The large diameter TA2 pipe was prepared by medium titanium forging 260MN and 680MN press units at 850℃, 870℃, 950℃ and 1000℃. The mechanical properties and metallography of TA2 pipe were analyzed at different extrusion temperatures. The results show that the mechanical properties and metallographic structure of the head, middle and tail of the pipe have good uniformity under different extrusion temperatures. When the extrusion temperature is below TA2 phase transition temperature, equiaxed microstructure and grain refinement effect are obvious. When the extrusion temperature is above the TA2 phase transition temperature, the grains are broken, but the grains grow up by dynamic recrystallization. Therefore, it is recommended that TA2 large diameter pipe be extruded at 40 ~ 60℃ below the TA2 phase transition temperature to obtain better mechanical properties and metallographic microstructure at room temperature.
TA2 industrial pure titanium is widely used in aerospace, shipbuilding, chemical industry, ocean engineering and other fields because of its excellent corrosion resistance, mechanical properties and welding properties. Titanium material thermal conductivity is low, in the process of extrusion produce great difference in temperature of billet surface and the core, when the extrusion temperature to 400 ℃, temperature can reach 200 ~ 250 ℃, in addition, the titanium material can produce a large amount of deformation in the process of plastic deformation heat, the above characteristics lead to titanium alloy pipe extrusion process prone to cracking defects such as uneven surface. Therefore, it is necessary to study the effect of extrusion temperature on microstructure and mechanical properties of TA2 large diameter pipe.
Large diameter titanium alloy pipes are produced by hot extrusion method, and the technological process is relatively mature. The key lies in whether there is a large extrusion machine. In this paper, the influence of extrusion temperature on the microstructure and properties of TA2 large diameter extrusion pipe is studied by using the 680MN and 260MN press units of Chinese Titanium forging, and the world's largest φ700mm pipe is successfully produced.
The test process
Test materials
The raw material for the test was industrial pure titanium TA2 melted in vacuum consumable arc furnace of Shenyang Zhongti Equipment Manufacturing Co., LTD. The chemical composition was shown in Table 1, which met the requirements of GB/T 3620.1-2016 standard. The β phase transition temperature was determined to be 904.6℃.
Table 1 Chemical composition of TA2 (%)
In the β phase region, the ingot was opened and upset for many times to fully break the coarse as-cast grains, and the extruded ingot was obtained. Its microstructure is shown in Figure 1. It can be seen from the figure that the grain distribution of the ingot was not uniform after many upsetting. Most of the grain morphology was serrated and coarse, and the grain size reached 150-300 μm, which was due to the uneven deformation of the material during upsetting. At the same time, a small amount of equiaxed grains appeared in the material, because the upsetting temperature reached the dynamic recrystallization temperature of TA2, resulting in dynamic recrystallization grains. In addition, a small number of twins were produced during upsetting.
Tube extrusion process
The pipe extrusion equipment is 260MN billet making machine and 680MN extruder of Qinghai Zhongtiqing Forging Equipment Manufacturing Co., LTD. The production process is heating, lubrication, billet, machining, coating, heating, lubrication, extrusion, cooling, heat treatment, sampling, machining, nondestructive testing, packaging storage. Extrusion process parameters are shown in Table 2. Annealing treatment at 650℃ is performed on the extruded pipe (FIG. 2).
Table 2 Extrusion process of TA2 large diameter pipe
Organizatioon and performance testinig
After the head and tail of the large-caliber pipe were cut off for 15 ~ 200mm, tensile and metallographic samples were prepared, and their microstructure and mechanical properties were observed by optical microscope and tensile machine.
Results and Discussion
Influence of extrusion temperature in microstructure of pipe
FIG. 3 shows the metallographic structure of TA2 pipe with large diameter at different extrusion temperatures. It can be seen from Figure 3(a) that when the extrusion temperature is 850℃, the large-caliber pipe has equiaxed structure and uniform grain distribution, with an average grain size of 35-55 μm. By figure 3 (b), the extrusion temperature to 870 ℃, the large diameter pipe axis of organizations such as tissue types are, part of the grain morphology is oblong or elongated, the average grain size was 60 ~ 80 microns,, because of the large extrusion forming process to produce a large number of hot deformation, metal extrusion tubes in the process of actual temperature is close to TA2 phase transition temperature, So there is a small amount of beta tissue in the tissue. FIG. 3(c) shows that when the extrusion temperature is 950℃ and the deformation temperature is higher than that of the TA2 phase transition temperature, the β microstructure is coarse, and the α phase is flat and needle-like, with a size of about 150-250 μm, and the microstructure is superheated.
By comparing Figure 1 and Figure 3(a), it can be seen that when the extrusion temperature is lower than the TA2 phase transition temperature, the extrusion deformation can fully break the grain, improve the microstructure and significantly reduce the grain size. By comparing FIG. 2 and FIG. 3(c), it can be seen that when the extrusion temperature is higher than the phase transition temperature of TA2, the grains are effectively broken under the condition of large deformation, but there are phase transition and dynamic recrystallization growth of grains during the deformation process, and the grain morphology is obviously changed, but the grain size is not significantly refined.
By comparing Figure 3(a), Figure 3(b) and Figure 3(c), it can be seen that the extrusion temperature has a significant impact on the grain and microstructure of TA2 large diameter pipe: when the extrusion temperature is lower than the phase transition temperature of TA2, the lower the extrusion temperature, the more complete the grain breakage; When the extrusion temperature is higher than the TA2 phase transition temperature, the extrusion process can break the grain, change the microstructure and morphology, but the grain refining effect is not obvious.
Influence of extrusion temperature on mechanical properties of pipe
Table 3 shows the mechanical properties of the head, middle and tail of annealed TA2 large diameter pipe.
Table 3 shows that the large diameter TA2 pipe has good mechanical properties at room temperature, all of which meet the requirements of GB/T 26058-2010 standard. The head, middle and tail of the pipe basically remain at the same level, with good stability. Wang Huailiu's research in "Brief Analysis of the Influence of Process Parameters on the Quality of TA2 extruded pipe" shows that when TA2 is extruded at 880℃, the tensile strength is 465MPa and the yield strength is 320MPa. The mechanical properties obtained in this paper are superior to the research results.
FIG. 4 shows that the extrusion temperature has little influence on the tensile strength, elongation after fracture and section shrinkage of the pipe, while the extrusion temperature has a significant influence on the yield strength of the pipe. Specifically, The phase transition temperature of TA2 has an obvious influence on the yield strength of the pipe. The yield strength of the pipe under and above the phase transition temperature of TA2 has a good stability. The reason is that the crystal structure of TA2 changes from densely packed hexagonal structure to body-centered cubic structure.
Conclusion
(1) Hot extrusion of TA2 large diameter pipe with 260MN and 680MN press unit is carried out. The grain is significantly refined under the TA2 phase transition temperature, while the grain is broken and the grain morphology is changed above the TA2 phase transition temperature, but the grain size is not significantly refined.
(2) The mechanical properties of the head, middle and tail of large diameter TA2 pipe produced by hot extrusion process have good stability and uniformity.
(3) The transition temperature of TA2 phase has obvious influence on the microstructure and properties of large diameter pipe.
