The difference between vacuum protection and vacuum degassing

Vacuum protection

Generally, metal materials are heated in the air, and due to the presence of oxidizing gases such as oxygen, water vapor, and carbon dioxide in the air, these gases will oxidize with metals. The reaction formula is as follows:

2M+O₂→2MO

M+H₂O⇋MO+H₂

M+CO₂⇋MO+CO

  As a result, an oxide film or oxide scale is formed on the heated metal surface, and the original metallic luster is completely lost. At the same time, these gases also react with the carbon in the metal to decarburize the surface. If the furnace contains carbon monoxide or methane gas, it will also carbonize the metal surface. For chemically active Ti, Zr, and refractory metals W, Mo, Nb, Ta, etc., heating in an air furnace will not only generate oxides, hydrides, and nitrides but also absorb these gases and diffuse them into the metal. , which seriously deteriorates the performance of metal materials. These disadvantages such as oxidation, decarburization, carbon increase, inhalation, and even corrosion are sometimes unavoidable when heated in a controlled atmosphere furnace or a salt bath furnace. In order to solve this problem, the usual practice is to leave a machining allowance before the heat treatment of the workpiece and then process it to remove the oxidation and decarburization layers after the heat treatment.

  With the development of science and technology, the requirements for the surface quality and dimensional accuracy of metal materials are getting higher and higher, so controllable atmosphere heat treatment, inert gas heat treatment, and vacuum heat treatment have been developed. Controlled atmosphere heat treatment needs to use dew point, infrared, or oxygen probes to control the composition of the atmosphere according to the carbon content of the treated steel, so as to meet the heat treatment requirements of ordinary carbon steel, alloy tool steel, and alloy structural steel. However, this atmosphere is not suitable for austenitic steels and nickel- and cobalt-based superalloys, let alone for the heat treatment of titanium alloys. Inert gases helium and argon are suitable for heat treatment of all metal materials. However, the price of helium is expensive, and it is unrealistic to use it as a protective atmosphere for heat treatment, and the price of argon is also more expensive and less used. Nitrogen is easy to produce and has low cost, and it is suitable for most steels except that it cannot be used as a protective gas for titanium, zirconium, and vanadium.

  Vacuum heat treatment is essentially heat treatment performed in an extremely rarefied atmosphere. According to gas analysis, the gas remaining in the vacuum furnace is H₂O, O₂, CO₂, and organic vapors such as grease. Since the content of these gases is very small, it is not enough to cause oxidation, decarburization, and carbonization of the treated metal material, so the chemical composition and original brightness of the metal surface can remain unchanged.

  It is very difficult and uneconomical to reduce the impurity content of inert gas to 1 part per million. However, to reduce the relative impurity content in the vacuum "atmosphere" to one millionth, that is, to achieve a vacuum degree of 10^-3 Torr, it only needs to be equipped with an ordinary mechanical pump and a booster pump. Vacuum heat treatment can make almost all industrial metal materials maintain the original surface finish, dimensional accuracy, and performance requirements, and can reduce the processing after heat treatment and the sand-blowing cleaning process before electroplating.

Vacuum degassing

When the metal material is smelted, the liquid metal must absorb H₂, O₂, N₂, CO, and other gases. The solubility of metals to gases increases with increasing temperature and decreases with decreasing temperature. Therefore, when the liquid metal is cooled to form an ingot, the solubility of the gas in the metal decreases. Because the cooling rate is too fast, the gas cannot be completely released and is left inside the solid metal, resulting in metallurgical such as pores and white spots (formed by H2). Defects or solid solution in the metal in the atomic and ionic state. Even if vacuum smelting is used, there is still a part of the gas inside the metal. In addition, these metal materials will inevitably absorb some gas during the subsequent processing such as forging, heat treatment, pickling, and brazing.

The absorbed gas is present in the metal in the form of:

1 Gas exists in metals in the form of atoms or ions. They usually exist as interstitial atoms (such as hydrogen) and replacement atoms (such as nitrogen);

2 The gas exists in the form of molecules in pores, white spots, and microcracks;

3 Gases and metals form separate phases on the surface and inside in the form of compounds, such as oxides and nitrides in steel;

4 Chemical and physical adsorption of gases on metal surfaces and internal pore surfaces.

  The electrical resistance, thermal conductivity, magnetic susceptibility, hardness, yield point, strength limit, elongation, section shrinkage, impact toughness, fracture toughness, and other mechanical and physical properties of metal materials that have absorbed gas are affected. Therefore, we must not only control the gas content of the raw materials in the metallurgical process, but also remove the gas absorbed in the hot working process, or improve the process to prevent the gas absorption.

