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Yield Strength of Small-diameter Seamless Pipe

Yield strength: It is the yield limit when the metal material yields, that is, the stress resisting the slight plastic deformation. For metallic materials without obvious yield, the stress that produces 0.2% residual deformation is defined as its yield limit, which is called the conditional yield limit or yield strength. External forces greater than this limit will permanently invalidate the parts and cannot be recovered. For example, the yield limit of low-carbon steel is 207 MPa. When an external force greater than this limit is applied, the part will undergo permanent deformation. If it is less than this, the part will return to its original state.

Seamless steel pipe with small outer diameters can be called small-diameter seamless pipe. Small-diameter seamless steel pipe can also be divided into: seamless small-diameter steel pipe and straight seam (also called welding) small-diameter seamless steel pipe, generally In the case where the outer diameter of the steel pipe is less than 89mm and more than 4mm; they can be collectively called small diameter seamless steel pipe. Some common standards for small diameter seamless steel pipe include GB/T8162, ASTM A53, ASME SA53, etc. Widely used in the manufacture of structural parts and mechanical parts, such as oil drill pipes, automobile transmission shafts, bicycle racks and steel scaffolds used in construction.

When the stress exceeds the elastic limit, the deformation increases rapidly after entering the yield stage. At this time, in addition to the elastic deformation, some plastic deformation also occurs. When the stress reaches point B, the plastic strain increases sharply and the stress and strain fluctuate slightly. This phenomenon is called yielding. The maximum and minimum stresses at this stage are called upper and lower yield points, respectively. Because the value of the lower yield point is relatively stable, it is used as an indicator of the material resistance, called the yield point or yield strength (ReL or Rp0.2).
Some steel (such as high-carbon steel) have no obvious yield phenomenon, and the stress when a small amount of plastic deformation (0.2%) occurs is usually used as the yield strength of the steel, which is called the conditional yield strength.
First explain the material deformation. The deformation of the material is divided into elastic deformation (the original shape can be restored after the external force is revoked) and plastic deformation (the original shape cannot be restored after the external force is revoked, and the shape changes, elongates or shortens). Yield strength is used as the basis for design stress in construction steel.

When the stress exceeds the elastic limit, the deformation increases rapidly after entering the yield stage. At this time, in addition to the elastic deformation, some plastic deformation also occurs. When the stress reaches point B, the plastic strain increases sharply and the stress fluctuates slightly. This phenomenon is called yielding. The maximum and minimum stresses at this stage are called upper and lower yield points, respectively. Because the value of the lower yield point is relatively stable, it is used as an indicator of the material resistance, called the yield point or yield strength (ReL or Rp0.2).
During the test, an automatic recording device was used to draw the force-chuck displacement diagram. It is required that the ratio of the force axis is less than 10N/mm2 per mm, and the curve must be drawn at least to the end of the yield stage. Determine on the curve the constant force Fe of the yield platform, the maximum force Feh before the force first falls in the yield stage, or the minimum force Fel that is less than the initial transient effect.

The yield strength, upper yield strength and lower yield strength can be calculated according to the following formula:
The formula for calculating the yield strength of small-diameter thick-wall seamless steel tubes: Re=Fe/So; Fe is the constant force at yield.
The formula for calculating the upper yield strength: Reh=Feh/So; Feh is the maximum force before the force first falls during the yield stage.
The formula for calculating the lower yield strength: ReL=FeL/So; FeL is the minimum force FeL that is less than the initial transient effect.

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