This paper focuses on the study of the Fe-Cr-Ni-Al alloy casting process. Through Procast software simulation and experimental research, the temperature field and cooling rate of the alloy are analyzed, and the effects of cooling rate on the as-cast microstructure, dendrite spacing, and microhardness are investigated. The results provide a theoretical basis and data support for the production process of this alloy.
1. Introduction
With the rapid development of China’s economy, especially the rise of the infrastructure industry, the demand for steel is increasing day by day, and there are stricter requirements for steel quality and products. Considering the lack of chromium and nickel resources in China and the comprehensive performance of materials, Fe-Cr-Ni-Al duplex stainless steel has attracted attention. At present, there have been many studies on alloy casting processes and numerical simulations, but there are still limitations in traditional casting processes for small castings. Therefore, this paper studies the filling and solidification process of Fe-Cr-Ni-Al alloy using Procast software to explore the relationship between cooling rate and microstructure and properties.
2. Experimental Materials and Methods
2.1 Material Preparation
Commercial Fe, Cr, Ni, and Al block raw materials were used. According to the proportion shown in Table 1, 40g of materials were weighed and placed in a copper crucible. The materials were repeatedly melted 4 times using a vacuum arc melting furnace, and then the uniformly melted metal was further melted and suction cast. After cooling, the mold was disassembled to obtain stepped specimens.
| Element | Fe | Cr | Ni | AI |
|---|---|---|---|---|
| Mass fraction/% | 71.4 | 17.6 | 8.8 | 2.2 |
2.2 Simulation and Experimental Procedures
The temperature field and cooling rate of the Fe-Cr-Ni-Al alloy solidification process were simulated using Procast software. The stepped specimens were longitudinally sectioned to obtain specimens of different thicknesses. These specimens were then embedded in an XQ-2 mounting machine, polished with sandpaper and a polishing machine, and corroded with aqua regia. The microstructure was observed under a metallographic microscope. Image Pro plus6.0 software was used to count and calculate the primary dendrite spacing and phase ratio, and a microhardness tester was used to measure and count the microhardness of different phase regions of the alloy.
3. Experimental Results and Analysis
3.1 Numerical Simulation Analysis
3.1.1 Influence of Thickness on the Cooling Rate of the Middle Part during Solidification
Three-dimensional models of the specimen and mold were established in Solidworks software. The models were meshed in a non-uniform manner, with the specimen mesh set to 0.5mm and the mold mesh set to 5mm. After checking the mesh quality, the surface mesh and volume mesh models were obtained. In the Procast software, the conditions were set in the Cast module. A gravity casting model was used, with the gravity direction consistent with the pouring direction (-Y axis). The pouring initial temperature was set to 1600°C, the mold initial temperature was set to 25°C, and the heat transfer interface coefficient was set to 1000 W/(m²·K). The boundary conditions were set, and the pouring speed was 2000mm/s. The temperature field distribution cloud diagram of the alloy during the filling and solidification process was obtained. By selecting the center points A, B, C, and D of different thicknesses on the longitudinal section of the casting, the temperature-time curves of different thickness center points were obtained. The cooling rates of the nodes corresponding to casting thicknesses of 1, 2, 4, and 8mm were calculated as 338, 185, 155, and 93K/s, respectively, indicating that the cooling rate decreases with increasing thickness.
3.1.2 Influence of Thickness on the Cooling Rate of the Edge Part during Solidification
The specimen and mold meshes were set to 1mm, and other boundary conditions were the same as the previous simulation. Twelve points were selected on the edge of the specimen longitudinal section, and the temperature-time curves of different thicknesses were obtained. The cooling rates of each point were calculated, and it was found that the cooling rate gradually increases with decreasing thickness.
3.2 Microstructure
3.2.1 XRD Analysis
XRD tests were carried out on the stepped specimen materials with cooling rates in the ranges of 65.2 – 90.5K/s and 50.7 – 59.3K/s. The results showed that the Fe-Cr-Ni-Al alloy microstructure mainly consists of austenite and δ-ferrite phases. With the increase in cooling rate, the intensity of austenite decreases, and the intensity of ferrite increases, indicating that the proportion of ferrite phase increases.
3.2.2 Metallographic Structure of the Edge Part of the Fe-Cr-Ni-Al Alloy
The experiment mainly analyzed the specimens with cooling rates in the range of 50 – 100K/s. The metallographic structure near the copper mold of the specimens showed that under the rapid cooling effect of the copper mold, the solidification structure grew directionally along the heat flow direction. With the increase in cooling rate, the primary dendrite morphology was refined, and the length increased, indicating that the cooling rate has an obvious inhibitory effect on dendrite growth.
