Annealing is a heat treatment process in which pure metals and alloys are heated to an appropriate temperature for a certain period of time and then cooled slowly.
What are the advantages of annealing?
1. Annealing can improve plasticity to facilitate processing.
2. Annealing not only refines grain but also eliminates microstructure defects caused by casting, forging and welding. It can even the structure and composition of metal and improve the performance of metal. Annealing is the preparation of microstructure for subsequent heat treatment.
3. Annealing removes internal stresses in metals to prevent deformation and cracking.
The purpose of homogenizing annealing is to reduce the segregation of chemical composition and the nonuniformity of structure of metal ingot, casting or forging blank.
It is an annealing process in which metals are heated to high temperatures, held for a long time, and then cooled slowly for the purpose of homogenizing chemical composition and microstructure.
Homogenizing annealing is obtained by atomic diffusion. In multicomponent alloys this process takes a long time. Sometimes it will result in coarse grain.
For example, when Nb-10Ti-10Mo-0.1C alloy is annealed at 2040 ℃, the grain size has grown up before the microstructure has reached obvious uniformity. So even after two arc furnace smelting alloy ingot should also be through hot working or cold working to change the excess phase and matrix contact condition before homogenizing annealing.
The homogenization temperature of niobium ingot is 1800~2000 °C. The holding time depends on the ingot size, generally 5~10 hours.
Recrystallization annealing is a process in which the metal after cold deformation is heated above the recrystallization temperature for an appropriate time. It can recrystallize the deformed grains into uniform equiaxed grains. Recrystallization annealing can eliminate deformation strengthening and residual stress.
The recrystallization temperature of niobium and niobium alloy is mainly related to the degree of deformation and the amount of alloying elements. The cold deformation of pure niobium in electron beam melting is 80~ 85%. Pure niobium begins recrystallization at 1070 °C ~ 1100 °C: 60% deformation. The addition of titanium has little effect on the recrystallization of niobium. The addition of molybdenum and tungsten can greatly increase the recrystallization temperature. For example, the recrystallization temperature of Nb-7Ti-0.8Zr alloy is the same as that of Nb-1Zr alloy. However, Nb-25W-5Mo-5Ti-1Zr alloy needs to be completely recrystallized at 1760 ℃ for 1 hour after 85% cold deformation..
When tantalum is heated to 600°C~1150°C, the stress caused by cold working is eliminated. Tantalum recrystallizes at 1200°C to 1500 °C. At 1600°C~1900°C, it can reduce the contamination caused by gas dissolution. The recrystallization of tantalum is related to the amount of deformation in cold working. When the deformation of tantalum is 40%, the recrystallization begins at the temperature of 1247 degrees Celsius and 140 minutes. The recrystallization of tantalum at 1297°C takes 35 minutes. It takes 14 minutes for tantalum to start recrystallizing at 1347 degrees Celsius. At 1397 degrees Celsius, tantalum takes 3.5 minutes to recrystallize.
| Temperature range of niobium alloy recrystallization | ||||
| Materials | Temperature | Materials | Temperature | |
| Nb-5Zr | 980~1300°C | Nb-4V | 930~1200°C | |
| Nb-10Ti-5Zr | 970~1150°C | Nb-5Mo-5V-1Zr | 1100~1370°C | |
| Nb-10W-1Zr-0.1C | 1150~1430°C | Nb-1Zr | 1000~1280°C | |
| Nb-27Ta-10W-0.1Zr | 1150~1430°C | Nb-15W-5Mo-1Zr-0.1C | 1540~1760°C | |
| Nb-10W-2.5Zr | 1150~1430°C | Nb-10W-10Hf | 980~1200°C | |
| Nb-10Hf-1Ti | 870~1200°C | Nb-28W-10Ta-1Zr | 1310~1370°C | |
| Nb-10W-1Hf | 980~1200°C | |||
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