Welding and fabrication of high temperature components for advanced power plant - Part 2

TWI Bulletin, March/April 1999

Trevor Gooch graduated in industrial metallurgy at the University of Birmingham, and continued to obtain the degrees of MSc(Eng) and PhD from the University of London. Dr Gooch joined TWI in 1965. In 1980, Dr Gooch became Head of the Materials Department, embracing the welding characteristics of virtually all metallic materials. In 1994, he was made Materials Technology Manager for TWI, involved with joining characteristics of all materials used for construction.

January's feature on manufacturing high temperature equipment for power generation plant resumes this issue. In Part 2 Adrienne Barnes, Richard Jones, David Abson and Trevor Gooch examine developments in welding processes, and take a long distance view of further research areas.

The welding processes used in the manufacture of both fossil-fired and nuclear power plant are similar, despite differences in system design and plant components. A common feature, particularly for steam generation and turbine generators, is that a large number of welds, often repetitive in nature, must be manufactured to high quality levels and rigorous quality assurance regimes. For this reason, the welding processes used have traditionally been based on conventional fusion techniques, predominantly arc welding. An accurate knowledge of the process techniques and the weld properties achievable, particularly at high temperatures, has enabled the required welds to be made consistently, and to an acceptable level of quality.

In recent years, welding process developments have been aimed at increasing productivity and automation, in order to reduce manufacturing costs and to improve plant reliability.

However, progress in their practical exploitation for the manufacture of power plant has been slow. This is due partly to the inherent conservatism that exists within this industry and the reluctance to adopt new technology. Furthermore, the fragmentation of the UK power generation industry has created a commercial climate which has restricted longer-term capital investments, such as are generally necessary to implement new welding methods. The current state of welding process technologies for boiler and turbine generator plant, together with their projected future developments, are summarised in the Table.

Table: Welding processes for high temperature power plant

Boiler plant components

Tube to tube welds in both the boiler panels and superheater/reheater elements are commonly welded by the tungsten inert gas (TIG) or flash butt processes. The latter welds are less easily inspected by radiography, and in some cases may be precluded from use for this reason. With regard to TIG welding, where practical, girth welds are made by rotating the tube beneath a stationary TIG welding head; the welding process is generally controlled automatically. In cases where the fabricated tube cannot be rotated, the orbital TIG method may be used for thin walled tubes, if adequate clearance for the welding head exists and sufficient numbers of welds are required to justify the initial set-up costs involved. Alternatively, these welds may be completed using manual TIG, and, for thicker walled tubes, manual metal arc (MMA) welding.

Fig.1 MMA welding of steam generating equipment

Thick section steam pipework and headers have very similar dimensions, and generally use the same materials, so the manufacturing methods are common to both. The use of TIG root runs and MMA fill runs is typical practice. Where the component can be rotated, and the pipe diameter permits ( ie >200mm OD), the joint can be filled with the SA process. Narrow gap techniques have found increasing use for such applications; the reduced weld volume facilitates faster joint completion rates, reduced consumption of consumables and less distortion. The narrow gap TIG process in particular has been found to be suitable for this application, and the associated low heat input is an advantage for welding Cr-Mo-V materials which may be susceptible to a degradation of mechanical properties with the higher heat input processes.

Turbine generator components

Site welding

Fig.2 Repair to a steam line support attachment (trunnion)

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.

Welding procedures for repair without PWHT

Fig.3 Example of a cold weld repair to a large attachment weld joining a wrought 0.5Cr-Mo-V intermediate pressure manifold to a forged 2.25Cr-1Mo anchor block

In order to permit adequate penetration of a second layer into the first, the half bead technique ^[27,28] 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 technique 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. ^{[10] , [29,30]} 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. ^{[6] , [31,32]} 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 ^[32] and an automated TIG repair procedure ^[33] 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. ^[34]

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. ^[35] 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. ^[35] It is beneficial to use consumables which overmatch the yield strength of the parent steels by as little as possible. ^[32] Hrivnak et al ^[36] described the relief of residual stresses in repair welds by over-stressing, with the optimum temperature for the pressure cycle being determined by evaluating the effect of test conditions on fracture toughness.

Code requirements

Developments in welding processes

Arc welding

The most likely trend in manufacturing methods for power generation plant over the next decade will be the increased use of mechanised and automated welding based on the more conventional process, ie TIG, MIG/FCAW and SA welding. The driving force for this will be the increasing need to reduce manufacturing costs and to increase plant reliability. A further factor will be the declining availability of skilled and fully qualified welders. There is presently a major shortage of such personnel, and the current welder training schemes which are aimed at giving the individual specific skills and not the broad skill range required of competent pressure vessel welders, will not make up this shortfall.

