Fracture Failure Analysis of SA453 Stud in Heat Exchanger Floating Head Cover

Failure Analysis of SA453 Stud of Heat Exchanger Floating Head Cover Chen Sunyi (Maoming Gravity Petrochemical Machinery Manufacturing Co., Ltd., Maoming, Guangdong 525024, China) 1I A batch of heat exchangers appeared two times before and after the high-strength studs in the assembly of the floating head cover. Chemical composition, mechanical properties, macroscopic fracture, metallographic observation, structural stress calculation and stud manufacturing process investigation, etc., comprehensive analysis of the fracture causes. The results show that the stress concentration of the stud and nut together is the main reason that the maximum shear stress exceeds the shear strength of the material and causes the stud to be broken. The maximum equivalent stress is obviously greater than the tensile strength, resulting in the material at the fracture. High strength and brittle fracture; the failure of the heat treatment process is the secondary cause of stud fracture, which is caused by the irregular operation of the stacking furnace heat treatment in the mass production of the stud; the stud is improved after vacuum tempering and heat treatment. Mechanical properties and hardness, after standardizing the fastener assembly operation process, meet the requirements for use.

Checked 28. 05mm; the calculated diameter of the thread is 3 found di=26.752mm; the original triangle height of the thread is H=3sin602.60mm. Calculated as As. 06mm2, the maximum allowable torque Tmai= 30=1.04x106Nm104kgm. In actual operation The torque applied by the torque wrench to the stud should not exceed the maximum allowable torque value even if the design value required for the seal is not reached.

If the safety factor is considered, the permitted torque is lower.

Shear stress check. In material mechanics, the shear stress 2 at any point on the cross section of the cylinder is m; p is the distance from any point on the cross section to the center of the circle. For the surface of the stud, the value is equal to the radius of the outer diameter of the external thread, ie 0.5 di =13. 376mm; / is the polar moment of inertia of the cross-section facing the centroid, (2) the shear stress of the stud is calculated to be 271.6MPa. However, due to the nut and the stud biting, the original cylindrical stud is transformed into a belt. For the shaft structure of the step, the stress concentration corner appears. According to the size of the nut and the stud, in the comparison 4 = 42.8/26.7521.60, the stress concentration factor 夂 = 2.8 is obtained, and the maximum shear position of the stud is broken. The stress is Tmai=1T Of. In addition, according to the VonMises yield criterion, there is also Tf=ay/, even if the maximum coefficient is 0.60, the Tf=0.60Rm=0.60x754 can be obtained according to the tensile strength detection value in Table 4. = 452.4 MPa, if calculated according to the yield strength detection value in Table 4, Tf = 0.60 ReL = 0.60 x 513 = 307.8 MPa, even if the maximum value of the tensile strength test in Table 4 is calculated, only Tf = 0.60 Rm = 1046=627.6MPa. Accordingly, T Tf, the maximum shear stress at the fracture exceeds the shear strength of the material, and the shear stress at other locations on the stud is lower than the allowable stress of the material. Theoretically, the stud is broken by shear stress.

Tensile stress check. According to the design calculation, the minimum stud load when the stud is pre-tightened is Wa=1843 847.9N, the total cross-sectional area of ​​the actual stud is Ab=29228.4, the average tensile stress of the stud is the same, and the stud is transformed into a stepped The tensile stress concentration also occurs in the shaft structure. According to the size of the nut and the stud, the stress concentration curve parameter of the control is calculated as -4c6.7521.60, and the stress concentration factor of the figure is 1.85. The sharp point of the root is used as the notch on the surface of the smooth cylinder, and the structure will cause stress concentration. According to the tooth size of the stud, the stress concentration curve of the control 4 is calculated separately from the -4b) in the comparison 4. The composite theoretical stress concentration coefficient = 1. =7.03, the maximum shear stress at the fracture position of the stud is calculated according to the test value of the yield strength in Table 4, L = 513 MPa, and the allowable stress is calculated as the safety factor of 1.5 = 513 / 1.5 = 342 MPa. Therefore, a bending stress. In practice, bolt-type parts are inevitably notched and subjected to skew loads. Sometimes the notch sensitivity of the forward load is not large, but it is more obvious in oblique stretching. H.5 finite element analysis of the sealing performance of the bolt-flange-gasket system, and found that internal pressure and high temperature The lower bolt is also subjected to a bending moment, the inner side is pulled and the outer side is pressed. Since the fracture occurs when the equipment is assembled, it is not subjected to internal pressure and thermal load, and normally there is no bending stress in the stud.

Equivalent stress. The VonMises plastic maximum equivalent stress at the break of the stud under the action of torque is composed of the maximum shear stress and the maximum tensile stress. The following formula is calculated. Therefore, the aeq 3.3 system analysis further checks the nut and finds that the nut material used for the column material is SA193 here nut material design and studs of the same material, the hardness is basically the same hardness as the stud, easy to damage the stud; the two with the dimensional accuracy is not enough, the nut pressing surface is not completely perpendicular to the stud center line; stud break The position of the middle right side is directly supported by the wrench. The nut on the left side is not locked by the tool; therefore, the force on both ends of the stud is significantly different, and the orientation of the force in the operation is incorrect. The complexity of the column body is a conditional factor in the occurrence of this failure; unknown factors. This mainly means that technicians and operators have insufficient knowledge of the hydraulic wrench, resulting in the failure to use the hydraulic wrench correctly.

