The first characteristic of titanium alloy heating is that titanium has a lower thermal conductivity than copper, aluminum, iron and nickel. The main difficulty in heating is that the heating time is quite long when the surface heating method is used. When the large billet is heated, the cross-section temperature difference is large. The thermal conductivity of copper, iron, and nickel-based alloys decreases with increasing temperature, and the thermal conductivity of titanium alloy increases with increasing temperature.
A second characteristic of titanium alloy heating is that they react strongly with air when the temperature is raised. When heated above 650 ° C, titanium reacts strongly with oxygen, while above 700 ° C, it also reacts with nitrogen while forming a deeper surface layer saturated by the two gases. For example, when the titanium billet having a diameter of 350 mm is heated to 1,100 to 1,150 by surface heating, it is necessary to keep the gettering layer having a thickness of 1 mm or more in the temperature range in which the titanium is strongly reacted with the gas for 3 to 4 hours. This gettering layer can deteriorate the deformation properties of the alloy.
When heated in an oil furnace having a reducing atmosphere, hydrogen absorption is particularly strong, and hydrogen can diffuse into the interior of the alloy during heating to reduce the plasticity of the alloy. When heated in an oil furnace having an oxidizing atmosphere, the hydrogen absorption process of the titanium alloy is significantly slowed down; when heated in a conventional box type electric furnace, hydrogen absorption is slower.
It can be seen that the titanium alloy blank should be heated in an electric furnace. When it is necessary to use flame heating, the atmosphere in the furnace should be slightly oxidized to avoid hydrogen embrittlement. Regardless of the type of furnace heating, the titanium alloy should not interact with the refractory material, and the stainless steel plate should be placed on the bottom of the furnace. A heat resistant alloy sheet containing more than 50% nickel may not be used to prevent the billet from being welded to the board.
In order to obtain a uniform fine-grained structure and high mechanical properties for forgings and die forgings, it is necessary to ensure that the blank has the shortest residence time at high temperatures during heating. Therefore, in order to solve the problem of low thermal conductivity of the titanium alloy during heating and serious inhalation at high temperatures, sectional heating is usually employed. In the first stage, the billet is slowly heated to 650-700 ° C and then rapidly heated to the desired temperature. Since titanium inhales less at temperatures below 700 ° C, the total penetration of the segmented heating oxygen in the metal is much less than in the usual heating.
The use of segmented heating can shorten the residence time of the billet at high temperatures. Although titanium has a low thermal conductivity at low temperatures, its thermal conductivity is close to that of steel at high temperatures. Therefore, when titanium is heated to 700 ° C, it can be heated to a higher temperature than steel.
For precision forgings requiring high surface quality, or important forgings with small margins (such as compressor blades, discs, etc.), the billet is preferably heated in a protective atmosphere (argon or helium), but this investment is large and cost It is high and there is still the danger of being polluted by air after it is discharged. Therefore, the coating is often coated with a glass-coated lubricant and then heated in a common box-type resistance furnace. The glass lubricant not only avoids the formation of scale on the surface of the blank, but also reduces the thickness of the alpha layer and can provide lubrication during the deformation process.
If the work is interrupted for a short time, the temperature of the furnace with the billet should be reduced to 850 ° C. When the work continues, the furnace temperature will be raised to the initial forging temperature at the possible speed of the furnace power. When the work is interrupted for a long time, the billet should be baked and placed on asbestos board or dry sand for cooling.