The effect of heat treatment temperature on the organization and properties of cold deformed Inconel 625 alloy pipe

The effects of heat treatment temperature on the organization, mechanical properties and corrosion properties of Inconel 625 alloy pipes were studied through metallographic analysis, tensile test, hardness test and nitric acid corrosion test.

Inconel 625 alloy is a multi-element alloy with a face-centered cubic structure, which shows good corrosion resistance in many media and mixed media. Also, it has excellent resistance to pitting, crevice corrosion, intergranular corrosion and stress corrosion cracking in chloride media. The material also has good processability and weldability and is widely used in many fields, such as aerospace, marine, chemical, petrochemical and nuclear industries. At present, Inconel 625 alloy has done much research; Liu Shuang et al. studied the organization of Inconel 625 alloy ingot and its organizational transformation during the heating process; the results show that the segregation in the material at 1185 ℃ is less, the organization is more uniform. Liu et al. probed the solid solution temperature of 1050 ℃ and above, Inconel 625 alloy sheet grain with the increase in temperature, and the holding time had a significant effect on the grain. The rapid growth of the holding time on the grain size is not obvious. Zhao Di’s research shows that the carbon content and heat treatment temperature on the quality of Inconel 625 alloy extrusion billet have a greater impact.
Although scholars have done much research on Inconel 625 alloy, only some people systematically study the heat treatment temperature on the cold deformation of Inconel 625 alloy organization, mechanical properties and resistance to intergranular corrosion properties. Because the corrosion resistance and mechanical properties of Inconel 625 alloy pipes are relatively good, they are commonly used as high-pressure heat exchanger materials in marine oil refineries. And about the influence of heat treatment temperature on the mechanical properties and intergranular corrosion resistance of the pipes in sensitive parts needs to be studied in depth. Therefore, this paper takes cold rolled state Inconel 625 alloy as the object to investigate the effect of heat treatment temperature on the organization, mechanical properties and intergranular corrosion resistance of steel pipe.

1. Test materials and methods

The test material is Inconel 625 alloy cold rolled pipe, specification 89 mm × 5.5 mm (deformation of 52.13%); its chemical composition is shown in Table 1. 150 mm × 25 mm × 5.5 mm, 40 mm × 25 mm × 5.5mm and 20mm × 10mm × 5.5mm rectangular specimens are cut along the direction of rolling Several. Heat treatment was carried out in a muffle furnace at temperatures of 960, 1000, 1040, 1080, 1120, and 1160°C, with a holding time of 15 min and cooling by water cooling. After heat treatment, an acid wash solution of hydrofluoric acid and nitric acid with a volume ratio of 1:6 was used to remove the oxide skin on the surface of the specimens. Then, the specimen was abraded and ultrasonically cleaned for 10 minutes to remove impurities and polishing solution. Surface corrosion with a mixture of hydrochloric acid and alcohol solution for 1-3 min, using an optical microscope (OM) to observe the organization of the specimen at different heat treatment temperatures and precipitation phases and analyze the effect of different temperatures on the organization and precipitation phases. With the TH150 Rockwell hardness tester for each sample hardness treatment, each sample hit 5 points, taking the average value, to study the effect of different heat treatment temperatures on the hardness of the material law. The tensile test was carried out on a ZWICK room temperature tensile testing machine to obtain tensile strength and elongation and to investigate the relationship between different heat treatment temperatures and the tensile properties of the material. Intergranular corrosion selected ASTM A262C method for the test; the corrosion medium is 65% nitric acid solution, the test temperature is 80 ℃, the test cycle is 5 cycles (a total of 120h), take the average value to analyze the effect of different heat treatment temperature on the corrosion rate of the specimen. Comprehensive results of the above tests, the cold deformation of Inconel 625 alloy pipe optimal heat treatment temperature.

