The technological process of QPQ treatment is:
Degreasing and cleaning → preheating → salt bath nitriding → salt bath oxidation → desalination and cleaning → drying (polishing → salt bath oxidation → desalination and cleaning → drying) → oil immersion.
QPQ (Quench-Polish-Quench) technology is a combination of nitriding and oxidation processes. It is a salt bath treatment that increases the wear and corrosion resistance of the substrate surface by combining nitrogen and oxidation.
This technology is often used as an alternative to carburizing and tempering, ion nitriding and chrome plating. Improves the wear and corrosion resistance of products while reducing the risk of hardening deformation.
QPQ technology has a wide range of applications in areas such as engineering machinery, instrumentation and light chemical industry. In this study, the wear and corrosion resistance of 40Cr steel was evaluated after undergoing QPQ treatment and compared with those of bright plasma nitriding, oxidation and chromium electroplating.
1. Test materials and methods
(1) Test materials and processes
The test material used in this study was 40Cr steel with a hardness of approximately 274HV after being quenched and tempered. The metallographic samples were prepared through wire cutting and had dimensions of φ30mm × 10mm for wear testing and φ10mm × 100mm for corrosion resistance testing. Samples were assigned separate numbers, as shown in Table 1.
Before undergoing heat treatment, the surface of the samples was sanded until reaching a roughness value of 1.6μm and cleaned with anhydrous acetone, rinsed with clean water and dried. The process parameters for each heat treatment are shown in Table 2.
After undergoing QPQ treatment and oxidation, the surface of the samples turned black, while after chrome plating it became silvery and shiny, and after ionic nitriding it became silvery gray.
Table 1 Sample No. of different heat treatments
| Sample Category | QPQ Processing | Oxidation | Chrome plating | Ion nitriding |
| Metallographic specimen | all | – | – | dl |
| Wear Test Sample | a2 | b2 | c2 | d2 |
| Corrosion Resistance Test Sample | a3 | b3 | c3 | d3 |
Table 2 Heat Treatment Process Parameters for 40Cr Steel
| Sample | Labor | Heat treatment process parameters |
| al,a2,a3 | QPQ Processing | Preheating (360 ℃ × 30min)+nitriding (630 ℃ × 120min)+oxidation (380 ℃ × 30min） |
| b2,b3 | oxidation | Tank solution (NaOH: NaNO2=2:1), oxidation (140C × 20min） |
| c2,c3 | Chrome plating | Bathing solution (CrO 3 : 250g/L+H 2 SO 4 : 3g/L), chrome plating (55C × 50A/dm 2 ） |
| d1,d2,d3 | Ion nitriding | Bright plasma nitriding (520℃×20h） |
(2) Microhardness test
Sand the metallographic samples (a1, d1) that have undergone several heat treatment processes with fine sandpaper until they are shiny. This is done for metallographic inspection and hardness measurement.
After metallographic embedding, measure the hardness gradient from the surface of the infiltrated layer to the matrix.
The microhardness tester used in the test has a testing force of 0.098 N (10 gf) and a retention time of 10 seconds.
Etch the prepared metallographic sample with a solution of 4% nitric acid and alcohol. After the sample has been dried, observe its structure using a 4XB metallographic microscope.
(3) Abrasion test
M-2000A Ring Block Wear Tester is used to perform sliding wear tests on wear samples (a2, b2, c2, d2) that have been subjected to various heat treatment processes.
The friction pair is a GCr15 steel test ring with a hardness of 57 HRC. The ring has an external diameter of 40 mm, rotation speed of 200 r/min, load of 100 N and total wear time of 30 minutes.
To prepare the worn sample, repeatedly clean the front and back of the sample with acetone and then dry it. Measure the weight loss of the sample using an electro-optical analytical balance accurate to 0.1 mg.
(4) Corrosion resistance test
Revised version:
Perform a neutral salt spray test on samples (a3, b3, c3, d3) with different heat treatment processes using the KD60 salt spray tester in accordance with GB/T10125.
The corrosion medium used for the test is a saline solution with 5% NaCl and a pH value of 6.7.
The temperature inside the test chamber is set to 35°C, the nozzle pressure is 83 kPa, and the observation period is 24 hours.
Intermittent spray time is 8 hours and stop time is 16 hours.
2. Test results and analysis
(1) Hardness of the carburized layer
Table 3 shows the hardness distribution of 40Cr steel after different processes.
As shown in Table 3, after undergoing QPQ, ion nitriding and chrome plating, the surface hardness reaches 711 HV, 525 HV and 703 HV, respectively. The hardness gradient gradually decreases from the surface to the substrate.
It is not possible to measure the hardness of the sample after oxidation treatment because the oxidation film is too thin. There is no oxide film present on the surface of the sample after it has been polished with fine sandpaper.
