Mechanism of thread locking in hydrogen-related pipelines and preventive measures
Thread Content
Hydrogen-related pipelines generally use S31603 stainless steel. Apart from welding, other connection methods include slip fittings or threads; the tightening is primarily achieved through the stainless steel threads, with a connection torque of 130–350 N·m, which is much higher than that of carbon steel threads. The high torque and the properties of stainless steel cause the threads to lock up during installation and removal. In mild cases, this leads to difficulties in removal, while in severe cases it makes removal impossible; destructive methods then have to be used to remove and replace the fittings and pipes, increasing the difficulty and cost of repairs. Therefore, it is necessary to analyze the locking mechanism of stainless steel threaded connections and the methods to improve it, as well as to take effective measures for prevention.
Damage analysis of the locked sample: To determine the cause of locking in stainless steel threads, a wire cutting machine was used to cut the stainless steel ferrule sample that had locked along its axis (Figure 1), in order to observe the damage pattern on the thread surfaces after locking occurred. The morphology of the internal and external thread connection under a laser confocal microscope (Figure 2). It can be seen that both the internal and external threads are severely worn; some threads have been worn down by about 70% in height, and wear debris has accumulated between the threads, forming a single mass. The cut ferrule samples were separated, showing the typical damage morphology of the ferrule samples (Figure 3). It can be seen that there is severe wear, with tears in the threads. Debris accumulates between the threads, causing obstruction to their tightening and loosening, which results in the threads becoming completely locked.
Locking process and experimental analysis: Severe wear occurred in the stainless steel threads during the locking process. To analyze the locking process of stainless steel threads, the wear characteristics under different numbers of removal cycles for coated and untreated threads were compared by using a repeated tightening and removal method, with 1, 5, and 10 cycles respectively. The collets in all tests did not lock up. After the test, wire cutting analysis was performed on the collar from the bottom of the thread in a direction perpendicular to the axis: (1) After 1 cycle of tightening and loosening, plow marks appeared on the thread surface, indicating abrasive wear on the thread surface. (Figure 4). (2) After 5 cycles of pre-tightening and removal, adhesive wear occurred on the thread surface, and tearing and peeling due to adhesive wear appeared at the top of the threads. (Fig. 5) (3) After 10 cycles of pre-tightening and loosening, adhesive wear intensified, and the volume of material experiencing tearing and delamination at the top of the threads increased. (Figure 6)
To address the locking issue, tests were conducted using fittings equipped with an anti-seize layer (silver-plated layer) and coated with anti-seize grease; the stainless steel threads of these fittings showed no signs of locking even after 10 cycles of tightening and loosening. Comparing the microscopic morphology (Figures 7 and 8) with Figures 4, 5, and 6, the degree of damage on the thread surface has been significantly reduced.
— 04 — Conclusions and Preventive Measures From the above analysis, it can be seen that as the number of tightening and loosening cycles increases, the wear mechanism on the thread surface shifts from abrasive wear to adhesive wear, with the degree of damage caused by adhesive wear gradually increasing. As the wear on the thread surface increases, the debris resulting from stripping and tearing gradually accumulates between the threads, which hinders the threading process and ultimately leads to locking. The occurrence of locking can be effectively reduced by using anti-seize layered fittings or applying anti-seize grease. To effectively resist stick-slip wear, ensure the anti-locking properties of stainless steel threaded connections, extend the service life of these threads, and reduce maintenance costs, the following four points must be taken into consideration during the installation and use of stainless steel pipe fittings: ①. Clean the surface of the stainless steel threads before installation, ensuring there are no stains or impurities on it ； ②Before installation, the fittings should be inspected for visual defects; any defects are not allowed ； ③. During installation, the thread should align with the centerline of the pipeline, with a deviation angle of ≤2° ； ④. Anti-galling grease or special sealing grease should be applied when installing uncoated pipe fittings.
Is it possible that having the stainless steel threads lock up is exactly what the design institute wants? Use its tendency to lock up to reduce the risk of leaks?
④. Anti-galling grease or special sealing grease should be applied when installing uncoated pipe fittings.