Application of partial pressure in vacuum heat treatment
Heat treatment under vacuum conditions is called vacuum heat treatment. Vacuum heat treatment is an ideal technology for bright heat treatment without oxygenation and decarburization. However, evaporation of metallic elements, depletion of alloying elements on the surface of the part, reduction of corrosion resistance, and improvement of surface brightness of the part are inevitable challenges in the vacuum heat treatment process.
I. Evaporation of metal elements under vacuum heat treatment Evaporation of metal elements under vacuum heat treatment The evaporation of metal elements in pure metals and alloys occurs at a certain temperature and vacuum level. It has a harmful effect on the quality of vacuum heat treatment of the parts and the pollution of the vacuum heating chamber, which cannot be ignored. According to the theory of phase equilibrium, the equilibrium pressure (vapor pressure) on the surface of the metal by evaporation of the metal is different at different temperatures. As the temperature increases, the vapor pressure also increases. When the external pressure is less than the vapor pressure of the metal element at that temperature, the metal element will evaporate (sublimate). The lower the external pressure, i.e., the higher the vacuum, the easier it is for the element to evaporate. Under the same conditions, elements with high vapor pressure are more likely to evaporate than elements with low vapor pressure. If the metal elements with high vapor pressure (such as Ag, Al, Mn, Cr, Si, Pb, Zn, Mg, Cu, etc.) are contained in the alloy (including the materials of the parts, the structural materials of the vacuum heating chamber and the materials of the jig) or the surface of the wire used for tying the parts, when heated in vacuum, when the vacuum is higher than the vapor pressure of the metal elements, the metal elements will evaporate, and the evaporated metal elements will be in the form of gas around the solid metal. The evaporated metal elements, in the form of gas around the solid metal, adhere to and contaminate the metal surface, causing mutual bonding between the parts or between the parts and the basket when cooling, and seriously causing a short circuit between the heating element and the furnace body. When evaporation is serious, it causes depletion of alloying elements on the surface of the part and affects the performance, and causes surface roughness and affects the surface brightness of the part. Therefore, in the vacuum heating, the choice of vacuum degree is a very important issue, must be given sufficient attention.
II. In order to reduce or avoid the evaporation of alloying elements, the vacuum heating chamber can be refilled with high-purity neutral or inert gases (such as nitrogen, argon, helium, etc.) to increase the pressure. The pressure in the vacuum chamber is adjusted in the range of 0.1 to 650 Pa. This vacuum atmosphere heat treatment method (also known as low-pressure gas protection method) prevents evaporation of alloying elements and provides a bright surface. Another advantage of this method is that it increases convective heat transfer, which is more conducive to uniform heating of the part. The purity of the gas used for vacuum protective atmosphere heat treatment should generally be greater than 99.99%. If the purity of the neutral or inert gas back to the charge is not enough, not only will not reduce the rate of evaporation of alloying elements, but will make it increase, resulting in depletion of alloying elements on the surface of the parts, corrosion resistance is reduced. Stainless steel contains a large number of Cr, Ni, AL, Mn, Ti and other alloying elements, its vapor pressure are relatively high. Vacuum solution treatment requires a higher vacuum ( 1.33 × 10 -2 ~ 1.33 × 10 -3 Pa ); to prevent the evaporation of alloying elements, should be used in vacuum partial pressure treatment, that is, the first heating chamber vacuum to 1.33 × 10 -2 ~ 1.33 × 10 -3 Pa, and then back to fill the high-purity neutral or inert gas, so that the inflatable pressure is maintained at 133 ~ 13.3 Pa or or higher. High-purity neutral or inert gas should always be kept back to the heating chamber, one is to make the gas to fully play a protective role to increase the pressure to reduce the evaporation of metal elements; the second is to increase the gas convection heat transfer effect, more conducive to uniform heating of parts. Vacuum aging can be used at a high vacuum level (1.33 × 10 -2 to 1.33 × 10 -3 Pa). Because of the long aging time, so the vacuum furnace pressure rise rate is preferably less than or equal to 0.67 Pa / h, otherwise, it is not easy to ensure the surface brightness of the parts.
III. The choice of rechargeable gas The choice of rechargeable gas for vacuum protective atmosphere heat treatment method. Vacuum heat treatment using partial pressure heating back to the gas should be selected from high-purity neutral or inert gas, hydrogen, helium, nitrogen, argon and other common gases, the cooling speed of the four gases from fast to slow hydrogen, helium, nitrogen, argon. If the cooling speed of air is 1, the ratio of the cooling speed of hydrogen, helium, nitrogen and argon to air is 7, 6, 0.99 and 0.7, respectively. Hydrogen has the highest heat transfer capacity and the highest cooling rate at any pressure, and can be used in vacuum furnaces where graphite is used as a heating and insulation element. However, for steels with high carbon content, there is a risk of slight decarburization during the high temperature phase of cooling (above 1050 °C); for high strength steels there is a risk of hydrogen embrittlement. The gas supply system with hydrogen as cooling medium should be closed and reliable. After the cooling operation, the hydrogen should be discharged and filled with nitrogen before the furnace door is opened. Helium gas is the most expensive. Since it has a certain cooling capacity at low pressure, it is usually used at an inflation pressure of 1 × 10 4 Pa or less. Nitrogen is the most commonly used gas and the cheapest. Forced circulation cooling at a pressure slightly below atmospheric pressure can increase the cooling intensity value by a factor of about 20. In the range of 200 ~ 1200 ℃, nitrogen is neutral to the general steel, titanium alloys, stainless steel, high temperature alloys are certain activity, if the vacuum heating and cooling under the partial pressure of nitrogen, the surface of the parts have Cr2 N formation, resulting in the deterioration of the surface properties, that is, the tensile strength increased, the decline in the rate of shrinkage of the section, corrosion resistance will also be affected to some extent. Therefore, it is not suitable to use nitrogen. Argon is more common, cheaper than helium, but more expensive than nitrogen, in the inappropriate use of nitrogen, such as stainless steel, high temperature alloys and titanium alloy vacuum heating and cooling, need to use argon.
IV. The choice of vacuum degree The surface brightness of the parts after vacuum heat treatment is closely related to the heating vacuum degree, heating temperature and other factors. At a certain temperature, the surface brightness of the parts after vacuum heat treatment increases with the increase of the vacuum level.