Terminology related to the mechanical properties of steel

Terminology related to the mechanical properties of steel:

1. Yield Point (σs)

When steel or a specimen is under tension, if the stress exceeds the elastic limit, and even if the stress no longer increases, the steel or specimen continues to undergo significant plastic deformation, this phenomenon is called yielding. The minimum stress value at which yielding occurs is called the yield point. Let Ps be the external force at the yield point s, and Fo be the cross-sectional area of ​​the specimen. Then, the yield point σs = Ps/Fo (MPa), where MPa is called megapascal and equals N (Newton) / mm² (MPa = 10⁶ Pa, Pa: Pascal = N/m²).

2. Yield Strength (σ₀.₂)

The yield point of some metallic materials is very indistinct, making measurement difficult. Therefore, to measure the yield characteristics of materials, the stress at which permanent residual plastic deformation equals a certain value (generally 0.2% of the original length) is defined as the conditional yield strength or simply the yield strength σ₀.₂.

3. Tensile Strength (σb)

The maximum stress a material reaches during tensile testing, from the start to fracture. It represents the steel's resistance to fracture. Related to tensile strength are compressive strength, bending strength, etc.

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

4. Elongation (δs)

The percentage of the plastic elongation of a material after fracture to the original specimen length is called elongation or extension rate.

5. Yield-to-Strength Ratio (σs/σb)

The ratio of the yield point (yield strength) to the tensile strength of a steel is called the yield-to-strength ratio. A higher yield strength ratio generally indicates higher reliability of structural components. Typically, the yield strength ratio for carbon steel is 0.6-0.65, for low-alloy structural steel it is 0.65-0.75, and for alloy structural steel it is 0.84-0.86.

6. Hardness

Hardness represents a material's resistance to indentation by a harder object. It is one of the important performance indicators of metallic materials. Generally, higher hardness indicates better wear resistance. Commonly used hardness indicators include Brinell hardness, Rockwell hardness, and Vickers hardness.

⑴ Brinell Hardness (HB)

A hardened steel ball of a certain size (typically 10mm in diameter) is pressed into the material surface under a certain load (typically 3000kg). After a period of time, the ratio of the load to the area of ​​the indentation is the Brinell hardness value (HB), expressed in kgf/mm² (N/mm²).

(2) Rockwell Hardness (HR)

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

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

HRB: Hardness obtained using a 100 kg load and a 1.58 mm diameter hardened steel ball; used for materials with relatively low hardness (such as annealed steel and cast iron).

HRC: Hardness obtained using a 150 kg load and a diamond cone indenter; used for materials with very high hardness (such as quenched steel). (3) Vickers Hardness (HV)

A diamond square indenter with a 136° apex angle is used to press into the material surface under a load of up to 120 kg. The Vickers hardness value (HV) is calculated by dividing the load value by the surface area of ​​the indentation.