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Detailed Guide to Vacuum Annealing of Steel Materials
Advantages of Vacuum Annealing of Steel Materials
Although vacuum annealing of tool steel and ordinary structural steel accounts for a small proportion of the total annealing, it is increasing day by day. Its main purpose is to obtain a bright surface. Vacuum annealing between the processes of thin plates and steel wires can restore and homogenize the deformed grains. At the same time, the remaining grease and oxides on the surface can be evaporated, and the dissolved gas can be eliminated. After vacuum annealing, the workpiece can obtain a smooth surface, so the degreasing and pickling processes can be omitted, and zinc and tin can be directly plated. This is particularly beneficial for high-speed galvanizing of steel wires. The copper coating of vacuum copper-plated steel wires is firmly bonded to the substrate and has a smooth appearance.
Annealing steel with a high content of impurity nitrogen at a temperature above 600°C can reduce the content of nitrogen and hydrogen, thereby reducing the brittleness caused by them. For precision workpieces and hypereutectoid steels whose surfaces must not be carburized or decarburized, it is difficult to avoid carburization or decarburization when annealing in a general protective atmosphere, but annealing under vacuum can obtain a high-quality surface.
Process parameters for vacuum annealing of steel materials
The main process parameters for vacuum annealing are heating temperature and vacuum degree, and the vacuum degree is selected according to the requirements for the surface state. General quality structural steel products only require a brightness of more than 60%, and annealing at a vacuum degree of 1.3~1.3×10-1Pa can meet the requirements. Coiled steel wire with a carbon content of 0.35%~0.60% can be annealed at 750~800℃ to obtain a good appearance. For important workpieces and tools (especially chromium-containing alloy tool steels) that undergo medium-temperature annealing, the chromium oxide film on the surface must be evaporated in a vacuum of more than 1.3X10-1~1.3X10-2Pa, and the workpiece can obtain a smooth surface. The treatment temperature should be slightly higher than the conventional annealing temperature. The brightness of general materials increases with the increase of annealing temperature. Annealing at a temperature above 950℃ can reach 80%. The rusted surface can be annealed at 1000℃, and the surface can be purified by evaporation. The rolled bearing steel with oxide scale is annealed at 780℃ and the high-speed steel is annealed at 840℃, and the surface can be purified. At the same time, it will be decarburized. If it is taken out of the furnace at a temperature above 300~500℃, the annealed and purified surface will be reoxidized. Therefore, in order to obtain a brightness of more than 70%, it must be taken out of the furnace at a temperature below 200℃. In order to shorten the processing cycle and improve the utilization rate of the furnace, it can be cooled by circulating human gas. The annealing process parameters of general steel are shown in the following table.
Vacuum annealing process parameters of steel
Stainless steel and heat-resistant alloys contain elements such as chromium, manganese, and titanium that have strong affinity with oxygen at high temperatures. When heated in air, the chromium on the surface oxidizes and the internal chromium diffuses outward, resulting in chromium depletion within a certain range. If annealed in a carbon-containing protective atmosphere, the stainless steel is prone to intergranular corrosion due to carbon addition. Annealing such alloys in a vacuum is easier to obtain a clean and high-quality surface and maintain corrosion resistance than annealing in the commonly used low-dew point hydrogen. When annealing thin sheets and filaments with small size, high precision, and large specific surface area, such as high-purity nickel wire, nickel alloy wire, ∅0.03mm nickel-chromium-nickel-aluminum thermocouple wire, etc., if oxidation occurs, the surface composition will change and its electrical properties will change. The practical significance of vacuum annealing such products is greater.
Surface brightness is an important indicator of stainless steel annealing. The surface brightness of low-temperature annealing at 6.7Pa is <60%, and the brightness increases with the increase of annealing temperature. The oxide film formed on SUS304 at 5.33Pa and below 900℃ will gradually evaporate during the heating process at a temperature above 900℃, and will completely disappear at 1050℃. The difficulty of reducing the oxide film is related to the CO partial pressure generated during reduction. The oxide film of steel with low partial pressure is difficult to reduce. Therefore, the brightness decreases in the order of martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, and heat-resistant cast iron. Vacuum annealing at 1.3X10-1Pa and 950℃ can achieve a brightness of more than 80% for the obtained stainless steel. It can be seen that annealing of the oxide film can be achieved for stainless steel at 1.3×10-1Pa and 1050℃. If the temperature and vacuum degree are too high, the bright surface cannot be obtained due to the evaporation of chromium, and the workpieces are easy to stick to each other.
Vacuum Annealing Furnace