Quench Polish Quench QPQ Hardening
This process is usually used when two or more of the following properties are required in a workpiece: wear resistance, corrosion resistance, lubricity, or fatigue strength. However, it is also used when superior corrosion resistance is required. Common applications are piston rods for 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 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 3 to 4 micrometre thick layer of iron oxide. It also gives the workpiece a black finish.