Study on the influence of thermal aging on the mechanical and magnetic properties of ferritic steel

Due to the extensive use of ferrite in nuclear power plants, the thermal aging issues of ferritic alloys have attracted significant attention. The present study investigates thermally aged FeCr binary ferritic alloys with different Cr contents using various mechanical and magnetic properties tests. The result indicates that the increase in Cr element content and prolonged thermal aging significantly enhances the strength of alloy materials, along with an important reduction in toughness. Meanwhile, the magnetic characteristic shows a considerable difference in different ferritic alloys with varying Cr contents under the influence of thermal aging. As a potential characterization method, magnetic measurement can be used to determine the thermal aging degree in ferritic alloys and the Cr concentration should be considered.


Introduction
The ferritic alloy is extensively employed in the nuclear industry due to its advantageous mechanical properties and corrosion resistance features.Similarly, by introducing a small amount of ferrite into the austenitic matrix, the material strength and corrosion resistance can be significantly improved, thus expanding the range of material applications.For example, Z3CN20.09Maustenite-ferrite duplex stainless steel, featuring a minor proportion of ferrite, is commonly used as the primary pipe material in nuclear power plants, as it exhibits excellent mechanical characteristics [1].The FeCrAl ferrite steel, known for its superior strength and outstanding oxidation resistance, is considered a key option for nuclear fuel cladding materials in the future [2].Furthermore, ferritic-martensitic stainless steel has broad application prospects in fourth-generation fission reactors and nuclear fusion reactors [3].
However, the chromium content is one of the key factors in determining the properties of ferrite, as well as the related ferritic alloy.Although high chromium content enhances strength, it can lead to a considerable decrease in toughness after prolonged exposure to a high-temperature environment [4].This is primarily attributed to the local enrichment of chromium caused by spinodal decomposition, which adversely affects the performance of steels like CF8 and Z3CN20.09Mwith a chromium content of 20 wt.% by reducing toughness and increasing the risk of failure [1].Similarly, the second-generation ATF FeCrAl cladding restricts the chromium content to 13 wt.%,significantly lower than the 22 wt.% in the first generation [2].Therefore, it is crucial to comprehend the thermal aging effect on the ferrite with various chromium content, which has a significant impact on the optimization of alloy design criteria and the enhancement of the service performance.
The mechanism of thermal aging embrittlement in duplex stainless steel has been widely studied [1].Generally, decomposition occurs in the ferritic alloy under thermal aging conditions, where the σ' phase appears and the composition fluctuation is greatly influenced by chromium content, aging temperature, and time [4,5].As a means to investigate the α' phase formed by spinodal decomposition, the ferritic alloys with varying chromium content have been widely studied [4][5][6].Danoix and Auger [5] pointed out that the α' phase formed by spinodal decomposition is significantly higher than that formed through nucleation and growth mechanism.Xiong et al. [4], building upon previous research on FeCr binary ferritic alloy, utilized the phase field simulation method to determine the spinodal line at approximately 20 wt.% Cr and 400℃.However, most of these studies have focused on the spinodal decomposition behavior at the micro-scale [4,5].Nevertheless, the properties influenced by thermal aging with varying chromium content need to be explored as well, which can provide a quantitative understanding of the mechanism and new method to probe the spinodal decomposition degree and the performance reduction during thermal aging.
Despite the importance, there is rarely reported literature concerning the impact of thermal aging on the mechanical and magnetic characteristics of ferritic alloy with various Cr content [7].In the present work, prolonged thermal aging is conducted on different FeCr binary alloys.Then mechanical and magnetic properties were systematically studied.Finally, we discuss the impact of thermal aging and Cr content on the physical properties of the ferritic alloys in the mentioned aspects.

Experiment procedure
As a primary mechanism of thermal aging damage, previous research on binary ferritic alloy shows that spinodal decomposition occurs under the condition that the Cr content mainly ranges from 10 wt.% to 38 wt.%.While the compositional fluctuation in the alloys is mainly driven by the nucleation-growth mechanism when Cr content is less than 10 wt.%.Specifically, the spinodal line is around 20 wt.%, which may vary in different studies but is always within 40 wt.% [4].Therefore, four ferritic Fe-Cr binary alloys with different Cr contents of 9.78 wt.%, 18.85 wt.%, 25.45 wt.%, and 37.61 wt.% were prepared by vacuum induction melting method, which is nominated as Fe-10Cr, Fe-19Cr, Fe-25Cr, and Fe-38Cr respectively.More details about the chemical composition are reported in previous studies [8].
Then different quenching processes were implemented to suppress austenite formation and obtain a fully ferritic microstructure in Figure 1.Despite the difference in grain size, the ferrite phase has been verified through X-ray diffraction analysis and FERITSCOPE FMP30 from Helmut Fischer GmbH.More details about the preparation and microstructure of the material in the virgin state have been reported in previous studies [8].The alloys are exposed to prolonged thermal aging at 400℃ for different times.Then the room temperature tensile and V-notch instrumented impact tests were carried out using respectively the SHIMADZU AG-IC 100KN and Zwick RKP 450 instruments at room temperature.It should be noted that the whole process adheres to the ASTM specification.
Besides, studies show that the dislocations interact closely with the magnetic domain walls inside the material, which affects the magnetic properties of materials [7].Therefore, the investigation of the magnetic parameters could provide a potential nondestructive testing approach to probe the thermal aging of the material.The materials were prepared into dimensions of 1 mm × 1 mm × 3 mm and the magnetic hysteresis loop was obtained using the Lake Shore 7407 vibrating sample magnetometer (VSM).Then the parameters such as the saturation magnetization and coercive force were further analyzed from the measured data.

