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The appearance of "banded structure" makes the mechanical properties of the material have obvious directionality, and the longitudinal and transverse mechanical properties vary greatly. Generally, the longitudinal tensile strength is higher, the elongation and the area shrinkage are better, and the transverse direction is The tensile strength is significantly reduced, and the elongation and reduction in area shrinkage are particularly pronounced. According to the data, if the same metal material has a banded structure in the matrix structure, if the life index of the transverse direction (90° with the fiber direction) is 1, then the longitudinal (consistent with the fiber direction) life is 2.5, which indicates the band shape. The anisotropy caused by the tissue is very serious. Its presence will result in a reduction in the overall mechanical properties of the part, resulting in reduced reliability, safety and service life of the gear.
2) The homogenization of carbon and alloying elements in austenite formed by heating in 20CrMnTi steel is an important stage in heat treatment. In this stage, the homogenization process of alloying elements in austenite lags behind the uniformity of carbon. In order to achieve homogenization of austenitic chemical composition, not only the normalizing temperature is high, but also the heating time is long. Generally, the homogenization of carbon must be above 950 ° C, and the homogenization of alloying elements can be completed above 1100 ° C, so that austenite grains are easily grown and coarsened. The grain size of 20CrMnTi steel cooled by 900 ° C for 3 h furnace is as shown.
The coarse austenite grains inevitably affect the microstructure and properties of its transformation products, which cause the yield strength, tensile strength, contact fatigue strength and impact toughness of the material to decrease, and the probability of microcrack formation is increased, so that the effect is affected. Gear performance and service life.
In summary, since the heat treatment temperature of 20CrMnTi steel is below 950 ° C, it is difficult to eliminate the band structure and obtain a fine and uniform matrix structure when only the conventional single heat treatment process is used, and it is difficult to further Improve and improve the intrinsic quality and external quality of gears made of 20CrMnTi steel.
In order to solve the above-mentioned defects which are easy to exist in the matrix structure of 20CrMnTi steel, the method of using the sample which has satisfied the thermodynamic conditions of the phase change, and then inputting the low-frequency pulse large current with a small duty ratio, is used in the substrate. During the transformation of the organization, the additional activation energy Δ is obtained, and the diffusion coefficient is increased to change the phase transition kinetic conditions of the material, which can increase the tissue transformation speed, refine the grain in a shorter time, and eliminate the defects in the steel. The microstructure provides an ideal matrix structure for the subsequent heat treatment process (carbonitriding, quenching, tempering). At the same time, the first phase of Yan Ligong Li Zeng Zhou Xiaoling eliminates the study of the band structure in the steel for automotive drive gears to save energy.
Test Methods Each sample has a size of <15 mm x 12 mm and is divided into several groups.
The two end faces of each sample were joined by wires. During the test, a sample was placed in a laboratory electric furnace at 900 ° C for heating, and its two wires were connected outside the furnace with a MY-2005 type pulse current generator designed and manufactured by itself as a carrier for the pulse current. The sample is heated to 900 ° C to start the heat preservation, and a pulse current of a certain intensity and a certain frequency is input at the same time. Different samples are input with pulse currents of 800A, 900A, 1000A and frequencies of 50Hz, 30Hz, 15Hz and the like. After a certain period of time (10 min), it is carried out according to the normalizing process, and the air is cooled. Subsequently, the metallographic examination and analysis of the above different groups of samples were carried out, and the best tissue and pulse parameters were selected.
The working diagrams of the experimental equipment such as the heating furnace, the pulse signal generator, the oscilloscope, and the electronic potentiometer are as shown.
Experimental results After a large number of experiments, it was found that the sample was subjected to a pulse current of a certain parameter in the heat preservation process under heating conditions of 900 ° C, and then air-cooled, which can eliminate the band structure and fine crystal in a short time (10 min). Granules, as shown in the metallographic photos. ～0 is 20CrMnTi steel in the austenite state, after inputting pulse current of different parameters, not only can the band structure be quickly eliminated in a short time, but also the metallographic structure can be refined in different degrees.
Analysis and Discussion 1) To study any phase change process, the thermodynamic and kinetic conditions of the phase change system must be studied. The phase transition critical temperature Ac of the material 20CrMnTi steel is 825 ° C. When heated and maintained at 900 ° C, the thermodynamic conditions of phase transformation are created for the complete austenitization of the sample, which can be achieved from the mixture of ferrite and pearlite. The spontaneous transformation of austenite, if the kinetic conditions of the phase change can be changed, then a tissue that is distinct from conventional heat treatment will be obtained.