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Article

Effect of Nickel Content and Cooling Rate on the Microstructure of as Cast 316 Stainless Steels

by
Lei Chen
1,
Yang Wang
2,
Yafeng Li
3,
Zhengrui Zhang
2,
Zhixuan Xue
2,
Xinyu Ban
2,
Chaohui Hu
2,
Haixiao Li
3,
Jun Tian
4,
Wangzhong Mu
5,6,
Kun Yang
1,* and
Chao Chen
2,*
1
College of Mechanical Engineering, Taiyuan University of Technology, Taiyuan 030024, China
2
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
3
Xinzhou Comprehensive Inspection and Testing Center, Xinzhou 034000, China
4
School of Iron and Steel, Soochow University, Suzhou 215137, China
5
Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, School of Metallurgy, Northeastern University, Shenyang 110819, China
6
Department of Materials Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden
*
Authors to whom correspondence should be addressed.
Crystals 2025, 15(2), 168; https://doi.org/10.3390/cryst15020168
Submission received: 31 December 2024 / Revised: 3 February 2025 / Accepted: 7 February 2025 / Published: 10 February 2025
(This article belongs to the Special Issue Microstructure and Mechanical Properties of Austenitic Stainless Steels: 2nd Edition)
Browse Figures
Versions Notes

Abstract

:
To meet the requirement of low magnetic permeability, which, in turn, lowers the ferrite content of castings, of special interest is 316 stainless steel, whose low ferrite content renders it suitable also for nuclear power applications. Therefore, the effects of the composition and cooling rate of 316 stainless steel castings on the ferrite content are investigated. Three 316 stainless steel continuous casting samples with different compositions (primarily differing in the Ni content) are studied, i.e., low-alloy type (L-316), medium-alloy type (M-316), and high-alloy type (H-316). The austenite-forming element nickel of three different industrial samples is 10%, 12%, and 14%, respectively. The effect of the cooling rate on the ferrite content and precipitation phases of the high Ni content of the 316 stainless steel casting (H-316) is studied by remelting experiments and different methods of quenching of liquid steel. In both cases, the ferrite content and the precipitate phases in the microstructure are analyzed using SEM and EBSD. The results indicate that compositional changes within the 316 stainless steel range lead to changes in the solidification mode. In the L-316 casting, solidified by the FA mode (ferrite–austenite mode), ferrite precipitates first from the liquid phase, followed by the formation of austenite, and the ferrite content is 11.2%. In contrast, the ferrite content in the M-316 and H-316 castings, solidified by the AF mode (austenite–ferrite mode), is 2.88% and 2.45%, respectively. The effect of the solidification mode on the ferrite content is more obvious than that of the composition. The microstructure of the L-316 casting is mainly composed of the austenitic phase and the ferritic phase. The microstructure of the M-316 casting is composed of austenite, ferrite, and a small amount of sigma phase, with a small amount of ferrite transformed into the sigma phase. The microstructure of the H-316 casting is basically composed of austenite and the sigma phase, with the ferrite has been completely transformed into sigma phase. Changes in composition have a greater influence on the precipitate phases, while the solidification mode has a lesser impact. In the remelting experiments, the ferrite content in the H-316 ingot obtained through furnace cooling and air cooling is 1.49% and 1.94%, respectively, and the cooling rates are 0.1 °C/s and 3.5 °C/s, respectively. Under oil- and water-cooling conditions, with cooling rates of 11.5 °C/s and 25.1 °C/s, respectively, the ferrite content in the ingot is controlled to below 1%. The effect of the cooling rate on the precipitation phase of the H-316L ingot is that the amount of precipitated phase in the ingot decreases with an increase in cooling rate, but, when the cooling rate exceeds a certain value (air cooling 3.5 °C/s), the change in cooling rate has little effect on the amount of the precipitated phase.

