Mechanical properties of seamless steel pipe

The mechanical properties of seamless steel pipes are critical indicators that ensure their performance in final applications. These properties are influenced by the steel's chemical composition and the applied heat treatment process. In steel pipe standards, depending on the intended use, various mechanical characteristics are specified, including tensile properties (such as tensile strength, yield strength or yield point, and elongation), hardness, toughness, and the material's behavior under high and low temperatures. This article will talk about mechanical properties of seamless steel pipe.

①Tensile strength (σb)
In the tensile process, the maximum force (Fb) that the sample bears when it breaks, divided by the original cross-sectional area (So) of the sample, is the stress (σ), which is called the tensile strength (σb), and the unit is N/mm2 (MPa). It represents the maximum ability of a metal material to resist damage under tensile force.

②Yield point (σs)
For a metal material with a yield phenomenon, the stress at which the sample can continue to elong without increasing the force (maintaining constant) during the stretching process is called the yield point. If the force drops, the upper and lower yield points should be distinguished. The unit of yield point is N/mm2 (MPa).
Upper Yield Point (σsu): The maximum stress before the specimen yields and the force drops for the first time; Lower Yield Point (σsl): The minimum stress in the yield stage when the initial transient effect is not taken into account.
The calculation formula of the yield point is: σs=Fs/So
Where: Fs--yield force (constant) during the tensile process of the sample, N (Newton) So--the original cross-sectional area of the sample, mm2.

③Elongation after breaking (σ)
In the tensile test, the percentage of the length of the gauge length increased after the sample is broken to the original gauge length is called the elongation. Expressed by σ, the unit is %. The calculation formula is: σ=(Lh-L0)/L0*100%
In the formula: Lh-the gauge length of the specimen after breaking, in mm; L0-the original gauge length of the specimen, in mm.

④Reduction of area (ψ)
In the tensile test, the percentage of the maximum reduction of the cross-sectional area at the reduced diameter of the sample after the sample is broken to the original cross-sectional area is called the reduction of area. Expressed in ψ, the unit is %. The calculation formula is as follows: ψ=(S0-S1)/S0*100%
In the formula: S0-the original cross-sectional area of the sample, mm2; S1-the minimum cross-sectional area at the reduced diameter of the sample after it is broken, mm2.

⑤ Hardness index
The ability of metal materials to resist the indentation of hard objects on the surface is called hardness. According to different test methods and scope of application, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, micro hardness and high temperature hardness. There are three commonly used pipes: Brinell, Rockwell, and Vickers hardness.
A. Brinell hardness (HB)
Use a steel ball or cemented carbide ball of a certain diameter to press 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 value is the quotient obtained by dividing the test force by the spherical surface area of the indentation. Expressed in HBS (steel ball), the unit is N/mm2 (MPa).
The measurement of Brinell hardness is more accurate and reliable, but generally HBS is only suitable for metal materials below 450N/mm2 (MPa), and is not suitable for harder steel or thinner plates. Among the steel pipe standards, Brinell hardness is the most widely used, and the hardness of the material is often expressed by the indentation diameter d, which is intuitive and convenient.

B. Rockwell Hardness (HR)
Rockwell hardness testing involves pressing an indenter, typically a steel ball or diamond cone, into the material's surface under a specified load and then measuring the depth of the indentation. The Rockwell hardness value is calculated based on the depth of the penetration. Unlike the Brinell method, Rockwell hardness testing is faster and doesn't require measuring the indentation diameter.

Rockwell hardness is divided into different scales based on the indenter type and test force, including HRB (using a steel ball for softer materials like copper alloys) and HRC (using a diamond cone for harder materials like hardened steel). The HRC scale is particularly suitable for steels harder than 450N/mm², making it widely used for steel pipes and other materials where higher precision is needed. Rockwell testing is simpler and more applicable for production environments due to its quick results and minimal sample damage.

C. Vickers Hardness (HV)
The Vickers hardness test uses a diamond pyramid indenter pressed into the material's surface under a specified load. The Vickers hardness value is determined by dividing the test force by the surface area of the indentation. This method provides high accuracy and can be used for both very hard and very soft materials. Unlike the Brinell and Rockwell tests, the Vickers method applies the same indenter for all materials, making it highly versatile across a broad range of hardness levels.

The Vickers hardness test is especially useful for measuring the hardness of thin materials or those with a highly variable hardness, such as coatings or surface-treated steel pipes. It is often applied in quality control for small or intricate parts due to its precision, but the test is slower and requires more elaborate equipment compared to Rockwell or Brinell methods.