## 4 Influence of Heat Treatment on the Properties  
### Solid Solution Treatment  
Solid solution treatment stands as the most prevalent heat treatment technique employed for AISI 321 stainless steel, effectively enhancing its overall performance by achieving a uniform microstructural organization within the material. When executing this treatment, the primary considerations lie in selecting the appropriate solid solution temperature and insulation duration, as these factors are of paramount importance. The steel displayed that the ferrite content significantly decreased as the treatment time increased at 1050 \({}^{\circ}\)C. Upon extending the holding time to 1 h, there was minimal change in the ferrite content and morphology (Figure 10b). When the holding time was increased to 5 h, the ferrite almost completely vanished in AISI321 stainless steel (Figure 10d) [38].  
Figure 9: Morphology of impact fracture surfaces of different specimens of AISI 321 stainless steel. (Indicate shallow pits with arrows), (**a1,a2**) solution annealed, aged at 750 \({}^{\circ}\)C for: (**b1,b2**) 700 h; (**c1,c2**) 1500 h [36].  
As the solution treatment time increases, the occurrence of elongated ferrite structures is virtually eliminated, thereby underscoring the profound influence of processing duration on the ferrite content [39]. From the preceding summary, it is evident that the presence of \(\delta\)-ferrite significantly enhances the mechanical properties along the rolling direction. However, as solution treatment progresses, the disappearance of \(\delta\)-ferrite and the subsequent growth of austenite grains impart changes in the properties that cannot be fully mitigated by a homogenized microstructure. Excessive sizing of austenite grains can lead to a decrease in the strength of the austenite phase. For instance, when AISI 321 stainless steel was held at 1075 \({}^{\circ}\)C for 80 min, it readily underwent transgranular precipitation following sensitization. Conversely, maintaining the steel at temperatures within the range of 800-900 \({}^{\circ}\)C had a negligible impact on its strength subsequent to sensitization treatment [40]. It is evident that, while solution treatment can enhance the overall properties of materials by homogenizing their structure, this method's application is confined to a specific temperature range, posing certain constraints for subsequent research endeavors. However, this limitation has been partially mitigated in recent times. Hu, J successfully mitigated grain coarsening by incorporating the rare earth element yttrium (\(\gamma\)) into AISI 321 stainless steel. Specifically, when the steel, devoid of yttrium (\(\gamma\)), was subjected to an increased holding temperature from 1050 \({}^{\circ}\)C to 1150 \({}^{\circ}\)C, the austenite grain size increased approximately fourfold. In contrast, with the addition of yttrium (\(\gamma\)), the change was minimal, enabling AISI 321 stainless steel to retain its superior comprehensive performance even when held at 1150 \({}^{\circ}\)C for 30 min (Figure 11) [39]. This breakthrough offers diversified avenues for solid solution treatment methodologies.  
Figure 11: The grain boundary map of (**a–c**) Y-free and (**d–f**) Y-containing simples at 1050 \({}^{\circ}\)C, 1100 \({}^{\circ}\)C, and 1150 \({}^{\circ}\)C [39].  
Figure 10: Microstructure of AISI 321 stainless steel at 1050 \({}^{\circ}\)C for different times, (**a**) 0.5 h; (**b**) 1 h; (**c**) 2 h; (**d**) 5 h [38].