Why Should Steel Parts Be Quenched and Tempered?

Quenching and tempering treatment: the heat treatment method of high temperature tempering after quenching is called quenching and tempering treatment.

High temperature tempering refers to tempering at 500-650 ℃.

Quenching and tempering can greatly adjust the properties and materials of steel, which has good strength, plasticity and toughness, and good comprehensive mechanical properties.

Tempered sorbite is obtained after quenching and tempering.

Tempered sorbite is martensite formed during tempering.

It can be distinguished by magnification of more than 500~600 times under optical metallographic microscope.

It is a composite structure with carbide (including cementite) particles distributed in ferrite matrix.

It is also a tempering structure of martensite, a mixture of ferrite and granular carbide.

At this time, the ferrite has basically no carbon supersaturation, and the carbide is also stable carbide, which is a kind of equilibrium structure at room temperature.

The quenched and tempered steel includes carbon quenched and tempered steel and alloy quenched and tempered steel.

Whether it is carbon steel or alloy steel, its carbon content is strictly controlled.

If the carbon content is too high, the strength of the quenched and tempered workpiece is high, but the toughness is not enough.

If the carbon content is too low, the toughness is improved and the strength is not enough.

In order to obtain good comprehensive performance of quenched and tempered parts, the carbon content is generally controlled at 0.30~0.50%.

During quenching, tempering and quenching, the entire section of the workpiece is required to be hardened to obtain a microstructure dominated by fine needle quenched martensite.

The microstructure of uniformly tempered sorbite was obtained by high temperature tempering.

It is impossible for small factories to carry out metallographic analysis for each furnace.

Generally, they only conduct hardness test.

That is to say, the hardness after quenching must reach the quenching hardness of the material, and the hardness after tempering shall be checked according to the drawing requirements.

Hardening and tempering of 45 steel

45 steel is a medium carbon structural steel with good cold and hot workability, good mechanical properties, low price and wide sources, so it is widely used.

Its biggest weakness is that workpieces with low hardenability, large section size and high requirements should not be used.

The quenching temperature of 45 steel is A3+(30~50) ℃. In actual operation, the upper limit is generally taken.

Higher quenching temperature can speed up workpiece heating, reduce surface oxidation, and improve work efficiency.

In order to homogenize the austenite of the workpiece, sufficient holding time is required.

If the actual charging amount is large, it is necessary to extend the holding time appropriately.

Otherwise, insufficient hardness may occur due to uneven heating.

However, if the holding time is too long, the defects of coarse grains and oxidation decarburization will also occur, which will affect the quenching quality.

We believe that if the furnace loading is greater than the provisions of the process document, the heating and insulation time should be extended by 1/5.

Because the hardenability of 45 steel is low, 10% brine solution with high cooling rate should be used.

After the workpiece is filled with water, it should be quenched, but not cooled.

If the workpiece is cooled in brine, it may crack.

This is because when the workpiece is cooled to about 180 ℃, the austenite rapidly transforms into martensite, causing excessive structural stress.

Therefore, when the quenched workpiece is rapidly cooled to this temperature zone, the slow cooling method should be adopted.

As the outlet water temperature is difficult to master, it must be operated by experience.

When the workpiece in the water stops shaking, the outlet water can be cooled by air (oil cooling is better).

In addition, the workpiece should be dynamic rather than static when entering the water.

It should move regularly according to the geometric shape of the workpiece.

The static cooling medium plus the static workpiece will lead to uneven hardness and stress, leading to large deformation and even cracking of the workpiece.

The hardness of quenched and tempered 45 steel parts should reach HRC56~59, and the possibility of large section is lower, but it should not be lower than HRC48.

Otherwise, it means that the workpiece has not been completely quenched, and sorbite or even ferrite structure may appear in the structure.

This structure is still retained in the matrix through tempering, which cannot achieve the purpose of quenching and tempering.

For high temperature tempering of 45 steel after quenching, the heating temperature is usually 560~600 ℃, and the hardness is required to be HRC22~34.

Because the purpose of quenching and tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide.

However, if the drawing has hardness requirements, the tempering temperature shall be adjusted according to the drawing requirements to ensure the hardness.

If some shaft parts require high strength, the hardness is required to be high;

However, some gears and shaft parts with keyways need to be milled and inserted after quenching and tempering, so the hardness requirements are lower.

As for the tempering and heat preservation time, it depends on the hardness requirements and the size of the workpiece.

We believe that the hardness after tempering depends on the tempering temperature and has little to do with the tempering time, but it must be back penetrated.

Generally, the tempering and heat preservation time of the workpiece is more than one hour.

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