1. Introduction

Stainless steels are widely used in daily life and in many industries [1,2,3,4,5,6]. One of the most common grades is the 316 stainless steel series [7,8,9,10,11], which is based on the 304 stainless steel with the addition of the Mo element. One of the applications of the 316 stainless steel grade is the materials for in-core and out-of-core components of Generation-IV fast reactors due to its excellent operating experience in light–water reactors [12,13,14,15]. The 316H stainless steel grade is also used for nuclear power pipeline systems [16,17,18]. The δ ferrite content, grain size, and secondary phase precipitation are key issues for nuclear applications [13]. A large number of studies have focused on the effect of δ ferrite on the hot deformation and recrystallization [14,19], mechanical properties [20,21,22], creep cavitation [23,24], fatigue crack [25], and corrosion behavior [26] of 316 stainless steels. However, this study is focused on the ferrite formation during crystallization and solidification of 316 stainless steels.
Since most 316 stainless steels are manufactured by continuous casting [27,28,29,30,31] or by casting and electro slag remelting [18], the δ ferrite is formed during the solidification processes and the residual ferrite is retained in the room-temperature microstructure of the stainless steel with austenite as the matrix [32,33,34,35,36]. While a small amount of residual ferrite can enhance the thermal cracking resistance of the material [37,38], excessive ferrite can significantly impact the high-temperature performance of austenitic stainless steel. Studies have shown that, when austenitic stainless steel with excessive ferrite is subjected to prolonged service at temperatures between 500 °C and 900 °C, the ferrite decomposes into the M23C6 carbide, the sigma (σ) phase, the Chi phase, and the Laves phase [39,40,41,42,43]. These phases generate a chromium-depleted area in the ferrite and in the vicinity of austenite grain boundaries, and, thus, accelerate the occurrence of intergranular corrosion [17]. Furthermore, Warren et al. [24] and Sasikala et al. [23] have found that the nucleation and growth of cavities preferentially occur at the δ/γ interface during creep, which is responsible for the reduction of the creep-rupture life in 316 steel.
Moreover, because of its low magnetic permeability, 316 stainless steel [44] is being increasingly used in medical materials and mobile phone frames. The presence of ferrite increases the magnetic permeability of 316 stainless steel, such a property being very unfavorable for its applications. In addition, for nuclear power applications, the residual ferrite area fraction of 316 stainless steel castings is required to be controlled below 1% [16]. Therefore, it is very important to control the ferrite content of 316 stainless steel during casting. As mentioned earlier, the control and formation of the ferrite during continuous casting of 304 stainless steel have been well studied [29,32,33,35,36]. Studies on ferrite formation and control during the continuous casting of 316 stainless steel are rare and urgent.
For austenitic stainless steels, the primary solidified phase can be ferrite or austenite. According to the phase transformation after solidification, the stainless steel solidification mode can be classified into A (austenite), AF (austenite–ferrite), FA (ferrite–austenite), and F (ferrite) modes. Due to the complexity of alloying elements, Cr and Ni equivalent (Creq, Nieq) formulas are proposed to simplify the composition of stainless steels [45]. A typical classification of the solidification modes of different stainless steel series is shown in Figure 1 (re-plotted based on ref. [46]). The solidification mode of different types of stainless steel depends on the Creq/Nieq ratio. In Figure 1, the solidification modes of 304 and 310S stainless steels are the FA and the A mode, respectively. It can be seen that the composition of 316 stainless steel is located near the transition point, and the solidification mode is highly sensitive to its composition and cooling rate [46]. In fact, both FA and AF modes are observed in 316 stainless steels [47,48]. The FA and AF modes refer to different solidification sequences in 316 austenitic stainless steel. In the FA mode, ferrite precipitates first from the liquid phase, followed by its transformation into austenite, whereas, in the AF mode, austenite precipitates first, followed by its transformation into ferrite. It is believed that the ferrite in 316 stainless steel solidified by the AF mode is less than that solidified by the FA mode.
In austenitic stainless steels, composition is the key factor affecting the ferritic content [49,50]. For low-carbon-content 316L stainless steels, Chun et al. [48] found that the composition of these steels is unique. The composition of 316L stainless steel is near the transition point, making its solidification process highly susceptible to compositional variations which has an important effect on the ferrite content. For the standard composition of AISI 316 stainless steel, the nickel content varies from 10% to 14%. In some 316 steels, the Ni content is close to 10% and is termed a lean alloy type. In addition, in some commercial steel grades, the nickel content is approximately 12%. A high nickel content, for example 14%, is produced for low-ferrite applications [28]. A recent study by the authors [51] demonstrates that two different solidification modes of 316L stainless steel exist with varying Ni contents of 10% and 14%. One of the motivations behind this study is to evaluate the solidification mode and ferrite content in 316 stainless steels with different nickel contents, for example, 10%, 12%, and 14%.
The cooling rate also has varying effects on ferrite content under different solidification modes [52,53,54]. Moreover, most earlier studies are focused on a high cooling rate of welding which differs significantly from actual continuous casting processes [53]. Only a few research papers have focused on the effect of cooling rate on ferrite content in 316 stainless steel under continuous casting conditions. Wang Q.Y. et al. [20] studied the 316H stainless steel and found that, under the AF mode of solidification, the cooling rate has no significant effect on ferrite content. In contrast, when it solidified by the FA mode, the ferrite content decreased with an increasing cooling rate and the studied cooling rate was limited to the relatively slow cooling rates which correspond to that of the continuous casting. Li et al. [16] studied the effect of the cooling rate on the FA mode over a broader range and found that, within this range, the ferrite content increased as the cooling rate increased. Additionally, composition and cooling rate have a significant impact on the formation of the sigma phase [42,55]. In summary, for the 316 stainless steel series solidified by the AF mode, the effect of the cooling rate on the ferrite content requires further studies. This is another motivation behind this study.
The effect of composition and cooling rate on the ferrite content of 316 stainless steel castings is investigated. First, industrial samples of three different compositions of 316 stainless steel castings are analyzed with respect to the ferrite content and precipitation phases. Specifically, the content of the austenite-forming element nickel in three different industrial samples is varied from 10% to 14%, approximately. The higher Ni content of the 316 stainless steel is assumed to be a candidate steel with a lower ferrite content. Second, the high Ni content of the 316 stainless steel casting (H-316) is further studied. The effect of the cooling rate on the ferrite content and precipitation phases of this H-316 steel is studied by remelting experiments and using different methods of quenching of liquid steel. This study can provide guidance for the design and production of low-magnetic-permeability 316 series austenitic stainless steel castings and contribute to the optimization of the microstructure of the 316 stainless steel series castings.
The manuscript is organized as follows. Section 2 includes the materials and methods, Section 3 the results following the analysis of ferrite morphology and precipitation phases, Section 4 the discussion with respect to the analysis of the solidification mode and ferrite content with the aid of both thermodynamics calculation and empirical Cr and Ni equivalent formulas, and, finally, the conclusions.

