The Failure Analysis of the Stainless Steel TP304 Tee (Part One)
Posted: 03/08/2022 09:58:56 Hits: 4
Abstract: The stainless steel TP304 tee used in the denitration facility of the power plant cracks. Through the composition analysis, the macroscopic and microscopic analysis of the fracture, the microstructure observation and the hardness test, combined with the production process, the causes of the crack of the tee are analyzed. The research results show that the low content of Ni in the material reduces the stabilization of austenite; the spherical non-metallic inclusions of grade 2.5 to 3.0 contained in the matrix cause tearing of the matrix and promote the initiation of fatigue cracks. Meanwhile, after the processing is completed, there is no solution treatment for the tee, which makes the internal stress of the material too great. These reasons lead to the reduction of the austenite content of the material, high hardness, and the great internal stress of the plastic deformation part of the material, resulting in a great stress concentration in the neck of the branch pipe. There is a lot of machining damage on the inner surface of the tee at the same time, which strengthens the stress concentration, and promotes the initiation of cracks in the tee under the action of fatigue load.
Stainless steel is widely used in industries such as food, petrochemicals and power. In particular, austenitic stainless steel has excellent comprehensive mechanical properties and corrosion resistance. The content of Cr in austenitic stainless steel is usually 18% to 20%, Ni between 8% to 10.5% and C less than 0.08%, and there is a small amount of Si, N, and Mn. High chromium content will promote the formation of a dense passivation layer on the steel surface and improve the material's corrosion resistance at room temperatures and high temperatures. The addition of Ni and Mn stabilizes the austenite structure. Low carbon content can effectively avoid the formation of chromium carbides at grain boundaries, and avoid embrittlement and sensitization.
A stainless steel tee is made by cold drawing (extrusion). Very significant work hardening happens for austenite. Therefore, construction specifications for ASME I Boiler and Pressure Vessel Code (Volume 1) Power Boiler require that stainless steel should be subjected to solution heat treatment after cold working. If the solution treatment is improper or not carried out, the performance of stainless steel will be greatly affected.
Since 2015, the stainless steel TP304 tee used in the denitration facility of the power plant has failed again and again. The plant adopts selective catalytic denitrification (SCR) process, using liquid ammonia as denitrification agent, and the ammonia zone has been put into operation since November 30, 2014. Since April 24, 2015, the nitrogen conversion tee, the rear tee of the adjustment door, and the adjustment bypass tee have been cracked one after another. When leakages occur, In case of leakage, the value of the alarm device for the ammonia leakage ranges from 15.18 mg/m to 75.9 mg/m3. When the crack is serious, white mist stuff can be seen at the crack. When the crack is small, soapy water should be used for detection.
1. Testing and results
The test sample is the tee behind the adjustment door at the entrance of the buffer tank, which is made of stainless steel TP304, and the tee has a size of DN80 X 15MM and pressure of PN2.5. The water bath temperature of the evaporation tank is 50 to 60℃, and the gas ammonia temperature is 50℃. The vibration at the outlet of the evaporation tank is 0.02 to 0.035 mm.
1.1 Chemical composition analysis
The chemical composition of the cracked tee was analyzed. The analysis equipment used was electric spark direct reading spectrum, and the analysis results were shown in Table 1. The results show that the main alloying elements Cr and Ni in the cracked tee are lower than that required in GB/T 12459-2005 implemented by the tee manufacturer.
1.2 Macro Analysis
The macrophotograph of the cracked tee is shown in Figure 1. Six obvious cracks can be seen in the neck of the branch pipe of the tee. It can be seen from the figure that there are repair welding marks at the position 1, and the material structure will be affected and changed. The samples were taken for positions 2, 5 and 6 and the fracture analysis is conducted. The macroscopic morphology of the positions 2 and 5 after sampling is shown in Figure 2. From the inner surface of the cracked sample, obvious damage caused by cold-working denaturation is observed, and the position 5 just overlaps with the position of deformation damage (Figure 2d).
