Discussion on welding quality control of titanium alloy pipe

With the development of the petrochemical industry, more and more new technologies have been applied to petroleum processing. At the same time, some strong acid or strong corrosive materials have been applied to petrochemical reactions due to their strong oxidation and good catalytic performance, but once leaked, it will cause very serious consequences. So how to make the containers of these strong acid and corrosive materials and the pipeline to transport them has become the focus of people’s attention.

Titanium and titanium alloy are valued by people for their excellent acid corrosion resistance, and are increasingly used to make equipment and process pipes, which are applied to the reactor with strong acid materials, conveying pipes. However, titanium and titanium alloy are easy to react with a variety of elements and compounds at high temperature and become embrittle. Especially in the welding process, once oxygen and nitrogen are absorbed in the air and dissolved in titanium metal, the alloy will significantly harden, reduce the ductility of titanium metal and make it brittle, resulting in pipe breakage and equipment damage. Therefore, it is of great significance to study the welding of titanium and titanium alloys for the construction and maintenance of the production equipment. The welding quality control of titanium alloy pipe is discussed in this paper based on the installation of titanium alloy pipe in acetic acid unit of a project.

1. Analysis of main factors affecting welding quality

1.1 Influence of high temperature heat and gas protection in forming welding area

At room temperature, titanium alloy performance is stable, high temperature, especially in the welding process, titanium alloy with oxygen, hydrogen, nitrogen reaction speed is very fast, according to the relevant information, titanium alloy in the welding process, the temperature above 300℃ can quickly absorb hydrogen, temperature above 600℃ can quickly absorb oxygen, temperature above 700℃ can quickly absorb nitrogen, Oxidation in air is the most sensitive. The test results show that when titanium alloy is welded, if oxygen, nitrogen and other harmful gases are intruded into the molten pool, the plasticity, toughness and surface color of the welded parts have obvious changes; Its mechanical properties decreased significantly; The tendency to overheat increases.

Bending experiments on the welding of Titanium Tubing have been conducted. Fracture occurred in the welding flesh, and obvious slag cracking occurred in the base metal around 10mm of the Titanium tubing. Therefore, when welding titanium alloy, the molten pool, molten drop and high temperature zone, whether positive or negative, should be comprehensive and reliable gas protection. The purpose of taking gas protection measures is to prevent harmful gas around the welding area from entering the molten pool; The second is to control the high temperature heat generation in the welding area after forming. If the welding area is not effectively protected by gas, it will certainly lead to the decline of welding quality and the scrapping of products in serious cases.

1.2 Influence of impurity gases and elements

Welding defects of titanium alloy are quite common, such as pores and cracks. These defects are mainly caused by hydrogen, iron, carbon and other gases and elements. The results show that hydrogen, iron, carbon and other gases and elements are related to the solubility of titanium alloy during welding. They mainly come from the humid air and environment. The purity of the protective gas; Moisture and oil on the surface of weldment and wire. The main reason for producing “pores” is from oil pollution and impurities in the protective gas; The cause of the “crack” is mainly from water and oil. Welding defects not only affect the sealing quality requirements of the weldment, but also reduce the strength, toughness, fatigue and other mechanical properties of the joint, which has a great impact on product quality. Therefore, welding should be controlled.

2. Control of welding quality

2.1 Clean the welds and welding materials before welding

The occurrence of welding defects is closely related to the cleanliness of the weldment and the surface of the welding wire. Therefore, before welding, the oil, water, oxide film and other stolen goods should be cleaned up in the range of no less than 40mm at the end edge of the pipe joint and on the surface of the welding wire. The tube end edge can be cleaned with 0.3mm thick electric or manual stainless steel wire brush until the metal light bright color, and the welding wire surface can be polished with 80-100 mesh diamond gauze until the metal light bright color. Clean white silk cloth dipped in acetone should be used to clean the above treated tube ends and welding wire surfaces.

