Microstructural mechanisms of cavity nucleation in ferritic/martensitic Grade 91 steels under creep conditions

Abstract

Premature failures of Grade 91 steel components in energy applications are strongly linked
to the nucleation of creep cavities, which grow and coalesce to form propagating macrocracks which eventually lead to failure. Although cavity interlinkage and crack propagation are better understood, the mechanisms of cavity nucleation have yet to be established. This thesis aims to pinpoint the damage susceptible microstructural factors in ferritic and martensitic Grade
91 steels and to reveal the underlying cavity nucleation mechanisms.

The Grade 91 material studied in this thesis originates from Barrel 2 (B2) of the Aberthaw
Power Station Unit 8, after 79,000 hours of service at a nominal steam outlet temperature of
570°C and a pressure of 16.5 MPa. The B2 section showed extensive damage compared to other
sections, with severe cracking that prompted the replacement of the header and extraction of B2
for inspection. Previous studies linked the poor performance of the B2 material to temperature
variations; however, temperature variations alone could not explain the increased damage
observed in Barrel 2, as other sections of the header also experienced elevated temperatures
but exhibited much less damage. Another damage contributing factor that was identified, was
the low N:Al ratio, which was associated with the formation of AlN precipitates. This, in turn,
reduced the number density of MX precipitates that contribute to precipitation strengthening.
However, it remained unclear whether the aforementioned factors were sufficient to cause such
extensive damage or if other metallurgical factors also played a role in the poor creep behaviour
and premature cracking of the B2 section of the header. Therefore, the aim of this research was
to investigate the microstructural parameters and reveal the underlying mechanisms responsible
for the creep-induced damage in the B2 Grade 91 material.

Ex-service B2 P91 samples in both ferritic and martensitic conditions, were creep tested and
provided by the Electrical Power Research Institute (EPRI). The creep tests yielded a series
of samples that were creep tested both until failure and interrupted at lower strain lives. The
interrupted samples provided insights in the early stages of cavity formation, while examination
of the failed samples contributed to understanding of creep damage evolution. The ferritic
samples that were produced exhibited strains of 4% (interrupted), 10% (interrupted) and 35.6%
(failed). The martensitic samples that were produced exhibited strains of 0.5% (interrupted), 1%
(interrupted) and 3.7% (failed).

The failed samples were investigated using a series of electron microscopy techniques and
data analysis software. Both continuous cavity nucleation and interlinkage were observed, with
cavities coalescing more easily in the ferritic condition than in the martensitic. Voids were also
observed prior to creep testing and the vast majority were associated with manganese sulphide
(MnS) inclusions. MnS inclusions were found to be significantly prone to damage both prior and
during creep testing.

A strong correlation of grain boundary misorientation and grain boundary junctions with
cavitation was observed for both conditions. In the ferritic condition although grain boundaries with a misorientation of 15-30° were relatively rare in the microstructure, they showed a notable
susceptibility to cavitation, while grain boundaries with misorientation angles >45° were the
prevalent type in the ferritic matrix and were frequently linked to damage. Cavity growth was
facilitated on the grain boundary junctions with the cavities found on single grain boundaries
being characterized by smaller sizes. In the martensitic condition grain boundaries with misorientations <40° were more prone to damage when involved in triple junctions, while higher-angle
grain boundaries > 40° often cavitated independently, without being part of triple junctions. The
majority of cavitation was found on prior austenite grain boundaries (PAGBs) with a substantially
lower amount of cavities observed intra-granularly in the martensitic samples.

In both ferritic and martensitic conditions, localized deformation indicators (KAM, GROD and
GOS) presented a modest correlation with damage which upon high-resolution EBSD imaging,
was found to be significantly higher. Most of ferritic and prior austenite grains were found
to be soft grains and the vast majority of cavities was observed on soft-soft grain boundaries.
Nevertheless, this was statistically expected, and thus a conclusion for the correlation between
the Schmid Factor and cavitation, could not be drawn with high statistical confidence.

Extensive TEM analysis on the interrupted samples, validated the damage susceptibility of
MnS inclusions. For the ferritic condition, it was revealed that damage formed on the inclusion matrix interface when other precipitates such as M₂₃C₆ carbides, Laves phase or MX-type
carbonitrides were present. In the martensitic condition, additional cavitation was observed
on the interface of MnS-Al₂O₃ particles while grains adjacent to the elongated MnS inclusions
were encircled by nano-sizes cavities due to local deformation induced by the inclusion. In both
conditions, precipitates such as M₂₃C₆ carbides, Laves phase and MX-type carbonitrides were
identified as secondary contributors to damage, whereas MnS inclusions acted as the primary
drivers of cavity nucleation. Tramp element segregation along grain boundaries was not observed.

A Plasma-FIB volume lift out and serial cross sectioning technique was adopted, to achieve 3D
reconstruction of the ferritic failed material. Image recognition and segmentation software was
utilised to visualise the cavitation and precipitation in a 3D perspective. The 3D data validated
previous 2D observations.

This thesis demonstrates that the structure of grain boundaries and the presence of inclusions significantly influence creep cavitation in Grade 91 steels. By employing a combination
of advanced microscopy techniques and comprehensive data analysis, deeper insights into the
underlying damage mechanisms are gained. The research strategy employed in this thesis, explains the severe degradation observed in Barrel 2, linking its premature failure not solely to
previously suggested factors such as operating temperature and the low N:Al ratio, but rather to
a combination of metallurgical features. Specifically, the presence of elongated MnS inclusions
forming during the manufacturing stage, their alignment along grain boundaries, and the presence of localized deformation sites appear to have collectively accelerated cavitation processes.
Compared to other examined barrels or tee piece sections of the Aberthaw header, Barrel 2 was
characterized by a higher sulphur level of 0.01 wt%, compared to 0.002 wt% for Tee Piece 1 and
0.0018 wt% for Barrel 6, which exhibited less pronounced damage. The elevated sulphur content
of Barrel 2, which is an unavoidable trace element that must be carefully controlled, may have
significantly contributed to the increased formation of MnS inclusions, which in this thesis are
shown to be strongly associated to damage. While the case of Barrel 2 was the focus of this thesis,
this research extends beyond a single component, contributing to a broader understanding of how
microstructural factors influence creep damage initiation and evolution in Grade 91 steels more
generally. The insights gained here are applicable to life prediction and integrity assessment
across similar high-temperature Grade 91 components.

Date of Award	25 Mar 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Peter E J Flewitt (Supervisor), Tomas L Martin (Supervisor) & Alan C.F. Cocks (Supervisor)