Heat Treatment

General heat treatment

It is a heat treatment method that simply uses temperature changes to improve the structure and properties of metals. include:

1 annealing:

That is, the metal is heated to a temperature above the critical point to obtain a high temperature phase, and then slowly cooled to cause a solid phase transition of the metal under equilibrium conditions to improve the metal during solidification, normal deformation, welding or previous heat treatment.

The structure can also be heated to a temperature at which no phase change occurs, allowing the metal to eliminate internal stress or to recover and recrystallize the metal undergoing cold deformation. Annealing is a heat treatment in which the internal structure of the metal approaches an equilibrium state.

If the medium is cooled after the high temperature phase is obtained, the metal is made to have a finer structure than the annealed structure to achieve an effect similar to annealing, and at the same time, to improve the performance, it is called normalizing.

2 quenching:

The metal is heated to a temperature above the critical point to obtain a high temperature phase, and then rapidly cooled to obtain an unbalanced structure to maintain a high temperature phase or form a metastable phase, thereby utilizing the good performance of the high temperature phase and preparing for the next heat treatment.

For example, only the second phase is dissolved at a high temperature (the substrate does not undergo a phase change), and then quenched to retain the high temperature solid solution state to room temperature. It is called solution heat treatment.

3 tempering:

The subsequent treatment of quenching heats the unbalanced structure after quenching to a temperature below the critical point, bringing the metal closer to the equilibrium structure; and controlling the progress of the phase transformation to obtain a suitable structure and properties of the metal.

The process of dissolving the second phase at room temperature or by heating to a lower temperature after solution treatment is referred to as "aging treatment".

Deformation heat treatment

In the metal plastic processing process, the metal's normal deformation and phase transformation law, especially the dynamic interaction of deformation and phase change, are used to control the internal structure of the metal and improve its comprehensive performance.

At present, important process developments include controlled rolling and post-forging residual heat quenching. These processes both improve the performance of the metal while reducing energy consumption.

Chemical heat treatment

Using the diffusion in the metal and the formation of the alloy phase, the metal can infiltrate different elements from the surface in a specific controllable medium, change the chemical composition and structure of the metal surface layer, and can cause the metal to occur in the subsequent heat treatment.

The phase change required to improve its chemical, physical or mechanical properties.

The elements that usually infiltrate are: carbon, nitrogen, boron, oxygen, sulfur, aluminum, chromium, antimony, vanadium, titanium, etc., and it is also possible to infiltrate various elements at the same time. The chemical heat treatment can be carried out in a gaseous, liquid or solid medium.

Quenching

Quenching is one of the basic means of strengthening steel. Quenching steel into martensite and then tempering to improve toughness is a traditional method for obtaining high comprehensive mechanical properties of steel.

In order to fully exploit the strength of the steel, the steel must first be completely transformed into martensite, that is, it must be cooled at a sufficiently fast rate to prevent the austenite from decomposing into a structure such as ferrite, pearlite or bainite during quenching.

This rate is referred to as the critical cooling rate and is also commonly referred to as the critical cooling rate.

From a process point of view, the choice of quenching temperature and quenching medium is an important factor affecting the quenching effect, and these depend on the properties of steel and alloy. The quenching heating temperature is referred to as quenching temperature, and the selection criteria should be based on the principle of obtaining fine and uniform austenite grains so as to obtain fine martensite after cooling.

If the steel contains strong carbide forming elements, the quenching temperature should generally be higher to accelerate the dissolution of carbides and increase the content of carbon and alloying elements in austenite, thereby improving the stability of supercooled austenite.

For steels with higher carbon and manganese, a lower quenching temperature should be used to avoid austenite grain coarsening. Oxidation and decarburization during quenching and heating process directly affect the service life of the workpiece after quenching.

For this reason, salt bath heating, controlled atmosphere heating or vacuum heating are adopted.

When the quenching cooling medium is quenched, it is necessary to obtain a portion of 100% martensite in the steel part, and the cooling rate (cooling rate) must be greater than the critical cooling rate, otherwise the hardening can not be sufficiently hardened and the required hardening depth is achieved.

However, the cooling rate is too large, and during the transformation of austenite to martensite, huge structural stress and thermal stress will be generated, which will deform the workpiece and have the risk of cracking.

In order to solve the above contradiction, the reasonable quenching and cooling process of steel usually requires the most unstable region of austenite in the pearlite transformation zone or bainite transformation zone, etc., to be rapidly cooled to prevent its decomposition, through the martensite transformation zone.

Slower cooling is required to reduce the stress that occurs when austenite transforms martensite.

In actual production, the cooling medium can be selected according to the characteristics of the steel. For example, the critical cooling rate of carbon steel is large. The medium with strong cooling capacity such as water and brine should be selected. The critical cooling rate of the alloy steel is small, and a relatively moderate medium can be used. Oil, etc.

The hardenability-hardenability of steel is one of the basic properties of steel.

It is different from hardenability, the latter refers to the hardness value of martensite, which is mainly determined by the carbon content in the steel. The curve of the effect of the carbon content at different martensite contents on the quenching hardness indicates the relationship between the hardness value and the carbon content in the hardened layer containing different percentages of martensite.

When the round bar of a certain size is quenched, the cooling rate of the surface and the core is different. It is obvious that the depth of the hardened layer depends on the critical cooling rate, so that the critical cooling rate of the steel can be lowered by adding alloying elements to increase the depth of the hardened layer of the steel.

The hardenability and quenching process ensures complete martensite structure during quenching of the workpiece.

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