Cause analysis and Countermeasures of 304 stainless steel pipe fitting crack
During the inspection of a new atmospheric and vacuum device (about six years idle), cracks were found in (06cr19ni10) 304 stainless steel pipe fittings (flange, elbow, tee). The analysis of the anatomy sampling of two flange with penetrating cracks was carried out, and the surface inspection, metallographic examination and hardness test were conducted for other pipe fittings. The results of various inspection and inspection are analyzed comprehensively. It is concluded that the main reason for the crack is that the solid solution heat treatment of the pipe fittings fails to meet the requirements, the metallographic structure is not uniform, the precipitate phase of chromium carbide and the suspected cracks are around the precipitate phase; 304 stainless steel (06cr19ni10) pipe fittings must strictly control the quality of solid solution heat treatment and metallographic inspection of the products.
304 stainless steel (06cr19ni10) has good heat resistance, corrosion resistance, cold and hot working performance and welding performance. It is widely used in the high temperature corrosion parts of refinery chemical plant, which is very important to ensure the safe operation of the unit. In the inspection of a new atmospheric and vacuum device, it was found that there were cracks in 304 stainless steel (06cr19ni10) pipe fittings (flange, elbow, tee), which seriously affected the safe operation of the device. In order to analyze the causes and mechanism of cracks in 304 stainless steel pipe fittings and eliminate the hidden dangers, the anatomy sampling and analysis of two flange with penetrating cracks were conducted, and surface inspection, metallographic examination and hardness test were carried out for other parts. The comprehensive analysis shows that the microstructure of austenite is uneven and the precipitate of chromium carbide is not uniform in metallographic structure, and the precipitate phase is granular, strip and chain like, and the cracks are suspected around the precipitate phase, which leads to the crack generation and expansion. If no cracks are found in the outer surface inspection, there is also the possibility of cracks on the inner surface or cracks in the process of operation. It is necessary to deal with them carefully.
Surface inspection of stainless steel pipe fittings
External surface penetration inspection
A total of 27 pipelines were tested, with 311 pieces flanges detection pieces. According to NDT and penetrant test (NBT47013.5), 14 pieces of defective pipe fittings were detected, 9 pieces flanges, 3 pieces large and small heads, 1 piece tee and 1 piece elbow. Example: the surface crack is found on the whole ring, flange side, base metal and near weld joint (see Fig.1) in flange size DN150.
11 pieces pipeline flanges and elbows are tested, numbered 1-11 ° in sequence. The metallographic examination is carried out according to the requirements of GBT 17455-2008 “metal phase complex technology for nondestructive testing surface inspection”. See Table 1 and figure 2-4 for the inspection results. The metallographic structure of flange is austenite + precipitate phase, and the precipitate is granular, strip and chain, among which, there are suspected cracks around the precipitate in 11 ° point structure.
Figure 1 p-107
Fig. 2 1 metallographic structure of flange (400 ×）
Fig. 3 6 metallographic structure of flange (400 ×）
Fig. 4 metallographic structure of 11 ° elbow (400 ×）
The Brinell hardness value of GB/T 20878-2007 stainless steel 304 is no more than 201. The results of inspection are within the qualified range. See Table 1 for hardness test results.
The ferromagnetic examination of all 304 stainless steel flange with magnet shows strong ferromagnetism, which shows abnormal austenite structure.
Chemical composition review
The component standard values of stainless steel according to GB/T 20878-2007 304 are shown in Table 2.
Table 1 Statistics of hardness test results of metallographic examination results
Four types of flange with through crack are taken for component test. See Table 2 for the results.
The average value of the test is compared with the standard component. The components in the sample are similar to the standard value, and Mn is about 0.57 lower than the standard value.
Table 2 test value of sample components%
Metallographic inspection of two flange with through crack
The microstructure of the base metal is fine and even, but there are a lot of precipitates on the grain boundary. By EDS analysis, the precipitate phase is Cr rich phase, which may be Cr23C6 carbide. Some black bars were observed at the grain boundary of the base metal by optical microscope. By SEM analysis, the strip was microcracks along the grain boundary. The metallographic photos of the weld and base metal of the sample are shown in Fig. 5.
The main crack and secondary crack are all intergranular cracks. The metallographic photos of the cracks are shown in Fig.6.
Fig. 5 metallographic picture of sample
Fig. 6 metallographic picture of flange crack
Analysis of crack cause of 304 stainless steel pipe fitting
- 1) The microstructure of the base metal is typical single-phase austenite, and there are many thin strip microcracks in the metallographic diagram. There are a lot of precipitates on the grain boundary of the base metal. By EDS analysis, the precipitate is Cr rich phase, which may be Cr23C6 carbide.
- 2) The weld structure of the weld is columnar austenite structure, and the austenite columnar crystal will deform to some extent near the fusion line.
- 3) According to the metallographic photos of the crack section, the crack form is preliminarily judged as intergranular cracking. There are a lot of carbon rich substances (about 70%) and precipitates at the grain boundary.
- 4) 304 stainless steel flange is in accordance with the current product standard, and the supply state is solid solution treatment. The metallographic structure of flange is austenite + precipitate. If solid solution treatment is carried out according to the standard requirements, the precipitate phase should be absorbed.
- 5) Ferrite mainly exists in precipitate phase, and there are many ferrite, so flange shows strong ferromagnetism.
- 6) The chemical composition and hardness test basically meet the standard requirements.
- Based on the above inspection and analysis, 304 stainless steel pipe fittings are not solid solution treated as per the standard requirements, and there are a lot of precipitates in the metallographic structure. The precipitate phase is distributed in granular, strip and chain shape, and the suspected cracks around the precipitate phase are the main factors for the crack generation and growth.
Conclusions and suggestions
- 1) After the installation of 304 stainless steel pipe fittings (not put into production), there is the possibility of cracks at room temperature.
- 2) The main reason for the crack of 304 stainless steel pipe fitting is the insufficient solution treatment and a large number of precipitates exist in the grain boundary of austenite structure.
- 3) The solid solution heat treatment of 304 stainless steel pipe fittings shall be strictly controlled and metallographic examination shall be carried out to ensure that the solid solution heat treatment meets the requirements.
- 4) For cylindrical pressure equipment, the inner wall stress is higher than the outer wall stress. When the outer wall is defective, the possibility of defects in the inner wall is not lower than that of the outer wall in theory. Therefore, the defects of the inner wall of the pipe should also be paid special attention.
Author: Tao Tao
Source: China Stainless Steel Pipe Fitting 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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