More and more users have put forward higher requirements on the welding quality of the fillet weld of the tube and tube sheet of the heat exchanger. The standards of GB150-1998[1] and GB151-1999[2] stipulate that welding procedure qualification must be performed before welding and penetration test after welding. Existing people advocate the use of a small focus circumferential X-ray machine or a small focus gamma source [3] for radiographic inspection of the weld.

Penetration method can only detect surface opening defects and cannot find internal defects, which is not effective in controlling its internal quality. Radiographic testing requires special-shaped film and thickness compensation corresponding to different tube diameters. The focal length of the transillumination is difficult to grasp, and it is inconvenient to place an image quality meter. At this time, the detection process can be used under the premise of agreement between the supplier and the buyer. But to be popularized, it will inevitably encounter obstacles in economy and practicality. This article introduces an easy-to-implement and popular method for internal welding quality control of tube-to-tube sheet fillet welds. This method can be applied to the heat exchanger products made by the author's company, which can meet the requirements.

1 Tube-to-plate welding sample production tube

The production requirements of tube-sheet welding specimens are completely the same as those of the upper tube-to-tube sheet weld of the product. The layout of the test plate tube holes is in a square arrangement (Figure 1), the thickness of the test plate is b1 ≮ 20mm, the center distance of the tube holes, the groove form and the assembly form of the tube and the tube plate are in accordance with the requirements of the standard or design drawing, and the welding method is manual argon For arc welding bottoming and cover, the welding process parameters (such as argon gas flow, welding voltage, welding speed, welding current and polarity, welding wire brand and diameter, whether to add wire and speed, whether to add pulse and Pulse parameters, welding torch angle, and the type of equipment selected, etc.) to improve welding quality. After the tube and tube sheet sample is welded, perform line cutting along the thickness of the tube sheet or finish machining from the opposite side of the weld to make the final thickness of the sample 7-10mm.

2 Non-destructive testing of welding samples

Permeate the sample first to ensure its surface quality, and then perform radiographic testing. When transilluminating, the transillumination area should be divided and marked (zone 1 and 2 in Figure 1). If the parameter of the heat exchange tube is 19mm×2mm, two sheets of 180mm×80mm specifications can be used for the sample as shown in Figure 1. The heat exchange tubes of other specifications can be determined according to the actual situation. During transillumination, the side with the weld seam is close to the film, and a protective lead plate or waste intensifying screen is placed on the back of the film to prevent scattered rays and improve clarity. Place corresponding image quality indicators and marks on the side facing upwards. Choose appropriate exposure conditions for transillumination, and the blackness of the film is about 2.5.

Various defects in the tube-to-tube sheet welds can be clearly observed on the film (such as pores, tungsten clamping, lack of fusion or incomplete penetration, etc., see Figure 2 in 1~3). When dissecting the test plate, the cutting direction can be determined according to the defect on the negative. After the dissection, it can be clearly observed and distinguished between incomplete fusion and incomplete penetration.

The weld surface and the reverse side of the test plate can be pre-made artificial defects (such as drilling). The maximum size of the artificial defect (diameter and depth of the hole) is the maximum size of the defect allowed in the weld. The shape and location of artificial defects can be clearly reflected on the radiographic film. This method proves that when the thickness of the sample is processed to 7-10mm, sufficient detection sensitivity is guaranteed.

3 Welding process improvement and quality control

When there are various defects in the welding seam of the test piece, and it is judged as unqualified according to relevant standards [4] or the customer's requirements, the welding process needs to be improved, and then the sample is made again, and the changes in the parameters are recorded. Benefit before and after comparison. Repeat the above steps until the weld quality meets the requirements. In this way, we can analyze the changes of welding parameters and welding quality, find the main and secondary reasons that affect welding quality, and take corresponding measures to correct and prevent them.

4 Conclusion

This method can be used to control product quality, evaluate and improve welding processes, and can also be used for welding skills training for welders to improve welders' skills in a targeted manner.