Effect of solid solution treatment on the organization and properties of Incoloy 825 alloy steel pipes

The effects of solid solution treatment on the organization and properties of Incoloy 825 alloy were studied by metallographic analysis, tensile test, and intergranular corrosion test. The results show that with the increase of solid solution temperature, the grains of Incoloy 825 alloy tend to grow. Still, the grain growth rate varies at different temperatures of solid solution, and when the solid solution temperature exceeds 1000 ℃, the grains grow rapidly and are accompanied by annealed twins. When the solid solution time is less than 30min at 950℃, the matrix appears to have mixed crystal organization, and after 60min of heat preservation, the mixed crystal state can be improved, and it is an equiaxed crystal with the average grain size of grade 7. With the increase of solid solution temperature and the extension of holding time, the tensile strength and yield strength have different degrees of decline, and elongation is generally a rising trend. Incoloy 825 alloy intergranular corrosion rate with the increase of solid solution temperature and the extension of holding time shows a trend of first decline and then stable after 60 min of solid solution at 950 ℃; the corrosion rate is stable at about 0.12mm/y. Incoloy 825 alloy in solid solution at 950 ℃ after 60 min, its mechanical properties and resistance to intergranular corrosion performance of the best integrated effect.

What is Incoloy 825 alloy?

Incoloy 825 alloy is a corrosion-resistant alloy developed by Huntington Materials Studio in 1952 for the sulfuric acid industry. It has good corrosion resistance in reducing and oxidizing corrosive media and excellent resistance to chloride ion stress corrosion cracking, pitting, crevice corrosion, and various corrosive solutions. It is widely used in many fields, such as petroleum, chemical, metallurgy, marine development, etc. Incoloy 825 alloy is a Ni-Fe-Cr type solid solution strengthening alloy, and after solid solution treatment can obtain good overall After solid solution treatment, Incoloy 825 alloy can obtain good mechanical properties and corrosion resistance, so Incoloy 825 alloy generally delivered solid solution. A domestic petrochemical hydrogenation unit with Incoloy 825 alloy steel tube products to reduce the tendency of field welding cracking and improve the resistance to intergranular corrosion and mechanical properties, the solid product solution state delivery, the average grain size control at about 7, room temperature tensile test to meet the requirements of ASME SB423 standard, and intergranular corrosion rate <0.3mm/y. Research shows that nickel-based alloy grains in The solid solution treatment process may grow abnormally and affect the material’s mechanical properties. At the same time, Incoloy 825 alloy in the heat treatment process may precipitate carbides on the grain boundaries; these carbides on the alloy’s resistance to intergranular corrosion properties have a negative impact; at present, the research on Incoloy 825 alloy is mainly focused on the quality control of welding, fewer people systematically study the solid solution process on Incoloy 825 alloy organization, mechanical properties and resistance to intergranular corrosion The impact of the ability to intergranular corrosion. In this paper, the solid solution treatment system on the Incoloy 825 alloy pipe fittings product organization and properties of the law of experimental analysis to determine the best solid solution process to meet the requirements of the material grain size, mechanical properties, and intergranular corrosion resistance of Incoloy 825 alloy pipe fittings for petrochemical hydrogenation plant products.

1. Test materials and methods

1.1 Specimen material preparation

Incoloy 825 alloy corresponds to the U.S. intermetallic grade UNS N08825; the test material is taken from the ASME SB423-2017 UNS N08825 size φ325mm × 10.31mm cold-drawn seamless steel pipe, Spectro test TXC03 full-element spectral analyzer is selected to measure the chemical composition, its measured data are shown in Table 1.
Table.1 Chemical composition of UNS N08825 seamless pipe (mass fraction, %)

C	Si	Mn	S	P	Ni	Cr	Mo	Ti	Cu	Al	Fe
0.014	0.21	0.46	0.001	0.016	39.2	21.5	2.8	1.24	1.84	0.095	32.25

1.2 Heat treatment test

Using wire cutting along the steel pipe longitudinal processing 70mm × 200mm Incoloy 825 alloy specimens, take 13 pieces, leaving a piece for the original comparison specimens, no heat treatment, the other 12 pieces for solution treatment process test, solution temperature of 950, 1000 and 1050 ℃, respectively, solution time of 10, 20, 30 and 60min. Solution treatment test in high temperature The cooling method is water-cooled, the water temperature is controlled at 20-40℃, and the solid solution cooling transfer time are less than 15s.

