The ferrule gets squeezed between the two and seals off any leaks. When you install the new faucet, many people will try to replace the ferrule and nut with new ones. You can do that if you want ...

The single- and double-ferrule fittings have both had a presence in ... brass, aluminum, nickel-copper, Hastelloy C, Alloy 600, Titanium, 6Mo, Incoloy 625 and 825, and 316 stainless steel.

Product Features. Suitable for the outer diameter of the pipe:1/4" 3/8" 1/2" Product Material: Brass. Used with plastic and copper tubing. Package Include:120PCS Compression Sleeve Ferrules.

Ferrule type flareless fittings seal by forcing the ferrule or sleeve cutting edge into the tubing wall to create an interference seal. ... usually copper, and low-pressure vibration-free systems.