According to Sievers' law, the solubility of gases such as H₂, O₂, and N₂ in metals is proportional to the square root of their partial pressure, namely:

S - solubility of a gas in metal; P - partial pressure of gas dissolved in metal in the atmosphere; K - Sievers constant, related to temperature.

  The above formula shows that as the partial pressure of the gas in the surrounding atmosphere decreases, the solubility of the gas in the metal also decreases, that is, it decreases to the equilibrium solubility corresponding to the partial pressure of the gas. If the amount of gas contained in the metal is greater than the equilibrium solubility, the excess gas will be released. Therefore, reducing the pressure and increasing the vacuum can achieve the purpose of reducing the gas dissolved in the metal.

  Vacuum degassing is usually divided into two types: one is called A-type degassing, that is, under vacuum conditions, the gas in the metal is released from the metal surface in a molecular form or a molecular state, and is then pumped away by a vacuum pump. ; The other is called B-type degassing, and the gas in the degassing process is removed by volatilizing the metal-generated compound vapor from the metal surface. For example, oxygen in Nb or Ta is evaporated from the metal surface in the form of vapor phase NbO₂, NbO, TaO, and TaO₂ vapor during vacuum degassing.

  In A-type degassing, gas molecules such as H₂O steam, N₂, CO₂, and other gas molecules are adsorbed on the metal surface in the form of physical or chemical adsorption. When the vacuum degree is pumped to 100 Torr, these gases will be desorbed and will be desorbed. draw away. Heating can speed up the desorption process of the gas.

  The gas exists in the metal lattice in the form of atoms or ions, or in the pores and cracks inside the metal in the form of molecules. The degassing process is as follows:

1. Dissolved in solid metal, gas atoms or ions located between metal lattices as interstitial atoms begin to pass through the gaps during vacuum degassing, and lattice defects such as dislocations, and low-angle crystals along grain boundaries or facets The boundary spreads to the surface;

2. Gas atoms or ions diffuse from the inside of the metal to the surface of the metal, and are adsorbed on the surface when they are separated from the metal lattice;

3. The atoms of the same gas adsorbed on the metal surface recombine into gas molecules;

[H] _adsorption + [H] _adsorption → H₂

Different gas atoms adsorbed on the metal surface combine into new gas molecules

[C] _adsorption + [O] _adsorption → [CO]

or [C]_adsorption+2[O]_adsorption→[CO₂]

The gas atoms adsorbed on the metal surface combine with the atoms of the basic lattice of the metal to form compounds.

[Ta]+[O]_adsorption→[TaO]

4. The recombined gas molecules are separated from the solid metal surface into the vacuum furnace chamber and pumped away by the vacuum pump, so as to achieve the purpose of degassing gas from the inside of the metal. Among them, compounds such as [TaO] are sublimated to gaseous TaO and are drawn away or condensed on the inner wall surface of the vacuum furnace.

  If the gas exists in the pores or cracks inside the metal in a molecular state, the vacuum degassing first decomposes the gas molecules into gas atoms or ions and dissolves them in the metal. The degassing steps are:

1) The gas molecules are in a physical adsorption state at the pores or cracks;

2) Change from physical adsorption to chemical adsorption and decompose into gas atoms or ions;

3) The decomposed gas atoms or ions dissolve in the metal lattice;

4) The gas atoms are adsorbed on the metal surface by diffusion and migration. Afterward, the gas is removed from the metal surface according to the degassing process described above.

  B-type degassing is also a purification process in which metal oxides are reduced or volatilized during vacuum heating. This purification effect will be described below.

The main reason why vacuum degassing can remove the gas inside the metal is under the condition of negative pressure, so the degree of vacuum used directly affects the speed and effect of vacuum degassing. The vacuum degassing process is the process of gas diffusion from the inside of the metal to the outside, and the diffusion constant increases with the increase of temperature, namely:

D=D₀exp(-E/RT)

Therefore, the second factor that determines the effect of vacuum degassing is temperature. Under a given vacuum condition, the higher the temperature, the better the degassing effect. When the temperature and vacuum degree of vacuum degassing has been determined, the longer the vacuum degassing time, the better the effect. Because the diffusion process requires a certain time, the vacuum degassing time is the third factor affecting the degassing effect. According to foreign literature reports, for metal materials with phase transitions such as steel, the effect of vacuum degassing is the best at the temperature near the phase transition point. The reason is that the lattice change is conducive to atomic migration during the phase transition.

Compared with conventional heat treatment, the mechanical properties of the metal material after vacuum heat treatment, especially the plasticity, and toughness, are significantly increased. The important reason is the degassing effect during vacuum heat treatment.

Vacuum protection and vacuum degassing are two basic physical phenomena that occur when the workpiece is heated in a vacuum furnace, which indirectly improves the inherent quality of the workpiece.