GB2546098B

GB2546098B - Methods of electron beam welding

Info

Publication number: GB2546098B
Application number: GB1600334.5A
Authority: GB
Inventors: P Lawler Steven; Clark Daniel
Original assignee: Rolls Royce Power Engineering PLC; Rolls Royce PLC
Current assignee: Rolls Royce Power Engineering PLC; Rolls Royce PLC
Priority date: 2016-01-08
Filing date: 2016-01-08
Publication date: 2019-06-05
Anticipated expiration: 2036-01-08
Also published as: GB201600334D0; GB2546098A

Description

METHODS OF ELECTRON BEAM WELDING

TECHNICAL FIELD

The present disclosure concerns a method of welding and/or a method of manufacturing a pressure vessel for a nuclear power plant.

BACKGROUND

Nuclear pressure vessels (e.g. reactor vessels) are generally large components, for example a typical pressure vessel would have a diameter in the region of at least 2 metres. The vessels are thick-walled components, e.g. greater than or equal to 200 mm thick, so as to meet process and regulatory requirements.

Nuclear pressure vessels (e.g. reactor vessels) are usually fabricated from a plurality of parts, e.g. multiple cylindrical central parts and two domed ends. Welding is used to join the parts of the vessel together. Due to the operating conditions of nuclear pressure vessels, and regulatory requirements, the welded joints need to have high integrity. US5491316 proposes using electron beam welding to weld the parts of the vessel together. Due to the size of the vessels, a local low pressure chamber is provided in the region of a joint between two parts, and an electron beam gun is carried by the chamber. The electron beam gun is moved relatively around the joint region to weld the entire joint.

Electron beam welding is advantageous over other types of welding because it does not require a filler material, so the chemical composition of the material of the weld is the material of the parent components. Further advantageously, electron beam welding offers the potential to weld thick sections of a pressure vessel in a single pass. Traditional welding processes in the nuclear industry require multiple passes of weld, requiring inter-stage examination and warming of the component prior to each weld pass. Conventional welding processes metal can cause the grooves to progressively contract, which complicates the welding process.

The high integrity required from nuclear pressure vessels means that if electron beam welding is to be used, a method of welding is required that ensures that the vessel is welded all the way along the joint both in terms of length and thickness.

SUMMARY

According to a first aspect there is provided a method of electron beam welding two components together along a joint, the method comprising: directing an electron beam at the joint and moving the electron beam relative to the joint sequentially along a plurality of oscillating paths that traverse the joint, each of the oscillating paths extending the length of the joint and being out of phase from one another, each of the oscillating paths being defined by a similar function and having the same amplitude and wavelength, wherein Vhe wavelength of the oscillating path greater than the diameter of an instantaneous weld pool formed when the electron beam contacts the components.

For example, the wavelength may be greater than or equal to twice the diameter of the instantaneous weld pool, or greater than or equal to three times the diameter of the instantaneous weld pool. A difference in phase between the oscillating paths may be selected to be less than or equal to a diameter of an instantaneous weld pool to enable the entire joint to be welded.

The oscillating path may be defined by a harmonic function that oscillates finitely. Alternatively, although in some examples less desirably the oscillating path may be defined by a square wave.

The method may comprise tilting the electron beam as it is moved relatively

The method may comprise providing a chamber around the joint, reducing pressure in the chamber, and mounting an electron beam gun to the chamber, and welding the components together using local vacuum electron beam welding.

The two components may be parts of a pressure vessel for a nuclear power plant, e.g. a reactor vessel.

According to a second aspect there is provided a method of manufacturing a pressure vessel (e.g. a reactor vessel) for a nuclear power plant, the method comprising providing a plurality of sections of the vessel and welding said sections together using the method according to the first aspect.

The method of the second aspect may comprise one or more optional features of the first aspect.

The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein.

DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with reference to the Figures, in which:

Figure 1 is a schematic of a nuclear power plant;

Figure 2 is a schematic sectional view of a reactor vessel of the power plant of Figure 1;

Figure 3 is a schematic of equipment used to weld the reactor vessel of Figure 2;

Figure 4 illustrates a method of welding the reactor vessel of Figure 2;

Figure 5 is a schematic of a path of an electron beam along a joint (not claimed);

Figure 6 is a schematic of a path of an electron beam along a joint; and Figures 7A to 7C are schematics of alternative paths of an electron beam along a joint.

