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Welding process of titanium tube

Technical Conditions and Standards for Design of 1.1 Titanium Tubes

1.1.1 Design Technical Conditions

1.1.1.1 Materials of pipes and fittings IN17850 3.7025,3.7035,3.7055. The chemical composition is as follows:

Chemical composition

DINl7850 (Grade) Ti C Fe N O H

1 3.7025 I margin ≤ 0.08 ≤ 0.20 ≤ 0.05 0.03~0.12 ≤ 0.013

2 3.7035 II margin ≤ 0.08 ≤ 0.25 ≤ 0.05 0.07 ~ 0.18 ≤ 0.013

3 3.7055 Ⅲ margin ≤ 0.10 ≤ 0.30 ≤ 0.05 0.15~0.25 ≤ 0.013

1.1.1.2 Pipe specifications: φ508 × 4.5,φ408 × 14,φ26.9 × l.5,φ21.3 × 2.6.

1.1.1.3 Titanium tube working conditions; Temperature 224 ℃, pressure 2.5MPa, medium acetic acid, bromide.

1.1.1.4 Pipeline quality requirements: welded joint coefficient 1, weld radiographic inspection 100, hydraulic test pressure 3.75MPa, air tightness test pressure 0.625MPa

1.1.2 Technical standards

1.1.2.1 LON1015E of welding specifications for titanium materials in pipeline engineering

1.1.2.2 Technical Conditions for Construction of Titanium Pipe Wood Company Standard

1.1.2.3 Specification for construction and acceptance of titanium pipe SHJ502-86

1.2 welding characteristics

Titanium pipe welding is the use of inert gas to effectively protect the welding area of the TiG welding process (black pole inert gas shielded welding). Due to the special physical and chemical properties of titanium, its welding process is quite different from other metals. When welding, it must be ensured that:(1) the metal in the welding area is not polluted by active gas N,0, H and harmful impurity elements C,Fe,Mn, etc. at 250 ℃. (2) can not form a coarse grain structure. (3) can not produce large welding residual stress and residual deformation. Therefore, the welding process must be based on a reasonable process, in strict accordance with the process quality management standards, the implementation of the whole process of quality control. The factors of man, machine, material and method are in a good controlled state, so as to ensure the welding quality of titanium pipe within a reasonable period of time.

2 Requirements for materials, equipment and tools

2.1 titanium pipes and fittings; shall have the factory certificate and quality certificate of the manufacturer. After re-inspection, its specifications, chemical composition, mechanical properties and supply status shall meet the requirements of DIN17850 standards.

2.2 welding materials

2.2.1 Welding wire: welding wire brand is ERTi-2. The selection of welding wire shall be in accordance with:(1) the chemical composition and mechanical properties of the welding wire shall be equivalent to that of the base metal;(2) if the welding part requires higher plasticity, the welding wire with higher purity than the base metal shall be used.

2.2.2 The welding wire shall be reinspected before use, and the factory certificate and quality certificate shall be checked. The surface of the welding wire shall be clean and free of oxidation color, crack, peeling, scar and slag inclusion, etc.

Defects. The chemical composition of the welding wire shall comply with the relevant provisions of AWS A5.16 -70.

2.2.3 Argon: industrial grade I pure argon, with a purity of not less than 99.98 and a water content of less than 50Mg/L. Before using argon, check the factory certificate on the bottle body to verify the purity index of hydrogen, and then check whether the bottle valve has gas leakage or failure.

2.2.4 Tungsten pole: φ2.0~φ3.0mm cerium tungsten pole is selected, and its chemical composition shall meet the following requirements:

Ingredients%

Grade W CeO Fe2O3 Al2O3 SiO2 Mo CuO Wce-20 balance 2.0 ≤ 0.02 ≤ 0.06 ≤ 0.01 ≤ 0.01

2.3 welding equipment

2.3.1 Welding machine: DC TiG welding machine is adopted. The welding machine shall ensure excellent working characteristics and regulation characteristics, and shall be equipped with a good ammeter and voltmeter.

2.3.2 Welding torch: QS -75 °/500 type water-cooled TiG welding torch shall be adopted. The welding torch shall have the characteristics of simple structure, light weight and durability, tight gun body, good insulation, stable air flow, firm tungsten clamp and suitable for welding in various positions.

