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Abstract: titanium and titanium alloy are valued by people for their excellent acid corrosion resistance, and are more and more used to make equipment and process pipelines, which are applied to reactors and conveying pipelines with strong acid materials.However, titanium and titanium alloy are easy to react with various elements and compounds at high temperature and embrittlement, especially in the welding process, once oxygen, nitrogen and other elements in the air are absorbed and dissolved in the titanium metal, the alloy will become significantly hard, 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 alloy for the construction and maintenance of the production equipment.

With the development of petrochemical industry, more and more new technologies are applied to petroleum processing, at the same time some strong acid or strong corrosive materials because of their strong oxidizing and better catalytic performance is applied to the petroleum and chemical reaction, but once the leak, can cause very serious consequences, so how to make these acid corrosive material containers and transport them to be the focus of attention.Titanium and titanium alloy are valued by people for their excellent acid corrosion resistance. They are more and more used to make equipment and process pipelines, which are used in reactors and conveying pipelines with strong acid materials.However, titanium and titanium alloy are easy to react with various elements and compounds at high temperature and embrittlement, especially in the welding process, once oxygen, nitrogen and other elements in the air are absorbed and dissolved in the titanium metal, the alloy will become significantly hard, 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 alloy for the construction and maintenance of the production equipment.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.

1.Analysis of main factors affecting welding quality

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

          Stable at room temperature, titanium alloy, high temperature, especially in the process of welding, titanium and oxygen, hydrogen, nitrogen reaction speed, according to the relevant data is introduced, the titanium alloy in the process of welding, the temperature above 300 ℃ can quickly absorb hydrogen, oxygen can rapidly when the temperature above 600 ℃, the temperature above 700 ℃ can quickly absorb nitrogen, and in the air oxidation is most sensitive.It is proved 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 part are obviously changed.Its mechanical properties decreased significantly.The tendency to overheat increases.After the bending experiment on the welding, the welding meat fracture phenomenon, the surrounding 10mm or so of the base material also appeared more obvious slag crack phenomenon.Therefore, in the titanium alloy welding, the pool, drop and high temperature zone, whether positive or negative, should be fully and reliably gas protection.The purpose of adopting gas protection measures is to prevent the harmful gas around the welding area from entering the weld pool.The second is to control the formation of high temperature heat generated in the welding area.If the welding area is not effectively protected by gas, it will lead to the deterioration of welding quality, and in serious cases, the product will be scrapped.

1.2 influence of impurity gas and element

Welding defects of titanium alloys are common, such as pores and cracks.The cause of these defects is mainly affected by hydrogen, iron, carbon and other gases and elements.Experimental results show that hydrogen, iron, carbon and other gases and elements are related to the solubility of titanium alloy during welding.Purity of protective gas;Water and grease on the surface of welding parts and wire.The reason of “blowhole” is mainly from the impurities in oil and protective gas.The cause of the “crack” is mainly from water and oil.Welding defects not only affect the sealing quality requirements of welded parts, but also reduce the strength, toughness, fatigue and other mechanical properties of joints, which has a great impact on product quality.Therefore, the welding should be controlled.

2. Control of welding quality

2.1 clean welding parts and welding materials before welding

The occurrence of welding defects is closely related to the cleanliness of welding parts and wire surface. Therefore, before welding, grease, water, oxide film and other stolen goods should be cleaned from the end edge of the pipe joint within the scope of no less than 40mm and the wire surface.Clean the edge of the pipe end with 0.3mm thick electric or manual stainless steel wire brush until the metallic light color, the welding wire surface can be polished to the metallic light color with 80 mesh ~ 100 mesh diamond gauze.Above the treatment of the pipe end and wire surface, use clean white silk cloth dipped in acetone wipe

2.2 welding temperature and gas protection

        When welding titanium pipe joint, 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 the formed welding meat.The main methods to control temperature are as follows: first, to the surface forming welding area, add protective gas towing cover;Second, the welded pipe will be filled with protective gas;The purity value of protective gas should be equal to or greater than 99.99%;The flow rate of shielding gas shall meet the specified value of welding technical requirements, and the protective effect of shielding gas shall finally meet the temperature control requirements of welding quality.In the process of welding, the interlayer problem shall be timely measured and interlayer intermittent welding shall be adopted to ensure that the interlayer temperature does not exceed 100℃.

2.3 welding method and requirements

        2.3.1 reasonable selection of welding joint type, welding parameters and gas protection mode according to technical requirements;

        2.3.2 construction personnel and welders shall wear clean white gauze gloves;

        2.3.3 with a stainless steel wire brush with a thickness of 0.30mm, brush the butt weld to a silver-white metal color, and then use a clean white silk cloth dipped in a little acetone to wipe the welding area clean. The processed welding area is strictly prohibited from touching or touching iron articles;

        2.3.4 the welding shall be carried out indoors as far as possible. The ambient wind speed shall be no more than 0.5m/s, and there shall be no overdraft.

        2.3.5 the welding equipment generally chooses “manual argon tungsten arc welding machine” whose performance meets the requirements of welding parameters;

        2.3.6 when positioning and spot welding of the pipe, the butt clearance is generally about 0.5mm;

        2.3.7 during the welding of the pipe, in order to make the welding quality meet the requirements of single-side welding and double-side welding, the welding of the welding seam shall be conducted twice: one is the back-sealing welding, the other is the forming welding, and the corresponding gas protection shall be carried out as required;

        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 smallest.

        2.3.9 during arc ignition of welding, air should be ventilated for 10s ~ 15s first. When arc ignition, the welding tool cannot be lifted immediately. Air supply should be continued to protect the welding area until the temperature drops to below 100℃.

        2.3.10 during welding, the welding tool shall not swing from side to side, and the melting end of the welding wire shall not move out of the gas protection zone;

        2.3.11 every weld should be welded as soon as possible. For the weld that must be welded, the joint should be cleaned before welding. The lap length of welded meat should be between 10mm and 15mm.

     2.4 qualified standards for weld and heat-affected zone

        2.4.1 residual height of welding seam: 0-1.5mm when the wall thickness is less than 5mm;When the wall thickness is larger than 5mm, it is 1.00-2.0mm.

        2.4.2 there shall be no edging or tungsten inclusion on the weld surface;

        2.4.3 color inspection of each weld pass shall be carried out before cleaning after welding, and the qualified standards are shown in the table;Qualified standard for color inspection of titanium weld bead

     2.5 post-welding treatment and detection

        The welding quality of titanium pipe joint should first be checked by the surface color of the welding area, and the protection effect above “good” is qualified.For the joint with unsatisfactory protection effect, necessary quality treatment should be carried out according to the actual situation.Under normal circumstances, when the oxidation effect is relatively light, the stainless steel wire brush can be used to brush the oxidation part to the metallic light color (silver white) or pickling treatment, in order to ensure that the internal metal is not further oxidation.100% radiographic inspection shall be carried out on the weld that passes the appearance inspection, and subsequent installation shall be carried out only after passing the inspection.

3. Control key points in the welding process

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

     3.2 the shielding gas used for welding shall be argon of high purity as far as possible, and the protection effect shall be good, and the flow value shall be within the specified range.

     3.3 the welding area and the surface of the welding wire shall be removed with scale, grease and other dirt. The welding joint shall be finished as soon as possible (within 8h).

     3.4 on-site welding shall meet the environmental requirements of titanium welding as much as possible.

4. Conclusion

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

Editor : Carrie Qi (rowlynti@rowlyn.com)