ABSTRACT: This paper describes the key control points of expansion welding process of T2 copper tube and T2 copper steel clad plate. The key control requirements are put forward according to the expanding and welding structure of tubesheet of Shell and Tube Heat Exchanger, and the control requirements and measures are put forward from the sealing principle of expanding structure, cleaning before welding, welding gap and environmental influence, and equipment adjustment. Keywords: Expansion Welding; T2 copper pipe and copper-steel clad plate; shell-and-tube Heat Exchanger; quality control of expansion welding

1. Introduction

The two components of a high capacity air conditioning, the water cooled water and water source heat pump units, are usually Shell and tube heat exchanger. Copper heat exchanger tubes with excellent thermal conductivity and corrosion resistance are preferred for shell-and-tube heat exchangers in refrigeration systems. In refrigeration industry, the connection between Copper Tube and tubesheet is mainly mechanical expansion. However, in order to satisfy such demanding fields as ships and nuclear power, both high sealing performance, strong vibration or fatigue loads, and the need to prevent crevice corrosion [1] are required. Therefore, the expansion welding and the connection structure are the mainstream connection between the pipeline and the tubesheet. In this paper, the expansion welding of composite tubesheet and T2 pipe is described mainly: the expansion welding of T2 pipe and T2 + steel, the expansion welding of copper pipe and composite tubesheet is first welded and then expanded. The expansion welding structure is shown in figure 1 below. In order to obtain good expansion joint and ensure the operation quality of air conditioning unit, the expansion welding process of copper tube should be strictly controlled. The following on the automatic pulse Argon fluoride welding process key control points are briefly introduced for the reference of peers.

1. Cleanliness control before welding

PRE-WELDING CLEANING: heat exchanger tubes and tubesheet welding, pre-welding cleaning work is very important, found during the welding process, once there is rust, oil, water or other dirt in the tubesheet, it is easy to form subcutaneous blowhole. Most of the pores have large openings and are deep enough to be difficult to repair. COUNTERMEASURES: Before Welding, tubesheet can be degreased cleaning, thorough cleaning, surface oil, impurities. After cleaning with Compressed air, clean with White non-woven cloth, White non-woven cloth non-discoloration is qualified.

2. Welding gap control

The thermal conductivity and thermal conductivity of T2 copper are 7 times higher than that of iron at 20 °C and 11 times higher than that of iron at 1000 °C. However, due to its high thermal conductivity, if the brazing process is similar to that of low carbon steel, the heat will move rapidly out of the heating zone. The larger gap makes the metal in the molten pool easier to crack during the crystallization process, lacks melting and has low permeability. At the same time, it may cause the oxidation of base metal in the front expansion zone and affect the expansion quality. Therefore, it is very important to control the welding gap by adopting the sealing structure of welding and strength expansion for the tubesheet of heat exchanger.

COUNTERMEASURES: To reduce or eliminate the gap between Tube and tubesheet by sticking expansion, to adjust the length of Tube expander, to keep the thickness of tubesheet reduced by 3 mm on the non-welded surface, and to control the thinning rate of about 2% , in order to prevent the expansion pipe lubricating oil from polluting the pipe hole, the non-viscous expansion volatile oil was used to stick expansion.

3. Welding Environment

Equipment adjustment, gas flow, tungsten needle grinding and other control welding environment of excessive wind speed, welding process will lose the protection of gas, thus reducing the protection of gas flow, ARC instability, easy to mix with the air and lose the protection role, cAUSES THE WELD surface to turn black or loses the metallic luster. Sometimes the tungsten electrode will burn out, affecting the normal welding. During the same welding process, adjusting and shifting the angle of the tungsten needle, or discharging the gas without gas protection, will splash out the metal of the welding pool during the explosion, and the tungsten electrode will burn, thus affecting the welding. However, if the flow rate of the protective gas is too high, the repulsion of the gas pressure impact will disturb the flow of the protective air, and the protective gas can not form laminar flow, so the protective effect is not good. So welding environment, gas flow and equipment adjustment is very important, must be strictly controlled, the following is the production of a few empirical parameters for peer reference.

3.1 welding environment

(1) the wind speed in the welding zone is less than 1m/s. 

(2) do not allow the fan to blow the welding area, feel the wind or weld blowhole, need to stop welding, find out the reason.