2.2 Welding temperature and gas protection

In order to prevent the whole welding area from being polluted by harmful gases and elements at high temperature, necessary welding temperature control must be carried out on the welding area and forming welding meat during welding of titanium pipe joint. The main methods of temperature control: one is to the surface forming welding meat area, add protective gas cover; The second is to be welded head tube, full of protective gas; The purity value of the protective gas should be equal to or greater than 99.99%; The flow rate of the protective gas should meet the specified value of the welding technical requirements, and finally make the protective effect of the protective gas meet the temperature control requirements of the welding quality; During the welding process, inter-layer problems should be measured in time, and intermittent welding between layers should be adopted to ensure that the inter-layer temperature does not exceed 100℃.

2.3 Welding methods and requirements

2.3.1 Reasonable selection of welding joint types, welding parameters and gas protection modes according to technical requirements;

2.3.2 Construction personnel and welders shall wear clean white fine gauze gloves;

2.3.3 With 0.30mm stainless steel wire brush, brush the butt weld to the color of silver metal, and then wipe the welding area with clean white silk dipped in a little acetone. The treated welding area is strictly prohibited to touch or touch the iron articles with hands;

2.3.4 Welding should be carried out indoors as far as possible. The ambient wind speed should be no more than 0.5m/s, and there should be no drafts;

2.3.5 Welding equipment is generally selected as “manual TIG welding machine”, whose performance meets the welding parameter requirements;

2.3.6 In the case of spot welding, the butt clearance is generally about 0.5mm;

2.3.7 When welding the pipe, in order to make the weld forming quality meet the requirements of single-side welding and double-sided forming, welding shall be performed twice: one is for the back cover welding, the other is for the forming welding, and corresponding gas protection shall be carried out according to the requirements;

2.3.8 The distance between the gas shield and the welding tool should be the shortest, and the gap between the gas shield and the pipe wall should be the minimum;

2.3.9 When welding arc initiation, it should be ventilated for 10s ~ 15s, and the welding tool should not be lifted immediately. It should continue to supply gas to protect the welding area until the temperature drops below 100℃;

2.3.10 During welding, the welding tool should not swing from side to side, and the melting end of the welding wire should not be moved out of the gas protection zone;

2.3.11 Each weld should be welded as much as possible at one time. The necessary welding seams should be cleaned before welding, and the length of the lap of the welding flesh should be between 10mm and 15mm.

2.4 Qualification standard for weld and heat affected zone

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

2.4.2 There shall be no edge biting or tungsten clamping on the weld surface;

2.4.3 The color check of each pass shall be carried out before the cleaning after welding. The qualification standards are shown in the table;

Titanium welding pass color inspection standard

2.5 Post-welding treatment and testing

The welding quality of titanium pipe joint should first be inspected by the surface color of the welding area, and the protection effect above “good” is qualified. For the joints with unsatisfactory protection effect, necessary quality treatment should be carried out according to the actual situation. In general, when the oxidation effect is light, the stainless steel wire brush can be used to brush the oxidation part to the metal bright color (silver white) or pickling treatment, to ensure that the internal metal will not be oxidized. If the weld is qualified in appearance inspection, it shall be subjected to 100% X-ray flaw detection. Only after qualified inspection can subsequent installation be carried out.

3. Control key points in welding process

3.1 Effectively control the temperature between welding layers to avoid the absorption and dissolution of titanium alloy with nitrogen, hydrogen, oxygen gas, iron and carbon elements.

3.2 As far as possible, the protective gas used for welding should be argon with high purity. The protective effect should be good, and its flow rate should be within the prescribed range.

3.3 The welding area and the surface of the welding wire shall be removed from oxidized skin, grease and other dirt, and the weldment joint shall be completed once (within 8h) as far as possible.

3.4 On-site welding of welds shall meet the environmental requirements of titanium welding as far as possible.

4. Closing remarks

Practice has proved that in titanium pipe joint welding, only in all aspects of good control, to ensure that the welding quality can meet the requirements of use. With the guarantee of welding quality, titanium materials can be used in more special environmental conditions.

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