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Seven Factors Affecting the Fatigue Strength of Seamless Pipe Materials

Date:2021-01-29    keywords:Fatigue Strength, Seamless Steel Pipe Materials
The fatigue strength of seamless steel pipe materials is extremely sensitive to various external and internal factors. External factors include the shape and size of the parts, surface finish and conditions of use, etc., and internal factors include the composition of the material itself, organization state, purity, and residual stress. The subtle changes of these factors will cause fluctuations or even substantial changes in the fatigue properties of the material.

The influence of various factors on fatigue strength is an important aspect of fatigue research. This research will provide the basis for the reasonable structural design of parts, the correct selection of seamless steel pipe materials and the rational formulation of various cold and hot processing techniques to ensure that the parts have high Fatigue performance.

1. The effect of stress concentration
The conventional fatigue strength is measured with carefully processed smooth specimens. However, the actual mechanical parts inevitably have gaps of 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 borne by the part, and the fatigue failure of the part often starts 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 obtained by elastic theory.
Effective stress concentration factor (or fatigue stress concentration factor) Kf: the ratio of the fatigue limit σ-1 of the smooth specimen to the fatigue limit σ-1n of the notched specimen.
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 it 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 notches, and is calculated by the following formula.
The data range of q is 0-1. The smaller the value of q, the less sensitive the seamless steel pipe material to the gap. Experiments show that q is not purely a material constant, it is still related to the gap size, only when the gap radius is greater than a certain value, the q value is basically independent of the gap, and for different materials or processing conditions, the radius value is also different.

2. The influence of size
Due to the inhomogeneity of the material itself and the existence of internal defects, the increase in size will increase the probability of material failure, thereby reducing the fatigue limit of the material. The existence of the size effect is an important problem in applying the fatigue data measured by the small sample in the laboratory to the actual size of the part. Because it is impossible to make the stress concentration and stress gradient on the actual size part completely similar to the small Reproduced on the sample, causing the disconnection between the laboratory results and the fatigue failure of certain specific parts.

3. The influence of surface processing
There are always uneven processing marks on the machined surface. These marks 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 can reduce the fatigue limit by 10%-20% or even more than longitudinal fine polishing. The higher the strength of the material, the more sensitive it is to surface finish.

4. The impact of loading times
In fact, no part works under absolutely constant stress amplitude conditions. The overload and secondary load in the actual work of the material will affect the fatigue limit of the material. Tests have shown that the material is generally exposed to overload damage and secondary load exercise.
The so-called overload damage refers to the reduction of the fatigue limit of the material after the material runs for a certain number of cycles under a load higher than the fatigue limit. The higher the overload, the shorter the cycles required to cause damage.

In fact, under certain conditions, a small number of overloads will not cause damage to the material, but will also strengthen the material due to deformation strengthening, crack tip passivation and residual compressive stress, thereby increasing the fatigue limit of the material. Therefore, the concept of overload damage should be supplemented and revised. The so-called sub-load exercise refers to the phenomenon that the material fatigue limit rises after the material runs for a certain number of times under a stress level lower than the fatigue limit but higher than a certain limit.
The effect of sub-load exercise is related to the performance of the material itself. For materials with good plasticity, the exercise cycle is generally longer and the exercise stress is higher to be effective.

5. The influence of chemical composition
There is a close relationship between the fatigue strength and tensile strength of materials under certain conditions. Therefore, under certain conditions, all alloy elements that can increase the tensile strength can increase the fatigue strength of the material. In comparison, carbon is the most important factor affecting material strength. 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 result in different microstructures. Therefore, the effect of heat treatment on fatigue strength is essentially the effect of microstructure. The material of the same composition can obtain the same static strength due to different heat treatments, but due to the difference in structure, the fatigue strength can vary within a considerable range.

At the same strength level, the fatigue strength of flake pearlite is significantly lower than that of granular pearlite. The same is granular pearlite, the smaller the cementite particles, the higher the fatigue strength.

The influence of the microstructure on the fatigue properties of materials is not only related to the mechanical properties of various organizations themselves, but also to the grain size and the distribution characteristics of the structure in the composite structure. Refining the grains can improve the fatigue strength of the material.

