Why do we need to carburize the surface of titanium alloy wire?

The excellent characteristics of titanium make its use continue to expand, and the demand for high-quality titanium pipes is also increasing. In the past, the thin titanium pipe processing methods mostly used the die drawing method, and there was fierce friction between the titanium pipe and the mold. The surface of the titanium tube is ablated and bonded, and the dimensional tolerances are difficult to control. In order to prevent these ills, it is usually necessary to perform oxidation treatment on the titanium material to form an oxide film on the surface for lubrication, and the amount of processing after each oxidation treatment It should not be too large, the oxide film is very hard, it is easy to cause mold wear, and there are problems in product size and surface quality. Therefore, it is eager to develop low-cost and high-quality titanium thin titanium tube processing methods.

According to the rolling theory of titanium tubes, the Q value (the ratio of the relative wall reduction to the relative reduction in diameter) has a great influence on the inner surface quality of the tube during the rolling process. In the three-roll rolling process, for a certain pass, after flaw detection and sampling to observe the cross-section, to ensure that there are no cracks on the inner surface, different Q values ​​(0.87, 1.00, 1.26) are selected for rolling. In the middle pass, ultrasonic flaw detection is performed on the pipe, and the cross section is sampled and observed to ensure that there are no cracks on the inner surface before proceeding to the next rolling. Results When the Q value was 0.87, the inner surface micro-cracks were very shallow, with a depth of about 5 μm, and there were fewer cracks; when the Q value increased to 1.26, the inner surface micro-cracks depth reached 50 μm. The micro-cracks appearing on the inner surface of the pipe are mainly due to the three-roll rolling that reduces the diameter first and then reduces the wall. The reduction is large and the reduction is too small, which causes the accumulation of materials during the reduction process and the formation of longitudinal micro-cracks on the inner surface. Therefore, when rolling titanium alloy pipes with a three-high rolling mill, the Q value should not exceed 0.87, otherwise cracks are prone to appear on the inner surface of the pipe.

During the cold-rolling process of small-size thick-walled titanium alloy pipes, micro-cracks are prone to occur on the inner and outer surfaces. The microcracks on the outer surface are usually removed by grinding and scraping, and the effect is very good; for the microcracks on the inner surface, in the industrial production process, the main method of boring is used to remove the inner hole larger than 13mm, and the inner hole smaller than 13mm Generally, no treatment is required, so the internal surface quality control is difficult.

(1) When rolling small-size thick-walled titanium alloy pipes, the deformation of the two-roll billet rolling is selected at 39%, and the inner and outer surfaces of the pipe are of better quality.

(2) When three-roll cold-rolling small-size thick-walled titanium alloy pipes, the Q value should not exceed 0.87, which can ensure that the inner surface of the pipe has a good quality and no cracks. Considering the good match of strength and plasticity, the deformation of three-roll rolling is selected at 30%, which can obtain better mechanical properties and microstructure.

(3) In the rolling process of titanium alloy pipes, for every 1 to 2 passes of rolling, degreasing, pickling, annealing, and straightening are performed, and then sandblasting + pickling is used to remove internal surface cracks. . Taking this measure can increase the flaw detection qualification rate of finished pipes to 35%-40%.

The microstructure of the finished pipe after being vacuum annealed at 750℃ under the conditions of 25%, 30%, 36% rolling deformation. It can be seen that the microstructure of the annealed titanium alloy tube is an equiaxed structure. As the amount of deformation increases, the degree of recrystallization becomes more complete and the crystal grains become finer. The room temperature mechanical properties of the finished pipe after vacuum annealing at 750℃ under the conditions of rolling deformation of 25%, 30%, and 36%. It can be seen that when the deformation is 25%, the yield strength of the finished pipe is 550MPa, the tensile strength is 675MPa, the elongation is 15.5%, and the elongation is slightly higher than the standard required value of 15%; when the deformation is 30%, the resistance The tensile strength is 670MPa, the yield strength is 535MPa, and the elongation is 17%; when the deformation is 36%, the tensile strength is 640MPa, and the yield strength is 517MPa, which is slightly higher than the standard required value of 515MPa, and the elongation reaches 19%. Considering the good match of strength and plasticity, comparing the mechanical properties and microstructure under different deformation conditions, it is reasonable to select 30% for the rolling deformation of the finished pipe.
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