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Mechanical Properties of Steel

   

  

1. Yield point (σs)

When the stress of steel or sample exceeds the elastic limit, even if the stress does not increase any more, obvious plastic deformation of the steel or sample continues to occur. This phenomenon is called yield, and the minimum stress value when yielding phenomenon occurs is the yield point. If PS is the external force at the yield point s and fo is the sectional area of the specimen, 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 it. Therefore, in order to measure the yield characteristics of materials, the stress when the permanent residual plastic deformation is equal to a certain value (generally 0.2% of the original length) is specified, which is called the conditional yield strength or yield strength σ 0.2 for short.

3. Tensile strength (σb)

The maximum stress value of a material from the beginning to fracture in the tensile process. It represents the ability of the steel to resist fracture. Corresponding to the tensile strength are compressive strength and bending strength. If Pb is the maximum tensile force of the material before breaking, and fo is the cross-sectional area of the sample, then the tensile strength σ B = Pb / fo (MPA).

4. Elongation (δs)

The percentage of elongation or elongation between the length of plastic elongation and the length of original sample is called elongation or elongation.

5. Yield ratio (σs / σb)

The ratio of yield point (yield strength) to tensile strength of steel is called yield ratio. The greater the yield ratio, the higher the reliability of structural parts. 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.

6. Hardness

Hardness is the ability of a material to resist hard objects pressing 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 common hardness indexes are Brinell hardness, Rockwell hardness and Vickers hardness.

(1) Brinell hardness (HB)

The hardened steel ball with a certain size (generally diameter of 10 mm) is pressed into the surface of the material with a certain load (generally 3000kg). After the load is removed, the ratio of the load to the indentation area is called the Brinell hardness value (HB).

(2) Rockwell hardness (HR)

When HB > 450 or the sample is too small, Brinell hardness test can not be used, but Rockwell hardness measurement is used. A diamond cone with a top angle of 120 ° or a steel ball with a diameter of 1.59 and 3.18 mm is pressed 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 by three different scales

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

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

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

(3) Vickers hardness (HV)

A diamond square cone indenter with a load less than 120kg and a top angle of 136 ° is pressed into the surface of the material. The Vickers hardness value (HV) is obtained by dividing the surface product of the indentation pits into the load value.

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