The fatigue strength of seamless steel pipe materials is extremely sensitive to various internal and external factors. External factors include the shape and size of parts, surface finish, service conditions, etc., and internal factors include the composition, organizational state, purity and residual stress of the material itself. Subtle changes in these factors will cause fluctuations or even large changes in the fatigue properties of materials.

The influence of various factors on fatigue strength is an important aspect of fatigue research. This study will provide the basis for the rational structural design of parts and the correct selection of seamless steel pipes. Materials, reasonably formulate various cold and hot processing processes to ensure that parts have high fatigue performance.

Seven factors affecting the fatigue strength of seamless steel pipe materials:

Conventional fatigue strength is measured on well-machined smooth specimens. However, actual mechanical parts inevitably have gaps in different forms, such as steps, keyways, threads, and oil holes. The existence of these gaps causes stress concentration so that the maximum actual stress at the root of the gap is much greater than the nominal stress of the part, and the fatigue failure of the part often starts from here.

Theoretical stress concentration factor Kt: Under ideal elastic conditions, the ratio of the maximum actual stress at the root of the notch to the nominal stress is obtained from the theory of elasticity.

Effective stress concentration factor (or fatigue stress concentration factor) Kf: the ratio of the fatigue limit σ-1 of the smooth sample to the fatigue limit σ-1n of the notched sample.

The effective stress concentration factor is not only affected by the size and shape of the component, but also by the physical properties of the material, processing, heat treatment, and other factors.

The effective stress concentration factor increases with the sharpness of the notch but is usually smaller than the theoretical stress concentration factor.

Fatigue notch sensitivity coefficient q: The fatigue notch sensitivity coefficient indicates the sensitivity of the material to the fatigue notch, which is calculated by the following formula.

The data range of q is 0-1. The smaller the q value, the less sensitive the seamless steel pipe material is to the notch. Experiments show that q is not a pure material constant, it is also related to the size of the gap. Only when the radius of the notch is greater than a certain value, the value of q has basically nothing to do with the notch, and the value of this radius is also different for different materials or processing states.

Due to the inhomogeneity of the material itself and the existence of internal defects, the increase in size leads to an increase in the failure probability of the material, thereby reducing the fatigue limit of the material. The existence of side effects is an important issue in the application laboratory for real-size parts because it is impossible for exactly similar stress concentrations and stress gradients to exist in real small-size parts. reproduced on samples, resulting in a disconnect between laboratory results and fatigue failure of some specific parts.

There are always uneven processing marks on the processed surface. These traces are equivalent to tiny gaps, which cause stress concentration on the surface of the material, thereby reducing the fatigue strength of the material. Tests have shown that for steel and aluminum alloys, rough machining (rough turning) reduces the fatigue limit by 10%-20% or more compared to longitudinal finishing. The stronger the material, the more sensitive it is to the surface finish.

In fact, no part works with an absolutely constant stress amplitude. The overload and secondary load in the actual work of the material will affect the fatigue limit of the material. Tests show that the material generally has overload damage and secondary load movement.

The so-called overload damage refers to the decrease of the fatigue limit of the material after a certain number of cycles is operated under a load higher than the fatigue limit. The higher the overload, the shorter the number of cycles required to cause damage.

In fact, under certain conditions, a small amount of overload not only does not cause damage to the material, but also strengthens the material due to deformation strengthening, crack tip passivation, and residual compressive stress, thereby increasing the fatigue limit of the material. Therefore, some additions and modifications should be made to the concept of overload damage.

The so-called partial load movement refers to the phenomenon that the fatigue limit increases after the material runs for a certain cycle at a stress level lower than the fatigue limit but higher than a certain limit. The effect of the secondary load exercise is related to the performance of the material itself. In general, materials with good plasticity require longer motion cycles and higher motion stresses to function.

Under certain conditions, there is a close relationship between the fatigue strength and the tensile strength of a material. Therefore, under certain conditions, any alloying element that can increase the tensile strength can increase the fatigue strength of the material. In contrast, carbon is the most important factor affecting the strength of the material. However, some impurity elements that form inclusions in steel have an adverse effect on fatigue strength.

Effect of heat treatment and microstructure Different heat treatment states will obtain different microstructures. Therefore, the effect of heat treatment on fatigue strength is essentially the effect of structure. Materials with the same composition can obtain the same static strength due to different heat treatments, but due to different structures, the fatigue strength can vary in a considerable range. At the same strength level, the fatigue strength of flaky pearlite is significantly lower than that of granular pearlite. It is also granular pearlite, the finer the cementite particles, the higher the fatigue strength.

