Background
Carburizing is a material surface modification technology, can improve the carbon content of a material surface layer within a certain range, ensures the toughness of a core part and simultaneously obtains higher surface hardness, and is widely applied to the field of gears and bearings. However, the corrosion resistance of the workpiece is generally reduced after carburization, the workpiece is more easily corroded in service under a complex working condition environment, and the service life of the workpiece is reduced to a certain extent.
In order to improve the corrosion resistance of the carburized layer and improve the carburized layer quality, various related process methods are proposed in the industry, such as an ion implantation method, (CN108179375A), a rare earth catalysis method (CN105483604A), a surface metal carbide coating method (CN109321875A) and the like, but the problems of high cost, material equipment limitation, complex process and the like still exist, so that the search for an effective method for improving the corrosion resistance of the carburized layer still has important significance.
Nitriding is also a common surface heat treatment method, can obtain higher hardness and residual compressive stress on the surface of a workpiece, but is often used as a final heat treatment process to improve the surface hardness of a diffusion layer because the diffusion layer is generally thinner and the nitriding temperature is lower. At present, no relevant research on the aspect of improving the corrosion resistance of a carburized layer by using nitriding as a pretreatment process before carburization exists.
Disclosure of Invention
The invention provides a pretreatment method capable of improving the corrosion resistance of a carburized layer, improving the quality of a carburized layer and prolonging the service life of the carburized layer.
The invention relates to a pretreatment method for improving corrosion resistance of a carburized layer, which comprises the following specific steps:
before carburizing treatment, setting nitriding temperature to be 300-1200 ℃ and heat preservation time to be 2-24 h according to chemical components of a base material, and performing pre-nitriding treatment on the base material to form a nitriding layer with the thickness of 20-100 microns on the surface of a workpiece.
And step two, performing high-temperature carburization treatment on the workpiece with the nitrided layer obtained in the step one, and controlling the specific carburization temperature to be 400-1200 ℃ according to the chemical components of the base material.
And step three, carrying out heat treatment after base material infiltration, and carrying out related processes such as high-temperature quenching, deep cooling, tempering treatment and the like on the workpiece subjected to the pre-nitriding treatment and the carburizing treatment.
Compared with the prior art, the nitriding process as pretreatment is more mature, the production cost is lower, the process is relatively simple, the limitation on materials and equipment is small, the corrosion resistance of the carburized layer can be effectively improved, the carburized layer quality is improved, and the service life of the carburized layer is prolonged.
Detailed Description
The following describes embodiments of the present invention in detail with reference to the following examples and accompanying drawings, and further verifies the beneficial effects of the present invention.
The first embodiment is as follows: taking case of case hardening steel 14Cr14Co13Mo5Ni
Taking carburized steel 14Cr14Co13Mo5Ni to be carburized, mechanically grinding the surface of the carburized steel according to a standard process, polishing the carburized steel by diamond polishing paste with the particle size of 2.5 mu m, and then ultrasonically cleaning and drying the carburized steel in absolute ethyl alcohol for later use. Wherein, the chemical composition of the carburizing steel 14Cr14Co13Mo5Ni is C0.14 percent in mass percentage; 13.96 percent of Cr; 12.73 percent of Co; 4.75 percent of Mo; 1.8 percent of Ni; v0.6%; 0.5 percent of W; the balance being Fe and unavoidable impurities.
And step two, putting the carburized steel 14Cr14Co13Mo5Ni treated in the step one into a vacuum furnace to carry out a pretreatment process before carburization, heating after vacuumizing, heating to 580 ℃, nitriding after the temperature is raised, wherein the specific nitriding time is 5h, a nitriding medium is ammonia gas, the ammonia gas decomposition rate is kept between 18 +/-2% in the nitriding process, and cooling to room temperature along with the furnace after nitriding is finished.