1.3 Performance test and tissue observation

After the solid solution treatment, microstructure observation, intergranular corrosion sensitivity, and mechanical properties testing. Metallographic specimens of 10mm×10mm×10mm were line cut from the solid solution-treated specimen block, and after mechanical polishing by sandpaper grinding, cold acid etching was used for 0.5-1min. The etching solution was 92% HCl+5% H₂SO₄+3% HNO₃ (mass fraction). The etched metallographic specimens were subjected to an optical microscope by GB/T6394- The average grain size was determined by the three-circle intercept method under the optical microscope by GB/T6394-2017 “Determination Method of Average Grain Size of Metals,” and the effect of solid solution process on grain size was analyzed. Room temperature tensile test is conducted by GB/T228.1-2010 “Tensile test of metal materials part 1: room temperature test method”. The solid solution treatment test block is processed into a longitudinal arc-shaped proportional specimen, the width of the specimen is 15mm, the minimum radius of the transition arc between the clamping end and the parallel length is 12mm, and the scale factor k is taken as 5.64. intergranular corrosion test By GB/T15260-2016 “corrosion of metals and alloys nickel alloy intergranular corrosion test method” D method of implementation, along the steel tube axial processing intergranular corrosion specimen, specimen size 30mm × 20mm × 3mm, after 675 ℃ × 1h sensitization treatment, grinder processing specimen surface, remove the oxide skin, the surface roughness Ra ≤ 0.8μm, choose the volume fraction of 65% nitric acid corrosion solution, intergranular corrosion test temperature of 80 ℃.

2. Test results and analysis

2.1 The effect of solid solution treatment on the microstructure

Figure 1 shows the microstructure of Incoloy 825 alloy before the solid solution. Figure 2 shows the microstructure of Incoloy 825 alloy after solid solution treatment by different processes. Figure 2(a) shows the microstructure of Incoloy 825 alloy after solid solution at 950°C for 10 min. The matrix organization is a fine equiaxed crystal with an average grain size of 8.5 grade. With the extension of solid solution time, the grains engulf each other, the grain boundaries move at high temperatures, and the grains grow. The metallographic photos of Incoloy 825 alloy by solid solution at 950°C for 20, 30, and 60 min are shown in Fig. 2(b-d), and the average grain size is 8, 8, and 7, respectively. In contrast, the grains grow, some grains grow significantly, showing a coarse/fine grain The average grain size of the original specimen without solid solution treatment was 8.5, as shown in Figure 1.

Figure.1 Microstructure of Incoloy 825 alloy before the solid solution
With the solid solution temperature increase, the atomic diffusion is intense, and the grain growth is more rapid; Figure 2(e) shows the metallographic photos of Incoloy 825 alloy after 10 min of solid solution at 1000°C. The average grain size changes from 8.5 to 5.5. The average grain size was 4.5, 4.5, and 3.5 with the increase of solid solution time. When the solid solution temperature is further increased, the grain growth is sharply intensified; as shown in Figure 2(i), the average grain size of Incoloy 825 alloy changes from grade 8.5 to grade 4.5 after 10 min of solid solution at 1050°C. With the solid solution time extension, the grain growth rate slows down. The metallographic photographs of 20, 30, and 60 min of solid solution at 1050°C are shown in Figure 2(j-l), and the average grain size was 4, 4, and 3, respectively. The number of annealed twins inside the grains increased significantly.
Figure 3 shows the variation pattern of grain size of Incoloy 825 alloy under different solid solution treatment regimes. With the increase of solid solution temperature, the grain of Incoloy 825 alloy grows gradually, and the grain size grades decrease. Still, the growth rate varies at different stages, and the grain growth is more moderate when the solid solution temperature is 950°C and grows rapidly when the solid solution temperature exceeds 1000°C.
This is because grain growth is mainly achieved by grain boundary migration, and the grain boundary migration process is the atomic diffusion process; the higher the temperature, the faster the grain growth rate. At 950℃, the solid solution’s temperature is low, grain boundary migration diffusion is slow, and grain growth is slow. When the solid solution temperature increases to 1000 ℃, the rapid growth of grains, the reason for the rapid growth of grains can be attributed to two aspects; on the one hand, the temperature increases, the atomic energy increases, diffusion is intense; on the other hand, some of the insoluble precipitation phase back to the matrix γ phase, reducing the resistance to atomic diffusion, the weakening of the role of nailing the grain boundaries, the new grains of grain boundary migration can be carried out freely, and thus the rapid grain size increases. When the solid solution temperature exceeds 1000°C, the grains grow rapidly and are accompanied by many annealed twins. Due to the low layer dislocation energy of Incoloy 825 alloy, the annealed twins may be formed when the grains grow through grain boundary migration at higher solid solution temperatures. As shown in Figure 3, to meet the requirement of controlling the average grain size of Incoloy 825 steel pipe for a petrochemical hydrogenation plant at about grade 7 and to avoid generating a large amount of mixed crystal organization, the recommended solid solution temperature of Incoloy 825 alloy is 950°C and the solid solution time is 60min.

