Discussion of Cracks of Reducers
Posted: 05/29/2023 11:44:12 Hits: 10
From cracks of the reducer of the cold high-resolution bottom sewage pipeline of the cracked hydrogenation unit, it can be judged that the cracks belong to stress corrosion cracking caused by H2S.
The conditions for cracking are as follows:
The conditions for cracking are as follows:
Hydrogen environment
There is a critical hydrogen concentration [ H] c, and the current medium is H2S plus H2O plus NH3. The concentration of H2S reaches 3%, far exceeding [H]c.
Critical stress σw is greater than σc
σc is related to the microstructure of the material, which is more sensitive to lower bainite, martensite, and tempered structure. At the same time, it is related to the residual stress. The stress concentration at the bottom of the pit due to pitting corrosion can be greater than σc.
Sensitivity of materials
Material sensitivity involves alloying elements, that is, chemical composition and molding process. At the same time, the stress corrosion cracking caused by the hydrogen sulfide environment is related to the purity of steel, especially sulfide inclusions. However, brittle oxide inclusions can also cause HIC.
Regarding the stress of the reducer, the pressure of the reducer is 15.6MPa, which is a high-pressure vessel. According to the hardness HB166 of the parent material at the crack, the tensile strength σb of the carbon steel reducer is about 75MPa, and the corresponding yield stress σs is about 45MPa. The working stress of the reducer is still very low relative to its material. Its working stress safety factor N is approximately equal to 3. In addition, judging from the variable section of the reducer, the tube wall is the thinnest, with a wall thickness of 9.5mm, while the crack occurs at the small end of the reducer, where the tube wall thickness is 12.5mm. It shows that the relationship between cracking and stress is not an important factor. Otherwise, cracking should occur at the small end. Corrosion is an important factor. The working environment of the sewage reducer is harsh. The average concentration of H2S is as high as 34248/106, and it is also acidic water. Eddy currents can be generated due to the sudden change of cross-section in the structure of the reducer. The pressure is reduced, and more hydrogen sulfide is precipitated; the partial pressure of hydrogen sulfide is increased. In this case, the inner surface of the reducer is corroded. Pitting occurs, and more hydrogen enters the reducer at the same time, resulting in hydrogen sulfide stress corrosion cracking. Inspecting the fracture of the fracturing source shows that the crack initiation starts from the pitting, and there are brittle inclusions in the crack initiation source. This is the main reason for the cracking of the reducer. Even though there is still 19599/106 NH3 in the cold high-resolution pipeline, some people think that adding NH3 to H2S can prevent HIC from happening. Under the working condition of the reducer, the existence of NH3 does not prevent H2S Stress corrosion cracking.
Because of the effect of the sewage eddy, on the one hand, the hydrogen sulfide here reaches a higher concentration; on the other hand, the corrosion on the surface of the reducer is aggravated, and small pitting is produced here. On the one hand, these pits further acidify the medium, and at the same time, the stress concentration at these pits causes the actual stress to reach the critical stress of HIC, making HIC occur. In addition, HIC was found to be related to inclusions when examining the fracture at the source of the fracture. There are two kinds of inclusions at the fracture source. One is a round inclusion (oxide) and the other is a square inclusion (AlN). Although these two inclusions are not M nS, they still cause HIC. Therefore, the reason for the cracking of the reducer of the cold high-pressure sub-bottom sewage pipeline is mainly due to the formation of eddy currents at the variable section of the reducer, which makes the concentration of hydrogen sulfide here higher than in other places, and pitting occurs. There are inclusions in the pits, which finally lead to HIC.
It is worth pointing out that HIC-resistant materials must be selected under the environment of hydrogen sulfide with high concentration and high partial pressure. Pure carbon steel should be the object of consideration, and the reducer is made from forged carbon steel. According to the API standard, in this case, the hardness of the steel must be controlled to be less than 240HB, and the hardness of the reducer is 166HB, which is much lower than 240HB. Although there is a welding seam at the big end of the reducer, the cracking occurs in the base metal. At the same time, the hardness of the welding seam is also lower than that of the base metal. Microstructural examination shows that there are ferrite and a small amount of pearlite. Sensitive microstructures such as martensite and bainite were not found. It shows that the manufacturing process and the final heat treatment of the reducer are normal.
However, HIC still happened. This shows that in an environment of high concentration and high pressure hydrogen sulfide and HIC should be avoided for steel; the sulfur content should reach a lower level. S should be less than 0.001%. In addition, MnS inclusions in steel can cause HIC, while other brittle inclusions substances such as SiO2 or Al2O3 and AlN can also cause HIC. Therefore, in the case of high-concentration and high-pressure hydrogen sulfide, not only is the steel required to have a lower sulfur content, but the content of plastic inclusions and brittle inclusions must also be strictly controlled. Only when these two inclusions have been controlled below 0.5, the occurrence of HIC can only be avoided.
5. Conclusions and recommendations
After the macroscopic and microscopic analysis of the reducer of the cold high-resolution bottom sewage pipeline of the cracked residual oil hydrogenation unit, there are the following conclusions:
(1) The cracking of the reducer of the cold high-resolution bottom sewage pipeline belongs to hydrogen sulfide stress corrosion cracking (SSCC).
(2) SSCC cracks originate in the big end of the reducer, where small pitting occurs due to the existence of eddy currents. Cracks originate from inclusions at corrosion pits and then propagate toward the outer axial wall.
(3) The reducer is made from carbon steel with high purity, but it still cannot prevent SSCC under such harsh conditions.
