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 γ′ 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.
Highlights
The microstructure of an austenitic stainless steel has a significant role in controlling the physical, chemical and mechanical properties
G phase evolution in austenite is associated with exposure to higher temperatures, within a nominal range of 500–800 °C [13], where the kinetics have been purported to be controlled by the rate of Si diffusion [21]
We describe the microstructure of fine-grained, Cr-enriched regions arising as a consequence of failure to homogenise the original cast microstructure during the thermo-mechanical fabrication of an ex-service Type 316 austenitic stainless steel component subject to a post-service heat treatment
Summary
The microstructure of an austenitic stainless steel has a significant role in controlling the physical, chemical and mechanical properties. The evolution of secondary phases during thermal ageing of Type 316 austenitic stainless steels has been observed experimentally [5,6,7,8,9] and predicted thermodynamically [10], driven by a favourable Gibbs energy [11] These precipitates will nucleate at discrete sites within the overall microstructure and grow over time. G phase precipitates form within the ferrite regions of duplex [21, 22] and nominally austenitic stainless steels (e.g. Type 300 series austenitic steels) [13, 15, 16, 19] during ageing within the temperature range of 250–500 °C [12, 14, 22]. Steels with high volume fractions of G phase have been found to be embrittled [19, 25] when subjected to room temperature fracture, and as such the phase promotes intergranular fracture
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