Explosion accident analysis of UNS S32205 duplex stainless steel pressure pipeline
Based on the analysis of a deflagration accident of a UNS S32205 duplex stainless steel pressure pipeline, the nondestructive testing, composition analysis, hardness testing, ferrite content testing, metallographic examination and mechanical property test were carried out on the UNS S32205 elbow. The influence of ferrite content of duplex stainless steel plate on the steel performance was studied. It was pointed out that hot forming was not considered in the design of 50% ferrite and 50% austenite respectively Or the effect of welding hot working on the decrease of microstructure and properties caused by the increase of ferrite content. It is suitable that the ferrite content of duplex stainless steel should be 45% and the austenite content should be 55%.
During the test run of a hydrocracking unit in a petrochemical project, an UNS S32205 duplex stainless steel elbow on the pipeline cracked during the process of hydrogen pressure rise, which resulted in the leakage and explosion of hydrogen medium and huge economic loss. The design pressure of the pipeline is 15.37mpa, the design temperature is 200 ℃, the operating pressure is 13.76mpa, the operating temperature is 154 ℃, and the medium is high temperature oil gas (main component hydrogen). The specification of accident elbow is Φ 609.6mm × 36mm, the material is ASTM A815 UNS S32205, and the material of straight pipe is ASTM A790 UNS S31803.
The field inspection shows that the continuous length of the cracked elbow is about 1.84M, and the circumference of the elbow is 1.91M. The cracking is within the scope of the elbow and almost runs through the whole circumference. The fracture features brittle fracture, as shown in Fig. 1 and Fig. 2. One end of the cracked edge of the elbow is close to the weld, and the edge of the cracked other end is about 160mm away from the weld. There is a 113mm crack on the fusion line of the welded joint on the other side of the cracked weld of the elbow, as shown in Figure 3, with a depth of 9.5mm and a wall thickness of 36mm. However, no crack is found on the deposited metal of the weld.
During the acceptance of the batch of elbow, 10% spectral inspection and 100% penetrant inspection were carried out. If they are put into installation after passing the acceptance, no defects were found in the NDT during the acceptance.
Figure.1 Picture of elbow cracking
Figure.2 Picture of elbow cracking
Figure.3 Picture of elbow cracking
Nondestructive testing and physical and chemical testing
NDT of elbow and welded joint
100% ultrasonic testing and penetrant testing were carried out on the body of accident elbow (hereinafter referred to as No1 elbow) and the body of the same kind of elbow (hereinafter referred to as NO2 elbow) without accident. 100% radiographic testing, magnetic particle testing and penetrant testing were carried out on the welded joint. The test results show that there are no other defects on the elbow body except for the cracks on the accident crack and the fusion line in Figure 3, which proves that there are no other defects on the elbow Therefore, the results of the previous nondestructive testing without defects are reliable.
Hardness and ferrite content test of pipe fittings
The body of No1 elbow and uncracked NO2 elbow was tested with the Richter hardness tester. The hardness test results showed that 210 points of No1 elbow were tested in total, and the hardness range was HB240-HB344, which exceeded the test value of product quality certificate HB 215-HB225, among which 119 points exceeded HB290 (the upper limit of ASTM A815 standard is HB290), and 93 points were tested in the outer bend of the elbow Point, hardness over HB290 has 69 points.
A total of 192 points were tested for the hardness of NO2 intact elbow, with the hardness range of HB240-HB325, all exceeding the measured value of product quality certificate HB 215-hb225, of which 74 points exceeded the hardness value of HB290 (upper limit of ASTM A815 standard is HB290), 93 points were tested for the outer bend of elbow, and 47 points exceeded the hardness limit.
|No.1 elbow||No.2 elbow||Measured value of product quality certificate||ASTM A815|
|In HB240-HB325, a total of 93 points were measured at the outer bend of the elbow, and 69 points with hardness over HB290.||In HB240-HB325, a total of 93 points were measured at the outer bend of the elbow, and 47 points were hardness higher than HB290.||HB215-HB225||≤ HB290|
Ferrite content detection
The content of ferrite in the outer wall of No1 elbow and NO2 elbow was measured at 72 points respectively. The content of ferrite in No1 elbow was 46.9% – 71.7%, that in No2 elbow was 41.6% – 69.4%, that in No1 elbow was 44.6% – 59.4%, that in No2 elbow was 44.6% – 56.5%, and that in No1 and NO2 elbow was more than 20% Over 60%. Compared with UNS S31803 straight pipe, the ferrite content of No1 elbow and NO2 elbow is uneven, with large fluctuation range. The higher limit exceeds the allowable value of 60% ferrite content, which does not meet the requirement of 40% – 60% ferrite content specified in article 5.9 of austenitic ferrite duplex stainless steel seamless pipe (GB / T21833-2008).