According to the above analysis results, the following suggestions are put forward for the reference of designers and manufacturers:
(1) Use carbon steel with higher purity, such as Japan's CR5 anti-HIC steel, whose sulfur content is less than 0.001%.
(2) If there is no Cl- in the medium, can austenitic stainless steel be used? However, if the austenitic stainless steel is used, attention must be paid to the welding of the reducer and the carbon steel pipe and the heat treatment after welding.
(3) Change the connection shape of the reducer, and change the "U" to "V" to eliminate the eddy current in the process of sewage discharge.
Regarding the stress of the reducer, the pressure of the reducer is 15.6MPa, which is a high-pressure vessel. According to the hardness HB166 of the parent material at the crack, the tensile strength σb of the carbon steel reducer is about 75MPa, and the corresponding yield stress σs is about 45MPa. The working stress of the reducer is still very low relative to its material. Its working stress safety factor N is approximately equal to 3. In addition, judging from the variable section of the reducer, the tube wall is the thinnest, with a wall thickness of 9.5mm, while the crack occurs at the small end of the reducer, where the tube wall thickness is 12.5mm. It shows that the relationship between cracking and stress is not an important factor. Otherwise, cracking should occur at the small end. Corrosion is an important factor. The working environment of the sewage reducer is harsh. The average concentration of H2S is as high as 34248/106, and it is also acidic water. Eddy currents can be generated due to the sudden change of cross-section in the structure of the reducer. The pressure is reduced, and more hydrogen sulfide is precipitated; the partial pressure of hydrogen sulfide is increased. In this case, the inner surface of the reducer is corroded. Pitting occurs, and more hydrogen enters the reducer at the same time, resulting in hydrogen sulfide stress corrosion cracking. Inspecting the fracture of the fracturing source shows that the crack initiation starts from the pitting, and there are brittle inclusions in the crack initiation source. This is the main reason for the cracking of the reducer. Even though there is still 19599/106 NH3 in the cold high-resolution pipeline, some people think that adding NH3 to H2S can prevent HIC from happening. Under the working condition of the reducer, the existence of NH3 does not prevent H2S Stress corrosion cracking.
Because of the effect of the sewage eddy, on the one hand, the hydrogen sulfide here reaches a higher concentration; on the other hand, the corrosion on the surface of the reducer is aggravated, and small pitting is produced here. On the one hand, these pits further acidify the medium, and at the same time, the stress concentration at these pits causes the actual stress to reach the critical stress of HIC, making HIC occur. In addition, HIC was found to be related to inclusions when examining the fracture at the source of the fracture. There are two kinds of inclusions at the fracture source. One is a round inclusion (oxide) and the other is a square inclusion (AlN). Although these two inclusions are not M nS, they still cause HIC. Therefore, the reason for the cracking of the reducer of the cold high-pressure sub-bottom sewage pipeline is mainly due to the formation of eddy currents at the variable section of the reducer, which makes the concentration of hydrogen sulfide here higher than in other places, and pitting occurs. There are inclusions in the pits, which finally lead to HIC.
It is worth pointing out that HIC-resistant materials must be selected under the environment of hydrogen sulfide with high concentration and high partial pressure. Pure carbon steel should be the object of consideration, and the reducer is made from forged carbon steel. According to the API standard, in this case, the hardness of the steel must be controlled to be less than 240HB, and the hardness of the reducer is 166HB, which is much lower than 240HB. Although there is a welding seam at the big end of the reducer, the cracking occurs in the base metal. At the same time, the hardness of the welding seam is also lower than that of the base metal. Microstructural examination shows that there are ferrite and a small amount of pearlite. Sensitive microstructures such as martensite and bainite were not found. It shows that the manufacturing process and the final heat treatment of the reducer are normal.
However, HIC still happened. This shows that in an environment of high concentration and high pressure hydrogen sulfide and HIC should be avoided for steel; the sulfur content should reach a lower level. S should be less than 0.001%. In addition, MnS inclusions in steel can cause HIC, while other brittle inclusions substances such as SiO2 or Al2O3 and AlN can also cause HIC. Therefore, in the case of high-concentration and high-pressure hydrogen sulfide, not only is the steel required to have a lower sulfur content, but the content of plastic inclusions and brittle inclusions must also be strictly controlled. Only when these two inclusions have been controlled below 0.5, the occurrence of HIC can only be avoided.
5. Conclusions and recommendations
After the macroscopic and microscopic analysis of the reducer of the cold high-resolution bottom sewage pipeline of the cracked residual oil hydrogenation unit, there are the following conclusions:
(1) The cracking of the reducer of the cold high-resolution bottom sewage pipeline belongs to hydrogen sulfide stress corrosion cracking (SSCC).
(2) SSCC cracks originate in the big end of the reducer, where small pitting occurs due to the existence of eddy currents. Cracks originate from inclusions at corrosion pits and then propagate toward the outer axial wall.
(3) The reducer is made from carbon steel with high purity, but it still cannot prevent SSCC under such harsh conditions.
According to the above analysis results, the following suggestions are put forward for the reference of designers and manufacturers:
(1) Use carbon steel with higher purity, such as Japan's CR5 anti-HIC steel, whose sulfur content is less than 0.001%.
(2) If there is no Cl- in the medium, can austenitic stainless steel be used? However, if the austenitic stainless steel is used, attention must be paid to the welding of the reducer and the carbon steel pipe and the heat treatment after welding.
(3) Change the connection shape of the reducer, and change the "U" to "V" to eliminate the eddy current in the process of sewage discharge.
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