There are two traditional methods to obtain bainitic steel:
(1) Isothermal quenching method: heat the steel to a certain temperature above AC1 or AC3 for austenitizing to obtain austenite structure, and then quench it to the medium temperature zone at a cooling rate greater than the critical speed. Heat preservation through salt bath, oil bath and other quenching media, carry out bainite transformation under isothermal conditions, obtain a certain amount of bainite, and then take out air cooling or water cooling to complete martensite transformation. Isothermal quenching temperature and holding time have a great influence on the microstructure and mechanical properties of Bainite / martensite multiphase steel. With the increase of isothermal quenching temperature, the hardness and strength of multiphase steel decrease, but the bremsstrahlung property will increase. This is because with the increase of isothermal time, the amount of bainite transformation will increase, and then the martensitic transformation during air cooling will decrease, resulting in the decrease of hardness and increase of impact bremsstrahlung of multiphase steel. The higher the isothermal quenching temperature, the lower the microhardness of bainite, but the microhardness of martensite is not affected by the isothermal temperature. The bainite / martensite multiphase structure obtained by isothermal quenching has better strength toughness coordination than that obtained by other quenching methods, but there are also many shortcomings: the isothermal quenching process is complex, generally requires equipment such as salt bath furnace, high energy consumption, quenching medium is easy to pollute the environment, and the quenching time is long when the workpiece size is large, Increased production cycle. Therefore, the process of obtaining Bainite / martensite multiphase structure by isothermal quenching is limited in industrial application.
(2) Air cooling quenching method, also known as alloying method. Using air as quenching medium, the austenitized steel is continuously cooled in air to obtain bainite structure. There are two types of air-cooled Bainitic Steels: one is that the alloy content is high enough to increase the hardenability of the steel, increase the incubation period of bainite and shift the pearlite C curve to the right, which can complete the bainite transformation under air-cooled conditions; Second, it contains a certain amount of Mo, V, B and other elements that increase the hardenability of the steel. The steel can also obtain bainite structure through continuous cooling under the condition of air cooling. The reason is that the C curve of ferrite is shifted to the right by adding alloying elements, which increases the incubation period of pearlite and has little effect on the C curve of bainite, Thus, the austenite structure can easily bypass the pearlite transformation zone and enter the bainite transformation zone during cooling. In addition, Mo, Cr, Ni and other raw materials are expensive, which increases the cost of air-cooled bainitic steel and limits the popularization and application of air-cooled bainitic steel to a certain extent.
Bainite martensite wear-resistant steel is obtained by continuous cooling. When the steel passes through the bainite transformation temperature zone during continuous cooling, the undercooled austenite decomposes into bainite, and the precipitated bainite divides and refines the austenite grains. When it is cooled to MS temperature, martensite appears. Compared with the single-phase martensite structure, the martensite lath size decreases with the increase of the precipitated bainite, and the twin martensite decreases, which has a better structure than the single martensite. Because martensite has high hardness and strength, while bainite has good bremsstrahlung, a certain amount of lower bainite is distributed on the high-strength martensite matrix. While maintaining the high strength of martensite, it can significantly improve the impact toughness, plasticity and fracture toughness of the material, so as to obtain better wear resistance. In order to obtain specific cooling speed, fog cooling, air cooling and other cooling methods are also used. Generally speaking, with the decrease of cooling rate, the content of bainite and martensite in bainitic Martian multiphase wear-resistant cast steel increases and decreases. With the change of microstructure, its mechanical properties change accordingly, as shown in the figure, the change of bainitic martensite morphology under different heat treatment processes.