In addition to increased mechanisation, it is envisaged that developments in process techniques and consumables will also be exploited. The introduction of vacuum-packed MMA low alloy steel electrodes with moisture-resistant electrode coatings has imparted significant benefits, especially for site welding. Very low weld metal hydrogen contents can be guaranteed, without the need for electrode baking before welding, thereby alleviating significantly the quality control requirements.

A significant development of the TIG process, originating from the Paton Welding Institute, Ukraine, is the use of activated fluxes which result in a significant enhancement of weld penetration. ^[41] Active fluxes are available commercially for a range of carbon and stainless steels. The process uses a square edge preparation, and no filler wire addition is necessary. The active flux is applied in the form of a paste, which is painted onto the joint surface, or by means of an aerosol spray.

During welding, the active flux causes a constriction of the arc, and the resultant concentrated heat source gives a two to three fold increase in penetration. In practical terms, welding of up to 10mm thick steel in the flat position can be achieved in a single pass, and up to 6mm thick is possible in-situ. Furthermore, the process appears to be unaffected by compositional variations which can arise between parent materials of the same specification.

The major application of the A-TIG process for power plant is tube to tube welding, for which significant improvements in welding productivity are envisaged. A-TIG welding can be carried out in a single pass on up to 6mm thick material in all positions. In comparison, conventional TIG welding requires the use of a bevel preparation, and up to six weld passes to complete the joint. Furthermore, the process is simple to apply, and requires no specialised equipment.

The gas-shielded flux-cored arc welding process is currently being developed for the welding of low alloy steel steam pipework as a more productive alternative to MMA welding. ^[42,43] A key requirement is to validate the long-term high temperature performance of FCA weldments. An extensive programme of creep rupture testing is currently underway among the principal UK power generators, to characterise their performance in comparison with MMA welds. Rutile flux-cored wires are preferred because of their superior positional welding characteristics. Further, preliminary studies on C-Mn structural steels at TWI ^[44] involving controlled thermal severity (CTS) testing have indicated that, for a given weld hydrogen level, rutile flux cored wires, may show a reduced sensitivity to HAZ hydrogen cracking compared to basic and metal cored wires. The application of rutile flux cored wires to thick section pipe welds appears to be most attractive where the metal deposition rate advantages can be fully exploited.

The increased use of narrow gap welding, particularly using TIG or SA for thick-sectioned butt joints, is anticipated. Improved welding procedures, together with more reliable welding equipment, make it possible consistently to manufacture high quality welds at higher production rates compared to conventional welding. Even unsophisticated joint tracking systems can give good assurance of sidewall fusion. The simple joint geometry involves the repeated deposition of similar weld passes, making it a relatively easy task to automate.

Narrow gap orbital TIG welding systems have also been developed, which are well suited to site applications. However, the undoubted advantages in terms of reduced welding time must be offset against the equipment set-up and more accurate requirements for pipe alignment, which can be difficult and time consuming to achieve in site conditions.

A number of sensor technologies have been developed for welding applications, in order to facilitate automated process control. Whilst the majority of these suffer from insufficient resolution and lack of robustness, the use of arc-based and vision-based systems has become well-established for joint tracking purposes. The latter approach, in the form of either optical or infra-red cameras, is being further developed to achieve top face weld penetration control. Changes in the weld pool geometry, and associated thermal profiles, can be used to monitor changes in weld penetration. Real time adjustment in welding parameters can then be made to maintain constant weld geometries. The introduction of such control systems is expected to have major impact on the reliability of automated welding systems. Future welding systems may be able to minimise the incidence of weld defects via the intelligent control of welding parameters, thereby reducing the need for traditional post-weld non-destructive examination.

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. These 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 4kW 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 to robotic systems for three-dimensional manipulation.

Recent work has resulted in the commercial availability of a 5kW 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 manufacture of power generation plant.

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.

Electron beam welding

Electron beam (EB) welding is a mature technology which has been used extensively in industry for more than 40 years. Conventionally, the process is operated at a vacuum pressure of better than 5 x 10 ^-3mbar, which dictates that the component to be welded must be wholly contained in a vacuum chamber. To an extent this has precluded the application of the process for the fabrication of large structures, although many components in the power generation industry, including rotors, turbine diaphragms and steam generating plant are routinely welded using EB at high vacuum outside of the UK. Attempts to use local sealing at high vacuum levels have been largely unsuccessful in the past, due to difficulties with leaks in the local sealing causing fluctuations in the vacuum pressure, and subsequent poor weld quality.

