Failure analysis and preventive measures of stainless steel flange bolt connection
- Objective: To study the failure mechanism of stainless steel flange bolt connection in low temperature environment.
- Methods: the chemical composition of stainless steel bolt material and the composition of corrosion products at fracture surface were analyzed by macro examination, micro detection, chemical composition analysis and energy spectrum analysis.
- Results: the main cause of bolt fracture was stress corrosion cracking in low temperature environment.
- Conclusion: according to the main influencing factors of stress corrosion of stainless steel bolts, the preventive measures of stress corrosion of bolts in low temperature filling system are put forward.
Flange bolts are identifiable by a ridge below the bolt head. This built-in washer distributes the clamping load over a greater area. Commonly used by assembly line workers in the automotive industry, these bolts are designed to provide the same holding power as a washer.
Flange bolts are also know as frame bolts and are designed to provide the holding power as if a washer was installed. The skirt is the distributing agent of the entire clamping load. Pipe flange bolts are widely used in various applications, especially pipe fittings and plumbing. With their multi-feature function, they help make jobs easier and faster. Flange bolts are made from a variety of types and grades to suffice for the various applications flange bolts are used.
Bolts are commonly used fasteners in industrial production. Compared with carbon steel bolts and alloy steel bolts, stainless steel bolts are widely used in petroleum, chemical, metallurgy, energy, aerospace engineering and marine development fields, no matter in low temperature and high temperature environment. The accidents of stainless steel bolt failure and fracture often occur in petrochemical enterprises. Once the pipeline valve bolts have problems, resulting in loose sealing and leakage of toxic or flammable and explosive media, if not found and handled in time, it is easy to cause major accidents. Therefore, the use safety of stainless steel bolts is the main content of safety management of petrochemical enterprises and low-temperature equipment . Because the mass fraction of nickel in austenitic stainless steel is more than 8%, it has strong austenite stability. When the temperature is lower than – 196 ℃, there will be no transformation from austenite to martensite, and it has high structural stability and dimensional stability of parts . Therefore, austenitic stainless steel has excellent structural stability and corrosion resistance at room temperature and low temperature. It is an ideal material for connecting parts. The cross section of bolt parts is perpendicular to the direction of maximum stress, and the stress is concentrated at the thread, so there is no obvious plastic deformation and fracture will not occur . During the commissioning of a low-temperature filling system, leakage occurred at the flange connection of the pipeline. It was found that one of the six bolts at the flange connection had been broken. In the process of removing the other five bolts, one bolt was broken under the touch, the other two were broken when the wrench was removed, and the four bolts were all broken at the root. The cross section of the bolt shows corrosion traces. The bolt material is M12 × 70, and its manufacturing standard is GB/T 5782-2000. In order to prevent the fracture from happening again, the failure reasons of stainless steel bolts in the alternating environment of room temperature and low temperature cycle were analyzed. The main influencing factors of stress corrosion of stainless steel bolts were obtained, and the preventive measures of stress corrosion of bolts in low temperature filling system were put forward.
Physical and chemical tests and results
Macroscopic inspection of bolts
Macro and micro analysis of bolt fracture is the most effective method to find out the fracture reason [4-7]. Six bolts (No.1-6) replaced and one unused bolt in the same batch were observed. The macro morphology is shown in the figure. It can be seen from Fig. 1 that most of the surfaces and cross sections of the 6 bolts are corroded to varying degrees, and there are gray black and brownish red corrosion products in the corrosion area. Among them, the fracture parts of 4 bolts (No. 1-4) are at the right angle between the head and the screw part. The fracture surfaces of four broken bolts were observed by stereomicroscope. It was found that the macro morphology of the four broken bolts was similar: the fracture was relatively flat, the fracture was perpendicular to the bolt axis, and the fracture was covered with brown corrosion products. The macro characteristics of the whole mouth were cleavage fracture, and there were obvious pitting pits and pitting holes on the nut, as shown in Fig. 2.
Fig.1 macroscopic appearance of broken bolt
Fig.2 macro morphology of bolt fracture
Micro inspection of bolt fracture
The fracture surface of bolt was analyzed by SEM, and the surface morphology and corrosion product composition were observed. The micro morphology of bolt fracture is shown in Fig. 3, and the energy spectrum of corrosion products on fracture is shown in Fig. 4.
