Sayee Raghunathan
TWI Ltd, Cambridge, UK
Paper presented at National Welding Seminar (NWS 2006) 24 - 26 November 2006, Chennai, India.
Abstract
Welding can be used to repair components which are still in manufacture or which have been in service for a long time. Repair welding techniques may also be used to modify a structure in service. However, repair welding can go badly wrong. An injudicious repair can shorten the life of a component, or even damage it beyond repair. Thus though the pressure to repair quickly is understandable, common sense suggests the need to investigate or try to understand what caused the failure before attempting the repair. Once there is an understanding of the root causes of failure, a repair procedure can be developed for the application, taking into account aspects such as the needs for remote repair techniques.
This paper provides an overview of recent weld repair techniques developed at TWI for various applications. It describes 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 international standards. Recent developments within process monitoring, seam tracking and adaptive control are presented.
Introduction
Many aspects have to be taken into consideration before arriving at the optimum repair technique: run-repair-replace decision, what-if scenarios, access for repair, operating conditions for in-service repair, location(shop/site/offshore), time to and for repair, applicable code/standard/specification for repair, desired properties of the repair joint, repair qualification requirements (including procedure qualification, welder/operator approval, simulation testing, validation and assessment of consumables), history of repair technique, avoidance of PWHT (by using controlled deposition repair techniques or austenitic/Ni based weld metal), equipment/process (including development of any specialised equipment for e.g. remote repairs and process comparison/optimisation), control of procedures/method statements and supervision. Even this list is not exhaustive. The general drivers influencing repair can be broadly classified as follows:
- Social: professional responsibility for safe operation.
- Technological: failure or degradation in service, life-extension, planned (preventative) maintenance (outages) and design improvements.
- Economic: minimise lost production/downtime (productivity of repair techniques), reduce waste (salvage during manufacture), minimise repair costs (cost of repair is generally the less important factor when considering repair, time to repair is the more important consideration).
- Environmental: spills/leaks/fallout, sustain natural resources.
- Political/Legislation: code compliance, health, safety and environment regulations.
This paper describes recent work carried out to evaluate the replacement of are Tungsten Inert Gas (TIG), Metal Inert Gas (MIG) and Manual Metal Arc (MMA) processes for repair by alternative joining processes such as advanced arcwelding 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. Laser processes are included, as they are very 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 discussed. Current practice for repair without post weld heat treatment is reviewed.
Developments in arc welding technology [1]
Background
The processes widely used for repair and refurbishment are TIG, MIG and MMA welding. Recent developments in the power source technology for the MIG and TIG processes, for example the VBC Interpulse, Lincoln STT, the Air LiquideToptig ® and the Fronius CMT ® technology have improved the performance and quality of these welding processes specifically for repair. By allowing accurate control of the waveform and/or wire feed, these processes have potential for customising for repair applications. The VBC Interpulse for example has reportedly been used for repair in the aerospace industry and for controlled deposition repair (manual TIG). The CMT process could potentially offer good heat-input control, thus proving beneficial for repair welding.
TIG Welding with active fluxes
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 the 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 6mmthickness 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
a) without flux
b) with flux
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 of passes or the increase in welding speed. 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.
Underwater repair techniques
There is occasional requirement for underwater repair in offshore applications and nuclear plants. Traditionally such repair has been carried out using MMA processes for wet welding and cutting. Recent work has been carried out to develop wet welding techniques using Flux Cored Arc (FCA) welding processes. The advantage of the FCA process is that it is 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 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 of a continually fedwire, 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 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 in Fig.2.
Fig.2. FCA wet weld in 8mm thick C-Mn steel plate welded in the vertical down position
a) General appearance of root pass
b) Cross section of weld
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.
Moving contact arc welding (MCAW) [2]
MCAW is another technique developed at TWI, which could offer benefits for repair or surfacing applications, and offers greater flexibility in application for remote processing than manual SMA welding. The process works as follows: the current supply is made with a sliding or rolling tool to the consumable via a narrow ridge which is part of the consumable core as shown in Fig.4. Initially an arc is struck, using a fuse or fine wire wool at the end of the electrode to ionise the consumable/substrate arc gap.
The flux underneath the consumable core ensures electrical insulation between base material and the RidgebackTM consumable and maintains a controlled arc length throughout the welding operation. The arc length can be changed by altering the thickness of the flux covering or by changing the shape of the metal core. The arc burns along the consumable electrode leaving a weld deposited onto the workpiece, as illustrated in Fig.3. An example of surface appearance of MCA weld made with Ridgeback TM consumable, BS970 grade 316L deposit onto BS970 grade 304L substrate is shown in Fig.4.
Fig.3. Basic principle of MCAW using Ridgeback TM consumable
Fig.4. Example of surface appearance of a MCA weld using Ridgeback TM consumable
Friction processes [3]
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 phasewelding 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.
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. The principle of friction taper stitch welding is shown in Fig.5.
Fig.5. Principles of friction taper stitch welding for crack repair
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.
The principle of Friction Hydropillar Processing (FHPP) is shown in Fig.6. The excavated hole can have straight or slightly tapered walls.
Fig.6. Friction Hydropillar Processing
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.7. 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 aswell 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 is being considered as material reprocessing technique, which would repair surface breaking, or near surface defects and has been applied to the manufacture of copper canisters for encapsulating high level nuclear waste and storage in deep level depositories.
Fig.7. Principles of FSW
Welding procedures for repair without PWHT [4]
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.
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 as shown in Fig.8. The intentionis 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.
Fig.8. Controlled deposition repair technique
The MMA process is commonly used, but MAG welding and TIG repair procedures 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.
Conclusion
Challenges in repair and refurbishment application are driving the use of more novel techniques and processes. For example, in nuclear repair, access is often restricted in order to reduce human exposure to radiation. 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. Occasionally welding and cutting for repair in nuclear applications or offshore applications need to be performed underwater. 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 repair.
References
- Fred Delany et al: 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.
- D Howse et Al: Novel joining techniques for repair in the power generation industry. EPRI welding and repair technology for Power Plant Conference, Point Clear, Alabama, USA, 26-28 June 2002.
- W Thomas, E D Nicholas: Emerging friction joining technology for stainless steel and aluminum applications. Productivity beyond 2000. IIW Asian Pacific Welding Conference, Auckland, New Zealand, February 1996.
- A Barnes, R Jones, D Abson, T Gooch: Welding and fabrication of high temperature components for advanced power plants Part 2 - TWI Bulletin, March-April 1999.