Before applying flux, what preparatory step should you take concerning the pipe ends?

The preparatory step that is essential before applying flux is to ensure that the pipe ends are properly prepared by removing any surface contaminants. This is crucial because contaminants like oil, grease, dirt, or oxidation can prevent the flux from adhering properly and may interfere with the brazing process. When the surfaces are clean, the flux can create a strong bond, allowing for effective heat transfer and proper joint formation during the brazing process.

Rounding the pipe end with an expander is not a standard preparatory step prior to flux application. While it's important for pipes to have proper fittings for effective connections, rounding the edges is typically more relevant for fitting and alignment purposes rather than for flux application.

By emphasizing cleanliness, you ensure that the brazed joint is strong and reliable, which is fundamental in brazing operations. Proper preparation can lead to improved joint integrity and overall success of the brazing process.