Unknown factors can be further divided into 63: (1) The concept is unclear. The pre-tightening torque of the mechanical bolt design, the flange pre-tightening torque of the pressure vessel bolt design, the flange operating torque and the pre-tightening force along the axial direction of the bolt are four different concepts. The output load of the hydraulic wrench pressure pump and the output load of the hydraulic wrench are also two different concepts. The former is the pressure and the latter is the torque. When the user adjusts the output load limit of the hydraulic wrench, the load required by the heat exchanger design is required. Limits are based on the recommended load data for the various bolts provided in the manual for the hydraulic wrench. Manufacturing enterprise personnel must clearly understand the differences between various load concepts when converting various data, and the basis for selecting the conversion formula must be reliable to avoid errors. (2) The mechanical nature of the tool is different. Non-standard tightening of the studs will cause the bolts to be preloaded and overloaded, causing stress corrosion of the bolts where stresses such as threads are concentrated. The data sheets for metric bolt preload and torque estimation are generally recommended in the hydraulic wrench product specifications of various brands; however, the friction coefficient selected in the data calculation, the ratio of the bolt allowable load to the yield limit, the bolt strength grade and The safety factors are different. When the hydraulic tools provide two separate instructions in Chinese and English, the Chinese manual is often provided by the middleman of the supply tool. The English manual is the original manual of the tool manufacturer. The content is comprehensive, accurate and detailed. . (3) Lack of management. As a tool with metering function, the hydraulic wrench was first used by foreign inventions and introduced into China. In the past 10 years, localized products appeared. The statutory verification period and testing institutions are not clear. The error after many years of use is unknown. In particular, when the initial preload is not sealed, the operator tends to self-adjust the output torque. In the case of this study, the actual torque output by the hydraulic wrench to the stud has not been identifiable for the above reasons.

A full visual inspection of the nuts and other unbroken stainless steel studs was performed and no defects were found.

4Technical countermeasures and effects According to the above comprehensive analysis, the phenomenon of stud fracture is individual, although it is not an accidental phenomenon, but the basic reason is clear, mainly because the torsional stress concentration makes the shear stress exceed the shear strength of the material, and the solution treatment When the batch studs are unevenly distributed or the temperature in the furnace is not uniform, a small part of the studs are not treated properly, and it is not necessary to perform more microscopic and detailed metallographic analysis on the fractured parts. Take the following countermeasures: Most of the first two batches of studs are not used. The slightly lower Ti content of individual materials has limited adverse effects on heat treatment dispersion, which can be satisfied by reheat treatment. Since the stud is already a finished product, there is no machining allowance in size, and in order to avoid heat treatment deformation, vacuum tempering is decided. Considering the hardness matching principle of nuts and bolts, the hardness of the nuts is slightly lower than that of the bolts (about 30 HBS) to protect the bolt threads; therefore, the appropriate stud hardness should be determined according to the measured hardness of the nuts to be about 295 HBW. According to these principles Adjust the heat treatment process, especially in the batch heat treatment production operation to avoid dense stud stacking, and strictly control the accuracy and uniformity of the heat treatment temperature.

Mechanical properties were tested on a per-batch basis.

Pay attention to the protection thread during inspection and use, and add some oil 08 during installation to avoid the formation of frictional occlusion source. After the stud is cleaned and then hardened and heat treated, the mechanical properties and hardness reach the standard index value, although the index is close to the lower limit; but it can meet the assembly and pressure test requirements.

It is better to phosphatize the surface of the stud. This chemical and electrochemical reaction forms a phosphate chemical conversion film, and the phosphate conversion film formed is called a phosphate film. The purpose is mainly to provide protection to the base metal, to prevent the metal from being corroded to a certain extent; to be used for primering before painting, to improve the adhesion and corrosion resistance of the paint layer; and to reduce friction and lubrication in the metal cold working process. There are many kinds of phosphating treatments, such as Dacromet or electroless Ni-P alloy plating; and Parkerizing phosphating surface treatment is to heat-treat the steel parts with iron manganese phosphate aqueous solution to form a phosphate film. Metal anti-corrosion method.

5 Conclusions Stress analysis shows that the torsional stress concentration caused by the stepped structure of the stray break position causes the maximum shear stress to exceed the shear strength of the material, causing the stud to be broken; the stepped structure at the fracture position and the sharp point of the thread root The combined stress concentration caused by the notch effect increases the tensile stress level. The maximum tensile stress is significantly greater than the allowable stress, but less than the yield strength. Although the tensile force does not play a major role in the fracture, it has a significant disadvantage to the flange sealability. influences. The maximum equivalent stress is significantly greater than the tensile strength, which is about 4 times the allowable stress, so that the material at the fracture completely exceeds the elastic state, and the brittle fracture is exhibited due to its high strength. In short, the high stress level is the main cause of this failure.

The shear stress and tensile stress at other locations on the stud are lower than the allowable stress of the material, and the equivalent stress at other locations is also lower than the yield strength of the material, so no fracture occurs.

Inadvertently causing damage to the stud thread and improper tightening operation during assembly, causing the stud and nut to bite together, forming a structural concentration point of torsional stress and tensile stress, is an external cause of this failure accident. The influence of low Ti content on the bulk structure of the bulk material is not the main cause of stud fracture, and the microstructure of the material has no obvious defects. It is recommended to standardize the fastener assembly process and improve the quality of work.

The two fractured studs are incomplete due to solution heat treatment and hardening heat treatment, and the strength and hardness of the stud materials are low, which is the individual factor and secondary cause of this failure accident. Stud manufacturing process monitoring should be strengthened to improve the quality of stud production.

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