Various factors affect the fatigue strength, which is an important aspect of fatigue research. This research provides a basis for the rational design of components, the correct selection of materials for seamless steel pipes, and the formulation of various cold and hot working processes to ensure that the parts have high fatigue performance.
Due to the unevenness of the material structure itself and the presence of internal defects, an increase in size increases the probability of material failure, thereby reducing the material's fatigue limit. The existence of the size effect is an important problem in applying fatigue data obtained from laboratory small samples to actual size parts. It is impossible to completely reproduce stress concentrations and stress gradients that exist on actual-sized parts on small samples, resulting in a disconnect between laboratory results and the fatigue failure of some specific parts.
The surfaces produced by machining always have uneven processing marks, which are equivalent to small defects, causing stress concentration on the surface of seamless steel pipes and thus reducing the fatigue strength of materials. Experiments have shown that for steel and aluminum alloys, roughing (coarse turning) can reduce the fatigue limit by 10%-20% or even more compared to longitudinal fine polishing. The higher the strength of the material, the more sensitive it is to surface smoothness.
In fact, no part works under absolutely constant stress amplitude conditions, and overloading and subloading in actual material work will affect the material's fatigue limit. Experiments have shown that materials commonly exhibit superload damage and subload training. The so-called superload damage means that seamless steel pipe materials running under loads higher than the fatigue limit for a certain number of cycles will cause a decrease in the fatigue limit. The higher the overload, the shorter the time required to cause damage.
In fact, under certain conditions, a few overloads will not damage the material. Due to the effects of deformation hardening, crack tip blunting, and residual stress, it can strengthen the material and thus increase the material's fatigue limit. Therefore, some supplements and revisions need to be made to the concept of overloading damage. The so-called sub-load training refers to the phenomenon that the fatigue limit of the material increases after the material runs for a certain number of cycles under a stress level lower than the fatigue limit but higher than a certain limit value. The effect of sub-load training depends on the properties of the material itself, and materials with good plasticity generally require longer training cycles and higher training stresses to be effective.
The fatigue strength of the material has a closely related relationship with the tensile strength under certain conditions. Therefore, any alloying element that can enhance the tensile strength under certain conditions can improve the fatigue strength of the material. Comparatively speaking, carbon is the most important factor affecting the strength of the material. Impurity elements that form inclusions in steel adversely affect the fatigue strength. The influence of heat treatment and microstructure is different. Different heat treatment states will result in different microstructures. Therefore, the influence of heat treatment on fatigue strength is essentially the influence of microstructure. Materials with the same composition can have a range of fatigue strengths due to different heat treatments, even with the same static strength due to different organizations.
Under the same strength level, the fatigue strength of lamellar pearlite is significantly lower than that of granular pearlite. Among the granular pearlite, the finer the carbide particles, the higher the fatigue strength. The influence of microstructure on the fatigue performance of materials is not only related to the mechanical properties of various structures but also related to the grain size and distribution characteristics of the structures in the composite structure. Refining the grain size can improve the fatigue strength of stainless steel seamless pipe produced by stainless steel seamless pipe suppliers.