316L belongs to austenitic stainless steel, corresponding to the domestic brand 022Cr17Ni12Mo2, which has good plasticity and corrosion resistance.
The addition of Mo element in this stainless steel greatly improves its pitting resistance.
Therefore, 316L stainless steel is widely used in petrochemical, pharmaceutical and other industries.
A 316L stainless steel coil used in an enterprise was found to have leakage and perforation on the pipe wall during the use.
The working medium inside the pipe is water vapor, with a working pressure of 0.9MPa.
The medium outside the pipe is strong alkali and copper powder, with a working pressure of 1.0MPa.
There is a pressure difference between the inner and outer walls.
The failure reason is analyzed and studied by experiment.
1. Macro analysis
1.1 Visual inspection
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 Fig. 1a.
On the inner wall, there are four longitudinally arranged small 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 macro morphology of leakage hole
1.2 Enlarged macro inspection
When observed with a variable magnification stereomicroscope, Fig. 2a is a macro photo of the inner wall leakage hole after 40 times magnification;
Fig. 2b shows the shape of the leakage hole after cutting.
The space inside the hole is relatively large, with a total volume of about 4mm3.
The hole wall is undulating and irregular.
Fig. 2 leakage hole morphology
2. Detection and analysis
2.1 Spectral chemical composition analysis
The full spectrum direct reading spectrometer is used to analyze the chemical composition of the samples around the leakage hole.
The results (average value) are shown in Table 1.
Compared with the composition of TP316L material in ASME SA213, it meets the requirements of the standard.
Table 1 chemical composition (mass fraction) analysis results (%)
2.2 Tensile test
The electronic universal testing machine is used to take samples on the coil for tensile test.
The results are shown in Table 2.
The fracture morphology of the sample is plastic fracture.
Through comparison, it can be seen that the tensile strength, yield strength and elongation after fracture of the material meet the ASME SA213 standard.
Table 2 mechanical property test results
|Type||Tensile strength Rm / MPa||Yield strength Rpo.2/MPa||Elongation after fracture A (%)|
2.3 Process performance test
The electronic universal testing machine is used to conduct the flaring and flattening test.
The results are shown in Table 3.
The appearance of the flaring sample after the test is shown in Fig. 3.
There is no crack on the inner wall of the pipe.
The morphology of the flattened specimen is shown in Fig. 4.
Cracks are generated on the tensile surface of the outer wall of the flattened specimen.
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
Pressing plate spacing / mm
No crack on the inner wall
Cracks at tensile deformation of outer wall
Fig. 3 flared sample
Fig. 4 crack on tensile surface of flattened specimen
2.4 Metallographic analysis
Micro metallographic samples shall be processed on the inner and outer walls and inside of the coil.
The cross-section of the samples shall be ground and polished, and then observed with a metallographic microscope.
It can be seen from Fig. 5a that there are “warped skin” cracks on the inner wall of the coil, and there are microcracks at the bottom of the “warped skin”;
It can be seen from Fig. 5b that there are a large number of microcracks on the outer wall, which originate at the defects or corrosion pits and expand along the inclusions;
It can be seen from Fig. 5c that there are a large number of granular, strip-shaped and massive inclusions inside, with the size of 5 ~ 25 μ m.
Fig. 5 different areas of coil materials
The microstructure of the corroded sample is austenite with an average grain size of 6.5, and precipitates are found on the grain boundary.
It can be seen from Fig. 6b that there are a large number of deformation slip lines on the inner wall, indicating the existence of residual stress;
It can be seen from Fig. 6c that there are transgranular and intergranular cracks on the outer wall.
Fig. 6 metallographic diagram
2.5 SEM analysis
Scanning electron microscope is used to observe the crack fracture. Fig. 7a is an old fracture.
It can be seen from the inner wall morphology of the leakage hole in Fig. 7b that there are “mud pattern” corrosion products on the surface.
Fig. 7 SEM appearance of failure part
2.6 Energy spectrum analysis
Energy spectrum analysis was carried out on the old fracture samples 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 mixed with Cu, Na, etc.
Table 4 energy spectrum analysis results (mass fraction) (%)
Fig. 8 energy spectrum of analysis point
3. Comprehensive analysis
Based on the above test data, the chemical composition, tensile strength, yield strength and elongation after fracture of coil TP316L stainless steel meet the requirements of ASME SA213.
The flaring test is qualified, cracks appear on the tensile surface of the outer wall of the flattened test tube, and the open end is an old crack.
Metallographic analysis and SEM + EDS show that the structure is austenitic stainless steel, but there are granular inclusions with different sizes, and there are “mud pattern” corrosion products on the surface, especially at the leakage hole.
The pore wall fluctuates irregularly, and there are a large number of microcracks at the “warped skin” and expand along the inclusions.
4. Conclusions and suggestions
The main causes of leakage of 316L stainless steel coil are a large number of inclusions, oxide corrosion products and microcracks in the material.
The continuity of the matrix is destroyed due to the presence of inclusions.
Under the joint action of residual stress and pressure difference between the inner and outer walls, cracks are easily generated and expanded at the inclusion.
However, the inclusion contacts with the high-temperature steam medium.
Since the corrosion resistance of the inclusion is far lower than that of the material itself, the inclusion is corroded, and eventually holes are formed in the pipe wall, resulting in perforation of the pipe wall and leakage.
In order to avoid similar leakage failure, it is recommended as follows:
1) For the service environment of the coil, the high-quality 316L stainless steel material with few inclusions should be selected.
2) Control the purity of the working medium to avoid the influence of corrosive substances on the pipe as much as possible.
3) The coil components shall be heat treated after bending and welding to effectively release the residual stress.