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what is the differences between blackening, bluing and QPQ surface treatments

Characteristics of QPQ vs Blackening vs Blueing Surface Treatment

QPQ processing: a modern, composite surface strengthening technique. It forms a high hardness compound layer on the surface of the workpiece by nitriding, and then obtains a black oxide film through oxidation. The core purpose is extremely high wear resistance, corrosion resistance, and good appearance.  

Process principle: Composite process: salt bath nitriding+salt bath oxidation+polishing+re oxidation. Firstly, nitrogen atoms are infiltrated to form a hardened layer, which is then regenerated into an oxide film.

Surface layer structure: 1. Outer layer: black oxide film (Fe304), 2. Inner layer: high hardness nitrogen compound layer (phase)

Core performance: Excellent comprehensive performance:

Extremely high hardness: HV500-1200 (tens of times darker than blackening).

Excellent wear resistance: more than twice that of hard chrome plating

Strong corrosion resistance: The neutral salt spray test can last for 100-400 hours or more.

Minimal deformation: The processing temperature is low, and the workpiece hardly deforms.

Appearance: Black or dark black, uniform, with a matte or semi matte finish. More delicate and sophisticated appearance

Environmental Protection and Safety: The process involves cyanide salt baths, requires large equipment investment, and has extremely high environmental requirements, which must be operated by professional factories.

Cost: High. The equipment and process are complex, and the cost of salt bath raw materials is high

Main applications:

Strict requirements for wear-resistant and corrosion-resistant parts:

Automobiles and motorcycles: crankshafts, camshafts, gears, valves. Mold: Plastic mold, Stamping mold Hydraulic parts: Piston pump Friction pair

Cutting tools and measuring tools

Blackening treatment with bluing (room temperature/high temperature): a traditional, low-cost chemical conversion coating treatment. The main purpose is to prevent rust and achieve a black appearance. The film layer is very thin (about 1-2 microns), almost unchanged in size, and has very low hardness and wear resistance.

Process principle: Chemical conversion coating: In alkaline (high temperature, about 140 ° C) (bluing) or room temperature solution (blackening), a layer of iron oxide (Fe304) thin film is formed on the surface of steel through chemical reaction

Surface layer structure: Single layer structure: Only one dense Fe304 oxide film, firmly bonded to the substrate.

Core Performance: Basic Protection:

Low hardness: The film layer itself is soft and cannot increase the hardness of the substrate.

Poor wear resistance: The film layer is easily scratched.

Corrosion resistance is generally average: The neutral salt spray test usually lasts for 1-2 hours (room temperature blackening) or several tens of hours (high-quality high-temperature blackening).

There are size changes: the film thickness is extremely thin, and the size is ignored

Appearance: Black or blue black, color uniformity is greatly affected by pre-treatment and material

Environmental Protection and Safety: Relatively Environmental Protection: High temperature Blackening: Using sodium hydroxide and nitrite, there are issues with alkali mist and wastewater. ·Room temperature blackening: Selenium, copper and other salts are used, and the wastewater contains heavy metals but no high-temperature steam.

Cost: Low. The equipment is simple, the solution cost is low, and it is suitable for large-scale processing.

Main applications: General rust prevention and appearance parts:

Standard parts (bolts, nuts)

Mechanical structural components.

Building hardware.

Firearms and tool casings.

Parts that do not withstand internal friction

Application scenarios of QPQ for blacking and blueing

Basic rust prevention function, pursuit of the lowest cost, no wear and tear on the workpiece, only requiring a black appearance, processing ordinary carbon steel parts such as screws, brackets, etc., choose blackening (bluing).  

Workpieces need to withstand severe wear and corrosion at the same time, maintain high precision without heat treatment deformation, be made of stainless steel (QPQ can greatly improve its wear resistance and maintain its natural color or turn black), have extremely high requirements for service life and reliability, and have sufficient budget, such as engine core moving parts, precision molds, high-end cutting tools, choose QPQ treatment.

Blackening treatment: commonly used for general mechanical parts, tools, fasteners, daily metal products, etc. that do not require high wear and corrosion resistance. It has a low cost and is suitable for large-scale production.

Blue processing: commonly used in situations where high requirements are placed on corrosion resistance, wear resistance, and appearance stability, such as military equipment, aerospace parts, precision instruments, high-end cutting tools, and firearm components.

The influence of two surface treatments on the dimensions of machined parts

QPQ processing

Reason for size change: growth of nitriding layer

Size impact: larger 10-25um (single-layer)

Film layer characteristics: The dense nitrogen-containing compound layer structure endows the workpiece with ultra-high wear resistance, anti bite force, and fatigue resistance. The Fe304 growth layer has a microporous structure, which can store oil agents, provide excellent corrosion resistance, and reduce friction

Deformation risk: extremely low

Requirement for design: Adequate machining allowance must be left

Blackening/Blueing

Reason for size change: growth of oxide film

Size impact: Extremely small 1-3um (single-layer)

Film characteristics: Blackening: The film has a relatively soft hardness, is easy to scratch, and has many pores, often requiring oil immersion or wax coating to assist in bluing: The film has a relatively hard hardness, fewer pores, is more wear-resistant, and has strong corrosion resistance

Deformation risk: moderate (high temperature bluing treatment)

Requirements for design: Usually there is no need to leave special machining allowance