Titanium Alloy Blank Forging Heating Method

Titanium has a low thermal conductivity and is difficult to heat. Generally, when the surface heating method is used, the heating time is quite long, and when the large billet is heated, the cross-sectional temperature difference is large. Unlike the thermal conductivity of copper, iron, and nickel-based alloys that decreases with increasing temperature, the thermal conductivity of titanium alloys increases with increasing temperature. In order to make the forging proceed smoothly and reduce the temperature drop of the metal during the forging process, especially to reduce the quenching of the blank surface, the forging titanium alloy mold needs to be preheated. Otherwise, the temperature drop and surface chilling of the metal will prevent the metal from filling the mold groove well and may cause many cracks. When the temperature is increased, they react strongly with air. When heated above 650 ° C, titanium reacts strongly with oxygen, while above 700 ° C, it also reacts with nitrogen, forming a deeper surface layer saturated with these two gases. For example, when using a surface heating method to heat a titanium blank with a diameter of 350 mm to 1100 to 1150, it is necessary to keep the temperature in the temperature range where titanium reacts strongly with the gas for more than 3 to 4 hours, and a getter layer with a thickness of more than 1 mm may be formed. This getter layer will deteriorate the deformation properties of the alloy. When heating in an oil furnace with a reducing atmosphere, hydrogen absorption is particularly strong, and hydrogen can diffuse into the alloy during heating, reducing the plasticity of the alloy. When heated in an oil furnace with an oxidizing atmosphere, the hydrogen absorption process of the titanium alloy is significantly slowed down; when heated in a common box electric furnace, the hydrogen absorption is slower.
Titanium alloy forging die preheating system is usually detachable, but sometimes also uses a heating device installed on the press. The detachable mold heating system is usually a gas heater, which can slowly heat the mold to the desired temperature range before the module is assembled into the forging equipment. The heating device fixed on the press usually adopts induction heating or resistance heating. In order to obtain a uniform fine-grained structure and high mechanical properties for forgings and die forgings, when heating, it is necessary to ensure that the residence time of the blank at high temperature is the shortest. Therefore, in order to solve the problems of low thermal conductivity of the titanium alloy during heating and serious inhalation at high temperatures, segmented heating is usually used. In the first stage, the billet is slowly heated to 650 ~ 700 ℃, and then quickly heated to the required temperature. Because titanium has less inhalation below 700 ° C, the total penetration effect of oxygen in metals by stage heating is much smaller than that of general heating. The titanium alloy blank should be heated in an electric furnace. When flame heating is necessary, the atmosphere in the furnace should be slightly oxidized to avoid hydrogen embrittlement. No matter what type of furnace is heated, the titanium alloy should not interact with the refractory material, and the bottom of the furnace should be placed with stainless steel plates. Do not use heat-resistant alloy plates containing more than 50% nickel to avoid blank welding on the plates. For precision forgings that require high surface quality, or important forgings with a small margin (such as compressor blades, discs, etc.), the blank is best heated in a protective atmosphere (argon or helium), but this requires a large investment and cost High, and there is still the risk of air pollution after it is released, so it is often used in the production of glass lubricant protective coating, and then heated in the ordinary box resistance furnace. The glass lubricant can not only prevent oxide scale from forming on the surface of the blank, but also reduce the thickness of the alpha layer, and can play a lubricating role in the deformation process.