Introduction to mechanical properties of steel

Yield point (σs)

When the steel or specimen is stretched, when the stress exceeds the elastic limit, even if the stress does not increase any more, and the steel or the specimen continues to undergo significant plastic deformation, the phenomenon is called yielding, and the minimum stress value when yielding occurs is To yield points.

Let Ps be the external force at the yield point s, Fo be the sample cross-sectional area, then the yield point σs = Ps/Fo(MPa), MPa is called MPa equal to N (Newton)/mm2, (MPa=106Pa, Pa: Pascal =N/m2)

2. Yield strength (σ0.2)

Some metal materials have a very low yield point and are difficult to measure. Therefore, in order to measure the yield characteristics of the material, it is specified that the permanent residual plastic deformation is equal to a certain value (generally 0.2% of the original length). The yield strength or simply the yield strength σ0.2.

3. Tensile strength (σb)

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

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

4. Elongation (δs)

After the material is broken, the length of the plastic elongation and the length of the original sample are called elongation or elongation.

5. Yield ratio (σs/σb)

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

6. Hardness

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

(1) Brinell hardness (HB)

Press a hardened steel ball of a certain size (usually 10mm in diameter) onto the surface of the material with a certain load (usually 3000kg) for a period of time. After the load is removed, the ratio of the load to the area of the indentation is the Brinell hardness value. HB), in kilograms force / mm2 (N / mm2).

(2) Rockwell hardness (HR)

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

HRA: It is a hardness obtained by using a 60kg load and a diamond cone indenter for materials with extremely high hardness (such as cemented carbide).

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

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

(3) Vickers hardness (HV)

Pressed into the surface of the material with a load of 120kg and a diamond square cone presser with a apex angle of 136°. The surface area of the material indentation pit is divided by the load value, which is the Vickers hardness value (HV).