*3.4. Precipitation*

Figure 5 shows the metallographic structure of the weld cross-section after application of the Trans-varestraint test for the 0% strain condition, as observed under scanning electron microscopy (SEM). It can be seen from the figure that a large number of precipitates appear in the intragranular and grain boundaries of the 441 material. The precipitates of 441Ce are relatively small. This indicates that the addition of the alloying element Ce reduces the generation of the precipitated phase and has a purifying effect on the grain boundary as well as the crystal inside, thus greatly influencing the hot crack sensitivity and strength of the material.

**Figure 5.** Metallographic structures of the weld cross-sections of (**a**) 441 and (**b**) 441Ce type steel.

The grain boundary of the metallographic structure of each weld cross-section was further magnified under SEM, and energy dispersive spectrometer (EDS) analysis was performed on the precipitation on the grain boundary. The results are shown in Figure 6. The precipitation of the 441 weld grain boundary is granular, and the size is approximately 1 μm. The EDS result shows that it is a composite precipitation rich in Fe, Cr, Ti, and Nb. The precipitation of the 441Ce weld grain boundary is relatively small, and the spectrum of the precipitation is similar to that of the 441 weld grain boundary. The thermodynamic calculation results in Table Table 2 indicate that Ce expands the solidification temperature range, increases the degree of undercooling during the solidification process, promotes heterogeneous nucleation and growth of the liquid metal during solidification, increases the grain boundary, and refines the grain, so that the precipitated phase is dispersed. In addition, rare earth elements can reduce the activity of C, increase the solubility of C, and reduce the precipitation of carbides of Ti and Nb during solidification [22]. Ce is easily segregated near the grain boundary because in the cooling process following the solidification of the weld, the radius of the Ce atom is large, which tends to segregate Ce near the grain boundary, thus causing lattice expansion near the grain boundary and increasing its energy. It is easy to nucleate the carbide [23] so that the precipitation of the grain boundary is fine and exhibits a dispersed distribution.

**Figure 6.** SEM images and EDS analyses of precipitations on the grain boundaries of (**a**,**c**) 441 and (**b**,**d**) 441Ce type materials, respectively.

Figure 7 depicts the phase-temperature equilibrium diagrams of 441 and 441Ce steel using Thermo-Calc software. When 441 ferritic stainless steel solidifies, it produces high melting point precipitates (Ti,Nb)(C,N). Some of these precipitations occur on the solidification grain boundary, destroying the liquid film that has not yet solidified at the boundary, improving the bonding strength of the boundary, and forming a "pinning" effect, as shown in Figure 8. This effect reduces the sensitivity of the weld solidification crack. Compared to the 441 stainless steel without added Ce, 441Ce increases the solidification temperature range, prolongs the total time of the weld in the solid–liquid mixing stage during the welding cooling process, and greatly increases the risk of solidification cracking. In addition, the Ce atoms are segregated on the grain boundary during solidification [23], which easily forms a low-melting liquid film. Because Ce promotes a solid solution of C [23], it is not conducive to precipitation of carbides of Ti and Nb, thus reducing such precipitations. Further, it reduces the pinning effect of precipitation, so the addition of Ce greatly improves the weld solidification crack sensitivity of 441 ferritic stainless steel.

**Figure 7.** Phase-temperature equilibrium diagrams for (**a**) 441 and (**b**) 441Ce type ferritic stainless steel materials.

**Figure 8.** Pinning effect of precipitation in the solidification grain boundary.

#### **4. Conclusions**

The sensitivity of the solidification crack of 441 ferritic stainless steel can be improved by adding the rare earth element Ce; the observations and conclusions of such an addition are as follows:

(1) Ce widens the solidification temperature range of 441 ferritic stainless steel, so in the actual welding process, it increases the duration of the weld metal in the solid–liquid mixing stage, thus improving the solidification crack sensitivity of stainless steel.

(2) The high temperature precipitates produced during solidification of 441 ferritic stainless steel weld metal, especially at the solid–liquid interface, can pin the interface, improve the bonding strength of the interface, and prevent the solidification cracks. After adding Ce to 441 ferritic stainless steel, the precipitates produced in the weld can be reduced and the weld structure can be purified. It is precisely because of this purifying effect that the pinning due to high temperature precipitates in the solidification process of the weld metal is weakened and solidification crack sensitivity is improved.

(3) In a future work, the influence of Ce on solidification temperature range and the precipitates will be studied in depth, and the improvement of the anti-solidification crack properties of Ce-containing ferritic stainless steel will be explored.

**Author Contributions:** Conceptualization: S.Z. and B.Y.; investigation: S.Z.; original draft preparation: S.Z.; review and editing: S.Z. and B.Y.

**Funding:** This research received no external funding.

**Acknowledgments:** This work was supported by the Baoshan Iron & Steel Co., Ltd.

**Conflicts of Interest:** The authors declare no conflict of interest.
