Microstructure and Properties of Incoloy 825 Alloy after Solid Solution at Different Temperatures

The microstructure, mechanical properties and corrosion resistance of Incoloy 825 alloy after solution treatment at different temperatures were studied by scanning electron microscope, transmission electron microscopy, Brinell hardness tester and tensile testing machine. The results show that when the solution temperature is between 980-1050 ℃, the grain size of the alloy does not change significantly. When the solution temperature is above 1050 ℃, the grain size increases at a faster rate; As the solution temperature increases, the hardness and tensile strength of the alloy gradually decrease, while the elongation continuously increases; The grain boundary precipitates are mainly composed of M23C6 carbides rich in chromium and molybdenum, as well as intermetallic compounds containing chromium, nickel, iron, and molybdenum. The number of grain boundary precipitates increases first and then decreases with the increase of solution temperature. When the solution temperature is 1015 ℃, the grain boundary precipitates are the most, while the alloy has the worst intergranular corrosion resistance.

0. Introduction

Nickel iron chromium alloy has good resistance to stress corrosion cracking, Crevice corrosion and pitting corrosion, oxidation resistance and reducing hot acid performance, and is mainly used in pipeline systems in offshore engineering, heat exchangers in petroleum processing, heating pipes in pickling equipment, etc. Incoloy 825 alloy is an austenitic nickel iron chromium alloy that has undergone titanium stabilization treatment. It has high hardness and strength in high-temperature environments. By adding alloy elements such as molybdenum and copper, it can be used in more complex and harsh corrosion environments that stainless steel cannot withstand. When the alloy composition is constant, the corrosion resistance and mechanical properties of the alloy mainly depend on its microstructure, composition and distribution of precipitates, etc. Different heat treatment processes will have a significant impact on the microstructure, grain size, composition, distribution, quantity and size of precipitates, etc. Therefore, it is necessary to study the heat treatment process of the alloy. Usually, the hot rolling temperature of Incoloy 825 alloy is 900-1150 ℃, and the cooling method is water cooling or rapid air cooling. In order to achieve good resistance to pitting corrosion and intergranular corrosion cracking in the alloy, solid solution treatment needs to be carried out between 1150 and 1250 ℃. At present, research on Incoloy 825 alloy mainly focuses on the analysis of intergranular corrosion causes, precipitation phase analysis, and other aspects. However, there is less in-depth research on the changes in mechanical properties and corrosion resistance after solution treatment at different temperatures, as well as the relationship between precipitation phase, microstructure, and corrosion resistance. Therefore, the author conducted a study on the microstructure, precipitates, corrosion resistance, and mechanical properties of Incoloy 825 after solution treatment at different temperatures, which is of great significance for optimizing heat treatment processes and studying the strengthening mechanism of alloy elements.

1. Sample preparation and testing methods

1.1 Sample Preparation

The material used in the experiment is a self-made Incoloy 825 alloy, which is melted in a vacuum induction furnace. The billet thickness is 120mm, the rolling temperature is 1100-1180 ℃, and the final rolling temperature is not less than 900 ℃. It is hot-rolled into a 12mm thick plate, and then hot pickled to remove the surface oxide scale. Its chemical composition is shown in Table 1. Take 5 sets of samples for solid solution treatment, with a sample size of 100mm × 12mm × 12mm, with solid solution temperatures of 980 ℃,1015 ℃,1050 ℃,1100 ℃,1200 ℃, and water quenching after 20 minutes of insulation.
Table.1 Chemical Composition (Mass Fraction)% of Incoloy 825 Alloy

C	Si	Mn	p	S	Ni	Cr	Mo	Ti	Al	Fe	Cu
0.023	0.064	0.4	0.002	0.0038	42.8	20.8	2.99	0.98	0.1	29.43	2.15

1.2 Test methods

According to GB/T 6394-2002 “Method for Evaluating the Average Grain Size of Metals”, the grain size of the heat treated specimens was rated using a Zeiss Axio Imager Z1m optical microscope; The microstructure and distribution of precipitates of the sample were observed by ZEISS EVO-18 scanning electron microscope. The corrosive agent was Copper(II) sulfate hydrochloric acid aqueous solution (2g Copper(II) sulfate, 10mL hydrochloric acid, 10mL distilled water); Adopting Zeiss Σ The inclusion analysis system INCA Feature equipped on the IGMA type field emission scanning electron microscope statistically analyzes the area fraction of precipitates on the surface of the sample; The precipitates in the sample were separated from the matrix using carbon extraction replica method, and the morphology, structure, and composition of the precipitates were characterized using JEM-2100F field emission transmission electron microscopy and the Oxford instrument INCA Energy 350 energy spectrometer.
According to GB/T 228.1-2010 and GB/T 231.1-2009, room temperature tensile test and Brinell scale are conducted on Model 5585H Tensile testing machine and Instron CLB3 Brinell hardness tester. The tensile sample is a round bar with a diameter of φ 8mm, and the average of three data is taken as the test result; Prepare 600mL boiling sulfuric acid Iron(III) sulfate solution with mass fraction of 50% according to method A in ASTM G28-2015, weigh the samples after solution treatment at different temperatures respectively, immerse them in boiling solution, keep them for 120h, wash, dry, weigh, and calculate corrosion rate.

