The method according to claim 1, wherein, for the first step, a 20-W fiber and pulse laser is used that is set at 20 kHz and at 50% of its power, defocused by 5 mm and subjected to a speed of advance of 300 mm/second.

5. The method according to claim 1, wherein, for the second step, a 20-W fiber and pulse laser is used that is set at 200 kHz and at 100% of its power, defocused by 5 mm and subjected to a speed of advance in the range 1 mm/second to 10 mm/second.

6. The method according to claim 1, wherein, for the second step, only a single pass is performed.

7. A method of welding a nitrided or nitrocarburized workpiece, comprising: subjecting at least a portion of the workpiece to a first step in which at least one laser beam is moved in at least one pass over said at least a portion, until a surface layer of the at least a portion is transformed such that a distribution of nitrogen concentration in an underlying diffusion zone is modified; subjecting the at least a portion to a second step in which at least one laser beam is moved in at least one pass over said at least a portion, thereby reducing the nitrogen concentration in the underlying diffusion zone; and forming a weld at the treated at least a portion of the workpiece.

8. The method according to claim 7, wherein the weld is formed by Tungsten Inert Gas welding.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) The invention is described in more detail below with reference to the figures of the accompanying drawings, in which:

(2) FIG. 1 shows a sample of a workpiece having its top face nitrided and having a zone (A) treated by implementing the first step of the method of the invention, and a zone (B) treated by implementing the first and second steps of the method of the invention;

(3) FIGS. 2, 2A, and 2B give curves showing nitrogen percentage relative to depth at the nitrided zone of the workpiece (FIG. 2), of the zone (A) treated by implementing the first step (FIG. 2A), and of the zone (B) treated by implementing both of the steps of the method (FIG. 2B);

(4) FIGS. 3 and 4 show the appearance of a TIG weld, one for a nitrided surface (FIG. 3) and the other for a nitrided surface having a zone that has been treated by implementing the first step of the method (FIG. 4);

(5) FIGS. 5 and 6 are endoscopic views showing a TIG weld, one for a nitrided surface (FIG. 5) and the other for a nitrided surface having a zone that has been treated by implementing the first step of the method (FIG. 6); and

(6) FIGS. 7 and 8 are endoscopic views showing a laser weld, one for a nitrided surface (FIG. 7) and the other for a nitrided surface having a zone that has been treated by implementing the first step of the method (FIG. 8).

DETAILED DESCRIPTION

(7) FIG. 1 shows a non-limiting example of a workpiece designated overall by reference 1, and having a nitrided face (1a). On the nitrided face (1a), a zone (A) has been treated by implementing a first step of the method of the invention and a zone (B) has been treated by implementing the first step and the second step of the treatment method. A weld bead (2) has been formed over the entire length of the workpiece (1) both at the nitrided zone (1a) and at the zones treated by implementing the method of the invention in one or two steps, characterized respectively by zone A and by zone B.

(8) It is recalled that an object of the invention is to modify the structure of the workpiece (1) by transforming the surface layer in order to enable any type of welding to be performed at the zone treated in this way. Thus, a zone (A) is subjected to a first step in which at least one laser beam is moved in at least one pass until the surface layer is completely removed from said zone of the nitrided face (1a). Advantageous results were obtained when a plurality of passes were made with an inter-line space between the passes that lay in the range 0.01 mm to 0.05 mm, and preferably with an inter-line space of 0.02 mm. This method is implemented by means of a 20-W fiber and pulse laser set at about 20 kHz and at 50% of its power, defocused by 5 mm and subjected to a speed of advance of about 300 mm/s.

(9) For TIG welding, reference is made to the curves of FIGS. 2, 2A, and 2B and to the metallographic sections of FIGS. 5 and 6. FIG. 6 shows a TIG weld at a zone (A) treated by implementing the first step of the treatment method, to be compared with FIG. 5 that shows a TIG weld at the non-treated nitrided surface. For laser welding, reference is made to the curves of FIGS. 2, 2A, and 2B and to the metallographic sections of FIGS. 7 and 8. FIG. 8 shows a laser weld at a zone (A) treated by implementing the first step of the treatment method, to be compared with FIG. 7 that shows a laser weld at the non-treated nitrided surface.

(10) A zone (B) of the workpiece (1) is previously treated by implementing the first step of the method of the invention as indicated above.

(11) This zone is then subjected to a second step in which the laser beam is moved in at least one pass so as to make it possible to modify the distribution of the nitrogen concentration at said zone in the underlying diffusion layer. For this second step, the same 20-W fiber and pulse laser is used with different settings.

(12) Thus, the laser is set at about 200 kHz and at 100% of its power defocused by 5 mm and subjected to a speed of advance lying in the range 1 mm/s to 10 mm/s. This second step is performed in a single pass.

(13) Reference is made to the curve in FIG. 2B that shows the additional reduction in the nitrogen content between the two steps of the method. This significant loss of nitrogen is difficult to achieve with the settings of the first step.

(14) These various tests show that the weld over a nitrided surface generated a non-uniform disturbed appearance having surface holes (FIG. 3), which were through blow holes (FIG. 5).

(15) These metallographic sections show that the invention gives good-performance results for TIG welding, and also, to a lesser extent, for laser welding.

(16) By associating a second step with the method, it was also observed that the distribution of the nitrogen concentration was modified. This modification was characterized by a reduction in the nitrogen content in the underlying diffusion layer.

(17) It results from the characteristics of the method of the invention that the treatment modifies the structure of the workpiece by transforming the surface layer and making it possible, as a result, to weld a nitrided workpiece.

(18) This laser operation is relatively inexpensive because a single operation suffices and makes it possible to treat all types of workpieces with very high accuracy, including specific zones or complex shapes.

(19) To sum up, and considering the fact that nitriding is not compatible with a welding operation, the treatment method makes it possible, in a first step, to transform the surface layer and to modify the distribution of the nitrogen concentration in the diffusion zone. Good-performance results were achieved not only for TIG welding but also for laser welding. This first step is sufficient to make the zone compatible with welding.

(20) In a second step of the treatment method, it is possible to modify the distribution of the nitrogen concentration while reducing the nitrogen content in the diffusion layer. Surprisingly and unexpectedly, it appeared that a reduction in the nitrogen concentration in the diffusion zone gave a reverse result, namely that the welding was of poorer quality. This reduction in the nitrogen content in the diffusion layer may be important for certain applications, e.g. for reducing the fragility of workpieces.