Cause Analysis of WP304WX Pipeline Leakage
Leakage occurred in stainless steel pipeline of a chemical enterprise after 5 years of service. The leakage location is the heat affected zone (the connection mode of stub end, tee and straight pipe is shown in Figure 1) near flanging of stub end and tee welding seam. The leakage location is about 50 mm to stub end side. The working medium in the leaking pipeline is 100% methanol at room temperature, the working pressure of the pipeline is 0.24MPa, the material of the tee is A403-WP304S, the material of the straight pipe is A312-TP304, and the material of the stub end is A403-WP304WX. In order to find out the cause of leakage, physical and chemical tests and analysis were carried out on the sampling of leakage location.
Fig. 1 Macroscopic appearance of leakage WP304WX pipeline
Fitting Classes for WP Grades
|CR||Corrosion resistant grade. S through WU Classes do not apply.|
Welded, RT or UT all pipe starting material welds made with filler metal. All fitting manufacturer welds except as excluded in A/SA403 paragraph 5.4.3.
If UT is substituted for RT, all welds must be examined 100% for their length as required by A/SA403 paragraph 5.6
|WP-WX||RT all welds with or without filler metal.|
|WP-WU||UT all welds with or without filler metal.|
|WPgrade-grade-class||Insert grades (304-304L) and class (S,W,WX, or WU). Applies to fittings meeting all chemical and physical requirements of both grades.|
The heat-affected zone (the box area in Fig. 1) near the flanging of stub end and tee weld seam was detected. It was found that there were cracks in this area. The direction of crack propagation was perpendicular to the axis of stub end pipeline, and the length was about 25 mm. The depth of crack penetrated the whole thickness of stub end. The surface of both sides of the crack was not seen. It is covered with corrosion products. The macroscopic morphology of the crack is shown in Fig. 2.
Fig. 2 Macro-morphology of cracks in leakage zone
1.2 Chemical Composition Analysis
The chemical composition of stub end, tee, stub end and base metal joints of the failure parts shown in Fig. 1 is analyzed. The results are shown in Table 1.
Table 1 Chemical composition (mass fraction) of samples
The chemical composition of stub end areas with leakage and stub end edges were analyzed, and the results of chemical composition detection at different stub ends positions were similar. Compared with the technical requirements of ASTM SA403-WP304, the carbon content of stub end base metal is slightly higher than the prescribed upper limit, while the chromium content is slightly lower than the prescribed lower limit. Other elements in this position meet the standard requirements; the chemical composition of tees and welds meet the standard requirements.
1.3 Vickers Hardness Test
Vickers hardness test was carried out at five locations of weld, heat affected zone on both sides, stub end base metal and tee base metal in the box area shown in Fig. 1. The test results are shown in Table 2.
Table 2 Vickers hardness test results
The Vickers hardness of WP304WX stainless steel should generally not exceed 200 HV. The results show that the Vickers hardness of flanged base metal is much higher than that of flanged base metal, and the Vickers hardness measured at other locations is normal.
1.4 Metallographic examination
The section is taken perpendicular to the direction of the crack at the leakage point, and the metallographic examination is carried out. Fig. 3 is the polished morphology of the crack position, showing that there are few non-metallic inclusions; Fig. 4, 5 and 6 are the microstructures after aqua regia erosion. The matrix structure is equiaxed austenite, and a large number of twins can be seen. Cracks occur at the grain boundary and propagate along the grain boundary, with lattices falling off, and the crack source area is not at the mechanical stress concentration. Nonmetallic inclusions were not found in the crack source region. In addition, the austenite grain boundaries of stainless steel are characterized by a large amount of carbides in Fig. 5.
Fig. 3 Polished morphology of cracks
Fig. 4 Microstructure of matrix and crack area after aqua regia erosion
Figure 5 Morphology of grain boundary carbides
1.5 Intergranular corrosion test
Four longitudinal specimens were taken at the weld joint of stub end and tee connection. According to the method E of GB/T4334-2008 “Test Method for Corrosion of Stainless Steel in Metals and Alloys”, i.e. stainless steel sulphuric Acid-Copper sulphate corrosion test method, the intergranular corrosion test was carried out. The test solution was 100g. The sulfuric Acid-Copper sulfate solution was prepared by dissolving pure copper sulfate in 700 mL distilled water, adding 100 mL high grade pure sulfuric acid and diluting it with distilled water to 1000 mL. After intergranular corrosion test, the specimens were bent at 180 degrees. All three specimens were ruptured after bending, accompanied by intergranular corrosion tendency.
1.6 Fracture Scanning Electron Microscope Analysis
Man-made cracks open, resulting in the fracture macro-granular, smooth fracture, no shear lip. Scanning electron microscopy (SEM) was used to observe the fracture morphology as shown in Fig. 7 and 8. It was found that the fracture morphology was ice sugar block with intergranular cracks, and there were a large number of secondary cracks at the grain boundary. No corrosion and oxidation products were found on the fracture surface.
Fig. 6 Micromorphology of cracks after aqua regia erosion
Figure 7 Low-power fracture morphology
Figure 8 High power fracture morphology
Through flaw detection, it is found that there is a crack perpendicular to the axis of stub end pipeline, which is the leakage part of pipeline. The results of chemical composition analysis showed that the carbon content of stub end base material was higher and the chromium content was lower. This state increases the hardness of stainless steel. At the same time, carbon is an element in the stable austenite phase zone, and its capacity is about 30 times that of nickel. However, carbon can form a series of carbides with chromium. During the slow cooling process of high temperature austenite, the carbides will precipitate along the grain boundary to form a third phase, resulting in serious chromium depletion and weakening near the grain boundary. The intergranular bonding force may lead to intergranular corrosion, which weakens the intergranular corrosion resistance of austenitic stainless steel. In addition, the high carbon content will make the weldability of stainless steel worse .
Microstructure analysis shows that the matrix structure of stub end heat affected zone of stub end and tee weld is equiaxed austenite grains, and a large number of twins can be seen. Twins are produced by austenite during cold deformation. The existence of twins indicates that there are large residual stresses in the material, and larger residual stresses will accelerate the growth of grain boundary cracks .
- (1) The main reason for the cracks in HAZ of stub end base metal of stainless steel pipeline is that this part is located in HAZ of weld seam. Under the action of welding, the poor chromium is caused by the defect of material itself, and the intergranular corrosion cracking is caused by the combined action of residual stress.
- (2) It is suggested that all weld heat-affected zones of pipelines should be inspected to find corrosion cracks, and cracks found should be polished and repaired.
- (3) It is suggested that the welding seam and its heat-affected zone should be treated by eliminating residual stress.
- (4) It is suggested to select suitable stainless steel, preferably low-carbon stainless steel; at the same time, after parts are processed and moulded, it is better to adopt solution strengthening treatment to eliminate internal stress and improve the anti-grain boundary corrosion ability of materials.
Source: China Steel Pipelines 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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