# Terms related to the mechanical properties of steel

1. Yield point (σs)

When the steel or sample is stretched, when the stress exceeds the elastic limit, even if the stress no longer increases, the steel or sample still continues to undergo significant plastic deformation. This phenomenon is called yielding, and the minimum stress value when yielding occurs is Is the yield point.

2. Yield strength (σ0.2)
The yield point of some metal materials is extremely inconspicuous, and it is difficult to measure. Therefore, in order to measure the yield characteristics of the material, it is stipulated that the stress when the permanent residual plastic deformation is equal to a certain value (usually 0.2% of the original length) is called the condition Yield strength or simply yield strength σ0.2.

3. Tensile strength (σb)

The maximum stress value reached by the material during the stretching process from the beginning to when it breaks. It indicates the ability of steel to resist fracture. Corresponding to the tensile strength, there are compressive strength and bending strength.

Let Pb be the maximum tensile force reached before the material is broken, and Fo is the cross-sectional area of ​​the sample, then the tensile strength σb = Pb/Fo (MPa).

4. Elongation (δs)

After the material is broken, the percentage of its plastic elongation length to the original sample length is called elongation or elongation.

5. Yield ratio (σs/σb)

The ratio of the yield point (yield strength) of steel to the tensile strength is called the yield ratio. The larger the yield ratio, the higher the reliability of structural parts. Generally, the yield ratio of carbon steel is 0.6-0.65, and that of low-alloy structural steel is 0.65-0.75 and alloy structural steel is 0.84-0.86.

6. Hardness

Hardness indicates the ability of a material to resist hard objects pressed into its surface. It is one of the important performance indicators of metal materials. Generally, the higher the hardness, the better the wear resistance. Commonly used hardness indicators are Brinell hardness, Rockwell hardness and Vickers hardness.

⑴Brinell hardness (HB) presses a certain size (generally 10mm in diameter) hardened steel ball into the surface of the material with a certain load (generally 3000kg), keeps it for a period of time, after unloading, the ratio of the load to the indentation area It is the Brinell hardness value (HB), the unit is kilogram force/mm2 (N/mm2).

⑵Rockwell hardness (HR)

When HB>450 or the sample is too small, Brinell hardness test cannot be used and Rockwell hardness measurement can be used instead. It uses a diamond cone with an apex angle of 120° or a steel ball with a diameter of 1.59 and 3.18mm, which is pressed into the surface of the measured material under a certain load, and the hardness of the material is obtained from the depth of the indentation. According to the hardness of the test material, it can be expressed in three different scales:

HRA: It is the hardness obtained with a 60kg load and a diamond cone indenter, used for extremely hard materials (such as cemented carbide, etc.).

HRB: It is the hardness obtained by using a 100kg load and a hardened steel ball with a diameter of 1.58mm, which is used for materials with lower hardness (such as annealed steel, cast iron, etc.).

HRC: It is the hardness obtained with a 150kg load and a diamond cone indenter, used for materials with high hardness (such as hardened steel, etc.).

⑶ Vickers hardness (HV) is pressed into the surface of the material with a load within 120kg and a diamond square cone indenter with an apex angle of 136°. The surface area of ​​the material indentation pit is divided by the load value to obtain the Vickers hardness value ( HV)