1. Preface
When a stainless steel threaded component is assembled on site, the nut and bolt of the component are locked and cannot be screwed.
Due to the limitation of installation space (see Fig. 1),
Fig. 1 installation conditions of threaded components
The operator uses the sleeve to cooperate with the long extension rod, and uses the ratchet wrench to pull and screw one-way, and the long extension rod drives the sleeve (see Fig. 2).
Fig. 2 thread assembly installation tool
Screw in the nut in the air.
The bite phenomenon occurs at the fifth thread from the nut to the end face of the bolt tail.
The bite bolt has obvious bending deformation, as shown in Fig. 3.
Fig. 3 bolt deformation of failed parts
The bolts and nuts are made of 1Cr18Ni9Ti steel with the strength of A2-70 and the surface treatment is electropolishing.
2. Macro morphology and mechanism analysis of failed parts
2.1 Macroscopic appearance of failed parts
After the failed part is transversely cut off at the bolt thread, the occluded part is cut along the axial direction by wire cutting.
The macroscopic appearance is shown in Fig. 4.
Fig. 4 macroscopic appearance of failed parts
It can be seen from Fig. 4 that the nut thread of the assembly and the thread of the bolt are asymmetric and biased to one side.
The thread teeth on the side with large clearance have defects, and the bolts that have been bitten have obvious bending deformation.
2.2 Microscopic observation and measurement of failed parts
The failed screw components were pressed, polished, and observed and measured by electron microscope.
Through observation, it is found that the thread engagement between the assembly nut and the bolt is asymmetric, and the thread teeth are biased to one side (see Fig. 5).
Fig. 5 side with large thread clearance
According to the measurement, the difference between the thread spacing on both sides of the failed part is 37.2μm (the larger side clearance is 58.53μm, and the smaller side clearance is 21.33μm), indicating that the axial deviation between the assembly nut and the bolt is too large.
There are two obvious defects on the thread of the bolt at the side with large clearance (see Fig. 6).
Fig. 6 damaged thread on the side with large thread clearance
Some thread teeth are worn off about 70% in the height direction, and there are accumulated superfluous materials near the damaged tooth shape (see Fig. 7).
Fig. 7 accumulation of superfluous materials
The results of energy spectrum analysis show that the deposited superfluous materials have the same composition as the matrix.
Microscopic inspection was carried out on the side with small thread clearance of the failed part. The microscopic morphology is shown in Fig. 8.
Fig. 8 micro morphology of side with small thread clearance
It can be seen from Fig. 8 that the thread material of the nut and the bolt is adhered at many places.
2.3 Thread morphology of failed parts
After cutting the thread components with bite, separate them with external force, and observe the threads of bolts and nuts respectively.
The macro morphology of the threads is shown in Fig. 9 and Fig. 10.
Fig. 9 external thread morphology of failed parts
Fig. 10 internal thread morphology of failed parts
It can be seen from the figure that there are obvious wear and tear marks on the external thread on the side with small clearance between the bolt and the nut, and there are accumulated metal residues on the internal thread.
The results of energy spectrum analysis of the superfluous substance showed that it was consistent with the matrix composition.
Due to the tight connection of the thread on the side with small clearance of the failed part, the material adheres.
After separation by external force, the material on the bolt surface is obviously worn, torn off and transferred.
The debris accumulates between the threads of the nut, which blocks the screw in and out of the thread.
2.4 Metallographic analysis of failed parts
The failed structural parts are separated by wire cutting and then subjected to sample pressing, polishing and nitric alcohol etching.
The microstructure is shown in Fig. 11 and Fig. 12.
Fig. 11 metallographic structure of failed bolt （200 ×）
Fig. 12 metallographic structure of failed nut （250x）
2.5 Measurement of dimension and hardness of failed parts
The dimension measurement of bolts not assembled in the same batch is shown in Table 1.
Table 1 measurement value of bolt size of the same batch
| Bolt size: | Major diameter / mm | Pitch diameter / mm | Tooth angle |
| Measured value | 5.881~5.905 | 5.233~5.252 | 59°24’~59°43’ |
| Standard value | 5.794~5.974 | 5.212~5.324 | 60° |
It can be seen from table 1 that the major diameter, pitch diameter and tooth angle of the bolt meet the requirements of the standard value.
In addition, m6-6g plug gauge is used to check whether the thread of the same batch of nuts is qualified, and the thread of the non-stop gauge is within 1.5 thread.
Vickers hardness measurement was carried out for bolts and nuts of failed structural parts. See Table 2 for the results.
Table 2 hardness measurement values of failed parts (HV)
| Serial number: | 1″ bolt | 2″ nut | 2″ bolt | 2″ nut | 3″ bolt | 3″ nut |
| 1 | 260 | 293 | 245 | 308 | 245 | 307 |
| 2 | 258 | 302 | 245 | 300 | 248 | 299 |
| 3 | 242 | 300 | 247 | 302 | 256 | 304 |
| 4 | 236 | 304 | 236 | 309 | 254 | 305 |
| 5 | 256 | 301 | 242 | 313 | 240 | 292 |
| Average value | 250.4 | 300 | 243 | 306.4 | 248.6 | 301.4 |
It can be seen from table 2 that the material hardness of bolts and nuts meet the standard requirements (standard value ≥ 210HV), but the hardness of bolts is nearly 50HV lower than that of nuts.
