A friction welding application in the nuclear power industry

TWI Bulletin, January 1982

by E D Nicholas, CEng, MIM, MWeldI and J Chapman

Mr Nicholas is a Principal Research Engineer in the Advanced Heavy Section Welding Department and Mr Chapman is in the Nuclear Engineering Department of Babcock Power.

A series of trials proved that friction welding could be used satisfactorily to complete a specific weld in nuclear power station boilers. This article describes the testing programme and the execution of the production welds at The Welding Institute, illustrating the close cooperation that exists between the Institute and its Research Members.

In constructing the boilers for Hartlepool and Heysham Nuclear Power Stations it was required to fit a system of thermocouple probes into selected boiler headers to record local steam temperatures. The probe tips are inserted into the holes of the tube sheets which form the base of the headers.

The Central Electricity Generating Board (CEGB) was responsible for the design concept of mounting the probes on six radial arms supported by a probe mast, through which the thermocouple leads are ducted to pass through the hemispherical closure dome of the header. The scheme adopted is as shown in Fig.1, so that the probe mast and closure dome junctions satisfied design code and weld inspection requirements. A design was evolved in which the probe mast is attached to the inside surface of the closure head by a friction weld. This weld is not a pressure containment weld and it serves only to support the hub and spokes carrying the thermocouples.

The Welding Institute Research Laboratory was approached by the boiler manufacturers, Babcock Power Ltd, to undertake development of the 'mast to closure dome' friction weld and subsequently to make the production welds.

Procedure optimisation trials

The header domes, forged from 316 grade austenitic stainless steel, were to be friction welded to tubular extensions machined from solid 69.8mm diameter 316 grade stainless steel bar stock obtained from a commercial stockist. The use of identical materials during optimisation and production was ensured by obtaining additional forged material cut from excess steel used during dome manufacture, so that test block simulations could be prepared. To assist welding the tubes, test blocks and domes were machined to the profiles shown in Fig.2. In view of the axial load, power and workholding required all welding was undertaken using The Welding Institute's FW3 machine rated at 1000kN maximum axial thrust, 75kW transmission power (hydraulic drive). To hold the domes for machining and friction welding a universal fixture was manufactured, which was capable of attachment to both a lathe and the welding machine (Fig.3).

Friction welding trials

To fulfil the production requirement of 13 completed superheater probe mast assemblies it was necessary to determine the optimum combination of welding variables and parts preparation which would provide a weld with acceptable metallurgical and mechanical properties. To establish the latter a simple hammer bend segment test was used. The test was carried out after the weld under evaluation was bored out to nominally 38mm and the external flash removed. The outside diameter was machined to 63.5mm for approximately 76mm above the block surface. The latter was also machined to 3mm below its initial surface. Eight individual segments were then cut through the tube. Alternate segments were hammered inwards and outwards until bend angles of ~90° were achieved, or failures resulted. Examples of such bend tested welds are shown in Fig.4. Weld strength was considered acceptable if all segments reached the 90° bend angle. Weld quality was further assessed by metallurgical examination of carefully prepared sections taken from the weld region.

After producing and evaluating 12 welds with the square preparation shown in Fig.2a, the combination of welding variables listed below was found to provide a weld of the required quality:

At this stage in the project the end preparation procedure was reviewed with the aim of reducing the total amount of machining required. The decision was thus taken to weld with tapered end preparations on both tube and block (Fig.2b). Clearly further trial welds were necessary to establish whether these changes would affect weld quality.

Two such welds were evaluated, and were found to be completely satisfactory, exhibiting similar bend test results to that demonstrated in Fig.4a, while their macrosections, as shown in Fig.5, revealed complete bond formation across the tube walls. As a result of the thermomechanical conditions existing during the process the weld lines display a curvature, while re-orientation of the micro-structure has also taken place.

Consistency trials

After discussion with Babcock Power it was agreed that 20 welds would be produced at the welding conditions given above, and that each would be assessed to determine weld quality. Eighteen welds were bend segment tested as described earlier, and all proved to satisfy the test criterion, as illustrated in Fig.6. Also, two out of the 20 welds were made on opposite faces of one block, then the assembly was machined to the tubular dimensions of the mast to provide a full size specimen for tensile test evaluation. The specimen, when tested in a 2000kN Baldwin machine, failed well away from the welds at a load of 1080kN, see Fig.7. Taking into consideration the tube cross section, the ultimate tensile strength achieved was 531 N/mm². The integrity of these consistency welds was further confirmed after sections from 18 of them had been examined microscopically.

