Co-dependent microstructural evolution pathways in metastable d-Ferrite in cast austenitc stainless steels during thermal aging

Journal Article · Thu Nov 01 00:00:00 EDT 2018 · Acta Materialia

DOI:https://doi.org/10.1016/j.jnucmat.2018.08.038· OSTI ID:1511266

Lach, Timothy G.  ^[1]; Devaraj, Arun  ^[1]; Byun, Thak Sang  ^[1]

---

1. BATTELLE (PACIFIC NW LAB)

Cast austenitic stainless steels (CASS) are good alloys for corrosive environments such as light water reactor coolant systems because they combine high corrosion resistance with high strength and toughness. However, they are susceptible to embrittlement upon long-term thermal aging at service temperatures. Thus, the microstructural evolution pathways during thermal aging need to be understood to predict and potentially prevent thermal aging embrittlement. Atom probe tomography was used to identify and quantify the microstructural evolution pathways of the d-ferrite in four different CASS alloys thermally aged for 1,500 and 10,000 hours at 290 °C, 330 °C, 360 °C, and 400 °C. The four steels – CF8, CF8M, CF3, and CF3M – which vary by Mo and C concentration, each experienced spinodal decomposition of the d-ferrite into Fe-rich a and Cr-rich a’, precipitation of Ni/Si/Mn rich G-phase clusters, and precipitation of Cu clusters attached to the G-phase. The amount – compositional amplitude and wavelength for decomposition and volume fraction for G-phase/ Cu precipitation – of each mechanism increased with aging time and aging temperature. However, the four alloys differed in the extent of these features due to their differences primarily in Mo and C concentration, with high Mo alloys experiencing greater spinodal decomposition, G-phase precipitation, and Cu co-precipitation, and low C alloys experiencing greater G-phase precipitation and Cu co-precipitation. Using radial distribution function analysis, the interactions of constituent elements was found to determine the extent of these features with Mo and C specifically influencing the movement of Cr, Ni, Si, Mn, and Cu atoms due to their miscibility, or lack thereof, with these elements. The results here will help inform predictive models for the use of duplex stainless steels for extended operation at high service temperatures.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1511266
• Report Number(s): PNNL-SA-134908
• Journal Information: Acta Materialia, Vol. 510
• Country of Publication: United States
• Language: English