Advanced joining processes for repair in nuclear power plants

Paper presented at 2005 International Forum on Welding Technologies in Energy Engineering September 21 - 23 Shanghai, China.

Abstract

This paper provides an overview of recent weld repair techniques developed at TWI for nuclear power plants.

It describes TIG welding with active fluxes (A-TIG), underwater flux cored arc welding, novel friction welding processes, laser welding and in-process monitoring.

Special welding procedures have been developed for repair welding without post weld heat treatment, which are now accepted by national standards.

Remote welding is benefiting from developments in penetration control, seam tracking and adaptive control. This is an area where TWI has developed considerable expertise.

Advanced NDT techniques such as Teletest enable the detection of defects in inaccessible pipework.

Introduction

Within the power generation industry, outage reduction is a key issue in terms of improving plant availability and increasing revenue. More specifically, within the nuclear industry, there is a need to remove personnel wherever possible from radiation areas in order to reduce dose uptake. There is, therefore, a general requirement to develop novel high productivity joining techniques that can, in some instances, also be used remotely. The paper describes recent work carried out to evaluate the replacement of conventional TIG, MIG and manual metal arc processes for repair in power plants by alternative joining processes such as advanced arc welding processes, laser welding and friction techniques. Although most procedures for repair rely upon conventional arc welding techniques, TWI has also pioneered work in other areas of arc welding. Advanced arc welding technologies such as the use of active fluxes for enhanced penetration underwater welding techniques, and in-process monitoring and control are discussed, as are laser processes, which are highly suitable for remote operation as light can be transmitted to the work area by fibre optic cable and focused at the point of application to produce deep penetration welds. The use of friction hydro pillar processing (FHPP), and friction stitch welding as a repair technique is described. Current practice for repair without postweld heat treatment is reviewed. Recent developments within process monitoring, seam tracking and adaptive control are presented. TWI is also active in the deployment of advanced NDT techniques for remote defect detection and accurate sizing.

1 Developments in arc welding technology

1.1 Background

The processes most widely used for repair and refurbishment in nuclear environments are Tungsten Inert Gas (TIG), Metal Inert Gas (MIG) and Manual Metal Arc (MMA) welding. Although both TIG and MIG processes are currently used for remote applications they both have limitations. The TIG process can be used remotely but, TIG welding does not give deep penetration welds, is relatively slow, and can require a number of welding passes to make a single welded joint. The process can also be prone to variable quality due to cast-to-cast variation and requires a separate wire feed system. MIG welding is a higher productivity welding process but also requires local wire feed. Recent developments in power source technology have improved the performance and quality of MIG welding to the point where they could be considered for high quality remote applications. In considering the use of novel processes for remote applications, particularly if they are to replace highly flexible manual welding techniques, the use of sensing technology and adaptive control must also be considered. Vision sensing, in-process control and offline programming techniques are all currently employed for robotic or automatic welding and there is a requirement to evaluate their suitability for remote repair.

1.2 TIG welding with active fluxes

Tungsten Inert Gas (TIG) welding is a widely used welding process by which an arc is struck between a non-consumable electrode and the workpiece creating the heat to make the joint. The main advantage of the process is to produce high quality welds, but it also has two major limitations. The first is that the deposition rates are lower than other consumable electrode arc welding processes, and that for stainless steels, the parent material composition can affect the depth of penetration achieved by altering the flow of the molten pool during welding. Active fluxes (A-TIG fluxes) that increase the penetration of TIG welds offer a means of significantly increasing the productivity of the welding process, and are capable of welding up to 6mm thickness carbon manganese or stainless steels in a single pass, without filler material.