2. Materials and Methods

Three 316 austenitic stainless steel castings with varying compositions are investigated in this paper. The standard composition of AISI 316 stainless steel and the composition of the three stainless steel castings are shown in Table 1. The Cr, Mo, and Mn content was similar among the three steels. The Cr content of the three castings was approximately 17%, while the Mo content was approximately 2.5%. The Ni content varied significantly, ranging from 10.00% in the low-Ni stainless steel (L-316) to 12.17% in the medium-Ni stainless steel (M-316) and 14.25% in the high-Ni stainless steel (H-316).
The three compositions of steel were produced from three industrial steel producers. The dimensions and sampling locations of the castings are shown Figure 2. The L-316 stainless steel is a lean alloy type continuous casting slab and the size was 220 mm thickness × 1200 mm width. As shown in Figure 2a, a sample at 1/12 of the width location, i.e., 100 mm from the corner, and at a 1/2 thickness location, i.e., 55 mm from the edge, was cut for analysis. The M-316 stainless steel is a round ingot casting and the diameter was 340 mm. As shown in Figure 2b, the sample was located at half of the radius of the ingot. Finally, the H-316 is a high-Ni-content stainless steel continuous casting billet with a size of 220 mm × 220 mm. As shown in Figure 2c, the sample was located at half of the width direction. Those sampling positions were all located in the center region (not the edge part with a high cooling rate) of the castings but not exactly in the center, where the macro segregation is severe. The size of all the samples was identical, i.e., 10 mm × 10 mm × 10 mm.
To analyze the effect of the cooling rate on the ferrite content of the 316 stainless steel series solidified by the AF mode, the H-316 casting billet was remelted and cooled by different methods. Specifically, the remelting experiment was conducted in a high-temperature tube furnace. After the base material was placed in an alumina crucible, it was positioned inside the tube furnace, evacuated, and filled with argon. The furnace was then heated from room temperature to 1590 °C at a rate of 0.2 °C/s, followed by a 1.5 h hold to ensure the complete melting of the steel. To prevent oxidation, both heating and insulation processes were performed in an argon atmosphere. Once the base material was fully melted, a cup sampler was used to collect a molten steel sample. The molten steel was then cooled using water, oil, and air cooling methods, respectively, while the remaining molten steel was allowed to cool in the furnace. Finally, four samples—furnace-cooled, air-cooled, oil-cooled, and water-cooled—were obtained from the H-316 casting billet.
The samples obtained from the casting and smelting process were first pre-ground and polished, followed by corrosion using an FeCl3 + HCl aqueous solution to reveal the ferrite structure. The ferrite morphology, grain size, and secondary dendrite arm spacing (SDAS) at different positions was observed using a ZEISS AX10 metallographic microscope (Carl Zeiss AG, Oberkochen, Germany). The ferrite area fraction and the SDAS of each sample were counted using the IPP (image-Pro Plus 6.0) software. The ferrite area fraction was used as an indicator to measure the amount of ferrite, and the SDAS was used to calculate the cooling rate of the studied ingot. To ensure the accuracy of the statistical measurements, the ferrite area fraction of at least five different locations within each sample was counted at 100× magnification; 120–150 dendrite arm spacings at five different locations wihtin each sample were selected for statistical analysis at 50× magnification [56].
For high-magnification imaging of the ferrite morphology, a JEOL JMT IT500 (JEOL, Tokyo, Japan) scanning electron microscope (SEM) was used. To further investigate the phase distribution in the microstructure, the samples were pre-ground and polished, followed by electrolytic polishing using a 10% perchloric acid alcohol solution. This step was performed to remove the residual stress induced by mechanical polishing. The polishing voltage was set to 25 V, with a polishing time of 20 s. The phase composition of the microstructure was examined using electron backscatter diffraction (EBSD).
Finally, thermodynamic calculations of the equilibrium solidification process of the three castings were calculated using the Thermo-Calc software (2025a) and the TCFE12 database.

