一、Terms relevant to the steel product mechanical property

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).

2、Yield strength（σ0.2）

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).

4、Extensibility（δs）

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.

5、Yield Ratio（σs/σb）

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.

6、Hardness

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.

⑴Brinell Hardness（HB）

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).

⑵Rockwell Hardness（HR） 

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).

⑶Vickers hardness（HV） 

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).

二、Terms relevant to the heat treatment of steel

1、Annealing of the steel

To heat the steel to a certain temperature and keep it for a period of time, and then cool it gradually, this is called the annealing. Steel annealing is the thermal processing method of heating the steel to the temperature of occurring phase transformation or partial phase transformation, and after heat preservation, cools it slowly. Purpose of annealing is to eliminate structural defects, improve organizations in order to leveling of the ingredients as well as refining of the grain, increase steel’s mechanical property, reduce residual stress, at the same time this could reduce the hardness, increase plasticity and tenacity, and improve tooling quality. Therefore annealing not only eliminate and improve the structural defects and internal stress remained in the last process, but also prepare for the subsequent handling, therefore the annealing belongs to the heat treatment of the semi-manufactured goods, it is also called as pre-heat treatment.

2、Steel’s normalizing

Normalizing is the thermal processing method of heating the steel above the limiting temperature, and makes all the steel changed into the even austenite, and then naturally cools it in the air. It could eliminate network cementite of the hypereutectoid steel. As for hypo-eutectoid steel, the normalizing could refine the crystal lattice, increase integrated mechanical property. For undemanding parts,normailizing repacing annealing is comparatively econamic. 

3、Steel’s quenching

Quenching is the thermal processing method of heating the steel above limiting temperature, preserving heat for a period of time, and then soon putting it in the quenching agent to suddenly reduce its temperature. This is the quick chilling in the speed exceeding the critical cooling rate, and obtains imbalanced structure mainly the martensite. The quenching could increase steel’s strength and hardness, but should reduce its plasticity. Common hardening agents are: water, oil, soda water and salt solutions and so on.

4、Steel’s drawing-back

To reheat the quenched steel to a certain temperature, and then use certain approaches to cool it is called drawing-back. Its purpose is to eliminate internal stress produced by quenching, reduce hardness and friability, in order to obtain expected mechanical property. Drawing-back is divided into high temperature tempering, average tempering and low temperature tempering. Tempering is always operated in junction with the quenching and normalizing.

⑴Thermal refining： 

The thermal processing method of high tempering after quenching is called the thermal refining. High tempering is to carry out tempering between the temperatures of 500 - 650℃. Refining adjusts the performance, material quality of the steel to its largest degree, its strength, plasticity and tenacity are all satisfactory, and possesses favorable integrated mechanical property.

⑵Aging treatment： 

In order to eliminate the changes in size, shape of the fine measuring instrument or mould, part during the long term usage, we frequently reheat the work piece to 100 - 150℃after low tempering ( low tempering temperature 150- 250℃),and before finish machining, then keep this for 5 - 20 hours. Such treatment of stabilizing the quality of the accurate work pieces is called the aging treatment. It is particularly important to undertake aging treatment to the steel products constructional elements under low temperature or dynamic load conditions is to eliminate residual stress, stabilized steel material’s organization and size.

5、Steel’s surface heat treatment

⑴Surface quenching： 

It is to quickly heat the steel piece’s surface above the limiting temperature, then rapidly cool it before the amount of heat reaches the centre, in this way we could quench the surface coat in the martensitic structure, while the centre does not experience the phase transformation, which realizes the purpose of superficial hardening and unaltered centre. This is suitable for medium carbon steel.

⑵Chemical heat treatment： 

It is a type of heat treatment technique of penetrating the atom of the chemical element into the surface coat of the work piece by using the atomic diffusion ability among the high temperature in order to change the chemical composition and structure of the work piece’s surface coat, hence achieve the result that the steel’s surface coat possesses special required organization and performance. According to different variety of penetrated element, the chemical heat treatment could be divided into carbonizing, nitrizing, cyaniding, and metallic cementation. 

Carburizing：It is the process of the carbon atom penetrating into the steel surface coat, it could also make the low carbon steel work piece possess the high-carbon steel surface coat, after quenching and low tempering, the surface coat of the work piece possesses high hardness and wearability, while the center of the work piece still keep the tenacity and plasticity of the low carbon steel.

Nitriding：It is also called as nitrogenization. It is the process of penetrating the nitrogen atom into the surface coat of the steel. Its purpose is to increase the hardness and wearability of the surface coat as well as fatigue resistance strength, corrosion resistance and so on. Currently we generally adopt the gas nitriding method in production.

Cyaniding: It is also called as carbonitriding, which is the process of penetrating the carbon atom and nitrogen atom into the steel. It makes the steel surface possess the characteristic of carburizing and nitrizing. 
Metallic Cementation: It is the process of penetrating the metal atom into the steel surface coat. it makes the alloying of the steel surface, and the work piece surface possesses possess the characteristic of alloy steel and special steel, such as heat resistant, wear resistant, resistant to oxidation, corrosion proof and so on. The commonly used in production are alitizing, chromizing, boriding, siliconizing, and so on.