3.2.3 Metallographic Structure of the Middle Part of the Fe-Cr-Ni-Al Alloy
The metallographic structure of the middle part of the specimens showed that the growth direction of dendrites became irregular, and the secondary dendrites grew significantly. With the increase in cooling rate, the alloy microstructure became more irregular due to the temperature gradient during cooling, resulting in uneven component distribution and unbalanced crystallization. At the same time, the increase in cooling rate led to an increase in the degree of supercooling and the nucleation rate of the solid solution alloy, resulting in grain refinement.
3.2.4 Dendrite Spacing
The primary dendrite spacing was measured manually in ImageProPlus6.0 software. The experiment mainly measured the stepped specimen section with a cooling rate in the range of 65.2 – 90.5K/s. Fifty groups of data were measured in the metallographic structure magnified 50 times, and the results were processed by weighted average. The results showed that within the cooling rate range of 65.2 – 90.5K/s, the primary dendrite spacing gradually decreased from 40.8, 34.8, 32.1 to 31.4μm.
3.2.5 Dendrite Growth Model
Based on the relevant theories of dendrite growth, the relationship between dendrite tip growth rate and supercooling degree, as well as the relationships between dendrite tip radius and growth rate, and primary dendrite spacing and tip radius were established. It was explained that with the increase in cooling rate, the supercooling degree increases, the dendrite growth rate increases, the tip radius decreases, and finally the primary dendrite spacing decreases.
3.2.6 SEM Analysis
SEM tests were carried out on the stepped specimens with cooling rates in the ranges of 50.7 – 59.3K/s and 65.2 – 90.5K/s. The results showed that the regions with relatively high Ni content and relatively low Cr content were judged to be austenite phases, and the regions surrounding the austenite phase with relatively high Cr content and relatively low Ni content were judged to be ferrite phases. The black granular substances unevenly distributed on the austenite and ferrite phases were σ phases.
3.3 Microhardness
The microhardness of the specimens with cooling rates in the ranges of 65.2 – 90.5K/s and 50.7 – 59.3K/s was tested. The results showed that with the increase in cooling rate, the hardness of both the ferrite and austenite phase regions increased. The hardness increase amplitude of the ferrite phase region was 29.74%, and that of the austenite phase region was 38.75%. The hardness increase amplitude of the austenite phase region was larger than that of the ferrite phase region, and the hardness of the ferrite phase was always greater than that of the austenite phase region. The change in hardness was due to the different phase transformation products under different cooling rates. The greater the cooling rate, the greater the instantaneous supercooling degree, the larger the nucleation rate, the more sufficient the alloy component diffusion, and the smaller the dendrite spacing, resulting in an increase in hardness.
4. Conclusions
- The stepped casting vacuum suction casting method was used to obtain the sub-rapid solidification casting process parameters of the Fe-17.6Cr-8.8Ni-2Al alloy, which can better fit the cooling crystallization speed conditions in the range of 47 – 248K/s.
- The main phases of the Fe-17.6Cr-8.8Ni-2Al alloy are austenite and δ-ferrite phases, with a small amount of σ phase distributed. With the increase in cooling rate, the proportion of ferrite phase increases, and the increase in supercooling degree leads to a decrease in the primary dendrite spacing of the alloy microstructure.
- With the increase in cooling rate, the hardness of both the austenite and ferrite phases increases. The hardness increase amplitude of the ferrite phase region is 29.74%, and that of the austenite phase region is 38.75%. The hardness increase amplitude of the austenite phase region is larger than that of the ferrite phase region, and the hardness of the ferrite phase is greater than that of the austenite phase region.
In future research, further optimization of the Fe-Cr-Ni-Al alloy casting process can be carried out based on these results to improve the performance of the alloy and expand its application range.
5. Discussion and Future Work
Although this study has obtained certain results on the Fe-Cr-Ni-Al alloy casting process, there are still some aspects that can be further explored. For example, the influence of other alloying elements on the microstructure and properties of the alloy can be studied. In addition, the optimization of the casting process can be further carried out to improve the quality and performance of the alloy. Future research can also focus on the application of the alloy in different fields and explore its performance and reliability in actual use.
6. Acknowledgments
The authors would like to express their gratitude to the National Natural Science Foundation of China (Project No. 52171037) for its financial support. Thanks also go to the colleagues who provided help and advice during the research process.
