Inconel 725

Alloy 725 is a Ni-Cr-Mo-Nb-Ti-A1 corrosion-resistant alloy that has both high corrosion resistance and age hardening to extremely high strength. The corrosion resistance of this alloy is basically the same as that of the widely used Inconel 625 alloy. The age hardening 7M alloy has twice the strength level of the 625 alloy and its plastic toughness remains at a high level. For applications in corrosive environments, both excellent corrosion resistance and high strength and high plastic toughness are required. Some large parts or parts with uneven cross-section dimensions cannot be cold-worked. This age-hardening nickel Base corrosion resistant alloy is the best choice. Alloy 725 has been successfully applied in the development of acid gas fields.

8. 3. 11. 1 Chemical composition and organization
The chemical composition of 00Cr20Ni60Mo8Nb3Ti(725) alloy is shown in Table 8-7. The Cr, Ni, Mo, Nb content of the alloy is basically the same as that of the 625 alloy. This is the fundamental reason why the corrosion resistance of this alloy is consistent with that of the 625 alloy. The difference between the two is that the carbon in the 725 alloy drops below 0.03% and is added. Ti and A1, the former reduces the carbide precipitation sensitivity and improves the resistance to intergranular corrosion, and the latter imparts the age strengthening function of the alloy.
00Cr20Ni60Mo8Nb3TiAl(725) alloy is a pure austenitic alloy in solution annealing state. Under the aging condition, carbide and intermetallic phase are formed. The carbide is mainly composed of M6C and MC carbide. The intermetallic phase of the alloy is [Ni3(Nb, Ti, Al)], the latter is the main age-strengthening phase of the alloy, and its function is to increase the strength of the alloy.
8. 3. 11.2 Mechanical properties of the alloy
The room temperature mechanical properties of 725 alloy are shown in Table 8-106. The high temperature transient mechanical properties of 5mm bar (solid solution + age hardening treatment) are listed in Table 8-107.

Table 8-106 Mechanical properties of room temperature 725 alloy

Table 8-107 Average High Temperature Instantaneous Mechanical Properties of 16 ~ 16. 5mm Bars Annealed + Aging A

1 Solution treatment +732 °C x8h aging at 56$/»! Cool to 621 °C x8h air cooling.

8.3.11.3 Corrosion resistance A general corrosion
In the acid solution, the corrosion resistance of 725 alloys with different heat treatment conditions is listed in Table 8-108. In order to compare the 625 and C-276 alloys in the literature, it is obvious that the corrosion resistance of 725 alloy can be 625 with the annealed state. It is comparable to C-276 alloy.

Table 8-108 Average corrosion rate of Alloy 725 in acidic solution

① is literature data

Table 8-109 shows the change in corrosion resistance of Alloy 725 in an acidic medium as a function of test time and medium temperature.

B stress corrosion
In the standard test for determining stress corrosion (hydrogen embrittlement) caused by sulfides (NACET M0177), the corrosion data of Alloy 725 is shown in Table 8-110. These data indicate that Alloy 725 is superior to Alloy 625 and Alloy 718.

1 5% NaCl + 0.5% CH3COOH, H2S saturated, room temperature, 100% / ^. 2 stress applied, carbon steel coupling.
2 315t, 1000h
3 Test stress is 84% Rp0.2: 1138MPa

The stress corrosion behavior of Alloy 725 in simulated acid well environment is shown in Table 8-111 and Figure 8-93. The SCC resistance of 725 alloy is better than that of cold processed 625 alloy, G-3 alloy, 925 alloy, 825 alloy and 718 alloy. , near C-276 alloy.

Table 8-111 SCC of Alloy 725 in Simulated Acid Well Environment

Note: C-ring sample, tested in autoclave, time 丨 4 days, stress = Rp0.2, medium: 25% NaCl + 0.5% CH3COOH + lg / L S + 827kPa H2S0
1 135 ° C;
2 There is one SCC in both samples.

C crevice corrosion resistance and corrosion fatigue
The crevice corrosion resistance of the aged 725 alloy is better than that of the annealed alloy 625 (Table 8-112), and the fatigue performance in seawater is consistent with that in air (Fig. 8-94).

able 8-112 Corrosion of 725 Alloy in Seawater 1

1 Tested in 303⁄4 flowing seawater for 30 days, a polypropylene plastic gasket was used to fix the gap on the alloy sheet sample.
2 The average value of the maximum depth of each gap, the maximum depth range is 0.02~0.66mm.

8.3.11.4 Heat treatment, thermoforming and welding properties
A heat treatment,
The strengthening of Alloy 725 is achieved by aging treatment phase precipitation, which should be combined with solution annealing before aging treatment. Solution annealing: 1040 ° C: solid solution, AC (air cooling). Aging treatment: For acid gas well applications, the following treatments are recommended, 730 ° C x 8 h, furnace cooling (56 / h) to 620 ° C x 8h, AC.

B Hot work

The suitable thermoforming temperature of B thermoformed 725 alloy is 899 -1121 °C: due to its high strength, thermoforming equipment should have sufficient deformation force. For the Ministry.The piece has uniform deformation and should be moderately reduced in the deformed low temperature zone (890~950 °C)The amount of compression. In order to avoid the mixed crystal structure, the alloy should be given a relatively uniform compression amount. For the open mold hot work, the final compression amount should be greater than 20%. For the closed mold hot work, the final deformation should be greater than 10%, and the air is cooled after hot work. In the hot work process, it is necessary to avoid overheating and avoid cold spots below 899 °C. Once surface cracks or other defects appear, they should be removed immediately. It is recommended to preheat the hot work tool and mold to 260 ° C, which is beneficial for eliminating surface cracks and defects on the part.
C welding
The most suitable welding method for Alloy 725 is GTAW and GMAW. SAW and SMAW are not recommended. The filling metal should be 725NDUR to ensure the strength of the weldment. Table 8-113 shows the slow tensile test data of 725NDUR wire surfacing 1 layer at 5% NaCl + 517kPa H2S + 2758kPa C02, 149 °C. The data in the table is the ratio of data obtained using the same slow tensile parameters in corrosive environments and air. Usually this ratio is acceptable above 0.90. Table 8-114 shows the room temperature mechanical properties of 725NDUR weld metal (GMA) and Table 8-115 for its impact properties.

Table 8-113 Slow stretching data ratio of 725 alloy surfacing layer

1 surfacing on A1SI4140 steel, 663 ° C x 2h, AC.
2 Surfacing on AIS 丨 4130 steel, 635 ° C x 2h, AC.

Table 8-114 Mechanical properties at room temperature of 725NDUR weld metal (GMA)