Impact of Solution Treatment on Microstructure and Mechanical Properties of S30432 Stainless Steel Pipe

Abstract: The effects of varying solution treatment temperatures and holding times on the microstructure and mechanical properties of S30432 stainless steel pipe were investigated using metallographic analysis, hardness testing, and room-temperature tensile testing. The test results indicate that the mechanical properties of S30432 steel pipe decline with increasing solution temperature, with the optimal properties observed at 1120°C. When the solution temperature exceeds 1160°C, the austenite grains grow significantly, and the mechanical properties decline with increasing solution time. Considering grain size, hardness, and mechanical properties, the solution temperature for industrial production of S30432 steel pipe should be controlled between 1100°C and 1160°C, with a solution time not exceeding 20 minutes. To reduce pollutant emissions, lower coal consumption, and improve thermal efficiency, ultra-supercritical (USC) power generation units have been widely adopted. The continuous increase in steam parameters in thermal power units has placed higher demands on the overall performance of materials, particularly for boiler superheater and reheater components. In coal-fired ultra-supercritical boilers, the superheater and reheater are critical components responsible for recovering energy from flue gas, heating steam, and enabling efficient energy conversion. These components operate under the highest pressure, the highest temperature, and the most demanding service conditions within the boiler. S30432 (10Cr18Ni9NbCu3BN) is a new type of austenitic stainless steel developed from the chemical composition of TP304H, with the addition of Cu, Nb, and N, and adjustments to the contents of Mo, V, and other elements. Thanks to its excellent high-temperature mechanical properties, as well as its good corrosion and oxidation resistance, this steel is widely used in the superheater and reheater components of coal-fired power units. The copper-rich phase in S30432 steel provides excellent strengthening effects in combination with other precipitates such as Nb(C,N), NbCrN, and M₂₃Co^₆.

With the rapid advancement of ultra-supercritical thermal power unit technology in China, the localization of S30432 stainless steel pipes has been successfully achieved. In domestic industrial production, the grain size, mechanical properties, and corrosion resistance of S30432 steel pipes have become focal points for many researchers. The performance of S30432 steel pipes depends largely on the final solution treatment. Zhengbin Zhong et al. studied the mechanical properties of S30432 austenitic stainless steel at various solution temperatures and found that the optimal solution temperature is 1120°C, which should not exceed 1160°C. Hansheng Bao et al. studied the performance of S30432 steel pipes and found that maintaining a solution temperature of at least 1100°C and a carbon content no higher than 0.083% can prevent the occurrence of intergranular corrosion. Shuping Tan found that when the solution temperature exceeds 1150°C, the room temperature strength of S30432 steel decreases significantly. Guangquan Si et al. studied the influence of grain morphology on the microstructure and properties of S30432 stainless steel pipes and found that elongated grains increase room temperature tensile strength and hardness, but also accelerate microstructural aging, thereby reducing high-temperature durability. This paper investigates the effects of different solution treatments on domestically produced S30432 steel pipes. It explores how varying solution temperatures and times influence the microstructure and mechanical properties of S30432 steel pipes, aiming to optimize the solution treatment process for industrial production and provide technical support to advance domestic manufacturing.

1. Sample Preparation and Test Methods

Industrial-grade S30432 austenitic stainless steel pipes were selected, with dimensions of 57 mm × 5 mm in the cold-rolled condition. The main production process includes tube billet heating, hot perforation, high-temperature softening, cold rolling, and solution treatment. Its chemical composition was measured using a SparkMAXx09A direct reading spectrometer. The results, shown in Table 1, meet the relevant requirements of ASME SA 213/SA 213M-2023, “Seamless Ferritic and Austenitic Alloy Steel Pipes for Boilers, Superheaters, and Heat Exchangers.”ASME SA 213/SA 213M-2023 specifies that the minimum solution treatment temperature for S30432 steel pipe shall not be less than 1100°C. The solution treatment temperatures selected for the test were 1100℃, 1120℃, 1140℃, 116℃, and 1180℃, with solution times of 5, 10, 15, 20, and 25 minutes, respectively, followed by water cooling.

