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Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

Why Vacuum Heat Treatment is a Good Idea

It has the following characteristics in high vacuum:

The chemical activity of the high vacuum atmosphere is extremely low. During vacuum heat treatment, the reactions that occur at the interface between the gas phase and the solid phase, such as oxidation, reduction, decarburization, carburization, etc., will not proceed to an influential level; the high vacuum atmosphere makes the gas The change in volume increase is very rapid, which can cause the metal or alloy to release dissolved gas or cause the metal oxide to decompose. It is precisely because of the characteristics of high vacuum atmosphere that in high vacuum atmosphere, because the partial pressure of oxygen is very low, oxidation will occur. is suppressed, so in order to achieve the purpose of no oxidation, the partial pressure of oxygen must be lower than the decomposition pressure of the oxide.
Degassing (degassing) action of vacuum The degassing action of vacuum is as follows. Metal degassing can improve the plasticity and strength of metal. Under vacuum conditions, a certain amount of dissolved gases (hydrogen, oxygen, nitrogen, etc.) in the metal workpiece will overflow and degas from the metal surface, which is beneficial to improving the plasticity and strength of the workpiece. , the higher the temperature, the more violent the movement of molecules, which is more conducive to the diffusion of gas dissolved in the metal to the surface, increasing the degree of vacuum. The lower the pressure, the more conducive to the overflow of gas diffused on the metal surface.

During the smelting process of metal materials, the liquid metal must absorb H2, O2, N2, CO and other gases. Considering that the solubility of the metal to the above gases increases with the increase of temperature, when the liquid metal is cooled into a steel ingot, the gas will be absorbed in the metal. The solubility in the metal is reduced, but because the cooling rate is too fast, the gas cannot escape (release) completely, and remains inside the solid metal, generating metallurgical defects such as pores and white spots (formed by H2) or solid solution in the atomic and ionic state. Metal interior.

In addition, during the thermal processing of metals such as forging, heat treatment, pickling, and brazing, gases will inevitably be reabsorbed. At this time, the resistance, thermal conductivity, magnetic susceptibility, hardness, yield point, strength limit, and elongation of the metal will Mechanical and physical properties such as rate, area shrinkage, impact toughness, and fracture toughness are all affected. Therefore, the gas content of the raw materials in the metallurgical process must be controlled, and at the same time, efforts must be made to eliminate the gas absorbed during the thermal processing, or through improvements process 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 vacuum. The speed and effect of 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 coarsening and metal phase change, the temperature cannot rise too high. For metal materials with phase change such as steel, vacuum degassing at a temperature near the phase change point has the best effect. , The reason is that the solubility of gases in metal materials decreases during phase change or the change in lattice facilitates the migration of gas atoms during phase change.

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

The oxidation reaction of metal is a reversible reaction. When the metal is heated, whether an oxidation reaction or a decomposition reaction of the oxide occurs 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 the oxide reaches equilibrium. If the decomposition pressure of oxygen is greater than the partial pressure of oxygen, the oxide decomposes, the oxygen produced is released, and what remains is the clean surface of the metal. Metal surface purification effect. There is very little residual oxygen in the vacuum, and the partial pressure of oxygen is very low. The higher the 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 metal oxide with a heating decomposition conditions.

Why Vacuum Heat Treatment is a Good Idea

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 during vacuum heating. The substances adhered to the surface of the workpiece are mainly oil stains, etc., which are carbon, hydrogen, and oxygen compounds with high vapor pressure. They are easy to volatilize or decompose during the vacuum heating process, and are removed by the vacuum pump, which has the effect of purifying the surface of the workpiece.

It should be noted that when the oxide on the metal surface is heated in a vacuum, it may also react with H2 and C diffusing outward from the inside of the metal material, thereby reducing the oxide on the metal surface. During the decomposition process of oxides, it is also accompanied by the removal of organic substances such as grease. Even without special cleaning to remove organic substances on the surface, the surface of the workpiece can still have a bright surface. The reason is that these greases and lubricants are all It is aliphatic and is a compound of carbon, hydrogen and oxygen. It has a high decomposition pressure, so it is easily decomposed into gases such as hydrogen, water vapor and carbon dioxide when heated in a vacuum, and is then pumped away by the vacuum pump to prevent it from contacting parts at high temperatures. If any reaction occurs on the surface, a clean surface without oxidation or corrosion can still be obtained. The purification effect of the vacuum increases the activity of the metal surface, which is beneficial to the absorption of C, N, Cr, Si and other atoms, making carburizing, nitriding and nitrogen carbon The speed of co-penetration is accelerated, and the penetration layer is more uniform.

Vacuum evaporation When the workpiece is heated in the vacuum furnace, the moisture in the furnace and the nitrogen, oxygen and carbon monoxide in the air will be evaporated and escaped at low temperatures. Hydrogen and nitrogen will be released from the surface of the workpiece above 800°C. And the decomposition gas of oxides completes the surface degassing, and the evaporation and escape caused by thermal decomposition makes the metal surface bright, which is a characteristic of vacuum heat treatment.

Another characteristic of vacuum heat treatment is the evaporation of metal surface elements, which is reflected in the mutual bonding between parts after heat treatment of high-chromium cold work mold steel or chromium stainless steel, or between parts and material baskets (tooling), and the surface appears. Orange peel shape, very rough, and the corrosion resistance is significantly reduced. This is the shortcoming of vacuum heat treatment – the evaporation of metal. Regarding the evaporation of metal, it is derived from the theory of phase balance. The equilibrium pressure of steam acting on the metal surface (steam There is a difference in pressure). When the temperature is high, the vapor pressure is high, and the evaporation amount of solid metal is large; when the temperature is low, the vapor pressure is low. If the temperature is constant, the vapor pressure will have a certain value. When the external pressure is less than the When the vapor pressure reaches a certain 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. Similarly, metals with higher vapor pressure are easier to evaporate.

It can be seen that the vapor pressure of different metals is different. Full attention should be paid to the evaporation problem according to the material of the workpiece, that is, according to the vapor pressure and heating temperature of the alloy elements of the workpiece being processed during heat treatment, the appropriate vacuum degree should be reasonably selected to prevent the surface from evaporating. Evaporation of 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 high vapor pressures and can easily produce vacuum evaporation during vacuum heating, causing workpieces ( Or adhere to each other with tooling). In fact, there is a certain corresponding relationship between vapor pressure and heating temperature. As long as the vacuum degree is appropriately selected, the evaporation of alloy elements can be prevented.

In addition, during vacuum heating, the type of metal material can be considered, and high-purity inert gas (i.e., reverse gas filling such as high-purity nitrogen, high-purity argon, etc.) can be introduced at a certain temperature to adjust the vacuum in the furnace. degree, using low vacuum heating method to prevent the evaporation of alloy elements on the surface of the workpiece. This measure is more effective for workpieces such as high-speed tool steel and high-alloy steel.