DETAILED DESCRIPTION

Referring to Figure 1, a nuclear power plant is indicated generally at 10. The plant includes a nuclear reactor 12, a primary circuit 14, a heat exchanger 16, a secondary circuit 18 and a turbine 20. The primary fluid in the primary circuit is heated by the nuclear reactor. The primary fluid then flows to the heat exchanger, where it heats secondary fluid in the secondary circuit. The heated secondary fluid is then used to drive the turbine 20.

Referring to Figure 2, a pressure vessel for use in the nuclear reactor 12 or heat exchanger 16 is indicated generally at 22. The vessel 22 is fabricated from multiple parts individually manufactured; including a plurality of cylindrical sections 24, and two domes 26, one dome being provided at each longitudinal end of the vessel. The thickness of the cylindrical sections and the dome sections is greater than 200mm, for example 350mm.

The parts of the vessel 22, e.g. two of the cylindrical sections 24, may be joined together by welding, in this example by electron beam welding. Referring to Figure 3, to weld the sections 24 together a chamber 30 is positioned around a joint 28 between the sections 24. The chamber 30 is then evacuated to create a local vacuum (or near vacuum) in the region of the joint 28; low pressure is desirable so as to minimise beam divergence. Due to the size of the vessel 22, it is not practical to position the entirety of the sections 24 into a vacuum chamber, and as such the local vacuum chamber provides a useful solution. The vacuum chamber used may be of the type described in US5491316 (e.g. illustrated in Figure 1, 6 and 8, and described in column 3, line 9 to 29 of US5491316), incorporated herein by reference.

An electron beam welding gun 32 is mounted to the chamber 30. The electron beam welding gun directs an electron beam (illustrated by dotted line 34) at the joint 28 between the sections 24. The welding gun and/or the sections 24 are moveable so as to move the welding gun along the joint 28 for welding of the two sections together.

The electron beam gun 32 may be provided inside or outside the vessel. Mounting the electron beam gun inside the vessel reduces the space requirements for the welding process and restricts x-ray emissions as the vessel itself acts as a partial shield. However, providing the electron beam gun on the inside of the vessel can make extraction of equipment when performing closure welds difficult.

Referring now to Figure 4, to weld two components together (e.g. to weld the sections 24 together) firstly the components are positioned adjacent each other so that a joint (or joint region) is defined between the two components (see block 36 of Figure 4). The chamber 30 is positioned around the joint 28 and the chamber is evacuated (see block 38 of Figure 4). The chamber is positioned and evacuated in a similar manner to that described in US5491316 (e.g. column 4, line 57 to column 5 line 27), incorporated herein by reference.

The electron gun 32 is then moved relative to the sections 24 so as to move the electron beam 34 along the joint (see block 40 of Figure 4). The beam from the electron gun is tilted by deflections induced by magnetic coils in the electron beam gun column (or in alternative embodiments the gun may be moved laterally to the joint) such that the electron beam 34 follows an oscillating path along the joint 28.

Fig. 5 illustrates an oscillating path 42 (not claimed). The oscillating path traverses the joint 28 at a high frequency (spatial pitch), e.g. such that the wave length of the path is less than (e.g. half of or a quarter of) the diameter of an instantaneous weld pool formed when the electron beam contacts the material of sections 24. In this way the likelihood of the sections being welded along the entire length of the joint is increased. However, due to the small wavelength of the path, the actual weld pool (i.e. the total region of molten material at a given point in time) is large relative to the electron beam source, e.g. 5 to 6 times the diameter of the electron beam. A large weld pool is advantageous because it means that the joint region will cool slowly. However, the size of potential defects is a function of the depth of the weld and the size of the weld pool, so using the oscillating path of Figure 5 could increase the size of potential defects, in particular because the weld depth will be large given the thickness of the sections 24. A weld path 44a, 44b, 44c is illustrated in Figure 6. When the electron beam contacts the material of sections 24 a weld hole (often referred to as a keyhole) is formed, and is indicated at 46 in Figure 6. A region 48 surrounding the keyhole 46 will be molten at the instance when the electron beam forms the keyhole, and is referred to as the instantaneous weld pool.

