Precipitation within localised chromium-enriched regions in a Type 316H austenitic stainless steel

Abstract

A Type 316H austenitic stainless steel component containing Cr and impurity element-rich localised regions arising from component fabrication was aged for a prolonged period during service at a temperature of approximately 550°C. These regions make up approximately 5% of the total volume of the microstructure. Previous work has shown that these regions contain ferrite and carbide precipitates and a finer austenite grain size than the adjacent matrix. The present study has used high-resolution transmission electron microscopy combined with compositional microanalysis to show that these regions have a highly complex microstructure containing G phase, chi phase and intragranular c 0 precipitates within the austenite grains. There is phosphorus migration to the chi austenite phase boundary, and the basis for this equilibrium impurity segregation is discussed. A Cr-depleted region was observed surrounding the chi phase precipitates, and the impact of this on the other precipitates is considered. The diversity of precipitates in these Cr-rich regions means that they behave significantly differently to the bulk material under long-term creep conditions leading to preferred nucleation and growth of creep cavities and the formation of localised creep cracks during service.

The Implications of Fabrication and Cast-to-Cast Variability on Thermal Aging in the Creep Range for AISI Type 316H Stainless Steel Components

“…Chi phase has been observed to have deleterious effects on corrosion resistance due to sensitization and a reduction in fracture toughness. [22,29] Given the observed pile-up of dislocations at austenite-chi-phase boundaries in the header examined in this study, [40] it is surprising that it has not been linked to modifications of the creep behavior in previous studies. [24,31] The Cr depletion around the chi-phase precipitates reduces the solute-strengthening effect in these regions, making the material weaker and further enhancing the influence of dislocation pile-ups.…”

“…[42] One of the examples presented is an AISI Type 316H which is compositionally similar to the boiler headers. Comparison between the predicted phases and those phases observed in the header interdendritic regions [40] showed significant discrepancies. This is presumably due to the localized, Cr-rich environment present in the interdendritic regions.…”

“…The TEM identification of the precipitate species in the header material has been extensively discussed in a previous paper. [40] Figure 3 Figure 4 shows EDX maps from bifurcations B and C. Electron diffraction characterized the secondary phases present in bifurcation A as ferrite and M 23 C 6 , while bifurcation C contained ferrite, G phase, and M 23 C 6 carbide. In both Figures 4(a) and (b), the G phase can be recognized by its higher silicon content, which is not present in the maps from bifurcation A.…”

“…Phosphorus segregation at austenite-ferrite boundaries has been predicted. [40] It should be noted that chromium depletion (À 1.96 wt pct) has been observed surrounding chi precipitates affecting the local precipitation behavior. [40] Bifurcation A showed phosphorous segregation Atomic resolution imaging in the header showed periodic dislocations at the austenite-chi-phase interface, Figure 6(a), along with increased dislocation density around chi-phase precipitates under a range of tilt conditions, Figure 6(b).…”

“…[40] It should be noted that chromium depletion (À 1.96 wt pct) has been observed surrounding chi precipitates affecting the local precipitation behavior. [40] Bifurcation A showed phosphorous segregation Atomic resolution imaging in the header showed periodic dislocations at the austenite-chi-phase interface, Figure 6(a), along with increased dislocation density around chi-phase precipitates under a range of tilt conditions, Figure 6(b). It is suggested that the greater lattice mismatch between the austenite and the chi phase than between austenite and ferrite (chi phase a = 0.795-0.845 nm [22] vs austenite a = 0.358 nm [41] and ferrite a = 0.285-0.289 nm) may play a role in favoring segregation of phosphorus to chi phase over ferrite.…”