BE1026209B1 - PROCESS FOR REPAIRING BY DEPOSITION OF LASER METALLIC POWDER - Google Patents

Abstract

The present invention relates to a laser metal powder deposition process that can be used for the repair of molded stainless steel components used in nuclear media. Internal defects found in a component (200) during routine maintenance and refurbishment operations are identified and then excavated to form a cavity (210). The cavity is filled with several layers (3101, 3102, ..., 310N) laser metal powder deposition and its integrity is then verified.

Description

Repair method by depositing metallic powder with laser

Field of the invention

The present invention relates to a repair method by depositing metallic powder with a laser, and it relates more particularly, although not exclusively, to the repair of nuclear components made of cast stainless steel.

Context of the invention

Laser-based technologies, such as laser plating or laser metal deposition, are now an established technology for repairing parts and modifying the surface of expensive components. Thanks to continuous improvements in laser sources, process technology and strategy, the range of applications for laser deposition processes continues to expand.

Document US-B-7169242 describes a method for eliminating molding defects from an article provided with an oriented microstructure. The method includes melting a locally defined molding defect by a heat source to a depth at least equal to that of the molding defect itself, and solidifying the molten material epitaxially with respect to the oriented microstructure. surrounding the article which is substantially free from molding defects. In this process, the material itself serves to fill the molding defect when it solidifies again.

Document US-A-2016/0243650 describes a process for recovering a component made from a base alloy which can be welded without fusion to eliminate a molding defect. The method includes forming a cavity in the component at the location of the molding defect and at least partially filling the cavity with a plurality of layers each comprising multiple laser powder deposition points. Each of the multiple laser powder deposition points is formed from a filler alloy.

The documents JP2001287062, US2017034933, US2016351281 and US20160351281 disclose a method comprising the steps of identifying the presence of at least one defect in a component; excavation of a region of the component which comprises the at least one defect identified to form a

BE2018 / 5802 cavity; and depositing an additive material inside the cavity according to a predetermined pattern.

Nuclear components made of cast stainless steel are regularly inspected using non-destructive testing techniques, and internal and / or surface defects are often detected during routine inspections. At present, it is impossible to repair these internal and / or surface defects using conventional welding processes because of the complex geometry of the components and the strict manufacturing tolerances associated with nuclear stainless steel components. mold. The result is that instead of repairing a component due to the presence of internal and / or surface defects, the only option is to replace the entire component. This discarded or discarded component then forms a source of secondary radioactive waste.

There is therefore a need to be able to repair internal and / or surface defects in molded stainless steel components so that their replacement is no longer necessary in the majority of cases.

Summary of the invention

An object of the present invention is to reduce secondary radioactive waste by repairing components of the nuclear environment instead of disposing of them.

Another object of the present invention is to provide a method of repairing molded stainless steel components suitable for use in a nuclear environment.

According to a first aspect of the present invention, there is provided a method for repairing a cast stainless steel component, the method comprising the following steps:

a) identification of the presence of at least one defect in the cast stainless steel component;

b ') cleaning of the cavity;

b) excavating a region of the molded stainless steel component which includes the at least one identified defect to form a cavity;

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c) depositing a metallic powder with the laser inside the cavity according to a predetermined pattern;

d) melting and melting the metal powder deposited with the laser inside the cavity by means of a laser; and

e) finishing the surface of the filled cavity so that it matches the surface of the cast stainless steel component.

The use of a laser metal powder deposition process to fill the excavated cavity allows rigorous control of the forces and stresses of the component, which leads to a reduction in deformations and distortions in the repaired component.

Step (b ') of cleaning the cavity before step c) makes it possible to guarantee that any free materials which may be inside the cavity are removed before filling so as to prevent the appearance of other defects during the laser metal powder deposition process.

In one embodiment, steps c) and d) are repeated several times.

Each repetition of steps c) and d) forms a layer inside the cavity. This ensures that the excavated cavity is completely filled.

In one embodiment, step b) may include the use of non-destructive testing to control the excavation of the region comprising the defect.

Using non-destructive testing in this way, precise testing is obtained to ensure that the defect is included in the excavation and that no more material than necessary is excavated from the area.