2.3.3 Argon pipe; semi-rigid plastic pipe, not rubber hose and other moisture-absorbing materials. It shall be used exclusively and shall not be used in series with pipes for conveying other gases. Argon pipe should not be too long, so as to avoid excessive pressure drop caused by airflow instability, generally not more than 30m.

2.3.4 Welding fixture: use austenitic stainless steel or copper pipe karan, locking bolts and other groups of titanium pipes and fittings. It should be ensured that there is a certain clamping force on the titanium pipe and fittings to ensure that the axis is consistent and the gap is uniform and appropriate.

2.3.5 Auxiliary equipment and tools: argon protective cover, polishing machine, special file, stainless steel wire brush, etc.

3 Welding process

3.1 pipe prefabrication stage

3.1.1 Pipe cutting and beveling; pipe cutting and beveling shall be carried out by mechanical processing method in special workplaces. When processing to use non-polluting media clean water for cooling, to prevent oxidation. Processing tools should be dedicated and kept clean to prevent iron contamination. The processed pipe orifice shall be smooth and free from defects such as cracks and heavy skin. The inclination deviation of the cut plane shall not exceed 1% of the pipe diameter.

3.1.2 Surface cleaning: Use a wire brush made of austenitic stainless steel to remove rust, paint, dirt, dust and sundries that can react with titanium materials on all welding surfaces of titanium pipes and within 100mm near the groove. Use grinding wheel to trim the processing surface and remove defects such as flash, burr, convex and concave.

3.1.3 Group pairs: align and clamp the titanium pipe and fittings, the axis shall not be offset, the gap shall be uniform, and the titanium pipe shall be prevented from being damaged and contaminated during assembly. Avoid forcing group pairs. The tack welding adopts the same welding process as the formal welding.

3.1.4 Degreasing treatment: use celluloid sponge to dip sulfur-free ethanol or sulfur-free acetone to degrease all welding surfaces and within 50mm near the groove, and the treated surface shall be free of any residue.

3.1.5 Welding: It shall be carried out under the conditions specified in relevant standards.

3.1.5.1 Welding procedure qualification

Before the titanium pipe is formally welded, the weldability test is carried out with φ252 × 14 TA2 pipe, and on this basis, the four welding process qualifications of φ36 × 4,φ252 × 14 vertical and horizontal fixed positions are carried out. Welding procedure qualification should be carried out in welding laboratory. Before the test, the same welding scheme as the actual construction of the project has been prepared, and the evaluation principles, requirements and methods shall be implemented in accordance with ASME IX. The qualified process parameters are as follows:

a. Groove conditions

Pipe wall thickness (mm) groove form groove angle to mouth clearance (mm) blunt edge (mm) cleaning range (mm)≤ 2 V 50 0~0.8 0~0.8 each side 50~100<2V 60 0.5~21~1.5 each side 50~100

B power type and characteristics: DC positive connection

c Welding specification

Wall thickness mm≤ 2 3~4 4~7 6~7 >7

Number of welding layers 1 1~2 2~3 3~4 4~5

Tungsten electrode diameter mm 2.0 2.0 2.0 3.0 3.0

Welding wire diameter mm 2.0 3.0 3.0 3.0 3.0

Torch diameter mm 10~12 16~20 16~20 16~20 16~20 16~20

Voltage V 10~12 12~14 12~14 12~14 12~14

Current A 40~70 80~110 110~140 120~180 120~180

Welding speed cm/min 7.5~10 10~15 10~15 10~15 10~15

Interlayer temperature ℃ <200 <200 <200 <200 <200

Line energy KJ/cm 2.4~6.7 3.9~9.3 5.3~11.8 5.8~15.1 5.8~15.1

Nozzle Argon l/min 8~12 12~15 15~20 15~20 15~20

Protective cover Argon l/min 16~25 25~30 35~45 35~45 35~45

Argon in tube l/min 6~10 8~15 10~20 10~20 10~20

Argon protection time S 30~60 >60 >60 >60 >60

Protection Zone Argon Filling Coefficient 21.8 21.8 21.8 21.8 21.8

3.1.5.2 Welder qualification

According to the process parameters provided by the welding procedure qualification, organize welders to learn under the guidance of experts, and invite experienced welders to demonstrate, and conduct operation skills training and examination for welders who will participate in welding. Results All the five welders who participated in the training passed the examination stipulated by the DIN8560 and participated in the welding of titanium pipes.