3. Comprehensive analysis and discussion
3.1 Failure mechanism analysis
From the observation of the thread morphology of the failed part, it can be seen that the thread surface material of the bolt and the nut is adhered, and the thread surface material is obviously worn, torn off and transferred after separation by external force.
It indicates that the solid-phase adhesion occurs on the thread contact surface of the bolt and the nut, and the external material transfers, that is, the adhesive wear occurs on the surfaces of the two.
The nuts and bolts of failed components are made of cold drawn 1Cr18Ni9Ti steel with the same hardness range (both ≥ 210HV), i.e. the same material and the same hardness.
According to the mechanism of adhesion wear and tribology theory, the mutual solubility of metal friction pairs has a great influence on adhesion wear.
The greater the mutual solubility, the greater the tendency of adhesion wear.
The wear amount of the same material during friction is much larger than that of different materials.
This is because the atoms of the same material have the same arrangement (lattice), the same atomic size, and strong mutual solubility.
Under the action of normal stress, the atoms of the contact surface are easy to diffuse and have a stronger tendency to adhere.
Due to the action of molecular force, the two surfaces are welded.
If the external force can overcome the bonding force of the welding point, the relatively sliding surface can continue to move.
If the shear force occurs on the original contact surface, the wear will not occur;
If the shear force occurs on the side of the metal with lower strength, the metal of the counterpart (failed bolt) will adhere to the surface of the material with higher strength (failed nut), causing the loss of the surface of the part and forming adhesive wear.
3.2 Failure cause analysis
According to the relevant data, the adhesive wear between the friction pairs is mainly related to the material, contact pressure and field assembly of the mating parts.
(1) Material characteristic analysis of failed parts
The miscibility of metal friction pair materials has great influence on adhesion wear. The stronger the miscibility, the greater the wear tendency.
The bolts and nuts of the failed assembly are made of the same material, both of which are cold drawn 1Cr18Ni9Ti steel, which are completely miscible friction pairs.
Meanwhile, the hardness test of the assembly shows that the hardness of the bolts and nuts is basically the same.
The material composition of the friction pair completely conforms to the condition of adhesive wear.
(2) Contact pressure analysis of failed parts
The operator shall use a ratchet wrench to tighten the components when assembling them on site.
From the bolt deformation of the failed part (see Fig. 3), it can be seen that the assembly is subjected to a great installation torque, which causes the bolt deformation, indicating that there is a great contact pressure between the threads during assembly.
When the friction speed is constant, the amount of adhesive wear increases with the increase of the contact pressure. When the contact pressure exceeds 1 / 3 of the material hardness, the amount of adhesive wear increases sharply, and in serious cases, the thread assembly will bite.
(3) Field assembly condition analysis
It can be seen from the installation conditions of threaded components (see Fig. 1) that the connected base blocks the screwing due to space constraints.
The installer uses the sleeve to cooperate with the long extension rod, and uses the ratchet wrench to pull and screw one-way.
The long extension rod drives the sleeve to hang and screw into the nut.
As the nut and sleeve, sleeve and long connecting rod, long connecting rod and ratchet wrench are used for screwing, there is a certain gap between the joints.
In addition, the ratchet wrench is subjected to one-way force, and the direction of force application is not perpendicular to the thread axis, affecting the coaxiality of installation, resulting in the deflection of nut and bolt during assembly.
Through observation, it is found that the thread engagement of the locked bolt and nut is obviously asymmetric, and the whole is biased to one side.
Such a large eccentricity difference is enough to cause the contact pressure on one side to be too large, and finally cause the whole thread to be locked.
(4) There are superfluous objects in the thread pair
The surplus between threads directly hinders the normal screwing between threads.
Through microscopic observation and energy spectrum analysis, it is found that there are accumulated surplus in the bolts and nuts of the failed parts, whose composition is consistent with the thread matrix, that is, the surplus is accumulated due to thread damage during assembly.
When it exceeds the fault tolerance capability of the thread pairs, it will cause serious deflection of the thread, increase the contact pressure between the threads, hinder the normal threading between the threads, and cause the thread pairs to bite.
4. Summary
In this article, the failure analysis of the thread bite failure of the thread assembly during field installation is carried out: the selection of the same material as the friction pair is the internal cause of the binding wear of the thread assembly.
During assembly, the thread assembly is suspended under the action of the ratchet wrench and is subjected to one-way force, which causes the actual force direction of the ratchet wrench to be not perpendicular to the thread axis.
The serious deflection between the nut and the bolt is the main cause of the binding wear of the bolt assembly.
In order to reduce the risk of thread biting, the following measures can be taken to reduce the occurrence of biting.
1) The thread accuracy and surface quality shall be strictly inspected.
The thread surface shall be inspected before each assembly, and there shall be no accumulated superfluous materials.
2) Change the material of thread components and try not to use the same series of materials as thread fitting parts.
3) Use a torque wrench to set a proper torque value and control the screwing speed and tightening torque.
4) Screw the nut to the fitting gasket and then screw it in with a torque wrench to avoid hanging assembly.
5) During installation, ensure that the matched parts are aligned to avoid eccentric installation.
