Failure analysis on corrosion cracking of bellows assembly of stainless steel globe valve
Objective: In view of the fact that the double-layer stainless steel 304 bellows component of the bellows stop valve in a refinery cracked, which caused the failure of the stop valve, the cause of corrosion cracking was found through failure analysis.
Method: The failure and cracking of stainless steel stop valve double-layer bellows assembly was inspected. The metallographic structure and chemical composition of the failure component were analyzed by metallographic microscope and direct reading spectrometer, and the fracture morphology and characteristics of the components were observed by electron microscope.
Result: Cleavage surface and cleavage steps can be observed on the fracture surface of outer pipe wall of bellows assembly, and the existence of corrosion products can be seen, which is a typical feature of stress corrosion fracture of austenitic stainless steel; there are dimples on the fracture surface of inner pipe wall, which belongs to mechanical fracture. The analysis of metallographic structure and chemical composition shows that the stainless steel used in bellows components meets the design and application requirements. The medium detection results of heat transfer oil show that the content of chlorine is 55 mg / kg, the total sulfur content is 350 mg / kg, and the content of harmful ions is high.
Conclusion: The failure mechanism of the inner wall and the outer wall of the double-layer bellows assembly is different: the outer wall is stress corrosion cracking caused by Cl -, and the inner wall is instability of the bellows assembly caused by the failure of the outer wall, resulting in a sharp decrease in compressive strength and life, and ductile fracture under the action of stress. It is suggested to reduce the content of harmful ions in the heat transfer oil and to use materials with better corrosion resistance.
Due to its simple and reasonable structure and reliable sealing performance, stainless steel globe valves are widely used in pipelines with flammable, explosive and toxic media such as steam and heat transfer oil. The bellows structure is adopted inside, and the lower end of the bellows assembly is welded to the valve stem. An axially movable metal shell is provided between the valve stem and the process fluid in the valve body to form a dynamic seal to prevent the process fluid from eroding the valve. Rod; The other end is placed between the valve body and the valve cover to form a static seal [1-2]. The bellows assembly bears the pressure, temperature and corrosion of the medium while withstanding the compression and extension of the valve stem, and is prone to cracking or fracture .
A bellows stainless steel globe valve of an oil refinery lost its sealing effect. After inspection, it was found that the double-layer bellows component of the globe valve cracked in a large area near the welding part of the bonnet, causing serious damage. The material grade of the bellows assembly is 06Cr19Ni10, the contact medium is a certain type of heat transfer oil, and the working temperature is 230 yi. In order to find the reasons for the corrosion and cracking of the bellows assembly, the failure analysis of the cracked bellows assembly was carried out.
Samples were taken from the fractures of the inner and outer pipe walls of the failed bellows assembly, and after degreasing and cleaning, the microscopic morphology of the fractures of the outer and inner pipe walls was observed with the LEO-1450 scanning electron microscope.
Samples were taken from the bellows components, inlaid with epoxy resin, and then polished with 240#, 360#, 600#, 800#, 1000# water sandpaper in turn, and then polished by a polishing machine. Rinse with distilled water, etch it in aqua regia alcohol solution for 3min , and observe its structure with 4XC metallurgical microscope.
Analysis of chemical composition and corrosion products of pipes
Samples were taken on the inner and outer pipe walls of the bellows assembly, and the composition was detected with the QSN750 direct-reading spectrometer of the German OBLF company. All the major elements in the stainless steel were sparked. An energy spectrometer was used to analyze the corrosion product composition on the fracture of the inner and outer pipe walls, and the chlorine distribution scan was performed on the side of the fracture on the outer pipe wall.