In one embodiment, step e) includes a mechanical finishing step.

Thanks to mechanical finishing, the surface of the component in the excavation region can be restored quickly to almost the state it was in before excavation.

In one embodiment, another step may include the step of verifying that the repair conforms to the rest of the cast stainless steel component.

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This is very important in a nuclear environment where integrity and dimensional tolerances are of great importance.

Brief description of the drawings

To allow a better understanding of the present invention, Reference will now be made, by way of example, to the appended drawings in which:

FIG. 1 represents a flow chart of a repair process for nuclear components made of molded stainless steel;

Figure 2 is a schematic illustration of part of a component that has been excavated to eliminate a defect;

Figure 3 is a side sectional view showing an excavation in the component to be repaired;

Figure 4 is a top view of the excavation of Figure 3;

FIG. 5 is similar to FIG. 3 but shows the passes of deposition of metal powder by laser in the excavation; and Figure 6 is similar to Figure 4 but shows the laser metal powder deposition passes in the excavation.

Description of the invention

The present invention will be described with reference to particular embodiments and in the light of certain drawings, the disclosure being however not limited thereto. The drawings described are only schematic and are not limiting. In the drawings, the size of some of the elements may have been exaggerated and not drawn to scale for illustration purposes.

Non-destructive testing (NDT) techniques are used to verify the integrity of nuclear components made of cast stainless steel. TND techniques use electromagnetic radiation, sound and other signal conversions to examine a wide range of items or components in terms of integrity, composition or condition without changing the condition of the item or of the component itself. Penetrant testing can also be used. Visual inspection, enhanced by the use of magnifying glasses, cameras or other optical devices, is the most common form of NDT. However, a visual inspection is limited to the surface regions or

BE2018 / 5802 external of controlled articles or components. A radiographic check using penetrating radiation, for example X-rays, neutrons and gamma radiation can allow a volumetric examination of the internal structure of an article or component. Ultrasonic testing using ultrasonic systems can also be used. For example, ultrasonic transducers emit sound waves for volumetric examination of articles or components, and the sound waves reflected by the controlled articles or components are evaluated in terms of amplitude variations or the like. It is also possible to test articles or components made of ferrous materials by applying iron particles to them and applying a magnetic field to the article or component to identify a possible leak in the magnetic field due to an internal fault. These techniques being widely known, no further description will be given below.

The expression “metallic laser powder” used in this document designates metallic powders used for three-dimensional printing or rapid prototyping where a high power density laser is used to melt metallic powders and merge them between they. Such a process is known as "selective laser melting" (or more broadly, "selective laser sintering") where thin layers of metal powder are evenly distributed on a substrate plate which has marks along it. 'a vertical axis. Once each layer has been distributed, each two-dimensional slice of the geometry of the part is fused by selectively melting the metal powder before repeating the operation for the following layers.

A typical stainless steel alloy that can be used for "selective laser melting" is 17-4 stainless steel (also known as UNS17400), a quality of precipitation-hardened martensitic stainless steel (which is magnetic ) containing 15 to 17.5% chromium and 3 to 5% nickel (sometimes called 15-5 stainless steel). A 3 to 5% fraction of copper can also be found in the alloy. This stainless steel alloy can be heat treated to achieve high levels of strength and hardness, with higher corrosion resistance and machinability than

BE2018 / 5802 of the most common stainless steel (UNS S30400, also called 18-8 stainless steel, which contains 18-20% chromium and 8-10.5% nickel and is austenitic).

However, other stainless steel alloys, such as austenitic stainless steel type 316L (UNSS31603), can also be used, depending on the material of which the component is made. Type 316L contains between 2 and 3% molybdenum, which increases its resistance to corrosion and gives it greater resistance at high temperatures.

The expression “deposition of metallic powder by laser” used in the present document designates the deposition of at least one layer of metallic powder by laser on a substrate, followed by melting and the melting of the layer of metallic powder by laser. Generally, several layers are deposited to fill an excavated cavity, as described in more detail below.

The term "defect" used in this document refers to both internal and surface defects that may be present in a cast stainless steel component.

