1. Introduction
Salt bath chromizing is a kind of valuable chemical heat treatment technology that is widely used for steels to obtain the required properties [
1,
2,
3]. It has some advantages over other chromizing technologies, such as a better surface quality, higher bonding strength and better wear resistance [
4]. After salt bath chromizing, a compound layer is formed, which can significantly improve the performance of metals [
5]. However, for obtaining the effective thickness of chromium carbide coating, a salt bath must be carried out at reasonable temperature and time [
6].
Researchers have derived the growth kinetics equation of chromium carbide coating on high chromium steel surface by TD heat treatment, and proposed that the structure of carbide coating is several or one of Cr
7C
3, (Cr,Fe)
7C
3 and Cr
23C
6; however, there is a lack of research on the structure growth process of carbide coating [
7]. Another researcher found that sub-micron CrN agglomeration area and CrN nano-crystal agglomeration area exist in the compound obtained by chromizing 20 steel in a low temperature composite salt bath for 12 h. There are a few defects in the CrN grain, but many (Cr,Fe)
7(N,C)
3 compounds with micro twins or stacking-fault are formed in the later stage of chromizing, which further supplement the structure of chromium carbide coating [
8].
TD processing equipment is open type, without vacuum conditions. The equipment is generally an independently designed silicon carbide rod type high temperature resistance furnace, which only needs to provide a stable and controllable heat source, and can process many workpieces each time, with a high production efficiency.
Due to its excellent mechanical properties and excellent polishing properties, P20 steel is widely used in injection mold, which is often used to make the cavity and core of the mold. In the production of automobile friction plates, it is difficult to meet the needs for factory production to improve the surface properties of P20 die steel by the electroplating process, while the TD chromizing process has great advantages in improving the surface properties of the die.
When the hot-pressing die is used to process the automobile friction plate, there will be many failure forms on the die surface, such as the friction and wear of the die surface and the corrosion of the friction plate forming material. Secondly, the defects of the chromium electroplating process are obvious: the coating is easy to peel off, the adhesion phenomenon appears in the process of production and processing, as does the pollution problem of industrial electroplating. Therefore, a TD salt bath chromizing process was developed to solve the problem of insufficient surface performance of the mold and replace the existing polluting chromium plating process, in order to improve the service life of hot-pressing die and comply with the requirements of energy saving, environmental protection and industrial green development.
In order to improve chromizing efficiency, the optimum salt bath temperature and time for salt bath chromizing were explored and compared with chromium plating coating. Meanwhile, the structure growth process of chromium carbide was analyzed and the improvement of its matrix performance was discussed.
2. Experimental
2.1. Materials
P20 was selected as the matrix material with the chemical composition (wt.%) of: 0.38 C, 1.3 Mn, 1.85 Cr, Mo 0.4, 0.082 S and balance Fe. The specimens were machined into a size of 25 mm × 25 mm × 10 mm. Then the mechanically polished specimens were treated with emery papers of different granulometry to achieve a mirror finish. Finally, the specimens were ultrasonically cleaned in anhydrous ethanol and dried before salt bath chromizing treatment.
2.2. Experimental
Before the experiment, the specimens should be polished, activated and preheated. The specimens were polished with water mill to ensure that the surface of the specimens is smooth without scratches, and then were dried and stored after alcohol cleaning. Before the experiment, 5% nitric acid solution was used to clean the surface of the specimens until gray white appeared on the surface of the specimens, and then the residual liquid was removed in alcohol. The rust and oil stains of specimens were removed in 5% hydrochloric acid solution, and then the residual liquid was removed in alcohol and washed and dried. The process is activation treatment, the purpose is to improve the surface activity of the specimens and it is easier to prepare the coatings in the salt bath treatment. The temperature of salt bath treatment is generally above 910 °C. Preheating treatment can prevent the surface cracks of specimens after rapid heating.
