13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 2. Material and experimental Since austenitic stainless steel grade, UNS S31035, was aimed for use in super-heaters and reheaters with metal temperatures up to 700°C, the design principles for this alloy were to achieve a stable microstructure, high strength by precipitation strengthening with stable nano precipitates and solution hardening, high corrosion resistance with high Cr content. Ni and N that can suppress the formation of sigma phase were added to reach sufficient structural stability and good fabricability. The chemical composition is shown in Table 1. Table 1 Nominal composition of austenitic stainless steel grade, UNS S31035 (wt%) Cmax Si Mn Cr Ni W Co Cu Nb N Fe 0.1 0.2 0.5 22.5 25 3.6 1.5 3.0 0.5 0.23 Bal. Figure 1 shows some fine precipitates observed in this newly developed austenitic stainless steel grade, UNS S31035 that can contribute to the improvement of the creep strength. Both intra- and intergranular M23C6 precipitates can be observed (Fig. 1a). In the grain boundaries, they have a <100>γ // <100>M23C6 coherent relationship to the austenite matrix. Laves phase was observed both randomly within the grains but also ordered on what appears to be former twin boundaries (Fig. 1b). Both coherent Laves phase precipitates with a [100]Laves // [100]γ orientation relationship, and incoherent Laves phase precipitates were observed. In the aged material the Laves phase are needle shaped but rather small. In the creep tested materials, these particles are fine and isometric. Copper rich nanoparticles can also be observed. They are round with a size up to 50 nm (Fig. 1c). In this material, a dense distribution of about 10 nm large precipitates was observed (Fig. 1d). Due to the limitation of the TEM, these particles could not be identified, but they are probably MX carbides or carbonitrides. Similar particles have been identified in other analyses. These particles were rather stable [5]. (a) (b)
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