Assessment of the Need to Consider HAZ Properties as Part of RPV Integrity Management

Abstract

Abstract A heat-affected zone (HAZ) forms adjacent to any weld; it is the region of the base metal that has not melted but rather has had its properties affected by the heat of welding. The physical size of the HAZ depends on the welding heat input and section thickness; generally, reactor pressure vessel (RPV) structural welds have a HAZ that extends into the base metal by ≈3 mm. Due to the complex microstructures in the HAZ, presence of residual stresses, and potential for undetected and (therefore) unrepaired indications HAZ regions have long been identified as a potential Achilles heel of component structural integrity. Consequently, HAZ toughness properties have long been monitored as part of RPV surveillance and their consideration in structural integrity assessments has been required by ASME Code and regulations. Considerable anecdotal evidence has accumulated over time suggesting that the toughness properties of the HAZ are, if anything, superior to that of the adjacent base metal and weld. As such, continued monitoring and assessment of these properties may no longer be needed to ensure the operating safety of RPVs. In this paper we provide a review of existing HAZ requirements in several countries and a comprehensive analysis of HAZ data in comparison with companion base and weld metal data. This analysis demonstrates that the HAZ material embrittles at a rate well modeled by existing predictive models for base metals, albeit with more uncertainty. This result suggests that in plants where the weld metal is more sensitive to the effects of embrittlement than the base metal (and, by association, the HAZ), the HAZ can never have the highest T41J value after embrittlement. For plants in which the base metal is more sensitive to the effects of embrittlement than the weld metal the only way for the HAZ transition temperature to exceed that of the base metal is via data scatter. Of the surveillance data examined the T41J of the HAZ exceeds that of the base metal less than 1% of the time.