Recent developments at TWI in EB welding equipment technology have resulted in a system which can be operated at a pressure in the range 0.1-10mbar. In consequence, this system can be used for welding large structures by using local vacuum sealing and a simple pumping arrangement. To date, the equipment at TWI has been used extensively for welding copper of over 70mm thickness for sealing nuclear waste containers. Furthermore, it has been demonstrated that steel of 150mm thickness and aluminium alloys of 70mm thickness can be welded reliably. Nevertheless, the experience of welding alloy steels, and in particular stainless steels, is limited, and the full performance capability of the process has yet to be proved.

Clearly there are many power plant applications which could benefit from the use of a low heat input, low distortion welding process which is capable of producing narrow parallel-sided welds of high integrity in a single pass. In particular thick section pipework, and header and steam chest components are viewed as candidate applications for this process. Furthermore, in the future, rotors and casings, which are currently manufactured as one-piece forgings, could be fabricated by using EB welding, with minimal requirements for subsequent machining.

In common with laser welding systems, the application of EB welding will require significant capital investment. However, the present demands to achieve short term payback on such investments, together with the uncertainties regarding the volume requirements for power plant over the next decade or so, are likely to be major obstacles to the practical exploitation of power beam technologies.

Areas for future research

Welding behaviour

Weldment properties

While creep resistance represents the sine qua non of advanced ferritic steels for high temperature power plant, it is essential also that materials display sufficient toughness at normal ambient temperature for pressure testing to be safely carried out. This has not been a particular problem with low alloy grades, but the situation is a little different with, for example, the modified 9Cr alloys. Most fabrication codes require toughness to be assessed by an impact test method, and considerable consumable development has been necessary to obtain weld metal compositions that allow typical code requirements to be met.

In general, impact toughness is not seen as a particular problem. However, it has become apparent from fracture mechanics tests on pre-cracked samples that modified 9Cr weld metals can display an appreciable tendency to 'pop-in' behaviour, with initiation and arrest of local brittle cracks during testing. The behaviour is influenced by material analysis, and further study is required to clarify the optimum consumable composition. Work to date indicates that changes such as reduced niobium or increased manganese relative to base steel are of benefit in respect of toughness, but are not necessarily consistent with achieving optimum creep strength. It is not known how far similar trends will be observed with other more recently developed alloys, and appropriate fracture mechanics testing is required.

PWHT procedures for high chromium creep-resisting steels were originally developed on the basis of achieving a reasonable degree of weld area softening from the as-welded condition, with the formation of a carbide distribution expected to be conducive to good creep properties. The precise thermal cycle used for PWHT can appreciably influence the weld area toughness, yet surprisingly little attention has been paid to optimising heat treatment conditions.

In essence, design of high temperature plant is based on control of the operative stress level to achieve the required life under creep conditions. Depending on the fabrication code involved, a factor may be applied to recognise the presence of welded joints. Creep ductility is not explicitly recognised. Nevertheless, it is desirable that weld metals display a reasonable capacity for accepting strain, for example, so that incipient failure from upset conditions or other cause can be identified before complete wall penetration takes place. Work on austenitic stainless steels has shown that inclusions in arc weld metal can appreciably reduce the rupture ductility, but there has been negligible analogous study on high temperature ferritic grades.

Service behaviour

In completed plant, modified 9Cr and similar steels will be used only for the high temperature regions, with lower alloy grades used elsewhere for reasons of economy. It will therefore be necessary to make a dissimilar metal joint between the two classes of steel at some point in the unit. When steels of varying chromium content are welded and exposed to elevated temperatures either during PWHT or in service, carbon migration takes place towards the higher alloy side in consequence of the lower carbon activity. The immediate consequence of this is that a decarburised zone is formed in the lower alloy material, with potentially much reduced properties relative to the unaffected base metal. The phenomenon of carbon migration in dissimilar metal joints has been well studied for welds between austenitic and ferritic steels, but much less information is available for welds between ferritic/martensitic alloys of different composition. It has been shown that development of a decarburised zone does not follow a direct parabolic relationship with time, ^[45] as would be expected if diffusion alone were controlling, but the rate of decarburisation tends to diminish with increased time. This is a consequence of carbide precipitation on the high alloy side and depletion of the matrix alloy content and hence the driving force for carbon diffusion. However, at present a comprehensive model has not been developed, and it is therefore difficult to predict the extent and consequences of carbon migration at dissimilar metal joints between steels of similar metallurgical structure.

Conclusions

References