Table.2 Ferrite content of elbow
|No.1 elbow||No.2 elbow||GB/T21833-2008|
|46.9% ~ 71.7%||41.6% ~ 69.4%||Ferrite content is 40% – 60%|
Component test, metallographic test and mechanical property test of pipe fittings
Cut off the UNS S32205 elbow and UNS S31803 straight pipe for chemical composition analysis. The analysis results (see Table 3) show that the chemical composition of UNS S32205 duplex stainless steel elbow and UNS S31803 straight pipe can meet the requirements of relevant standards.
Table.3 Analyze the chemical composition of cut-off UNS S32205 elbow and UNS S31803 straight pipe (wt%)
Through metallographic inspection on No1 elbow, it is found that the metallographic structure is abnormally coarse (the grain size reaches 2-6mm, as shown in Figure 4), the grain size is lower than grade 00, which is about 100 times larger than the normal grain size, showing typical overheated structure characteristics. The metallographic structure of NO2 elbow also has certain superheat characteristics (as shown in Figure 5), and the grain size is smaller than that of No1 elbow. The metallographic structure of the straight pipe section connecting the No1 and NO2 elbows is normal.
Mechanical property test
The elongation of three of the four tensile specimens cut from the outer arc of the accident UNS S32205 elbow does not meet the requirements of ASTM A815 that the elongation should be greater than 25%. The tensile test results of the four tensile specimens are 20%, 22%, 24% and 27% respectively, among which the elongation of 20% and 22% of the specimens are near the outer arc of the UNS S32205 elbow.
The impact energy of straight pipe is 292j, 292j, 293j, and that of No1 elbow is 149j, 174j, 160J. The impact energy of the elbow after hot forming is obviously lower than that of the straight pipe. The impact energy shall be greater than 250j.
According to the provisions of TSGD7002-2006 type test rules for pressure piping components, the corresponding pipe fittings shall be subject to the type test (technical document review, sample inspection test and safety performance test) specified in TSGD7002-2006 to verify whether the safety performance of the products formed by hot processing meets the safety technical requirements. However, the type test report provided by this batch of pipe fittings is cold forming process, which is inconsistent with the actual hot forming process of this batch of pipe fittings. That is to say, the actual hot processing process of this batch of pipe fittings has not been verified by the type test specified in TSG D7002 safety technical specification, and the safety performance and quality of pipe fittings cannot be guaranteed.
Analysis and discussion
NDT for formed defects
The ultrasonic and penetrant tests were carried out for the duplex steel pipe parts before and after the construction acceptance. The results of the ultrasonic and penetrant tests before and after the accident of No1 elbow and NO2 elbow are consistent. That is to say, before cracking, there is no crack and other defects in the UNS S32205 elbow. For the 113mm long crack found on the fusion line on the other side of the cracked UNS S32205 elbow weld, it is found that there is no crack at this position before the accident through the pre accident radiograph, and the crack is considered to be caused in the accident. The results of NDT before and after comprehensive comparison can be concluded that the results of NDT before the accident are credible. The UNS S32205 elbow has no formed defects such as cracks, incomplete penetration, incomplete fusion, etc., but later there is a 1.91M long brittle large opening crack in the elbow body. Therefore, the NDT only aims at the possible formed defects (such as cracks, incomplete penetration, incomplete fusion, etc.) It’s not enough, it’s limited.
Non destructive testing for unqualified (unformed defects) components
During the installation, the construction company accepted the pipe fittings according to the requirements of SH / T3501, including 10% of quality certificate, appearance, size specification, mark, spectrum and 100% of surface inspection. The spectrum is for the inspection of components, the surface inspection is for the inspection of formed defects, the inspection of type test certificate is not in accordance with the provisions of safety technical specification TSGD7002-2006, and the inspection is not in accordance with ASTM A815 and GB / T21833- In 2008, the microstructure and properties of pipe fittings were tested, that is, the ferrite content and hardness were not tested.
