Corrosion failure analysis of UNS S32750 duplex stainless steel hexagon head bolt

Duplex stainless steel combines the performance characteristics of ferritic stainless steel and austenitic stainless steel, with excellent corrosion resistance and good overall mechanical properties, and has been more widely used in various fields.

An enterprise pump operation 2a (years) after the bolt was found to have corrosion failure, bolt material UNS S32750 duplex stainless steel, the final heat treatment state for the solution treatment. In order to identify the cause of the bolt corrosion failure, to avoid the recurrence of similar failure mode, the author carried out the inspection and analysis.

Physical and chemical inspection

1. Macroscopic analysis

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Figure.1 macroscopic shape of the bolt
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Figure.2 macroscopic shape of the failed bolt

The macroscopic shape of UNS S32750 stainless steel hexagonal head bolt is shown in Figure 1, and the macroscopic shape of the bolt with corrosion failure is shown in Figure 2. The rest of the threads and heads did not show any obvious corrosion traces.

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Figure.3 Macroscopic appearance of the nut
Figure 3 shows the macroscopic shape of the nut with the failed bolt, which shows that the inner thread profile of the nut is basically intact, and individual pitting corrosion pits can be seen.
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Figure.4 Corrosion of lighter areas of low times the shape
Figure 4 shows the low magnification of the surface of the failed bolt in the light corrosion area, which shows that there are corrosion pits with diameters of 0.5~2mm at the bottom of the threads and a large number of pitting corrosion defects around the pits.
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Figure.5 Macroscopic morphology of the serious corrosion area
Figure 5 shows the surface morphology of the serious corrosion area of the failed bolt, which will be divided into three areas, A, B and C, for description.

2. Scanning electron microscopy and energy spectrum analysis

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Figure.6 Microscopic morphology of section A area
Figure 6 shows the microscopic morphology of the corrosion A area in Figure 5, it can be seen that there is a significant surface coverage, no typical fracture characteristics.
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Figure.7 Section A area energy spectrum analysis results
Figure 7 shows the results of the A area energy spectrum (EDS) analysis, it can be seen that the region in addition to the matrix elements there is a large number of oxygen elements, and a small amount of sulfur, chlorine, calcium, potassium, magnesium, aluminum and other elements.
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Figure.8 Microscopic morphology of section B
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Figure.9 Results of energy spectrum analysis of section B area
Figure 8 shows the microscopic morphology of section B. The elements of chlorine, potassium, magnesium and aluminum can be seen in long stripes of grains.
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Figure.10 Microstructure of section C
Figure 10 shows the microscopic morphology of section C. The long strip-like grain structure can be seen.
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Figure.11 The results of energy spectrum analysis in the C region of the cross-section
Figure 11 shows the results of energy spectrum analysis in the C region, which shows that in addition to the matrix elements, there are some oxygen elements and small amounts of sulfur, chlorine, potassium, calcium, aluminum and other elements in the region.

3. Metallographic analysis

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Figure.12 Microstructure of the intact thread
Figure 12 shows the microstructure of the intact thread of the failed bolt, and no obvious discontinuity defects such as folding and cracking were found.
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Figure.13 Non-metallic inclusions of the failed bolt
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Figure.14 Non-metallic inclusions of the intact bolt
Figure 13 and Figure 14 show the shape of non-metallic inclusions of failed bolts and intact bolts respectively. Oxide inclusions (fine system) 1, class D spherical oxide inclusions (coarse system) 0.5.
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Figure.15 Corrosion area microstructure morphology
After light aqua regia etching, the corrosion area microstructure of the failed bolt is shown in Figure 15, which shows that the corrosion preferentially expands along the phase interface.
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Figure.16 Microstructure of the longitudinal section of the failed bolt
After electrolytic etching, the microstructure of the longitudinal section of the failed bolt matrix is shown in Figure 16, and the ferrite content is calculated to be about 45.6% (volume fraction, the same below), and there is an obvious black σ phase at the phase interface and ferrite grain boundary.
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Figure.17 Microstructure of the longitudinal section of the intact bolt
After electrolytic etching, the microstructure of the longitudinal section of the intact bolt is shown in Figure 17, and the ferrite content is calculated to be about 50.1%, and no obvious black σ phase is found.
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Figure.18 Microstructure of the cross-section of the nut
After electrolytic etching, the cross-sectional microstructure of the nut is shown in Figure 18, and the calculated ferrite content is about 44.6%.

4. Mechanical performance test

The mechanical properties of the intact bolt test, the results are shown in Table 1, it can be seen that all indicators meet the relevant standards for UNS S32750 stainless steel technical requirements.

Table.1 Mechanical properties of intact bolts test results

Project Tensile strength Rm/Mpa Yield strength Rp0.2/Mpa Hardness/(HBW2.5/187.5)
Measured value 889 620 289 ,293,289
Standard value ≥800 ≥550 ≥310

5. Chemical composition analysis

Chemical composition analysis of the failed bolt and the supporting nut, the results are shown in Table 2, in line with the chemical composition requirements of UNS S32750 duplex stainless steel provided by the client.