The main treating process was composed of three steps: salt bath chromizing, oil quenching and tempering. Firstly, the specimens were chromized in salt bath chromizing medium at different temperatures for various times, and then were quenched in oil. Finally, the specimens were tempered at 220 °C to reduce the stress concentration after quenching [
9]. The formula of salt bath chromizing was 80% anhydrous borax, 10% Cr
2O
3 powder, 5% Al powder and 5% NaF activator. The effective temperature of TD salt bath chromizing coating is generally between 910 and 1050 °C [
10]. In order to explore the effect of salt bath temperature and time on salt bath chromizing, two groups of experiments were designed and compared with chromium plating coating. The process parameters are given in
Table 1.
In Group 1, three temperature gradients of 930, 960 and 980 °C were set, and the salt bath holding time was set as 5 h. In Group 2, the three groups of specimens were kept in the salt bath for 3, 4.5 and 6 h, respectively, and the holding temperature of the salt bath was set as 960 °C. The above specimens and the specimens with electroplating coating were cut into 10 mm × 8 mm small specimens. After polishing the coating section, 4% nitric acid alcohol solution was used for etching for 5–10 s, and then the specimens were rinsed with ethanol and dried by cold air.
2.3. Characterization of Modified Surface Layers
The cross-sectional microstructure was observed by optical microscopy and the thickness of coatings was measured by scanning electron microscopy (SEM). Five points at different positions on the coatings were selected for thickness measurement and the average value was taken. The element contents were determined by energy dispersive X-ray analysis (EDS). The equipment of SEM-EDS is the field emission scanning electron microscope with X-MAX 50 X-ray energy spectrometer produced by Zeiss, Jena, Germany, and the model is Zeiss ultra plus. Hardness measurements were made in a HX-1000TM micro-hardness tester (Shanghai optical instrument factory, Shanghai, China), with the test load of 2.94 N and holding duration of 15 s. Each hardness value was determined by averaging at least 5 measurements.
The corrosion resistance of the specimens was tested by corrosion test. In the process of forming a friction plate, the punch and die are corroded by the decomposition products of chlorine, fluorine and their compounds. In the actual working condition, it is found that the die surface has a pitting corrosion phenomenon after a long time of use. Therefore, it is necessary to explore the corrosion resistance of the two kinds of coatings. Due to the long period of salt spray test, the corrosion resistance of the two coatings cannot be obtained in a short time. In order to speed up the experimental progress and obtain reasonable experimental data, a full immersion corrosion test was used. Because hydrofluoric acid has poor stability, has a low boiling point and is difficult to control, the corrosion effect of hydrochloric acid is similar to that of nitric acid, so nitric acid solution was used as a corrosive agent. At room temperature, the full immersion corrosion test was carried out on the chromium plated, salt bath chromized and non-chromized specimens in 65% nitric acid. The corrosion time was 6, 12, 24, 36 and 48 h. By comparing the corrosion of three groups of samples, the difference of corrosion resistance of the three specimens was obtained. The mass of the specimens before and after corrosion was measured by AL204-IC electronic balance (METTLER TOLEDO, Zurich, Switzerland, accuracy: 0.1 mg) for five times and then averaged.
4. Conclusions
The temperature and time of salt baths have important influences on the growth of coating. The thickness of the coating increases with the increase of the treatment temperature and the treatment time, and the thickness of the coating does not obviously increase to a certain extent. The optimum process of salt bath heat treatment for P20 steel is 960 °C and 6 h. The hardness of chromium plating coating is less than 700 HV, and that of chromium carbide coating is 1500 to 1600 HV. The bonding mode of chromium carbide coating and substrate is matrix bonding, but the electroplating chromium coating is physical bonding. Therefore, the adhesion of chromium carbide coating is stronger than that of electroplating coating, so it is not easy to peel off in practical use, and the matrix is less likely to be corroded. The corrosion resistance of the specimens with chromium carbide coating or chromium plating coating is obviously better than that of untreated specimens. The corrosion resistance of the two coatings is basically the same in the early stage of the corrosion test, and the corrosion resistance of the chromium plating coating decreases with the corrosion time. In terms of the corrosion amount, the corrosion per unit area of untreated P20 steel is 15 times as much as that of electroplated specimen and more than 18 times that of Chromizing coating, which indicates that the coating can effectively protect the matrix and improve the corrosion resistance of the specimen.