1.3 Fracture analysis
Fracture analysis was performed for position 2. The fracture surface of the crack can be divided into two typical areas, marked as zone I and zone II (Figure 3a); the color of zone I is slightly darker, and the color of zone II is lighter. According to the "cross-section oxidation color method", it can be determined that the area with darker color is generally the area with earlier cracking time. The fracture area is further subdivided into zones A, B, and C. A and B are slow expansion zones, and zone C is a fast fracture zone, showing an obvious brittle fracture morphology (Figure 3b). According to the judgment of the direction of the radial ridges, it can be determined that the crack starts in area A and expands to area B, and area A is close to the inner surface of the tee, that is, the crack originated on the inner wall of the tee, and continued to expand from the inside to the outside, and finally formed cracks on the outer surface.
Stainless steel is widely used in industries such as food, petrochemicals and power. In particular, austenitic stainless steel has excellent comprehensive mechanical properties and corrosion resistance. The content of Cr in austenitic stainless steel is usually 18% to 20%, Ni between 8% to 10.5% and C less than 0.08%, and there is a small amount of Si, N, and Mn. High chromium content will promote the formation of a dense passivation layer on the steel surface and improve the material's corrosion resistance at room temperatures and high temperatures. The addition of Ni and Mn stabilizes the austenite structure. Low carbon content can effectively avoid the formation of chromium carbides at grain boundaries, and avoid embrittlement and sensitization.
A stainless steel tee is made by cold drawing (extrusion). Very significant work hardening happens for austenite. Therefore, construction specifications for ASME I Boiler and Pressure Vessel Code (Volume 1) Power Boiler require that stainless steel should be subjected to solution heat treatment after cold working. If the solution treatment is improper or not carried out, the performance of stainless steel will be greatly affected.
Since 2015, the stainless steel TP304 tee used in the denitration facility of the power plant has failed again and again. The plant adopts selective catalytic denitrification (SCR) process, using liquid ammonia as denitrification agent, and the ammonia zone has been put into operation since November 30, 2014. Since April 24, 2015, the nitrogen conversion tee, the rear tee of the adjustment door, and the adjustment bypass tee have been cracked one after another. When leakages occur, In case of leakage, the value of the alarm device for the ammonia leakage ranges from 15.18 mg/m to 75.9 mg/m3. When the crack is serious, white mist stuff can be seen at the crack. When the crack is small, soapy water should be used for detection.
1. Testing and results
The test sample is the tee behind the adjustment door at the entrance of the buffer tank, which is made of stainless steel TP304, and the tee has a size of DN80 X 15MM and pressure of PN2.5. The water bath temperature of the evaporation tank is 50 to 60℃, and the gas ammonia temperature is 50℃. The vibration at the outlet of the evaporation tank is 0.02 to 0.035 mm.
1.1 Chemical composition analysis
The chemical composition of the cracked tee was analyzed. The analysis equipment used was electric spark direct reading spectrum, and the analysis results were shown in Table 1. The results show that the main alloying elements Cr and Ni in the cracked tee are lower than that required in GB/T 12459-2005 implemented by the tee manufacturer.
1.2 Macro Analysis
The macrophotograph of the cracked tee is shown in Figure 1. Six obvious cracks can be seen in the neck of the branch pipe of the tee. It can be seen from the figure that there are repair welding marks at the position 1, and the material structure will be affected and changed. The samples were taken for positions 2, 5 and 6 and the fracture analysis is conducted. The macroscopic morphology of the positions 2 and 5 after sampling is shown in Figure 2. From the inner surface of the cracked sample, obvious damage caused by cold-working denaturation is observed, and the position 5 just overlaps with the position of deformation damage (Figure 2d).
1.3 Fracture analysis
Fracture analysis was performed for position 2. The fracture surface of the crack can be divided into two typical areas, marked as zone I and zone II (Figure 3a); the color of zone I is slightly darker, and the color of zone II is lighter. According to the "cross-section oxidation color method", it can be determined that the area with darker color is generally the area with earlier cracking time. The fracture area is further subdivided into zones A, B, and C. A and B are slow expansion zones, and zone C is a fast fracture zone, showing an obvious brittle fracture morphology (Figure 3b). According to the judgment of the direction of the radial ridges, it can be determined that the crack starts in area A and expands to area B, and area A is close to the inner surface of the tee, that is, the crack originated on the inner wall of the tee, and continued to expand from the inside to the outside, and finally formed cracks on the outer surface.
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Next: The Failure Analysis of the Stainless Steel TP304 Tee (Part Two)
Next: The Failure Analysis of the Stainless Steel TP304 Tee (Part Two)
