What are the mechanical properties of steel?

The mechanical properties of steel are important indicators to ensure the final performance (mechanical properties) of steel. It depends 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 are specified according to different usage requirements, as well as high and low temperature properties required by users. ​​

① Tensile strength (σb)

During the stainless steel alloy closing circlip stretching process, the maximum force (Fb) that the sample endures when it breaks is the stress (σ) obtained by the original cross-sectional area (So) of the sample, which is called tensile strength (σb). The unit is N/mm2 (MPa). It represents the maximum ability of a metal material to resist damage under tension. The calculation formula is: Where: Fb–the maximum force endured when the sample is broken, N (Newton); So–the original cross-sectional area of the sample, mm2. ​​​​

②Yield point (σs)

For metal materials with yield phenomenon, the stress at which the sample can continue to elongate without increasing (remaining constant) force during the stretching 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 before the specimen yields and the force decreases for the first time; Lower yield point (σsl): the minimum stress in the yield stage when the initial instantaneous effect is not considered. The calculation formula of the yield point is: Where: Fs–yield force (constant) during the tensile process of the specimen, N (Newton); So–original cross-sectional area of the specimen, mm2.

③Elongation after break (σ)

In the tensile test, the percentage of the increased length of the gauge length of the specimen after it is broken to the original gauge length is called elongation. Expressed by σ, the unit is %. The calculation formula is: Where: L1–the gauge length of the sample after breaking, mm; L0–the original gauge length of the sample, mm. ​ ​

④Reduction of area (ψ)

In the tensile test, the percentage of the maximum reduction in the cross-sectional area at the reduced diameter after the specimen is broken and the original cross-sectional area is called the area shrinkage rate. Expressed by ψ, the unit is %. The calculation formula is as follows: Where: S0–the original cross-sectional area of the sample, mm2; S1–the minimum cross-sectional area of the reduced diameter after the sample is broken, mm2. ​​​​

⑤Hardness index

The ability of a metal material to resist the indentation of the surface by hard objects is called hardness. According to different test methods and application scope, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness and high temperature hardness. There are three commonly used hardnesses for pipes: Brinell, Rockwell, and Vickers hardness. ​

A. Brinell hardness (HB)

Use a steel ball or carbide ball of a certain diameter to press into the sample surface with the specified test force (F). After the specified holding time, remove the test force and measure the indentation diameter (L) on the sample surface. Brinell hardness value is the quotient obtained by dividing the test force by the surface area of the indented sphere. Expressed in HBS (steel ball), the unit is N/mm2 (MPa). The calculation formula is: F–the test force pressed into the surface of the metal sample, N; D–the diameter of the test stainless steel constant force clockwork spring ball, mm; d–the average diameter of the indentation, mm. ​ ​

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 steel pipe standards, Brinell hardness is the most widely used. The indentation diameter d is often used to express the hardness of the material, which is both intuitive and convenient. ​ ​

Example: 120HBS10/1000130: It means that the Brinell hardness value measured by using a 10mm diameter steel ball under the test force of 1000Kgf (9.807KN) for 30s (seconds) is 120N/mm2 (MPa). ​ ​

B. Rockwell hardness (HK)

The Rockwell hardness test, like the Brinell hardness test, is an indentation test method. The difference is that it measures the depth of the indentation. That is, under the sequential action of the initial test force (Fo) and the total test force (F), the indenter (cone or steel ball of the steel mill) is pressed into the surface of the sample. After the specified holding time, the main force is removed. Test force, use the measured residual indentation depth increment (e) to calculate the hardness value. Its value is an anonymous number, represented by the symbol HR, and the scales used include 9 scales, including A, B, C, D, E, F, G, H, and K. Among them, the scales commonly used for steel hardness testing are generally A, B, and C, namely HRA, HRB, and HRC. The hardness value is calculated by the following formula: When testing with A and C scales, HR=100-e When testing with B scale, HR=130-e Where e–the residual indentation depth increment, which is specified The unit is 0.002mm, that is, when the axial displacement of the indenter is one unit (0.002mm), it is equivalent to a change in Rockwell hardness by one number. The larger the e value, the lower the hardness of the metal, and vice versa. The applicable range of the above three scales is as follows: HRA (diamond cone indenter) 20-88 HRC (diamond cone indenter) 20-70 HRB (diameter 1.588mm steel ball indenter) 20-100 Rockwell hardness test is currently widely used method, in which HRC is used in steel pipe standards second only to Brinell hardness HB. Rockwell hardness can be used to measure metal materials from extremely soft to extremely hard. It makes up for the shortcomings of the Brinell method. It is simpler than the Brinell method and the hardness value can be read directly from the dial of the hardness machine. However, due to its small indentation, the hardness value is not as accurate as the Brinell method.

