Evaluation of metallurgical creep risk factors for 9 wt.% Cr steels

9 wt.% Cr steels are essential for components in powerplant applications, such as combined cycle or coal-fired systems, typically operating at 600~650°C. These steels have a martensitic microstructure strengthened by stabilizing precipitates. However, inclusions, and the evolution of second-phase particles can introduce metallurgical risks related to creep. This research is aimed at developing a deep and quantitative understanding of these metallurgical risk factors in a range of 9 wt.% Cr steels and establishing their correlation with creep behaviour. The study involved extensive microstructural characterisation of both as-received and creep tested samples. This data can facilitate the correlation of microstructural changes with creep performance, offering valuable insights to alloy designers and steel manufacturers for optimizing the composition and fabrication process.

As-received and creep tested 9 wt.% Cr steel samples were prepared and characterized using a range of complementary microscopy techniques, including focused ion beam (FIB) microscopy, field emission gun scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD). The experimental aspect of this study focused on examining microstructural changes in 9 wt.% Cr steels in their as-received conditions compared to after creep exposure. To investigate the phase stability and the effects of various alloying elements on 9 wt.% Cr steels, thermodynamic calculations were performed using their chemical compositions as input data. This analysis predicted the influence of several key alloying elements, such as B, C, N, Mo, W, Ta, and Nb, on the formation of secondary phases, such as M23C6, Laves phase, Z phase, and MX. Additionally, the iii calculations explored the conditions under which inclusions would form, providing a deeper understanding of the material's behaviour.

Second phase particles are rigorously identified and quantified using various techniques in this study, and the results are systematically compared using different data collection methods. No association was observed between creep cavities and M23C6 or Laves phase particles. Additionally, this study establishes a link between creep cavities and inclusions. It uses a comprehensive approach to identify and quantify both cast pores and creep cavities, revealing their growth and coalescence during creep exposure. In-depth 3D investigations provide overwhelming evidence confirming a strong association between Ta-enriched particles and creep cavities, demonstrating the value of 3D microstructure reconstructions as a powerful tool for better understanding creep risk factors.

A coherent linkage between distinct second-phase particles and the damage that occurs during creep tests in these 9 wt.% Cr steels during creep tests was established through a meticulously crafted research methodology. Furthermore, this approach has the potential to be applied more broadly to various steel alloys.