Thick-walled valves, steam chests, and casings suffer service damage from thermal stresses due to the significant through-thickness temperature gradients that occur during operating transients. Fatigue is the primary damage mechanism, but recent examination of turbine casings has revealed extensive sub-surface creep cavitation. The low primary stress levels for these components are unlikely to cause creep damage, so detailed inelastic analysis was performed to understand the complex stress state that evolves in these components. This illustrates that fatigue cycles can cause elevated stresses during steady operation that cause creep damage. This paper will explore a case study for a 1CrMoV turbine casing where the stress-strain history during operating transients will be related to damage in samples from the turbine casing. This will also highlight how service affects the mechanical properties of 1CrMoV, highlighting the need for service- exposed property data to perform mechanical integrity assessments. Finally, the consequences for repair of damage will be discussed, illustrating how analysis can guide volume of material for excavation and selection of weld filler metal to maximize the life of the repair. This, in turn, will identify opportunities for future weld repair research and material property data development.

The current research adopts a novel approach by integrating correlative microscopy and machine learning in order to study creep cavitation in an ex-service 9%Cr 1%Mo Grade 91 ferritic steel. This method allows for a detailed investigation of the early stages of the creep life, enabling identification of features most prone to damage such as precipitates and the ferritic crystal structure. The microscopy techniques encompass Scanning Electron Microscopy (SEM) imaging and Electron Back-scattered Diffraction (EBSD) imaging, providing insights into the two-dimensional distribution of cavitation. A methodology for acquiring and analysing serial sectioning data employing a Plasma Focused Ion Beam (PFIB) microscope is outlined, complemented by 3D reconstruction of backscattered electron (BSE) images. Subsequently, cavity and precipitate segmentation was performed with the use of the image recognition software, DragonFly and the results were combined with the 3D reconstruction of the material microstructure, elucidating the decoration of grain boundaries with precipitation, as well as the high correlation of precipitates and grain boundaries with the initiation of creep cavitation. Comparison between the 2D and 3D results is discussed.

Laser additive manufacturing (AM) is being considered by the nuclear industry to manufacture net- shape components for advanced reactors and micro reactors. Part-to-part and vendor-to-vendor variations in part quality, microstructure, and mechanical properties are common for additively manufactured components, attributing to the different processing conditions. This work demonstrates the use of microstructurally graded specimen as a high throughput means to establish the relationship between process-microstructure-creep properties. Through graded specimen manufacturing, multiple microstructures, correlated to the processing conditions, can be produced in a single specimen. The effects of a solution annealing heat treatment on the microstructure and creep properties of AM 316H are investigated in this work. Using digital image correlation (DIC), the creep strain can be calculated in these graded regions, allowing for multiple microstructures to be probed in a single creep test. The solution annealing heat treatment was not sufficient in recrystallization of the large, elongated grains in the AM material; however, it was sufficient in removing the cellular structure commonly found in AM processed alloys creating a network of subgrains in their place. The resulting changes in microstructure and mechanical properties are presented. The heat treatment was found to generally increase the minimum creep rate, reduce the minimum creep rate, and reduce the ductility. Significant amounts of grain boundary carbides and cavitation were observed.

This paper presents three recent example cases of cracking in Grade 91 steel welds in longer-term service in high temperature steam piping systems: two girth butt welds and one trunnion attachment weld. All the cases were in larger diameter hot reheat piping, with the service exposure of the welds ranging from approximately 85,000 to 150,000 hours. Cracking in all cases occurred by creep damage (cavitation and microcracking) in the partially transformed heat-affected zone (PTZ, aka Type IV zone) in the base metal adjacent to the welds. The location and morphology of the cracking are presented for each case along with operating conditions and potential contributors to the cracking, such as system loading, base metal chemical composition, and base metal microstructure.

