Flange high temperature stud failure analysis

Sinopec Jinling Branch PX combined plant deuteration reactor is the main equipment of the deuteration device, the design pressure is 36MP, a working pressure is 321MP, a design temperature is 505 working temperature is 477 °C, equipment specification is ¢4 16010nn equipment main The material of the S-387CriCI internal component is 321 and the medium in the container is hydrogen, benzene, benzene hydrocarbon and light hydrocarbon. After the equipment was commissioned for one week in November 2008, the leakage of the leaking studs in the lower oil and gas outlet pipe flange of the equipment was not completely eliminated. After parking, it was found that some of the studs were tightly stuck in the nut, so that the stud could not be unscrewed normally. Finally, the stud was broken under the action of large external torque.

1 stud technical characteristics equipment lower oil and gas outlet line (see) inner diameter 0 is non-standard flange, material is 14ClMV grade forgings; matching fastener studs and nuts are 25CrMOA twisted studs to break in the nut and Broken in the nut 20 mainly. Stud size M48nmX350nn pitch 3n thread is formed by rolling process. The stud base metal is quenched at 860 ° C, then tempered at 620 ~ 640 °, and the furnace is cooled for 2 h. In order to thoroughly understand the cause of the failure of the batch of studs, the construction unit entrusted Sinopec Nanjing Equipment Failure and Prevention Research Center to carry out physical and chemical properties testing and microscopic metallographic analysis of the material of the studs. The crack will be referred to below, and the crack is cracked on the outer surface of the root of the thread, and the width is gradually narrowed in the radial direction, but the length is longer than that shown, the tip is sharper, and finally the filament extends toward the center of the stud.

There are many point inclusions present, the diameter of the inclusions is different, and the degree of density is also a certain gap.

From the middle, it can be found that the size and density of the inclusions in the fractured stud are not as high as those in the stud material, and some inclusions have formed pits inside the material. In addition, it can be seen from the figure that there is a relatively obvious black network grain boundary structure in the material. For fasteners used under high temperature conditions, the structure appearing in the material will affect its high temperature performance.

The microscopic observation of the 252 stud radial surface material is the radial surface material of the twisted stud and the unbroken stud and the relative chemical composition. Therefore, it is often used to analyze the presence or absence of the fracture during the analysis of the fracture morphology. Harmful elements and their relative content.

The metallographic structure of the radial surface of the stud material twisted in the nut is the scan of the fracture surface of the nut 20 mm away from the nut. The chemical composition of the fracture surface of the fracture surface of the nut is broken. Table 4 shows The corresponding chemical composition, as can be seen from the data listed in the table, does not contain harmful elements that affect the performance of the stud and may promote the fracture. The elements such as CaAN present in the cross section are brought into the sampling process. .

Table 4 shows the chemical composition of the scan position of the twisted fracture surface at 20 mm from the nut. The fracture profile of the 262 twisted in the nut is the scan line that is broken in the fracture surface of the nut. Table 5 shows the corresponding chemistry. Composition, as can be seen from the data listed in the table, the material also contains no harmful elements that affect the performance of the stud and may promote the fracture. The fracture is relatively clean and has fewer other elements.

3 failure analysis of high-quality steel, its ideal organization is uniform tempered sorbite, the steel not only has excellent comprehensive mechanical properties such as high strength, ductility and toughness, but also has good heat strength and anti-relaxation ability. Commonly used in high temperature and high pressure environments, it is the most widely used stud material in thermal equipment.