Macroscopic morphology of fracture
The macroscopic morphology of the fracture of the failed stop valve and its corrugated components is shown in Figure 1. The fracture zone of the corrugated pipe component is located at the welding place of the corrugated pipe component and the upper end of the valve stem and bonnet, as indicated by the arrow in Figure 1a. The corrugated component is composed of two thinner pipe walls inside and outside, with a single layer thickness of 0.3mm. It can be seen from Figure 1b that serious corrosion has occurred on the outer surface of the outer pipe wall, and obvious pits and cracks can be seen; there are no obvious corrosion marks on the inner pipe wall surface. At the same time, it can be observed that the degree of deformation of the bellows assembly shifts in the axial direction, and the phenomenon of instability appears, and the bellows assembly breaks at the position of the first corrugated wave crest of the bellows obliquely to the trough, and the fracture shape is irregular.
Fig.1 Macrograph of the failed sample: a) the failed sealed valves, b) the bellows subassembly
Microscopic morphology of fracture
Observe the samples taken from the fracture of the inner and outer pipe wall of the failed corrugated component. The micro morphology of the fracture of the outer pipe wall is shown in Figure 2. Figure 2a shows that there is a loose corrosion product film on the side of the fracture surface close to the medium, which has obvious corrosion characteristics; Figure 2b shows the river-like pattern on the fracture surface, and the fracture surface is clearly visible and the cleavage steps are clear. The presence of corrosion products can be observed, which is a typical feature of stress corrosion cracking in austenitic stainless steel; in Figure 2c, obvious signs of secondary cracks can be observed, cracks branching and spreading, intersecting each other, forming brittle fractures, these are also Typical characteristics of stress corrosion cracking fracture.
Fig.2 Micrograph of the fracture at the outer wall of bellows subassembly: a) fracture surface of outer wall before cleaning, b) the cleavage fracture on the outer wall, c) the secondary crack on the outer wall
The microscopic morphology of the fracture of the inner pipe wall is shown in Figure 3. The dimples on the fracture are clearly visible, indicating that the fracture of the inner pipe wall of the corrugated component is a ductile fracture.
Fig.3 Micrograph of the fracture at the inner wall of bellows sub-assembly
Corrosion products and chlorine distribution
The energy spectrum analysis of the corrosion products on the fracture of the inner and outer pipe walls is carried out, and the results are shown in Table 1. From the data in Table 1, it can be seen that the section steel is 18-8 stainless steel. In addition to Fe, Cr, Ni, Si, Mn, the corrosion products also contain O from oxides and O adsorbed from the air. The presence of Cl element was detected on the fracture of the inner pipe wall, and the mass fraction was only about 0.1%; the content of Cl element on the fracture of the outer pipe wall was higher than that of the inner pipe wall, and the mass fraction was close to 2%.
A surface scan was performed on the side of the outer fracture to analyze the distribution of Cl element near the fracture. The result is shown in Figure 4. It can be seen from Figures 4a and b that the Cl element is more densely distributed on the side near the fracture and where the secondary cracks appear, and enrichment occurs at the cracks, which fully shows that the cracking of the outer wall of the bellows assembly is caused by Cl- Stress corrosion cracking.
Tab.1 EDS analysis results of corrosion products at the fracture
|Position||Mass fraction /%|
|Fracture of outer pipe wall||25.07||0.57||1.94||23.36||0.66||39.48||8.92|
|Fracture of inner pipe wall||19.07||0.62||0.09||20.08||0.88||50.74||8.52|
Fig.4 Chlorine distribution on the side face of fracture at the outer wall of bellows subassembly: a) micrograph of the fracture at the outer wall, b) EDS mapping of Cl element at the fracture
Metallographic structure and chemical composition
In order to verify whether the failed material is a qualified stainless steel material, the metallographic structure and chemical composition are analyzed. First, take a metallographic sample near the fracture of the bellows assembly, perform rough grinding, fine grinding, and polishing. After etching with an aqueous solution of copper sulfate and hydrochloric acid, observe the metallographic structure under a metallographic microscope , as shown in Figure 5. From Figure 5, the austenite structure can be observed, with a small amount of twins, and the grain size is relatively uniform.
Fig.5 Metallographic microstructure of the bellows subassembly
Secondly, a direct-reading spectrometer was used to analyze the chemical composition of the bellows components. The analysis results are shown in Table 2. It can be seen from Table 2 that the chemical composition of the corrugated component meets the requirements of 06Cr19Ni10(304) in GB/T20878-2009 “Stainless steel and heat-resistant steel grades and chemical composition”.
