Low-temperature Repair of Cracks in Stainless Steel Canister using Vaporizing Foil Actuator Welding

Background and Motivation Materials and Methods

Future work

Results

References

Conclusions

Jianxiong Li, Anupam Vivek , Glenn Daehn

Department of Materials Science and Engineering, The Ohio State University, OH, 43210, USA

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• VFA welding and its variants present high potential in crack repair of 304 SS UNF canisters. These crackrepair techniques are low-temperature operations with no melting and no Heat affected zoneformation.

• VFA patch welding, preformed flyer VFAW, and combined shearing and welding methods involving flattarget and inclined target are all working well. Inclined VFAW can minimize the unwelded area, which isthe preferable technique.

• Of many austenitic stainless steels, 0.5-in 304L stainless steel (Fe-18 Cr-8 Ni) is the most frequently used UNF canister material and will be the focus of this proposedinvestigation.

• Different repair strategies using VFA welding or shearing are developed in this work such as such as patch welding, combination of shearing and welding, spotwelding with preformed flyer, and inclined VFAW.

[1] T. Mintz, L. Caseres, X. He, J. Dante, G. Oberson, D. Dunn, T. Ahn, Atmospheric Salt Fog Testing to Evaluate Chloride-Induced Stress Corrosion Cracking of Type 304 Stainless Steel, Corrosion. 2012, 11-15.[2] C.R. Bryan, D.G. Enos, nuclear fuelUnderstanding the risk of chloride induced stress corrosion cracking of interimstorage containers for the dry storage of spent : Evolution of brine chemistry on the container surface, NACE -International Corrosion Conference Series, 2016, pp. 3502-3516.[3] A. Vivek, G.A. Taber, J.R. Johnson, S.T. Woodward and G.S. Daehn, “J. Mater. Process. Technol., 2013, 213: 1311-1326.[4] A. Vivek, S.R. Hansen, B.C. Liu and G.S. Daehn. Vaporizing foil actuator: A tool for collision welding. J. Mater. Process.Technol., 2013, 213: 2304-2311.

AcknowledgementThe authors would like to thank DOE NEUP program office for providing funding andsupport. We are also thankful to our research group members and Dr. Ramirez’s groupfor providing background support and materials to work with.

2019 AWS Professional Program, November 11-14, 2019

Contact:li.8266@osu.eduImpulse Manufacturing LaboratoryThe Ohio State University, www.iml.osu.edu

• Used nuclear fuel (UNF) canisters made of 304 stainless steel (SS) have the possibility of forming atmosphericchloride-induced pitting (CIP) and chloride induced stress corrosion cracking (CISCC) due to their serviceconditions and material characteristics.

• The formation of stress corrosion cracking could lead to confinement loss and structural strength reduction of thecanisters during fuel storage and transportation.

• 304 SS has been shown to be sensitive to both CIP and CISCC when exposed in chlorides [1]. UNF canisters aremostly made by arc welding, containing significant residual stresses, one of the major reasons for CISCC. These drystorage UNF canisters are cooled by atmospheric air convection as well as served in salt-bearing environments,which facilitates the formation of chlorides on the canister surface [2].

Fig. 1. Storage of Spent Nuclear Fuel Fig. 2. Example Vertical Dry Cask

• Once removed from the reactor, spent fuel is transferred into a spent fuel pool which provides shielding and heatremoval as shown in Fig. 1. Spent fuel assemblies are then transferred into nuclear regulatory commission (NRC)licensed dry casks and stored within an independent spent fuel storage installation (ISFSI). Fig. 2 shows anexample vertical dry cask with canisters made of welded austenitic stainless steel.

Fig. 3. Chloride induced pitting corrosion in 304 SS Fig. 4. Stress corrosion cracking in stainless steel

• Repair and mitigation technologies must exhibit low heat input, no spark sources and acceptable externalforces such as tensile residual stresses, which can avoid severe strength reduction, mitigation of penetration ofhydrogen into the canister and prevention of deformation and distortion of parts.

• The DOE canister repair project aims to evaluate the suitability of four processing technologies to combat againstCIP and CISSC in 304 SS, one of the commonly UNF canister materials. These four processing technologies aretemperature controlled FSW, cold spray deposition, vaporizing foil actuator (VFA) welding, and soldering. Thisposter focuses on the VFA welding method.

Fig. 5. Schematic diagram of VFA welding process. (A) electrically driven vaporizing metallic foils [3] ; (B) VFA welding process [4]

• Using this technique, a thin sheet of stainless steel in similar composition to the canister material can belocally welded over a pitted, cracked, or degraded region.

• VFAW can provide compressive stresses on the cracking areas which prevents the initiation or propagation ofSCC. This repair strategy could be used to prevent future CISCC during the service life.

• VFA welding utilizes a high-pressure pulse generated by the vaporization of aluminum foils to launch a flyersheet toward a stationary target to create welds.

Fig. 7. VFA patch welding

Fig. 9. Combined VFA shearing andwelding

Fig.8. VFA welding setup using preformed 304 SS flyer.

Fig. 10. Inclined VFAW setup for combinedVFA shearing and welding method

Fig. 14. (A) Inclined VFA weld appearance with circularsheared part. (B) Interfacial microstructure of inclined VFAweld.

• For VFA patch welding, the extra materials at both sides were machined away. The patch weld showed clean and airtight edges which can prevent electrolytes fromcoming into the welds.

• For VFA welding using preformed 304 SS flyers, after peeling, failure occurred at the SS flyer base metal, which means the weld is stronger than the base metals. Butthe extra materials need to be removed by machining.

• For VFA combined shearing and welding method, oval-shape or circular shape sheared parts can be used depending on the type of cracks.• For inclined VFA welding, a 10˚ angled target block was used to place the target sheet. The circular sheared parts are accelerated at this angle to collide with the

target and formed a solid-state collision weld. Elongated and refined grains are shown near the interface. No melting and HAZ formation are shown in themicrostructure.

Fig. 12. VFA weld appearance after peeling. (A) VFAweld at input energy of 6 kJ; (B) VFA weld afterpeeling.

Fig. 13. Flat VFA shearing andwelding with oval-shape shearedparts.

Fig. 6. 304 SS base metal microstructure

• Equiaxed grains with grain sizes ofEquiaxed grain structure, grain size~35 µm 12 µm (ASTM 6.5)

• Stringers aligned along rollingdirection.

• Avoid extra materials remainingaround the oval-shape sheared part.

• May contain unbonded zone in themiddle.

• Failure occurred at base metals afterpeeling. This indicates reliable welding.

• Extra materials remaining could be anissue and post weld machining is needed.

• Avoid extra materials remaining around theoval-shape sheared part.

• Larger welding area compared to flat weldingsetup due to preset impact angle.

• Elongated grains, and shear bands formed onboth sides.

• EDM notches as mock-up cracks• 4-point bend testing fixture

Fig. 11. Patch VFA weld after machining.

• Extra materials on both sides aremachined.

• Middle welded area is air-tight, preventingelectrolytes coming into the repaired area.

Repaired area

Flyer

• Four-point bend test in boiling MgCl2 submersion testing at DNV (ASTM G36)