7 Factors affecting the fatigue strength of seamless steel pipe materials
Keywords: metal fatigue strength, seamless pipe materials
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 the part, surface finish and use conditions, etc., and internal factors include the composition, tissue state, purity and residual stress of the material itself. Subtle changes in these factors can cause fluctuations or even substantial changes in the fatigue properties of materials.
The influence of various factors on fatigue strength is an important aspect of fatigue research. This research will provide the basis for the rational structural design of parts, the correct selection of seamless steel pipe materials, and the rational formulation of various hot and cold processing processes to ensure that the parts have high fatigue performance.
Seven factors affecting the fatigue strength of seamless steel pipe materials：
1. The effect of stress concentration
The conventional fatigue strength is measured with carefully machined smooth specimens. However, actual mechanical parts inevitably have different forms of notches, 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 larger than the nominal stress of the part, and the fatigue failure of the part often starts from here.
Theoretical stress concentration factor Kt: the ratio of the maximum actual stress to the nominal stress at the root of the notch, obtained from the elastic theory under ideal elastic conditions.
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 fatigue notch, and 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 purely a material constant, it is still related to the size of the notch. Only when the radius of the notch is greater than a certain value, the value of q is basically independent of the notch, and the value of this radius is also different for different materials or processing states.
2. The influence of size factors
Due to the inhomogeneity of the material itself and the existence of internal defects, the increase in size leads to an increase in the probability of material failure, thereby reducing the fatigue limit of the material. The existence of the size effect is an important issue in applying the fatigue data measured by the small sample in the laboratory to the actual size parts, because it is impossible to completely similar the stress concentration and stress gradient existing on the actual size parts in the small size. reproduced on the sample, resulting in a disconnect between the laboratory results and the fatigue failure of some specific parts.
3. The influence of the surface processing state
There are always uneven machining marks on the machined surface. These marks are equivalent to tiny gaps, causing 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 surface finish.
4. Effects of Loading Experience
In fact, no part works under the condition of absolutely constant stress amplitude. The overload and secondary load in the actual work of the material will affect the fatigue limit of the material. The test shows that the material generally has overload damage and secondary load exercise phenomenon.
The so-called overload damage refers to the decrease of the fatigue limit of the material after the material has been operated under a load higher than the fatigue limit for a certain number of cycles. The higher the overload, the shorter the number of cycles required to cause damage.
In fact, under certain conditions, a small number of overloads will not only cause no damage to the material, but also strengthen the material due to deformation strengthening, crack tip passivation and residual compressive stress, thereby increasing the fatigue limit of the material. Therefore, some supplements and modifications should be made to the concept of overload damage. The so-called sub-load exercise refers to the phenomenon that the fatigue limit of the material increases after a certain cycle of operation at a stress level lower than the fatigue limit but higher than a certain limit. The effect of the second load exercise is related to the performance of the material itself. Generally speaking, the material with good plasticity needs a longer exercise cycle and a higher exercise stress to be effective.
5. The effect of chemical composition
There is a close relationship between the fatigue strength and the tensile strength of the material under certain conditions. Therefore, under certain conditions, all alloy elements that can improve the tensile strength can improve the fatigue strength of the material. In comparison, 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.
Influence of heat treatment and microstructure Different heat treatment states will obtain different microstructures. Therefore, the influence of heat treatment on fatigue strength is essentially the influence of microstructure. Materials of the same composition, due to different heat treatments, can obtain the same static strength, but due to different structures, the fatigue strength can vary within a considerable range.
At the same strength level, the fatigue strength of flaky pearlite is significantly lower than that of granular pearlite. The same is granular pearlite, the finer the cementite particles, the higher the fatigue strength.
The influence of microstructure on the fatigue properties of materials is not only related to the mechanical properties of various structures, but also to the grain size and the distribution characteristics of the structures in the composite structure. Grain refinement increases the fatigue strength of the material.
6. Effects of Inclusions
The inclusions themselves or the holes generated by them are equivalent to tiny gaps, and under the action of alternating loads, stress concentration and strain concentration will occur, which will become the crack source of fatigue fracture, and will adversely affect the fatigue performance of the material. The influence of inclusions on fatigue strength depends not only on the type, nature, shape, size, quantity and distribution of the inclusions, but also on the strength level of the material and the level and state of the applied 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.) are more harmful.
Inclusions with a larger expansion coefficient than the matrix (such as sulfide) have little influence due to the compressive stress in the matrix, while inclusions with a smaller expansion coefficient than the matrix (such as alumina, etc.) have a greater influence due to the tensile stress in the matrix.
How tightly the inclusions bond to the base metal also affects the fatigue strength. Sulfide is easy to deform and is closely combined with the base metal, while oxide is easy to detach from the base metal, resulting in stress concentration. It can be seen that from the type of inclusions, the influence of sulfides is small, while oxides, nitrides and silicates are more harmful.
Under different loading conditions, the influence of inclusions on the fatigue properties of the material is also different. Under high load conditions, regardless of the existence of inclusions, the applied load is sufficient to cause plastic rheology of the material, and the influence of inclusions is small. The fatigue limit stress range of the material and the presence of inclusions cause local strain concentration to become the controlling factor for plastic deformation, which strongly affects the fatigue strength of the material. That is to say, the existence of inclusions mainly affects the fatigue limit of the material, and 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 melting, vacuum degassing and electroslag remelting, etc.) can effectively reduce the impurity content in the steel and improve the fatigue performance of the material.
7. Changes in surface properties and effects of residual stress
In addition to the surface finish mentioned above, the influence of the surface state also includes changes in the mechanical properties of the surface layer and the influence of residual stress on the fatigue strength. The change of the mechanical properties of the surface layer can be caused by the difference in the chemical composition and structure of the surface layer, or it can be caused by the deformation and strengthening of the surface layer.
In addition to increasing the wear resistance of parts, surface heat treatment such as carburizing, nitriding and carbonitriding is also an effective means to improve the fatigue strength of parts, especially corrosion fatigue and galling.
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, the ratio of surface hardness to core hardness, layer depth, and the magnitude of residual compressive stress formed by surface treatment. distribution and other factors. A large number of tests have shown that as long as the notch is processed first and then chemically heat treated, generally speaking, the sharper the notch, the more the fatigue strength is improved.
Under different loading methods, the effect of surface treatment on fatigue performance is also different. During axial loading, since there is no uneven distribution of stress along the depth of the layer, the stress on the surface and under the layer is the same. In this case, the surface treatment can only improve the fatigue performance of the surface layer, and the improvement of 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 the surface treatment and this applied stress are superimposed, so that the actual stress on the surface is reduced. Fatigue strength in torsional condition.
Contrary to chemical heat treatment such as carburizing, nitriding and carbonitriding, if the parts are decarburized during the heat treatment, the strength of the surface layer is reduced, which will greatly reduce the fatigue strength of HSCO carbon steel pipe materials. Similarly, the surface coating (such as Cr, Ni, etc.) is fatigued due to the notch effect caused by cracks in the coating, the residual tensile stress caused by the coating in the base seamless steel pipe, and the immersion of hydrogen during the electroplating process to lead to hydrogen embrittlement and other reasons. Intensity decreases.
Induction quenching, surface flame quenching and thin-shell quenching of low hardenability steel can obtain a surface hardness layer of a certain depth, and form a favorable residual compressive stress on the surface layer, so it is also an effective method to improve the fatigue strength of parts.
Surface rolling and 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.
sales@hu-steel.com
English
Español
