Vacuum heat treatment characteristics

Aug 24, 2022

In high vacuum, it has the following characteristics:

The chemical activity of a high-vacuum atmosphere is extremely low. During vacuum heat treatment, the reactions that occur at the interface of the gas phase and the solid phase, such as oxidation, reduction, decarbonization, and carburization, will not be carried out to the extent that they have an impact.;

The high vacuum atmosphere increases the volume of the gas very rapidly, which can cause the metal or alloy to release dissolved gas or decompose the metal oxide. It is precisely because of the characteristics of the high vacuum atmosphere that in the high vacuum atmosphere, the partial pressure of oxygen is very low and the oxidation effect is suppressed. Therefore, in order to achieve the purpose of non-oxidation, the partial pressure of oxygen must be lower than the decomposition pressure of the oxide.

Bright heat treatment is a heat treatment method that can prevent the oxidation reaction of metal workpieces during heat treatment and still obtain a bright metal surface. Bright heat treatment can also be carried out in a protective atmosphere and inert gases such as argon, helium and nitrogen, which can also achieve the purpose and requirements of preventing oxidation.Vacuum heat treatment can achieve all metal materials to maintain the original surface finish, dimensional accuracy and performance requirements. For workpieces that need to be ground again, the processing margin before heat treatment can be greatly reduced, and the surface cleaning process (such as pickling, sandblasting, shot blasting, etc.) is eliminated. Therefore, vacuum heat treatment is the most promising process method and the most ideal heat treatment “atmosphere”. Its share in heat treatment equipment has reached more than 20%, especially in aviation, aerospace, electronic components, textiles, tooling and other fields. It has been widely used.

The degassing (degassing) effect of vacuum The degassing effect of vacuum is as follows.Metal degassing can improve the plasticity and strength of the metal. Under vacuum heating, a certain amount of gas (hydrogen, oxygen, nitrogen, etc.) dissolved in the metal workpiece will overflow and degass from the metal surface, which is conducive to improving the plasticity and strength of the workpiece. The higher the temperature, the more intense the molecular movement, which is more conducive to promoting the diffusion of the gas dissolved in the metal to the surface, so that the degree of vacuum is increased, and the lower the air pressure, it is conducive to the overflow of the gas diffused on the metal surface.

In the smelting process of metal materials, liquid metals absorb H2, O2, N2, CO and other gases. Taking into account that the solubility of the metal to the above gases increases with the increase of temperature, when the liquid metal is cooled into steel ingots, the solubility of the gas in the metal decreases, but due to the cooling speed is too fast, the gas cannot be completely overflowed (released), but remains inside the solid metal, generating metallurgical defects such as pores and white spots (formed by H2) or solidly dissolving in the metal in an atomic and ionic state.

In addition, in the thermal processing process of metal forging, heat treatment, pickling, brazing, etc., gas will inevitably be reabsorbed. At this time, the resistance, heat conduction, magnetization, hardness, yield point, strength limit, elongation, cross-section shrinkage, impact toughness, fracture toughness and other mechanical and physical properties of the metal are affected, so control the gas content of raw materials in the metallurgical process, but also try to eliminate the gas absorbed in the thermal processing process, etc., or by improving the process flow to prevent gas absorption.

The diffusion speed of gas molecules in the solid phase often determines the speed of degassing. The reason why vacuum degassing can remove the gas inside the metal is that the gas in the metal can be removed under negative pressure conditions, so the state of vacuum in the furnace affects the speed and effect of vacuum degassing.Another factor that determines the degassing effect is the temperature in the furnace. The higher the temperature, the better the degassing effect.The third factor is time. The longer the degassing time, the better the degassing effect.Taking into account the influence of factors such as grain thickening and metal phase transition, the temperature cannot rise too high. For metal materials with phase transition such as steel, vacuum degassing at the temperature near the phase transition point has the best effect. The reason is that the metal material reduces the solubility of the gas during the phase transition or is conducive to the migration of gas atoms due to lattice changes during the phase transition.

Compared with conventional heat treatment, the mechanical properties (especially plasticity and toughness) of metal material workpieces after vacuum heat treatment have increased significantly. The reason is that the vacuum heat treatment has a good degassing effect.Surface purification and degreasing are used to heat the workpiece in a vacuum state. The oxide film, slight rust, nitrides, hydrides, etc. on the surface are reduced, decomposed or evaporated and disappear, so that the metal obtains a smooth surface. This is a feature of vacuum heat treatment.

The oxidation reaction of a metal is a reversible reaction. When the metal is heated, whether it produces an oxidation reaction or an oxide decomposition reaction depends on the relationship between the partial pressure of oxygen in the heating atmosphere and the decomposition pressure of the oxide.