3. Results

3.1. Effect of Composition on the Microstructure and Ferrite Content

Figure 3a, b, and c are the metallographic observation results of the L-316, M-316, and H-316 castings, respectively. Samples at the same position on the edge of the three castings were selected. The gray matrix is the austenite structure, and the color darker than the gray matrix is the ferrite structure [57]. As it can be seen from Figure 3, the morphology and distribution of the ferrite of the three ingots were significantly different. In the L-316 casting, ferrite was distributed around the center of the dendrite in a continuous skeleton shape, and some blocky ferrite also appeared. In the M-316 casting, ferrite was distributed in the austenite grain boundary in the shape of short rods. Its morphology and distribution were similar to those of the ferrite in the H-316 casting, but they were quite different from the ferrite morphology and distribution in the L-316 casting. This shows that the M-316 casting was solidified in the AF mode, and the two different solidification modes have a great influence on the morphology and distribution of ferrite. In the H-316 casting, ferrite was also distributed in the austenite grain boundaries in the form of short rods.
The ferrite content at different positions within the three castings was measured. In the L-316 casting, the ferrite content was 11.2%, which is the highest among all the three castings, while in the M-316 and H-316 castings the ferrite content was 2.88% and 2.45%, respectively. The ferrite content in the L-316 casting was significantly higher than that in the other two castings, whereas the difference in ferrite content between the M-316 and H-316 castings was not significant. Further SEM observations were carried out to examine the ferrite morphology at high magnification. Figure 4a–c show the high-magnification secondary electron (SE) images of ferrite in the three castings. As seen in Figure 4a, the ferrite in the L-316 casting generally retained its complete morphology. However, in the M-316 and H-316 castings, there were features of ferrite decomposition. As indicated by the yellow box in Figure 4b, part of the ferrite in the M-316 casting decomposed into a white and gray coupled structure, while some regions still maintained the complete ferrite morphology. Precipitates were also observed at the ferrite–austenite phase interface. In the H-316 casting, the complete ferrite structure was almost unrecognizable, with the ferrite inside showing a clustered morphology.
EBSD analysis was performed on the microstructure of the three castings to determine the phase composition in the microstructure, as shown in Figure 5a–c. The microstructure of the L-316 casting consisted of the ferrite phase and austenite phase. The microstructure in the M-316 casting was basically composed of the austenitic phase and ferrite phase, but it can be seen that a portion of the ferrite had been transformed into the sigma phase, forming a coupled structure of ferrite and sigma phase, which corresponds to the white precipitated phase in Figure 4b. However, the ferrite phase could not be observed in the microstructure of the H-316 casting, and the microstructure was mainly composed of the austenitic phase and the sigma phase. This is consistent with the results in Figure 4c. It was basically impossible to observe the intact ferrite morphology, and the ferrite was completely decomposed into the sigma phase. At the same time, it can be seen from Figure 5c that part of the austenite phase was also wrapped inside the sigma phase. This indicates that the decomposition behavior of ferrite is formed by the eutectoid reaction (δ → σ + γ2) [18]. A large portion of the ferrite was transformed into the sigma phase and a portion of the ferrite was transformed into the secondary austenite phase; finally, the coupled structure of the sigma phase and the secondary austenite was formed.

3.2. Effect of Cooling Rate on the Microstructure of High-Ni-Content 316 Stainless Steel

The high-Ni-content 316 stainless steel casting (H-316) was remelted and cooled by different methods. The metallographic results are shown in Figure 6. In Figure 6a, short rod-like black ferrite structures are still visible, and the ferrite content was measured to be 1.94%, which is close to the content in the continuous casting billet. Under the air-cooling condition, the ferrite content was 1.49%. Under the air-cooling condition, as shown in Figure 6c, ferrite was almost completely absent. Under the oil-cooling and water-cooling conditions, the microstructure of the cast ingot became fully austenitic, and no ferrite phase could be observed; the ferrite phase had essentially disappeared.
The cooling rate range within the continuous casting slab varied only slightly. In the remelting experiment, different cooling methods were employed to obtain solidification structures within a larger range of cooling rates. To study the effects of different cooling methods on the microstructure, it was necessary to further determine the cooling rates corresponding to these methods. In this study, the cooling rate distribution within the ingot is indirectly measured using the empirical formula initially proposed by Cicutti et al. [58] and further applied to stainless steels [20,59]. The formula is given as follows:
λ2 = 44ν−0.38
where λ2 is SDAS and ν is the cooling rate.
The cooling rates for four different cooling methods—furnace cooling, air cooling, oil cooling, and water cooling—were calculated to be 0.1 °C/s, 3.5 °C/s, 11.5 °C/s, and 25.1 °C/s, respectively. Additionally, the grain sizes of the ingots with four cooling rates were calculated as 145, 45, 33, and 30 μm, respectively. Further EBSD analysis was conducted on the ingots under the four cooling rates. Figure 7a, b, c, and d show the EBSD results for the ingots cooled by furnace cooling, air cooling, oil cooling, and water cooling, respectively. The results show that, for the ingot solidified by furnace cooling (0.1 °C/s), the microstructure consisted of austenite, ferrite, and sigma phases. Some of the ferrite had decomposed into the sigma phase, with only a small portion of ferrite retained, which is similar to the microstructure of the casting in Figure 5c. For the ingot solidified by air cooling (3.5 °C/s), as shown in Figure 7b, the microstructure was predominantly ferrite, with only a very small amount of ferrite transforming into the sigma phase. In the EBSD results for the ingots solidified by oil cooling (11.5 °C/s) and water cooling (25.1 °C/s), only a very small amount of ferrite remained, with a ferrite content of 0.26% and 0.36%, respectively. Due to the low ferrite content, it is difficult to distinguish between the ferrite and austenite phases in the metallographic image. The EBSD results show that the ferrite content dropped below 1%. Furthermore, the phase distribution in the ingots solidified by oil and water cooling was similar to that of the ingot solidified by air cooling, with only a small amount of ferrite transforming into the sigma phase.
The cooling rate, ferrite content, and phase constitution results for the 316 stainless steel continuous castings and remelting ingots are summarized in Table 2. From Table 2, it can be seen that there were significant differences in the ferrite content and phase constitution of continuous castings with different compositions and remelting ingots solidified with different cooling methods. Both composition and cooling rate had a significant impact on the microstructure. For the 316 continuous castings with similar cooling rates, changes in composition (mainly an increase in the Ni content) led to a decrease in ferrite content from 11.2% to 2.45%, and the phase composition changed from δ + γ to δ (small amount) + γ + σ. In the ingots with the same composition (H-316), an increase in cooling rate reduced the ferrite content, but had a smaller effect on the phase constitution. The specific effects of composition and cooling rate on the ferrite content and phase constitution are discussed in detail in Section 4.

