In the feed preheater used in pressure vessels in the refined terephthalic acid industry, due to the high temperature (280 ° C), high pressure (8.0MPa) and corrosive media and other environments, the pipe used in this field must have high strength, thickness and Thick wall Gr.3 titanium tube with good corrosion resistance is widely used in this application field.
Thick-walled titanium tubes, especially thick-walled titanium tubes with diameter to thickness ratio iDIS, ^ 10, are prone to surface defects during cold rolling, especially internal surface cracks and folds. The mechanical properties of pure titanium depend largely on the content of interstitial elements, especially the oxygen content. The material with reduced oxygen content has good plasticity and good processing performance, but simply adopting this method does not eliminate the defects of the inner surface of the pipe, such as cracks and folds, and it is difficult to ensure the strength of the pipe. Therefore, it is necessary to analyze the rolling deformation process of thick-walled pipes under different oxygen contents to find out the causes of defects. For titanium materials, due to the effect of work hardening, there is a positive correlation between the degree of deformation and its strength and hardness. Therefore, studying the microhardness and
metallographic structure on the deformed cross-section can indirectly show different parts of the cross-section The degree of deformation, so as to study and analyze the rolling process.
The relationship between the hardness of low-oxygen pipe and the amount of deformation. The hardness of the radial layers of the pipe changes continuously with the increase of e. Although there are multiple peaks on the curve, the hardness is gradually increasing. The peaks on the curves of the layers do not always appear at the same time, and the curve has Staggered, indicating that the thick-walled tube is deformed unevenly during the rolling process in the radial direction; when the amount of deformation is below 7.5%, the hardness relationship is: Out> Mid> In, view the deformation curve data, and the outer diameter of the cross section is In> Mid, the metal is in the initial stage of wall reduction; when the deformation is 11.5% ~ 20%, the hardness relationship is: In> Out> Mid, the hardness of the inner and outer layers of the pipe is higher than that of the middle layer, indicating that the wall thickness in the initial stage of the blank is along the radial direction The deformation is uneven, and the pipe is not “rolled through”. Later, as the rolling progresses, as the amount of deformation continues to increase and the tube wall thins, the unevenness of the tube wall hardness along the radial distribution gradually decreases.
When e exceeds 38.9% (in this case 5.61mm, the tube blank wall reduction is 2.39mm), the hardness value of the wall thickness of the pipe along the radial direction is not much different, indicating that the deformation distribution of the pipe wall along the radial direction is gradually uniform. When the amount of deformation is below 15.3%, the hardness of the inner and outer layers of the pipe is always higher than that of the middle layer; when the amount of deformation is below 11.2%, the hardness relationship is: Out> Mid> In, the metal is in the reduced deformation section and the hardness curve They are consistent with each other; the uneven distribution of tube wall hardness in the late milking period gradually decreases. When e exceeds 34.8%, the hardness value of the tube wall thickness in the radial direction has not changed much. When the amount of deformation is below 7.5%, the hardness relationship is: Out> Mid> In, which is in the stage of air reduction; when the amount of deformation is 7.5% ~ 10%, the relationship of hardness is: Out> In> Mid, the metal is decreasing The beginning of wall deformation also coincides with the hardness curve; moreover, the hardness peaks appear almost simultaneously, indicating that as the deformation progresses and the wall thickness decreases, the deformation has gradually become uniform.
Microstructure of the near-outer wall and near-inner wall of each pass of the low-oxygen pipe rolled. The deformed fiber structure of the near inner wall after each rolling of the pipe is finer than that of the outer layer. The hardness value of the inner wall point in the hardness curve during the rolling process is greater than the outer wall point, which is basically consistent. This also reflects from the side The uneven deformation along the thickness direction on the section during the deformation.
- 1) From the analysis of the hardness distribution curve, the thick-walled Gr.3 titanium pipe deforms unevenly along the wall thickness during the deformation process. Increasing the oxygen content makes this unevenness more complicated. In the case of a large deformation rate (more than 35%) and a low oxygen content, the deformation of the thick-walled tube at the breaking surface of the rolling process will gradually become uniform. But when the oxygen content is high, even if the tube rolling meets the condition of large deformation rate, the deformation on the cross section is difficult to be uniform.
- 2) During the deformation of thick-walled pipes, the curve, especially the curve of the inner hole, should be gentle, and the feeding amount should be small.