In 2012, the total contract price of West Lake Saline-alkali Chemical Pipeline corridor prefabrication project was about 90 million yuan; Which involves nickel base, titanium base two kinds of non-ferrous metal pipe material; The welding amount of nickel base is more than 900 dynes and the welding amount of titanium base is more than 2100 dynes. This paper mainly describes the welding technology and process requirements of titanium tube.
Titanium Tubing of (ASME-B861 Ti2) is of 10 sizes ranging from Φ25*2 to Φ325*5. Ti2 is an industrial pure titanium with a strength of σb 450-600mpa. It has good plasticity, toughness and corrosion resistance, especially good low-temperature performance, so titanium based materials are widely used in chemical projects. After the KICK OFF of the pipeline prefabrication project, we made various welding tests, and tested with different gas protection parameters. Finally, we obtained the welding process with the best protection effect. Moreover, we evaluated the welding process of titanium pipe materials, and compiled detailed welding process cards, so as to ensure the welding quality.
Two, titanium tube welding process
1. Weldability analysis
Titanium and its alloys have strong chemical activity. When the temperature exceeds 400℃, they begin to react with oxygen, nitrogen, hydrogen and carbon. When the temperature is higher than 600℃, the reaction is violent. The increase of oxygen, nitrogen, hydrogen and carbon content will lead to the embrittlement of titanium and its alloy weld metal, so the key issue is gas protection during the welding of TA2 titanium tube. At the same time, the temperature of the weld and heat affected zone is controlled to avoid the coarse grain and superheated structure due to overheating, which will lead to the mechanical properties of the metal will be reduced.
2. Welding defects prone to occur in welding
1) Stoma problem
When welding titanium and its alloys, it is often found that there are clustered pores near the fusion line after the weld RT. The main pores are hydrogen pores. Because the solubility of hydrogen in titanium decreases with the increase of temperature, the high temperature near the fusion line during welding will cause the hydrogen to dissolve out. If the hydrogen partial pressure in the atmosphere around the welding zone is high, the hydrogen in the molten metal is not easy to precipitate, so it will gather to form hydrogen pores.
2) Crack problem
Because the content of sulfur and phosphorus impurities in the welding titanium material is very small, there will be few hot cracks. However, it is very possible to have cold crack and delay when welding titanium material. Mainly due to the poor thermal conductivity of titanium, heat loss is slow, easy to appear weld grain coarse; When the content of gas impurities is high, the plasticity of welded joints decreases, especially when more hydrogen dissolved in the weld will form hydrogen embrittlement.
3. Gas protection
Due to the high purity requirement of gas, argon gas with high purity of 99.999% was selected for welding titanium materials. The range of argon gas to protect is the molten pool, heat affected zone and base metal in each 10mm zone outside the fusion line on both sides. For this reason, special gas protection devices need to be developed (as shown below); The interior of the pipeline is protected by an argon chamber device.
4. Selection of welding materials
SAF-5.16ERTi-2 with a good matching size of Φ2.0/2.4 is selected according to the group of base materials. The chemical composition is shown in the following table:
TABLE Chemical Composition
AWS
Classification
UNS Number
Weight Percent
C
O
N
H
Fe
Al
Other Element
Amonut
ERTi-2
R50120
0.03
0.08-0.16
0.015
0.008
0.12
--
--
--
5. Main welding parameters
5.1. Argon flow rate has a direct impact on the protection effect of the weld during the welding process. According to the verification results, gas flow parameters that can meet the requirements can be obtained:
Argon flow rate of welding joint
10--16L/Min
Note: 1. The back and supporting cover shall supply air in advance according to the pipe diameter;
2. At the end of welding, the gas can be stopped for 10s--30s to protect the weld at high temperature.
Gas flow rate in bracket
12-20 l/Min
The back protects the gas flow
15-20 l/Min
5.2. The welding current directly affects the heat input in the welding process, so welding parameters that can meet the requirements can be obtained according to the verification results:
Weld Passes
Process
Filler Metal
Current (A)
Range(V)
Other
Classification
Dia.mm
T&P
Range(A)
1
GTAW
ERTi-2
Φ 2.0/2.4
DCEN
50-120
13 to 18
NA
2
GTAW
ERTi-2
Φ 2.0/2.4
DCEN
80-180.
13-20
NA
3 *
GTAW
ERTi-2
Φ 2.0/2.4
DCEN
60-160.
13-20
NA
Note: In the welding process, as far as possible to use small wire energy, current size should be selected appropriately, and have a fast welding speed; It is necessary to ensure that the weld is not overheated, no coarse crystal is produced and the hardening phenomenon is prevented.
Iii. Weld quality assessment
After the completion of welding, the quality of the weld is mainly judged by the appearance and surface color of the weld. The color of the weld surface is mainly directly related to the argon gas protection and the cleanliness of the break. See the following table to determine the quality of the weld according to the surface color:
Weld surface color
Weld protection and cleanliness
Weld quality determination
Silver white
good
Non-polluting weld
Golden brown
good
Almost pollution-free welds
blue
general
The surface of the weld is slightly polluted but has no effect on the whole property of the weld
Blue and purple
poor
The weld contamination degree is larger, the joint plasticity decreases and becomes brittle
Dark grey
poor
If blue or cyan purple appears on the weld surface in the welding process, welding should be stopped immediately, find out the reason and improve welding in time
Measures; If dark gray appears on the surface of the weld, welding should be stopped immediately for repair, all dark gray parts will be removed, and welding again.
Iv. Conclusion
To sum up, the key issues of titanium welding are to choose the correct welding method, take effective protective measures, clean the welding area before welding, and select the appropriate welding process parameters. Only by mastering these key points of attention, can the welding quality of titanium be guaranteed.