4. Discussion

4.1. Solidification Mode Predicted by Empirical Models

The solidification mode of the austenitic stainless steel refers to the various solidification sequences that occur during its cooling process. Austenitic stainless steel experiences a three-phase region during the solidification process. Due to the different compositions and cooling rates, ferrite or austenite precipitate as the primary precipitate phase. Therefore, the solidification mode plays a crucial role in the formation of ferrite, and the precipitation sequence of different phases subsequently affects the morphology and content of ferrite. Given the complex composition of austenitic stainless steel, various elements are often simplified as Cr and Ni equivalents using the Creq and Nieq formulas. Many researchers have proposed different Creq and Nieq formulas, as reviewed in ref. [29]. Formulas (1) and (2) are the most widely used ones, proposed by Hammar and Svensson [60]:
ω(Creq) = ω(Cr) + 1.37 × ω(Mo) + 1.5 × ω(Si) + 0.5 × ω(Nb)
ω(Nieq) = ω(Ni) + 22 × ω(C) + 0.31 × ω(Mn) + 14.2 × ω(N) + ω(Cu)
Based on the Creq and Nieq formulas mentioned above, the Creq/Nieq values for the three castings were calculated as follows: L-316 (1.74), M-316 (1.48), and H-316 (1.37). According to the different Cr and Ni equivalents, the solidification modes can be categorized into four types:
A mode: L → L + γ → γ  (Creq/Nieq) < 1.25
AF mode: L → L + γ → L + γ + δ → γ + δ  1.25 < (Creq/Nieq) < 1.48
FA mode: L → δ + γ → L + δ + γ → δ + γ  1.48 < (Creq/Nieq) < 1.95
F mode: L → L + δ → δ → δ + γ  1.95 < (Creq/Nieq)
where L represents the liquid phase, δ represents ferrite, and γ represents austenite.
From this, it can be observed that the solidification mode of the L-316 casting was the FA mode, the solidification mode of the M-316 casting was between the AF and FA modes, and the solidification mode of the H-316 casting was the AF mode. The differences in the solidification modes of these castings are primarily attributed to variations in their composition, especially in relation to the Ni content.

4.2. Equilibrium Solidification by Thermodynamic Calculations

The equilibrium solidification processes of the three castings were further calculated using the thermodynamic software Thermo-Calc and the TCFE12 database, as shown in Figure 8. From Figure 8, it can be seen that the solidification processes of the three castings differed significantly. In L-316, ferrite precipitated first from the liquid phase, followed by the precipitation of austenite. During solidification, the maximum mass fraction of ferrite can reach 67.05%. After solidification ends, the ferrite content rapidly decreases and transforms into austenite. In the solidification process of M-316, the maximum mass fraction of ferrite was 30.55%, which is much lower than that in L-316. However, ferrite still precipitated first from the liquid phase. In contrast, for H-316, austenite precipitated first from the liquid phase, and ferrite precipitated in a negligible manner during solidification. The maximum mass fraction of ferrite was only 1.11%. Based on the above thermodynamic calculations, the solidification modes of the three castings were FA, FA, and AF, respectively. The thermodynamic calculation results are similar to the empirical model calculation results. However, in L-316, the precipitation temperatures of ferrite and austenite were 1456 °C and 1430 °C, respectively. The temperature difference was relatively large at 26 °C. In M-316, the precipitation temperatures of ferrite and austenite were 1430 °C and 1424 °C, respectively. The temperatures of two phases were closer. Due to the close precipitation temperatures of the two phases, the equilibrium solidification process in M-316 was inconsistent with the actual solidification process. Previous studies have found that an increase in cooling rate inhibits the nucleation and growth of ferrite, causing the solidification mode to change from FA to AF [18]. Observing the ferrite morphology in the M-316 casting, the ferrite morphology was short, rod-shaped, and distributed at the austenite grain boundaries, a phenomenon which is consistent with the ferrite morphology in H-316. This morphology is a typical product of the AF solidification mode. Therefore, in the M-316 casting, the solidification process was not consistent with the equilibrium solidification process, and the solidification mode was AF. Based on the above analysis, the solidification modes of the three castings were regarded as FA, AF, and AF.