Fig.3 Analysis of fracture morphology of bolt
Fig.4 energy spectrum analysis of bolt material
It can be seen from Fig. 3 that the micro morphology of the bolt fracture is intergranular, and the secondary cracks between grains can be seen. The fracture material energy spectrum analysis results show that the composition of the fracture surface and the corrosion area on the bolt surface are basically the same, mainly containing Fe, Cr, Mn, O, s, CL elements, and the area without corrosion products mainly contains Fe, Cr (9.8%), Mn (17.6%) and Cu (1.2%) were compared with the fracture surface without corrosion products. It was found that the corrosion products of austenitic stainless steel were mainly enriched in Cl and s, indicating that chloride and sulfide directly promote the corrosion of austenitic stainless steel.
Inspection of bolt material composition
The chemical composition range of austenitic stainless steel (GB/T 1220-2007) for manufacturing failed bolts is shown in Table 1. Meanwhile, the chemical composition analysis results of fractured bolts are given.
Table.1 chemical analysis results of bolt materials
|Ingredients required in GB/T 1220-2007||≤0.1||≤1||≤2||≤0.05||≤0.03||15～20||8～19||≤4|
|Measured value of failure bolt||0.42||0.42||17.8||0.016||0.007||9.32||0.07||1.16|
It can be seen from table 1 that the element composition and content of the materials used for failed bolts deviate from the requirements in GB/T 1220-2007. In the measured values, the mass fraction of Ni and Cr is 0.07% and 9.32% respectively, while the mass fraction of Ni and Cr in the standard are 8% – 19% and 15% – 20%, respectively, and the content of Mn is 17.8%. In stainless steel, the equivalent ratio of chromium and nickel is an important parameter to determine the structure, mechanical properties and corrosion resistance of the material. The nickel content in the steel is far lower than the standard composition, and the austenite is extremely unstable, so it is easy to form martensite structure with high brittleness during cooling. Under tensile stress, brittle fracture occurs easily. Chromium is the main element to enhance corrosion resistance. The combination of chromium and oxygen forms a corrosion-resistant Cr2O3 passivation film, which can prevent corrosive ions from invading the substrate, and improve the corrosion resistance of stainless steel. Therefore, the higher the content of Cr, the stronger the repair ability of passive film of stainless steel, and the greater the density of passivation film In order to have obvious corrosion resistance, the mass fraction must be more than 12%. On the other hand, Ni is the main stable element of austenite in stainless steel. When the content of nickel is low, the stability of austenite decreases, the content of ferrite and martensite increases, and the plasticity and toughness of the material decrease, so the electrochemical properties of the matrix are not uniform. The unqualified elements of the failed bolts are the main reasons that lead to the substandard materials of the bolts, and also the main reasons for the unqualified mechanical properties and corrosion properties of the materials. The main reason of intergranular weakening is that the impurity elements in steel exceed the standard value. The main chemical composition content of the problem bolt does not meet the standard content of the chemical composition of austenitic stainless steel fastener, which leads to the decline of the corrosion resistance of the material and the low temperature performance of the bolt.