Table 3 Microhardness Test Results
| Distance from surface/um | 0 | 100 | 200 | 300 | 400 | 500 |
| Surface hardness of HV ionic nitriding | 525 | 462 | 375 | 310 | 274 | 274 |
| Surface hardness QPQ HV | 711 | 303 | 300 | 274 | 273 | 270 |
| HV chrome surface hardness | 703 | 274 | 274 | 273 | 274 | 273 |
| Oxidation surface hardness HV | – | 274 | 274 | 274 | 273 | 274 |
(2) Metallographic structure of the infiltration layer
Figures 1a and 1b show the metallographic photos of 40Cr steel after ionic nitriding and QPQ treatment, respectively.
There is no need to examine the metallography of the sample after chrome plating and oxidation treatment. The coating structure of the sample after chrome plating is pure chromium, and the surface after oxidation is a very thin black Fe 3 Ó 4 oxide film.
Fig. 1 Metallographic Structure of 40Cr Treated by Different Processes
As seen in Figure 1, the nitriding layer of the QPQ and ionic nitriding treatments is composed of a composite layer and a diffusion layer, with the white band in the figure representing the composite layer.
In the case of QPQ treatment, the amount of oxide layer on the surface is too large to be observed under a metallographic microscope.
Although the ionic nitriding treatment time is seven times longer than that of the QPQ treatment, the thickness of the composite layer formed is approximately half that of the composite layer of the QPQ treatment.
In terms of homogeneity of the composite layer, Figure 1 shows that the structure of the nitrided layer after QPQ treatment is more uniform, while the structure of the sample after ion nitriding is less uniform.
(3) Sliding wear test results
Table 4 compares the wear values of samples treated by different processes under the described wear test conditions.
As seen in Table 4, the wear value of the QPQ-treated sample in the 30-minute test is at least 1.9 mg.
The wear resistance of the QPQ-treated sample is 1.45 times that of the chrome-plated sample, 4.32 times that of the ion-nitrided sample, and 7.9 times that of the oxidized sample.
It is clear that the wear resistance of the QPQ-treated samples was significantly improved.
Table 4 Comparison of Wear Values from the Sliding Wear Test
| Serial number | Processing method | HV hardness | Wear value/mg | Relative wear ratio |
| 1 | QPQ Processing | 711 | 1.9 | 1 |
| two | Chrome plating | 703 | 2.75 | 1.45 |
| 3 | Ion nitriding | 525 | 8.2 | 4.32 |
| 4 | Oxidation | – | 15 | 7.9 |
(4) Permeation layer corrosion performance results
Table 5 shows the results of the neutral salt spray test on samples treated by different processes under the described corrosion resistance test conditions.
As seen in Table 5, the salt spray corrosion resistance of the QPQ-treated 40Cr steel sample is 3.2 times that of the chrome-plated sample, 8 times that of the ion-nitrided sample, and 32 times that of the ion-nitrided sample. that of the sample with ionic nitridation. oxidized sample.
This demonstrates that the corrosion resistance of steel parts after QPQ treatment has improved significantly.
Table 5 Corrosion resistance comparison of neutral salt spray test
| Serial number | processing method | Rust start time/h | Comparison of relative corrosion resistance |
| 1 | QPQ Processing | 256 | 1 |
| two | Chrome plating | 80 | 0.31 |
| 3 | Ion nitriding | 32 | 0.13 |
| 4 | Oxidation | 8 | 0.03 |
(5) Analysis of results
In the QPQ treatment process, the surface of 40Cr steel forms a high concentration of Fe2~3N nitride layer and dense Fe3O4 oxide film. This type of composite layer has high hardness and wear resistance, but the bonding strength of the chromium layer is not strong enough.
During the sliding wear test, the chrome layer is prone to peeling off, resulting in lower wear resistance compared to the QPQ treatment. However, the wear resistance of chromium plating is better than the low nitrogen alloy structure on the surface after ion nitriding.
The surface after oxidation has only a thin Fe3O4 oxide film, which has low hardness and only provides anti-corrosion properties.
The high corrosion resistance of the sample after QPQ treatment is mainly due to the high corrosion-resistant Fe2~3N compound layer and the dense oxide film on the surface. Oxygen can penetrate the deeper layer of the compound, further passivating it and providing greater corrosion resistance to the surface.
Conclusion
QPQ treatment of 40Cr steel results in the formation of a high concentration of Fe2～3N nitride and a dense Fe3O4 oxide film on its surface, which greatly improves its surface microhardness, wear resistance and corrosion resistance.
The surface wear and corrosion resistance of 40Cr steel decreases in the order of QPQ, chrome plating, ionic nitriding and oxidation treatment.