Table 1 Chemical composition of S30432 steel pipes

Metallurgical samples were cut along the pipe’s longitudinal direction, sequentially polished with sandpapers ranging from No. 240 to No. 1500, mechanically polished, and then electrolytically etched using nitric acid. The microstructure was examined using an Olympus GX53 inverted metallographic microscope, and the grain size was determined according to the three-circle intercept method specified in GB/T 6394-2017, “Method for Determination of Average Grain Size of Metals.”The Brinell hardness test was conducted on full-wall ring specimens in accordance with GB/T 231.1-2018, “Metallic Materials Brinell Hardness Test Part 1: Test Method.” The test instrument used was a Huayin 400HBS-3000A Brinell hardness tester, equipped with a 2.5 mm diameter indenter, applying a test force of 1,838.75 N for a holding time of 12 seconds. Measurements were taken at three positions in different directions, with three points tested in total; the average value was recorded as the final result. Two full-wall arc tensile specimens were cut along the longitudinal direction of the pipe and subjected to room temperature tensile testing in accordance with GB/T 228.1-2021, “Metallic Materials — Tensile Test — Part 1: Room Temperature Test Method.” The tensile test was performed using a Sansi UTM 5305X universal testing machine, and the average value was recorded as the final result. The dimensions of the tensile specimens are shown in Figure 1.

Figure 1 Dimensions of Tensile Test Specimens

2. Test Results and Analysis

2.1 Effect of Solution Treatment on Microstructure

The microstructure of S30432 steel pipe after solution treatment at 1100℃, 1120℃, 1140℃, 1160℃, and 1180℃ for 5 minutes is shown in Figure 2. As shown in Figure 2, the microstructure of the S30432 steel pipe after solution treatment consists of a single-phase austenitic structure. The recrystallization process is complete, the austenite grains are equiaxed, and a small amount of secondary phase is present. The grain size of the austenitic structure was evaluated using the three-circle intercept method; at a solution temperature of 1100°C, the average grain size is 9 (Figure 2a). As the solution temperature of the S30432 steel pipe rises to 1140°C, the average grain size decreases to 8.5 (Figure 2c); when the temperature further increases to 1160°C, the average grain size decreases to 8, but then increases again at higher temperatures (Figure 2d).

When the solution temperature rises to 1180°C, the average grain size reaches 7, and the austenite grains grow significantly (Figure 2e). The internal energy of the S30432 steel pipe increases after cold rolling deformation, driving a recovery process from an unstable high free energy state toward a more stable low free energy state. As the solution temperature increases, the austenite grains undergo recovery, recrystallization, and growth. Recrystallization is complete at solution temperatures of 1100°C and 1160°C. When the solution temperature exceeds 1160°C, the grains of the S30432 steel pipe grow significantly. This behavior occurs because, in steel containing Nb, grain growth is slow when the austenitizing temperature is below the critical temperature but accelerates rapidly once the temperature exceeds this critical threshold. The microstructure of the S30432 steel pipe after solution treatment at 1120°C for various holding times is shown in Figure 3. It can be observed that, following solution treatment, the deformed grains from the cold-rolled state undergo recrystallization nucleation. When the solution time is 5 minutes, the recrystallized grains are finer, with an average grain size of 8.5, as shown in Figure 3(a). As the solution time is extended to 15 minutes, the grain size remains largely unchanged, with an average grain size of 8.5, as shown in Figures 3(b) and 3(c). When the solution time is extended to 25 minutes, the grain size gradually increases, with an average grain size of 8, as shown in Figure 3(e). As observed above, the grain size grade of S30432 steel pipe tends to decrease with increasing solution time. As the solution time extends, the recrystallization nucleus interface migrates toward the surrounding deformed areas, driven by the strain energy difference between the undistorted new grains and the surrounding distorted old grains.