FIG. 1 represents the flowchart 100 of a method for detecting and repairing internal faults in molded stainless steel components and in particular in molded stainless steel components used in a nuclear medium, for example a cooling pump reactor. The first step, step 110, includes a component CND to identify possible internal faults. These internal faults may appear after initial manufacturing and / or subsequent repair. In addition, these internal faults may have developed inside the component since the last CND inspection and are identified during the following CND inspection. Depending on the type of component, several of the NDT techniques described above can be used.

Having identified an internal defect, a region comprising the defect is excavated, step 120, to form a cavity. These excavations can be carried out by machining or grinding to eliminate unacceptable defects. Generally, a set of excavations is used, for example, geometric sets, which allow the fault to be reached from the surface of the

BE2018 / 5802 component. The excavation is controlled by a CND examination which makes sure that all of the defect (s) have been eliminated during the excavation operation.

In one embodiment, described below with reference to FIGS. 2 to 6, the geometrical assembly can comprise a series of rounded oblongs or elongated circles, that is to say circles which have been divided into two along a diameter and elongated to include a rectangle between the two halves of the divided circle of different sizes. This series of rounded oblongs or elongated circles can form a stage whose base has the smallest dimensions and whose upper surface has the largest dimensions with a gradual change of dimensions from the base to the upper surface to form a graduated or inclined wall. A typical stage can be circular or elliptical (or made up of elongated circles as described above).

The excavated region or cavity is cleaned, step 130, to ensure that any foreign matter likely to have entered the excavated region or cavity is removed before depositing several layers of metallic powder deposit with the laser, step 140, in the region or the excavated cavity. The laser metal powder deposition step deposits a 316L stainless steel metal powder as described above in the region or the excavated cavity according to specific paths or patterns and the powder deposited is then heated and melted by a laser in a neutral gas atmosphere to fill the excavated region or cavity. It normally takes several passes to ensure that the excavated region or cavity is completely filled with the filler material, that is, the molten powder, by the accumulation of several layers. Visual monitoring is used during laser metal deposition to ensure that the cavity is properly filled.

Naturally, the laser metal powder used is chosen so as to correspond to the material from which the initial molded component was made.

Once the excavated region or cavity has been filled by depositing several layers of metallic powder with the laser, a finish of the surface of the component is implemented, step 150, so that it conforms to the

BE2018 / 5802 rest of the component. The finishing step includes mechanical finishing, for example polishing or grinding, to meet the required surface criteria.

After finishing, a verification again by CND, step 160, of the region where the initial fault has been detected is implemented. This ensures that the repair meets the criteria with dimensional and geometric controls intended to guarantee the maintenance of strict tolerances for the particular component.

The process can then be repeated for other defects identified in the article or component.

Figure 2 shows a plan view of a portion of an article or component 200 in which an excavation region or cavity 210 has been created to eliminate a defect. Here, the excavation region or cavity 210 on the surface of the component can be considered to be an elongated circle as described above. However, other suitable profiles can be used for regions or excavation cavities.

Figure 3 shows a sectional side view of the portion of the article or component 200 showing the region or the excavation cavity 210 in more detail. Here, the excavation region or cavity 210 has a base part 220 which extends to a surface part 230 by means of an inclined wall part 240.

In Figure 4, the component 200 and the region or the excavation cavity are shown in a plan view. As seen, the base part 220 also comprises an elongated circle but whose dimensions are smaller than those of the elongated circle forming the surface part 230, and is located in the center inside the surface part 230 and connected to this by the inclined or sloping wall part 240. Although, in this example, the base part 220 is represented as located in the center inside the surface part 230, the presence of this central location n is not essential and there may be a certain offset between the base part and the surface part.

FIG. 5 is similar to FIG. 3 but also represents a number of laser metal powder deposition passes 3001, 3002, 3003, ...,

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300n, where N is represented as being 4 in this specific example, on the base part 220 and the wall part inclined or sloping 240 to the surface part 230. Naturally, N can be any suitable value , and several layers of passes are necessary to fill the region or the excavation cavity 210. Each pass follows a path which is similar to the shape of the base portion 220 of the cavity 210, that is to say the '' one of a series of rounded oblongs or elongated circles which jointly fill the cavity 210.