Fig.1. The characteristic appearances of the conventional TIG arcs and TIG with active fluxes and the comparative depths of penetration in 6mm thick stainless steel:

The active flux process can be applied in both manual and mechanised welding operations. However, because of the need to maintain a short arc length to achieve deep penetration, it is more often applied in mechanised applications. Specific advantages claimed for the active flux process, compared with the conventional TIG process, include:

• Increased depth of penetration e.g. up to 12mm thick stainless steel can be welded in a single pass compared with typically 3mm with conventional TIG.
• Overcomes the problem of cast to cast variation e.g. deep penetration welds can be produced in low sulphur (less than 0.002%) content stainless steels which would normally form a wide and shallow weld bead with conventional TIG.
• Reduces weld shrinkage and distortion e.g. the deep narrow weld in a square edge closed butt joint will produce less distortion than a multi-pass weld in the same thickness material but with a V-joint.

The claims for a substantial increase in productivity are derived from the reduction in the welding time either through the reduction in the number, passes or the increase in welding speed. Economic data for the process can be seen in Table 1.

Table 1: Costing analysis of conventional mechanised TIG welding compared with TIG welding with active fluxes for 6.0mm thickness stainless steel.

Disadvantages of using a flux include the rougher surface appearance of the weld bead and the need to clean the weld after welding. In mechanised welding operations, the as-welded surface is significantly less smooth than is normally produced with the conventional TIG process but in manual welding operations, the surface roughness is similar. On welding, there is a light slag residue on the surface of the weld which often requires rigorous wire brushing to remove.

Fluxes are now commercially available from various suppliers for C-Mn steel, stainless steel, titanium and some nickel alloys. TWI has developed a low cost flux which is suitable for welding nuclear power plant materials.

2 Underwater repair techniques

2.1 Arc processes

Within the nuclear industry there is also some occasional requirement for underwater repair, work has been carried out to develop wet welding techniques to replace MMA processes for wet welding and cutting using flux cored arc (FCA)welding processes. The advantage of the FCA process compared to MMA is that it is much more suited to being automated and, therefore, has potential for remote application.

The E O Paton Institute has recently developed an innovative wet welding technique, based on the self-shielded flux-cored arc (FCA) process, which can also be used for cutting. The FCA wires have been developed specifically for operating in direct contact with water, and the novel wire feed system can be completely immersed. When used in either welding or cutting operations, the FCA process offers potential for significant productivity benefits through use ofa continually fed wire, compared with MMA where the rod electrodes must be changed at frequent intervals. Furthermore, it is claimed that the combination of flux formulation and wire composition produces the desired slag-gas forming reactions which will not only improve the weld bead profile but also reduce the pick up of hydrogen and oxygen in the weld metal.

As the FCA process appears to offer substantial benefits for cutting and welding operations, a series of welding trials was carried out at TWI to evaluate the FCA process (consumables and equipment). This was to substantiate claims for wet underwater welding and cutting with regard to the benefits in weld bead characteristics and productivity. Results of the trials were collated over a period of six months using TWI welders and welder-divers from the UK. Several applications carried out in the former Soviet Union countries have been used to illustrate the benefits of the process for underwater welding.

Examples of the welds are shown. As the wire is essentially a rutile type, molten metal transfers from wire to weld pool by the short circuiting mode of metal transfer and is protected during cooling by a vapour gas bubble which surrounds the arc. This bubble is more stable in the flat position than vertical but vertical welding can also be carried out.

Fig.2. FCA wet weld in 8mm thick C-Mn steel plate welded in the vertical down position:

The FCA process can also be readily used for cutting operations. A 2.4mm diameter wire is normally used which generates a more forceful arc and gas 'jet'. The process appears to work equally well in the vertical-down and horizontal-vertical positions. However, when cutting in the vertical-up position, it was significantly more difficult to maintain the cut opening.

It was concluded that, based on TWI's evaluation tests at Cambridge and the practical experience in the former Soviet Union, there is no doubt that the FCA system offers a substantial advantage over conventional MMA for wet welding and cutting operations, especially in those situations where a large amount of welding/cutting must be carried out. Potential savings from use of FCA welding operations compared with MMA welding should be approximately 50%. The savings will be realised from reduction in the ancillary operations e.g. electrode changing, and the slightly higher deposition rates. Although the process was designed for manual welding, it also opens up the possibility of remote operation using an ROV. However, successful application of automatic techniques will depend upon the ability of the ROV to mimic the manipulative skill of a human welder. Irrespective of the type of operation (manual or automatic), reliability of the system will be crucial in order to ensure that the economic benefits derived from continuous operation, can indeed be realised.

