Mechanical performance indicators of seamless steel pipes
The mechanical properties of steel are important indicators to ensure the ultimate usability (mechanical properties) of steel, which depend on the chemical composition and heat treatment system of the steel. In steel pipe standards, tensile properties (tensile strength, yield strength or yield point, elongation), hardness and toughness indicators, as well as high and low temperature properties required by users, are specified according to different usage requirements.
① Tensile strength（ σ b）
The maximum force (Fb) that the specimen bears during the tensile process at break, divided by the stress obtained by dividing the original cross-sectional area (So) of the specimen（ σ）， Called tensile strength（ σ b) , in N/mm2 (MPa). It represents the maximum resistance of metal materials to failure under tensile force.
② Yield point（ σ s）
The stress at which a metal material with yield phenomenon can continue to elongate without increasing (maintaining a constant) force during the tensile process is called the yield point. If the force decreases, the upper and lower yield points should be distinguished. The unit of yield point is N/mm2 (MPa).
Upper yield point（ σ Su): The maximum stress of the sample before yielding and the first decrease in force; Lower yield point（ σ SL): The minimum stress in the yield stage when the initial instantaneous effect is not considered.
The formula for calculating the yield point is:
In the formula: Fs – yield force during the tensile process of the specimen (constant), N (Newton) So – original cross-sectional area of the specimen, mm2.
③ Elongation after fracture（ σ）
In a tensile test, the percentage of the length increased by the gauge length of the specimen after breaking compared to the original gauge length is called elongation. with σ Represented in%. The calculation formula is: σ= （Lh-Lo）/L0*100%
In the formula: Lh – the gauge length of the specimen after fracture, mm; L0- Original gauge length of the sample, mm.
④ Reduction of area（ ψ）
In a tensile test, the maximum reduction in cross-sectional area at the reduced diameter of the specimen after fracture, as a percentage of the original cross-sectional area, is called the reduction in area. with ψ Represented in%. The calculation formula is as follows:
In the formula: S0- Original cross-sectional area of the sample, mm2; S1- The minimum cross-sectional area at the reduced diameter of the specimen after fracture, mm2.
⑤ Hardness index
The ability of metal materials to resist surface indentation by hard objects is called hardness. According to different test methods and application scope, hardness can be divided into Brinell hardness hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high-temperature hardness. There are three commonly used types of hardness for pipes: Brinell, Rockwell, and Vickers.
A. Brinell hardness (HB)
Press a steel ball or hard alloy ball of a certain diameter into the surface of the sample with the specified test force (F), remove the test force after the specified holding time, and measure the indentation diameter (L) on the surface of the sample. The Brinell hardness hardness number is the quotient obtained by dividing the test force by the spherical surface area of the indentation. Expressed in HBS (steel ball), in N/mm2 (MPa).
The calculation formula is:
In the formula: F – test force pressed into the surface of the metal sample, N; D – diameter of the steel ball used in the test, mm; D – Average diameter of indentation, mm.
Brinell hardness measurement is more accurate and reliable, but generally HBS is only applicable to metal materials below 450N/mm2 (MPa), not applicable to harder steel or thinner plates. In the steel pipe standards, Brinell hardness hardness is the most widely used. The indentation diameter d is often used to represent the hardness of the material, which is intuitive and convenient.
For example: 120HBS10/1000/30: it means that the Brinell hardness value measured with a 10mm diameter steel ball under the action of a 1000Kgf (9.807KN) test force is 120N/mm2 (MPa) after 30s (seconds).