And step three, setting the vacuum carburizing temperature to be 960 ℃ according to the chemical components of the carburized steel 14Cr14Co13Mo5Ni and the carburizing temperature commonly used in actual production development. And (3) loading the carburized steel 14Cr14Co13Mo5Ni subjected to nitriding treatment in the step two into a vacuum furnace, vacuumizing, starting to heat, heating to 960 ℃, starting to carburize, setting the carburized surface concentration to be 1.2%, using acetylene as a carburizing medium, setting the specific carburization time (carburization + diffusion) to be 13h, and cooling the oil to room temperature after the carburization is finished.
And 4, heating the carburized steel 14Cr14Co13Mo5Ni subjected to carburization in the third step to 1065 ℃ in a vacuum furnace, preserving the temperature for 1h, performing oil quenching, taking out, and performing cryogenic treatment (-190 ℃), tempering treatment (500 ℃) and secondary cryogenic treatment and secondary tempering treatment at the same temperature, wherein the specific heat treatment process is shown in figure 1.
Comparative example one:
the comparative example is different from the first example in that the pretreatment process of the second step before carburization is not performed after the first step, and is otherwise the same as the first example.
Example two: case of case hardening steel 20CrMnTi
Taking 20CrMnTi of carburized steel to be carburized, mechanically grinding the surface of the carburized steel according to a standard process, polishing the carburized steel by diamond polishing paste with the particle size of 2.5 mu m, and then ultrasonically cleaning and drying the carburized steel in absolute ethyl alcohol for later use. Wherein, the chemical components of the carburizing steel 20CrMnTi are C0.21 percent in mass percentage; 1.28 percent of Cr; 1.03 percent of Mn; 0.07 percent of Ti; 0.01 percent of Cu; 0.01 percent of Ni; 0.26 percent of Si; p is 0.016 percent; 0.003% of S and the balance of Fe and inevitable impurities.
And step two, filling the carburized steel 20CrMnTi processed in the step one into a vacuum furnace for a pretreatment process before carburization, heating after vacuumizing, heating to 530 ℃, nitriding, wherein the specific nitriding time is 7h, a nitriding medium is ammonia gas, the ammonia gas decomposition rate is kept between 18 +/-2% during nitriding, and cooling to room temperature along with the furnace after nitriding is completed.
And step three, setting the vacuum carburization temperature to 920 ℃ according to the chemical components of the carburized steel 20CrMnTi and the carburization temperature commonly used in actual production and development. And (2) loading the carburized steel 20CrMnTi subjected to nitriding treatment in the step two into a vacuum furnace, vacuumizing, heating to 650 ℃, keeping the temperature for 30min, preheating, heating to 920 ℃, then starting carburizing, setting the carburized surface concentration to be 0.75%, using acetylene as a carburizing medium, setting the specific carburizing time (carburizing and diffusing) to be 3h, cooling to 830 ℃ along with the furnace after carburizing, keeping the temperature for 30min, and cooling oil to room temperature.
And 4, carrying out cryogenic treatment (-190 ℃) and low-temperature tempering treatment (180 ℃) on the carburized steel 20CrMnTi subjected to the carburization treatment in the third step, wherein the specific heat treatment process is shown in figure 2.
Comparative example two:
the comparative example is different from the second example in that the pretreatment process of the second step before carburization is not performed after the first step, and the other steps are the same as the second example.
The self-corrosion current density I of each case of carburized steel was obtained by Tafel extrapolation of the polarization curves of FIGS. 3 and 4corrAnd self-etching potential EcorrAs shown in Table 1, it can be seen that the autogenous corrosion current density of the carburized steel 14Cr14Co13Mo5Ni and 20CrMnTi carburized layer after pretreatment is reduced and the autogenous corrosion potential is significantly increased, which indicates that the carburized steel after pretreatment is corrodedThe corrosion rate is reduced and the corrosion tendency is also significantly reduced. In addition, as can also be seen from fig. 3 and 4, the resistive arc diameters of the pretreated carburized steel 14Cr14Co13Mo5Ni and 20CrMnTi carburized layer are larger, which also indirectly reflects the corrosion rate of the carburized steel. Therefore, it can be seen that, by nitriding the base material before carburizing, the corrosion resistance of the subsequent carburized layer can be significantly improved.
TABLE 1 polarization curve fitting results of carburized steel after full process treatment