Welding quality control of titanium tube in vinyl acetate plant

1. Overview

Titanium has good corrosion resistance, and its corrosion resistance is superior to that of stainless steel in oxidizing, neutral and chloride ion media, sometimes even more than 10 times that of 1Cr18Ni9Ti stainless steel. Successful applications in chemical, metallurgical, paper, pesticide, shipbuilding, desalination, marine engineering, power station condensers and medical devices.

The main corrosive medium in the vinyl acetate plant is acetic acid and mixtures thereof, which are highly corrosive to metallic materials. The correct selection and use of metallic materials in contact with corrosive media is important to improve the safety, stability and reliability of the production process. Generally, industrial stainless steel is difficult to resist the corrosion of acetic acid and its mixture. By using titanium as the corrosion-resistant metal material of the vinyl acetate device, it can have a good anti-corrosion effect.

Our company carried out the construction of a vinyl acetate plant in 2010. The highest design pressure of the corrosive medium is 0.5MP a, the highest design temperature is 150 °C, the main corrosion-resistant pipeline is Titanium Tube, the material is TA2, the diameter of the pipe is 25.4~ 711.2mm (1 ′′~28′′), wall thickness ranging from 2.8 to 7 mm, using manual tungsten argon arc welding . This paper combines engineering practice to explain the quality control measures of the whole process of titanium tube welding .

2. Titanium weldability

(1) Effect of gas Titanium is an active metal that can form a dense oxide film with oxygen at normal temperature to maintain high stability and corrosion resistance. The oxide film formed at high temperature is not dense, and has a strong affinity with oxygen, nitrogen and hydrogen at high temperature, and the reaction speed is fast. Rapid hydrogen absorption above 300 °C, rapid oxygen absorption above 600 °C, rapid nitrogen absorption above 70 °C, and air contains a lot of oxygen and nitrogen. The increase of nitrogen, oxygen and hydrogen makes the titanium weld be brittle. Its plasticity is seriously degraded. Therefore, in order to avoid air pollution, the welding of titanium materials requires protection of the welding zone and the temperature above 300 °C.

(2) Weld hole of the weld Titanium is particularly sensitive to impurities such as moisture, grease and oxide in the weld zone. The pores are often encountered in welding . The pores formed are mainly hydrogen pores. The formation of pores in the weld metal mainly affects the joint. The fatigue strength.

3. Preparation before welding

(1) Welding process evaluation First, before the construction of the titanium pipe, it is necessary to prepare a welding procedure qualification guide according to the welding procedure qualification standard specified in the engineering technical requirements, combined with the actual titanium pipe specification and welding method in the project, and carry out the welding process evaluation. Next, the titanium tube before welding, welding procedure in accordance with need, various titanium tubes for welded joints, the preparation of a welding process instruction, welding the welding process in strict accordance with the instructions of the welding process.

(2) Certified welders Welders engaged in the welding of titanium pipes shall hold the corresponding welder qualification certificates. The items of the qualification certificates shall meet the requirements of the corresponding welding methods, welding materials, specification thickness and welding position. The domestic welding qualification certificate is “special Equipment Operator Certificate (welder).

(3) Welding materials First of all, the incoming welding consumables should have qualified quality certification documents, and the physical objects should be consistent with the batch number on the certificate. The surface of the wire should be clean and free of oxidative color. There should be no defects such as cracks, peeling, spots and inclusions. Secondly, the temperature in the warehouse is kept above 5 °C, the humidity is not more than 60%, and the welding consumables are stored off the ground. Before welding, the use of sulfur acetone or ethanol to the wire degreasing treatment. It is strictly forbidden to use chloride solvents such as trichloroethylene and carbon tetrachloride, and to avoid attaching cotton fibers to the surface.

(4) Welding equipment Welding equipment should be complete with double meter (current meter, voltmeter) and good performance. The argon arc welding machine should have a high-frequency arc-ignition and current-attenuating device, a pre-air supply and a delayed gas-breaking function, and a water-cooling and air-cooling system.