According to the data provided by the above-mentioned high-temperature equipment technology research laboratory of Nanjing University of Technology, it can be seen that the results of the relevant fractures indicate that the stud material does not contain harmful elements affecting its performance; the hardness measurement results show that Except for the Rockwell hardness of the stud in the nut is higher than the standard hardness, the hardness of the other parts of the 25C2MOA steel meets the requirements; the requirements of the 25C2MVA material in the 1999 alloy structural steel standard, the extension of other studs The rate basically meets the requirements, but the values ​​of the yield strength and the impact energy of the twisted stud are larger than those of the unbroken stud material. Therefore, in general, in addition to the elongation, the mechanical properties such as tensile strength and yield strength of the batch of stud materials can meet the design and related specifications. It should be noted that there is a certain difference between the properties of the twisted stud material and the unbroken stud material; the macroscopic and microscopic features of the twisted fracture indicate that the final fracture zone of the fracture surface has fibrous characteristics and cracking of the stud material. The opening has obvious toughness characteristics; the axial section of the twisted stud and the unbroken stud is found to be 115CM4VA steel which belongs to the quenched and tempered 擂c brewing. There are a large number of inclusions distributed along the axial direction in the twisted snail. Inside the axial plane of the column, cracks that are cracked at the root of the thread and extend in the radial direction are also found. On the radial section of the twisted stud and the unbroken stud, not only the presence of black point inclusions but also the presence of a black net grain boundary in the metallographic structure is observed. The service time in the environment is short, only about one week. It can be ruled out that the existence of the network boundary is related to the service process in the high temperature environment, but should be related to the poor quality of the heat treatment of the steel.

These factors will adversely affect the performance of 25C2MOA steel in high temperature and high pressure environments and in the presence of large tightening conditions, which will seriously affect the service life of the fastening studs made of this type of steel.

In addition, the following adverse effects may exist during the design and construction of the site. The studs and nut materials selected for the design are all 25CrMVA steel. It is difficult to ensure the hardness difference between the studs and the nuts. The average hardness value of the material is about HB170, which is lower than the average hardness value H250 of the nut material. In addition, there is no gasket between the stud and the nut, which causes the heat to be tight, because the external load is too large, and the nut is embedded in the flange. Part 2 ~ 3mm part stud and nut bite; 6MPa working pressure is 321MPa design temperature is 505 working temperature is 477 °C, oil and gas outlet line flange is evenly distributed by 24 sets of studs, according to GB150-1998 Equation 9 - 4 calculates the minimum load of the bolt in the pre-tightening state is 7. 105N Calculated according to the formula 9 - 5, the minimum bolt load is 2 484X106N, so the minimum pre-tightening force of the batch of studs provided by Luoyang Design Institute is 1.035 The maximum pre-tightening force of X105N is 1651X105N. In the process of multiple nitrogen gas tightness in the field and the hot state evaluation of the furnace reverse system, the hydraulic wrench and hydraulic tensioner have repeatedly exceeded the design of the stud due to the poor tightness test of the port. Maximum Tight pressure applied, resulting in excessive stud bearing; (4) furnace reverse system oven end, due to the poor air tightness, during the disassembly process, the studs and nuts were killed, the workers hammered with a sleeve , causing the stud to break.

According to the above analysis, when the stud is put into use, during the tightening of the stud, due to the excessive preload force applied, the stud of the local position is subjected to excessive load, resulting in the presence of 25CrMOA steel. Stress concentration in parts such as inclusions or processing defects becomes a source of cracks and induces cracks. Due to the continuous stress and the high hardness and high strength of the stud material, crack propagation is easier and the material carrying capacity is reduced. Because some of the studs and nuts are bitten. "After the flange leaks, some studs are found to be unable to be unscrewed normally. Therefore, they can only be twisted under the action of a large external force. The metallographic part of the twisted part indicates that the part exists. There are cracks that break at the root of the thread.

4 The torsion fracture of the conclusion stud is due to the fact that the material itself contains more inclusions and obvious network crystals, which leads to a certain degree of decline in material properties. During the installation and disassembly process, a large external force is encountered and a transient fracture occurs. Detailed analysis of the stray failure event will provide useful help for the selection of high-temperature flange fasteners, raw material inspection, re-inspection, and quality control of the construction process.

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