Automatic argon tungsten arc welding process for titanium tube and tube sheet

Through the analysis of weldability, welding protection and reasonable welding specification parameters of titanium tube and titanium tubesheet, and through Mock up simulation test, develop a reasonable automatic tungsten arc welding process parameters and welding bevel form, tube and tube plate welding specimens by nondestructive testing, macroscopic test, microscopic test, hardness test to prove its welding quality qualified, and the application of the product to promote. 

Because titanium has good corrosion resistance, in recent years, titanium heat exchanger in PTA, brine heat exchange, sewage treatment and other industries have been widely used. Titanium heat exchanger tube and tube plate welding quality is a key factor in determining the service life of the equipment, often after a period of operation of the equipment, due to excessive leakage of tube plate welding joints, resulting in equipment scrap, so improve the quality of tube plate welding is the top priority in the manufacture of heat exchanger equipment, automatic tungsten arc welding with a high degree of mechanization, the advantages of good welding quality, so the titanium heat exchanger tube plate welding using automatic Automatic tungsten tigrid arc welding process for titanium heat exchanger tube plate welding has the significance of universal promotion [1,2].

1. Introduction of automatic tungsten arc welding machine for tube and tube plate

The automatic tungsten arc welding machine used in this test is produced by a domestic company, the welding machine head is hand-held operation, compared with the gantry type automatic pipe and tube plate welding machine, has the characteristics of flexible operation, which can improve efficiency. The head circumference has three support rods and tube plate plane contact at the same time, the head center rod inserted into the tube through the gas expansion chuck fixed to ensure that the head and the tube concentric, to ensure that the head all-position welding can get a high-quality weld, the basic technical parameters of the welder as shown in Table 1.

2. Titanium welding

2.1 The weldability of titanium

Titanium is a chemically very active metal, at higher temperatures and many elements and compounds react, especially with hydrogen, oxygen and nitrogen in the air, resulting in embrittlement of the weld metal, the performance of the weld decreases. Titanium welding has a certain sensitivity to porosity, when the hydrogen atoms in the weld metal reaches a certain concentration, it is very easy to form hydrogen porosity. Titanium should avoid iron ion pollution, iron ions and titanium will form a hard and brittle TiFe phase, reducing the corrosion resistance of titanium, and TiFe phase will also accelerate the rate of hydrogen absorption of titanium. Due to the low carbon, sulfur, phosphorus and other impurity elements in titanium, few below-melting point co-crystals are produced at the grain boundaries, while the crystallization temperature interval of titanium is narrow, and the shrinkage of titanium liquid metal during solidification is small, so titanium has low susceptibility to thermal cracking [3].
Table.1 Basic parameters of the welding machine

Technical parameter
Main power distribution	PC50 series digital automatic welding host
Welding rotation speed	0.15-18.52 r/min
Torch inclination angle	0°-30°
Maximum welding current	230A (temporary load rate 60%)
Motor code R/current	Main swing 84/0.8A; Wire feeding 144/0.6A; Arc length swing 30/0.5A
Maximum wire feeding speed	1 200 mm/min
welding wire	0.8 mm, 0.9 mm, 1.0 mm
Welding shielding gas	Industrial pure argon
Welding machine cooling mode	Water-cooling
Diameter of weldable pipe	External diameter φ 16- φ 48mm

2.2 Protection of titanium welding

Titanium welding should be protected by high-purity argon gas, the tail of the welding gun should be protected by an argon gas torch cover, and the back is protected by argon filling to prevent titanium from absorbing hydrogen, oxygen and nitrogen at high temperatures. Titanium welding before the welding area should be cleaned with acetone to prevent porosity in the welding process. Titanium welding site should be a clean area, should not be in contact with carbon steel, low-alloy steel and other materials to avoid iron ion contamination, titanium surface blue dot detection should be used before welding to determine whether the titanium surface is contaminated.

3. Pipe beveling

Tube plate bevel form is appropriate to use J-type bevel, compared to V-bevel, J-type bevel root gentle, easy to spread the liquid metal during the welding process, the root of the bevel is easier to weld through. The test tube plate material SA-182F304L + SB-265Gr.1 titanium composite plate, the tube material SB-338Gr.3, the tube plate for the outreach structure, the specific structure and bevel form as shown in Figure 1.