Table 2 Chemical composition of the failed bolt and nut (mass fraction)

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6. Simulation verification test

Two specimens of the same size (50 mm × 25 mm × 4 mm) are intercepted on the intact bolt and marked as No. 1 and No. 2 respectively. Two specimens were simulated at different temperatures of solid solution treatment, and carried out different temperature 72h ferric chloride pitting corrosion test, the results are shown in Table 3.

Table.3 Ferric chloride pitting corrosion test results

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It can be seen that UNS S32750 stainless steel 1050 ℃ solid solution treatment specimens of iron trichloride pitting corrosion resistance is significantly better than the 950 ℃ solid solution treatment specimens.
Figure 19 shows the microstructure of the matrix of the solid solution treated specimen at 950℃, the ferrite content is 51.4%, and there is an obvious black σ phase at the phase interface. Figure 20 shows the microstructure of the matrix of the solid solution treated specimen at 1050℃, the ferrite content is 50.6%, and no obvious black σ-phase is found.
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Figure.19 Microstructure of solid solution treated specimens at 950℃
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Figure.20 Microstructure of solid solution treated specimens at 1050°C
The above test results show that: compared with 1050 ℃ solid solution treatment, 950 ℃ solid solution treatment is more likely to produce σ phase; duplex stainless steel after the production of σ phase its resistance to ferric chloride point corrosion performance will be significantly reduced.

Comprehensive analysis

The surface morphology of the failed bolt shows that the corrosion area is concentrated in the end of the bolt about 13 buckle thread range, the rest of the thread and head are not found obvious corrosion traces, indicating that the bolt service process from the end of about 13 buckle length of thread contact with the corrosive medium, the rest of the thread and head are isolated from the corrosive medium. The residual metal diameter in the area of serious corrosion is only about 1/2 of the original diameter, and there is a large amount of metal loss in the corrosion process; the area of slight corrosion can be seen as a clear macroscopic corrosion pit morphology, and there are a large number of pockmarked corrosion pits around the macroscopic corrosion pit of the bolt, and the macroscopic corrosion morphology of the bolt is consistent with the characteristics of pitting corrosion.
Scanning electron microscope morphology analysis results show that the serious corrosion area and the corrosion pits at the bottom of the thread can be seen as long stripes of grain structure, and the microstructure of the corrosion area can be seen that corrosion occurs preferentially along the phase interface, and the scanning electron microscope morphology is consistent with the characteristics of metallographic organization. Preferential corrosion along the phase interface is consistent with the significant characteristics of duplex stainless steel pitting corrosion, so the bolt failure mode is judged to be pitting corrosion. Austenitic stainless steel in different corrosive media system will occur in different forms of corrosion failure, and the most serious local corrosion failure. Austenitic stainless steel local corrosion failure form mainly pitting corrosion, crevice corrosion, intergranular corrosion, stress corrosion cracking, etc.. In general, the occurrence of pitting corrosion must have the following conditions.

  • ① Ssensitive materials, such as easily passivated materials or materials with passivated films on the surface.
  • ② Media containing corrosive ions, such as halogen ions, especially the common Cl.
  • ③ With certain oxidizing conditions, such as the medium contains a certain amount of oxygen.

Energy spectrum analysis results show that there is a large amount of oxygen in the bolt corrosion area, as well as a small amount of sulfur, chloride, potassium, calcium, aluminum, magnesium and other elements, indicating that the bolt contact medium contains dissolved oxygen with sulfides, chlorides and other salts.
Metallographic analysis results show that there is an obvious σ phase in the microstructure of the failed bolt, the harmful phase is Fe-Cr-Mo intermetallic compounds, supporting nuts and intact bolt microstructure is not found in the obvious σ phase. When the duplex stainless steel is heated below 1000 ℃ σ phase precipitation from the ferrite, σ phase precipitation leads to depletion of the adjacent matrix chromium, corrosion resistance is reduced, which is consistent with the test results simulating the corrosion resistance of the specimens after solid solution treatment at different temperatures. Intermetallic compounds usually become pitting nucleation, duplex stainless steel containing σ phase has obvious selective corrosion characteristics, in the electrochemical corrosion process, the σ phase is the first to dissolve, followed by the surrounding chromium-poor area, followed by the ferrite phase, and finally the initial austenite phase and secondary austenite phase. σ phase not only reduces the material’s pitting resistance and passivation film stability, but also changes the material after electrochemical corrosion. With the gradual increase in the amount of σ phase precipitation, corrosion morphology from the hemispherical shape into a small mouth cavity, internal damage serious morphology, and then gradually transformed into a wide shallow corrosion morphology, which coincides with the corrosion morphology of the failed bolt.

Conclusions and recommendations

The UNS S32750 duplex stainless steel hexagonal head bolt failure mode is pitting corrosion; the root cause of the bolt pitting corrosion is improper heat treatment process, resulting in the presence of significant intermetallic compounds σ phase in the bolt microstructure, while the presence of corrosive media containing Cl in the service environment. It is recommended that the finished product of duplex stainless steel fasteners should be tested for pitting corrosion resistance and harmful intermetallic compound test before being put into use.

Author: Xu Jiankang

Source: China Flange Bolt Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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