Carbon Steel Pipe Mechanical Properties

Carbon Steel Pipe Mechanical Properties

The mechanical property of carbon steel pipe is an important index to ensure the final service performance of steel. It depends on the chemical composition and heat treatment system of steel. In the steel pipe standard, according to the different use requirements, stipulate the tensile properties (tensile strength, yield strength or yield point, elongation) and hardness, toughness index, as well as user requirements of high and low temperature performance.

• Tensile strength (σb)

During the tensile process, the maximum force (Fb) endured by the specimen at fracture divided by the stress (σ) obtained by the original cross-sectional area (So) of the specimen is called tensile strength (σb), and its unit is N/mm2(MPa). It represents the maximum ability of a metal material to resist destruction under tension. The calculation formula is:

In the formula: Fb — the maximum force borne by the specimen when it is broken, N(Newton); 

So — original cross-sectional area of the sample, mm2.

• Yield point (σs)

For metal materials with yield phenomenon, the stress at which the specimen can continue to elongate without increasing force (keeping constant) 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 (σsu): the maximum stress of the sample before the first decrease of yield and force; Yield down point (σsl): the minimum stress in the yield phase without considering the initial transient effect.

The calculation formula of the yield point is: σs=Fs/So
In the formula: Fs–yield force (constant), N(Newton) of the specimen during tensile process. So–the original cross-sectional area of the sample, mm2.

• Elongation after breaking (σ)

In tensile test, after the specimen is broken, the percentage of the length increased by the standard distance to the original standard distance length is called elongation. It is expressed as σ, the unit is mm. The calculation formula is: (LH-L0)/L0*100%.

In the formula: L1 – the standard length of the specimen after fracture (unit: mm); L0 – the original measured length of the sample in millimeters.

• Reduction 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 mm2. Calculated 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 after the sample is broken, mm2.

• 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.

Vickers hardness method can be used for very thin metal materials and surface layer hardness. It has the main advantages of Brucellosis and Roche method, but overcomes their basic disadvantages, but not as simple as the Roche method. Vickers is rarely used in steel pipe standards.

• Brinell hardness(HB)

With a certain diameter of steel ball or carbide ball, with the prescribed test force (F) pressed into the style surface, after the prescribed holding time to remove the test force, measurement of the surface of the specimen indentation diameter (L). The Brinell Hardness value is the quotient of the test force divided by the surface area of the indentation sphere. Expressed in HBS (steel ball), the unit is N/mm2 (MPa).

• Rockwell hardness(HK)

Rockwell hardness test, like Brinell hardness test, is an indentation test method. The difference is that it measures the depth of the indentation. In other words, the indenter (cone or steel ball) was pressed into the surface of the sample under the action of the initial test force (Fo) and the total test force (F). After the specified holding time, the main test force was removed, and the hardness value was calculated using the measured increment of residual indentation depth (E). Its value is A nameless number, represented by the symbol HR, and the ruler used is A, B, C, D, E, F, G, H, K and so on. Among them, the scales commonly used for steel hardness samples are A, B, C, namely HRA, HRB, HRC.

• Vickers hardness(HV)

Vickers hardness test is also an indentation test method. It is used to press a regular four-pyramidal diamond indenter with an Angle of 1360 on the opposite surface into the test surface with a selected test force (F). After a specified holding time, the test force is removed and the two diagonal lengths of the indentation are measured.

• Impact toughness index

Impact toughness reflects the resistance of metal materials to external impact load, and is generally expressed by impact toughness value (AK) and impact work (AK), whose units are J/cm2 and J (joule) respectively.

Impact toughness or impact power test (referred to as “impact test”), because the test temperature is different and divided into normal temperature, low temperature and high temperature impact test three kinds; If the sample notch shape can be divided into “V” notch and” U “notch impact test two.