Figure.1 Tube plate structure and bevel form

4. Welding specification parameters to determine

4.1 Peak current and the base value current

Peak current as the welding current, the root of the tube plate weld through and weld fusion play a key role, the base current as the maintenance arc current to ensure stable arc combustion. Titanium pipe plate bottoming welding, the peak current should not be too large or too small, the peak current is too small, can not guarantee the root weld through, and affect the arc stability, the peak current is too large, titanium melting too much liquid metal, due to the good liquid metal fluidity of titanium, pipe plate down slope welding, liquid metal by gravity flow quickly to the front of the arc, hinder the arc on the root of the bevel melting, resulting in the root not welded through. The base value current is too small, the arc can not be stable combustion, the pulse phenomenon is not obvious when the base value current is too large, and will lead to the base value when the welding is also carried out. Titanium pipe plate for filler and cover welding, the peak current and base value current can be increased, but too much current will cause oxidation discoloration of the titanium weld and heat-affected zone [4,5].

4.2 Peak time and base time

Peak time and base time indicate the duration of peak and base currents. The peak time has an effect on the crystallization of the molten pool tissue, the shorter peak time has a certain stirring effect on the molten pool and facilitates the escape of pores from the molten pool, and the longer peak time, which increases the welding heat input. The base time and pulse characteristics have a great relationship, if the base time is too long, the pulse characteristics are not obvious, the base time is too short, during the pulse current pause, the amount of wire melting is not enough, the melt drop transition is not stable.

4.3 Pre-melting current and pre-melting time

Pre-melting current and pre-melting time is to preheat the wire end and base material, in preparation for the peak current welding melt drop transition. Pre-melting current and pre-melting time are usually determined according to the tube wall thickness, the tube wall thickness is thin, pre-melting current is small, pre-melting time is short; tube wall thickness is thick, pre-melting current is increased, pre-melting time is slightly longer.

4.4 Welding gun angle

Titanium pipe plate welding gun angle should be 10 ° -15 ° against the side of the tube is appropriate, such as the angle of the welding gun is too large, welding arc heat too much on the side of the tube, the tube wall thickness is thin, slow heat dissipation, easy to cause the tube wall burn through, the angle is too small, tungsten electrode is difficult to align the bevel root position.

4.5 Tungsten electrode extension length

The tungsten electrode extension length of 3-6mm is appropriate, the tungsten electrode extension length is too short, the tungsten electrode can not be close to the root of the bevel, the tungsten electrode to the root of the bevel is best to ensure the distance of 1.5-2mm, the tungsten electrode extension length is too long, the nozzle protective gas on the tungsten electrode and the welding pool protection effect is reduced.

4.6 Advance blowing and lagging gas break time

Titanium pipe plate welding advance blowing and lag time compared to other materials to be longer when welding, advance blowing time is long to ensure that the welding area less harmful gases, longer lag time can ensure that the shielding gas on the weld has a longer cooling time, can effectively avoid the weld absorption of hydrogen, oxygen, nitrogen.

4.7 Wire Feeding Stroke and Welding Stroke

Welding stroke must be greater than the wire feed stroke, but the wire feed stroke should not be less than 360 °, that is, a week of the pipe circle, usually the wire feed stroke 360 °-365 ° is good, welding stroke control in 365 °-370 ° is good.

4.8 Wire Feeding Speed

Wire feeding speed should be properly matched with the welding current, too low wire feeding speed leads to a reduction in the amount of weld filler metal, too large wire feeding speed may lead to welding unfused, or even wire directly into the molten pool, resulting in sticky wire, unable to properly apply welding.

5. Pipe plate Mock-up simulation test

Foreign chemical giants and engineering companies require heat exchanger tube plate welding before the Mock-up test, that is, to simulate the product tube plate welding test, after passing the test, in accordance with the qualified Mock-up process for welding, and only through the Mock-up test welders can carry out product welding, so this project of titanium tube plate automatic tungsten argon arc for Mock-up test.

5.1 Pre-welding preparation

Before welding the tube plate and tube acetone cleaning to remove oil, and blue dot test to ensure that the tube plate and tube are not contaminated by iron ions. The back of the titanium pipe plate welding needs to be filled with argon gas protection, argon gas purity should reach 99.999%, Mock-up test tube plate back to increase the argon gas protection cover, as shown in Figure 2.

Figure.2 Back gas shield

5.2 Titanium pipe plate bottoming welding

Titanium pipe plate bottoming welding using automatic tungsten argon arc self-melting (no wire) welding, because titanium has good weldability, self-melting welding can also ensure that no cracks, and self-melting welding to ensure root penetration, self-melting bottoming welding after welding as shown in Figure 3.

Figure.3 Self-fusing bottoming welding

5.3 Titanium tube sub-pipe plate filler cover welding

Titanium pipe plate filler, cover welding are used automatic tungsten argon arc wire welding, welding material ERTi-3/φ0.9mm, cover welding after the welding seam forming state and surface color as shown in Figure 4.

