Everything You Need To Know About QPQ Coating

There are many surface treatments used to improve some properties when manufacturing metal parts. The QPQ treatment is one of them, especially to improve the properties of the surface of the metal part.

Here you will learn everything you need to know about QPQ coating.

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What is QPQ Meaning?

QPQ stands for “Quench Polish Quench”. It refers to a process used to increase corrosion resistance.

Yet, other properties can be improved by performing the QPQ treatment, including:

Wear resistance.
Capacity to reduce friction, known as “lubricity”.
Fatigue resistance.

Advantages of QPQ Coating

The QPQ coating offers several benefits. First of all, it improves the properties mentioned above.

Also, for many industries and applications, it is essential to increase corrosion resistance. So, quench polish quench becomes a very helpful treatment for them.
Some of the most relevant advantages of applying QPQ treatment include:

Improved corrosion resistance.
Improved wear resistance.
Improved fatigue resistance.
Improved heat resistance.
Reduced friction between moving parts.
Better and smoother surface finish.
Reduced distortion of the metal structure.
Short cycle times.
Results are predictable and consistent.
Applicable to a wide variety of materials.
Applicable to complex pieces.
Environmentally friendly.
Appealing black surface result.

Disadvantages of QPQ Coating

Unfortunately, the QPQ coating is not a perfect process.

The QPQ treatment will help you improve those properties you need. But, there are a few drawbacks to consider.

The most important disadvantages of QPQ coating are:

Sometimes post washing or post polishing is required, and this increases turnaround times.
Higher energy consumption when compared to other processes.
Pieces are exposed to very high temperatures. So, previously treated parts may be prone to suffer distortion.
It should be performed at the end of the production process. This is because it makes it hard to process the piece afterward.
It can be considered a costly process when compared to other treatments.

Available Materials For QPQ Coating

Several materials can be treated by applying the QPQ coating.

Ferrous materials are the most commonly treated by this process. This is because it is mainly used to improve corrosion resistance.

That said, here are some of the QPQ coating applicable materials:

Carbon steels.
Stainless steels.
Alloy steels.
Austenitic steels.
High-speed steels.
Structural steels.
Pure iron.
Cast iron.
Sintered iron.

A few tests have proved that the QPQ process may reduce the corrosion resistance of some stainless steels. So, caution is needed with this material.

QPQ Coating Common Applications

Many applications can benefit from a treatment like QPQ coating.

Many industries want their products to be stronger and more durable. So, they may find the QPQ process very attractive.

Common industries that take the benefits provided by the QPQ treatment include:

Automotive
Weapons manufacturing.
Metal manufacturing.
Oil and gas.
Construction equipment manufacturing.
Food
Pharma
Chemicals

Therefore, common application examples are:

Hydraulic valves.
Ejector Pins & Ejector Sleeves for injection mold
Piston rods.
Manifolds
Shock absorbers.
Hydraulic jacks.
Brake rotors.
Camshafts
Clutch discs.
Gears
Pneumatic components.
Seals
Bushings
Bearings
Dies
Forging tools.
Extrusion tools.
Drill bits.
Milling cutters.
Golf club heads.
Power tool parts.
Electrical switches.

How to do QPQ Process?

The QPQ process is performed in three steps. Also, it requires high temperature, mechanical processing, and chemical reactions.

Like with any other treatment, precise control of important parameters is necessary.

The 3 steps of the QPQ process are:

Step 1

The first “quench”. It requires preheating the piece to perform a special type of nitrocarburizing. Here, the piece is submerged in a salt bath containing nitrogen and carbon for about 2 hours. Then, the piece is oxidized for about 30 minutes. Afterward, it is rinsed and cooled off at a temperature like the preheating temperature, around 650-750 °C.

Step 2

The second step is the mechanical process. The piece is polished by using one of many options. Common processes are vibratory polishing, centerless grinding, and lapping. These processes help to reduce surface roughness.

Step 3

Finally, the piece is submerged in a new oxidizing salt bath. After that, it is quenched and rinsed with water one more time. That is why the process is called “quench polish quench”. This second oxidizing and quench process is very important. It helps to recover the corrosion protection layer removed during the polishing process.

Similar Coatings Comparison

There are many other processes used to treat metal parts which are like QPQ Coating. Among the most popular ones, we can find Nitride, DLC Coating, and Black Oxidize.

The main similarity is that they are all used to improve the properties of the finished part. However, there are many other similarities.

For example, Nitride is similar to QPQ Coating in the fact that both are considered case hardening processes. DLC Coating is used to increase wear resistance and lubricity. Meanwhile, black oxidized is used to increase corrosion resistance. Clearly, they are similar, so the question is what makes them different?

QPQ Coating vs. Nitride

While both QPQ coating and Nitride create a layer of ε iron nitride on the material, they are not the same.

They both indeed belong to the group of thermochemical heat treatments. Yet, there are important differences in parameters. These are the temperature used, the completion time, and the steps to complete the process.

Take look at the table to better understand how these two processes compare.

Parameter	QPQ Coating	Nitride
Hardness	HRC60 – HRC70	HRC14 – HRC65
Thickness	0.04mm – 0.45mm	0.0127mm – 0.6096mm
Temperature	480°C – 630°C	400°C – 590°C
Completion time	30 min – 5 hours	Up to 90 hours

QPQ Coating vs. DLC Coating

The most obvious difference between the QPQ Coating and DLC coating is the appearance of the surface treated.

Diamond-Like Carbon (DLC) coating results in a very reflective polished surface. This result makes it different from the QPQ Coating since it results in a more “mate-like” surface finish.

Other differences can be seen more clearly in the table below.

Parameter	QPQ Coating	DLC Coating
Hardness	HRC60 – HRC70	1200HV – 7000HV
Thickness	0.04mm – 0.45mm	1µm – 10µm
Temperature	480°C – 630°C	<200°C
Completion time	30 min – 5 hours	From minutes to hours depending on part sizes

QPQ Coating vs Black Oxidized

The Black Oxidized process can be performed at different temperatures. This is an obvious difference with QPQ coating. The different temperatures make the process take different names. Such names are hot black oxidized, mid-temperature black oxidized, and cold black oxidized.

Another big difference is that Black Oxidized can be applied to non-ferrous materials such as copper and zinc.

Even so, the most important difference is that Black Oxidized does not increase the hardness of the part. Black Oxidized treatment only provides corrosion resistance, reduced light reflection, and desired appearance.

For a better view of other differences, take a look at the table below.

Parameter	QPQ Coating	Black Oxidized
Hardness	HRC60 – HRC70	N/A
Thickness	0.04mm – 0.45mm	<1µm
Temperature	480°C – 630°C	Room temperature (cold), 104°C -118°C (mid-temperature), 141°C (hot)
Completion time	30 min – 5 hours	From minutes to hours depending on part size

Conclusion

Quench Polish Quench or QPQ coating is a sophisticated heat treatment for ferrous metals such as steels and iron.

The main idea to use the QPQ process is to achieve very high corrosion resistance and lubricity. This makes it ideal for many different applications.

Besides, keep in mind that QPQ treatment is an environmentally friendly process. Also, it provides low distortion results.

And of course, there are many other advantages that you can get which were described above.

However, the QPQ coating is limited to ferrous materials. So, if you are thinking about treating other types of material, you may want to look for a different process.

You must also consider your budget. Higher energy consumption and the equipment required make it more expensive than others.

It is up to you to choose the surface treatment. Yet, the recommendation is to always consult with an expert. This way you can make sure you are investing your money wisely.

And you can get the help you need right here. Contact us at quote@dekmake.com and get the expert consultation you need.