PSI - Issue 42

3

Guocai Chai et al. / Procedia Structural Integrity 42 (2022) 155–162 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 1. Schematic specimen for the SSRT testing.

The grain structure was investigated using a Carl Zeiss ΣIGMA FEG -SEM equipped with a Symmetry electron backscatter diffraction (EBSD) detector supplied by OXFORD Instruments. EBSD measurements were carried out with an accelerating voltage of 20 keV, aperture size of 120 µm and scanning step size of 0.5 µm. The fracture and deformation structures were studied using a FIB SEM with FEG tip manufactured by Carl Zeiss. Electron channel contrast image (ECCI) technique was performed by collecting backscattered electron (BSE) images at an accelerating voltage of 10 keV and aperture size of 120 µm. It should be noted that both the SEMs used in this study are equipped with Gemini Column technology. To obtain good results, specimens were subjected to polishing with colloidal silica in the final step. 3. Results and discussion 3.1. Influence of tempering on microstructure Fig. 2a and 2b show the microstructures of the as-delivered material. It has a pre-austenitic grain size of about 25  m and a width of martensitic lath of about 0.36  m with homogeneous crystal orientation distribution. Few nano NbC particles (red precipitates in Fig. 2b) can be observed. Thermodynamic calculation shows that 0.003 at% of NbC would precipitate in the as-delivered material. SEM/EBSD analysis shows that NbC precipitated in the material is about 0.0019 at%, which is close to the theoretical calculation. Fig. 2c and 2d show the microstructures of the tempered material. They are similar to those in the as-delivered material. The main difference is the precipitates. According to the thermodynamic calculation, four types of precipitates can appear at 510°C. They are Cu-rich phase (0.0308 at%), NbC (0.003 at%), M 23 C 6 (0.0011 at%) and austenitic phase (0.067 at%). All these phases have FCC structures. Total amount of these precipitates is 0.102 at%. SEM/EBSD analysis shows that total amount of the precipitates is only about 0.003 at%, much lower than the theoretical one. It may depend on the following two reasons. One is that Cu rich phase in nano size is too small to be identified by the EBSD technique used, same for other nano phases. The other is that no M 23 C 6 precipitate was observed. Since amount of NbC phase changes little during the tempering, the identified precipitates increased by the tempering are mainly austenitic phase as shown in Fig. 2d (red ones). They precipitated mainly at pre-austenitic grain boundaries (black lines) or martensitic lath boundaries (light lines).