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Titanium pipeline titanium and titanium alloy with its excellent acid corrosion resistance is valued by people, more and more are used to manufacture to set up equipment and process pipeline, applied to the reactor with strong acid material, pipeline. However, titanium and titanium alloys are easy to react with a variety of elements and compounds at high temperatures and embrittlement. In particular, during the welding process, once the oxygen, nitrogen and other elements in the air are absorbed, dissolved in titanium metal will make the alloy obviously harden, reduce the ductility of titanium metal and make it brittle, causing pipe fracture and equipment damage. Therefore, it is of great significance to study the welding of titanium and titanium alloys for the setting and maintenance of consumer devices in which they are applied.

With the growth of petrochemical industry, more and more new skills are applied to petroleum processing. At the same time, some strong acids or strong corrosive materials are applied to petrochemical reactions due to their strong oxidation and good catalytic performance. However, once leaked, it will cause very tense consequences. Therefore, how to manufacture the containers of these strong acids and strong corrosive materials and the pipelines for transporting them have become the focus of attention. Titanium and titanium alloy titanium pipe is valued by people for its excellent acid corrosion resistance, and is increasingly used to manufacture equipment and process pipes, and is applied to reactors and pipelines with strong acid materials. However, titanium and titanium alloys are easy to react with a variety of elements and compounds at high temperatures and embrittlement. In particular, during the welding process, once the oxygen, nitrogen and other elements in the air are absorbed, dissolved in titanium metal will make the alloy obviously harden, reduce the ductility of titanium metal and make it brittle, causing pipe fracture and equipment damage. Therefore, it is of great significance to study the welding of titanium and titanium alloys for the setting and maintenance of consumer devices in which they are applied. In this paper, combined with the installation of titanium alloy pipeline in acetic acid plant of a project, the welding quality control of titanium alloy pipeline is discussed.

Analysis of Main Factors Affecting Welding Quality

Influence of high temperature heat and gas shield in forming welding zone of 1.1 titanium pipeline

At normal temperature, the performance of titanium alloy is relatively stable. At high temperature, especially in the welding process, the reaction rate of titanium alloy with oxygen, hydrogen and nitrogen is extremely fast, which accounts for the relevant data first. During the welding process, titanium pipeline titanium alloy can quickly absorb hydrogen when the temperature is above 300 ℃, can quickly absorb oxygen when the temperature is above 600 ℃, and can quickly absorb nitrogen when the temperature is above 700 ℃, and oxidation in air is sensitive. Tests have proved that when titanium alloys are welded, if harmless gases such as oxygen and nitrogen are invaded into the molten pool, the plasticity, toughness and surface color of the welded parts will have significant changes; the performance of the machine will obviously decrease; the overheating bias will increase. After the bending test of the welding, the welding meat has fracture signs, and the base material around 10mm also has a significant slag crack sign. Therefore, in the holding of titanium alloy welding, the molten pool, droplets and high temperature zone, whether it is the front or the back, should be held a comprehensive and reliable gas cover. The goal of accepting the gas cover step: one is to prevent the harmless gas around the welding area from entering the molten pool; the second is to control the high temperature heat generation of the welding meat area after forming. If the welding area is not an effective cover of gas, will lead to a decline in the quality of welding, tension will lead to product scrap.

Influence of 1.2 impurity gases and elements

The welding defects of titanium alloy are rare, such as pores and cracks. The causes of these defects are mainly affected by hydrogen, iron, carbon and other gases and elements. Experiments show that hydrogen, iron, carbon and other gases, elements in the welding and the solubility of titanium alloy, they are mainly derived from the humid air, the environment; cover the purity of the gas; weldment and welding wire on the surface of the moisture, oil and so on. The reason for the "porosity" is mainly from the impurities in the oil and cover gas; the reason for the "crack" is mainly from the moisture and oil. Welding defects in addition to affect the quality of weldment sealing requirements, but also low the strength of the joints, toughness, and other machine performance, the product quality has a greater impact. Therefore, welding should be controlled.

2 titanium pipe welding quality control

Cleaning of 2.1 weldments and welding materials before welding

The occurrence of welding defects is closely related to the cleanliness of the surface of weldments and welding wires. Therefore, oil stains, water, oxide films and other stolen goods within the range of not less than 40mm at the end edge of pipe joints and the surface of welding wires should be cleaned up before welding. The edge of the pipe end can be cleaned with 0.3mm thick electric or manual stainless steel wire brush to the bright color of the metal, and the surface of the welding wire can be polished with 80-100 mesh diamond gauze to the bright color of the metal. The above treated and punished pipe ends and welding wire surfaces shall be cleaned with clean white silk cloth dipped in acetone.