Figure 2. Microstructure of S30432 steel pipe after solution treatment at different temperatures for 5 minutes

Figure 3. Microstructure of S30432 steel pipe after solution treatment at 1120°C for various holding times

2.2 Effect of Solution Treatment on Mechanical Properties

The hardness of S30432 steel pipe after solution treatment at different temperatures and durations was measured, and the variation is shown in Figure 4. As shown in Figure 4, at solution temperatures ranging from 1100°C to 1180°C, the hardness of the S30432 steel pipe decreases with increasing solution time. After the solution time exceeds 20 minutes, the hardness decreases slightly and then stabilizes. This gradual stabilization indicates that recrystallization in the matrix is complete, resulting in the formation of undistorted equiaxed grains.

During the solution treatment of the S30432 steel pipe, the Nb(C,N) phase disperses and precipitates, pinning the grain boundaries and providing precipitation strengthening. This effect increases the hardness of the steel pipe, counteracting the softening caused by grain growth and causing the hardness to stabilize near a steady value. When the solution temperature exceeds 1140°C, the hardness of the S30432 steel pipe decreases significantly. At a constant solution time, the hardness also declines as the solution temperature increases. ASME SA 213/SA 213M-2023 specifies that the hardness of S30432 steel pipe must not exceed 219 HBW, the solution temperature must be at least 1100°C, and the solution time must be no less than 5 minutes. Under these conditions, the hardness of S30432 steel pipe meets the standard requirements. However, GB/T 5310-2023, “Seamless Steel Pipe for High-Pressure Boilers,” specifies a minimum hardness of 150 HBW. Therefore, when the solution temperature exceeds 1160°C, there is a risk that the hardness may fall below the required standard.

Figure 4. Relationship Between Hardness of S30432 Steel Pipe and Solution Temperature and Holding Time

The relationship between the mechanical properties of S30432 steel pipe and solution temperature and holding time is shown in Figure 5. As observed from the figure, both tensile strength and yield strength generally decrease with increasing solution time. As shown in Figure 5(a), at a solution temperature of 1100°C, tensile strength and yield strength exhibit only slight changes as the solution time increases. When the solution time exceeds 20 minutes, the tensile strength shows a downward trend, decreasing slightly from 665 MPa to 659 MPa, while the yield strength decreases marginally from 364 MPa to 361 MPa. As shown in Figure 5(b), at a solution temperature of 1120°C, the tensile strength decreases from 690 MPa to 661 MPa, and the yield strength decreases from 388 MPa to 363 MPa as the solution time increases. At a solution temperature of 1140°C, both tensile strength and yield strength decrease compared to those at 1120°C. These properties decrease further at 1160°C, as shown in Figures 5(c) and 5(d). When the solution temperature is between 1100°C and 1160°C, the room temperature mechanical properties of S30432 steel pipe meet the requirements of ASME SA 213/SA 213M-2023, which specify a tensile strength of at least 590 MPa, yield strength of at least 235 MPa, and elongation (A) of at least 35%. Based on the microstructure, hardness, and room temperature tensile test results of S30432 steel pipe, the solution temperature should be controlled between 1100°C and 1160°C, and the solution time should not exceed 25 minutes.

Figure 5. Relationship Between Mechanical Properties of S30432 Steel Pipe and Solution Temperature and Holding Time

The relationship between strength and grain size can be expressed by the Hall-Petch equation (Equation 1):

Here, a represents the yield strength, c is a constant independent of grain size, h is a material constant, and ddd denotes the grain size. The smaller the grain size, the higher the strength. As the solution temperature increases and the solution time is extended, the grain size of S30432 steel pipe grows, leading to a decrease in room temperature strength.

3. Conclusion

• As the solution temperature increases and the holding time is extended, the grain size of S30432 steel pipe exhibits a tendency to grow. Grain growth becomes significantly more pronounced when the temperature exceeds 1160°C.
• With increasing solution temperature and holding time, the mechanical properties of S30432 steel pipe show an overall decreasing trend, with the optimum properties observed at 1120°C.
• During the industrial production of S30432 steel pipes, the solution temperature should be maintained between 1100°C and 1160°C, and the holding time should not exceed 25 minutes.