1 、 Yield Point（σs）
Stretching the steel product or sample, when the stress exceeds the limit of elasticity, even if stress no longer increases, the steel product or sample still keeps on the obvious plastic deformation; this phenomenon is called as the yielding, while the minimum stress value that occurs in this yield phenomenon is the yield point.
We suppose Ps as the external force on the yield point s, Fo is the sample section area, then the yield point σ s = Ps /Fo( MPa), the MPa is called as the mega Pascal, which equal to N( Newton /mm2),( MPa =106Pa, Pa：Pascal =N /m2).
Yield point for some metallic materials are not so obvious, which causes trouble in measurement, therefore in order to measure the material’s yield characteristic, specify stress when producing permanent remainder plastic deformation equal to certain value (generally is 0.2% of the original length), this is called the condition yield strength or for short yield strength σ 0.2.
3、Strength of extension（σb）
The maximum stress value reached from the outset to occurrence of the breaking during the stretching course. It demonstrates the extent of the steel product resisting the cracking. Corresponding to the strength of extension, there is also the compression resistance, bending resistance and so on.
We suppose Pb is the maximum tension stress before the cracking of the material, Fo is the sample section area, then strength of extension σ b =Pb/ Fo (MPa).
After bursting of the material, the percentage between the length of the plasticity stretched and the original sample length is called as the extensibility or expansion rate.
The ratio of the product's yield point (yield strength) and the strength of extension’s ratio are called as the yield ratio. The bigger the yield ratio is, the higher reliability the structural parts are. Generally yield ratio of the carbon steel is 0.6- 0.65, yield ratio of the alloy of low percentage structural steel is 0.65 - 0.75, the alloy constructional steel is 0.84 - 0.86.
Hardness shows the capability of resisting hard objects penetrate into its surface. It is one of the most significant performance indices of the metallic materials. Generally the higher of the hardness is, the better of the wearability is. Commonly used hardness indices are Brinell hardness, Rockwell Hardness and Vickers Hardness.
It takes certain loading( generally 3000kg),and press the quenched steel ball of certain size( generally diameter is 10mm)into the surface of the material, after a period of time, remove it, then the ratio of the load and its area of indentation is called as the Brinell Hardness( HB), its unit is kilogram force / mm2( N / mm2).
When the HB > 450 or the sample is too small, we can not adopt the Brinell hardness test, while measure by the Rockwell hardness. It is using a diamond cone with the 120°vertex angle or a steel ball with the 1.59, 3.18mm diameters, and presses them into the surface of the material under certain loading, and determines the material’s hardness by the depth of the indentation. According to different test material hardness, we divide it into three kind different scales for demonstration:
HRA：Hardness acquired by adopting the 60kg loading and diamond cone forcer; this is used in super high hardness materials (for instance hard alloy and so on).
HRB：Hardness acquired by adopting the 100kg loading and hardened steel ball with the 1.58mm diameter, and used in lower hardness materials (for instance annealed steel, cast iron and so on).
HRC：Hardness acquired by adopting the 150kg loading and diamond cone forcer, and used in very high hardness materials (for instance hardened steel and so on).
It takes the loading within 120kg and the diamond square cone forcer with the 136°vertex angle to press into the surface of the material, the loading value is divided by surface area of the material compression mark dent, the result is called as the Vickers hardness (HV).