The technological process of QPQ treatment is:
Degreasing and cleaning → preheating → salt bath nitriding → salt bath oxidation → desalting and cleaning → drying (polishing → salt bath oxidation → desalting and cleaning → drying) → oil immersion.
QPQ technology is a composite process consisting of nitriding and oxidation processes.
It is a nitrogen and oxidation salt bath composite treatment technology to improve the wear resistance and corrosion resistance of the substrate surface.
It is often used to replace carburizing quenching, ion nitriding, chromium plating and other heat treatment and surface strengthening technologies to improve the wear resistance and corrosion resistance of products and solve the problem of hardening deformation.
This technology is widely used in engineering machinery, instrumentation, light chemical industry and other fields.
In this article, 40Cr steel was treated by QPQ, and its wear resistance and corrosion resistance were compared with those of glow plasma nitriding, oxidation and chromium electroplating.
1. Test materials and methods
(1) Test materials and process
The test material is 40Cr steel (the hardness of the matrix after quenching and tempering is about 274HV).
The metallographic sample is processed by wire cutting, and the size of the wear test sample is φ30mm × 10mm, size of corrosion resistance test sample φ10mm × 100mm, number each sample separately.
The number information of each sample after different heat treatment is shown in Table 1.
The surface of the sample shall be ground to make the surface roughness value reach 1.6μm.
The sample shall be cleaned with anhydrous acetone, rinsed with clean water and dried before heat surface treatment.
The process parameters after different heat treatment are shown in Table 2.
After QPQ treatment and oxidation treatment, the surface of the sample is black, after chrome plating, it is silver bright, and after ion nitriding, it is silver gray.
Table 1 Sample No. of Different Heat Treatment
|Sample category||QPQ processing||Oxidation||Chrome plating||Ion nitriding|
|Wear test specimen||a2||b2||c2||d2|
|Corrosion resistance test sample||a3||b3||c3||d3|
Table 2 Process Parameters of Heat Treatment for 40Cr Steel
|Sample||Workmanship||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||Bath solution (CrO3: 250g/L+H2SO4: 3g/L), chromium plating (55C × 50A/dm2）|
|d1，d2，d3||Ion nitriding||Glow plasma nitriding (520 ℃ × 20h）|
(2) Microhardness test
Grind the metallographic specimens (a1, d1) that have undergone various heat treatment processes with fine sand paper until they are shiny, which is used for metallographic inspection and hardness measurement.
After metallographic inlay, the hardness gradient from the surface of the infiltrated layer to the matrix is measured.
The microhardness tester used in the test has a test force of 0.098N (10gf) and a holding time of 10s;
Corrosion the prepared metallographic sample with 4% nitric acid and alcohol solution.
After the sample is dried, observe the sample structure with a 4XB metallographic microscope.
(3) Abrasion test
M-2000A ring block wear tester is used to carry out sliding wear test on wear samples (a2, b2, c2, d2) that have undergone various heat treatment processes.
The friction pair worn against it is GCr15 steel test ring, with a hardness of 57HRC, an outer diameter of 40mm, a rotating speed of 200r/min, a load of 100N, and a total wear time of 30min.
The front and back of the worn sample shall be cleaned repeatedly with acetone and then dried, and the weight loss of the sample shall be measured with an electro-optical analytical balance with an accuracy of 0.1 mg.
(4) Corrosion resistance test
Use KD60 salt spray tester to conduct neutral salt spray test on samples (a3, b3, c3, d3) with different heat treatment processes according to GB/T10125.
The corrosion medium used in the test is saline solution with 5% NaCl and pH value of 6.7.
The temperature of the test chamber is 35 ℃, the nozzle pressure is 83kpa, and the observation period is 24h.
The intermittent spray is 8h, and the stop time is 16h.
2. Test results and analysis
(1) Hardness of carburized layer
The hardness distribution of 40Cr steel after different processes is shown in Table 3.
It can be seen from Table 3 that after QPQ, ion nitriding and chromium plating, the surface hardness reaches 711HV, 525HV and 703HV respectively, and the hardness gradient gradually decreases from the surface to the substrate.
