Understanding the performance of 11–12 wt. % Cr creep strength enhanced ferritic steels, through microstructural characterisation and investigation of precipitate quantification techniques

To improve power generation efficiencies, offset carbon capture and storage efficiency penalties, and reduce CO2 emissions, the steam temperatures and pressures within boilers and heat recovery steam generators (HRSGs) must increase. This demands new or optimised materials, capable of withstanding long-term operation within increasingly aggressive environments, whilst being affordable, inspectable, repairable, and manageable in service. The development of a family of 9–12 wt. % Cr creep strength enhanced ferritic (CSEF) steels aims to meet such demands, through the consideration of creep strength, thermal fatigue, corrosion and steam oxidation resistance, manufacturability, weldability, and cost.

Two 11–12 wt. % Cr CSEF steels, VM12-SHC and Thor® 115, are candidates for boiler and HRSG tubing and piping applications at such conditions. Their increased Cr addition is designed to improve steam oxidation resistance compared to mainstay 9–10 wt. % Cr steels, for which this is a life-limiting property at steam conditions approaching, or above, 600°C. Modelling and characterisation studies of samples aged at service-like conditions, are critical to assessing the effectiveness and safety of Thor® 115 or VM12-SHC in application. However, very few independent studies currently exist. Additionally, a range of characterisation techniques and experimental procedures are used throughout literature, with limited investigation to justify their selection.

Consequently, through thermodynamic equilibrium modelling and microstructural characterisation of a range of service-like exposed and simulated heat affected zone (HAZ) samples, this thesis aims to improve our understanding of the performance of these steels, during service and welding procedures. Additionally, as any improvements to characterisation techniques and procedures will contribute towards achieving a better understanding of long-term material performance, these are investigated in order to recommend best practice. Ultimately, this thesis contributes to knowledge in five key areas; precipitate quantification procedures, microscopy techniques, materials modelling, materials characterisation, and welding; with a focus on Thor® 115 and VM12-SHC steels throughout.

Firstly, Laves phase and M23C6 carbide imaging and quantification procedures are investigated from respective scanning electron microscope (SEM) backscattered electron detector (BED) and focused ion beam (FIB) imaging, respectively. Setting selections for the best compromises between statistical representativity of the bulk, minimising noise influences, and improving accuracy of results are studied and discussed. Amongst other things, this led to recommendations of 75 nm (2 pixel (px)) minimum particle equivalent circular diameter (ECD) and 48 µm horizontal field width (HFW), and 37.5 nm (3 px) minimum ECD and 12.8 µm HFW, for Laves phase and M23C6 carbide quantification, respectively.

In addition, with these settings, the number of images per analysis location required to return acceptably low uncertainties on mean measured precipitate values was investigated. From this, at least 15 images (corresponding ≈ 250 µm2¬¬ total area of analysed Laves phase particles and ≈ 5,000 M¬23C6 carbides in service-like aged and as-tempered 11–12 wt. % Cr steel, respectively) are required per analysis location. This investigation contributes towards a longer-term industry led initiative to standardise procedures, given a material type and state.

Secondly, a novel microscopy technique for quantifying precipitates, which was discovered as part of this research, is discussed and compared to FIB imaging techniques. The novel field emission SEM (FESEM) in-column upper secondary electron detector (USD) technique returns images from first order secondary electrons (SE1s), which enables imaging and separate quantification of M23C6 and Z-phase from a single micrograph. The advantages and pitfalls of this novel technique are highlighted and future optimisation work to enable the quantification of M23C6, Laves phase, and Z-phase from a single microscope and image, is outlined. Such a development would significantly reduce the time and costs associated with research to make it far more accessible, whilst providing a pathway towards automated larger area mapping, which could lead to significant advances in materials research.

Thirdly, for both materials, the major phase transformations and temperatures, stable secondary phases, their compositions, and their sensitivity to compositional and temperature variation, are predicted via thermodynamic equilibrium modelling throughout service, tempering, and normalisation temperature ranges. The amounts of major stable secondary phases; M23C6 and modified (Cr,Fe)(V,Nb)N Z-phase in both steels, M2N in Thor® 115, and W-rich Laves phase in VM12-SHC at service temperatures, with V-rich and Nb-rich MX at normalisation temperatures; are predicted and their influences are discussed. Key control elements for phase amounts, such as C in M23C6 or Nb, N, and V in modified Z-phase, and their likely influences on material performance during service are also discussed. With most of this work being unpublished, it improves current understanding of the expected microstructural influences within both materials.

Fourthly, via application of micro-hardness analysis, X-ray fluorescence (XRF), electron backscattered diffraction (EBSD) mapping, transmission electron microscopy (TEM), SEM, and FIB imaging; the grain structure, precipitate populations, and chemical distributions are characterised and compared across a range of as-tempered, service-like thermally aged, and creep strained Thor® 115 and VM12-SHC samples. This shows both materials to exhibit tempered martensitic microstructures, strengthened by fine M23C6 carbide distributions along grain boundaries. The influence of thermal ageing, which severely coarsens precipitates and decreases hardness with increasing time and temperature, is then analysed and discussed.

By comparing similar characterisation results to true strain and reduction in area across creep tested gauges, a quantitative assessment of creep failure processes is provided. This work highlights that a critical point relating to strain exists in both materials, above which carbides coarsen, damage instigates, recrystallisation occurs, and hardness decreases, leading to failure. This therefore enables industry to better understand the key processes which lead to failure, thus improving their ability to prevent in-service failures. Further material characterisation and additional work to identify the measured values (i.e. reduction in area and strain) at this critical point in real time, would improve the management of these materials during service and return a fuller understanding of their performance.

Lastly, the HAZ microstructure in Thor® 115 and VM12-SHC steel has yet to be comprehensively characterised, before or after post weld heat treatment (PWHT). Therefore, well controlled simulated HAZ thermal cycles were instrumented with dilatometry to determine the austenite transformation temperatures, Ac1 and Ac3. Following simulated HAZ welding thermal cycles, four PWHT conditions were applied in the allowable PWHT range (740°C to 780°C for 1 or 3 hours) to Thor® 115 and the specified PWHT (770°C/0.5 h) was applied to VM12-SHC. All resultant samples were microstructurally characterised using hardness, EBSD, and FIB imaging, with prior publications utilised to compare the HAZ sub-zones.

The HAZ in Thor® 115 and VM12-SHC was thus classified for a simulated single weld thermal cycle into three distinct regions, based on the observed prior austenite and sub-grain structure and dissolution of precipitates. The three regions included; 1) the completely transformed zone (CTZ), 2) the partially transformed zone (PTZ), and 3) the over tempered zone (OTZ). A new definition for the PTZ is highlighted to accommodate ‘high temperature’ and ‘low temperature’ transition zones which are not bound by Ac1 or Ac3. The characteristics of each pre- and post-PWHT region are described, providing more clarity on the classification of the HAZ for 11–12 wt. % Cr CSEF steels. This should lead to further work to compare the simulated HAZ with real-world, creep tested HAZ microstructures.