The complete guide to Quench Polish Quench (QPQ) Coating

Table of Contents

I Introduction to Quench Polish Quench (QPQ) Coating

Over the years, the coating has been established as an effective way of fortifying and securing materials from corrosion and wearing effects when exposed to severe conditions. However, with so many coating methods available, deciding on an effective coating method for protective projects can sometimes take time and effort. Nevertheless, versatile coating techniques like Quench Polish Quench (QPQ) coating make it easy to coat surfaces, effectively improving their durability. Quench Polish Quench (QPQ) coating involves the nitrocarburizing of metal substrates, a layer of protective compound to enhance its anti-corrosion properties and toughness. This guide is the holy grail of Quench Polish Quench coating. So, if you’re ready to know more, let’s begin.

II. The QPQ Coating Process

QPQ coating consists of thermal and chemical treatment of surfaces and substrates to provide more protection and increase their lifespan. However, these treatments are done in three phases, including nitrocarburizing the surface with nitrogen and carbon, surface polishing, and quenching processes.

  1.   Nitrocarburizing: Quench Polish Quench coating begins with nitrocarburizing the surface, which often involves using gaseous substances or salt solution to diffuse carbon and nitrogen on the metal surface. This process often forms a rigid material with anti-corrosion properties on the metal surface. The two methods used in nitrocarburizing are gas and salt bath nitrocarburizing.

 Gas nitrocarburizing: The metal component or substrate requires a mixture of carbon and nitrogen gas under extreme temperatures to achieve nitrocarburizing for the metal surface.

Salt bath nitrocarburizing: Salt bath, on the other hand, requires a salt solution containing carbon and nitrogen compounds with the metallic substrate immersed in it to achieve nitrocarburizing.

  1.   Polishing: The next step after achieving nitrocarburizing is polishing, ensuring no roughness on the surface, and giving it a fine, sleek, and pleasant surface finish. You’ll have to use either mechanical or chemical-assisted polishing to achieve a smooth finish.

Mechanical polishing: Fine abrasive materials are used in removing any imperfections on the surface of the coated material when using mechanical methods to achieve the polishing process. It often leaves a crease-free surface that is pleasant to the sight.

Chemical-assisted polishing: For this method, chemical compounds are poured on the rough surface to dissolve the uneven surfaces left after the complete nitrocarburizing.

  1.   Quenching: The last step in the QPQ coating process is quenching the excessive heat generated during the coating process. Cooling is often done rapidly to achieve a firm coating surface. Two major methods are often used to quench this extreme heat: water and polymer quenching.

Water quenching involves simply submerging the coated material into water or brine to achieve rapid cooling and a rigid coated surface.  

  Polymer quenching: In this case, cooling involves using a polymer solution in which you submerge the coated material. This method is often better than water quenching, considering it is better controlled.

The QPQ process flow typically starts with pre-cleaning the metal components to eliminate any surface impurities, followed by nitrocarburizing, polishing, and quenching. Process parameters and optimization are critical in attaining the required attributes of QPQ coatings. Temperature and time management, gas flow and concentration, polishing techniques and parameters, quenching procedures, and cooling rates are all critical aspects to consider in the QPQ process.

III. QPQ Coating Materials

To be effective, the QPQ coating process requires specific coating materials. However, the choice of base materials is critical since it determines the final properties of the coated product. Low-carbon steels are commonly utilized because of their superior nitriding capabilities. Other materials that can be utilized include alloy steels, cast irons, tool steel, and several nonferrous metals.

Also, the nitrocarburizing chemicals utilized in the process comprise nitrogen, ammonia, hydrocarbon gases, and some additional unique compounds. These compounds aid in absorbing nitrogen and carbon into the substrate metal material, forming solid and wear-resistant nitrides. The concentration and movement of these gases must be continuously monitored to ensure that proper chemical reactions occur.

Furthermore, in the QPQ coating process, polishing, and quenching agents, including silicon carbide, aluminum oxide, and diamond particles, are required for mechanical polishing. Also, oxidizing acids function as agents for chemical-assisted polishing. However, depending on the exact application requirements, the quenching agents might be water-based or polymer-based. Furthermore, some industries prefer using a polymer quenching agent to provide extra corrosion protection.