The hardness of the sample after oxidation treatment cannot be measured because the oxidation film is very thin and there is no oxide film on the surface of the sample after polishing with fine sandpaper.
Table 3 Microhardness Test Results
|Distance from surface/um||0||100||200||300||400||500|
|Surface hardness of ion nitriding HV||525||462||375||310||274||274|
|QPQ surface hardness HV||711||303||300||274||273||270|
|Chromium plated surface hardness HV||703||274||274||273||274||273|
|Oxidation surface hardness HV||–||274||274||274||273||274|
(2) Metallographic structure of infiltration layer
Fig. 1a and b are the metallographic photos of 40Cr steel after ion nitriding and QPQ treatment respectively.
Since the coating structure of the sample after chromium plating is pure chromium, and the surface after oxidation is a very thin black Fe3O4 oxide film, it is unnecessary to look at the metallography of the sample after the two processes.
Fig. 1 Metallographic Structure of 40Cr Treated by Different Processes
It can be seen from Figure 1 that the nitriding layer of QPQ and ion nitriding is composed of compound layer and diffusion layer from the surface to the inside, and the white band in the figure is the compound layer.
Only the oxide layer on the surface of QPQ is too much to be observed under the metallographic microscope.
It can be seen from the figure that the ion nitriding treatment time is 7 times that of the QPQ treatment, but the thickness of the formed compound layer is about half of that of the QPQ treatment compound layer.
In terms of the homogeneity of the compound layer, it can be seen from the figure that the structure of the nitrided layer of the sample after the QPQ treatment is more uniform, while the structure of the sample after the particle nitriding is less uniform.
(3) Sliding wear test results
Under the above wear test conditions, the comparison of wear values of samples treated by different processes is shown in Table 4.
It can be seen from Table 4 that the wear value of the sample treated by QPQ in the 30min test is at least 1.9mg.
The wear resistance of sliding wear is 1.45 times that of chromium plating, 4.32 times that of ion nitriding and 7.9 times that of oxidation.
It can be seen that the wear resistance of QPQ treated samples has been greatly improved.
Table 4 Comparison of Wear Values of Sliding Wear Test
|Serial No||Processing method||Hardness HV||Wear value/mg||Relative wear ratio|
(4) Corrosion Performance Results of Permeation Layer
Under the above corrosion resistance test conditions, the neutral salt spray test results of samples treated by different processes are shown in Table 5.
It can be seen from Table 5 that the salt spray corrosion resistance of 40Cr steel sample treated by QPQ is 3.2 times that of chromium plating, 8 times that of ion nitriding and 32 times that of oxidation.
This shows that the corrosion resistance of steel parts after QPQ treatment has been greatly improved.
Table 5 Comparison of Corrosion Resistance of Neutral Salt Spray Test
|Serial No||processing method||Rust start time/h||Comparison of relative corrosion resistance|
(5) Result analysis
In the four processes: QPQ treatment, the surface of 40Cr steel formed a high concentration of Fe2~3N nitride layer and a dense Fe3O4 oxide film.
This kind of compound layer has high hardness and wear resistance, but the binding force of the chromium plating layer is not strong enough.
In the sliding wear test, the chromium plating layer is easy to peel off, so its wear resistance is not as good as QPQ.
However, compared with the low nitrogen alloy structure on the surface after ion nitriding, the wear resistance is better.
After oxidation, the surface is only Fe3O4 oxide film, which has very low hardness and only anti-corrosion effect;
In addition, 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, and the oxygen can extend into the deeper compound layer, which further passivates the compound layer to make the surface have higher corrosion resistance.
(1) The surface of 40Cr steel treated by QPQ has formed high concentration Fe2 ～ 3N nitride and dense Fe3O4 oxide film, and its surface microhardness, wear resistance and corrosion resistance have been greatly improved.
(2) After QPQ, chromium plating, ion nitriding and oxidation treatment, the wear resistance and corrosion resistance of 40Cr steel surface become lower and lower.