1. The influence of thread valley shape and radius size.
When the bolt is stressed, stress concentration will occur at the thread valley, and its value depends to a large extent on the shape of the thread valley. Changing the shape of the valley, for example, the smoother the valley groove of the thread, the smaller the stress concentration and the higher the fatigue strength. In general, flat-bottomed threads have low fatigue strength. If rounded valleys are used instead of flat-bottomed valleys, the fatigue strength of the bolt can be improved. For example, the elastic stress concentration coefficient of the flat-bottom thread valley is 2.54, while the improved arc groove is 1.52, that is, the latter's valley stress concentration coefficient is 40% lower than the former, which can increase the fatigue strength by at least 20%; if The fatigue strength of quenched and tempered 40CrNiMo steel bolts with M6-1.0 flat-bottomed valleys is 95MPa. When using arc-shaped valleys with a large radius of 0.1mm, the fatigue strength can be increased to 120MPa, which is an increase of 26 %. The fatigue strength of CD (critical design for fracture) bolts newly developed by Japan's Nippon Steel Corporation has been increased even more, up to 100%. The main feature of CD bolts is that the crest height of the internal thread of the nut gradually decreases to allow it to bear force. More uniform.
2. The influence of thread surface roughness.
The surface roughness of the thread has a great influence on the fatigue life of the bolt. For example, when the roughness of a 40CrNiMo steel bolt with an M6-1.0 thread is reduced from 0.08 to 0.16 to 0.63 to 1.35, the fatigue strength is reduced by 33%; for a bolt with an M12-1.5 thread, the surface roughness is reduced from 0.08 to 0.16 to When 0.16~0.32, the fatigue strength decreases by 21%.
3. Influence of thread rolling process.
Rolling threads will produce a deformation strengthening layer and high residual compressive stress, which plays a great role in preventing the initiation and early expansion of fatigue cracks; at the same time, it will also reduce the surface roughness of the valley, thus benefiting the fatigue strength of the bolt. improvement. However, if the thread is rolled and then heat treated, the above beneficial factors will disappear. Therefore, from the perspective of improving the fatigue performance of bolts, the threads should be rolled after heat treatment. But there is another problem at this time, that is, the hardness of bolts, especially high-strength bolts, is usually higher after heat treatment, which reduces the life of the thread rolling die. In addition, if the quality of the thread rolling is not good enough and micro-cracks or spalling phenomena similar to contact fatigue occur on the surface or root of the thread, the effect of improving the fatigue performance of the bolt will not be obvious, and the fatigue performance will even be reduced.
4. The influence of metallurgical defects in steel.
Decarburization on the surface of raw materials is usually caused by the lack of effective protection of the blank surface during the rolling and heating process. If the decarburization layer is shallow and the finished product needs to undergo sufficient cutting processing, the decarburization layer will be removed, thereby eliminating the impact of this decarburization. However, some bolts are no longer machined after cold heading or cold drawing, so the surface defects of the raw materials remain on the surface of the finished parts.
The severe decarburization layer on the surface of the bolt is a weak area on it. During the thread rolling process after cold heading, due to the large deformation of the steel surface, most of the decarburization layer will be squeezed into the top area of the thread. The strength and hardness of this decarburized layer are very low, so it is prone to wear and tripping (threads are sheared), and it can easily become a source of fatigue cracks, causing early fatigue failure.
Inclusions in steel, especially large hard and brittle inclusions, destroy the continuity of the matrix material. Under the action of internal and external stress, high stress concentration is easily generated at the interface between the inclusions and the matrix, leading to the early initiation of fatigue cracks. Significantly reduces the fatigue resistance of high-strength bolts.
