Development of PWHT-Free, Reduced Activation Creep-Strength Enhanced Bainitic Ferritic Steel for Large-scale Fusion Reactor Components

Using reduced-activation steels will be highly beneficial for fusion reactor vacuum vessels and structural rings, operating in a temperature range of 400-550°C, to address high volume recycling and/or disposal considerations of large-scale structural components in the simplest waste streams. Low Cr containing Cr-W bainitic ferritic steels, based on Fe-3Cr-3W-0.2V-0.1Ta-0.1C (ID: 3Cr-3WVTa), meet these property requirements due to high-temperature strength that exceeds ferritic-martensitic steels up to 550°C, improved toughness with low DBTT, reduced-activation characteristics by control of alloying additions, and inexpensive material cost compared to high Cr-containing RAFM steels or austenitic stainless steels. A modified 3Cr-3WVTa bainitic steel was recently proposed, targeting a post-weld heat treatment (PWHT)-free steel with an advanced alloy design concept. The approach balances high hardenability and low hardness in the as-welded conditions through compositional modification as well as minimized microstructural impact at the heat affected zone. The modified steel with increased Mn and reduced C additions has successfully validated the proposed design concept by achieving improved creep performance across the weldments up to 550 °C and higher room-temperature impact toughness of the weld metal without PWHT. The scope and technical achievements of the past decades support three different research directions in 3Cr-3WVTa bainitic steels at ORNL: (1) property improvement of modified 3Cr-3WVTa bainitic steel weldments by process optimization, (2) application of the developed “PWHT-free” bainitic steel concept in component production through additive manufacturing (using wire-direct energy deposition), and (3) exploration of “Mn-free” modified bainitic steel to minimize irradiation-induced embrittlement. The details of all manufacturing and property aspects from computational and experimental approaches will be discussed. This research was sponsored by the Office of Fusion Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with UT-Battelle LLC.