Figure.2 Microstructure of Incoloy 825 alloy after solid solution treatment by different processes

(a) 950 ℃ × 10 min; (b) 950 ℃ × 20 min; (c) 950 ℃ × 30 min; (d) 950 ℃ × 60 min; (e) 1000 ℃ × 10 min; (f) 1000 ℃ × 20 min; (g) 1000 ℃ × 30 min; (h) 1000 ℃ × 60 min; (i) 1050 ℃ × 10 min; (j) 1050 ℃ × 20 min; (k) 1050 ℃ × 30 min; (l) 1050 ℃ × 60 min

Figure.3 Effect of solid solution treatment on the average grain size of Incoloy 825 alloy

2.2 Mechanical properties

GB/T20801-2020 “Pressure Piping Code for Industrial Piping Part 2: Materials” stipulates that the upper operating temperature of Incoloy 825 alloy is 538°C. Incoloy 825 alloy is generally used in low and medium-temperature environments and is seldom used in high temperature environments, so this test selects room temperature tensile. The room temperature tensile properties of Incoloy 825 alloy are shown in Table 2. The tensile strength of the original specimen without solid solution treatment is 670 MPa, and after solid solution at 950, 1000, and 1050°C for 60 min, the tensile strength decreases to 627, 557, and 552 MPa, respectively. This is because the average grain size of the specimen without solid solution treatment is about 8.5, the grain size is fine, and the number of grain boundaries is high when the tensile loading deformation is applied; the dislocations are blocked at the grain boundaries. Due to the dislocation difference between the grains, the slip band terminates near the same time. To coordinate the deformation, the grains are required to carry out multi-system slippage, and the multi-system slippage will occur at the intersection of dislocations, further enhancing the material’s tensile strength. After solid solution treatment, the average grain size of the material decreases, the grains grow, the number of grain boundaries decreases, and the original fine grain strengthening effect decreases, decreasing the material’s tensile strength. On the other hand, with the increase of solid solution temperature, the solubility of alloying elements in the matrix increases, the atomic diffusion is intense, the dissolution of precipitated phases, and the nailing effect on dislocations is weakened, which also leads to the decrease of tensile strength to a certain extent.
Table.2 Tensile properties of Incoloy 825 alloy after solid solution by different processes

Solid solution temperature/℃	Solid solution time/min	Rm/MPa	Rp0.2/MPa	A/%
950	10	664	456	42.2
	20	653	450	41.4
	30	649	447	40.3
	60	627	420	43.9
1000	10	600	395	43.5
	20	587	383	43.7
	30	585	375	44.2
	60	557	326	45.4
1050	10	587	380	43.1
	20	572	350	44.5
	30	560	345	44.9
	60	552	314	45.8
Untreated		670	462	42.7

From the test data, it can be seen that the effect of solid solution temperature and solid solution time on the yield strength of Incoloy 825 alloy is obvious, the yield strength of the original specimen without solid solution treatment is 462 MPa, and after 60 min of solid solution at 950, 1000 and 1050°C, the yield strength decreases to 420, 326 and 314 MPa respectively, which is because the yield strength is closely related to the slip transfer from the first plastic deformation of This is because the yield strength is closely related to the transfer of slip from the first plastic deformation of the grain to the adjacent grain, and whether this transfer can occur depends mainly on the degree of stress concentration in the dislocation plugging group near the grain boundary of the slipped grain, and whether the dislocation source in the adjacent grain slip system can be excited to open up, to carry out more slip. The dislocation plugging near the grain boundaries of small grains that have been slipped causes a relatively small stress concentration; the opportunity to excite the adjacent grains to plastic deformation is much smaller than the large grains, the need for a larger applied stress to make the adjacent grains plastic deformation, so the finer the grain, the higher the yield strength. The grain size of the original specimen without solid solution treatment is 8.5, the grains are small, the stress concentration caused by dislocation plugging is small, and the probability of excitation of plastic deformation of adjacent grains is low, which requires large external stress to occur in concert with the plastic deformation, showing a higher yield strength. As the solid solution temperature increases and the holding time increases, the average grain size decreases, the grains coarsen, and the fine grain strengthening effect decreases, resulting in a decrease in yield strength.
The elongation of Incoloy 825 alloy is related to the solid solution treatment process. When the solid solution is applied at 950°C, the elongation decreases and increases as the solid solution time increases. This is because after 10 min of solid solution, there is a mixed distribution of course/fine grains in the matrix, and the plastic deformation is not uniform, which easily causes stress concentration and leads to lower elongation. With the extension of solid solution time, the grain size tends to be consistent; the mixed crystal state can be improved, continue to extend the solid solution time, the precipitates dissolve more fully, and elongation can be improved. 1000 ℃ and 1050 ℃ solid solution, with the extension of solid solution time, elongation is a rising trend. This is due to the increase in the solubility of alloying elements in the matrix, the intense atomic diffusion, and the large amount of dissolution of precipitates, as well as the small degree of miscibility in the matrix organization, which does not easily cause stress concentration and can withstand larger deformation before the material fracture, thus showing a higher elongation.

