How is the stainless steel bolt broken?
UNS S43100 stainless steel is a kind of martensitic ferritic stainless steel, which is introduced into ASTM standard. It is characterized by retaining corrosion resistance of ferritic stainless steel and high strength of martensitic stainless steel, so it is widely used in industry. Generally, the materials used for bolts are 40CrNiMoA, 42CrMo and 12cr13. However, the corrosion resistance of the materials in seawater is not as good as that of UNS S43100 stainless steel. Therefore, UNS S43100 stainless steel is the preferred material for bolts in seawater environment. However, the heat treatment process of UNS S43100 stainless steel is very strict, which is easy to cause corrosion cracking. In this paper, the broken UNS S43100 stainless steel bolt is taken as the research object, and the causes of bolt fracture are analyzed.
Fracture
The broken UNS S43100 stainless steel bolt in a project is a forged bar produced by electroslag remelting, with a specification of 40mm × 300mm. Its chemical composition is WC = 0.137%, WMN = 0.15%, WSI = 0.58%, WS = 0.006%, WP = 0.024%, WCR = 16.88%, WNI = 2.32%. The heat treatment process of bolts is quenching: 1020 ℃ heat preservation, oil cooling to below 200 ℃, out of furnace air cooling to room temperature, tempering: 530 ℃ heat preservation and air cooling to room temperature. Then, the specimens are processed into rod-shaped tensile samples with gauge distance of 30mm and diameter of parallel parts of 6mm, and tensile tests at room temperature are carried out on rsa250 universal testing machine according to GB / T 228-2002 “tensile test methods for metallic materials at room temperature”. The measured mechanical properties and standard technical requirements are shown in Table 1.
Table.1 Mechanical Properties of UNS S43100 stainless steel
| Project | Standard | Sample 1 | Sample 2 | Sample 3 |
| Tensile strength /MPa | 931 | 1021 | 1026 | 1012 |
| Yield strength /MPa | 833 | 872 | 887 | 892 |
As shown in Fig. 1, the broken part of stud bolt is located at the joint of screw and threaded bolt head.
Figure.1 photo of broken bolt
Fracture cause analysis
Before cleaning the fracture surface of the broken bolt, there was no obvious shear lip on the whole section, and thick corrosion products existed on the fracture surface of the bolt. SEM / EDX element analysis shows that the corrosion products mainly contain Fe, Cr, O, C and Cl, and a small amount of S and Si elements, indicating that Cl – and other corrosive ions are contained in the actual service environment.
The fracture morphology after pickling and removing corrosion products with brush is shown in Fig. 2. The micro characteristics of the fracture surface after removing corrosion products were observed by scanning electron microscope. The results show that the fracture surface of fractured bolt presents typical intergranular fracture characteristics, and the fracture surface presents the characteristics of rock sugar.
Fig. 2 fracture morphology of broken bolt
Fig. 3 shows SEM photos of UNS S43100 stainless steel after heat treatment. It can be seen from Fig. 3 that the UNS S43100 stainless steel after heat treatment is mainly composed of sorbite, which is uniformly distributed with massive ferrite, and a large number of carbides are dispersed and continuously distributed in the grains and grain boundaries.
（a）
（b）
Fig.3 SEM photos of S4313 stainless steel after heat treatment
Another 3 bolts were processed into rod-shaped tensile specimens with gauge distance of 35mm and diameter of parallel part of 5mm. The stress corrosion test of metals and alloys was carried out on a slow strain rate tensile stress corrosion testing machine at a strain rate of 1.0 × 10-6s-1 according to the standard GB/T15970.7-2000 corrosion stress corrosion test of metals and alloys The results of slow tensile test (SSRT) are listed in Table 2. Generally, the stress corrosion sensitivity is between 0 ~ 1, and the greater the value, the higher the stress corrosion sensitivity of the material. It can be seen from table 2 that the UNS S43100 stainless steel after quenching and tempering has different degrees of loss, and the discrete type is very large, among which the stress corrosion sensitivity of sample 6 is the largest, reaching 0.44.
Table 2 slow tensile test results of UNS S43100 stainless steel
| Project | Sample 4 | Sample 5 | Sample 6 |
| Tensile strength /MPa | 892 | 903 | 915 |
| Yield strength /MPa | 770 | 748 | 795 |
| Stress corrosion sensitivity | 0.25 | 0.09 | 0.44 |
Fig. 4 shows the fracture and section morphology of sample 4 after slow strain rate test. It can be seen from Fig. 4 that the fracture mode is brittle fracture with no obvious plastic deformation, and it is characterized by intergranular fracture. The observation of cross-section morphology shows that the crack propagates along the grain boundary, indicating that the fracture property is stress corrosion cracking.
(a) Section
(b) of fracture
Fig. 4 fracture and section morphology of UNS S43100 stainless steel after SSRT test
UNS S43100 stainless steel needs heat treatment to obtain the corresponding mechanical properties. However, when the steel is tempered at about 530 ℃, a large number of Cr containing carbides are precipitated on the grain boundary, thus forming a CR poor zone, thus reducing the corrosion resistance of the material.
Improvement measures
In the past, the yield strength of UNS S43100 stainless steel bolt was 833mpa, but recently it has been changed to 686MPa when it is used in foreign compressor. In view of the large amount of Cr containing carbides precipitated near 530 ℃ in UNS S43100 stainless steel, which reduces the corrosion resistance of the material, the tempering temperature of UNS S43100 stainless steel bolt is changed to 600 ℃ according to the foreign experience and the above inspection results.
Another 6 bolt forgings of the same batch were quenched: 1020 ℃ heat preservation, oil cooling to below 200 ℃, air cooling to room temperature, tempering: heat treatment process of air cooling to room temperature after heat preservation at 600 ℃. Mechanical properties of three samples were tested. The results showed that the yield strength of sample 7 was 806mpa, that of sample 8 was 811mpa, and that of sample 9 was 794mpa. The remaining 3 samples were tested for stress corrosion sensitivity, and the results of slow tensile test (SSRT) were all 0, indicating that the stress corrosion sensitivity of UNS S43100 stainless steel was very small at this time. Then, the fracture scanning inspection was carried out on the sample No. 8, as shown in Fig. 5, no obvious corrosion fracture occurred.
(a) Section
(b) of fracture
Fig. 5 fracture and section morphology of UNS S43100 stainless steel after heat treatment
Conclusion
(1) After heat treatment, UNS S43100 stainless steel is mainly composed of sorbite with massive ferrite, and a large number of carbides are dispersed and continuously distributed in the grains and grain boundaries.
(2) The fracture of UNS S43100 stainless steel bolt is a typical intergranular fracture.
(3) When UNS S43100 stainless steel is tempered at about 530 ℃, a large amount of Cr containing carbides are precipitated on the grain boundary, resulting in the formation of Cr poor zone, thus reducing the corrosion resistance of the material.
Authors: Chen Wei, Wang Dongying, Liang Ying
Source: China Fasteners 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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