2. Test results and discussion

2.1 Organization analysis

Figure 1 shows the microstructure of Inconel 625 alloy at different temperatures. 960 ℃ insulation after 15 min, the organization is basically fine isometric crystals, the average size of the grain size of 43.1μm; there are many fine precipitation phases in the matrix, see Figure 1(a). As the heat treatment temperature increases, the grains engulf each other, the grain boundaries begin to move, and the grains begin to grow; the number of precipitated phases within the matrix gradually decreases with the increase in temperature, but the size of some of the precipitated phases tends to grow. The temperature is 1040 ℃ and the precipitated phases have significant growth, see Figure 1(c). When the temperature rises to 1080 ℃, the average size of the grain is 69.1 μm, and the size of the precipitated phase is also significantly reduced, see Figure 1 (d); when the temperature is higher than 1080 ℃, the grain grows rapidly, 1120 ℃ average size of the grain is 95.2 μm, 1160 ℃ average size of the grain is 107.5 μm. The growth rate of grain growth rate appeared after the first slow and fast due to the cold deformation of the alloy base and the existence of a large number of residual energies in the body of the residual energy of the alloy. There are a large number of residual energy and dislocations in the alloy matrix after cold deformation. When the matrix is heated up, part of the residual energy is released, and recrystallization is easy to occur where dislocations exist. Because there are precipitation phases in the matrix and pinned grain boundaries, grain growth is limited, so grain growth is not obvious at the beginning of heat treatment. When the temperature continues to rise, on the one hand, the residual energy within the matrix can be completely released; the atomic activity increases and the diffusion phenomenon is more intense; on the other hand, the precipitation phase within the matrix is also dissolved, the pinning effect on the grain boundary is reduced, these two factors together lead to a high-temperature section of the grain grows rapidly.

Figure.1 Microstructure of Inconel 625 alloy under different heating temperatures
(a) 960 ℃; (b) 1000 ℃; (c) 1040 ℃; (d) 1080 ℃; (e) 1120 ℃; (f) 1160 ℃
Table.1 Chemical composition of Inconel 625 alloy (mass fraction, %)

C	P	S	Cr	Mo	Ti	Nb	Fe	Ni
0.024	0.0088	0.0042	21.79	8.34	0.126	3.18	3.47	Bal.

Since the temperature has a great influence on the grain size of Inconel 625 alloy, the grain growth process can be described by the Arrhenius formula, which is.

D² – D₀² = A.exp(-Q/RT) (1)

In the formula.

• D is the average grain size at a certain temperature, μm;
• D₀ is the original grain size, μm;
• A is a pre-exponential factor;
• Q is the activation energy of grain growth, kJ/mol;
• R is the gas constant, R = 8.314 J/(mol.K);
• T is the heat treatment temperature, K.

Since the initial grain is very small, D₀²≤D², the equation becomes.

InD =InA’ – Q/2RT (2)

The inverse of temperature 10000/T and the logarithm of grain size lnD are plotted and linearly fitted. The results of the fit between the inverse of different temperatures and the logarithm of grain size for the heat preservation of 15 min are shown in Fig. 2. As can be seen from Fig. 2, R_b²=0.9542, are close to 1 fitting degree is high. The fitting equation for the B curve in Fig. 2 is.

InD =10.72 – 8670/T (3)

According to Eq.(2) and Eq.(3), the activation energy of grain growth of Inconel 625 alloy at 960-1160 °C for 15 min is calculated to be Q = 144.14 kJ/mol. With the increase of temperature, on the one hand, the atoms diffuse violently and the grains grow rapidly ; on the other hand, some undissolved precipitated phases are basically dissolved back into the matrix, which reduces the resistance of atomic diffusion, weakens the pinning effect on grain boundaries, and accelerates grain growth.