Figure 6 is similar to Figure 4 but shows a number of laser metal powder deposition passes 300i, 3002, 3003, ..., 300n, in the region or the excavated cavity 210. Again, N is shown as being equal to 4 but can be any suitable value necessary to fill the region or the excavation cavity 210.

To perform the steps or passes of depositing metal powder with the laser, the minimum laser speed required is at least 1000 mm / min (or around 16.5 mm / s). The time taken to fill a cavity 20 mm long by 10 mm wide and 5 mm deep is between 1 and 4 minutes with additional time necessary for the preparation of the filling, the positioning of the laser relative to the component to be repair, programming of the laser and establishment of a neutral gaseous medium in which the deposition of metal powder by laser is carried out, etc.

The laser can be used vertically, that is to say perpendicular to the surface of the component to be repaired, but other angles are also possible in combination with the direction of injection of the metal powder with the laser.

An example where the method of the present invention can be used is that of the refurbishment of cooling pumps for nuclear reactors. The refurbishment process includes decontamination, disassembly and inspection, followed by repair of the components, or their replacement if necessary, then reassembly. Depending on the results of the inspection, some components may turn out to have defects that require repair, usually using a welding process.

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However, in a nuclear environment, the main risk involved in repairing a component by means of a welding process is deformation of the component to be repaired. As the tolerances on the components used in the nuclear environment are strict, welding can produce a deformed component which exhibits localized deformation due to local heating at the welding site. This localized deformation can lead to incompatible deformations. The result of these incompatible deformations is that the component can no longer be used without undergoing other modifications and that it is often discarded. In addition, since a welding operation generally requires a large volume of material, machining after welding is compulsory for the restoration of an acceptable surface condition and dimensions of the repaired component.

Laser metal powder deposition for repair of RCP components is advantageous over conventional welding repair since it allows the deposition parameters to be closely monitored or controlled to ensure quality is maintained. As the area affected by heat (HAZ) is limited, the forces and stresses exerted on the component during deposition of metal powder with the laser are much lower than those exerted during conventional welding processes. In addition, thanks to the limitation of forces and stresses, the component to be repaired does not deform as much as during conventional welding processes. In addition, the amount of post-machining required after depositing metal powder with the laser is less than that required after welding. By repairing a component by depositing metal powder with a laser instead of disposing of it, contaminated waste is reduced.

Thanks to the possibility of repairing nuclear components in cast stainless steel, there is an increased availability of components since it is no longer necessary to replace the component, which avoids long delays in manufacturing a replacement component.

Because the component is repaired instead of being scrapped, secondary radioactive waste is greatly reduced. In addition, the management of spare parts is improved.

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Other components used in the nuclear environment and which can be repaired by the process of the present invention include, but are not limited to, diffusers, valves and shafts.

Claims (6)

claims 1. Method for repairing a nuclear component (200) made of cast stainless steel, the method comprising the following steps: a) identification of the presence of at least one defect in the nuclear component (200) of cast stainless steel, characterized in that the method further comprises the following steps: b) excavating a region of the molded stainless steel nuclear component (200) which includes the at least one defect identified to form a cavity (210); b ') cleaning the cavity (210) c) depositing a metallic powder by laser (3001, 3002, 3003 ..., 300N) inside the cavity (210) according to a predetermined pattern: d) melting and melting of the powder deposited inside the cavity (210) by means of a laser; and e) finishing the surface of the filled cavity so that it corresponds to the surface of the cast stainless steel (200) nuclear component. 2. The method of claim 1, further comprising repeating steps c) and d) several times. 3. The method of claim 2, wherein each repetition of steps c) and d) forms a metal layer inside the cavity (210). 4. Method according to any one of claims 1 to 3, in which step b) comprises the use of a non-destructive test to control the excavation of the region comprising the defect. 5. Method according to any one of claims 1 to 4, wherein step e) comprises a mechanical finishing step. 6. Method according to any one of claims 1 to 5, further comprising the step of verifying the conformity of the repair with the rest of the nuclear component (200) of cast stainless steel.