Quench polish quench (QPQ) is a specialized type of nitrocarburizing case hardening that increases corrosion resistance. It is sometimes known by the brand name of Tufftride, Tenifer or Melonite.Three steps are involved: nitrocarburize ("quench"), polish, and post-oxidize ("quench").
This process is often used when two or more of the following properties are required in a workpiece:
Common applications of the process are for piston rods of shock absorbers, cylinders and rods for hydraulic systems, pumps, axles, spindles and valves.
The process starts with a standard salt bath nitrocarburizing cycle, which produces a layer of ε iron nitride. Next, the workpiece is mechanically polished; typical polishing processes include vibratory finishing, lapping, and centerless grinding. Finally, the workpiece is re-immersed into the salt quench bath for 20 to 30 minutes, rinsed, and oil dipped. This last step optimizes the corrosion resistance by creating a layer of iron oxide about 3 to 4 micrometers thick.It also gives the workpiece a black finish.