IV. Process Parameters and Optimization

The QPQ coating process necessitates precise control of several factors to obtain the necessary coating qualities. The following are the essential process parameters that must be optimized:

  1.  Temperature and time control: As with mold design considerations the nitrocarburizing stage necessitates a temperature range of 500-570°C and a holding duration of 30-90 minutes, depending on the material type and thickness. Temperature and time management are also required during polishing to guarantee effective material removal without surface damage.
  2.  Gas flow and concentration: The flow rate, nitrocarburizing gas mixture composition, and pressure all impact the coating composition and depth. The concentration and optimal gas flow should be calculated for every type of material and thickness.
  3.  Polishing techniques and parameters: This procedure helps to achieve the required surface finish and remove the damaged surface layer. For mechanical polishing, careful selection of the abrasive type, pressure, and size is essential. On the other hand, chemical-assisted polishing necessitates the selection of suitable etchants and concentrations.
  4.  Quenching techniques and cooling rates: The cooling rate and quenching method are used to influence the microstructure and characteristics of the coating. In the case of water quenching, you can expect maximum surface hardness. In contrast, polymer quenching produces the highest toughness and ductility.

V. QPQ Coating Properties

With the Quench Polish Quench (QPQ) coating technique, material characteristics are often enhanced, making them more robust and resistant to corrosion, wear, and fatigue. The essential features of materials enhanced by QPQ coating are highlighted below.

  1.       Hardness and wear resistance of the surface: Perhaps the most significant enhancement is that the QPQ method forms a strong, wear-resistant coating on the surface of materials, greatly improving their resistance to abrasion, friction, and pressure. This is accomplished by diffusing carbon and nitrogen into the material’s surface, forming a tougher compound layer than the primary material.
  2.   Corrosion resistance: QPQ coatings are highly successful in protecting materials from corrosion, especially in hostile situations such as high humidity, marine, or chemical conditions. The nitride layer created during the QPQ coating operation is a corrosion barrier, preventing corrosive substances from penetrating the material.
  3.   Fatigue strength: Generally, materials that undergo the QPQ process have greater fatigue resistance, often seen as the tendency of materials to fracture or break in the face of repetitive stress or repeated loading. The QPQ method forms a compressive stress layer on the material’s surface, which enhances fatigue resistance.
  4.   Friction and lubricity: The QPQ process may also increase a material’s lubricity and friction, making it more wear-resistant and decreasing friction between moving components. The way to accomplish this is by smoothing the material’s surface to minimize the degree of roughness and increase the surface’s capacity to retain lubricants.
  5.   Dimensional and surface finish control: QPQ coatings may be applied with great accuracy, providing tight tolerances and exceptional quality monitoring for the material’s surface. One great application is in situations requiring exact measurements and flawless surface finishes.

VI. QPQ Coating Applications

Being a versatile coating method, Quench polish quench (QPQ) coatings have several applications based on their several advantages, including increased wear resistance, enhanced fatigue strength, and good corrosion resistance.

These characteristics make them suitable for application in a wide range of industries. We have highlighted a few of the most popular ones here.

1. Transportation and automobile industries

Along with advanced production techniques like CNC machining, QPQ coatings have become standard in the transportation and automotive sectors. This attraction is based on the coating’s ability to improve corrosion and wear resistance, which makes it perfect for components subjected to hostile conditions. Transmission and Engine components, brake systems, steering and suspension components are examples of these components.

2. The Defense Industry and Firearms

Used extensively in the gun manufacturing and defense sectors, QPQ is a preferred coating method for increasing the durability of materials used to manufacture arms and ammunition. The coating is corrosion and wear-resistant, making it appropriate for weapons parts such as bolts, barrels, and triggers. QPQ coatings also give weapons a sleek, black finish that is highly sought after.

3. Power transmission components such as gears and bearings

It is not uncommon to find QPQ coatings in bearings, gears, and other power transmission elements. Because of the coating’s strong wear and corrosion resistance, it is suitable for these components, which are frequently subjected to tremendous loads and hostile conditions.

