Case sharing: leakage failure analysis of 316L stainless steel pipe
316L belongs to austenitic stainless steel, corresponding to domestic brand 022cr17ni12mo2, which has good plasticity and corrosion resistance. The addition of Mo element in the stainless steel greatly improves its pitting resistance. Therefore, 316L stainless steel is widely used in petrochemical, pharmaceutical and other industries. The 316L stainless steel pipe used in an enterprise was found to have leakage and perforation on its wall. The working medium in the pipe is water vapor, the working pressure is 0.9mpa, the medium outside the pipe is strong alkali and copper powder, the working pressure is 1.0MPa, and there is pressure difference between the inner and outer walls. In this paper, the failure reason is analyzed and studied.
From the external macroscopic visual inspection, it is found that the leakage hole is a small hole with a diameter of about 2mm on the outer wall, as shown in Figure 1a. In the inner wall, there are four longitudinal arranged holes with a diameter of about 1mm, and a large number of “warped skin” cracks can be seen, as shown in Fig. 1b.
Fig.1 macroscopic appearance of leakage hole
Enlarge macro inspection
According to the observation of zoom stereomicroscope, figure 2a is the macro picture of the inner wall leakage hole after being magnified by 40 times; figure 2b is the shape of the leakage hole after being cut, the space inside the hole is relatively large, the total volume is about 4mm3, and the wall of the hole is uneven and irregular.
Fig.2 appearance of leakage hole
Detection and analysis
Composition analysis the chemical composition of the samples around the leakage hole is analyzed by the full spectrum direct reading spectrometer. The results (average value) are shown in Table 1. Compared with the TP316L material composition in ASME SA213, it meets the standard requirements.
Table.1 analysis results of chemical composition (mass fraction) (%)
The electronic universal testing machine is used to sample the steel pipe for tensile test, and the results are shown in Table 2. The fracture morphology of the sample is plastic fracture. Through comparison, the tensile strength, yield strength and elongation after fracture of the material are all in accordance with ASME SA213 standard.
Table.2 test results of mechanical properties
Process performance test
The electronic universal testing machine is used for expanding and flattening test, and the results are shown in Table 3. After the test, the morphology of the expanded sample is shown in Figure 3, and there is no crack on the inner wall of the tube. The morphology of the flattened sample is shown in Figure 4. Cracks appear on the tensile surface of the outer wall of the flattened sample. The outer wall of the crack opening end is an old fracture, and the crack tip is a new fracture. It can be seen that there are old cracks on the outer wall of the tube.
Table.3 process performance test results
Fig.3 flared sample
Fig.4 crack on tensile surface of flattened sample
The inner and outer walls of the steel pipe and the inner parts are processed with micro metallographic samples. The cross section of the samples is grinded and polished, and then it is observed after being magnified with metallographic microscope. It can be seen from Fig. 5A that there are “warped skin” cracks on the inner wall of the steel pipe and microcracks at the bottom of the “warped skin”; from Fig. 5b, it can be seen that there are a lot of microcracks on the outer wall, which sprout at the defects or corrosion pits and expand along the inclusions; from Fig. 5C, it can be seen that there are a lot of granular, strip and block inclusions in the inner wall with the size of 5 ~ 25 μ M.
Fig.5 different areas of steel pipe materials
After corrosion, the microstructure of the sample is austenite, with an average grain size of 6.5. It is found that there are precipitates on the grain boundary, as shown in Fig. 6A. From Fig. 6B, it can be seen that there are a large number of deformation slip lines on the inner wall, indicating the existence of residual stress; from Fig. 6C, there are transgranular and intergranular cracks on the outer wall.
Scanning electron microscope was used to observe the crack fracture. Figure 7a is an old fracture. It can be seen from the inner wall morphology of the leakage hole in Figure 7b that there are “mud pattern” corrosion products on the surface.
Fig.7 SEM appearance of failure position
Energy spectrum analysis
The energy spectrum analysis is carried out for the old fracture sampling in Fig. 7a, as shown in Fig. 8. The results are shown in Table 4. The corrosion products are relatively complex, mainly composed of oxides, and also mixed with Cu, Na, etc.
Table.4 energy spectrum analysis results (mass fraction) (%)
Figure.8 energy spectrum of analysis point
Based on the above test data, the chemical composition, tensile strength, yield strength and elongation after fracture of TP316L stainless steel pipe meet the requirements of ASME SA213 standard. The expansion test is qualified, cracks appear on the tensile surface of the outer wall of the flattened test tube, and the open end is old cracks. Metallographic analysis and SEM + EDS show that the structure of austenitic stainless steel is austenitic, but there are granular inclusions of different sizes, and there are “mud pattern” corrosion products on the surface, especially at the leakage hole. There are a lot of microcracks in the “warped skin” and expand along inclusions.
Conclusions and suggestions
The main reason of 316L stainless steel pipe leakage is that there are a lot of inclusions, oxide corrosion products and microcracks in the material. The matrix continuity is destroyed by inclusion. Under the action of residual stress and pressure difference between inner and outer walls, the inclusion is easy to crack and expand. The inclusion contacts with high temperature water vapor medium. Because the corrosion resistance of the inclusion is far lower than the material itself, the inclusion is corroded, and finally a hole is formed inside the pipe wall, leading to the perforation and leakage of the pipe wall. In order to avoid similar leakage failure, it is recommended as follows:
- 1) For the service environment of the steel pipe, 316L stainless steel with low inclusions should be selected.
- 2) Control the purity of working medium and avoid the influence of corrosive substances on the pipe as much as possible.
- 3) After bending and welding, the steel tube members are heat treated to effectively release the residual stress.
Source: China Steel Pipe 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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