Remote crack repair - the laser solution

TWI Bulletin, March/April 2001

Nettem Sekhar, a Project Leader at TWI, has been involved in the development of procedures for welding and cutting ferrous and non-ferrous alloys using Nd:YAG lasers. He has been active in the application of the procedures to industrial context. He is currently pursuing a PhD at the University of Cambridge, UK.

Fibre delivered Nd:YAG laser systems offer possibilities for remote processing applications. In this work Nettem Sekhar and Paul Hilton investigate the feasibility of performing an in situ repair on a low silicon content mild steel component, used in the core region of a gas cooled Magnox reactor.

Uranium in the UK's Magnox type reactor fuel cells is sealed into cans to prevent the escape of fission products. It is necessary to have a system that will provide a warning of any developing leaks in the fuel cans at an early stage. In Magnox reactors, a 'burst can detection' (BCD) system monitors the coolant gas as it emerges from the top of the reactor core. Inside the reactor pressure vessel (RPV), BCD gas sampling lines are suspended, allowing the gas to be sampled at various locations. The sample lines are supported by free hangers. Figure 1 shows four BCD hangers in situ, taken by a remote camera. The BCD hangers are constructed from two plates of low silicon content mild steel, of thickness about 4mm, arc welded to each side of a spacer of 6mm thickness.

The hangers are susceptible to oxide jacking at the interface between the side plates and the spacer, which can cause the welds to crack. Oxide jacking is a phenomenon caused by the build-up of oxide on mild steel in a carbon dioxide atmosphere at high pressure and temperature. As the oxide builds up between the parts, stresses can be generated that force the components apart. This can lead to cracking in welds or fracture in bolts. Repair of components may be required in-situ, to prolong the service life.

Various methodologies ^[1-4] have been developed for the repair of similar components in nuclear power plants. Most involve arc welding methods but the use of fibre delivered laser systems has also been investigated ^[5] due to the advantages that lasers could provide. The objective of the work reported here was to develop procedures for crack repair on a BCD hanger, using a fibre delivered Nd:YAG laser beam.

The approach

Dummy samples were manufactured, oxidised and cracked, for the repair experiments, which were conducted in a laboratory environment. The samples were generally covered in an oxide layer of approximately 0.4mm thickness. Test samples had been provided oxidised with the external oxide layer removed. A 4kW Nd:YAG laser, capable of producing a 0.6mm focused spot, was used for the repair. The fibre end was mounted on a six axis robot while the workpiece was held stationary. The specimens were held vertically in a vice. The welding head was positioned in relation to the workpiece as shown in Fig.2. A18 mild steel MIG filler wire, 1.2mm diameter, was used. Helium gas (at flow rates of 50 litre/min) was used.

Two possibilities for repairing the cracks and thus increasing the service life of the component were evaluated. The first involved the laser beam remelting the crack formed in the weld metal of the original arc welds and the second using filler wire to reinforce the original weld. Complete repair can only be obtained by fully remelting the crack and the surrounding region. A major difficulty in achieving this was that the origin and the direction of propagation of the cracks were not known and differed from sample to sample. Five techniques were evaluated: use of linear single and double pass autogenous remelting of the cracked region, use of linear single and double pass remelting with wire feed addition and use of a weave pass technique with wire feed.

Results and analysis

A technique to remove the oxide layer from the sample surface was developed. When using a laser power of 1kW at the workpiece, the laser beam focus 20mm above the surface and scanning speeds of 2m/min, the 0.4mm thick oxide spalled off as flakes, over a width of ~15mm on each side of the impingement point of the laser beam. No signs of melting of the base metal were observed. The differential thermal cycling of the oxide and the substrate (base metal) and the difference in the thermal expansion coefficients between the oxide and the substrate led to the delamination of the oxide layer when the laser was scanned over the sample.

The results of a single repair pass without wire feed addition, on a sample with oxide, indicated incomplete healing of the crack ( Fig.3) and unacceptable undercut. In samples with the oxide layer removed, the penetration of the laser beam was in fact lower, attributed to the fact that the oxide layer assisted in the coupling of the laser beam.

In a single pass repair using filler wire addition on a sample with oxide removed, a highly convex and wider bead profile was obtained but with less penetration ( Fig.4). The reduced penetration was due to the use of a defocused laser beam, filler wire addition and absence of oxide. Downward progression of the filler was observed in the welds. In the absence of complete knowledge of the position of the crack, it was not possible to repair it with a single pass due to the limited size of the molten pool available. In an attempt to overcome this problem, repairs were also made using two passes.

In two pass welds on oxidised samples using filler wire addition, larger melt areas were observed. Although full healing of the crack was obtained, minor inclusions were seen at the root of the first pass. As an unacceptable degree of undercut in the resultant top bead could be seen further work using a weaving laser pattern was undertaken.

It was noticed that the aspect ratio (depth/width) of the repairs made with the laser focused at the surface was of the order of two. It was believed that such a high aspect ratio would not be required for repair so a defocused laser beam was used with the weave technique to produce a wider weld.

A section of a repair made on a sample with oxide using the weave technique ( Fig.5) and with filler wire addition is shown in Fig.6. From the cross section it can be concluded that remelted welds with no observable cracks had been produced. Distributed inclusions and minor undercut at the levels seen, were deemed to be acceptable.

Hardness surveys were carried out on a sample repaired using the weave pattern technique. Sections were made at two different positions along the length of the weld and traverses were made at the root and cap of the welds. An increase in hardness (~90%) was observed in the weld metal of the repaired hanger compared to the base metal. The maximum hardness values obtained (210HV) suggest that there is no brittle phase (martensite) formation in the weld. The higher hardness values in the weld can be attributed to the composition of the weld metal and the weld thermal cycle. No significant variation was observed between the hardness values obtained at the root and cap of the weld nor between the two locations along the weld length. The hardness values were very similar to those obtained in samples repaired by existing MIG welding technique.

Discussion

A major advantage of the laser approach is the need to deploy only one package into the reactor. A more compact laser head would need to be developed, to gain better access to the components requiring repair. As the beam quality of this type of laser is constantly improving, this is not thought a problem.

Conclusions

• Using a laser power of 1kW with the workpiece surface 20mm below the beam focus, effective oxide removal was possible at 2m/min.
• Repairs made using a weave pattern technique successfully remelted the crack with no appreciable change in the hardness values recorded at various locations along the weld length and from the root to the cap of the weave pass weld.
• Generally, better penetration in the repaired sections could be seen on samples covered in oxide due to increased absorption of the laser energy in the oxide covered areas compared to the base material of the hanger.
• Compared to a MIG repair process, reactor downtime could be less using the laser process as it has the potential to avoid two additional steps required in MIG.

References