3.2 equipment adjustments

(1) adjust the position of the tungsten electrode of the welding torch so that the welding arc is located at the junction of the tubesheet and the heat exchange tube. 

(2) the tungsten electrode should be replaced in time if it is burned. 

3.3 protective gas replacement

 (1) the welding protective gas flowmeter must be vertical up.

 (2) when the protective air pressure is less than 1.0 MPA, it must be replaced immediately.

3.4 Tungsten Needle grinding and replacement

When grinding tungsten electrode, the grain of grinding can not be helical, it should be in the direction of Extension Generatrix, and the angle a should be changed according to the welding current The tungsten tip shall be ground to a platform with a diameter D of 0.2-0.8, depending on the welding current. Tungsten needle grinding requirements, when the tip of Tungsten Needle Burn, there is a base metal residue, must be replaced in time, welding process, observe tungsten needle, at most 5 pipe hole welding, to observe 1 time.

4. Expansion sequence

If the connection between heat transfer Tube and tubesheet adopts expansion welding structure, the sequence of expansion welding has great influence on the welding quality. The experience shows that the welding temperature field will affect the expanding area when first expanding and then welding. In effect, it causes the expansion area to relax. According to National Standard 151-1999"shell-and-tube heat exchanger" , 15mm non-expansion area is reserved to reduce the influence of welding and expansion. However, because the weld is closed at 15mm, the dust and gas in the space will expand rapidly after heating, and there is no smooth discharge path, so when the pressure reaches a certain limit, it will be discharged explosively into the molten pool. This will create a through hole in the weld. Due to the lack of pressure, there will be some residue in the Weld, resulting in a subcutaneous hole [2] . During the explosion, the metal in the pool will often splash, resulting in tungsten electrode burn and affect the welding. On the contrary, if the first welding and then expansion, then the possibility of the above phenomenon is very low. Statistically, the former is 40-60 times more likely to have pores than the latter. In addition, the stress distribution of the welded pipe after expansion is more complex after welding, but the welding stress is relatively stable before expansion. In order to ensure the overall quality of the heat exchanger, expansion welding and structural pipe and tube-sheet welding should be carried out first, even considering reducing the porosity of the weld and stress state of the welded joint.

6. Expansion joint principle of copper tube and tube sheet

After the Copper Tube and the tube sheet are expanded, the copper tube has plastic deformation, the tube sheet hole has a little rebound, and the residual stress between the copper tube and the inner wall of the tube hole forms contact surface. The expansion joint directly affects the service and life of the air conditioning unit. The effective expansion length determines the pulling force, which directly reflects the ability of the unit to withstand the temperature difference and differential pressure stress, the sealing performance is directly determined by the fitting degree between the contact surfaces. The expansion length described in this paper is based on the thickness of Tubesheet, the thickness of tubesheet is reduced by 3mm at each end, and the expansion tube is adjusted. Because the front end of the tubesheet has been welded, the welding seam will be squeezed by external force, which will lead to the cracking of the welding seam and affect the quality of the welding seam. At the same time, the welding seam will be formed irregularly and irhorizontally, and the uniformity of the force on the whole expanded joint surface will be affected when it is expanded, therefore, the front end of the tube can be reduced by 3 mm only to avoid the welding seam. The back end of the tube sheet can be reduced by 3 mm to avoid excessive expansion. The end of the tube hole is cut off from the copper tube and damages the copper tube.

7. Concluding remarks

The production practice has proved that the welding of t 2 copper pipe and t 2 copper steel clad plate using tungsten electrode argon helium mixed Gas metal arc welding, the cleanliness before welding and the control of welding clearance are all important technological parameters for the welding of heat exchanger tube and tube plate, they directly affect the quality of welding. The welding environment, equipment adjustment, gas flow and tungsten needle grinding are the direct factors that affect the welding seam. If the welding process is not adjusted in time and spot check is formed, the batch welding will be abnormal, and even the welding seam can not be repaired and the unit will be scrapped, it's a huge financial loss. For expansion and welding heat exchangers, the sequence of expansion and welding has a significant effect on the quality of welded joints. Post Weld expansion not only reduces the occurrence of weld defects, but also ensures a good stress distribution in welded joints. The control method and process measures proposed in this paper can ensure the quality of the heat exchanger with expansion welded structure, and meet the safety and long-term use requirements of shell-and-tube heat exchanger. 

Source: Chinanews.com, by Longzhongyun

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