Component analysis (including spectral detection) belongs to component detection, ferrite content test belongs to tissue detection, and hardness test belongs to performance detection. In the metallographic structure, the hardness of martensite is higher than that of pearlite, pearlite is higher than that of ferrite, and the hardness of ferrite is higher than that of austenite. Therefore, through the hardness value, the metallographic structure of the material, the change of structure and the effect of heat treatment can be roughly understood. The composition test of No1 elbow and NO2 elbow conforms to the standard, but the ferrite content exceeds 60% of the standard, and the hardness value exceeds HB290, which proves that although the chemical composition of No1 elbow and NO2 elbow meet the requirements, the organizational performance has deteriorated. Before construction, the type test must be carried out according to the requirements of professional standard TSGD7002-2006, ASTM A815 and GB / T21833-2008 And structure and property test (ferrite content test, hardness test).
Processability and durability
According to the standard, the solution treatment temperature of UNS S32205 duplex stainless steel is 1020 ℃ ~ 1100 ℃. When it is heated to over 1200 ℃, the high temperature elongation of UNS S32205 duplex steel will be greatly increased, reaching 1200%, which is very convenient for processing and forming. However, after cooling, the grains will be sharply coarsened and the properties will be sharply deteriorated. The hardness is more than hb290, the ferrite content is more than 60%, and the material is brittle. Therefore, during the hot working (including welding) of UNS S32205 duplex stainless steel, not only the convenience of processing, but also the durability after forming should be considered, which is determined by the structure and performance. The ferrite content of UNS S32205 duplex stainless steel is less than 60%, close to the middle value of 50%, the hardness is lower than hb290, close to the hardness of raw materials, which is the key factor to ensure the durability (avoid cracking) of UNS S32205 duplex stainless steel.
Failure mechanism of elbow
From the test and inspection data, it can be seen that no 1 cracked elbow and the same batch of pipe fittings are thermally processed by using the process not verified by type test, resulting in coarse grain, hardness exceeding standard, ferrite content exceeding standard, elongation decreasing, impact toughness decreasing and material embrittlement, resulting in no 1 bending hair cracking, while the same batch of pipe fittings have the possibility of cracking due to material deterioration due to the same process Yes.
With the increase of solution treatment temperature, the proportion of ferrite phase increases and that of austenite phase decreases. The grain size of solid solution ferrite increased significantly above 1200 ℃. According to the literature, the two-phase ratio of 2205 duplex stainless steel at different solution temperature is shown in Table 4
It can be seen from table 4 that the ferrite content increases with the increase of solution temperature. When the ferrite content is 65.4%, the corresponding solution temperature is 1300 ℃, which greatly exceeds the requirements of ASTM a815 and GB / t21833-2008 for solid solution treatment of UNS S32205 duplex stainless steel in the range of 1020 ~ 1100 ℃. When the content of ferrite is more than 60% and the hardness is more than HB290, it can be revealed that the solution treatment temperature of dual phase steel is more than 1100 ℃.
Duplex stainless steel has the two-phase structure of ferrite + austenite, which is usually controlled at 50% / 50% (volume fraction), so it has the properties of both ferritic stainless steel and austenitic stainless steel. Compared with ferritic stainless steel, duplex stainless steel has higher toughness, lower brittleness transition temperature, significantly improved resistance to intergranular corrosion and welding performance, and retained the characteristics of ferritic stainless steel, such as high thermal conductivity, small coefficient of linear expansion, superplasticity, etc.; compared with austenitic stainless steel, duplex stainless steel has higher strength, especially higher yield strength, and intergranular corrosion resistance Corrosion resistance, stress corrosion resistance, corrosion fatigue resistance and other properties have been significantly improved.
