## 2 The Effect of Alloying Elements on Microstructure  
### Carbon (C)  
In AISI 321 stainless steel, carbon (_C_) plays an indispensable role, heavily influencing the development of various microstructures based on its concentration and distribution. More precisely, the concentration of carbon exerts a significant influence on both the corrosion resistance characteristics and the dynamic recrystallization behavior of 321 stainless steel [11]. Carbon serves as a pivotal stabilizing element within the austenitic phase, exhibiting an approximately 30-fold greater capacity for austenite formation compared to nickel (_N_I). Furthermore, AISI 321 stainless steel possesses the capability to dissolve up to 2.11 wt.% of carbon (_C_), thereby ensuring its resistance to phase transformations in environments characterized by elevated carbon concentrations [12]. In scenarios where the carbon (_C_) content is excessively low, there is a notable contraction in the austenite phase region, leading to an increased tendency for \(\delta\)-ferrite formation. Moreover, the solidification temperature range of AISI 321 stainless steel is extended by approximately 10 \({}^{\circ}\)C in higher carbon (_C_) variants compared to those with lower carbon (_C_) content [13]. When the carbon content (_C_) increases from 0.02 wt.% growth to 0.08 wt.% The temperature at which the liquid phase disappears significantly decreases (Figure 1). This wider temperature range is beneficial to the diffusion and assimilation of carbon, aiding in the formation of a more uniform microstructure.  
A higher carbon (_C_) content can also avoid excessive softening of AISI321 stainless steel. Radionova, L.V studied the hot deformation behavior of austenitic Ti-modified AISI 321 steel with relatively high carbon (_C_) content (0.07 wt.%) and titanium (_T_I_) content  
Figure 1: AISI 321 stainless steel JMatPro phase diagram simulation, (**a**) 0.02 wt.% C; (**b**) 0.08 wt.% C.  
(0.50 wt.%) in the temperature range of 1000-1280 \({}^{\circ}\)C and strain rate range of 0.01-1 s\({}^{-1}\). As the Zener Hollomon (Z) parameter increases, the proportion of dynamically recrystallized grains in the total volume of the material decreases monotonically. This is because the dispersed particles of precipitated Ti (C, N) inhibit the growth of new grains [14]. After hot deformation, only a few small grains can be seen near the black band structure (Figure 2). However, in 321 steel with low carbon (C) content (0.04 wt.%) and titanium (_Ti_) content (0.32 wt.%), deformation occurs at a temperature of 1000 \({}^{\circ}\)C, complete dynamic recrystallization occurs at a strain rate of 0.01 s\({}^{-1}\)[15]. Dynamic recrystallization constitutes a crucial softening mechanism, implying that an increased carbon (C) content in AISI 321 stainless steel leads to an enhancement in its high-temperature strength.  
Moreover, an augmentation in carbon content results in a decrement in corrosion resistance. [PERSON], A executed a dual loop electrochemical potential kinetic reactivation experiment to assess the corrosion sensitivity of AISI 321 stainless steel with varying carbon concentrations at temperatures ranging from 500 to 800 \({}^{\circ}\)C. The findings confirmed that 321 stainless steel demonstrates superior corrosion resistance when the carbon (C) content is maintained below 0.03 wt.%. At this concentration level, fluctuations in corrosion resistance arise due to alterations in the titanium content. However, when the carbon content surpasses 0.03 wt.%, even an increase in titanium content cannot adequately counteract the decrement in corrosion resistance. This implies that a carbon (C) content within the range of 0.03-0.04 wt.% in AISI 321 stainless steel serves as the baseline requirement to guarantee that corrosion resistance remains its primary characteristic or performance attribute [16].