Fig.3. Wet welding trials at TWI underwater technology tank

2.2 Friction processes

The recorded use of frictional heat for solid-phase joining techniques dates back over a hundred years. The friction welding process, however, to a large extent has been restricted to round, square, or rectangular bars. In addition to the applicability of these techniques to form attachment to structures, TWI has been working on techniques which now allow solid-phase friction welding as a viable option for plate fabrication in a range of materials. Solid phase welding is thought to be less sensitive to Helium cracking than conventional arc welding, which affects repair of irradiated stainless steels. Of particular interest are three techniques that have potential for the repair of defects, friction taper stitch, friction hydropillar processing and friction stir welding. Friction processes have been successfully applied to produce sound welds underwater.

2.2-1 Friction taper stitch welding

Friction taper stitch welding is particularly suited to repair of cracks. This is a solid phase welding process and involves drilling a tapered hole through the full thickness of a plate at the location of the defect. A tapered plug with a similar included angle is then friction welded into the hole. By using a series of inter-linking holes long defects can be repaired. The process is portable and will run from power supplied by mobile generators. The hole plugging weld cycle time in 8mm thickness stainless steel is ~0.5 seconds.

Fig.4. Principles of Friction taper stitch welding for crack repair

2.2-2 Friction hydropillar processing

Friction hydro pillar processing (FHPP) is a comparatively recent solid-phase welding technique. Invented at TWI, this technique is the focus of considerable R&Dmp;D interest because of its potential in fabrication and manufacturing where it offers a number of novel production routes. The FHPP technique is still under development, but already shows promise for joining and repairing thick plate in ferrous and non-ferrous materials. Conventional fusion welding of thick section fabrications involves lengthy processing sequences and with some process large volumes of consumable material. In contrast, use of the FHPP welding technique should provide a reduction in joint preparation and weld filler metal, which will lead to significant cost savings.

The FHPP technique involves rotating a consumable rod co-axially in a circular hole, under an applied load to continuously generate a plasticised layer. The layer consists of an almost infinite series of adiabatic shear surfaces. The main features of the process are illustrated. During FHPP the consumable is fully plasticised at the frictional interface across the bore of the hole. This interface travels through the thickness of the workpiece. The plasticised material develops at a rate faster than the feed rate of the consumable rod. This means that the frictional rubbing surface rises along the consumable to form the dynamically recrystallised deposit material. The plasticised material at the rotational interface is maintained in a sufficiently viscous condition for hydrostatic forces to be transmitted, both axially and radially, to the bore of a parallel sided hole enabling a metallurgical bond to be achieved. Since this material is being forced hydrostatically into the surrounding bore, the diameter of the deposit material is nominally greater than the feed stock material.

Fig 5. Friction Hydropillar Processing (FHPP). Principles and example. The excavated hole can have straight or slightly tapered walls

2.2-3 Friction Stir Welding and processing

Friction Stir Welding (FSW) is a continuous hot-shear process involving a non-consumable, rotating probe of harder material than the substrate itself. The basic principle of the process is shown in Fig.6. Essentially, a portion of a specially shaped rotating tool is entered between the abutting faces of the workpiece (i.e. the joint). The tool's rotary motion generates frictional heat which creates a plasticised region(a local active zone) around the immersed portion of the tool, the contacting surface of the shouldered region on the tool and the workpiece top surface. The shouldered region provides additional friction treatment to the workpiece as well as preventing plasticised material from being expelled. The tool is then steadily moved along the joint line, with the plasticised zone cooling behind the tool to form a solid-phase joint as the tool moves forward.

The FSW process has been applied to the manufacture of copper canisters for encapsulating high level nuclear waste and storage in deep level depositories. FSW is also being considered as material reprocessing technique which would repair surface breaking or near surface defects.