Figure.4 Cover welding seam

5.4 Welding parameters

Pipe tube plate welding parameters are shown in Table 2.
Table.2 Welding parameters

	Backing weld bead	Filler weld bead		Cover weld bead
Pre melting current/A	80	80	80	80
Pre melting time/S	1	1	1	1.5
Peak current (Ip1)/A	125	130	135	145
Peak time/S	0.1	0.1	0.12	0.12
Base value current (Ib1)/A	80	80	80	80
Base value time/S	0.1	0.1	0.12	0.12
Advance blowing time/S	50	5	5	5
Welding stroke/(0)	370	370	370	370
Welding speed/(‰)	300	300	300	300
Wire feeding stroke/(°)	/	361	361	370
Wire feeding speed/(‰)	/	200	350	400
Delay blowing time/S	10	10	10	10
Direction of rotation	Anti-clockwise

6. Tube plate physical and chemical tests and qualified indicators

Titanium tube pipe plate welded joints after welding need to be PT, RT nondestructive testing, as well as macro and micro tests, hardness tests, pull-off force test to verify the quality of welded joints and process reliability.

6.1 PT testing

FULL-PT test is carried out after welding of the pipe plate in accordance with ASMEVIII-1 mandatory Appendix 8, and the results are qualified.

6.2 RT Inspection

RT inspection of the pipe plate refers to ISO-15614.8 clause 7.1.4 for RT inspection of pipe plate slices, as shown in Figure 5. After RT inspection, there are no cracks, porosity and unfused defects.

Figure.5 RT test of tube and tubesheet

6.3 Pull-off force test

Tube plate in the press for pull-off force test, pull-off force test as shown in Figure 6, 2 pull-off force specimens are broken in the tube, tensile strength of 580MPa, 575MPa, respectively, greater than the lower limit of SB-338Gr.3 tube tensile strength of 450MPa, proving that the welded joint has the strength to meet the requirements.

Figure.6 Pull-off force test

6.4 Macroscopic test

Pipe tube plate macroscopic specimens cut by wire cutting method, cut specimens polished by polishing machine, and then polished by 80 #, 200 #, 600 # metallographic sandpaper fine polishing, put into aqua regia solution erosion, 20 times magnification under the electron microscope observation, no cracks, porosity, unfused and other defects, macroscopic specimens qualified. Measured under the electron microscope fusion depth ag value greater than 2.8mm, ac value (throat height) greater than 1.4 times the tube wall thickness, the minimum leakage channel r value greater than 0.9 times the tube wall thickness, to meet the design requirements. Macroscopic test is shown in Figure 7.

Figure.7 Macroscopic specimen section

6.5 Microscopic test

Microscopic test of the tube plate weld and heat-affected zone on both sides, the matrix organization is α-phase, no microscopic defects found, microstructure as shown in Figure 8-10.

Figure.8 Microstructure of weld seam

Figure.9 SB-265 Gr.1 side heat-affected zone microstructure

Figure.10 SB-338 Gr.3 side heat-affected zone microstructure

6.6 Hardness test

The tube plate weld and both sides of the heat-affected zone hardness test, hardness values as shown in Table 3, the weld, both sides of the heat-affected zone hardness value does not exceed 200HV5 qualified indicators, proving that the welding process, good welding protection, has not been hydrogen, oxygen, nitrogen pollution, causing the weld and heat-affected zone hardness increase.
Table.3 Hardness test (HV5)

SB-265 Gr.1 side heat affected zone	Weld line	SB 338 Gr.3 side heat affected zone
119, 126, 116	165, 150, 172	165, 148, 148

7. Product site welding management

Mock-up test after passing, titanium pipe plate automatic tungsten arc welding in the product has been applied, but automatic tungsten arc welding process is susceptible to the impact of equipment stability, so every day before the product is welded in the adjustment board debugging, welders each shift, welding gun replacement tungsten electrode, welding machine to replace the wire disc, welding parameters are adjusted in the adjustment board debugging welding first, to prove the stability of the welding machine, after Then we can weld on the product. Through the above-mentioned site management, the quality of stable pipe pipe plate welded joints, the product pipe pipe plate formal welding appearance quality as shown in Figure 11.

8. Conclusion

Mock-up test of automatic tungsten arc welding of titanium pipe sub-pipe plate, through nondestructive testing, physical and chemical testing to verify the reliability of the welding process, the welding process of titanium pipe sub-pipe plate fully protected and effective management of on-site welding to ensure its welding quality, the automatic tungsten arc welding of titanium pipe sub-pipe plate effective promotion, improve the production efficiency of the product.

Figure.11 Appearance quality of tube sheet we lding

Author: Liu Yuxiang
Source: China Titanium Tubes Manufacturer: www.titaniuminfogroup.com

References:

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