4.3. Effect of Composition on the Microstructure

Samples from the three castings were selected for metallographic observation. By using Equation (1), the cooling rates at the selected positions in the three castings were calculated to be 0.5 °C/s, 0.4 °C/s, and 0.5 °C/s, respectively. The cooling rates of the three castings were similar, ranging between 0.4 °C/s and 0.5 °C/s, yet there were significant differences in their ferrite content. The Creq/Nieq ratio and the ferrite content of different compositions of 316 castings are shown in Figure 9. From Figure 9, it is evident that there was a correlation between the Creq/Nieq ratio and the ferrite content in continuous castings with different compositions. As Creq/Nieq decreased, the ferrite content also decreased. The ferrite content in the L-316 casting differed greatly from that in the other two castings. The ferrite content difference between the M-316 and H-316 castings was much smaller, at only 0.43%. When combined with the solidification modes of the three castings, as shown in Figure 9, it is suggested that the effect of Creq/Nieq on the ferrite content was primarily through its influence on the solidification mode. For the 316 castings with the same AF solidification mode, such as the M-316 and H-316 castings, changes in Creq/Nieq had little impact on the ferrite content. However, for castings with FA and AF solidification modes, such as L-316 and the other two castings, the difference in ferrite content was significant. Zhai et al. [43] also found, in their study, that the ferrite content of FA-mode castings is higher than that in AF-mode counterparts, and it is difficult to eliminate through subsequent heat treatment [61].
At the same time, Creq/Nieq also plays an important role in the precipitation behavior of the sigma phase. From the EBSD results, it can be seen that the way in which Creq/Nieq affected the precipitated phases differed from its effect on the ferrite content. The influence of Creq/Nieq on the ferrite content mainly occurred by altering the solidification mode, whereas its effect on the precipitation behavior of the phases was more pronounced. In the M-316 casting, only a small amount of ferrite transformed into the sigma phase, while, in the H-316 casting, almost all the ferrite transformed into the sigma phase. A comparison of the composition of the two castings shows that the main difference was the Ni content, and an increase in Ni content promoted the decomposition of ferrite into the sigma phase. As shown in Figure 5c, the sigma phase in the H-316 casting was surrounded by some secondary austenite phase, indicating that the sigma phase formed via the eutectoid reaction (δ → σ + γ2). In the H-316 casting, the higher Ni content in the matrix promoted the formation of secondary austenite, further enhancing the formation of the sigma phase.

4.4. Effect of Cooling Rate on the Microstructure

The discussion above focused on the influence of Creq/Nieq on the microstructure, but the cooling rate also plays a significant role in the microstructure, in addition to the factor of composition. In the H-316 casting, the cooling rate was 0.5 °C/s, and in the remelting experiments the cooling rate was in the range of 0.1–25.3 °C/s. The ferrite content in the H-316 continuous casting billet was 2.45%, which is higher than that in the furnace-cooled and air-cooled ingots, but its cooling rate was between the two. This might be because the ferrite content is also influenced by the macrostructure and macrosegregation behavior in the continuous casting [29]. Therefore, this study focuses only on the microstructure obtained from the same experimental conditions. The cooling rate and ferrite content of the remelting ingots with different cooling methods are shown in Figure 10. It can be clearly seen that, as the cooling rate increased, the ferrite content generally decreased. The results from Figure 10 show that, at cooling rates of 0.1 °C/s and 3.5 °C/s, the ferrite content was 1.94% and 1.49%, respectively. As the cooling rate decreased, the ferrite content decreased. For the oil-cooled and water-cooled ingots, with cooling rates of 11.5 °C/s and 25.1 °C/s, the ferrite phase was no longer visible in the metallographic structure. Therefore, the cooling rate had a minimal effect on the ferrite content at those rates. The EBSD results also show that, at these cooling rates, the ferrite content was only 0.26% and 0.36%, respectively, both below 1%, which satisfies the nuclear industry requirements for non-magnetic properties. For the 316 stainless steel solidifying in the AF mode, the primary precipitated phase is the austenite phase. As solidification proceeds, the ferrite-forming elements in the remaining liquid phase become concentrated. Ferrite precipitates at the austenite grain boundaries at the end of solidification. The effect of the cooling rate on the formation of ferrite can be roughly divided into two processes: one is the effect of the cooling rate during the solidification stage on ferrite formation, and the other is the effect of the cooling rate after solidification on ferrite transformation. As the cooling rate increases, the diffusion capability of the elements decreases, reducing the degree of ferrite-forming element concentration in the remaining liquid phase and inhibiting the nucleation and growth of ferrite [32]. As the cooling rate increased, the grain size of the four castings decreased. After the solidification process had ended, the reduction in grain size lowered the diffusion distance required for the solid-state phase transformation, thereby facilitating the transformation of ferrite into austenite. Thus, as the cooling rate increased, the ferrite content gradually decreased. The effect of the cooling rate on the microstructure was also quite evident. At 0.1 °C/s, a large portion of ferrite transformed into the sigma phase. At a cooling rate of 3.5 °C/s, only a small amount of ferrite transformed into the sigma phase. At cooling rates of 11.5 °C/s and 25.1 °C/s, a small portion of ferrite also transformed into the sigma phase. In the cooling rate range above 3.5 °C/s, the effect of the cooling rate on the sigma phase precipitation became minimal.