Tab. 2 The chemical composition of bellows subassembly
|Sample type||Mass fraction /%|
|Standard sample ||≤0.08||≤1.00||≤2.00||≤0.030||≤0.045||18.00-20.00||8.00-11.00|
Discussion of results
The microscopic fracture morphology of the inner and outer pipe walls of the corrugated pipe presents different fracture characteristics: dissociation fractures and secondary cracks appear on the fracture of the outer pipe wall, and there are obvious signs of corrosion at the same time, which occurs with austenitic stainless steel in a chlorine-containing medium. The characteristics of stress corrosion cracking are the same ; but dimples appear on the fracture of the inner pipe wall, which conforms to the characteristics of ductile fracture .
Analysis of failure factors
Generally speaking, the stress corrosion cracking of materials is the result of the three effects of material, environmental media and stress, while ductile fracture is generally related to material and stress, and has little to do with media. For this reason, the fracture failure of the corrugated component can be analyzed from the following three aspects, so as to know the cause of its failure and fracture.
From the metallographic microstructure (see Figure 5) and chemical composition analysis (see Table 2), it can be seen that the grain size distribution on the metallographic structure of the bellows component is uniform, there are a few twins, and its chemical composition is in the national standard. Within the range of chemical composition, the stainless steel used should be 18-8 type 06Cr19Ni10 austenitic stainless steel, which meets the design and use requirements.
Therefore, it is believed that the failure and cracking of the bellows components have nothing to do with the improper use of materials. However, this type of 18-8 austenitic stainless steel is prone to local corrosion in some environments, so material factors cannot be ruled out.
After inspecting the inner and outer pipe wall surfaces of the bellows assembly, it was found that there were obvious corrosion pits and cracks on the outer surface of the outer pipe wall (see Figure 1b), but no obvious macroscopic corrosion and cracks were seen on the inner pipe wall surface. Under working conditions, the outer pipe wall of the corrugated pipe assembly is in contact with the working medium.
The working medium is heat transfer oil. The test and analysis results provided by the manufacturer show that the chlorine content in the heat transfer oil is 55mg/kg, the total sulfur content is 350mg/kg, and it contains a small amount (about 0.06%) of moisture. From the corrosion product analysis results (Table 1), it can be seen that the presence of Cl was detected on the surface of the fracture, and the mass fraction of chlorine on the fracture of the outer pipe wall reached 1.94%. From the perspective of the element distribution on the side of the fracture (Figure 4), the presence of chlorine is Enrichment occurs near the fracture and secondary cracks. Considering that the bellows assembly belongs to the 18-8 type austenitic stainless steel, this type of stainless steel is easy to form pitting corrosion in the aqueous medium containing chlorine and H2S, is sensitive to stress corrosion, and easily constitutes the origin of cracks, so it is believed that The working medium is most likely the medium factor for the failure of the bellows assembly.
For the outer pipe wall, there are cleavage fractures and secondary cracks on the fracture surface (Figure 2), and there are obvious corrosion marks. These are the typical features of stress corrosion cracking of austenitic stainless steel, indicating that the failed parts have withstood A certain amount of stress. From the analysis of the force, the lower end of the bellows assembly is welded to the valve stem, and the upper end is connected to the valve cover. When the valve is opened or closed, the bellows are in compression and tension respectively, and bear axial compressive stress and tensile stress; at the same time , The working temperature of the bellows assembly is 230 yi, and the steam pressure of the heat transfer oil is about 1 MPa. When the bellows assembly is in a stretched state, the corrugated sidewall section is inclined, and the diameter of the wave crest remains unchanged. When it is large, the angle of inclination of the side wall section is larger, and the connection between the side wall and the arc of the crest is simultaneously subjected to the bending moment and load caused by the inclination, forming a local stress concentration [9-10]; in addition, it will be formed during the processing Residual stress . When the three are superimposed to reach the critical stress scc that causes stress corrosion cracking of the bellows, the stress condition of stress corrosion is satisfied. For the inner pipe wall, there is a dimple morphology on the fracture (see Figure 3), which is a ductile fracture, indicating that after the inner pipe wall undergoes a certain stage of plastic deformation, the crack expands to a certain length, which reduces the effective section of the component. , Resulting in a one-time rapid fracture . When there is no crack in the outer pipe wall, the stress condition of the inner pipe wall and the outer pipe wall is the same; but when the outer pipe wall cracks due to stress corrosion cracking, the meridian bending stress and circumferential direction generated by the internal pressure Under the action of the resultant force of the membrane stress, the bellows component undergoes plastic deformation, and the plane is unstable, and the displacement is concentrated on one or several corrugations, which greatly reduces the life and compression resistance of the bellows component . Therefore, the inner pipe wall also satisfies the stress conditions for cracking.
