*3.1. Microstructure*

The XRD diagrams of samples processed by different technologies are shown in Figure 1. It can be seen that the samples prepared by SLM are mainly composed of ferrite phase, whereas the solution treated samples at 1050 ◦C were mainly composed of austenite and the σ phase, and mainly composed of ferrite and austenite phases for samples treated at 1100–1200 ◦C, respectively. The TEM bright-field image in Figure 2a shows that dislocations were found in the S1 sample, suggesting that the high stress caused by rapid cooling led to the appearance of dislocations. Dislocations provide a driving force for recrystallization during solution annealing [14]. The scanning transmission electron microscope-energy dispersive spectrometer (STEM-EDS) mapping of Figure 2b,c shows that the nanoscale Cr2N were found at the ferrite grain boundary.

**Figure 1.** The XRD diagrams of samples processed by different technologies.

(**c**) 

**Figure 2.** TEM morphology of the S1 sample (**a**) TEM bright field image; (**b**,**<sup>c</sup>**) scanning transmission electron microscope-energy dispersive spectrometer (STEM-EDS) mapping image.

The EBSD inverse pole diagram and phase distribution diagram of samples with different solution annealing processes are shown in Figure 3. Table 3 shows the proportion of phases and grain size of samples with different solution annealing processes. As shown in Figure 3, 10% of the σ phase and 88% of the γ2 phase are precipitated from ferrite by eutectoid transformation [16,17]: α →σ + γ2 at 1050 ◦C. In the temperature range of 1100–1200 ◦C, the grain size and the ferrite content increased gradually, while the austenite content decreased gradually with the increment in solution annealing temperature. The γ2 phase nucleated only along the recrystallized ferrite grain boundary at 1200 ◦C [18]. The nitrogen content of HDSS powder decreased from 0.36% in powder state to 0.24% in selective laser melted state [15]. Nitrogen is an austenite-forming element, suggesting that its decrease resulted in the decrease of the austenite content after solution annealing, and it is not possible to achieve 50-50% ferrite-austenite ratio due to the nitrogen loss. Figure 4 shows the relationship between nitrogen content and austenite content of UNS S32707 HDSS simulated by JMatPro software. The simulated result shows that the decrease in nitrogen content reduced the austenite content by 10%.

**Figure 3.** The electron backscatter diffraction (EBSD) diagram of different solution annealing processes (**<sup>a</sup>**–**d**) inverse pole figure and (**<sup>e</sup>**–**h**) phase distribution figure; (**<sup>a</sup>**,**<sup>e</sup>**) S2; (**b**,**f**) S3; (**<sup>c</sup>**,**g**) S4; (**d**,**h**) S5.


**Table 3.** The proportion of phases and grain size of samples with different solution annealing processes.

**Figure 4.** The relationship between nitrogen content and austenite content of UNS S32707 HDSS simulated by JMatPro software.
