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QPQ Coating

Black Phosphate Coating

black phosphate coating process introduction

Phosphating is a process in which a workpiece is treated in a phosphoric acid salt solution containing manganese, iron, and zinc to form a layer of insoluble phosphate film on the surface of the workpiece. It is also known as phosphate treatment. Phosphating can be divided into four types: high temperature, medium temperature, low temperature, and room temperature phosphating, depending on the operating temperature.

The thickness of the phosphating film is about 3-20 microns, and it is gray or dark gray in color. It has good adhesion to the metal substrate and is stable under atmospheric conditions. The corrosion resistance of phosphating and metal oxides in acidic and alkaline environments is not good, and it is important to note that the corrosion resistance of phosphating film is about 2-10 times higher than that of oxide film.

 

Therefore, metal oxide and phosphating films are generally used as pre-treatments for surface treatment. In other words, after oxide or phosphating treatment, powder or paint is sprayed on the surface to increase corrosion resistance, even in humid and acidic environments.
In addition to its good adhesion to the substrate, the phosphating film also has good adhesion to paint coatings. This means that after phosphating, the paint film is not easily peeled off. There is a significant difference in the salt spray test between pre-treatments with and without phosphating.
The electrical insulation of the phosphating film is high, and it can withstand 1000-1200V after being coated with insulation paint, making it less susceptible to electrochemical corrosion.
In fact, there are many scenarios in which the atmosphere is humid and hot, such as outdoor iron parts. Even if they are treated with blueing or blackening, they are still prone to rust in humid environments after rain, as the metal oxide and phosphate films are not resistant to acidic and alkaline corrosion. This is why oxide and phosphating are used as pre-treatments, and then paint is sprayed on the surface to improve the adhesion between the pre-treated surface and the paint coating. The surface after painting has strong corrosion resistance in various environments.

QPQ Coating

What is qpq coating？QPQ Treatment Process Introduction

Hot-dip nitriding of steel parts (QPQ treatment) is a surface hardening method that involves immersing steel parts into a nitriding solution and allowing nitrogen atoms to penetrate the surface of the steel parts at high temperatures. This results in a hard, low-friction, and wear-resistant surface layer, as well as an internal nitrided layer, which increases the strength and wear resistance of the entire steel part. QPQ treatment can also improve the corrosion resistance of steel parts while reducing deformation and discoloration.

The QPQ treatment process stands for “Quench-Polish-Quench”, which is a combination of heat treatment and corrosion protection technologies. It involves immersing black metal into two different types of salt baths to form a composite layer on the metal surface by the infiltration of multiple elements, thus achieving the purpose of surface modification of parts. It does not involve an actual quenching step, but it achieves the same effect as surface quenching.

The characteristics of the QPQ treatment process are

Excellent Wear Resistance and Fatigue Resistance

QPQ can greatly improve the surface hardness and wear resistance of various black metal parts, as well as reduce friction coefficients. After being treated with QPQ, the wear resistance is more than 16 times higher than conventional quenching and high-frequency quenching, more than 9 times higher than 20 copper carburizing quenching, and more than 2 times higher than hard chrome plating and ion nitriding. Fatigue tests show that QPQ can increase the fatigue strength of medium carbon steel by more than 40%, which is better than the effects of high-temperature nitriding and gas nitriding. QPQ technology is especially suitable for complex-shaped parts, solving the problem of deformation.

Excellent Corrosion Resistance

Under the same test conditions, samples of different materials and process treatments were subjected to continuous spray tests according to the ASTM B117 standard. The salt spray test temperature was 35+2℃, the relative humidity was >95%, and the 5% NaCl solution was sprayed. The test results showed that the corrosion resistance of parts treated with QPQ was 5 times that of 1Cr18Ni9Ti stainless steel, 70 times that of hard chrome plating, and 280 times that of blackening. After being treated with QPQ, some materials can achieve a neutral salt spray test of 100-300 hours.