Fig.6. Principle of friction stir welding

3 Laser welding

Lasers are attractive manufacturing tools for a range of welding, cutting and heat treatment tasks, due to their associated reduced manufacturing costs and increased flexibility. For welding applications, the primary advantages are controlled and predictable distortion, high joint completion rates and consistent, reliable weld quality, although there will be significant capital outlay. At present, two main laser types are available for materials processing. There are the CO ₂ gas laser and the Nd:YAG solid state laser. CO ₂ lasers with powers up to 15kW and Nd:YAG lasers with powers up to 6kW are currently being used in production. Although power is limited, the Nd:YAG laser beam has the advantage of being able to be delivered to the workpiece by flexible optical fibres, at least up to 200m long, giving substantial production advantages over CO ₂ laser beams which can only be manipulated via moving mirror systems. The Nd:YAG laser is thus suited to difficult access or remote applications. Furthermore, the fibre optic can also be easily attached torobotic systems for three-dimensional manipulation. Recent work has resulted in the commercial availability of a 6kW Nd:YAG laser source, with even higher power systems under development. With this laser, welds in 10mm thick steel can be made, as well as high speed processing of thin materials. This new high power laser technology, and expected future developments, will considerably expand the opportunities for laser exploitation in the repair of power generation plant.

Fig.7. Cracked burst can detector hanger welds. Arrow shows location of crack.

Work carried out at TWI demonstrated the feasibility of using such an approach to repair cracked burst can detector (BCD) hangers. The laser used was a 4kW Nd:YAG system transmitting the beam to the workpiece via a 0.6mm optical fibre and focussed to produce a 0.6mm spot size at the workpiece. Trials were carried out using linear, single and double autogenous re-melting of the cracked region, use of wire feed additions and use of a weave pass technique with wire feed autogenous laser welding to completely re-melt the crack defect. Results showed that cracks could easily be repaired in a single operation by using a deep penetration re-melt procedure. An example of a cross section of an acceptable weld repair is shown in Fig.8.

For boiler plant fabrication, the fin to tube weld is well suited to laser welding. Low distortion, high speed welding should be possible. Laser welding, in combination with narrow gap-TIG for example, could also offer production advantages for thick section weldments, including pipework, headers, and turbine generator components. However, its main application in the power generation industry, at present, is for remote repair operations.

Fig.8. Cross-section of a weave pass Nd:YAG laser weld on cracked BCD hanger welds. Wire feed rate 1.6m/min, laser power 3.9 kW, travel speed 0.2m/min, laser focus 5mm above workpiece surface

A novel generation of lasers, the Yb-Fibre lasers, are now available and combine the high power of the CO ₂ lasers with the fibre delivery mechanism of the Nd-Yag lasers. TWI is equipped with a 7kW source, capable of producing narrow single pass welds in 12mm steel. The fibre lasers offer advantages of higher beam quality which gives the option of a longer stand-off distance, improved power conversion efficiency, operation reliability and smaller size.

Its compact design, easy set-up, minimal cooling requirement and fibre optic delivery over long distances makes it an ideal laser source for power plant repair welding and cutting.

Fig.9. View of TWI's Yb-Fibre laser set up, showing a compact design and fibre optic beam delivery.

4 Repair welding without post-weld heat treatment (PWHT)

In a wide variety of structures, vessels and pipework, defects may be found on final inspection after PWHT has been carried out. The presence of defects of a sufficient size to warrant repair raises the question of whether it is necessary, after completion of the repair welding, to carry out a further PWHT operation. Relevant fabrication codes may not make provision for repair welding without PWHT. Hence, its omission will normally require the agreement of all interested parties (which is likely to include the certifying authority, insurers, operators, and owners) and that a fitness for purpose analysis be carried out, with data generated in one or more procedure qualification tests. However, recent studies have shown that repair welding without subsequent PWHT will be viable in many situations.