4. Hydraulic and pneumatic components

These elements are frequently exposed to corrosive conditions, which makes coating against corrosion and wear critical to their durability. QPQ coatings are advantageous for these components given their superior anti-corrosion and wear resistance provisions, extending their lifespan.

5. Petroleum and gas industry

In the oil and gas sector, QPQ coatings protect components like underground pipes and tubing from unfavorable conditions encountered during drilling and production. Also, the coating is perfect for members such as pipelines, valves, and pumps due to its great corrosion and wear resistance.

6. Medical and dental supplies

Medical and dental equipment also benefit from QPQ coatings. As a result of its great anti-corrosion properties, the coating is excellent for equipment exposed to harsh chemicals used in sterilizing operations. Furthermore, the anti-wear characteristics of the coating help to extend the life of these components.

Given their unique qualities, QPQ coatings have an extensive spectrum of applications in various sectors. QPQ coatings are gaining popularity in multiple sectors, from boosting the wear resistance of automotive components to protecting weapons against corrosion.

VII. Quality Control and Testing of QPQ Coatings

The QPQ coating process relies heavily on quality control and testing like other standard manufacturing processes including mold manufacturing. However, this procedure usually guarantees that the coating satisfies the specified standards and functioning criteria using several approaches and techniques. Some of the most frequent quality control and testing procedures in QPQ coating are as follows:

  1. Surface hardness evaluation: Surface hardness is one of the essential features of QPQ coatings, and it can be tested using a variety of methods, including Vickers, Rockwell, and Brinell hardness tests. These examinations guarantee that the coating has the necessary hardness to withstand abrasion, wear, and other types of damage.
  2. Wear resistance testing: One other equally important and key attribute of QPQ coatings is wear resistance testing. It often involves several testing methods, such as ball-on-disk, pin-on-disk, or sand slurry tests, to ascertain the coating’s ability to withstand impact. These tests imitate practical wear circumstances and give significant and usable data on the coating’s capacity to endure wear and tear.
  3. Corrosion resistance testing: A major advantage of QPQ coating is its good corrosion resistance which protects the underlying substrate from deterioration. Cyclic corrosion, salt spray, and electrochemical tests are performed to assess the coating’s corrosion resistance and performance in severe conditions.
  4. Dimensional inspection: This procedure is often required to confirm that the thickness of the coating alongside other dimensions satisfies the required criteria. Some prevalent techniques include ultrasonic thickness measurement, Micrometer, optical microscopy, and profilometry.
  5.   Metallographic examination: In order to evaluate the microstructure and composition of the coating, a metallographic examination, which is critical to its qualities and functionality, is essential. Energy-dispersive X-ray spectroscopy (EDX) is one of the most popular metallographic examinations, along with SEM and X-ray diffraction (XRD). These methods help in investigating the chemical content, structure, and crystallographic orientation of the coating.

VIII. QPQ Coating Equipment and Setup

Knowing the required equipment and the appropriate setup for an effective QPQ coating process is critical to the quality of the final product. Consequently, QPQ professionals often ensure that the following equipment is in good working condition and is set up correctly.

   1. Heating Systems and Furnaces

The furnace is one of the most critical pieces of equipment in any QPQ coating process. They heat the items to the specified nitrocarburizing temperature. However, the furnace must be capable of attaining and maintaining the right temperature for an extended time. Furthermore, you’ll require a variety of furnaces to carry out QPQ coating, including electric furnaces and gas-fired furnaces.

   2. Gas Distribution and Control Systems

A regulated environment of nitrocarburizing gases is also critical for QPQ coating. The gas supply and control systems should effectively supply the furnace with a steady and precise flow of gases. Nitrogen, ammonia, and methane are common gas mixes used for QPQ coating.

   3. Polishing Supplies

To prepare the surfaces of the items for nitrocarburizing, mechanical polishing machinery or chemical-assisted polishing equipment are utilized. The polishing procedure is crucial for producing a high-quality QPQ coating.

   4. Quenching Systems and Tanks

After nitrocarburizing and polishing, the pieces are quenched in a liquid tank. The type of quenching liquid employed might differ based on the finished product’s intended qualities. The most often used quenching agents are water and polymer solutions.