Creep strength enhanced ferritic (CSEF) steels have shown the potential for creep failure in the weld metal, heat affected zone (HAZ) or fusion line. Details for this behavior have been frequently linked to metallurgical risk factors present in each of these locations which may drive the evolution of damage and subsequent failure. This work is focused on three weld samples fabricated from a commercially sourced Grade 92 steel pipe section. These weld samples were extracted from the same welded section but were reported to exhibit failure in different time frames and failure locations (i.e., HAZ of parent, fusion-line, and weld metal). The only variables that contribute to this observed behavior are the post weld heat treatment (PWHT) cycle and the applied stress (all tests performed at 650 °C). In this work detailed microstructural analysis was undertaken to precisely define the locations of creep damage accumulation and relate them to microstructural features. As part of this an automated inclusion mapping process was developed to quantify the characteristics of the BN particles and other inclusions in the parent material of the samples. It was found that BN particles were only found in the sample that had been subjected to the subcritical PWHT, not those that had received a re-normalizing heat treatment. Such micron sized inclusions are a known potential nucleation site for creep cavities, and this is consistent with the observed failure location in the HAZ of the parent in the sample where these were present. In the absence of BN inclusions, the next most susceptible region to creep cavitation is the weld metal. This has an intrinsically high density of sub-micron sized spherical weld inclusions and this is where most of the creep damage was located, in all the renormalized samples.

This study investigates the role of grain-boundary precipitates in enhancing creep rupture strength of Ni-based alloys through analysis of Ni-15Cr-15Mo and Ni-15Cr-17Mo (at.%) model alloys. The investigation focused on the “Grain-boundary Precipitation Strengthening (GBPS)” effect from the thermally stable TCP phase, a phenomenon previously observed in Fe-Cr-Ni-Nb austenitic heat-resistant steels. Through multi-step heat treatments, specimens were prepared with varying grain boundary coverage ratios (ρ) of TCP P phase (oP56) and consistent grain-interior hardness from GCP Ni2(Cr, Mo) phase (oP6). In the 15 at.% Mo alloy, specimens with a higher coverage ratio (~80%) demonstrated significantly improved creep performance, achieving nearly four times longer rupture time (3793 h vs. 1090 h) at 300 MPa and 973 K compared to specimens with lower coverage (~35%). However, the 17 at.% Mo alloy showed unexpectedly lower performance despite high coverage ratios, attributed to preferential cavity formation at bare grain boundaries. These findings confirm that GBPS via thermally stable TCP phase effectively enhances creep properties in Ni-based alloys, with grain boundary coverage ratio being more crucial than intragranular precipitation density.

Dissimilar metal welds between T91 ferritic steels and TP347H austenitic alloys are commonly used in fossil power plants in China. Premature failure of such dissimilar welds can occur, resulting in unplanned plant outages that can cause huge economic losses. In this article, microstructural evolution of T91/TP347H dissimilar welds after different service conditions were studied, mechanical properties before and after service were also analyzed, a full investigation into the failure cause was carried out. The results show, the dissimilar metal welds in the as-welded condition consists of a sharp chemical concentration gradient across the fusion line, failure is attributed to the steep microstructural and mechanical properties gradients, formation of interfacial carbides that promote creep cavity formation.

Contrary to expectations, long-term performance of creep stress enhanced ferritic steels (CSEF) falls short of predictions based on short-term data. This discrepancy is attributed to the formation and growth of creep voids, leading to reduced ductility. This study investigates cavities in creep-tested P92 steel, revealing an association with large ceramic particles (1-2 μm) in standard samples. Three distinct particle compositions were identified: boron nitride (BN), manganese sulfide, and γ-Al 2 O 3 . Statistical analysis showed a strong correlation between BN particles and cavity formation. Using a 3D “slice and view” technique with a focused ion beam/field emission gun scanning electron microscope (FIB-FEGSEM), the study revealed irregular shapes for both cavities and associated particles. Furthermore, analysis of the head-gauge transition area (lower stress exposure) showed small cavities near BN particles, suggesting preferential nucleation on these hard, irregular features. These findings strongly support the hypothesis that BN particles play a key role in cavity nucleation, impacting the long-term performance of P92 steel.

As long term laboratory creep data became available the original estimates of the allowable stresses for creep strength enhanced ferritic steels (CSEF) had to be reduced. Thus, even in properly processed steel, the long term performance and creep rupture strength is below that originally predicted from a simple extrapolation of short term data. One of the microstructural degradation mechanisms responsible for the reduction in strength is the development of creep voids. Nucleation, growth and inter linkage of voids also result in a significant loss of creep ductility. Indeed, elongations to rupture of around 5% in 100,000 hours are now considered normal for long term creep tests on many CSEF steels. This relatively brittle behaviour, and the associated creep void development, promotes burst rather than leak type fracture in components. Moreover, the existence of significant densities of voids further complicates in-service assessment of condition and weld repair of these steels. The present paper examines background on the nucleation and development of creep voids in 9 to 12%Cr martensitic steels and discusses factors affecting brittle behavior.