ABSTRACT

Nickel alloy UNS N10276 is nominally a single-phase, corrosion-resistant alloy used in a wide variety of applications, particularly in the chemical process industries. Rich in alloying elements like Mo and Cr that promote corrosion resistance, the grain boundary precipitation of mu (µ) phase is avoided since it is typically associated with sensitization. Recent work by the present authors to create an atlas of microstructures for this alloy included experimental validation of calculated Time-Temperature-Transformation (TTT) curves as a function of variation in alloying elements within the composition space specified for the alloy. Precipitation of the intermetallic mu (µ) phase was experimentally determined to be the exclusive result of the heat treatment regimens applied to verify the positions and shapes of calculated TTT curves. The present work is an empirical investigation of the sensitization behavior of alloy UNS N10276 as a function of mu precipitation - the first step toward generating Time-Temperature-Sensitization (TTS) curves that can be directly compared to empirical TTT behavior.

INTRODUCTION

Nickel based alloys are widely used across process industries where corrosion resistance and good mechanical strength at elevated temperatures are valued. Unlike their stainless steel counterparts that undergo the ferrite-to-austenite transformation upon heating and/or cooling, nickel alloys like UNS N10276 maintain a fully “austenitic” structure from solidification to room temperature. However, thermal stability is measured by more than just the base metal microstructure as intermetallic phases can and do precipitate, particularly at grain boundaries. In fact, this alloy was developed as an improvement to another alloy in this class, which exhibited severe susceptibility to intergranular corrosion in the heat affect zones as-welded condition1. With the advent of argon oxygen decarburization (AOD) technology in the 1960s, carbon and silicon levels could be significantly lowered thereby significantly reducing the precipitation potential of the “new” UNS N10276. Since its creation, this alloy continues to be widely used across a variety of industries in corrosive applications, particularly in the chemical processing industry. The microstructural development of UNS N10276 has been examined by numerous authors in the technical literature including intermetallic precipitation2-4 and weld solidification behavior5-7 with the focus on Mo-rich phases M6C (diamond cubic), µ (hexagonal), and P (orthorhombic). Similarities in intermetallic phase composition5 and morphology makes positive phase identification difficult without the use of electron backscattered diffraction (EBSD)3 or thin film evaluation in a transmission electron microscope (TEM)8. Work published recently3 by the present authors resolved that the phase precipitating in the UNS N10276 plate under investigation in the present study is exclusively the µ phase though previous authors have reported M6C4 and P2 phases as well. The details of phase identification are beyond the scope of the present paper, but the authors acknowledge that the technical literature is not unanimous on the identity of the intermetallics that form in UNS N10276 though it is important to note large differences in heat treatment schedules observed in many applicable studies. The authors also note some apparent disagreement in the reported crystal structure for the µ phase, with some authors reporting rhombohedral4 and others5 reporting hexagonal crystallography. The reason(s) for this disparity may be associated with differences in reported composition though further work is necessary to provide a more conclusive determination on this point. The backscattered diffraction patterns in the present study were found to be consistent with the hexagonal µ phase and not the rhombohedral form as shown below in Figure 1 and more completely elsewhere3. The absence of intermetallic phases other than µ is likely attributed to the low carbon content in the alloy though the role of grain boundary character on intermetallic nucleation kinetics is being examined by the present authors in future work.

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