Manufacture of tungsten and molybdenum and addition of alloying elements
The preparation of tungsten, molybdenum products is mainly realized by powder metallurgy. After the ore is refined and then reduced (the oxides of W and Mo are reduced in hydrogen) to obtain the corresponding metal powder (purity above 99.9%).
These powders (usually 3~5 μm in diameter) are pressed into billets (using oil pressure instead of isostatic pressing), and the subsequent sintering is not carried out in a mixing furnace, but directly heated by electric heating in a movable frame furnace or a single furnace. Tungsten and molybdenum billets are sintered respectively in hydrogen atmosphere at temperatures above 2000°C or 1700°C. Of course, sometimes sintering can be realized in high temperature vacuum furnace. If the above method is used to prepare the sintered body, the density must be above 90% of the theoretical density value (W is 19.21g/cm3, Mo is 10.19g/cm3).
Then, after the rolling of tenon billet and the rotary hammer, the corresponding rod and wire products can be obtained by drawing. As for the plate, it can be made by heating forging and calendering the plate blank.
The crystalline structure of tungsten and molybdenum used as heat-resistant materials belongs to body centered cubic type and the related intrusive impurities may have a certain impact on their processability, thus resulting in embrittlement due to ductile brittle migration when the temperature drops.
1. Add alloy elements
Rhenium is the most commonly used additive element for tungsten and molybdenum. Taking molybdenum as an example, when the addition of re reaches 20~50%, it can be found that with the increase of Re addition, DBTT will gradually decrease from room temperature to-273°C. It is estimated that the improvement of molybdenum grain boundary brittleness is due to the promotion of Re in the gas phase impurity.
In addition, tungsten-rhenium, as the most common alloying element in tungsten, can rapidly exhibit high elongation in the heat treatment environment centered on 2000°C. Recently, it has been reported that carbon and other elements which can effectively strengthen the grain boundary can also improve the low temperature ductility if properly combined with rhenium.
2. Single crystal
In addition to the alloying element addition method, avoiding grain boundary single crystallization can also improve the low temperature ductility. The possibility of large-scale production of tungsten and molybdenum can be easily realized by means of secondary recrystallization with the growth of crystalline particles at high temperature.