Tensile behavior
The true stress-strain curves of the various ferritic alloys after prolonged thermal aging are compared in Figure 2. By comparing the tensile properties of ferritic alloys in the virgin state, it can be observed that the yield strength exhibits a significant increase as the Cr content rises, while the ultimate strength shows an upward trend.However, the elongation experiences a sharp decrease due to diminished toughness.The presence of higher concentrations of Cr element exerts a substantial influence on both the strength and toughness, resulting in a more brittle characteristic.
After long-term thermal aging, all the ferritic alloys show a strength increase, which is evident from the rise in both yield strength and tensile strength, with an enhancement effect observed with increasing Cr content.However, due to the substantial reduction in elongation and toughness, high Cr-content ferritic alloys experience brittle fracture before reaching their ultimate tensile strength or even theoretical yield strength after thermal aging.Notably, the decrease in toughness is particularly pronounced in ferritic alloys with Cr contents of 19 wt.% and 25 wt.%.

Charpy impact test
The Charpy impact curves of the thermally aged ferritic alloys are compared in Figure 3.By comparing the Oscillo graphic impact properties of the ferritic alloys in the virgin state, it is shown that the Cr content has little impact on elastic deformation, as the curves exhibit similar behavior at the linear elastic stage.Nonetheless, the unloading displacement experiences a substantial decrease as the increasing Cr content.This observation can be attributed to a notable reduction in toughness, which correlates with the decreasing impact energy.
Specifically, both the plastic deformation and final fracture mode during the impact process are affected by the different Cr contents.In both Fe-10Cr and Fe-19Cr ferritic alloys, plastic deformation occurs before reaching the ultimate load, followed by partial stable crack propagation, while the process of stable crack growth is notably more prominent in Fe-10Cr.Finally, the Fe-25Cr and Fe-38Cr ferritic alloys reach the ultimate load, exhibiting a negligible stable crack growth process.Furthermore, the slope of the unloading process is also strongly affected by different Cr content.The unloading angle experiences a significant increase and the Oscillo graphic curve progressively sharpens and steepens during the unloading process, indicating a rapid brittle fracture with instantaneous crack propagation due to reduced toughness.Compared to the virgin state, the impact toughness of the ferritic alloys significantly decreases with prolonged thermal aging time until the saturation plateau, particularly for the high Cr content alloys.This could be attributed primarily to the increase in material hardness and toughness decrease caused by spinodal decomposition [4][5][6][7].

Magnetic properties
According to microscopic magnetic theory and thermodynamic principles, a stable magnetic state corresponds to a state where the Gibbs free energy within ferromagnetic materials is minimized [9].In ferromagnetic materials, when a magnetic domain wall is located at the position of a dislocation, the Gibbs free energy decreases and the dislocation acts as a pinning point, hindering the movement of the domain wall and affecting the magnetic characteristics of the material.
The comparison of magnetic hysteresis loops (B-H curve) of the ferritic alloys with varying Cr contents after prolonged thermal aging is shown in Figure 5. Globally, all the materials show a soft magnetic characteristic, featuring narrow hysteresis loops that are symmetric concerning the origin.The materials exhibit high magnetic permeability, low coercive force (Hc), and residual magnetization (Br).However, as can be observed from the enlarged plot near the zero point, there are still differences in the coercive force and residual magnetization of the materials after being subjected to different thermal aging times.By analyzing the hysteresis loop, conventional magnetic characteristic parameters such as coercive force, residual magnetization, and saturation magnetization can be obtained.The following analysis mainly focuses on the variations of the magnetic characteristic parameters of the ferritic alloys during the thermal aging process.The coercive force represents the magnitude of the external magnetic field required for the demagnetization process to reach a magnetic field intensity of zero.It reflects the resistance encountered by domain walls in overcoming defects during the demagnetization process, which means that a higher coercive force indicates greater resistance encountered during demagnetization.Studies show that the coercive force exhibits complex transformation characteristics due to multiple factors such as defects and lattice distortions, during the damage process such as thermal aging [7].This poses challenges in the characterization of the damage extent by coercive force, as confirmed by the results of this experiment.
As shown in Figure 6, the variation in coercive force during the thermal aging process differs significantly among the various ferritic alloys.When the Cr element content is within the range of 19-25 wt.%, the coercive force shows regular responsiveness to the thermal aging time.Particularly, the coercive force of Fe-19Cr ferritic alloy exhibits a monotonic decrease with prolonged thermal aging time.
Therefore, the Cr content level should be carefully considered when the extent of thermal aging in ferritic alloys is assessed by coercive force, with an optimal Cr content of around 19 wt.%.Previous research indicates that saturation magnetization (Bs) is related to spontaneous magnetization and insensitive to the microstructure of material [7].However, the residual magnetization is closely associated with the microstructure, defects, and internal stress.From the present experiment results, it can be concluded that the saturation magnetization remains relatively constant with prolonged thermal aging.Therefore, the variation in residual magnetization ratio (Br/Bs) is discussed afterward.
In Figure 7, it can be observed that the variation in the residual magnetization ratio follows a similar trend as the coercive force during thermal aging.For the Fe-19Cr and Fe-25Cr ferritic alloys, the residual magnetization ratio rapidly decreases along with the occurrence of thermal aging damage and then stabilizes.However, ferritic alloys with excessively low or high Cr contents exhibit a significant increase in the residual magnetization ratio after a thermal aging time of 1000 hours.Hence, it is important to consider the reasonable range of compositions when the extent of thermal aging damage is characterized by the residual magnetization ratio.

