What are the mechanical properties of steel?

1. Yield Point (σs)

During the tensile process of steel or sample, when the stress exceeds its elastic limit, the steel or sample will continue to undergo significant plastic deformation even if the stress is no longer increased. This phenomenon is called yield, and the minimum stress value at which the yield phenomenon occurs is the yield point. Let Ps be the external force at the yield point s and Fo be the specimen sectional area, then the yield point σs=Ps/Fo(MPa).

2. Yield strength (σ0.2)

The yield point of some metal materials is not obvious, so it is difficult to measure. In order to measure the yield characteristics of the material, the stress generated when the permanent residual plastic deformation is equal to a certain value (generally 0.2% of the original length) is called the conditional yield strength or simply the yield strength σ0.2.

3. Tensile Strength (σb)

The value of the maximum stress reached by the material during the tensile process from the beginning to the fracture. It indicates the steel's ability to resist breaking. Corresponding to the tensile strength are compressive strength, bending strength and so on. Pb is the maximum tensile force achieved before the material is broken, Fo is the specimen cross-section area, then the tensile strength σb= Pb/Fo (MPa).

4. Elongation (δs)

After the material is pulled off, the percentage of the length of the plastic elongation and the length of the original sample is called the elongation or elongation.

5. Yield ratio (σs/σb)

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

Step 6: Hardness

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

(1) Brinell Hardness (HB)

A hardened steel ball of a certain size (diameter is generally 10mm) is pressed into the surface of the material with a certain load (generally 3000kg) and maintained for a period of time. After loading, the ratio of the load to the indentation area is the Brinell hardness value (HB).

(2) Rockwell Hardness (HR)

When HB> 450 or the sample is too small, can not use Brinell hardness test and use Rockwell hardness measurement. It is a diamond cone with a top Angle of 120° or a steel ball with a diameter of 1.59 and 3.18mm, pressed into the surface of the material to be measured under a certain load, and the hardness of the material is determined by the depth of the indentation. According to the hardness of the test material, it is divided into three different scales:

HRA: The hardness obtained by using a 60kg load and a diamond cone indenter is used for extremely hard materials (such as carbide, etc.).

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

HRC: The hardness obtained by using a 150kg load and a diamond cone indenter is used for very hard materials (such as hardened steel, etc.).

(3) Vickers Hardness (HV)

With a load of less than 120kg and a diamond square taper indenter with a top Angle of 136°, the surface area of the material indentation pit is divided by the load value, which is the Vickers hardness value (HV).