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Melonite – QPQ (DHQN)

Melonite® – QPQ® Thermochemical Quench-Polish-Quench (DHQN)

IBC Coatings Technologies uses a thermochemical quench-polish-quench (QPQ® see registered trademarks disclaimer at the bottom of the page) nitriding process equivalent to Melonite® to harden, strengthen, and improve the corrosion resistance of metal components and equipment. Unlike some other finishes and treatments, Melonite® has a predictable and consistent outcome that significantly strengthens the surfaces of carbon steel, stainless steel, alloy steel, and austenitic steel products. It can also be highly effective when used for cast iron and sintered iron, as well.

Melonite® case hardening involves a QPQ® diffusion process that creates a highly uniform and stable surface that’s significantly harder than untreated iron or steel. Thus, parts, components, and equipment treated with this form of nitrocarburizing (also called liquid nitriding) are far more resistant to normal wear and tear, even under the harshest conditions.

This process involves multiple steps, including molten salt baths, polishing, and rinsing, during which nitrogen, carbon, and oxygen are diffused into the surface of the metal, creating the case hardening effect that customers want for the parts they depend upon day in and day out.

Across industries, companies require components, parts, and equipment to perform to a certain standard, even under harsh, corrosive conditions with massive amounts of friction. Thanks to Melonite® (also called Tenifer® or Tufftride®), you can get the finest case hardening process to ensure the best performance for your products for years to come.

Melonite® processing produces a hardened, corrosion-resistant surface that will out-perform metal plating (hard chrome, zinc, or nickel), gas nitriding, Parkerizing®, and black oxide or bluing. While these traditional surface finishing processes were used in many industries for many years, they are each being methodically replaced by a superior finishing process – Melonite®.

How Does QPQ® Work?

The first step in the Melonite® process is to place the applicable parts or components in a preheat furnace. Once these parts are pre-heated to the appropriate temperature, they’ll be immersed in a molten, aerated solution of salts containing nitrogen and carbon. This step, which is referred to as nitriding, will generally last up to two hours, depending on the parts and the conditions they will be exposed to.

After nitriding, the parts are treated in an oxidation bath, usually for a period of up to 30 minutes, forming a surface layer of corrosion resistant magnetite (Fe3O4). The parts are then fully quenched and rinsed using a water bath.

The next phase of the process, polishing, is specifically designed to restore the part’s original surface finish. Polishing techniques and processes vary, but this step may be completed with centerless or vibratory polishing, lapping, glass bead blasting, or by another method of surface polishing.

Once the polishing is finished, the part will be immersed once more in an oxidizing salt bath, followed by a second water quench and rinse. This is why the process is referred to as quench-polish-quench, or QPQ®.

This unique process creates a much harder, more uniform surface condition that protects parts, components, and equipment from corrosion, friction, heat, and other wear and tear far better than coatings like nickel or chrome plating.

In addition to being harder and more wear resistant, when the QPQ® process is complete, the parts will have a smooth black finish that is clean and attractive with no need for painting or further finishing treatments.

Advantages of Melonite® Case Hardening:

Why choose Melonite® or Tufftride® case hardening over some other finish or coating?

Consider a few advantages that this process has over other alternatives:

  • Improved wear and corrosion resistance
  • Improved heat and friction resistance
  • Increased fatigue strength
  • An attractive black finish

Depending on the Melonite® process you choose for your parts, other advantages also include:

  • Less friction between moving parts
  • Smoother surfaces
Melonite QPQ Total Case Depth Core50HV
Figure 1: Typical case depths achieved by QPQ® salt bath nitriding on three representative materials, 1018 mild steel, 4140 alloy steel, and H13 tool steel.
Cross section of 1018 steel after QPQ salt bath nitriding, showing the hard and wear resistant compound layer of iron nitride formed at the surface.
Figure 2: Cross section of 1018 steel after QPQ® salt bath nitriding, showing the hard and wear resistant compound layer of iron nitride formed at the surface.
1018 mild steel400300200
4140 alloy steel800650350
416 stainless steel1000600300
H13 tool steel1200750500
Table: Typical surface and core hardness results after QPQ® salt bath nitriding

Uses for Melonite® Finishing Across Industries

  • Oil and gas
  • Automotive
  • Firearms and armaments manufacturing
  • Industrial tooling
  • Hydraulics
  • Construction equipment manufacturing

In these and other industries, Melonite® QPQ® processing has a number of applications, including:

  • Treatment of hydraulic parts and components, such as piston rods, hydraulic jacks, valves, and tubes.
  • Hardening of numerous automotive parts, such as brake rotors, transmission clutch plates, drive shafts, gearbox actuators, wheel hubs, caliper pistons, camshafts, engine valves, and more.
  • A more cost effective replacement for stainless steel to create numerous parts and components.
  • Producing longer lasting, more corrosion resistant slides and barrels for firearms.
  • Creating longer lasting, more accurate tool dies, as well as injection molds, forming and forging tools, and extrusion tools.

These are just a few of the ways that Melonite® QPQ® finishing is used in multiple industries for numerous purposes. Tufftride®, or Melonite®, processing can vastly improve the performance of parts and equipment in any industry that requires its parts to have high surface resistance to corrosion, wear, friction, heat, and other factors.

Melonite® is less expensive to produce than stainless steel, and at the same time it is more effective at resisting rust and other corrosion. If your business produces parts and/or equipment that must be functional in harsh and/or wet conditions, you’ll want to turn to a finishing process that will extend the life of your products for many years. Quench-polish-quench case hardening is the very best process for this purpose, and it’s proven itself over and over again across many industries and in many uses.

Are you interested in learning more about QPQ® finishing and how it can help your business create more durable, longer lasting steel or iron parts, components, and/or equipment? Contact us at IBC Coatings online at or via phone at (765) 482-9802. We are standing by to take your call, and we will be more than happy to answer all of your questions about Melonite®, Tenifer®, and the QPQ® case hardening process. Get in touch today and let us help you produce higher quality products with more effective finishes tomorrow.

Registered trademarks disclaimer:

The TUFFTRIDE® process is known in English-speaking and Asian countries under that name, in Europe and German-speaking countries as TENIFER® and in the USA as MELONITE®.

TUFFTRIDE®, QPQ®, TENIFER® and MELONITE® are registered trademarks of Durferrit GmbH.