Experiment Study On Fracture Failure Of Brazed Tube-sheet Joints Of M11 Tubular Heat Exchanger

In this paper, experimental study on fracture failure of brazed tube-sheet joints of M11 tubular heat exchanger was conducted, all the failed tubular heat exchangers all crack along the interface of the low-carbon steel/brazing seam, which leads to the leaking channels and the failure of the products. Statistics shows that the two tube-sheet joints on both sides of the same product may leak at the same time, and more than one leakage may form in various positions.SEM results show that the fracture of the failed brazing tube-sheet joints are brittle fracture morphology, which is closely related to the brazing quality and the stress state of the joints on the heat exchanger. Incomplete penetration defects and continuous brittle compounds Fe₂P on the interface are the internal causes of the failed joints, on the other hand, working load such as pressure pulse and shock load impact of the working medium, vibration, shock load impact of the engine, which lead to the stress concentration at the corner under the surface of the tube-sheer joints, those external factors accelerate the fracture.Research results show that the copper coating quality and heating process is the key factors to the quality of the brazing tube-sheet joints of the M11 tubular heat exchangers. When the copper layer on the surface of the low-carbon steel flange is less than 10μm, it will completely dissolved in the liquid brazing filler metal during the heat process, then continuous brittle compound layer appears on the brazing interface, when the thickness of the copper layer is up to 20²5μm, about 5¹0μm residual copper layer exist on the interface and prevent the diffusion of P element, so that no brittle compounds generate on the interface. on this basis, the properties of copper layer is improved, including thickness, uniformity and the bonding strength; furthermore, the heating device and brazing process were also improved to improve the properties of the brazing joints.Products of high quality is possible if the quality of copper layer and brazing process are all improved, so the quality and reliability of the heat exchangers can be ensured.