Abstract
Life prediction is critical for safe operation of plant within the creep regime. This study investigates the influence differences in specific composition and fabrication routes have on creep degradation for components with compositions within the design specification. Four ex-service AISI 316H austenitic stainless steel components exhibiting different manufacturing techniques and a range of compositions were used in the study. The boiler header component was found to contain localized chromium-rich regions arising from the original cast microstructure which result in a wide range of phases and show enhanced creep cavitation. Chi-phase precipitates contribute a significant role in creep damage accumulation via phosphorus segregation to the chi-austenite inter-phase boundaries. Tubular components with a homogenous microstructure and similar composition to the header showed limited secondary phase evolution. In one tubular specimen, creep cavitation was linked to phosphorus segregation at M23C6—austenite inter-phase boundaries. The remaining two tubular components, with a higher (but within specification) silicon content, showed homogeneous microstructures with limited creep cavitation and no phosphorus segregation was observed.
Highlights
AUSTENITIC stainless steels such as the Type 316 series are frequently used to manufacture high-temperature plant components, since they possess high strength and corrosion resistance.[1,2] As such, these steels are frequently fabricated into components that have a wide range of shapes—from thin-walled pipes to large cast components and beyond
The Transmission electron microscopy (TEM) identification of the precipitate species in the header material has been extensively discussed in a previous paper.[40]
All fall within compositional design specification, and would be expected to behave from the perspective of creep damage forecasting
Summary
AUSTENITIC stainless steels such as the Type 316 series are frequently used to manufacture high-temperature plant components, since they possess high strength and corrosion resistance.[1,2] As such, these steels are frequently fabricated into components that have a wide range of shapes—from thin-walled pipes to large cast components and beyond. The influence of a range of fabrication affects such as welding[3,4] and cold work[5] on in service creep has been the subject of a large number of previous studies. These studies have considered factors such as the introduction of residual stresses,[6,7] changes in grain structure,[8] enhanced diffusion kinetics,[5] and secondary phase evolution.[8,9,10] Many studies make the assumption that these materials have a uniform microstructure and that findings can be Manuscript submitted January 9, 2018.
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