After normal normalizing, the microstructure and properties of the steel material are heated and forged at 1200 ~ 1250 °C to form a billet, and the air is cooled at room temperature at 1050 ~ 1000 °C to room temperature. At this time, the microstructure is pro-eutectoid ferrite and Wei's microstructure ferrite, pearlite and bainite, and the crystal grains are coarse (grade 2 to 3) and the hardness is high (220 to 280 HBW). . The forging billet cooled to room temperature is heated and kept at (930±10) °C for 1 hour, and then air-cooled. The hardness is 168-230HBW, the hardness difference is 30-62HBW, and the metallographic structure is granular bainite, ferrite and pearlite. (B FP), the grain size is 3 to 4 grades. Obviously it is difficult to meet the structural and performance requirements of forged blanks. From 20CrMnTi steel Figure 1
The continuous cooling transition (CCT) curve of the CCT diagram of 20CrMnTi steel shows that different cooling rates have different microstructures and hardnesses, and it can be seen that bainite structure appears in a large range of cooling rates. The bainite forming temperature is slightly higher than that of the upper bainite, and is a bainite distributed in the bulk ferrite.
Since bainite is granular, it is called "grain", which is hard and tough, and deteriorates the machinability of steel. The normal normalized structural transformation is done under continuous cooling conditions, and the cooling conditions of different parts and different parts of the same part (such as surface and core) are different, so the uniformity of microstructure transformation, hardness and stress distribution is poor. The same blank has a hardness difference of 30 to 62 HBW. This makes it difficult to control the dimensional accuracy of parts during the cutting process (especially during high-speed machining). In particular, it is difficult to control the tooth profile, tooth direction and cumulative error during high-speed shaving and spline tooth decay [1]. Therefore, it is not suitable to use normalizing as a preliminary heat treatment for a 20CrMnTi steel tooth blank.
The carburized and quenched structure of the air-cooled part after forging is made into a smooth sample after forging air-cooled steel, and carburized and quenched according to the listed process on the carburizing automatic line. The austenite grain size was determined by the hot etching method, and the crystal grains of different sizes were obtained in each sample, and the order was 8 grades and 3 grades. It is shown that the coarse austenite grains with non-equilibrium structure such as bainite are hereditary, while the parts with balanced tissues F and P do not undergo tissue inheritance, and the grains are remarkably refined. This indicates that the heritability of the coarse austenite grains after forging can be cut off by obtaining an equilibrium structure before reheating the austenite [2].
Table 1 sample carburizing quenching process parameters heating diathermy carburizing diffusion pre-cooling quenching area 12345 oil tank temperature / °C80090094095083060 gas flow RX5434/m3?
h-1C3H80.140.180.151.4 Forging waste heat isothermal normalizing structure and properties After experimental measurement, the initial forging temperature is 1200 ~ 1250 ° C, most parts are at the upper limit of the initial forging temperature; the final forging temperature is 1050 ~ 1000 ° C, most of In the middle limit, the time required for forging from air to 800 °C for final forging is about 3.5 min. The isothermal furnace uses a 15 kW box-type resistance furnace at a furnace temperature of 650 ° C. Considering the space separation distance between the workshops in actual production, the experiment is actually carried out in the experimental site. The temperature was 850 ° C and was placed in an isothermal furnace. Experimental data at different furnace temperatures and different isothermal times.
Figure 2 isotherm and normalizing schematic diagram through the set different furnace temperature, the final determination of the workpiece after forging to 800 ° C into the isothermal furnace is the lower limit of the furnace temperature. By setting different isothermal times, it is finally determined that 65 min in the 650 ° C isothermal furnace is the lower limit of the holding time. The initial forging temperature is too high, so that the forged structure is obviously overheated, but there is no overheating, since this isothermal normalizing can completely disappear.
Different furnace temperature and different isothermal time experimental data number furnace temperature / °C isothermal time / min metallographic grain hardness / HBW remarks 18065 equiaxed F P7 ~ 8 grade 160 qualified 275065F S multi-needle Wα4 ~ 6 (mixed crystal) 179 unqualified 370045F S more acicular Wα4 ~ 6 grades are 3 grades mixed crystal 184 unqualified 480045 untransformed forged air-cooled tissue equiaxed FP, the two organizations have obvious boundaries 2 to 3 7 ~8 grade mixed crystal 164 is not qualified Note: F-ferrite P-pearlite Wα-Weisite S-Sorstite (fine pearlite) In addition to this structure, it is transformed into a qualified structure, while the grains are also It is refined, then, there is no need to reduce the initial forging temperature. Alloy carburized steel with coarse austenite grains has two ways to produce non-equilibrium structure during forging hot isothermal normalizing. First, the actual temperature of the billet after stop forging is lower than BS point or MS, then there is bainite. Or martensite appears; second, the cooling rate is moderate after stop forging, and the ferrite Wα is precipitated from austenite before isothermal. If a pearlite transformation occurs subsequently, it is an α-Fe Wei's tissue. Although the equiaxed ferrite plus pearlite structure can be obtained with a lower cooling rate, the hardness is too low and does not meet the requirements of normalizing technology. Therefore, the key is to increase the cooling rate to suppress the precipitation of Wα. If a faster cooling rate is used after stop forging and the minimum cooling temperature is controlled above the BS point, the slab does not undergo a phase change before the isothermal temperature, and an isothermal transformation occurs before the lower temperature FP transition, thereby avoiding non-equilibrium tissue. The formation of the large nucleation rate and the growth rate ratio of the large subcooling phase transformation can be used to form an FP structure with moderate grain size and hardness [2].
After carburizing and quenching deformation of gears with different preheating treatments, the gears after normal normalizing and forging isothermal normalizing are respectively subjected to carburizing and quenching, and the deformation amount of the gears is measured.
Table 3 Different pre-heat treated gears Carburizing and quenching deformation μm measurement item Pre-heat treatment deformation amount 12345678910 Average deformation fluctuation amount Boss end spline bottom hole Normal normalizing-59-46-56-57-30-49-39-51-58 -41-48.829.0 Isothermal Normalizing-36-40-30-25-35-33-36-31-38-34.4-34.414.0 Spokes Spline Bottom Aperture Normal Normalizing-52-62-43-20 -49-35-33-50-44-51-44.042.0 Isothermal normalizing-44-35-25-28-32-38-40-33-31-3434.019.0 Common normal length Normalizing 4530263528401927384232.026.0 Isothermal normalizing 1916131018141317151215.09.0 It can be seen that the pre-heat treatment adopts the isothermal normalized gear. Because the structure is uniform, the carburizing and quenching deformation is smaller than the ordinary normal fire, especially the deformation fluctuation range is small, which is very advantageous for controlling deformation. Therefore, for alloy carburized steel parts, only the isothermal normalizing can be used to obtain the microstructure and hardness required for heat treatment reliably and stably, in order to more effectively control the heat treatment deformation [3].
Conclusion 1) The automobile gear blank adopts the conventional normal normal fire to easily generate non-equilibrium structure, which not only affects the cutting performance, but also increases the carburizing and quenching deformation of the gear. Normal normalizing is not suitable as pre-heat treatment. 2) Control the cooling rate and temperature of the gear blank after forging to perform isothermal normalizing, easy to obtain uniform microstructure and appropriate hardness, and the deformation amount after carburizing and quenching is significantly reduced. 3) Using forging waste heat instead of normalizing heating can achieve energy saving and improve production efficiency.
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