3.1.5.3 Welding Material

The welding wire is the ERTi-2 wire provided by Germany, and the specifications are determined as φ2.0 and φ3.0 after process evaluation. Before using the welding wire, the surface shall be cleaned and degreased according to the groove cleaning method, and the end of the welding wire shall be removed 10~20mm long during welding.

3.1.5.4 Welding environment

Titanium pipe construction shall be carried out in the prefabricated workshop. When welding the fixed port on site, rainproof and windproof sheds shall be set up as required to ensure that the welding environment meets the process requirements. If one of the following conditions occurs, welding is not allowed.

3.1.5.5 Interlayer Cleaning and Protection

For multi-layer weld bead, before the next layer of welding, first check the degree of surface oxidation. If there is any abnormal situation, immediately carry out surface treatment or repair treatment. Special austenitic stainless steel wire brush and grinding wheel must be used during treatment.

3.1.5.6 Pickling and passivation treatment of weld surface

After titanium welding, the weld and heat affected zone shall be pickled and passivated after the surface color inspection. After pickling, it must be rinsed thoroughly with water immediately. To remove the acid solution remaining on the weldment. The temperature of the whole pickling process should be controlled below 40 ℃. Pickling passivation solution shall be formulated according to the following proportion;

3.1.6 Welding inspection

All welds shall be qualified by visual inspection, X-ray inspection, coloring inspection and pressure test.

3.2 pipeline installation stage

The qualified prefabricated pipe wiring single-line diagram and installation quality shall be required for pipe installation. During installation, iron tools and supports shall not be in direct contact with the titanium pipe, and the titanium pipe shall not be fixed by forced method. Fix the welded joint on site during installation, and the welding process is the same as that of prefabricated pipes. The welder shall be qualified for all-position welding training examination.

4 Welding quality control

Quality control of 4.1 group pairs: during group pairs, the direction length deviation, spacing deviation, angle deviation, distance deviation between bolts adjacent to flange, vertical deviation between flange surface and pipe center, etc. shall be controlled within the specified range.

4.2 welding quality control: welding focus on the control of welders, welding materials, welding process and non-destructive testing of several major links. Mainly take the following measures;

4.2.1 Strengthen the protection of argon on the welding zone

Titanium hydrogen absorption above 250 ℃, oxygen absorption above 400 ℃, nitrogen absorption above 600 ℃, the infiltration of these elements will greatly reduce the mechanical properties of titanium. Therefore, the protection of argon on the hot weld is a key factor in the quality of welded joints. The welding protection of titanium pipes is carried out by passing argon inside the pipe and using a special protective cover outside the pipe. The form of the protective cover generally has the whole cover structure, the half cover structure and the elbow cover structure.

a. Whole cover structure

In order to avoid iron ion contamination, all structural materials for protection are made of copper. The structure consists of an argon pipe, a buffer, a protective cover, a sealing port, etc.

The argon gas pipe is made of φ6mm copper pipe. A row of φ1~1.5mm small holes are drilled on a horizontal pipe into the mixing chamber. The hole is forward to the upper wall, which mainly reduces the impulse of the argon gas population and distributes it evenly. Then it enters the protection cover through the second layer of 20~40 mesh copper wire mesh. The diameter of the protective cover should be about 40mm larger than the diameter of the titanium tube, the length is generally 80~120mm, and the length of the sealing opening is about 20mm, and it is advisable to maintain a uniform gap of 1~2mm with the outer diameter of the titanium tube. So on the one hand to avoid the loss of argon too much, on the other hand can ensure that the protection zone micro positive pressure, can prevent the outside air mixed. The nozzle wire insertion opening is about 25~40mm. If it is too small, it will affect the line of sight to see the weld, and it will be difficult to ensure the quality. If it is too large, the loss of argon will be serious and the benefit will be affected.

B. Half cover structure

The principle of the half-cover structure is basically the same as that of the full-cover structure, and this structure is adopted when the diameter is larger than Dg150.

The length of the half cover is generally 230~250mm, the width is 90~100mm, the height is 40mm, and the sealing mouth is 40mm long.

