The effects of repetitive thermal cycles on the microstructure of repair welds in Zirconium 702

ABSTRACT

In the current scenario of the chemical industry, deeply understanding assets and their failure modes reduces machine downtime, production loss and maintenance costs. Zirconium 702 appears as an excellent material for manufacturing nuclear reactor components due to its resistance to corrosion at high temperatures. This material is also widely used in the manufacture of process equipment that requires high resistance to corrosion. This study addressed the effects of consecutive thermal cycles on the microstructure of the material when subjected to GTAW (tungsten-gas arc welding), repair welding by surveying metallographic aspects and mapping the hardness of the HAZ (heat-affected zone). The HAZ was identified as a transition region with a gradual increase in hardness between 5% and 20%, directly related to the increase in the geometry of the Zirconium grains, generated by the morphological change from phase α to phase β at temperatures above 863°C. It is worth noting that the uneven cooling of this region causes the formation of fine lamellar martensitic recrystallization structures, which have good toughness but low plasticity. Such characteristics present potential risks to repetitive welding, especially from the third repair sequence onwards, generating points of attention to the repair process in equipment manufactured with Zirconium 702.

Keywords
Zirconium 702; Repair welding; Thermal cycles; Hardness; Microstructure