Figure.4 Shows the effect of solid solution treatment on the tensile properties of Incoloy 825 alloy.
Incoloy 825 alloy is widely used in petrochemical engineering because of its excellent corrosion resistance, and its mechanical properties are also important considerations. Combined with the tensile test, it can be seen that with the increase of the solid solution temperature and the extension of the holding time, the tensile strength and yield strength have different degrees of decline. Therefore, when developing the solid solution treatment process, the solid solution temperature and holding time should be reduced as much as possible to reduce the degree of grain growth to ensure a sufficient solid solution. The test results show that after the solid solution of Incoloy 825 alloy at 950℃ for 60min. However, the tensile strength and yield strength indexes have decreased. Still, the rate of decrease is not as obvious as that of solid solution at 1000℃ and 1050℃, with higher Tensile strength and yield strength; At the same time, elongation increased compared with the original specimen; the comprehensive mechanical properties of the material are better at this time.

2.3 Intergranular corrosion resistance

The intergranular corrosion test is divided into five cycles, each cycle of 48h, using the weighing method to calculate the corrosion rate, the determination of intergranular corrosion susceptibility related to chromium carbide precipitation, analysis of the solid solution process on the corrosion rate of the specimen, the intergranular corrosion rate calculation formula as shown in formula (1).

γ_corr=(K·Δm)/(A·t·ρ) (1)

In the formula:

• K is a constant, taken as 8.76 × 10⁴.
• Δm is the mass loss, g; A is the surface area of the specimen, cm².
• t is the corrosion time, h; ρ is the density, taken as 8.14g/cm³.

The intergranular corrosion rates of Incoloy 825 alloy after solid solution treatment by different processes are shown in Table 3 and Figure 5. It can be seen that, with the increase of the solid solution temperature and the extension of the holding time, the intergranular corrosion rate of Incoloy 825 alloy shows a trend of first decreasing and then stable, and the intergranular corrosion rate is stable after the solid solution time exceeds 30 min. Research shows that the intergranular corrosion performance of Incoloy 825 alloy is mainly affected by the MC phase (TiC) and M₂₃C₆ precipitates; TiC is a high-temperature precipitation phase, about from 800 ℃ to form, the fastest formation rate at about 900 ℃, with the increase in solid solution temperature, TiC and began to dissolve, from 900 ℃ heating to 1200 ℃, the number of TiC decreasing. The starting precipitation temperature of M₂₃C₆ precipitates is related to the carbon content. When the carbon content is 0.01%, the starting precipitation temperature of M₂₃C₆ is about 843°C. With the increase of solid solution temperature, the M₂₃C₆ precipitates will dissolve into the matrix, and the quantity decreases. The solid solution temperature selected in this test is within the range of solid solution temperature of TiC and M₂₃C₆ precipitates. With the increase of solid solution time, the precipitated phase in the matrix decreases, the chromium-poor phenomenon at grain boundaries decreases, and the resistance of the alloy to intergranular corrosion is enhanced. After the solid solution at 950 ℃ for 60 min, the precipitated phase in the matrix was completely dissolved; with the increase of solid solution temperature, the precipitates in the matrix showed no significant changes, and the intergranular corrosion rate also showed no significant difference, basically stable at about 0.12mm/y.
Comprehensive analysis of the solid solution treatment on Incoloy 825 alloy organization, mechanical properties, and intergranular corrosion resistance: with the increase of the solid solution temperature and the extension of the holding time, Incoloy 825 alloy grain size is decreasing trend, tensile strength, and yield strength also gradually decreased, the corrosion rate decreased and finally stabilized, the intergranular corrosion resistance can be improved. It can be seen that the solid solution treatment can effectively improve the organization and properties of Incoloy 825 alloy. When the solid solution temperature is above 1000°C, the grain size grows significantly, and the elongation and intergranular corrosion resistance increase with the increase of solid solution time, but the tensile strength and yield strength decrease significantly. The solid solution temperature is 950℃; the grain growth is slow; the tensile strength and yield strength are slightly decreased but still have high values. After 30min of solid solution at 950℃, the matrix can easily appear as mixed crystal organization, leading to a decreased elongation and high corrosion rate. Therefore, the best solution temperature of Incoloy 825 alloy is 950℃, and the solid solution time is 60min.
Table.3 Intergranular corrosion rate of Incoloy 825 alloy after solid solution treatment of different processes (mm/y)