   5. Control and monitoring systems

Monitoring and control systems are critical for assuring the consistency and repeatability of the QPQ coating process. Sensors, controls, and software are often used in these systems to enable real-time monitoring and change of process parameters.

IX. Health, Safety, and Environmental Considerations

Health, safety, and environmental concerns are crucial for a sophisticated coating procedure like QPQ coating, especially since chemicals are required during operation. Aside from the use of hazardous materials and chemicals, there is also the presence of high temperatures and equipment that can endanger employees and the environment. As a result, it is critical to take the appropriate safeguards to maintain worker safety while minimizing environmental damage.

Personal protection equipment (PPE) such as masks, gloves, and goggles are required for employees while operating machinery and handling chemicals. Furthermore, proper ventilation and filtration systems should be built to avoid exposure to hazardous chemicals and gases. Again, implementing a working dangerous waste management system should be a priority to appropriately dispose of harmful chemicals and other substances utilized during the process.

X. Challenges and Limitations of QPQ Coating

As with many innovative technologies and techniques, its numerous benefits often come with challenges and limitations, and QPQ coating has its fair share. Material compatibility and substrate restrictions are two examples of such constraints. Specific materials may not be appropriate for QPQ coating, while some other substrates may necessitate additional pre- or post-treatment procedures. There is a chance that it might increase the overall complexity and cost of the manufacturing process.

Another difficulty of QPQ coating is process control and reproducibility. Several factors must be carefully regulated to produce the appropriate coating qualities, including duration, temperature, and chemical composition. Any variation from the ideal values might lead to coating quality and performance inconsistencies.

Coating in mass and with large sizes might also pose difficulties for QPQ coating. The method can be tedious and laborious, making bulk manufacturing quite challenging. Furthermore, the expense of QPQ coating equipment and supplies might make it less economically viable for high-volume manufacturing.

Lastly, economic and financial reasons may restrict the application of QPQ coating. While the procedure has several advantages, it might be more pricey than other coating processes. Therefore, in evaluating if QPQ coating is the best solution for their application, you may need to compare the possible benefits against the higher price tag.

XI. Innovations and Future Developments in QPQ Coating

QPQ coating’s vast uses in various sectors continue to propel technological advancements. Here are some of the most recent QPQ coating advancements and upcoming developments:

  1. Hybrid and advanced QPQ techniques: Significant progress has been made in establishing improved QPQ procedures for advancing the surface characteristics of treated materials. Laser-assisted QPQ and plasma-assisted QPQ are two examples that can increase the coating’s thickness, wear resistance, and hardness. There is also the rise of hybrid procedures integrating QPQ coating with different surface treatment technologies like PVD and electroplating.
  2. Environmentally friendly and sustainable QPQ processes: The need for sustainable and environmentally friendly manufacturing processes has led to the development of QPQ techniques that reduce or eliminate hazardous waste and emissions. These include the use of water-based and ion exchange-based QPQ processes that do not require toxic chemicals.
  3. Tailor-made QPQ solutions for specific applications: Customized QPQ coating for projects with special requirements are beginning to increase, and the versatile nature of QPQ makes adapting it to fit the needs of specific applications easy. Applying various quenching media, such as oil, air, or water, for example, might change the characteristics of the coating. Customized QPQ coatings have applications ranging from the aerospace to the automotive industry.
  4. Emerging applications and trends in QPQ coating: QPQ coating is finding new applications in sectors such as rapid prototyping additive manufacturing, where it helps improve the surface qualities of 3D printed products. The increased use of QPQ coating for non-metallic materials such as plastics and ceramics is gradually growing.

XII. Conclusion

In a nutshell, QPQ coating has numerous advantages, including its adaptability and efficient surface treatment that can improve the quality and longevity of many metal components. These advantages largely explain its attractiveness to many manufacturers and industries. Its added benefits, such as enhanced hardness, corrosion resistance, wear resistance, and fatigue strength, also contribute to its preference in various sectors. Fortunately, the speedy technological advances pave the way for more opportunities, including more sophisticated QPQ techniques, environmentally friendly procedures, and bespoke solutions for specific applications. The future holds enormous promises for QPQ technology as more research and development might lead to even more substantial breakthroughs. But what’s still holding you back from adopting QPQ coating in your projects?


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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