Understand the Advantages of Vacuum Heat Treatment

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Vacuum heat treatment removes the air from the furnace so that the workpiece is heated and cooled in a vacuum state.

Compared with vacuum heat treatment, controlled atmosphere heat treatment has lower cost and is easy to use. It is one of the current development directions of heat treatment.

Its advantages are:

(1) Prevent oxidation and decarburization of the workpiece when heated, achieve bright quenching, bright annealing, and improve product quality.

(2) Carry out controllable gas carburizing or carbonitriding to strictly control the surface carbon amount, carbon concentration distribution and carburizing layer depth to ensure the quality of the carburized layer.

(3) Carry out recarburization treatment on workpieces that have undergone surface decarburization to restore and improve the performance of the workpieces.

(4) Facilitate the mechanization and automation of the vacuum heat treatment process.

Gases in vacuum furnace

When a vacuum pump is used to evacuate the gas in the furnace to a certain degree of vacuum, the pumping cannot be stopped, because in addition to the remaining air, there are many sources of gas in the furnace, such as:

① Gas released from the workpiece;

②Gas released from the surface of furnace building materials:

③ Gas leaking into the device from the outside, etc.

It is generally believed that in low vacuum, the residual gas is mainly air; while in medium and high vacuum, the main component of the residual gas is water vapor released from the surface of the material in the furnace.

Characteristics of vacuum heat treatment

1.Vacuum protection function

When metal is heated in a vacuum, the partial pressure of oxygen is very small, which can protect the metal from oxidation and decarburization.

2.Surface purification effect

When the metal surface is heat treated in vacuum, it can not only prevent oxidation, but also when the oxidized surface is heated in vacuum:

Oxides can also be removed.

This is a reversible reaction. When the partial pressure of oxygen is less than the decomposition pressure of the oxide, the reaction proceeds to the left and decomposition of the oxide occurs.

Solution, reducing the metal.

3.Degreasing effect

The grease on the surface of the workpiece decomposes into hydrogen, water vapor, carbon dioxide and other gases during vacuum heating and is removed by the vacuum pump.

4.Degassing

Trace gases in metals such as hydrogen, oxygen, etc., their solubility (S) in the metal is proportional to the square root of their partial pressure (P)

(Sivers’ law)S=K·P ^1/2

Degassing occurs when the partial pressure of a gas becomes very small in a vacuum.

Degassing is carried out in the following three steps:

①The gas in the metal diffuses to the surface;

② Gas is released from the metal surface;

③The gas is removed from the vacuum furnace.

5.De-elementation phenomenon

During vacuum heating, when the pressure in the furnace is lower than the vapor pressure of some alloy elements, volatilization loss of these elements will occur.

① Causes the reduction of alloying elements on the metal surface;

② Causes the metal surface to become rough and the finish to deteriorate:

③Produces vacuum evaporation. Bond closely adjacent workpieces to each other.

Elements with high vapor pressure include chromium, manganese, zinc, nickel, aluminum, etc., which are prone to evaporation when heated in a vacuum.

Therefore, the vacuum degree cannot be too high during vacuum heating. Generally, 10^-2~10^-4 Torr is more suitable.

Learn More:

What Is Aluminum Vacuum Brazing

Titanium alloy vacuum brazing process

Why Switch To Brazing In A Vacuum Furnace