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In preheaters for pressure vessels used in the phthalic acid industry, due to high temperature (280°C), high pressure (8.0MPa) and corrosive medium, pipes used in this field are required to have high strength, thickness and good corrosion resistance.

In the cold rolling process, the thick-walled titanium tube, especially the thick-walled titanium tube with a diameter/thickness ratio of 10 or more, is prone to surface defects, especially internal surface cracks and folds.

The mechanical properties of pure titanium depend largely on the content of interstitial elements, especially oxygen.The material has good plasticity and good processing performance to reduce oxygen content, but this method alone does not eliminate the inner surface cracks and folds of the pipe in a large area, and it is difficult to guarantee the strength of the pipe.Therefore, it is necessary to analyze the rolling deformation process of thick wall tubes with different oxygen content and find out the cause of defects.

For titanium, due to the influence of work hardening, the degree of deformation is related to its strength and hardness. Therefore, the study of microhardness and metallographic structure on the deformed section can indirectly show the degree of deformation of different parts on the section, so as to study and analyze the rolling process.

The relationship between hardness and deformation of hypoxic tubing.The radial hardness of each layer of the pipe keeps changing with the increase of e. Although there are several peaks on the curve, the hardness on the whole gradually increases. The peaks on the curves of each layer do not always appear at the same time, and the curves are staggered, which indicates that the deformation of the thick wall pipe along the radial direction is uneven.When the deformation amount is below 7.5%, the hardness relationship is: Out>Mid>In. Check the deformation curve data. At this time, the outer diameter of the section is In>Mid, and the metal is at the beginning of wall reduction.When the deformation amount is between 11.5% and 20%, the hardness relationship is In>Out>Mid. The hardness of the inner and outer layer of the pipe is higher than that of the middle layer, which indicates that the wall thickness In the initial stage of billet is uneven In radial deformation, and the pipe is not “rolled through”.With the rolling process, with the increase of deformation and the thinning of the tube wall, the irregularity of the radial distribution of the tube wall hardness decreases gradually.

When e exceeds 38.9%(5.61mm and 2.39mm), the hardness value of the pipe wall thickness along the radial direction has little difference, indicating that the deformation distribution of the pipe wall along the radial direction has become uniform.When the deformation is less than 15.3%, the hardness of the inner and outer layer is always higher than that of the middle layer.When the deformation is below 11.2%, the hardness relationship is as follows: Out>Mid>In, and the metal deformation section at diameter reduction is also consistent with the hardness curve.When e exceeds 34.8%, the hardness value of the pipe wall thickness along the radial direction has little difference.When the deformation amount is below 7.5%, the hardness relationship is: Out>Mid>In, In the air reduction phase;When the deformation amount is between 7.5% and 10%, the hardness relationship is :Out>In>Mid, and the metal In the initial deformation section of the wall is also consistent with the hardness curve.Moreover, the peak value of hardness appeared almost at the same time, indicating that the deformation was gradually uniform as the deformation progressed and the wall thickness decreased.

Microstructure of the near outer wall and the near inner wall of the hypoxic tube rolled at each pass.The fiber microstructure near the inner wall of the tube after each pass of rolling is finer than that of the outer wall, and the hardness increment of the inner wall point in the hardness curve during rolling is basically the same as that of the outer wall point, which also reflects the uneven deformation along the thickness direction of the section of the thick wall tube during the deformation process

1) according to the hardness distribution curve analysis, there is an uneven deformation along the thickness of the thick wall gr.3 titanium pipe during the deformation process, and the increase of oxygen content will make this uneven phenomenon more complicated.Under the condition of large deformation rate (more than 35%) and low oxygen content, the deformation of thick wall tube in the interrupted surface of rolling process will gradually become even.However, when the oxygen content is high, even if the tube rolling meets the condition of large deformation rate, the deformation on the section is difficult to be uniform.

2) in the process of deformation of thick-walled pipe, the curve, especially the inner hole curve, should be gentle and the feeding amount should be small.

Editor : Carrie Qi (rowlynti@rowlyn.com)