Molybdenum base alloy processing
Plastic processing is not only a means of forming molybdenum alloy, but also can improve the strength and low temperature plasticity of molybdenum alloy. Molybdenum and its alloys can be produced by conventional plastic processing methods for molybdenum sheet, molybdenum strip, molybdenum foil, molybdenum tube, molybdenum bar, molybdenum wire and profiles. The characteristics of molybdenum alloy processing are that each hot deformation process has obvious influence on the final properties of the product. Molybdenum oxidizes rapidly above 600℃, and the oxidation products volatilize and appear in liquid phase at about 725℃, so molybdenum and its alloys are usually protected by hydrogen or other reducing atmosphere when heated. Because the contamination layer of molybdenum is very thin, it is easy to melt alkali to wash away, so the hot processing can be carried out under atmospheric conditions, but it is advisable to fast. Cold machining of molybdenum and its alloys should be carried out above the plastic-brittle transition temperature.
The billets of molybdenum and its alloys are mainly produced by powder metallurgy process and can also be produced by smelting process. Generally small size billets mostly use powder metallurgy process, large size billets both can be used. The process to be adopted depends on the performance requirements of the final product. The suitable density of powder metallurgy billets is about 93 ~ 96% of the theoretical density. Vacuum consumable arc smelting and electron beam smelting are the main smelting methods for molybdenum and its alloys in industry. The ingots with coarse grains must be extruded before they can be processed.
Used to break coarse as-cast grains, improve the processing performance of ingot, also can be used to produce pipes, bars and profiles. In order to make the as-cast grains fully broken, the extrusion ratio should not be less than 4, and the extrusion temperature is usually between 1100 ℃ and 1315℃. If products and intermediate products are obtained directly by extrusion, larger extrusion ratios and higher extrusion temperatures should be used. In order to prolong the mold life and ensure the product size and surface quality, zirconium dioxide or aluminum oxide refractory coating mold should be used, lubricated with glass lubricant during extrusion.
Including rotary forging and ordinary forging. Rotary forging is mainly used for the production of fine rod with diameter above 2.5 mm and the blank of wire drawing material. The blank used is 10 ~ 30 mm square sintered bar. The open forging temperature of pure molybdenum rotary forging is usually about 1400℃, and the pass deformation is generally 10 ~ 20%, but also up to about 30%. As the diameter decreases, the forging temperature decreases gradually, and can be reduced to about 800℃ at 3 mm diameter. For ordinary forging, hammer forging is better than press forging. Large size billets and large forgings can be obtained by ordinary forging. The opening forging temperature of pure molybdenum is about 1400℃, and the opening forging temperature of extruded billet can be lower. Free forging should pay attention to safety, to prevent the workpiece or fragments from flying out of harm.
Used for the production of sheet, strip, foil and bar. The initial rolling temperature of the billet provided by rolling smelting and extrusion is generally between 1200 ~ 1250℃; The initial rolling temperature of billets provided by POWDER metallurgy is generally about 1400℃. In order to reduce the uneven deformation, the pass deformation should be between 20 and 40% during the initial rolling, and the total rolling deformation should be about 75% after each heating. When the total deformation exceeds 85% (the thickness of the plate is about 6 mm), the rolling temperature can be reduced to 700 ~ 900℃. When the plate thickness is 1 ~ 2 mm, the rolling temperature can be reduced to 200 ~ 400℃. Depending on the plastic-brittle transition temperature of the material, the thickness of the sheet transferred to cold rolling is 0.5 ~ 1 mm. The anisotropy of the product can be improved by cross rolling. In order to obtain sufficient work hardening and improve low temperature plasticity, the appropriate amount of cold processing for the final product should be about 70%.
Pipe processing Molybdenum pipe is mainly cast ingot or sintered ingot as extrusion tube blank, using warm processing technology, through rolling, drawing or spinning pressed into a variety of pipe.Small diameter molybdenum tube is produced by warm rolling in China. The initial rolling temperature is generally about 650℃, and the final rolling temperature is about 350℃. Warm rolling pass processing rate is generally between 20 ~ 35%, the maximum can be more than 40%. For 8 mm diameter molybdenum tube with 0.5 mm wall thickness, the rolled tube can be up to 6500 mm. Warm rolled molybdenum tubes have good internal and external surfaces, good room temperature plasticity and can be further drawn into capillaries. Thin-walled tubes with larger diameters are generally made of extruded or sintered tubes by spinning.
Recrystallization annealing and stress relief annealing are commonly used. Recrystallization annealing is used in extrusion, forging and hot rolling processes. Stress relief annealing is to eliminate work hardening. Because recrystallization annealing increases the plastic-brittle transition temperature of materials, it is not conducive to the next processing. Generally, processed products are delivered and used in stress-relieved annealing state.