The effect of microstructure on the fatigue properties 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. Grain refinement increases the fatigue strength of the material.

The inclusions themselves or the holes they produce are equivalent to tiny gaps. Under the action of alternating loads, stress concentration and strain concentration will occur, which will become the crack source of fatigue fracture and have an adverse effect on the fatigue performance of the material. Material. The influence of inclusions on fatigue strength not only depends on the type, nature, shape, size, quantity, and distribution of inclusions but also depends on the strength level of the material and the level and state of the stress.

Different types of inclusions have different mechanical and physical properties and have different effects on fatigue properties. Generally speaking, easily deformable plastic inclusions (such as sulfides) have little effect on the fatigue properties of steel, while brittle inclusions (such as oxides, silicates, etc.). Both are more harmful things.

The degree to which inclusions bond to the base metal also affects fatigue strength. Sulfides are easy to deform and are closely combined with the base metal, while oxides are easy to separate from the base metal, resulting in stress concentration. It can be seen that from the perspective of the types of inclusions, the influence of sulfides is small, while the hazards of oxides, nitrides, and silicates are greater.

Under different loading conditions, inclusions have different effects on the fatigue properties of materials. Under high load conditions, the applied load is sufficient to induce plastic rheology of the material, regardless of the presence of inclusions, and the influence of inclusions is small. The fatigue limits the stress range of the material and the presence of inclusions causes local strain concentration to become the controlling factor of plastic deformation, which strongly affects the fatigue strength of the material. That is to say, the presence of inclusions mainly affects the fatigue limit of the material, but has little effect on the fatigue strength under high-stress conditions.

The purity of the material is determined by the smelting process. Therefore, the use of purification smelting methods (such as vacuum smelting, vacuum degassing, and electro slag remelting, etc.) can effectively reduce the impurity content in steel and improve the fatigue performance of materials.

In addition to the above-mentioned surface finish, the influence of the surface state also includes the change of the mechanical properties of the surface layer and the influence of residual stress on the fatigue strength. Changes in the mechanical properties of the surface may be caused by differences in the chemical composition and structure of the surface, or by deformation and strengthening of the surface.

In addition to increasing the wear resistance of parts, surface heat treatments such as carburizing, nitriding, and carbonitriding are also effective means to improve the fatigue strength of parts, especially corrosion fatigue, and strain.

The effect of surface chemical heat treatment on fatigue strength mainly depends on the loading method, carbon and nitrogen concentration in the infiltrated layer, surface hardness and gradient, ratio of surface hardness to core hardness, layer depth, and residual compressive stress after surface treatment. distribution and other factors. A large number of tests have shown that as long as the notch is processed first, and then chemical heat treatment is performed, generally speaking, the sharper the notch, the greater the increase in fatigue strength.

Under different loading methods, the effect of surface treatment on fatigue performance is also different. During the axial loading process, since the stress is not unevenly distributed along the depth direction, the stress in the surface layer and under the layer is the same. In this case, the surface treatment can only improve the fatigue performance of the surface layer, but the improvement of the fatigue strength is limited because the core material is not strengthened. Under bending and torsion conditions, the stress distribution is concentrated on the surface layer, and the residual stress formed by surface treatment is superimposed on this applied stress so that the actual stress on the surface is reduced. Fatigue strength under torsional conditions.

Contrary to chemical heat treatments such as carburizing, nitriding, and carbonitriding, if the parts are decarburized during the heat treatment process, the surface strength will decrease, which will greatly reduce the fatigue strength of the HSCO carbon steel pipe material. Similarly, the surface coating (such as Cr, Ni, etc.) is fatigued due to the notch effect caused by coating cracks, the residual tensile stress of the coating in the substrate seamless steel pipe, and the immersion of hydrogen during processing. The electroplating process leads to hydrogen embrittlement and other reasons. Reduced strength.

Induction quenching, surface flame quenching, and thin shell quenching of low hardenability steel can obtain a certain depth of surface hardness layer and form favorable residual compressive stress on the surface layer, so it is also an effective method to improve the fatigue strength of parts.

Surface rolling shot peening can form a certain depth of deformation hardening layer on the surface of the sample, and at the same time generate residual compressive stress on the surface, so it is also an effective way to improve the fatigue strength.

The above introduces the factors that affect the fatigue strength of seamless steel pipe materials. If you want to know more or want to buy seamless carbon steel pipes, please contact us.

Baolai is a professional custom seamless pipe manufacturer. As a first-class manufacturer, we are specialized in production, trade and import and export. Our seamless steel tubing is available in sizes 1/2"-36" and complies with API, ASTM, BS, DIN, and JIS standards for liquid and gas transportation and construction.