Quench polish quench

Quench polish quench (QPQ) is a specialized type of nitrocarburizing case hardening that increases corrosion resistance, originally developed by the Kolene Company. The name refers to the three steps involved: nitrocarburizing, polishing, and post-oxidizing.

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.

Process

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 3 to 4 micrometre thick layer of iron oxide. It also gives the workpiece a black finish.

Corrosion resistance

The two charts below compare the corrosion resistance of QPQ to other common corrosion resistance coatings.

File:QPQ field immersion comparison chart.png

A comparison of corrosion resistance of the QPQ process to other surface treatments based on field immersion tests. Test conditions for the immersion test are full immersion in 3% sodium chloride plus 3 g/L of hydrogen peroxide for 24 hours.

File:QPQ ASTM B117 comparison chart.png

A comparison of the corrosion resistance of surface treated steel automotive steering columns based on the ASTM B117 salt spray test.

References

Bibliography

• Davis, Joseph R. (2002), Surface Hardening of Steels: Understanding the Basics, ASM International, ISBN 9780871707642, http://books.google.com/?id=eIfcHJtv_kcC.