2. Test results and discussion

2.1 Effect of solid solution temperature on microstructure

From Figure 1, it can be seen that Incoloy 825 alloy has a single austenitic structure; After solution treatment at 9801015105011001200 ℃, the grain size levels are 9.5, 9.5, 9.0, 3.5, and 1.5, respectively. This indicates that the change in grain size is not significant when the solid solution temperature is between 980 ℃ and 1050 ℃. When the solid solution temperature is 1100 ℃, the austenite grains grow rapidly, and when the solid solution temperature is 1200 ℃, the grains continue to grow. This is because as the solid solution temperature increases, the dislocation density decreases, the migration rate of grain boundaries accelerates, and the growth rate of grains also accelerates.

Figure.1 Microstructure of Incoloy 825 alloy after solid solution at different temperatures

2.2 Effect of Solid Solution Temperature on Mechanical Properties

From Figure 2 (a), it can be seen that there is a good correspondence between the Brinell hardness of the alloy and the average grain size. As the solution temperature increases, the Brinell hardness decreases; When the solid solution temperature is between 980 and 1050 ℃, the decrease in hardness value is relatively small. When the solid solution temperature is above 1050 ℃, the Brinell hardness sharply decreases, which is consistent with the changes in average grain size and microstructure. From Figure 2 (b), it can be seen that as the solid solution temperature increases, the elongation after fracture gradually increases, and the trend of tensile strength is basically consistent with the changes in grain size and Brinell hardness. This indicates that there is a certain correlation between the hardness, strength, and plasticity of the alloy and the grain size of the alloy. When the solid solution temperature exceeds 1050 ℃, the austenite grains grow rapidly, the number of grain boundaries decreases sharply, and the bonding force between grains weakens, resulting in a rapid decrease in tensile strength. However, at this time, the solid solution temperature is higher, the number of dislocations inside the grains is lower, and the internal stress is smaller, so the elongation after fracture, which reflects plasticity, increases rapidly.

Figure.2 Mechanical properties and average grain size of Incoloy 825 alloy after solid solution at different temperatures

2.3 Effect of Solid Solution Temperature on Corrosion Resistance

From Figure 3 (a), it can be seen that when the solid solution temperature is between 980 and 1200 ℃, the corrosion rate and precipitation phase content (area fraction) of the alloy show a trend of first increasing and then decreasing with the increase of the solid solution temperature, and the change pattern of the two is basically consistent. From Figures 3 (b) and (c), it can be seen that when the solid solution temperature ranges from 980 to 1015 ℃, a large number of grain boundary precipitates appear in the microstructure, and the grain size of the alloy is basically the same. This indicates that the pinning effect of precipitation relative to grain boundaries inhibits grain growth, and the appearance of a large number of precipitates in this temperature range leads to an increase in intergranular corrosion rate. When the solid solution temperature increases to 1050 ℃, the grains grow slightly, as shown in Figure3 (d). This is mainly due to the dissolution of precipitates on the grain boundaries, which reduces the number of precipitates and weakens their pinning effect on the grain boundaries. When the solid solution temperature is 1100 ℃, the grain size significantly increases, as shown in Figure 3 (e). There are no obvious precipitates at the grain boundaries, and the corresponding intergranular corrosion rate decreases. When the solid solution temperature is 1200 ℃, as shown in Figure 3 (f), compared with the solid solution temperature of 1100 ℃, the grain boundary precipitates are basically the same, and the grain size increases, but there is no significant change in the intergranular corrosion rate. This indicates that the grain size has no significant effect on the intergranular corrosion rate.

Fig.3 Precipitated phase content, intergranular corrosion rate and Secondary electrons image of Incoloy 825 alloy after solution at different temperatures

2.4 Micromorphology of Precipitated Phase

From Figure 4, it can be seen that the precipitates of the experimental alloy after solution treatment at 980 ℃ are mainly M23C6 carbides with a face centered cubic structure, M is mainly composed of chromium and molybdenum elements, as well as intermetallic compounds rich in chromium, nickel, iron, molybdenum, etc; Both of these precipitates are rich in chromium, and the precipitates are distributed along grain boundaries. Due to the lower diffusion rate of chromium towards the grain boundary than its aggregation rate towards the grain, a chromium deficient zone is formed around the grain boundary, which is prone to corrosion. This can well explain the phenomenon of increasing precipitation phases in Figure 3 (a) while corresponding to an increase in grain boundary corrosion rate.

Figure.4 TEM bright field images and elemental surface distribution of two precipitates in Incoloy 825 alloy after solution treatment at 980 ℃

3. Conclusion

• (1) As the solid solution temperature increases, the grain size of Incoloy 825 alloy shows an increasing trend. When the temperature is between 980 ℃ and 1050 ℃, the increase in grain size is not significant. When the temperature is above 1050 ℃, the grain size increases at a faster rate; The hardness and tensile strength of Incoloy 825 alloy gradually decrease, while the elongation continuously increases.
• (2) After solid solution treatment, the precipitates at grain boundaries of Incoloy 825 alloy are mainly composed of carbides rich in chromium and molybdenum, as well as intermetallic compounds containing chromium, nickel, iron, and molybdenum.
• (3) As the solid solution temperature increases, the number of grain boundary precipitates in Incoloy 825 alloy first increases and then decreases; The intergranular corrosion rate shows a trend of first increasing and then decreasing. When the solid solution temperature is 1015 ℃, the precipitation phase at the grain boundary is the most, and the corrosion rate is the highest.

Author: Hong Huimin