Mechanical properties terms for steel pipes-Boiler Tubes,Heat-Exchanger Tubes,Superheater Tubes,Supplier,Beite

Mechanical properties terms for steel pipes

Mechanical properties terms for steel pipes

This article introduces the third part of common terms in the technical requirements of steel pipes --- Mechanical properties terms for steel pipes

The mechanical properties of steel are important indicators to ensure the end-use performance (mechanical properties) of the steel, and it depends on the chemical composition and heat treatment system of the steel. In the steel pipe standard, according to different usage requirements, 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.

1. Tensile strength (MPa)

The maximum force (Fm) that the sample is subjected to during the tensile breaking process is the stress obtained from the original cross-sectional area (So) of the sample, which is called the tensile strength (Rm), and the unit is N / mm2 (MPa). It represents the maximum ability of a metal material to resist damage under the action of tension. The calculation formula is:

Rm = Fm / So

Where Fm is the maximum force that the specimen is subjected to when breaking, N (Newton);

So--sample original cross-sectional area, mm2.

2.Yield point

For a metallic material with a yield phenomenon, the stress at which the specimen can continue to stretch without increasing the force during the tensile process (which remains constant) 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 (ReL): the maximum stress before the sample yields and the force decreases for the first time;

Lower yield point (σsL): The minimum stress in the yield stage when the initial transient effects are not taken into account.

Yield Strength

Yield strength is the yield limit when a metal sample yields, that is, the stress that resists a small amount of plastic deformation. For metal materials with no obvious yield phenomenon, the stress value that produces 0.2% residual deformation is specified as its yield limit, which is called the conditional yield limit or yield strength.

3. Elongation after breaking

In a tensile test, the percentage of the length of the gauge after the sample is broken off and the length of the gauge length is called the elongation.

Bit is%. The calculation formula is: L1 / L0

In the formula: L1—gage length after the sample is broken, mm; L0—the original gage length of the sample, mm.

4.Section shrinkage

In the tensile test, the maximum reduction of the cross-sectional area at the reduced diameter of the sample after the sample is cut off is the percentage of the original cross-sectional area, which is called the cross-section shrinkage.

Rate. the unit is%. The calculation formula is as follows: S1 / S0

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 breaking, mm2.

5. Hardness

Hardness is a mechanical property index that measures the degree of softness and rigidity just now. It represents the ability to resist deformation in a local volume on the metal surface. Hardness is not a simple physical concept, but a comprehensive indicator of mechanical properties such as elasticity, plasticity, strength and toughness. According to different test methods and applicable scopes, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, micro hardness and high temperature hardness. Brinell, Rockwell and Vickers hardness are commonly used for pipes.

(1) Brinell hardness (HB)

Brinell hardness (HB) is generally used when the material is soft, such as non-ferrous metals, steel before heat treatment or after annealing. Rockwell hardness (HRC) is generally used for materials with higher hardness, such as hardness after heat treatment and so on.

Brinell hardness (HB) is a test load of a certain size. A hardened steel ball or cemented carbide ball of a certain diameter is pressed into the surface of the metal to be measured, held for a specified time, and then unloaded to measure the diameter of the surface indentation. The cloth hardness value is the quotient obtained by dividing the load by the indented spherical surface area. Generally: Press a hardened steel ball of a certain size (typically 10mm in diameter) into the surface of the material with a certain load (generally 3000kg) and keep it for a period of time. After unloading, the ratio of the load to the indentation area is Brinell. Hardness value (HB) in kilogram-force / mm2 (N / mm2).

(2) Rockwell hardness (HRC)

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 action of the initial test force (Fo) and the total test force (F), the indenter (Jingang Plant cone or steel ball) is pressed into the sample surface, and after the prescribed holding time, the main body is removed. For the test force, use the measured residual indentation depth increment (e) to calculate the hardness value. Its value is an unnamed number, which is represented by the symbol HR. The scales used are A, B, C, D, E, F, G, H, K, and other 9 scales. Among them, the scales commonly used for steel hardness testing are generally A, B, and C, that is, HRA, HRB, and HRC.

The hardness value is calculated using the following formula:

HR = 100-e when tested with A and C scales HR = 130-e when tested with B scales

In the formula, e--increase of the residual indentation depth, which is expressed in a prescribed unit of 0.002mm, that is, when the axial displacement of the indenter by one unit (0.002mm), it is equivalent to a change in Rockwell hardness. The larger the value of e, the lower the hardness of the metal, and vice versa.

The applicable range of the above three scales is as follows: HRA (diamond round cone indenter) 20-88 HRC (diamond round cone indenter) 20-70 HRB (diameter 1.588mm steel ball indenter) 20-100

The Rockwell hardness test is a widely used method at present, in which HRC uses the steel pipe standard after Brinell hardness HB. Rockwell hardness can be applied to the determination of extremely soft to extremely hard metal materials. It makes up for the Brinell method and is simpler than the Brinell method. It can read the hardness value directly from the dial of the hardness machine. However, because its indentation is small, the hardness value is not as accurate as the Brinell method.

(3) Vickers hardness (HV)

The Vickers hardness test is also an indentation test method. A regular quadrangular pyramid diamond indenter with an opposite angle of 1360 is pressed into the test surface with a selected test force (F), and the test is removed after a specified retention time. Force, measure the two diagonal lengths of the indentation.

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 the Brinell and Rockwell methods, and overcomes their basic disadvantages, but it is not as simple as the Rockwell method. The Vickers method is rarely used in steel pipe standards.

6. Impact toughness

Impact toughness is a reflection of the resistance of metal to external impact loads. It is generally expressed by impact energy (Ak), and its unit is J (Joule).

Impact work test (referred to as "impact test"), which is divided into three types: normal temperature, low temperature and high temperature impact tests due to different test temperatures; if the notch shape of the sample is divided into "V" notch and "U" notch Experiment with both.

Impact test: A specimen of a certain size and shape (10 × 10 × 55mm) (with a "U" or "V" notch in the middle of the length direction, the notch depth is 2mm) is subject to impact load on a specified test machine Experiment of breaking at the notch.

Shock absorption work Akv (u)-The work absorbed by a metal pattern with a certain size and shape when it is broken under the impact load. The unit is Joule (J).

The normal temperature impact test temperature is 20 ± 5 ℃; the low temperature impact test temperature range is <1 ～ -192 ℃; the high temperature impact test temperature range is 35 ～ 1000 ℃.

The cooling medium used in the low temperature impact test is generally a non-toxic, safe, non-corrosive metal and a liquid or gas that does not solidify at the test temperature. Such as anhydrous ethanol (alcohol), solid carbon dioxide (dry ice) or liquid nitrogen atomized gas (liquid nitrogen).