The importance of brazing holding time

The brazing holding time is an important stage for the filler metal to fill the gap and control the alloying effect, and the effect on the joint strength is similar to that of the brazing temperature. A certain holding time is necessary for the mutual diffusion of the solder and the base metal to form a firm bond. But too long a holding time can also lead to the overdevelopment of some processes and the opposite.

The choice of brazing holding time is mainly determined by the interaction between the brazing filler metal and the base metal. When the brazing filler metal and the base metal have strong dissolving, forming brittle phase, causing intergranular infiltration and other unfavorable interactions, the brazing holding time should be shortened as much as possible. On the contrary, if the brittle phase or low-melting structure in the brazing joint can be eliminated through the interaction of the two, the brazing holding time should be appropriately extended, as shown in the figure. The brazing seam of the high-pressure I-stage turbine blade cover plate of an engine works at a high temperature of about 850℃, and the adhesive tape brazing material (BNi75CrSiBCo) is used, and the gap is not easy to control. The diffusion brazing temperature is 1190±10C. After holding for 1h, the brazing joint structure is a nickel-chromium solid solution, and there is no intermetallic compound.

The brazing holding time is also related to the size, thickness and brazing gap value of the weldment. The holding time of large and thick pieces is longer than that of small and thin pieces to ensure uniform heating. When the brazing gap is large, in order to ensure the necessary interaction between the brazing filler metal and the base metal, there should be a longer brazing holding time. Some components are not very thick, but the brazed joints are partially or completely shielded and cannot be directly heated by radiation. The holding time should be appropriately extended to minimize the temperature difference between the surface and the inside of the heated workpiece.

In addition, the heat treatment requirements of the base metal should also be considered. For example, for austenitic stainless steel, when heated to more than 900 cores, the carbides are quickly dissolved, and the brazing holding time does not need to be too long. Martensitic stainless steel needs to be fully solutionized before complete martensite can be obtained during subsequent quenching, so the brazing holding time should be correspondingly longer. For heat-resistant alloys, the alloying elements must be fully dissolved in order to provide conditions for solid solution strengthening and dispersion strengthening, so the brazing holding time should be longer.

It should be pointed out that the brazing temperature and brazing holding time should not be determined in isolation to determine that there is a certain complementary relationship between them, and can be changed within a certain range accordingly. Therefore, the selection should be determined through experiments according to the above principles.