At present, the basic heat treatment processes used by titanium alloy rod manufacturers in my country mainly include stress relief annealing, complete annealing, solid solution and aging treatment. Other annealing processes, such as isothermal treatment, multi-stage annealing, and multi-stage aging, are rarely used in production.
Stress relief annealing. The purpose of stress relief annealing is to eliminate the internal stress generated in the process of cold working, cold forming and welding. Recovering primarily occurs during this process, sometimes referred to as incomplete annealing. The stress relief annealing temperature should not be too high, generally lower than the recrystallization temperature. Temperatures below the recrystallization temperature can be used for heat-treatable β-type titanium tubes and titanium alloys. However, if it exceeds a certain time at this time, the aging effect will often occur or the inoculation effect will be caused to the aging. Therefore, solution treatment is sometimes used to eliminate stress.
In order to choose a reasonable annealing process, we first observed the effects of heating temperature and cooling method on the microstructure and mechanical properties of titanium rods and TC4 titanium alloy rods. The test materials are hot-rolled titanium rods and TC4 titanium alloy rods at 920 °C. The total deformation rate of hot rolling is about 80%, and the a+β/β transformation point is 980-990 °C. The samples were heated and kept at 1000°C, 950°C, 930°C, and 830°C for 1 hour, and then air-cooled, water-cooled and furnace-cooled, respectively. Different annealing methods have effects on the microstructure and mechanical properties.
The cooling rate has a great influence on the microstructure and mechanical properties of the above four temperatures. When water-cooled, the β-phase components in equilibrium at 1000°C, 950°C and 930°C all undergo martensitic transformation, and the β-phase transforms into martensite a` needles. At 1000 ℃, it showed obvious Widmanrite structure, and its mechanical properties were comparable to the data of 1000 ℃ air cooling. On the water-cooled samples at 950°C and 930°C, the microstructure is similar to that of air cooling, but the equiaxed primary a phases are β+ martensite a` needles. At this time, the corresponding comprehensive performance is high, and the creep resistance is better than that of the air-cooled structure. When the temperature was kept at 830 °C, the equilibrium β-phase composition could not touch the M line, but after water cooling, very fine needle-like transformation products were also found in the intergranular β-phase, which could only be distinguished by electron microscopy. However, the structure of the needle-like product has not been determined. At this time, the tensile strength and the reduction in area are very low. As for furnace cooling, due to the slow cooling rate of the sample and the long residence time at high temperature, the polymorphic transformation is fully carried out, and all a-phases become coarse. After furnace cooling at 1000 °C, coarse a-phase and inter-sheet β-phase are generated in the original β grains, and there is a thick network formed by strip-like a-phase on the original β-grain boundary, which is generally called a basket-like structure. After furnace cooling at 950°C, 930°C and 830°C, since the a-phase tends to nucleate and grow at the interface of the original a-phase, the microstructures are all equiaxed a-phase and intergranular β-phase. The tensile strength after furnace cooling at 1000 °C is lower than that of air cooling and water cooling, and the tensile plasticity is higher. The comprehensive performance after furnace cooling at other temperatures is also lower than that of water cooling and air cooling.
To sum up, in order to obtain good strength and plastic comprehensive properties for titanium rods and TC4 titanium alloy rods, as well as good creep resistance and fracture toughness, air cooling (or water cooling) can be used after holding at 950 °C for 1 hour. Annealing process. In order to facilitate subsequent processing, when the metallurgical plant leaves the factory, both titanium rods and TC4 titanium alloy rods are air-cooled at 700-800 °C for 1 hour. For some large-sized forgings, in order to ensure the uniformity of performance, furnace cooling is sometimes used.
Nicole
Company: Baoji Dynamic Trading Co., Ltd
Cuntry:China
Add:Baoti road,Jintai,Baoji city,Shaanxi,China
Cel:+86 13369210920
Gmail:nicole@jmyunti.com
Website:www.jm-titanium.com
