## 1 Introduction  
Incoly 800H (or Alloy 800H, Unified Numbering System No. N08810) is one of a few code-qualified materials used at temperatures up to 760 \({}^{\circ}\)C for fabricating in-core and out-of-core components operating in high-temperature reactors, e.g., reactivity control rods and intermediate heat exchangers. The alloy is an iron-based alloy with a nominal composition of Fe-32.5 Ni-21 Cr [1]. To improve the creep rupture properties, Alloy 800H maintains a carbon content of at least 0.05%. Alloy 800H must undergo solution annealing at temperatures above 1050 \({}^{\circ}\)C, such as 1093 \({}^{\circ}\)C, to achieve a stable austenitic structure. It is intended for use at temperatures > 593 \({}^{\circ}\)C [1]. The annealing treatment at temperatures above 1050 \({}^{\circ}\)C is known to produce an ASME grain size number of 5 or coarser, i.e., coarser than 65 \(\mu\)m. This alloy is currently approved under ASME Section III, Division 5 for nuclear service up to 760 \({}^{\circ}\)C [2]. There are efforts to extend the ASME-allowable stresses of Alloy 800H beyond 800 \({}^{\circ}\)C, preferably up to 950 \({}^{\circ}\)C, and to achieve a service lifetime of 60 years. These efforts are based on the concepts of longer lifetimes and higher operation temperatures, including under off-normal and accident conditions, for various components used in the development of modern high-temperature gas-cooled reactors [3].  
Welding processes are required for assembling components in the application of Alloy 800H. Achieving high reliability in a long-term, high-temperature service environment requires the welds to possess high creep resistance [4]. However, a suitable filler material to improve the high-temperature creep resistance of the Alloy 800H weldments is required in the high-temperature application. Cross-weld creep testing of most Ni-based Inconel and stainless steel alloys indicates that weld strengths are usually in the range of 50-100% of those of the Alloy 800H base material [4; 5; 6]. Commonly used filler materials for Alloy 800H welding, such as Inconel 82, 182, 188, and SS 304 alloys, exhibit slightly lower creep rupture stresses than Alloy 800H base metal, in the range of 538 to 760 \({}^{\circ}\)C, regardless of whether the exposure time is short or long [5; 7; 8]. A recent investigation at the Idaho National Laboratory using Inconel 617 filler material did not demonstrate an improvement in creep rupture properties [9]. In addition, the high content of cobalt in Alloy 617 (10-15 wt.%) makes it unsuitable due to the activation of the cobalt under neutron irradiation. Haynes 230, a solid-solution-strengthened Ni-Cr-W alloy, exhibits more enhanced creep rupture strength than Alloy 800H at temperatures higher than 475 \({}^{\circ}\)C. It also demonstrates better corrosion resistance at elevated temperatures and contains a lower cobalt concentration [10]. Inconel 625, a Ni-Cr-Mo alloy with additional Nb, has an excellent combination of corrosion resistance and superior creep resistance at high temperatures over 600 \({}^{\circ}\)C [11; 12]. In this paper, Inconel 625 and Haynes 230 were selected as candidate filler materials to produce Alloy 800H weldments (WMs) using the gas tungsten arc welding technique. The high-temperature tensile and creep properties of these weldments were then compared and analyzed. The microstructures before and after the creep test and fracture mode were reported. The creep mechanisms, dislocation structure, and precipitates evolution were explicated.