The decomposition pressure of oxygen is the partial pressure of oxygen produced after the decomposition of oxides reaches equilibrium. If the decomposition pressure of oxygen is greater than the partial pressure of oxygen, the oxide decomposes and the oxygen produced is released. What is left is the clean surface of the metal, which achieves the effect of purifying the metal surface.There is very little residual oxygen in a vacuum, and the partial pressure of oxygen is very low. The higher the degree of vacuum, the lower the partial pressure of oxygen, which is lower than the decomposition pressure of the oxide. The reaction proceeds to the right, so the vacuum provides the decomposition conditions of the metal oxide when heated.

In addition, under the premise that the oxygen partial pressure in the furnace is very low, metal oxides can be decomposed into suboxides, which are easily sublimated and volatilized in vacuum heating.The substances that adhere to the surface of the workpiece are mainly oil stains, etc., which are carbon, hydrogen, and oxygen compounds. The vapor pressure is high. They are easily volatile or decompose during the vacuum heating process and are pumped away by the vacuum pump to purify the surface of the workpiece. Effect.

It should be noted that when the oxide on the metal surface is heated in a vacuum, it may also react with the diffusion from the inside of the metal material to H2 and C, reducing the oxide on the metal surface.In the process of decomposition of oxides, it is also accompanied by the removal of organic substances such as oils and fats. That is, without special cleaning to remove organic substances on the surface, the surface of the workpiece can also have a bright surface. The reason is that these oils and lubricants are aliphatic and are compounds of carbon, hydrogen and oxygen. The decomposition pressure is high, so it is easy to decompose into hydrogen, water vapor, carbon dioxide and other gases when heated in a vacuum, and then pumped away by a vacuum pump, it will not have any reaction with the surface of the parts at high temperatures, and a non-oxidizing and non-corrosive clean surface can still be obtained. The purification effect of vacuum enhances the metal surface activity and is conducive to the The absorption of C, N, Cr, Si and other atoms accelerates the rates of carburizing, nitriding, and nitrogen-carbon co-infiltration, and the infiltration layer is more uniform.

Evaporation of vacuum When the workpiece is heated in a vacuum furnace, the moisture in the furnace and the nitrogen, oxygen, and carbon monoxide in the air will be evaporated and dissipated at low temperatures. Above 800℃, hydrogen, nitrogen, and oxide decomposition gases will be released from the surface of the workpiece to complete the surface degassing effect, and the evaporation formed by thermal decomposition and escape make the metal surface bright. This is the characteristic of vacuum heat treatment. The vacuum coating process uses this principle to make the coated glass put into commercial applications in the 1990s.

Another feature of vacuum heat treatment is the evaporation of metal surface elements. This is reflected in the heat treatment of high—chromium cold-work die steel or chromium stainless steel. After the heat treatment, the parts are bonded to each other, or between the parts and the material basket (tooling). The surface is orange peel-like and very rough. At the same time, the corrosion resistance is significantly reduced. This is the disadvantage of vacuum heat treatment-metal evaporation. Regarding the evaporation of metal, the equilibrium pressure (vapor pressure) of steam acting on the metal surface is different. If the temperature is high, the vapor pressure is high, and the evaporation of solid metal is large; if the temperature is low, the vapor The pressure is low. If the temperature is certain, the vapor pressure has a certain value. When the external pressure is less than the vapor pressure at that temperature, the metal will evaporate (sublimate).The smaller the external pressure, that is, the higher the degree of vacuum, the easier it is to evaporate, and similarly, the higher the vapor pressure of the metal, the easier it is to evaporate.

It can be seen that the vapor pressure of different metals is different. According to the material of the workpiece, full attention should be paid to the evaporation problem, that is, according to the vapor pressure and heating temperature of the alloying elements of the treated workpiece during heat treatment, the appropriate degree of vacuum should be reasonably selected to prevent the evaporation of the surface alloying elements.

Commonly used elements in steel such as Mn, Ni, Co and Cr, as well as elements such as Zn, Pb and Cu, which are the main components of non-ferrous metals, have a high vapor pressure. When heated in vacuum, it is easy to produce vacuum evaporation and cause the workpiece (or tooling) to stick to each other.In fact, there is a certain correspondence between vapor pressure and heating temperature. As long as the vacuum degree is selected appropriately, the evaporation of alloying elements can be prevented.

In addition, when vacuum heating, the types of metal materials can be considered, and high-purity inert gases (that is, reverse inflation such as high-purity nitrogen, high-purity argon, etc.) can be introduced at a certain temperature to adjust the degree of vacuum in the furnace, and low-vacuum heating can be used to prevent the evaporation of alloying elements on the surface of the workpiece. This measure is more effective for high-speed tool steels, high-alloy steels and other workpieces.