5. Conclusions

  • Three different compositions of 316 stainless steel continuous casting samples are studied: L-316, M-316, and H-316. The primary difference among the three castings is the Ni content, which is 10%, 12%, and 14%, respectively. The ferrite content of the L-316, M-316, and H-316 castings is found to be 11.20%, 2.88%, and 2.45%, respectively. The microstructure of the L-316 casting is mainly composed of the austenitic phase and ferritic phase. The microstructure of the M-316 casting is composed of austenite, ferrite, and a small amount of the sigma phase, with a small amount of ferrite transformed into the sigma phase. The microstructure of the H-316 casting is basically composed of austenite and the sigma phase, and the ferrite is completely transformed into the sigma phase.
  • Based on Creq/Nieq and ferrite morphology, the solidification mode of the 316 stainless steel castings with different composition is determined. It is found that the solidification mode is FA in the L-316 casting and AF in the M-316 and H-316 castings. However, the thermodynamic calculation results indicate that the solidification modes of the three castings are FA, FA, and AF, respectively. It is found that the influence of composition on the ferrite content manifests mainly through the altering of the solidification mode. In the 316 stainless steel solidifying in the FA mode, the ferrite content is much higher than that in the 316 stainless steel solidifying in the AF mode. Moreover, for the 316 stainless steel solidifying in the AF mode, changes in Creq/Nieq have little impact on the ferrite content.
  • The high-Ni-content 316 stainless steel remelting ingots obtained by different cooling methods are studied. It is found that the ferrite content of the ingots obtained under furnace cooling (0.1 °C/s) and air cooling (3.5 °C/s) is 1.94% and 1.49%, respectively. The content of ferrite is less than 1% in both oil-cooled and air-cooled ingots. In the furnace-cooled ingot, most of the ferrite is transformed into the sigma phase. In the other three cooling methods, only a very small amount of the ferrite is transformed into the sigma phase. It is found that the ferrite content of the 316 stainless steel solidified in AF mode decreases with the increase in the cooling rate. In the studied H-316 steel, the ferrite content can be controlled below 1% when the cooling rate is above 11.5 °C/s.
  • Through the above remelting experiments, it is found that the cooling rate has a great influence on the ferrite content and the formation of the sigma phase. With the increase in the cooling rate, the ferrite content in the H-316 ingot decreases. The solidification mode of the H-316 casting is AF and the increase in the cooling rate reduces the element diffusion rate during solidification, reduces the element segregation behavior at the grain boundary, and makes the formation of ferrite at the grain boundary more difficult. At the same time, the increase in the cooling rate also inhibits the transformation of ferrite into the sigma phase by decreasing the diffusion time of the element.

Author Contributions

Conceptualization, L.C., C.C. and K.Y.; methodology, L.C., K.Y. and C.C.; software, W.M., Y.W., Z.Z. and Z.X.; validation, X.B. and C.H.; formal analysis, L.C., Y.L., H.L. and J.T.; investigation, W.M., Z.X., X.B., C.H., J.T. and Y.L.; resources, W.M., J.T. and H.L.; data curation, Y.L. and H.L.; writing—original draft preparation, L.C., Y.W. and Z.Z.; writing—review and editing, W.M., K.Y. and C.C.; visualization, Y.W., Z.Z., Z.X., X.B. and C.H.; supervision, K.Y. and C.C.; project administration, C.C.; funding acquisition, K.Y. and C.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the research project supported by the Applied Fundamental Research Programs of Shanxi Province, grant number 202303021221036; the Shanxi Scholarship Council of China, grant number 2022-040; the “Chunhui Plan” Collaborative Research Project by the Ministry of Education of China, grant number HZKY20220507; and the Undergraduate Innovation and Entrepreneurship Training Program of Shanxi Province, grant number 20230135.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

Those who have participated in the experiments are sincerely acknowledged. They are Tao Liu, Qiji Yan, Qi Zhao, Haozheng Wang, Junxian Wang, and Yunzhe Zhang.

Conflicts of Interest

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.