6. The influence of inclusions
The inclusion itself or the hole created by it is equivalent to a tiny gap, which will produce stress concentration and strain concentration under the action of alternating load, which will become the source of fatigue fracture and have a negative impact on the fatigue performance of the material. The influence of inclusions on fatigue strength not only depends on the type, nature, shape, size, number and distribution of inclusions, but also depends on the strength level of the material and the level and state of applied stress.

The mechanical and physical properties of different types of inclusions are different, and the difference between the properties of the base metal is different, and the effect on the fatigue performance is also different. Generally speaking, easily deformable plastic inclusions (such as sulfides) have little effect on the fatigue performance of steel, while brittle inclusions (such as oxides, silicates, etc.) have greater harm.
Inclusions with a larger expansion coefficient than the matrix (such as sulfides) have little effect due to compressive stress in the matrix, while inclusions with a smaller expansion coefficient (such as alumina, etc.) have a greater influence due to tensile stress in the matrix.
The tightness of the combination of inclusions and base metal also affects the fatigue strength. Sulfides are easy to deform and bond closely with the base material, while oxides are easy to separate from the base material, causing stress concentration. It can be seen that from the type of inclusions, sulfides have less influence, while oxides, nitrides and silicates are more harmful.

Under different loading conditions, inclusions have different effects on the fatigue properties of materials. Under high load conditions, regardless of the presence of inclusions, the external load is sufficient to cause the material to produce plastic rheology, and the influence of inclusions is small. The fatigue limit stress range of the material, the presence of inclusions causes the local strain concentration to become the controlling factor of plastic deformation, which strongly affects the fatigue strength of the material. In other words, the presence 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 smelting, vacuum degassing and electroslag remelting, etc.) can effectively reduce the impurity content in the steel and improve the fatigue performance of the material.

7. The effect of surface property changes and residual stress
In addition to the surface finish mentioned above, the influence of the surface condition also includes the change in 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 layer can be caused by the chemical composition and organization of the surface layer, or the surface layer can be caused by deformation strengthening.

Surface heat treatments such as carburizing, nitriding, and carbonitriding can increase the wear resistance of the parts, and are also an effective means to improve the fatigue strength of the parts, especially to improve corrosion fatigue and biting corrosion.

The effect of surface chemical heat treatment on fatigue strength mainly depends on the loading method, the carbon and nitrogen concentration in the infiltration layer, the surface hardness and gradient, the ratio of surface hardness to core hardness, the depth of the layer, and the magnitude and magnitude of the residual compressive stress formed by the surface treatment. Factors such as distribution. A large number of tests have shown that as long as the notch is processed first and then subjected to chemical heat treatment, generally speaking, the sharper the notch, the more the increase in fatigue strength.

Under different loading methods, surface treatment has different effects on fatigue performance. During axial loading, since there is no uneven distribution of stress along the depth of the layer, the stresses on the surface and under the layer are the same. In this case, the surface treatment can only improve the fatigue performance of the surface layer. Since the core material has not been strengthened, the increase in fatigue strength is limited. Under bending and torsion conditions, the stress distribution is concentrated on the surface. The residual stress formed by the surface treatment and this applied stress are superimposed to reduce the actual stress on the surface. At the same time, due to the strengthening of the surface material, it can effectively improve the bending and Fatigue strength under torsion conditions.

Contrary to chemical heat treatment such as carburizing, nitriding and carbonitriding, if the parts are decarburized during the heat treatment process, the strength of the surface layer is reduced, and the fatigue strength of the seamless steel pipe material of Permanent Steel Manufacturing Co.,Ltd will be greatly increased. reduce. Similarly, surface coatings (such as Cr, Ni, etc.) due to the notch effect caused by cracks in the coating, residual tensile stress caused by the coating in the base seamless steel pipe, and hydrogen embrittlement caused by the infiltration of hydrogen during the electroplating process, make fatigue The intensity is reduced.

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

Surface rolling and shot peening treatments 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, which is also an effective way to improve the fatigue strength.

Tips: ASTM A53 covers seamless and welded steel pipe with nominal wall thickness. The surface condition is usually black and hot-dipped galvanized. ASTM A53 is produced mainly for pressure and mechanical applications, and is also used for transport of steam, water, gas line pipes.

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