Custom CNC machining service: Unveiling the Benefits of QPQ (Quench-Polish-Quench) Surface Treatment

1. QPQ Process Overview

The QPQ process is not a traditional "quench-hardening" method but a controlled thermochemical diffusion treatment. It typically consists of three key stages:

Step 1: Salt Bath Nitriding

The component is immersed in a molten salt bath at around 570–590°C containing nitrogen-bearing salts. This creates a hard compound layer (usually ε-Fe₂–₃N) on the surface, along with a diffusion zone beneath. This layer significantly increases surface hardness and reduces friction.

Step 2: Surface Polishing

After nitriding, the component undergoes mechanical or chemical polishing to remove surface oxides, improve roughness, and ensure uniformity. This step also enhances the visual finish and functional smoothness of the part.

Step 3: Post-Oxidation Treatment

The polished part is then immersed in an oxidizing salt bath at approximately 400–450°C, forming a dense black Fe₃O₄ (magnetite) layer. This layer improves corrosion resistance and provides a stable black surface finish.

2. Suitable Materials for QPQ

QPQ is compatible with a wide range of ferrous alloys, particularly those that can form a stable nitride layer. Common examples include:

 Carbon steels – e.g., 1045, 1050
 Alloy steels – e.g., 4140, 4340, 8620, 52100
 Tool steels – e.g., D2, H13, O1
 Martensitic stainless steels – e.g., 410, 420, 17-4 PH (under specific conditions)

 Note: Austenitic stainless steels such as 304 and 316 are generally not recommended for QPQ, as they do not form a hard nitrided layer and show limited improvement in surface properties.

3. Key Characteristics of QPQ-Treated Components

Surface Hardness

The compound layer reaches hardness values between 900–1200 HV, significantly improving resistance to abrasive and adhesive wear.

Corrosion Resistance

The post-oxidation stage forms a compact Fe₃O₄ film, enhancing corrosion resistance to levels comparable to or better than black oxide or hard chrome plating in many environments.

Dimensional Stability

Due to the diffusion-based nature of the process, dimensional growth is minimal (typically 10–20 microns), making QPQ suitable for precision components.

Fatigue Resistance

The treatment induces compressive residual stress on the surface, helping to suppress crack initiation and improve fatigue performance under cyclic loads.

Surface Appearance

QPQ-treated parts exhibit a uniform black finish that is not only functional but also aesthetically suitable for exposed components.

4. Advantages and Limitations

Advantages

- Enhanced wear and corrosion resistance in one integrated process
- Minimal distortion, suitable for precision CNC machining parts

- Uniform black surface finish
- Potential to replace traditional coatings such as hard chrome in certain use cases
- Environmentally safer compared to some plating processes

Limitations

- Applicable only to ferrous alloys with suitable chemistry
- Process requires specialized salt bath equipment and strict control
- Not effective for non-ferrous metals or austenitic stainless steels
- Complex geometries may require masking to ensure uniform treatment

5. Common Applications of QPQ

Thanks to its combined mechanical and chemical benefits, QPQ is widely applied across multiple industries:

 Automotive – piston pins, crankshafts, valve components, suspension parts
 Firearms and defense – barrels, slides, bolts, and other critical moving parts
 General machinery – shafts, bushings, pins, gear components
 Mold and die industry – plastic injection molds, die inserts, wear plates
 Energy sector – hydraulic components, downhole tools, valve stems

FAQ:

Q: How is QPQ different from standard gas nitriding?
A: QPQ is a salt bath-based process, which allows faster diffusion, a more uniform compound layer, and better corrosion resistance due to the final oxidizing step.

Q: Does QPQ treatment alter part dimensions?
A: The process results in minimal dimensional change, typically within 10–20 microns, making it suitable for close-tolerance components.

Q: Can QPQ replace coatings like hard chrome?
A: In many wear- and corrosion-critical applications, QPQ is a reliable alternative to hard chrome plating, without the environmental concerns of hexavalent chromium.

Q: Is post-treatment machining required?
A: In most cases, no post-machining is needed, but surface polishing is integrated into the process to meet specific roughness or appearance requirements.

If your application involves steel components subject to high wear, corrosive environments, or cyclic loading, QPQ surface treatment may offer a practical and cost-effective solution. For more technical consultation or quotation, feel free to contact our engineering team.