Small Deformation After Product Treatment

The workpiece hardly deforms after being treated with QPQ, effectively solving the problem of hardening deformation that is difficult to solve with conventional heat treatment methods. For example, after QPQ treatment, the surface hardness of a 2Cr13 stainless steel thin plate with a size of 510*460>1.5mm is greater than HRC60, and the unevenness is less than 0.5mm. Currently, QPQ technology has been applied very successfully to many types of shaft parts and slender rod parts, effectively solving the contradiction of hardening and product deformation that has existed for a long time.

Can Replace Multiple Heat Treatment and Corrosion Protection Processes, Short Time Period

After being treated with QPQ, the workpiece has improved hardness and wear resistance, increased corrosion resistance, and a black, beautiful appearance. It can replace multiple processes such as conventional quenching, tempering, and blackening (chromium plating), shortening the production cycle and reducing production costs. A large amount of production data shows that compared with carburizing quenching, QPQ treatment can save 50% of energy, and compared with hard chrome plating, it can save 30% of costs, with high-cost performance.

The commonly used substrates for QPQ treatment

Various structural steels

Mild steel, Q235, 20, 20Cr, 20CrMnTi, 20CrNiMo, 35CrMo, 42CrMo, 45, 40Cr, 50CrV, 65Mn, and 38CrMoAl.

Various tool sheets of steel

T7~T12, 5CrMnMo, 5CrNiMo, 3Cr2W8V, GCrl5, HI3 (0.35% C, 1.5% Mo, 5% Cr, 1% Si, 1% V), Cr12MoV, and various high-speed sheets of steel.

Various stainless steels

0Cr3~4Crl3, 201, 301, 304, 316, 1Cr18Ni9Ti, 0Crl8Nil2MoTi, 4Cr9Si2, and 5Cr21Mn9Ni4N.

Various cast irons

Gray iron, malleable iron, ductile iron, and wear-resistant alloy cast iron.

Various iron-based powder metallurgy components.

0Cr3~4Crl3, aTypical applications of QPQ treatment include engine valves, crankshafts, cylinder liners, gears, camshafts, bearings, spindles, sliders, steering arms, windshield wiper ball joint shafts, guide rails, hydraulic cylinders, universal joints, connecting pins, various molds, pistons, threaded screws, bolts and nuts, pump bodies, high-speed steel drill bits, gun barrels, various cutting tools, flanges, keys, gaskets, housings, and so on.201, 301, 304, 316, 1Cr18Ni9Ti, 0Crl8Nil2MoTi, 4Cr9Si2, and 5Cr21Mn9Ni4N.

Surface hardness and case depth comparison of conventional materials after QPQ treatment:

Material Type Representative Steel GradePrior TreatmentNitriding Temperature (℃)Nitriding Time (h)（μm）Compound Layer (μm)Total Diffusion Layer (mm)
Medium Carbon Steel45#Not treated or normalized5651.5170.3-0.5
Medium Carbon Steel40CrNot treated or normalized5651.5150.2-0.4
Cold Work Die SteelCr12MnMoHigh-temperature quenching5402.5100.15-0.2
NitridingSteel38CrMoAlNormalization5702120.25-0.4
Casting Die Steel3Cr2W8VQuenching and high-temperature tempering5702.5110.15-0.3
Hot Work Die SteelH13Normalization5653110.15-0.3
Hot Work Die Steel5CrMnMoQuenching5703110.15-0.3
High Speed SteelW6Mo5Cr4V2(Tool) Quenching5500.2520.05-0.1
High Speed SteelW6Mo5Cr4V2(Tool) Quenching5502.5110.15-0.3
Stainless Steel1Cr13；4Cr13Quenching5702.5100.1-0.2
Stainless Steel1 Cr18Ni9TiQuenching5702.5100.1-0.2
Stainless SteelOCr18Ni12Mo2TiQuenching5702.580.1-0.2
Carbon Structural Steel08；10；10FAnnealing5702.5150.3-0.5
Carbon Structural Steel35；40Not treated or normalized5702.5150.3-0.5
Bearing SteelG Cr15Quenching and annealing5652100.15-0.3
Alloy Structural Steel50 MnAnnealing5202120.2-0.4
Alloy Structural Steel30CrMo-35CrMoQuenching and annealing5652150.25-0.4
Alloy Structural Steel38CrMoAlA-Quenching and annealing570120.25-0.4
38CrWVAlA
Alloy Structural Steel38CrAlA-Quenching and annealing5702120.25-0.4
40CrNiMoA
gray cast ironHT200Quenching and annealing5703150.15-0.3
Ductile IronQT20-60Not treated5703150.15-0.3

Note 1: Prior to undergoing the QPQ salt bath composite process, complex parts need to undergo tempering treatment at a temperature of at least 580℃, followed by slow cooling. To compensate for the slight expansion after the process, precision parts should have a machining allowance of 10±2μm in diameter before treatment.

