Friction Stir Welding and Preliminary Characterization of Irradiated 304 Stainless Steel

Abstract

The mitigation of helium induced cracking in the heat affected zone (HAZ), a transition metallurgical zone between the weld zone and base metal, during repair welding is a great challenge in nuclear industry. Successful traditional fusion welding repairs are limited to metals with a maximum of a couple of atomic parts per million (appm) helium, and structural materials helium levels in operating nuclear power plants are generally exceed a couple of appm after years of operations. Therefore, fusion welding is very limited in nuclear power plants structural materials repairing. Friction stir welding (FSW) is a solid-state joining technology that reduces the drivers (temperature and tensile residual stress) for helium-induced cracking. This paper will detail initial procedural development of FSW weld trials on irradiated 304L stainless steel (304L SS) coupons utilizing a unique welding facility located at one of Oak Ridge National Laboratory’s hot cell facilities. The successful early results of FSW of an irradiated 304L SS coupon containing high helium are discussed. Helium induced cracking was not observed by scanning electron microscopy in the friction stir weld zone and the metallurgical zones between the weld zone and base metal, i.e. thermal mechanical affected zone (TMAZ) and HAZ. Characterization of the weld, TMAZ and HAZ regions are detailed in this paper.

Microstructure and Mechanical Performance of the Friction Stir Welds Made on Neutron-Irradiated Steel with Helium

“…Although promising, the FSW had not been used for joining irradiated materials until Oak Ridge National Laboratory's (ORNL's) team, including the present report co-authors, who also were the major contributors to FSW and characterization efforts, successfully demonstrated FSW on irradiated 304L SS for the first time in late 2017 [6,8,9].…”

“…Up to now, FSW has been successfully applied on three irradiated 304L SS coupons that contained ~5, ~10, and ~20 weight parts per million (wppm) of boron before irradiation, respectively. Preliminary characterization showed no macro helium-induced cracking observed in a metallographic specimen removed from the center of these joints [6,8,9]. Currently, the remaining welded specimens, as well as leftover coupon blocks, are stored at ORNL to support the current work and future investigations.…”

“…During FSW, the material is heated by the friction between the welding tool and welded material and by plastic deformation of the material being welded. Heated material flows/deforms around the welding tool and forms a joint due to mechanical mixing and recrystallization [6]. Tool wear during high-temperature materials FSW is often only mentioned as a minor side effect [7].…”

“…FSW is arguably the most recent invention in welding technologies [6]. It has been widely studied and developed with different materials, from nonferrous metals to ferrous metals and steels to high-alloy refractory materials.…”

“…This development of irradiated 304L SS FSW and subsequent initial characterization [6,8] was a joint effort of several organizations, programs, and projects, including the US Department of Energy (DOE) Light Water Reactor Sustainability (LWRS) Program. Within this effort, custom 304L SS specimens, with a controlled amount of boron, were irradiated at the High Flux Isotope Reactor (HFIR) at ORNL to receive a controlled amount of transmuted helium from boron.…”