Subsequent analysis of the instrumentation records taken during this stage revealed that the burn-off times (heating durations) varied within the range 7.7-8.3sec. This reflects slight variation of metal displacement rate during heat generation but from the test results was shown not to affect weld properties.

In view of the satisfactory results obtained from the consistency trials a pre-production dome/mast assembly was fabricated. Prior to friction welding, both the mast and dome were machined at their weld surfaces to the preparation dimensions shown in Fig.2c. After welding the assembly was machined to remove the flash and provide another weld preparation in accordance with a Babcock Power drawing, see Fig.8, to facilitate a cosmetic TIG welding operation. To confirm weld soundness, at least at the periphery, a dye penetrant test was carried out. To fill the weld preparation and provide sufficient metal for a generous radius to be machined in this area, 17 weld runs were needed.

The main objective of this extra work was to ensure a more acceptable profile to enhance fatigue life. On completion of metal deposition, dye penetrant testing of the weld region again demonstrated the absence of surface breaking defects. The assembly in its finally welded and machined state was carefully checked for dimensional accuracy. Although slight distortion of the mast had taken place the unit just satisfied the acceptance limits. For the further work needed to complete the overall assembly, i.e. the attachment of a tubular extension, it was despatched to Babcock Power's works at Tipton.

Production welding

Before commencing production welding of 12 assemblies, two pre-production test welds were produced with blocks and their integrity was evaluated by the procedures described above. This was considered necessary since in the interim period between the consistency trials and start of production the friction welder had been reset for another project. These test welds proved to be fully acceptable when welded at the optimised machine settings.

Before friction welding, the 12 domes were machined internally to provide the necessary weld preparations. Use was made of a locating peg in the fixture when setting up the domes in the lathe prior to machining. During setting up care was taken to ensure that the flats on the outside surfaces of the domes were located firmly against the rear of the holding fixture. Before machining a check was made of the main bore and weld preparation dimensions of the dome.

With weld preparations completed the holding fixture was loaded with a dome while it was separated from the friction welding machine, and then it was fitted to the axial moving but non-rotating carriage of the machine. It was ensured that the dome was accurately located in the holding fixture and subsequently in the machine. The mast was loaded into a collet attached to the rotating spindle of the machine and the weld was made. This procedure was repeated to produce each of the assemblies, see Fig.9. The instrumentation records, when checked, indicated an increase in heat duration of approximately 1sec when compared with the consistency and pre-production test welds, although the welding pressure levels and burn-off setting recorded were correct. This suggested that the increased thermal mass of the dome in comparison with the smaller test block was having an effect on the thermomechanical conditions existing at the weld interface, which resulted in a marginal lowering of the metal displacement rate.

The welded assemblies were machined to remove the flash and to provide the weld preparation in readiness for the TIG cosmetic runs. Prior to TIG welding all the friction welds were again dye penetrant checked and found to be free from surface defects.

During this production phase careful attention was given to: a) maintaining positive identification of the assemblies; b) recording the results of dye penetrant testing; and finally c) applying detailed tolerance measurement checks of the completed closure domes to the inspection and acceptance procedures laid down by Babcock Power Construction Division. Also throughout the duration of the development and production programme, there was close 'cooperation with a representative of the CEGB's PITB inspection branch, who visited on a number of occasions.

Concluding remarks

It is gratifying that friction welding - a solid phase bonding process - has successfully been applied to assemblies which are to be used in nuclear power plant. This project has served to demonstrate the close liaison and cooperation that can be achieved between The Welding Institute and its Research Members. Technically, the welding trials showed that sound joints can be achieved by continuous drive friction welding between 63.5mm OD x 36.6mm ID stainless steel tube (316 grade) produced from bar stock and forged stainless steel test blocks. Such joints exhibited acceptable bend, tensile and metallurgical properties.

Acknowledgement

The authors are grateful to Babcock Power Ltd, the CEGB and the National Nuclear Corporation for permission to publish this article. Thanks are also due to Messrs M Camping, R Lilly, D Wilson, D Patten and J Dobbs who were responsible for the efficient progress of this project.