For the part other than the half cover, in order to reduce the loss of argon gas and increase the protection effect, and to fix the half cover and meet the needs of all-position welding, an arc-shaped protective belt with a width of 30~50mm and a concave middle belt shall be made. Both ends of the belt are connected to the protective cover, one end is fixed, and the other end is connected with a live buckle. In the actual welding, the ideal protection effect can be achieved, and some technical problems in all-position welding can be solved.

C, elbow cover structure in order to achieve the purpose of argon protection elbow weld, must add an elbow shape additional cover. During fixed welding, the elbow additional cover does not move, and the straight tube protective cover is movably matched with the elbow additional cover, which can rotate around the elbow additional cover to achieve the purpose of argon protection.

d. Argon filling protection in the tube adopts the method of segmental argon filling. In the 200~300mm area on both sides of the weld joint, make a sealing device and seal it for argon. The sealing device is sealed with rubber and stainless steel splint. The air outlet adopts φ4mm aperture, and the air inlet adopts φ8mm aperture, which can ensure that the air in the tube is driven clean to achieve the purpose of argon protection. On the other hand, positive pressure in the tube is ensured to prevent external air from entering, which is conducive to weld formation. However, the positive pressure should not be too large, which will cause the molten pool metal to spray out or produce concave defects on the back. The air inlet side shall be tightly sealed to prevent the argon protection from being damaged by bringing air from the gap when argon is introduced. For all-position welded joints, it is difficult to charge argon, because the distance between the two ends of the joint is long, and there are elbows, etc. Nitrogen is still filled by sections. Before the welding seam is aligned, the sealing gasket (composed of stainless steel splint and rubber, the argon filling pipe is a soft rubber pipe, and a stainless steel wire with certain strength is connected in parallel) is placed 200~300mm away from the welding seam in the pipeline. After the weld assembly is completed, take out the sealing gasket.

4.2.2 Strengthen the training of welders and improve the quality of all-position joints

The welding of all-position welded joint of titanium pipe is an important breakthrough in the field construction. Due to the blocked operation (to drive the protective cover to rotate together, the internal wire filling method cannot be used) and poor sight, the formation and crystallization of the molten pool cannot be accurately grasped, and defects such as concave and poor fusion are easy to occur. Therefore, welders must master the essentials, operate skillfully, achieve uniform rhythm, fast wire filling and control the temperature of the molten pool to ensure the welding quality. Welders on the job must pass the training and examination, and on this basis, welders with good quality and abundant physical strength shall be selected for welding of all-position pipeline welds.

4.2.3 Strictly implement the welding process and improve the first pass rate of the weld

Due to the special requirements and process characteristics of titanium welding, the toughness of the material will be greatly reduced when the weld is heated for many times. Therefore, it is necessary to carefully organize and manage the welding process, strictly implement the process system, and possibly improve the welding qualification rate of the welder. In particular, to eliminate excessive oxidation, cracks, not fusion, incomplete penetration, concave, shrinkage and other defects. In the whole welding process, the process elements such as interlayer temperature, welding line energy, welding material and welding environment should be specially controlled. Special personnel shall be responsible for measuring and recording the process data, and special personnel shall be responsible for keeping and cleaning the welding wire.

5 Technical and economic effects

5.1 technical effect: the construction practice proves that the technology is mature, advanced, the equipment is simple, easy to master, and has a wide range of practicability.

5.2 quality effect; in the whole PTA project titanium pipe construction, strict implementation of welding construction technology, obtained good quality effect, all welded joints by the following items inspection, have achieved satisfactory results.

5.3 Appearance inspection: according to the LON1015 inspection, all welded joints weld surface shape is good, smooth transition with the base metal, no surface pores, cracks, dents and other defects. The color of heat affected zone is mostly silver white and golden yellow, and a few welds are purple. Results The excellent rate was 98% and the qualified rate was 100%.

5.4 X-ray radiography inspection: the inspection shall be carried out according to DIN54 III, the photo quality shall be in accordance with 54-109, and the identification and classification of the film shall be in accordance with llW. Results: The first pass rate was 98% and the second pass rate was 100%.

5.5 Coloring inspection: all welds shall be inspected according to AD-MerKbeattHPS/3. The result was 100% qualified.

5.6 pressure test: all prefabricated pipes shall be subjected to water compression test and air tightness test in the prefabricated yard, and the system shall be subjected to water pressure test and air tightness test after installation. Test pressure: hydraulic strength 3.75MPa. Air tightness 0.625MPa. The test result was 100% acceptable.