The material of the bellows is 06Cr19Ni10(304) austenitic stainless steel, which has a strong tendency to stress corrosion cracking under the combined action of tensile stress and appropriate active medium. The content of Cl and S in the heat transfer oil is high. The presence of Cl- will destroy the passive film on the surface of the stainless steel, causing the surface to form corrosion pits, causing stress concentration, and accelerating the initiation and propagation of cracks. Experiments have shown that for 18-8 type stainless steel, stress corrosion will occur when the Cl- content in the medium reaches 25mg/kg[14-15], and the Cl content in the medium where the component is located reaches 55mg/kg, which is higher than this. condition. At the same time, the working temperature of the bellows assembly is 230 yi, and the temperature of stress corrosion of this material stainless steel is 70~300 yi. Generally, the higher the temperature, the greater the stress corrosion sensitivity . In addition, when the axial stress on the wave crest is superimposed with the residual tensile stress and reaches the critical stress scc that causes stress corrosion cracking of the bellows, the stress condition of stress corrosion is satisfied. Therefore, under the combined action of Cl-, S and axial residual tensile stress in the heat transfer oil medium, 18-8 stainless steel, which is sensitive to stress corrosion cracking, experienced stress corrosion cracking and failed.
For the outer wall of the corrugated pipe, its outer surface is in contact with the corrosive medium, the corrosive medium contains Cl-, and the working temperature is in the stress corrosion sensitive range of austenitic stainless steel, which meets the conditions of stress corrosion of stainless steel, and the fracture surface It conforms to the characteristics of stress corrosion, so it can be inferred that the fracture of the outer wall of the bellows is the stress corrosion cracking caused by Cl-. For the inner wall of the corrugated pipe, it is not in direct contact with the corrosive medium, and its fracture morphology is ductile fracture morphology. Due to the rupture of the outer pipe wall, the bellows assembly becomes unstable, and the displacement is concentrated on the corrugation near the valve cover. The pressure resistance and service life of the bellows are drastically reduced, which will cause the bellows to break quickly or open the valve several times. Fracture occurs, so it is believed that the fracture of the inner pipe wall is caused by the decrease of service life and compression resistance caused by the instability of the bellows.
The failure and fracture modes of the inner and outer pipe walls are different: the failure of the outer pipe wall is caused by stress corrosion cracking caused by Cl-; the inner wall cracking is caused by the instability of the bellows and the reduction of the service life and compressive strength. The occurrence of stress between chlorine and sulfur and components in the heat transfer oil medium is the main media factor and mechanical factor leading to its failure. In order to prevent this kind of cracking failure of the bellows assembly under actual working conditions, it is recommended to reduce the content of harmful ions in the heat transfer oil; avoid the concentration or existence of stress; use a material with better corrosion resistance, such as 316L stainless steel. Bellows assembly.
Authors: Ming Wei Ping 1,2, Zhang Yi Yue 3, Wang Chuan Zhi 1,2, Xu Yuan 1,2, Liu Xin Fang 1,2, Xiang Jin Ping 1,2
Source: China Valve Manufacturer – Yaang Pipe Industry Co., Limited (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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