Figure.2 Inconel 625 alloy ln D-10000/T relationship curve

2.2 Mechanical properties analysis

As shown in Figure 3 (a), as the temperature increases, the tensile strength of Inconel 625 alloy gradually decreases, and the rate of decrease increases. The heat treatment temperature is between 960-1080 ℃, and the tensile strength of the alloy decreases from 803MPa to 792MPa; The heating temperature is 1080-1160 ℃, and its tensile strength decreases from 792MPa to 780MPa. Cold-deformed Inconel 625 alloy contains a large number of dislocations and distortion energy, which forms smaller-sized new equiaxed crystals after heating. These newly formed equiaxed crystals are prone to nucleation and growth in defective positions, which is completed by consuming distortion energy and dislocations in the matrix. At the beginning of heat treatment, the grain size formed is relatively small, and the strength of the alloy decreases slowly. With the continuous increase of heat treatment temperature, newer equiaxed crystals will be formed, and the previously formed new equiaxed crystals will grow. This process will consume a large amount of distortion energy and dislocations, reduce dislocation density, increase grain size, and significantly reduce the tensile strength of the alloy. From Figure 3 (b), it can be seen that as the temperature increases, the hardness of the alloy gradually decreases, and the elongation increases. The two curves intersect at 1080 ℃. This is due to the high content of dislocations and the pinning effect of precipitates in Inconel 625 alloy after cold rolling. During the heat treatment process, on the one hand, dislocations begin to move, distortion can be released, and the alloy completes recrystallization and growth, weakening the fine-grain strengthening effect. On the other hand, the solubility of alloy elements in the matrix increases, atoms diffuse violently, and a large amount of precipitates dissolve, resulting in a decrease in hardness and an increase in the plasticity of the alloy. When the heat treatment temperature is between 960-1080 ℃, the hardness of the alloy decreases with the increase of the heat treatment temperature, from 178HBW to 173.5HBW. The elongation of the alloy shows a gradual upward trend, reaching 64.3% at 960 ℃ and 67.4% at 1080 ℃. When the temperature is between 1040 and 1080 ℃, the hardness value and elongation remain basically unchanged, indicating that the mechanical properties of the alloy are relatively stable at this time. When the heat treatment temperature continues to increase, the hardness of the alloy decreases significantly, with a hardness value of 169HBW at 1160 ℃, but the elongation of the alloy continues to increase.

Figure.3 Effect of Heat Treatment Temperature on Mechanical Properties of Inconel 625 Alloy
Plot the average grain size of Inconel 625 alloy at different heat treatment temperatures with tensile strength and hardness values, and perform linear fitting to obtain the relationship between the steel tensile strength, hardness, and the reciprocal square root of the average grain size, as shown in Figure 4. This linear relationship basically conforms to the Hall Petch relationship. The effect of heat treatment temperature on the microstructure and mechanical properties of steel is that as the heat treatment temperature increases, the strength and hardness of the steel decrease, while the grain size and plasticity increase. Based on this experiment, it can be seen that the Inconel 625 alloy after cold deformation has a large amount of precipitates that dissolve back at a temperature of 1080 ℃. The grain size grows rapidly, and the tensile strength and hardness decrease, while the elongation increases. Moreover, at a temperature of around 1080 ℃, there is a intersection point between the hardness value and elongation. At this time, the comprehensive mechanical properties of the steel are good.

2.3 Fracture morphology analysis

Figure 5 shows the tensile fracture morphology of Inconel 625 alloy at 960-1160 ℃ heat treatment temperature. From Figure 5(a), it can be seen that with the heat treatment temperature of 960 ℃, the alloy fracture morphology consists of more deconstructed surfaces and a small number of small tough nests. This is due to the existence of a large number of dislocations, precipitation phases and distortion energy in the alloy after cold deformation; the distortion energy in the matrix is not completely released at the beginning of the temperature rise and the reduction of dislocations is small, and the strength of the matrix is relatively high. Hence, the fracture morphology as a whole shows deconvoluted fracture. As the temperature increases, the distortion energy within the matrix is completely released, the grain grows, the dislocations are consumed and the precipitated phase is gradually dissolved back. The toughness of the alloy is significantly improved. Figure 5(b, c) shows the fracture morphology after heat treatment at 1000℃ and 1040℃, respectively, and it can be found that the number of tough fossae at the fracture increases, and the disintegrated surface decreases, which shows quasi-disintegrated fracture. Observing the fracture morphology at 1080°C, it can be found that there are a large number of tough nests at the fracture, and the deconstructed surface is basically not observed, which is a typical toughness fracture, as shown in Fig. 5(d). From Fig. 5(e, f), it can be seen that there are a large number of uniformly distributed and deeper toughness nests at the fracture, showing obvious ductile fracture, which is due to the alloy by the external force, the formation of voids in the internal organization. With the increase in temperature, the precipitation phase back dissolves, the matrix slip effect is enhanced, and the formed cavities are gradually growing, which may be related to the fusion of adjacent cavities to form a larger tough nest. As the heat treatment temperature increases, the fracture morphology also changes; the deconvoluted fracture gradually evolves into a toughness fracture, thus improving the plasticity of the material.

