PRODUCT CATEGORIES

Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

Effect of vacuum heat treatment on microstructure of alloy pipes

Clamp (Mo) has the characteristics of high melting point, high strength, high hardness, etc., and has excellent thermal conductivity, electrical conductivity, corrosion resistance and low thermal expansion coefficient. It is widely used in aerospace, metallurgy, nuclear energy, machinery, chemical industry and other fields. However, molybdenum has a ductile-to-brittle transition temperature higher than room temperature, has poor processability and welding performance at room temperature, and has recrystallization brittleness, which greatly limits its application. Therefore, it is generally necessary to strengthen and toughen silver alloys through alloying, microstructure control, and second phase dispersion, improve the structure of molybdenum metal, improve its performance, and expand its application scope.

There are many commonly used alloying elements in molybdenum alloys, such as C, K, Si, Cu, W, Ti, Re, etc. Among them, the most prominent alloying element is rhenium (Re). Rhenium has high solubility in silver. Rhenium with a mass fraction of 5% to 50% can not only increase the strength and plasticity of molybdenum alloys at the same time, improve its processing performance and welding performance, but also make the alloy have a lower ductile-brittle transition temperature. Molybdenum-rhenium alloy is widely used in electric vacuum devices, such as gates, heat shields and other electronic components due to its good room temperature plasticity, high temperature strength and the ability to maintain good plasticity after recrystallization.

1.1 Materials

The raw materials are high-purity molybdenum powder (>99.95%, mass fraction) and rhenium powder (>99.98%, mass fraction), of which the average particle size of the two powders is less than 5um. Alloy pipes are obtained by powder metallurgy vacuum sintering and rolling. High-purity molybdenum powder and rhenium powder are first mixed at a mass ratio of 86:14. After mixing, the molybdenum and rhenium powders are relatively evenly distributed. Then perform cold isostatic pressing (pressure greater than 150MPa) forming, and finally undergo pre-sintering and high-temperature hydrogen sintering at a temperature greater than 2000C to prepare a molybdenum-rhenium alloy with a relative density of >90%. The obtained molybdenum-rhenium alloy bar billet is subjected to pressure processing such as forging (temperature greater than 1200C), rolling (three passes), and finally Mo-14Re pipes are prepared. Microstructure of vacuum sintered Mo-14Re raw materials and rolled Mo-14Re pipes. After testing, the relative density of the rolled Mo-14Re pipe is 99.7%, which has reached densification.

1.2 Vacuum heat treatment and characterization

Samples were taken from the prepared Mo-14Re pipes and marked as samples No. 0, 1, 2, and 3 respectively. Sample No. 0 was not treated in any way and was kept in its original rolling state as a comparison sample. Samples No. 1, 2, and 3 were processed at 900C respectively. Vacuum heat treatment is carried out under three temperature conditions: 1100C and 1300C. The vacuum degree in the vacuum heat treatment furnace is maintained to be better than 1×10-2Pa for 1 hour, and then slowly cooled with the furnace.

Four samples were sampled using wire EDM equipment, and tensile properties tests and microstructure analysis were performed on each of the four samples.

2 Vacuum heat treatment process results and discussion

2.1 Effect of vacuum annealing temperature on pipe structure and room temperature tensile properties

The picture below shows the microstructure of Mo-14Re alloy pipe after heat treatment at different temperatures.

Figure (a) shows the metallographic structure of the original rolling state. It can be seen that after rolling with a large deformation, the grain structure of the Mo-14Re alloy pipe is elongated and fibrous along the rolling direction, and the grain length is relatively long. big. The rolled Mo-14Re alloy pipe samples were subjected to vacuum heat treatment at three different annealing temperatures. The corresponding microstructures are as follows (b), (c) and (d). As can be seen from (b), after the Mo-14Re alloy pipe is subjected to vacuum heat treatment at 900C, the local flat fiber structure becomes dense. This is because atoms are activated at high temperatures and the internal defects of the material begin to migrate, resulting in internal sub-surface The structure changes, but most of the structure still maintains the state before annealing, showing an elongated fibrous structure, which is a typical recovery phenomenon. At this temperature, the grain structure has no obvious change ratio. When the heat treatment temperature is 1100C, it can be seen that the fibrous structure begins to decrease. At the same time, a large number of fine grains are produced at the boundaries of the elongated fibers (see the arrow in Figure (c)), indicating that at this annealing temperature Under the condition, the fibrous tissue widens and the aspect ratio decreases. As shown in Figure (d), as the annealing temperature increases, the fiber flat structure in the original rolling state has completely disappeared. The recrystallized grains have replaced the original structure, and the grain boundaries have begun to become regular and clear, but they can still be seen. There is a tendency for recrystallized grains to be distributed along the rolling direction, and the grain morphology is unevenly distributed, indicating that the grains have not grown sufficiently. From the above analysis, it can be concluded that after high-temperature annealing at 1300C, grain equiaxing has been fully realized and growth has occurred. Due to the existence of the “rhenium effect”, the grains of molybdenum-rhenium alloy are refined. Although high-temperature heat treatment is performed, the grains do not grow abnormally, and the grains are between 20 and 70um.

3Impact of vacuum heat treatment process

(1) After vacuum annealing of Mo-14Re alloy rolled pipes, as the annealing temperature increases, the microstructure changes from the original elongated fiber structure to an equiaxed recrystallized structure. The recrystallization temperature is between 1100~1300C. time, it has completely recrystallized at 1300C.

(2) As the vacuum annealing temperature increases, the mechanical properties of Mo-14Re alloy pipes also change significantly. Generally speaking, the strength decreases and the plasticity first increases and then decreases. When the annealing temperature is 1100C, the elongation after fracture reaches the maximum value of 36.5%, and the plasticity is improved. However, beyond the annealing temperature, the plasticity decreases significantly.

(3) After annealing at 900~1100C, the fracture surface of Mo-14Re alloy pipe mainly shows a wood grain tearing morphology, and the plastic deformation characteristics are obvious: when the annealing temperature is 1300C, the fracture surface shows quasi-cleavage fracture. After recrystallization, Mo- 14Re alloy still has excellent plastic deformation ability. Comprehensive analysis shows that the optimal heat treatment temperature for Mo-14Re alloy rolled pipes should be controlled between 1100 and 1300C.