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Figure 1. Solidification sequence of stainless steel under different Ni/Cr equivalent ratios.
Figure 1. Solidification sequence of stainless steel under different Ni/Cr equivalent ratios.
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Figure 2. Schematic diagram of casting dimensions and sampling. (a) L-316, (b) M-316, (c) H-316.
Figure 2. Schematic diagram of casting dimensions and sampling. (a) L-316, (b) M-316, (c) H-316.
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Figure 3. The ferrite morphologies of L-316, M-316, and H-316 analyzed by optical microscopy. (a) L-316, (b) M-316, (c) H-316.
Figure 3. The ferrite morphologies of L-316, M-316, and H-316 analyzed by optical microscopy. (a) L-316, (b) M-316, (c) H-316.
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Figure 4. Ferrite morphology of L-316, M-316, and H-316 at high magnification observed by SEM. (a) L-316 casting, (b) M-316 casting, (c) H-316 casting.
Figure 4. Ferrite morphology of L-316, M-316, and H-316 at high magnification observed by SEM. (a) L-316 casting, (b) M-316 casting, (c) H-316 casting.
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Figure 5. EBSD analysis of the phase constitution of the three castings. (a) L-316 casting, (b) M-316 casting, (c) H-316 casting.
Figure 5. EBSD analysis of the phase constitution of the three castings. (a) L-316 casting, (b) M-316 casting, (c) H-316 casting.
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Figure 6. The ferrite morphologies of the H-316 remelting sample observed by optical microscopy. (a,b) furnace-cooled ingot, (c,d) air-cooled ingot, (e,f) oil-cooled ingot, and (g,h) water-cooled ingot.
Figure 6. The ferrite morphologies of the H-316 remelting sample observed by optical microscopy. (a,b) furnace-cooled ingot, (c,d) air-cooled ingot, (e,f) oil-cooled ingot, and (g,h) water-cooled ingot.
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Figure 7. EBSD analysis of the phase constitution of the H-316 remelting samples. (a) Furnace-cooled ingot, (b) air-cooled ingot, (c) oil-cooled ingot, (d) water-cooled ingot.
Figure 7. EBSD analysis of the phase constitution of the H-316 remelting samples. (a) Furnace-cooled ingot, (b) air-cooled ingot, (c) oil-cooled ingot, (d) water-cooled ingot.
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Figure 8. Equilibrium solidification process of the castings by Thermo-Calc calculation. (a) L-316 casting, (b) M-316 casting, (c) H-316 casting.
Figure 8. Equilibrium solidification process of the castings by Thermo-Calc calculation. (a) L-316 casting, (b) M-316 casting, (c) H-316 casting.
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Figure 9. Creq/Nieq, ferrite content, and solidification modes of the 316 castings with different compositions.
Figure 9. Creq/Nieq, ferrite content, and solidification modes of the 316 castings with different compositions.
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Figure 10. Cooling rate and ferrite content of the 316 remelting ingots with different cooling methods.
Figure 10. Cooling rate and ferrite content of the 316 remelting ingots with different cooling methods.
Crystals 15 00168 g010
Table 1. Chemical composition of the AISI 316, L-316, M-316, and H-316 stainless steel (mass fraction %).
Table 1. Chemical composition of the AISI 316, L-316, M-316, and H-316 stainless steel (mass fraction %).
CSiMnPSCrNiMoN
AISI 316≤0.080≤1.00≤2.00≤0.045≤0.03016.00–18.0010.00–14.002.00–3.00≤0.100
L-3160.0220.521.140.0350.00116.5810.002.010.043
M-3160.0490.371.610.0150.00117.5012.172.620.062
H-3160.0590.581.200.0270.01117.1314.252.730.046
Table 2. The cooling rate, ferrite content, and phase constitution of the 316 stainless steel continuous castings and remelting ingots.
Table 2. The cooling rate, ferrite content, and phase constitution of the 316 stainless steel continuous castings and remelting ingots.
SampleCooling Rate (°C/s)Ferrite Content Average (%)Ferrite Content Ranges (%)Phase Constitution
Continuous
Casting		L-3160.511.210.2–13.2δ + γ
M-3160.42.92.6–3.3δ + γ + σ (small amount)
H-3160.52.52.2–2.7δ (small amount) + γ + σ
H-316
Remelting
Ingot		Furnace-Cooling0.11.91.5–2.1δ + γ + σ
Air-cooling3.51.51.3–1.8δ + γ + σ (small amount)
Oil-cooling11.50.3-δ + γ + σ (small amount)
Water-cooling25.10.4-δ + γ + σ (small amount)
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Chen, L.; Wang, Y.; Li, Y.; Zhang, Z.; Xue, Z.; Ban, X.; Hu, C.; Li, H.; Tian, J.; Mu, W.; et al. Effect of Nickel Content and Cooling Rate on the Microstructure of as Cast 316 Stainless Steels. Crystals 2025, 15, 168. https://doi.org/10.3390/cryst15020168

AMA Style

Chen L, Wang Y, Li Y, Zhang Z, Xue Z, Ban X, Hu C, Li H, Tian J, Mu W, et al. Effect of Nickel Content and Cooling Rate on the Microstructure of as Cast 316 Stainless Steels. Crystals. 2025; 15(2):168. https://doi.org/10.3390/cryst15020168

Chicago/Turabian Style

Chen, Lei, Yang Wang, Yafeng Li, Zhengrui Zhang, Zhixuan Xue, Xinyu Ban, Chaohui Hu, Haixiao Li, Jun Tian, Wangzhong Mu, and et al. 2025. "Effect of Nickel Content and Cooling Rate on the Microstructure of as Cast 316 Stainless Steels" Crystals 15, no. 2: 168. https://doi.org/10.3390/cryst15020168

APA Style

Chen, L., Wang, Y., Li, Y., Zhang, Z., Xue, Z., Ban, X., Hu, C., Li, H., Tian, J., Mu, W., Yang, K., & Chen, C. (2025). Effect of Nickel Content and Cooling Rate on the Microstructure of as Cast 316 Stainless Steels. Crystals, 15(2), 168. https://doi.org/10.3390/cryst15020168

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