Fig.10. Repair to a steam line support attachment carried out without PWHT

4.1 Welding procedures for repair without PWHT

In principle, two alternative approaches may be adopted, the half bead technique and the temper bead technique. Each requires the careful placement of a regular arrangement of uniform layers of beads, with each bead laid down so as to overlap the previous bead. The intention is to produce a smooth overall profile for the inner and outer surfaces of the layer, with the substantial overlap of beads giving a high proportion of grain-refined HAZ microstructure, together with a measure of tempering and softening. The extent of bead overlap is important, with an overlap amounting to 50% typically being required. As weld bead shape is influenced by deposition technique and by welding parameters, welding procedures must be devised with care. A second layer is required, to replace any grain-coarsened HAZ with grain-refined HAZ.

In order to permit adequate penetration of a second layer into the first, the half bead technique requires the removal, by grinding, Of half of the depth of each layer before the next layer is deposited. These procedures specify particular pre-heating and post-heating requirements. Largely because of the difficulty in determining, controlling and monitoring the depth of metal removed, this techniques now finds little, if any application, having been superseded by the temper bead technique.

The temper bead technique was devised to generate a fine-grained HAZ in low alloy (Cr-Mo-V) steels which suffered reheat cracking sensitivity. The principle of this technique is that a first layer of overlapped beads is deposited, essentially as for the half bead technique; appropriate penetration of a second layer into the first is achieved by using a higher arc energy, which is typically 1.5 to 2.5 times that of the first layer. The intention is that a second layer deposited in this way will generate a region of coarse-grained reheated weld metal which is contained within the first layer, with the corresponding grain-refined region re-austenitising any coarse-grained HAZ in the underlying parent steel.

In hardenable steels, further layers may be deposited to temper the grain-refined HAZ. The MMA process is commonly used, but MAG welding and an automated TIG repair procedure have also been adopted. Where ferritic consumables are used, preheating is commonly required when welding thicker sections, particularly in the more hardenable steels, However, nickel-base electrodes have been used without preheat.

Fig.11. Controlled deposition repair welding

4.2 Residual stresses

While it is commonly assumed that yield magnitude residual stresses are present in the weld and HAZ of an as-welded repair, the actual levels are not necessarily always of this magnitude. Nevertheless, consideration must be given to the consequences of any residual stresses for the defect tolerance of the newly-repaired fabrication, particularly when it is first subjected to its service loading. Where the fabrication operates at elevated temperature, and where C-Mo or Cr-Mo steel consumables are used, some relaxation of residual stresses may occur during early service, particularly if the lower carbon variants are chosen.

Clearly, the extent of any residual stress reduction will depend on the service temperature and on the parent steels and weld metals used. It is beneficial to use consumables which overmatch the yield strength of the parent steels by as little as possible.

4.3 Code requirements

Several national standards and procedures devised by the major utilities now permit PWHT to be omitted, provided that certain criteria are met. Such National codes include BS 1113, BS 2633, ASME VIII, and ASME XI. In addition to restrictions on the parent material types and wall thickness, the limitations imposed include, variously that the fabrication was previously given a PWHT, that the repair does not exceed a particular depth, and that particular preheat and post-heat requirements are met. As noted above, for the half-bead technique ASME XI specified particular pre-heat and post-heat requirements. However, this code now allows repair by the temper bead technique followed by post-heating. In the National Board Inspection Codes(NBIC) in the USA covering repair after service exposure, the 1977 issue was the first to include weld repair procedures without PWHT for C, C-Mn, C-Mn-Si, C-0,5Mo and 0.5Cr-0.5Mosteels.

In the light of data provided by several research programmes, the 1995 issue extended the list of steels to include ASME P4 and P5 (Cr-Mo) steels. In view of the high cost, and in some cases impracticability, of carrying out PWHT after repair, it is likely that similar changes will be adopted more widely, as supporting data are generated.

5 In process monitoring and control

Another area which has become increasingly important in applying any welding technology for remote applications is sensing and control technology. The advances in this area of welding have largely come about through reduced cost of camera and image processing systems. These systems use data collected in real time to derive information regarding the state of the process and control the parameters to maintain weld quality for various environmental changes.

This is of particular interest where the process has to be robust enough to cope with variable joint fit-up and manual welding cannot be applied. Camera based systems can be used to either view the joint to be welded illuminated with a laser, 'laser strips' systems. These types of systems are particularly suited to tracking joint lines and maintaining height position.

