For VM12-SHC 11-12 wt. % Cr steel, there have been no systematic investigations to define the regions or characterise the microstructures within the heat-affected zone (HAZ) of weldments. In similar steels, these regions relate to the Ac 1 and Ac 3 transformation temperatures and can affect weldment performance. In this study, controlled thermal cycles were applied to VM12-SHC parent metal using a dilatometer and the Ac 1 and Ac 3 temperatures were measured for various heating rates. The Ae 1 and Ae 3 temperatures were also calculated by thermodynamic equilibrium modeling. Through dilatometry, thermal cycles were then applied to simulate the microstructures of the classically defined HAZ regions. The microstructural properties of each simulated material were investigated using advanced electron microscopy techniques and micro-hardness testing. It was found that the simulated HAZ regions could be classified as; (1) the completely transformed (CT) region, with complete dissolution of pre-existing precipitates and complete reaustenitisation; (2) the partially transformed (PT) region, exhibiting co-existing original martensite with nucleating austenite microstructures with partial dissolution of precipitates; and (3) the over tempered (OT) region, with no phase transformation but precipitate coarsening and decreased hardness.

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.

Grade 92 steel has been widely applied in the power generation industry for use as steam pipes, headers, tubes, etc. owing to a good combination of creep and corrosion resistance. For the welding of thick section pipes, a multi-pass submerged arc welding process is typically used to achieve sufficient toughness in the weld. To relieve the internal stress in the welds and to stabilise their microstructures, a post weld heat treatment (PWHT) is commonly applied. The heat treatment conditions used for the PWHT have a significant effect on both the resulting microstructure and the creep behaviour of the welds. In this study, interrupted creep tests were carried out on two identical Grade 92 welds that had been given PWHTs at two different temperatures: 732°C and 760°C. It was found that the weld with the lower PWHT temperature had a significantly reduced stain rate during the creep test. In addition, microstructural examination of the welds revealed that the primary location of creep damage was in the heat affected zone in the sample with the lower PWHT temperature, whereas it was in the weld metal in the sample with the higher PWHT temperature. To understand the effect of the different PWHT temperatures on the microstructure, initially the microstructures in the head portions of the two creep test bars were compared. This comparison was performed quantitatively using a range of electron/ion microscopy based techniques. It was apparent that in the sample subjected to the higher PWHT temperature, larger Laves phase particles occurred and increased matrix recovery was observed compared with the sample subjected to the lower PWHT temperature.

To address current energy and environmental demands, the development and implementation of more efficient power plants is crucial. This efficiency improvement is primarily achieved by increasing steam temperatures and pressures, necessitating the introduction of new materials capable of withstanding these extreme conditions. Nickel-based alloys emerge as prime candidates for high-temperature and high-pressure applications, offering significant creep strength and the ability to operate at metal temperatures above 750°C. This research focuses specifically on steam header and pipework systems, which are critical components carrying steam from boilers to turbines under severe operating conditions. The study emphasizes the importance of selecting suitable materials for these components and developing methodologies to predict their safe operating lifetimes, thereby ensuring the reliable and efficient operation of next-generation power plants.

Creep strength enhanced ferritic (CSEF) steels, particularly modified 9Cr steels Grade 91 and 92, are increasingly used in advanced coal-fired power plants for header and steam piping construction. While these materials typically enter service after receiving a standard high-temperature normalizing treatment followed by lower temperature tempering to achieve optimal microstructure, practical situations like welding operations may expose components to additional heat treatment exceeding the Ac 1 , and potentially the Ac 3 , temperature before returning to tempering temperature. This research examines the effects of simulated post weld heat treatments (PWHT) on Grade 91 and 92 materials using dilatometer-controlled heating and cooling rates, with peak temperatures below Ac 1 , between Ac 1 and Ac 3 , and above Ac 3 , followed by heat treatment at 750°C for 2 hours. Hardness measurements revealed significant reduction when exceeding the Ac 1 temperature, while advanced electron microscopy, including electron back scatter diffraction, was employed to analyze changes in martensite laths and grain structure, along with detailed carbide size distribution analysis using both scanning and transmission electron microscopy. The findings are discussed in terms of how such PWHT overshoots might affect mechanical properties during high-temperature service.