The weld path 44a, 44b, 44c of Figure 6 has a greater wavelength than that shown in Figure 5. The wavelength of the weld path 44a, 44b, 44c is selected such that it is greater than the diameter of the instantaneous weld pool 48, for example greater than or equal to twice or three times the diameter of the instantaneous weld pool. The weld path 44a, 44b, 44c traverses the weld joint with a constant wavelength and amplitude. Due to the wavelength of the path, in one pass there will be portions of the joint that are not molten and not welded together. However, once the welding gun has travelled along a first oscillating path 44a along the entire joint, the gun’s start position is offset along the axis of the joint line and moves along a second oscillating path 44b. The second oscillating path 44b has the same form, including amplitude and wavelength as the first path 44a, but is out of phase with the first path. Once the electron beam gun has travelled along the entire length of the joint 28 following the path 44b it is again offset along the axis of the joint line and follows a third path 44c. The third path has the same form, including amplitude and wavelength as the first and second paths but is out of phase with the first and second paths. This process of the electron beam gun following an oscillating path along the length of the joint, being offset in a direction along the joint line, and following another oscillating path continues until the entirety of the joint has been welded together.

In the example shown in Figure 6, the electron beam path is defined by a harmonic (e.g. a sine wave), but in alternative embodiments the electron beam path may be defined by any suitable oscillating function, for example a more complex harmonic function or a square wave. Example alternative oscillating paths are indicated at 50, 52 and 54 in Figures 7A to 7C respectively. Further alternative oscillations include curlicues, rotary oscillations and figure of eight style oscillations. The direction (clockwise/anticlockwise or left-to-right then right-to-left) may be varied between welding passes. In some examples, one oscillation mode may be superimposed on another.

The oscillating nature of the electron beam path of Figures 5 to 7C provides increased certainty that the entirety of the joint between the two components has been welded, compared to welding along a path parallel to the joint. In Figure 5 (not claimed), oscillation of the electron beam achieves this by increasing the effective width of the weld pool and so widening the fused area.

The use of a larger wavelength oscillating path (as illustrated in Figures 6 to 7C) that repeats at an offset position along the length of the joint means that the weld cools progressively and therefore slowly. Slow cooling of the weld promotes toughness in the metallurgical phase transformation product of the weld metal and heat affected zone which resists hydrogen induced cracking, and as such the quality of the weld can be increased.

The larger wavelength oscillating path reduces the volume of molten metal compared to the path shown in Figure 5, which reduces the number and size of potential defects. Furthermore, the volume of molten metal at any point along the joint region is small so the size of any flaws is reduced and also the management of any liquid spillages is simplified. Further, by divorcing the requirement to sustain a wide weld pool from the width of the oscillation the lateral width (i.e. perpendicular to the joint line) of the oscillation can be increased, which further ensures that the entire joint line is welded.

The time and position of the heat input and keyhole relative to the joint line may be biased to improve the quality of weld between dissimilar alloys wherein there is a difference in properties such as melting point, thermal conductivity and/or magnetic susceptibility.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made within the scope of the claims. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

Claims

1. A method of electron beam welding two components together along a joint, the method comprising: directing an electron beam at the joint and moving the electron beam relative to the joint sequentially along a plurality of oscillating paths that traverse the joint, each of the oscillating paths extending the length of the joint and being out of phase from one another, each of the oscillating paths being defined by a similar function and having the same amplitude and wavelength, wherein the wavelength of the oscillating paths is greater than the diameter of an instantaneous weld pool formed when the electron beam contacts the components.

2. The method according to claim 1, wherein a difference in phase between the oscillating paths is selected to be less than or equal to a diameter of an instantaneous weld pool to enable the entire joint to be welded.

3. The method according to any one of the previous claims, wherein the oscillating path is defined by a harmonic function that oscillates finitely or by a square wave.

4. The method according to any one of the previous claims, comprising tilting the electron beam as it is moved relatively along the joint so as to define the oscillating path.

5. The method according to any one of the previous claims, comprising providing a chamber around the joint, reducing pressure in the chamber, and mounting an electron beam gun to the chamber, and welding the components together using local vacuum electron beam welding.

6. The method according to any one of the previous claims, wherein the two components are parts of a pressure vessel for a nuclear power plant.

7. A method of manufacturing a pressure vessel for a nuclear power plant, the method comprising: providing a plurality of sections of the vessel and welding said sections together using the method according to any one of the previous claims.