TWI has developed an adaptive process control system to adjust the process parameters during welding and achieve consistent weld quality at optimum productivity. A laser vision system was integrated with a robot controller to provide control over travel speed, tracking position, wire feed speed, weave amplitude and weave frequency during arc welding. The adaptive control system compensates for variation in joint volume, joint gap and weld reinforcement by adjusting the pass area to maintain a fixed layer height using constant wire feed speed. This multimillion pound development programme was targeted at the fabrication industry, but the experience gained, and the lessons learnt about joint finding, joint following, and adaptive control for remote autonomous robotic arc welding, are invaluable for nuclear repair situations.

Fig.12. Remote, automatic welding using TWI adaptive process control and seam tracking

6 Advanced inspection

Inspection of nuclear components by non-destructive testing (NDT) presents many challenges. Direct access is often difficult if not impossible because of the radiation hazard. NDT has to detect, size and record defects of concern with reliability, accuracy and repeatability. TWI is helping the nuclear industry to meet these challenges.

• Automated and robotic techniques such as Teletest ^® can reach inaccessible regions and reduce operator dose.
• Ultrasonic time of flight diffraction (TOFT) and P-SCAN are accurate sizing techniques where scans can be recorded for later comparisons to see if defects have grown.
• Specialist procedures such as phased array are being developed and tested for new applications.

TWI has won a two year Project for development of in-service robotic inspection methods for nuclear applications, known as Rimini. The project has a total value of 2 million Euros.

Safe operation of nuclear power plants depends on regular in-service inspection, particularly of the reactor pressure vessel (RPV), which contains the nuclear fuel. RPVs are made of thick steel sections welded together. This steel becomes brittle with age and is therefore more susceptible to the rapid growth of cracks. The water containment also means that the steel is susceptible to damage due to stress corrosion cracking.

Working with large enterprises and several SMEs, TWI will address the drawbacks of existing inspection methods notably: time consuming manual intervention requiring operators to work in radiation hazardous areas. This also increases inspection time, which carries a huge economic cost. Developments will include novel inspection methods which can speed up inspection times, improve defect detectability and reduce operator exposure whilst working inside reactor containments.

TWI's NDT experts have nuclear industry experience and can provide advice on most ultrasonic, electromagnetic and radiographic methods. TWI was recently asked to develop a technique to inspect the inner wall of a double walled containment. However, not all problems require a high-tech solution as a recent project to plan the inspection of a large number of nuclear waste storage cylinders showed - it was found that prioritisation by design could reduce the amount of work.

The demonstration of inspection reliability through inspection qualification and performance demonstration has its roots in nuclear safety. The European methodology for inspection qualification is now a standard way of providing the necessary confidence. TWI has experience in all parts of the process:

Fig.13. Long range ultrasonic testing of pipes

• Preparing and assessing technical justification
• Setting and invigilating NDT performance trials
• Membership and chairmanship of inspection qualification

NDT performance trails require test specimens containing realistic defects. There is a range of techniques available at TWI for producing suitable defects of a given morphology and size. Test specimens have been manufactured for many laboratory and performance trials including some of the large test pieces used for the qualification of the inspection of the Sizewell B pressurised water reactor, and more recently, for submarine nuclear plant.

Fig.14. Mock up of a nuclear stream generator with artificial defects manufactured by TWI for the inspection validation of Sizewell B PWR

7 Conclusion

Nuclear plants present many challenges when it comes to repair. Access is often restricted in order to reduce human exposure to radiation. Occasionally welding and cutting need to be performed underwater. Remote access requires the deployment of special welding processes that can be readily automated. Automatic joint trading, and adaptive process control are required to achieve consistent weld quality. It is sometimes challenging to achieve optimum weld metal properties, especially when it is not possible to carry out post weld heat treatment. Overcoming these restrictions require innovative approaches to welding processes, welding metallurgy, in process monitoring and NDT. This paper reviews some of TWI's recent contribution in the repair of nuclear power plants.

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