Why do stainless steel bolts and nuts bite?

According to the use performance characteristics and functions of fasteners, the fasteners used in many fields will be made of stainless steel, and the most common disadvantage of stainless steel fasteners is that in the process of use, the bolt and nut can not be unscrewed after matching, which is called "seizure". In general, we can also call it "locking" or "seizure". Why do stainless steel bolts and nuts have this problem? How can we do a good job of prevention to avoid the phenomenon of stainless steel fasteners.

In the process of using bolt and nut, it is difficult to separate the two kinds of fasteners once they are locked. In general, we will cut off the bolt or break the nut to realize the disassembly of accessories, which will directly affect the speed of disassembly of accessories and the reusability of fasteners. Therefore, we need to analyze the principle and cause of the stainless steel fastener to find out the most reasonable preventive measures to prevent the fastener bolt and nut from seizing.

1. The principle of stainless steel bolt and nut seizing
Stainless steel fastener, with rust resistance, will produce a thin oxide layer on the metal surface when the surface is damaged (in the case of austenitic stainless steel, chromium oxide) to prevent further deeper corrosion. When the stainless steel fastener is locked, the pressure and heat generated between the teeth will destroy and wipe off the chromium oxide layer between them, so that the metal teeth will be blocked / sheared directly, and then adhesive phenomenon will occur. When the adhesion continues to occur, the stainless steel fastener will be completely locked, and can no longer be removed or locked. Usually this series of blocking → cutting → sticking → locking takes place in a few seconds.

2. Causes of stainless steel bolt and nut seizing
Through the analysis of the principle of the phenomenon of stainless steel fastener seizing, it can be seen that the seizing of stainless steel fastener is caused by the direct contact of metal materials and the further development of the oxide layer damaged by the pressure or heat between the teeth, so that the excessive pressure or heat between the teeth often causes the phenomenon of stainless steel fastener seizing. For the following reasons, too much pressure or heat is often generated between the teeth, and then the stainless steel fastener will be killed.

1) The teeth are rough or stained with foreign matters. If there are solder joints and other metal chips between the teeth, it will often lead to seizure.
2) Too much tightening force or too fast tightening speed. If the tightening force is too large, the pressure between the teeth will be produced and the teeth will be killed; if the tightening speed is too fast, the temperature between the teeth will rise rapidly and the teeth will be killed.
3) The angle of force application direction is wrong. When tightening, the tightening direction is not perpendicular to the bolt axis, which is easy to cause seizure.

3. Disassembly method of stainless steel bolt and nut
Once the stainless steel bolt and nut are bitten, it is difficult to separate them, and they can only be removed in a destructive way. The bolt is often cut off and the bolt and nut are replaced. But in some cases, if the bolt diameter is large, it is inconvenient to cut off, or the bolt is embedded structure. Once the bolt is damaged, it is inconvenient to replace it. Therefore, it is of practical significance to find some methods that can quickly separate the bitten fastener or remove the nut without destroying the bolt.

The following combined with the actual application, gives several stainless steel bolts and nuts in the case of seizure of the disassembly method.
1) Use thread loosening agent (such as kerosene, lubricating agent, etc.) for lubrication. During the operation, spray the loosening agent on the threaded connection part, let the loosening agent penetrate into the threaded connection part as much as possible, and use the rubber hammer and other tools to tap the nut circle lightly to make the threaded connection vibrate. After spraying the loosening agent for about 30 minutes, try to remove the nut. Through practical application, it can be found that this method has obvious effect on the removal of corroded fasteners, but has limited effect on the removal of threaded fasteners.
2) For the nut which is still unable to be disassembled after spraying the loosening agent, we can try to use the thermal expansion method, that is to use the welding gun and other tools to heat the nut, so that the nut will expand, and then remove the nut. The use of this method is limited to some extent. It is necessary to consider whether there is space for hot work around fasteners and whether fasteners and connected parts can withstand high temperature and high heat.
3) Special tools (such as nut splitters, etc.) can be directly used to remove the bitten bolts and nuts. When the nut cleaver is used for operation, the nut cleaver is sleeved on the nut, and the cutter head is pushed through mechanical force or hydraulic pressure to split the nut, so as to separate the nut from the bolt. The method is convenient and fast, especially using hydraulic system to push the cutter head, saving time and labor. However, it should be noted that some improvements should be made in the operation method if the bolts are to be kept intact and only the nuts are to be removed. Because at the moment when the cutter head splits the nut, because of inertia, the cutter head also leaves indentation on the bolt stud. In the process of real-time application, try to control the feed speed of the cutter head, and still leave a certain trace on the bolt. Through the field test, it is found that only the nut is extruded, and it stops immediately after a small amount of deformation. The nut has visible deformation and can be removed, but there is almost no damage to the bolt thread.