Solid solution temperature/℃	Solid solution time/min	1st cycle	2nd cycle	3rd cycle	4th cycle	Cycle 5	Average value
950	10	0.1933	0.1524	0.1157	0.1645	0.1856	0.1623
	20	0.1203	0.1217	0.1156	0.1925	0.2015	0.1503
	30	0.1185	0.1241	0.1703	0.1056	0.1231	0.1283
	60	0.1206	0.121	0.1213	0.1204	0.1206	0.1207
1000	10	0.1252	0.1023	0.1237	0.1125	0.1785	0.1284
	20	0.1325	0.1124	0.1009	0.1384	0.1224	0.1213
	30	0.1245	0.1206	0.1106	0.1155	0.1301	0.1202
	60	0.1124	0.1204	0.1178	0.1237	0.1258	0.12
1050	10	0.1243	0.1105	0.1248	0.1295	0.1574	0.1293
	20	0.1148	0.124	0.1265	0.1232	0.1247	0.1226
	30	0.1229	0.1193	0.1156	0.122	0.1215	0.1202
	60	0.1298	0.1109	0.1182	0.1207	0.1194	0.1198
Untreated		0.1875	0.1897	0.1752	0.1454	0.143	0.1681

Fig.5 Effect of solid solution treatment on the intergranular corrosion rate of Incoloy 825 alloy

3. Conclusion

• 1) With the increase of solid solution temperature, the grain growth of Incoloy 825 alloy, the grain degree grade is decreasing. The grain growth rate of Incoloy 825 alloy at different solid solution temperatures is different; when the solid solution temperature exceeds 1000 ℃, the grain growth is rapid and accompanied by annealing twins. At 950°C, the grain growth is slow, and when the solid solution time is less than 30min at 950°C, the matrix appears to have mixed crystal organization. After solid solution at 950 ℃ for 60 min, the mixed crystal state can be improved; the matrix is equiaxed crystal, with an average grain size of 7, to meet the grain size requirements of the petrochemical hydrogenation plant steel pipe.
• 2) With the increase of solid solution temperature and the extension of holding time, Incoloy 825 alloy tensile strength and yield strength have different degrees of decline in 950 ℃ solid solution. However, the tensile strength and yield strength decreased, but the rate of decline is not 1000 ℃ and 1050 ℃ solid solution but still has a high tensile strength and yield strength. Incoloy 825 alloy Elongation with the increase of solid solution temperature and the extension of holding time, the overall trend is rising, but in 950 ℃ solid solution, with the extension of holding time, the elongation first decreased and then increased, after 60 min of solid solution at 950 ℃, the elongation increased compared to the original specimen, at this time the comprehensive mechanical properties of the material is better, the material tensile properties in line with the requirements of ASME SB423 standard.
• 3) With the increase of solid solution temperature and the extension of holding time, the intergranular corrosion rate of Incoloy 825 alloy showed a trend of first decreasing and then smoothing. After solid solution at 950 ℃ for 60 min, the corrosion rate is stable at about 0.12 mm/y, which meets the requirement of an intergranular corrosion rate of less than 0.3 mm/y for petrochemical hydrogenation equipment. Therefore, to meet the requirements of grain size, mechanical properties, and intergranular corrosion resistance of Incoloy 825 alloy steel pipe for a petrochemical hydrogenation plant, the optimal process of Incoloy 825 alloy solid solution treatment is determined as 950°C solid solution 60min.

Authors: Yucheng Zhang, Haomei Jia