Discussion and analysis
Based on the above data of macro morphology, micro morphology and chemical composition, it is shown that the failure fracture of flange connecting bolt in low temperature filling system is stress corrosion fracture under low temperature environment. The causes of failure mainly come from environment and material itself, including temperature, chloride and sulfide in working environment, unqualified material of production bolt and so on. Austenitic stainless steel generally has excellent low-temperature toughness and is the main structural material in low-temperature environment. However, the phenomenon of low-temperature embrittlement is found in austenitic stainless steel with high manganese content . The former Soviet scholar borahe B  found that face centered cubic to face centered tetragonal transformation occurs in high manganese austenitic alloy at about – 100 ℃, accompanied by a sharp decrease in elastic modulus. Near this temperature, the impact toughness decreases significantly, from plastic fracture to brittle fracture. Therefore, low temperature phase change is the main reason for low temperature embrittlement of high manganese austenitic stainless steel. Chai Shousen  also confirmed that in Fe Mn alloy, with the increase of manganese content in the alloy, the cold brittle transition occurs
With the increase of temperature, the fracture appears as intergranular brittle fracture. It is found that Mn segregates along the grain boundary, and the higher the Mn content, the greater the degree of segregation. Preliminary analysis shows that Mn enrichment at grain boundary is the cause of cold embrittlement. Based on the first principle, Fu Ruidong  theoretically predicted the grain boundary doping effect of impurities or solute atoms in fe-38mn austenitic stainless steel by using material studio material calculation software. The results show that the grain boundary segregation of impurity atoms such as oxygen, sulfur, selenium, silicon and phosphorus reduces the intergranular fracture work of fe-38mn austenitic steel, showing a tendency to weaken the grain boundary and promote the occurrence of intergranular fracture. Although the weakening ability of Mn to grain boundary is small, it can also promote the intergranular embrittlement of fe-38mn austenite alloy. In the failed bolts, the manganese, sulfur and other elements are significantly higher than the specified values in the standard, and the content of MNS inclusions is significantly higher than that of qualified materials. Muto [11-12] and others studied the chemical composition of inclusions and the surface layer of passive film by means of photoelectron spectroscopy. The results show that sulfide inclusions are the initial source of pitting corrosion of stainless steel. Henthorne  found that chromium rich sulfide can resist the dilution of oxidizing acid, and pointed out that MNS inclusions are more likely to cause pitting corrosion than CRS inclusions. H. Krawiec  and others used electrochemical methods to study and found that MNS inclusions are usually dissolved along the interface at a very fast speed in electrochemical tests, but under natural corrosion conditions, inclusions are not uniformly dissolved along the interface, and many inclusions are dissolved separately and independently. S. From the atomic level, J Zheng  and others found that MNS inclusions were wrapped with nano mncr2o4 octahedral crystals, which formed mncr2o4-mns micro area nano battery, resulting in MNS dissolution and pitting corrosion. The pitting corrosion mechanism of stainless steel is occluded micro cell corrosion effect . The anode is mainly the anodic dissolution reaction of Fe, Cr, Ni and other metals, while the cathodic reduction reaction occurs on the adjacent metal surface to form pitting pits. The chloride outside the hole moves into the passive film / stainless steel interface, which results in the increase of chloride ion in the hole, which forms metal chloride MCL with metal cations in the hole, and the metal chloride has a large volume ratio. This makes the passivation film rupture, forming an erosion ion into the channel, plus the occurrence of corrosion, the formation of microcracks. Under the action of axial tensile stress, microcracks propagate. Because the material of the failed bolt is formed by the unqualified high manganese austenitic stainless steel, the corrosion mainly occurs at the grain boundary where the impurities segregate. Under the synergistic effect of tensile stress, the crack propagates rapidly along the grain boundary and forms obvious grain boundary fracture.
Based on the above analysis and discussion, it can be concluded that the causes of stress corrosion failure of flange connecting bolts in low temperature filling system and the preventive measures are as follows:
- 1) The high manganese content increases the ductile brittle transition temperature of austenitic stainless steel, which increases the brittleness and fracture tendency of austenitic stainless steel.
- 2) The inclusion formed by sulfur and manganese weakens the intergranular bonding energy, which is one of the factors causing material embrittlement, and the existence of inclusions is also the main metallurgical factor causing pitting corrosion.
- 3) The chromium content in the failure bolt material is lower than the value in the standard, which reduces the passivation ability of stainless steel, makes pitting grow and become microcracks; the nickel content is far lower than the value in the standard, and the austenite structure is extremely unstable. In the low temperature environment, the steel matrix changes from face centered cubic to body centered tetragonal structure, which reduces the toughness of the material.
- 4) According to the working conditions of the filling system, it is suggested to select A2-70 austenitic stainless steel with high nickel and low manganese content, which meets the standard range, so as to improve the low temperature ductile brittle transition temperature of the material. When the temperature is high, the upper limit value of chromium content should be taken to increase the corrosion resistance of the material.
Source: China Pipe Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading flange 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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