Mechanical Properties of Steel Explained: Yield Point, Strength, Hardness and More

1. Yield point

When the stress of steel or sample exceeds the elastic limit during tension, even if the stress is no longer increased, the steel or sample continues to have obvious plastic deformation. This phenomenon is called yield, and the minimum stress value when yield phenomenon occurs is the yield point. Let P be the external force at the yield point and F be the cross-sectional area of the sample, then the yield point =P/F(MPa).

2. Yield strength

The yield point of some metal materials is not obvious, so it is difficult to measure. Therefore, in order to measure the yield characteristics of materials, it is required to produce the stress when the permanent residual plastic deformation is equal to a certain value (generally 0.2% of the original length), which is called conditional yield strength.

3. Tensile strength

The maximum stress of a material from the beginning to the time of fracture in the tensile process. It indicates the ability of steel to resist fracture. Compressive strength and flexural strength are also corresponding to tensile strength. Let P be the maximum tensile force reached before the material is broken, and F be the cross-sectional area of the sample, then the tensile strength = P/F (MPa). 

4. Elongation

The percentage of the length of plastic elongation of a material after breaking to the length of the original sample is called elongation or elongation.

5. Yield ratio

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

6. Hardness

Hardness indicates the ability of a material to resist the pressure of a hard object into its surface. It is one of the important performance indexes of metal materials. Generally, the higher the hardness, the better the wear resistance. The 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 (generally 10mm in diameter) into the material surface with a certain load (generally 3000kg) and keep it for a period of time. After unloading, the ratio between the load and its indentation area is the Brinell hardness value (HB).

2) Rockwell hardness (HR)

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

– HRA: it is the hardness obtained by using 60kg load and diamond cone press. It is used for materials with extremely high hardness (such as cemented carbide, etc.).

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

– HRC: it is the hardness obtained by using 150kg load and diamond cone press. It is used for materials with high hardness (such as quenched steel, etc.).

3) Vickers hardness (HV)

Press the material surface with a load within 120kg and a diamond square cone press with a top angle of 136 °, and divide the surface product of the material indentation pit by the load value, which is the Vickers hardness value (HV).