C. Vickers hardness (HV)

The Vickers hardness test is also an indentation test method. It presses a square pyramidal diamond indenter with an included angle of 1360 between opposite surfaces into the test surface at a selected test force (F), and removes it after the specified holding time. Force, measure the length of the two diagonals of the indentation. ​​

Vickers hardness value is the quotient obtained by dividing the test force by the indentation surface area. The calculation formula is: ​ ​ ​ ​ ​ Type: HV–Vickers hardness symbol, N/mm2 (MPa); ​ ​ ​ ​ F– test force, N; ​ ​ ​ d– The arithmetic mean of the two diagonals of the indentation, mm. ​ ​

The test force F used in Vickers hardness is 5 (49.03), 10 (98.07)

, 20 (196.1), 30 (294.2), 50 (490.3), 100 (980.7) Kgf (N) and other six levels, the measurable hardness value range is 5 ~ 1000HV. ​ ​

Example of expression method: 640HV30/20 means that the Vickers hardness value measured with a test force of 30Hgf (294.2N) for 20S (seconds) is 640N/mm2 (MPa). ​ ​

The Vickers hardness method can be used to determine the hardness of very thin metal materials and surface layers. It has the main advantages of Brinell and Rockwell methods and overcomes their basic shortcomings, but it is not as simple as Rockwell method. Vickers method is rarely used instainless steel special shaped switch spring pipe standards. ​

⑥Impact toughness index ​

Impact toughness reflects the resistance of metal to external impact loads. It is generally expressed by impact toughness value (ak) and impact energy (Ak), whose units are J/cm2 and J (Joule) respectively. ​ ​

Impact toughness or impact energy test (referred to as “impact test”) is divided into three types: normal temperature, low temperature and high temperature impact test due to different test temperatures; according to the shape of the sample notch, it can be divided into “V” shaped notch and “U” shaped notch. There are two types of notch impact tests. ​ ​

Impact test: Use a sample of a certain size and shape (10×10×55mm) (with a “U” or “V” notch in the middle of the length direction, a notch depth of 2mm) to automatically test the specimen under impact load on the specified testing machine. Experiment of breaking at the notch. ​​

A. Impact absorbed work Akv(u)–the work absorbed when a metal pattern with a certain size and shape breaks under the action of impact load. The unit is Joule (J) or Kgf. m. ​

B. Impact toughness value akv(u)–the quotient obtained by dividing the impact absorbed energy by the cross-sectional area of the bottom of the notch of the specimen. The unit is Joule/cm2 (J/cm2) or kilogram force. m/cm2 (Kgf. m/cm2). The calculation formula is: Where: Akv(u)–the work absorbed when the sample is broken, Kgf. m (J); S–the bottom cross-sectional area of the sample notch, cm2. ​

The normal temperature impact test temperature is 20±50C; the low temperature impact test temperature range is <15~-1920C; the high temperature impact test temperature range is 35~10000C. The cooling medium used in the low-temperature impact test is generally a liquid or gas that is non-toxic, safe, does not corrode metal, and does not solidify at the test temperature. Such as anhydrous ethanol (alcohol), solid carbon dioxide (dry ice) or liquid nitrogen atomization gas (liquid nitrogen), etc.