Conclusion
In summary, to investigate the thermal aging effect on the mechanical and magnetic properties of ferritic alloys, four FeCr ferritic alloys were exposed at 400℃ for up to 2000 h.Material mechanical property tests, including tensile and Charpy impact experiments, indicate that an increase in Cr element content and prolonged thermal aging significantly enhance the strength of alloy materials, along with a significant reduction in toughness.Meanwhile, there are considerable differences in the magnetic characteristics of different ferritic alloys with varying Cr contents after prolonged thermal aging.As a potential characterization method, VSM magnetic measurement can be used to determine the thermal aging degree of ferrite and the Cr concentration should be carefully considered.

Figure 3 .
Figure 3.Comparison of impact property after prolonged thermal aging for various ferritic alloys (a) Fe-10Cr, (b) Fe-19Cr, (c) Fe-25Cr, and (d) Fe-38Cr.The impact energy of the ferritic alloys has been further analyzed and the effect of thermal aging is shown in Figure 4.In the virgin state, a global decrease in impact energy along with the rising Cr content is observed, which indicates a substantial increase in material brittleness as the Cr content rises.Compared to the virgin state, the impact toughness of the ferritic alloys significantly decreases with

Figure 4 .
Figure 4. Effect of thermal aging on the impact energy for various ferritic alloys.

Figure 5 .
Figure 5.Comparison of magnetic hysteresis loop after prolonged thermal aging for various ferritic alloys (a) Fe-10Cr, (b) Fe-19Cr, (c) Fe-25Cr, and (d) Fe-38Cr.The coercive force represents the magnitude of the external magnetic field required for the demagnetization process to reach a magnetic field intensity of zero.It reflects the resistance encountered by domain walls in overcoming defects during the demagnetization process, which means that a higher coercive force indicates greater resistance encountered during demagnetization.Studies show that the coercive force exhibits complex transformation characteristics due to multiple factors such as defects and lattice distortions, during the damage process such as thermal aging[7].This poses challenges in the characterization of the damage extent by coercive force, as confirmed by the results of this experiment.As shown in Figure6, the variation in coercive force during the thermal aging process differs significantly among the various ferritic alloys.When the Cr element content is within the range of 19-25 wt.%, the coercive force shows regular responsiveness to the thermal aging time.Particularly, the coercive force of Fe-19Cr ferritic alloy exhibits a monotonic decrease with prolonged thermal aging time.Therefore, the Cr content level should be carefully considered when the extent of thermal aging in ferritic alloys is assessed by coercive force, with an optimal Cr content of around 19 wt.%.

Figure 6 .
Figure 6.Effect of thermal aging on the coercive force for various ferritic alloys.Previous research indicates that saturation magnetization (Bs) is related to spontaneous magnetization and insensitive to the microstructure of material[7].However, the residual magnetization is closely associated with the microstructure, defects, and internal stress.From the present experiment results, it can be concluded that the saturation magnetization remains relatively constant with prolonged thermal aging.Therefore, the variation in residual magnetization ratio (Br/Bs) is discussed afterward.In Figure7, it can be observed that the variation in the residual magnetization ratio follows a similar trend as the coercive force during thermal aging.For the Fe-19Cr and Fe-25Cr ferritic alloys, the residual magnetization ratio rapidly decreases along with the occurrence of thermal aging damage and then stabilizes.However, ferritic alloys with excessively low or high Cr contents exhibit a significant increase in the residual magnetization ratio after a thermal aging time of 1000 hours.Hence, it is important to consider the reasonable range of compositions when the extent of thermal aging damage is characterized by the residual magnetization ratio.

Figure 7 .
Figure 7. Effect of thermal aging on the residual magnetization ratio for various ferritic alloys.