QPQ – Quench Polish Quench

QPQ is an acronym for Quench, Polish, Quench. It is a finishing designation specific to the salt bath nitriding (SBN) process where the parts will be “quenched” in an oxidizing salt to add an oxide layer on top of the compound layer, and then polished by mechanical means, either with vibratory polishing equipment or various types of bead blasting (glass, steel shot or aluminum oxide), and then “re-quenched” into the oxidizing salt to produce a deeper oxide layer that both adds to corrosion protection but will also deepen the darkness of the finish color on the parts.

The SBN process alone will leave a texture on the surface of the parts not unlike that of an egg hell. There is a small amount of roughness imparted due to the nature of the compound layer that generally matches the surface roughness of the part itself. It can also be compared to the general roughness of flat paint. Some parts require a smooth finish, and mechanically polishing them after SBN will leave a smooth, shiny surface on the parts. A second “quench” builds the oxide layer and deepens the finish color on the parts. Combined with a finishing/protective oil afterwards will produce a darker black finish. Be sure to check out our interactive finish tool to see what finishes that TS Tech Nickel can produce for your parts.

The QPQ process is also known by other names, such as, Melonite QPQ or Black Nitriding. It is generally paired with the SAE Aerospace Material Specification 2753 when specified on component drawings.

Components manufactured by the firearm and defense industries are typically large users of the QPQ process because of the deep black finish that can be produced. Some manufacturers prefer a smooth, glossy, black finish, some prefer a matte black finish, and yet others prefer a textured deep black finish. TS Tech Nickel’s QPQ process can produce any of these variations. Check out our interactive finish tool to get a look at examples of each of these finishes.

Aside from the firearms industry, parts can benefit from the QPQ process with its double oxidation process. This can enhance the corrosion resistance of the parts with the additional oxide thickness from the second submersion in the oxidizing salt. TS Tech Nickel recommends using a protective/finishing oil to both deepen the shade of black and to further enhance the corrosion resistance properties of the treatment.

Yes! This is accomplished with a mechanical blasting step prior to doing the salt bath nitriding process. TS Tech Nickel offers options such as glass bead, steel shot, and aluminum oxide media blasting to produce a textured finish on parts that will remain after SBN treatment. QPQ, or Quench, Polish, Quench is then applied to the parts to remove the flat paint feel of the process and leave a smooth, textured finish. Combined with a finishing/protective oil as a last step will produce a lighter black finish. Check out our interactive finish tool to get an idea of the different finishes that TS Tech Nickel can put on your parts.

TS Tech Nickel recommends that you use SAE’s Aerospace Material Specification 2753 for the salt bath nitriding process as a standalone note on your part print. For the QPQ process specifically, use a note directly after AMS2753 specifying a polish step to achieve a smooth finish. It is also helpful to know if you require a particular color, such as the light black, as this will indicate that you’d like the finishing/protective oil to be applied or not. Without the finishing oil, the parts will have a shiny dark grey color after QPQ. For more assistance specifying your desired outcome, we encourage you to try our interactive finishing tool to see an example of results, depending on the finishing options you specify.

All nitriding processes, including salt bath nitriding, are intended to be a finishing process and therefore you should see little to no dimensional change with the process. While the process is technically a form of heat treatment, the processing temperatures are typically between 950-1075°F, which for all forms of steel are below the critical temperature where crystal structures start to change and reform (known as the austenite phase). Staying below the austenite phase means that the shape of the parts will not be affected as the steel has not started to plasticize, as with heat treatments requiring hotter temperatures such as neutral hardening or carburizing.

Having said that, the compound layer that builds into and on the surface of the parts can cause a minimal amount of growth. Typically, the compound layer will be somewhere between 10-20 microns of depth, and a general rule of thumb is that half of that depth will be above the original surface, so you can expect somewhere between 5-10 microns of growth on all surfaces. Very fine thread pitches can sometimes become brittle, but for the vast majority of applications, this is not a problem. If you are concerned about brittle thread pitches, you can mask threaded holes using the corresponding bolt, but a high-temperature anti-seize compound (rated to 1200°F minimum) must be applied to the threads prior to salt bath nitriding, or removal becomes difficult to impossible.