QPQ process flow

  • Load basket
  • Remove oil and clean
  • Rinse
  • Preheat
  • Nitriding
  • Air-cooling
  • Oxidizing
  • Hot water cleaning
  • Clean water rinsing
  • Dehydration and drying
  • Rough polishing
  • Fine polishing
  • Load basket
  • Preheat oxidation
  • Hot water cleaning
  • Clean water rinsing
  • Dehydration and drying
  • Fine polishing
  • Dip in hot rust preventive oil or emulsion rust preventive oil
  • Wipe off the oil.

Process and parameter requirements:

Use a dedicated material basket, and control the amount of loading within 35% of the salt weight in the crucible, otherwise it will affect the thickness of the nitrided layer.
After cleaning the oil, the cleaning agent residue must be rinsed off, otherwise corrosion spots will occur during preheating.

Preheating

Preheat temperature at 320 ~ 350°C, hold for 0.30 ~ 1.0h. Note that the color should be blue with a yellow tint, and the color of the whole basket should be consistent.

Nitriding

Nitriding at 575°C (560 ~ 580°C), hold for 2.0 hours. Start counting the holding time from 565°C. Note that the furnace temperature should not be lower than 530°C after the workpiece enters the furnace, otherwise, corrosion spots will occur.

Pre-cooling

After the co-diffusion, pre-cool to around 480°C (note: the salt must not solidify) before entering the oxidation furnace. If the workpiece enters the furnace too early, it will oxidize and turn yellow (light rust).

Oxidation

The first oxidation (after nitriding) at 380420°C, hold for 1530 minutes; the second oxidation (after polishing) at 410-430°C, hold for 45-90 minutes.

QPQ Treatment Production Equipment Display

Production Equipment Display of QPQ Treatment
Production Equipment Display of QPQ Treatment

QPQ Treatment Testing Equipment Display

QPQ Processing Inspection Equipment Display
QPQ Processing Inspection Equipment Display

Appearance Inspection

The surface of the workpieceafter QPQ treatment is black or dark blue. Under an illumination of 500Lx and observed by the naked eye at a distance of 300mm from the light, the surface color should be relatively uniform and consistent, without obvious mottling, rust, or reddening.

Salt Spray Test

Conducted in a salt spray test chamber for 24 hours continuously. The salt spray test temperature is 35±2°C, the relative humidity is >95%, and 5% NaCl water solution is sprayed. The duration of NSS (neutral salt spray test) should be more than 100 hours.

Wear Resistance Test

Generally, two methods, rolling wear test and sliding wear test, are used for wear resistance testing.

Case Depth Hardness Test

The surface hardness of the case depth is usually measured by microhardnessmeasurement under a load of 100g. Low-load Vickers hardness measurement can also be used, but not large-load hardness testers such as Rockwell. Hardness cannot be measured with a cone-shaped tool. The specimen used for hardness measurement should be polished with No. 4 or No. 5 sandpaper before QPQ treatment, and then gently polished with the same sandpaper after treatment until the metal gloss is seen. Then measure the hardness on the polished surface. If the sandpaper is ground too lightly, the porous layer on the surface of the case depth will not be removed, and the hardness will be lower, sometimes much lower. If the sandpaper is ground too heavily, the case depth will be reduced too much, which will also result in a lower hardness value. Therefore, in the initial stage of hardness testing or for novices, sandpaper should be repeatedly ground lightly and heavily several times to obtain the highest hardness value.

QPQ Processing Sample Cases

QPQ treatment sample case display
QPQ treatment sample case display

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