Figure.4 Average grain size of Inconel 625 alloy versus strength (a) and hardness (b)

2.4 Analysis of Intergranular Corrosion Properties

The intergranular corrosion rates of Inconel 625 alloy after holding for 15 min at different heat treatment temperatures are given in Table 2 and Figure 6. It can be seen that, with the increase of heat treatment temperature, the intergranular corrosion rate of Inconel 625 alloy shows a trend of decreasing and then stable, and the corrosion rate basically stabilizes in a certain range when the temperature reaches 1080 ℃ or higher. The corrosion performance of Inconel 625 alloy is mainly affected by the effect of carbide precipitation. On the one hand, niobium carbide precipitation increases, the precipitation of chromium carbide at the grain boundary will be reduced, the grain boundary will not form a chromium-poor zone and the matrix intergranular corrosion resistance will be improved; on the other hand, as the temperature rises, the matrix precipitation phase decreases, the chromium-poor phenomenon is weakened, and the alloy’s resistance to intergranular corrosion ability to enhance. Cold deformation of the alloy began to heat; the base body precipitated phase content is more, so the corrosion rate is relatively high; with the increase in temperature, the precipitated phase is gradually reduced and the corrosion rate of the alloy is also gradually reduced. When the temperature rises to 1040 ℃, the precipitated phase content is reduced, but its size has increased, so the corrosion rate relative to the 1000 ℃ change is not obvious. When the temperature rises to 1080 ℃, the precipitated phase in the matrix has been basically completely dissolved; at this time, the corrosion rate of the matrix reaches 0.036mm/month. With the continuous increase in heat treatment temperature, the precipitates in the matrix have no significant changes, and there is no significant difference in the corrosion rate.

Figure.5 Tensile fracture morphology of Inconel 625 alloy at different heat treatment temperatures
(a) 960 ℃; (b) 1000 ℃; (c) 1040 ℃; (d) 1080 ℃; (e) 1120 ℃; (f) 1160 ℃
Table.2 Intergranular corrosion rate of Inconel 625 alloy (mm/month)

Temperature/℃	First Cycle	Second Cycle	Third Cycle	Fourth Cycle	Fifth Cycle	Average Value
960	0.063	0.055	0.051	0.053	0.058	0.056
1000	0.053	0.044	0.040	0.044	0.051	0.046
1040	0.51	0.040	0.034	0. 051	0.045	0.044
1080	0.044	0.030	0.038	0.029	0.040	0.036
1120	0.046	0.036	0.032	0.039	0.034	0.037
1160	0.034	0.032	0.036	0.040	0.033	0.035

Comprehensive analysis, heat treatment temperature on the Inconel 625 alloy organization, mechanical properties and corrosion resistance: with the heat treatment temperature, on the one hand, the strength and hardness of Inconel 625 alloy gradually decreased, grain size and plasticity has increased significantly; on the other hand, heat treatment of the alloy’s resistance to intergranular corrosion performance is also gradually reduced, and finally stabilized in a certain range. It can be seen that heat treatment can effectively improve the comprehensive performance of Inconel 625 alloy. Heat treatment temperature of 1160 ℃, the alloy’s elongation and corrosion resistance, although the optimal performance, but coarse grain size, strength and hardness is low. While heat treatment at 1080 ℃, intergranular corrosion resistance and elongation are slightly worse than 1160 ℃, but the strength and hardness are more appropriate. Therefore, the optimum heat treatment temperature for cold deformed Inconel 625 alloy under the condition of holding temperature for 15min is 1080℃.

Fig.6 Effect of heat treatment temperature on the corrosion rate of Inconel 625 alloy

3. Conclusion

• 1) Cold deformation of 52.13% of Inconel 625 alloy pipe by 960-1160 ℃ insulation 15 min, the organization is basically isometric crystal, the grain growth rate was first slow and then fast trend, the temperature of 1080 ℃ when the grain grows rapidly, the growth of the activation energy of 144.14 kJ/mol.
• 2) Inconel 625 alloy pipe tensile strength hardness with the heat treatment temperature increases and decreases, and in the temperature range of 960-1160 ℃, tensile strength, hardness and grain size to meet the Hall-Pecth equation. When the temperature is about 1080℃, the hardness and elongation have an intersection and the mechanical properties are optimal. With the increase in temperature, the fracture mode of Inconel 625 alloy pipe is transformed from deconvolution fracture to toughness fracture.
• 3) The intergranular corrosion rate of Inconel 625 alloy pipe shows a trend of decreasing first and then stabilizing as the temperature rises. The corrosion rate is stable when the temperature reaches 1080°C and above in order to ensure that the Inconel 625 alloy pipe has excellent mechanical properties and good resistance to intergranular corrosion to determine the optimal heat treatment process for 1080 ℃ insulation 15 min.

Author: Qin Xingwen