With the increase of solution temperature in hot working, the content of ferrite in the structure of UNS S32205 duplex stainless steel increases gradually. When the content of ferrite exceeds 60%, the grain size of duplex steel increases sharply, the hardness increases sharply, and the material embrittlement. The relevant standards do not consider the influence of the increase of ferrite content after hot working such as welding on the decrease of microstructure and properties. Therefore, the value of ferrite phase compared with austenite should be adjusted from 50%: 50% to 45%: 55%. The space for the increase of ferrite phase should be reserved for the hot forming and welding of pipe fittings, so as to prevent the material embrittlement caused by the increase of ferrite phase after hot working. Welding is a heat cycle process of rapid heating and cooling. GB50517, SH3501 and SH / T3558 are suitable to regulate the ferrite content of weld metal as 30% ~ 60%. The welding overheated zone has experienced a sharp thermal cycle, and there are also problems of coarse grains, increased hardness, and increased ferrite phase due to overheat. Therefore, the fusion line cracks in Figure 3 are generated in the fusion line.
Nondestructive testing based on failure mechanism
The definition of modern nondestructive testing: on the premise of not damaging the test piece, it is a method to check and test the structure, property and state of the internal and surface of the test piece by means of physical or chemical methods with the help of advanced technology and equipment. According to the definition, the four conventional nondestructive testing, acoustic emission, eddy current, hardness, ferrite content, spectrum, metallographic testing are also non-destructive testing.
Generally, non-destructive testing refers to radiographic testing, ultrasonic testing, magnetic particle testing, penetrant testing, acoustic emission testing, eddy current testing, visual testing, TOFD testing and phased array testing, mainly for the detection of discontinuities (defects) in materials. Generally, people pay attention to the detection of formed defects (such as cracks, incomplete penetration, etc.) and continue to pay attention to the improvement of detection technology However, there are no unformed defects (unqualified composition, structure and performance in materials) to be concerned about, and the unqualified composition, structure and performance of materials are more likely to be converted into serious accidents such as forming defects and fracture.
8.2.1 of TSG21-2016 Supervision Regulation on safety technology of stationary pressure vessel stipulates that inspection scheme shall be formulated according to the use condition, damage mode and failure mode of pressure vessel, that is to say, by consulting and studying the previous similar data, according to the use condition of the equipment, the failure mode of the equipment and material shall be grasped in advance, and the inspection and detection scheme shall be formulated with emphasis and pertinence to avoid the absence Drop the arrow, causing missed inspection. The omission of targeted methods in the inspection scheme cannot be made up by other inspection methods.
The failure mechanism of UNS S32205 duplex stainless steel is that when the hot working temperature exceeds 1100 ℃, the grains will be coarse, the ferrite content will be more than 60%, the hardness will be more than hb290, and the materials will be embrittled, but the cracks will not occur immediately. Therefore, the quality certificate inspection, type test certificate inspection, appearance inspection, size and specification inspection, and mark inspection shall be carried out for the failure mechanism during the pipe fitting acceptance Spectrum test, 100% surface test, 100% hardness test, 100% ferrite test, 100% thickness test. Nondestructive testing should not only aim at the formed defects, but also at the unqualified component structure and performance of the unformed defects. The formed defects are defects, and the unformed component structure and performance are also defects. Sometimes the unformed defects are more harmful than the formed defects. Only the nondestructive testing based on the failure mechanism is an effective nondestructive testing.
- The generalized nondestructive testing should not only include the testing of the formed defects, but also the non-destructive testing of the unformed defects (unqualified component structure and performance). The conventional non-destructive testing cannot (unqualified component structure and performance). The non-destructive testing based on the failure mechanism can detect the unformed defects and realize the early warning of failure.
- As a method of rapid performance testing, hardness testing should be widely used in the detection of material failure warning.
- During the inspection of UNS S32205 duplex stainless steel pipe fittings, ferrite content test and hardness test shall be adopted to realize the early warning of the failure of structure and performance. The ferrite content of UNS S32205 dual phase steel should be strictly controlled below 60% and close to the middle value of 50%, the hardness should be strictly controlled below HB290, the hardness value should reach or exceed HB290, the ferrite content should reach or exceed 60%, which is very easy to crack.
- If the ferrite content is more than 60% and the hardness value is more than HB290, it indicates that the heat treatment temperature of UNS S32205 duplex stainless steel is more than 1100 ℃ and the embrittlement of the structure.
- Considering the influence of the increase of ferrite content caused by welding and hot working forming on the decrease of microstructure and properties, it is advisable to control the ferrite content of UNS S32205 dual phase steel at about 45% (30% ~ 60%), the ferrite content of weld joint at 30% ~ 60%, and the embrittlement cracking failure easily occurs when the ferrite content is close to or over 60%.
Source: China Duplex Stainless Steel Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.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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