Effects of alloying and processing modifications on precipitation and strength in 9% Cr ferritic/martensitic steels for fast reactor cladding

Abstract

P92 was modified with respect to alloying and processing in the attempt to enhance high-temperature microstructural stability and mechanical properties. Alloying effects were modeled in ThermoCalc® and analyzed with reference to literature. ThermoCalc® modeling was conducted to design two low-carbon P92-like low-carbon alloys with austenite stabilized by alternative alloying; full conversion to austenite allows for a fully martensitic structure. Goals included avoidance of Z-phase, decrease of M23C6 phase fraction and maintained or increased MX phase fraction. Fine carbonitride precipitation was optimized by selecting alloying compositions such that all V and Nb could be solutionized at temperatures outside the -ferrite phase field. A low-carbon alloy (LC) and a low-carbon-zero-niobium alloy (0Nb) were identified and fabricated. This low-carbon approach stems from the increased creep resistance reported in several low-carbon alloys, presumably from reduced M23C6 precipitation and maintained MX precipitation [1], although these low-carbon alloys also contained additional tungsten (W) and cobalt (Co) compared to the base P92 alloy. The synergistic effect of Co and W on the microstructure and mechanical properties are difficult to deconvolute. Higher solutionizing temperatures allow more V and Nb into solution and increase prior austenite grain size; however, at sufficiently high temperatures δ-ferrite forms. Optimal solutionizing temperatures to maximize V and Nb in solution, while avoiding the onset of the δ ferrite phase field, were analyzed in ThermoCalc®. Optical microscopy showed ThermoCalc® predicted higher δ-ferrite onset temperatures of 20 °C in P92 alloys to nearly 50 °C in the designed alloys of the critical temperature. Identifying the balance where maximum fine precipitation is achieved and δ-ferrite avoided is a key factor in the design of an acceptable P92-like alloy for Generation IV reactor cladding. Processing was further modified utilizing thermomechanical processing (TMP) simulations with the Gleeble® 3500. Hardness increased substantially in thermomechanically processed alloys, with increased hardness strongly correlating to decreased TMP temperature. The most significant difference between low- and high- temperature thermomechanically processed specimens was an increase in crystallite size at the higher temperature. The fundamental reason for higher strength in the TMP conditions is higher dislocation density, as precipitate volume fraction was not specifically improved in TMP conditions. Thermal stability of the base P92 and of the experimental alloys was analyzed by aging the alloys for times ranging from 500 to 10,000 h at 550, 600, 650, and 700 °C. Results suggest the hardness and thermal stability of LC is greater than that of 0Nb at lower aging temperatures and shorter times, with 0Nb surpassing LC microhardness at 10,000 h at 650 °C and for most conditions aged at 700 °C. Small- and wide-angle x-ray scattering (SAXS/WAXS) was conducted at Argonne National Laboratory (ANL). Atom probe tomography (APT) and scanning transmission electron microscopy high-angle annular dark field (STEM-HAADF) in conjunction with EDS were used to elucidate x-ray findings. These microstructural characteristics were then correlated with mechanical properties, including Vickers microhardness testing, elevated-temperature tensile testing, and creep rupture testing. The designed alloys exhibited less stable microstructures leading to less favorable mechanical potencies, as compared to the base P92 alloy. It is posited that factors other than inclination towards MX over M23C6 precipitation are important in generating thermal stability and high-temperature strength, i.e. perhaps the solid solution or diffusion controlling effects of Co in the low-carbon variation of Taneike’s alloys [1] delay martensite recovery. The refined thermal profiles, however, put both P92 and LC creep strengths beyond those found in literature.