2.2 welding temperature and gas shield

During welding of titanium pipe joints, in order to prevent the entire welding area from being purified by harmless gases and elements at high temperatures, it is necessary to control the welding temperature required for the welding area and the formed welding meat. The main essentials of temperature control: first, add a cover gas to the surface forming and welding meat area; The second is to fill the welded joint with cover gas. The purity value of cover gas should be or greater than 99.99; The flow of cover gas should meet the specified value required by welding skills, and ultimately make the cover effect of cover gas meet the temperature control requirements of welding quality. In the process of welding, timely conduct inter-layer problem measurement, accept inter-layer intermittent welding, and ensure that the inter-layer temperature does not exceed 100 ℃.

2.3 welding essentials and requirements

2.1 Reasonably select the type of welded joint, welding parameters and gas shielding method according to the skill requirements;

2.2 construction personnel and welders wear clean white fine yarn cloth gloves;

2.3 Use a stainless steel wire brush with a wire thickness of 0.30mm, brush the butt weld to silvery white metal color, and then use a clean white silk cloth dipped in a little acetone to clean the welding area. It is strictly forbidden to touch and touch iron articles by hand in the welding area punished by disposal;

2.4 titanium pipeline welding will probably be held indoors. The ambient wind speed should not be greater than 0.5 m/s, and there should be no wind;

As usual, "manual tungsten argon arc welding machine" is selected for 2.5 welding equipment, and its performance meets the requirements of welding parameters;

When the 2.6 is spot-welded to the nozzle, the butt gap is about 0.5mm as usual;

2.7 When the nozzle is welded, in order to make the forming quality of the weld meet the requirements of single-sided welding and double-sided forming, two welding events shall be held for the forming of the weld: one for back cover welding and the other for forming welding, and corresponding gas shield shall be held as required;

The interval between the 2.8 gas shield and the welding tool should be short, and the gap in contact with the pipe wall should be small;

2.9 When applying welding and starting arc, the welding tool should be ventilated for 10s ~ 15s first. When stopping arc, the welding tool should not be lifted immediately, and the welding area should be protected by continuous gas supply until the temperature drops below 100 ℃;

During 2.10 welding, the welding tool shall not swing left and right, and the melting end of the welding wire shall not move out of the gas shield area;

2.11, each weld shall be welded at one time. For the weld that must be welded, the joint shall be cleaned before welding, and the lap length of welding meat shall be between 10mm and 15mm.

2.4 titanium pipe weld and heat affected zone pass standard

2.4.1 Weld residual height: 0-1.5mm when the wall thickness is less than 5mm; 1.0-2.0mm when the wall thickness is greater than 5mm;

2.4.2 The weld surface shall be free of undercut and tungsten entrapment;

2.4.3 The color inspection of each weld bead shall be conducted before cleaning after welding, and the passing standard is shown in the table;

Titanium weld bead color inspection pass standard

2.5 titanium pipeline post-welding disposal punishment and detection

The welding quality of titanium pipe joints shall be first checked for the surface color of the welding area, and the cover effect shall be passed if it is above "excellent. For the joints with unsatisfactory cover effect, the necessary quality disposal punishment should be held according to the actual environment. Under the same normal environment, when the oxidation effect is relatively light, a stainless steel wire brush can be used to brush the oxidized part to the bright color (silver white) of the metal or to hold pickling treatment punishment to ensure that the internal metal is not continuously oxidized. 100% radiographic flaw detection is required for welds that pass the appearance inspection, and subsequent installation can only be held if they pass the inspection.

3 Key points of control in welding process

3.1 effectively control the welding interlayer temperature, to prevent titanium alloy and nitrogen, hydrogen, oxygen gas and iron, carbon absorption and dissolution.

The shield gas used for 3.2 welding should accept high-purity argon as roughly as possible, and the shield effect should be excellent, and its flow value should be within the specified range. 3.3 the welding area and the surface of the welding wire shall be removed from the oxide scale, grease and other dirt, and the weldment joint shall be welded as soon as possible (within 8h).

3.4 site weld welding, to try to meet the environmental requirements of titanium welding.

4 Conclusion

Practice has proved that in the holding of titanium pipe joint welding, as long as the key to be well controlled, in order to ensure that the quality of welding to meet the use requirements. With the guarantee of welding quality, titanium can be used in more special environmental conditions.