Foreword Aobei ductile iron (ADI) is a new type of engineering material with excellent mechanical properties. Because it not only has high strength, high hardness, high wear resistance, but also good plasticity and low temperature toughness. It has aroused great attention and attention from the engineering and technical circles around the world. All kinds of gears are important application fields of ADI. The ADI has made the rear axle gear of the truck and the end gear of the tractor have achieved good economic effects and technical effects. ADI can replace forged steel as the material for manufacturing gears because it has the following characteristics compared with forged steel gears: Aobei ductile iron gears have better root bending fatigue strength than quenched and tempered steel, and are shot peened. The tooth root bending fatigue strength of the strengthened Aube ductile iron gear can be further improved to reach the best level of steel. The tooth surface contact fatigue strength of the Aube ductile iron gear is better than that of the quenched and tempered steel and the nitrided steel. The wear resistance of austempered ductile iron is significantly better than that of quenched and tempered steel and austempered steel. Since the ADI density is about 10% smaller than forged steel, the overall weight can be reduced by about 10%. Due to the presence of graphite in the ADI gear, it has good shock absorption and low noise during machine operation.
Jiangsu University of Science and Technology cooperated with FAW Wuxi Diesel Engine Factory to develop the work of replacing 40Cr quenched and tempered steel gear with Aobei ductile iron gear, aiming to reduce the noise of diesel engine, improve the service life of diesel engine gear, and explore the practicality suitable for FAW Xichai. Production process of Aobei ductile iron gear production process.
The test conditions and methods are melted using a 100kW medium frequency thyristor electric furnace. The melting temperature is controlled at 1500L1520E. The spheroidizing method is used for spheroidizing. The spheroidizing agent is made of W5 rare earth magnesium ferrosilicon alloy. The spheroidizing agent is added in an amount of 1.5A~1.7%. The spheroidization temperature is controlled by the 1480-Si alloy as the inoculant, and the inoculum is 0.5%. The chemical composition of the molten iron is controlled at 3.6~3.9C; 0+=[0;0+3M. After the spheroidization of the molten iron , pour 25mmY test block and gears. Both the mechanical property test specimen and the heat treatment test specimen were taken on the Y-shaped test block. The austenitizing temperature is 900E, austenitizing is carried out in a carburizing furnace, and titrated with methanol to create a protective atmosphere, and the isothermal quenching is performed after 60 m-heating. The isothermal temperature was:; the isothermal time was 60 min. The microstructure was observed on a MM6 metallographic microscope and a XA-840A scanning electron microscope. The sample size was 015×8 mm. The tensile test bar was a national standard short test bar, which was effective. The size is 010x50mm. At the same time, the Aube ductile iron gear is replaced by the 40Cr quenched and tempered steel gear for the installation test, and the noise and wear resistance are tested.
Test results and analysis Table 1 Effect of isothermal temperature on mechanical properties and metallographic structure of test bars No. 1 Isothermal temperature (E) Tensile strength (MPa) Elongation Impact toughness (/cm2) HRC Metallographic spheroidization grade 455240985 Extra-external 2246. Effect of isothermal temperature on mechanical properties and metallographic structure The austenitic isothermal transformation temperature is the key to the mechanical properties of austempered ductile iron. The suitable austenitic isothermal transformation temperature and time can give the ductile iron the best plasticity and toughness as well as the desired mechanical properties. However, since the austenite isothermal transformation parameters are affected by the ductile iron composition, the austenite isothermal transformation time and temperature are different for ductile iron of different compositions. Therefore, for ductile iron of different compositions, the parameters of the isothermal transformation must be specifically selected. When the test bar composition is 2.5% <0.1% Mn, the effect of isothermal temperature on its properties and metallographic structure is shown in Table 1. It can be seen from Table 1 that the tensile strength is the highest at 270E isothermal, and then with the increase of isothermal temperature. The elongation and the impact toughness increase with the increase of the isothermal temperature, but the elongation reaches the maximum at 370E, and the impact toughness is highest at 400E; the hardness decreases with the increase of the isothermal temperature. As the isothermal temperature increases, the bainite needle becomes longer and thicker, and it changes from lower shell to upper shell, and the paralympic increases.
The effect of Mn and Si on mechanical properties and metallographic structure is seen. The test bars with higher manganese content have higher strength and hardness than test bars with lower manganese content. It can be seen from the relationship between the elongation and the austempering temperature that the manganese content is low and the elongation is relatively high. From the relationship between austempering temperature and impact toughness, it can be seen that when the austempering temperature is lower than 340C, the impact toughness of the austenitic ductile iron with lower manganese content is higher, while the austenite ductile iron with higher manganese content is higher. The impact toughness is lower; when the austempering temperature is higher than 340C, the impact toughness of austenite with higher manganese content is higher than that of manganese. This is due to the high amount of Paralympics. Under different austempering temperatures, the hardness of austenite with higher manganese content is also higher.
Austempering temperature (X can be seen, the change of silicon content has no effect on the tensile strength under the test conditions. Under the condition of austempering temperature lower than 340C, the increase of silicon content makes the elongation of austempered ductile iron When the austempering temperature exceeds 340C, the elongation is decreased when the silicon content is high. Under the test conditions, the influence of silicon content on the impact toughness of the sample is: the austempering temperature is lower than Under the condition of 340C, the impact toughness of the test strip with higher silicon content is relatively higher; when the isothermal quenching temperature is higher than 340C, the impact toughness of the test strip with lower silicon content is relatively higher. In addition, the high silicon content is higher. The test bar has a relatively high hardness.
Effect of alloying elements copper and molybdenum on properties In order to investigate the effect of alloying elements copper and molybdenum on the properties of the rod after quenching, a test of adding molybdenum and copper and molybdenum was also carried out in the test. The effect of alloying elements copper and molybdenum on the properties is shown in Table 2. It can be seen from Table 2 that in the case of similar silicon and manganese phases, the effect of single addition of molybdenum on the properties and hardness of the test bars is not significant, but the addition of copper and molybdenum composites The hardness has a great influence. When the isothermal temperature is 3,703, the hardness is 4.5HRC higher than that of the single molybdenum. When the temperature is 3403, the hardness is increased by 1.9HRC. The influence of the alloying elements copper and molybdenum on the properties of the chemical composition (6c) isotherm. Spheroidized mechanical properties No. Temperature class 2.4 Work hardened ~40% retained austenite, which has different work hardening characteristics than ordinary ductile iron. This test is based on austempered ductile iron, 35CrMoA steel and QT800-2 ductile iron. The samples were subjected to different pressures on a Brinell hardness tester, and then the HRC (1471 N) hardness was measured at the center of the indentation to test the degree of hardening under different pressures. The test results are shown in Table 3. The hardness of the tested materials was increased to varying degrees after compression. At lower pressures (250kN), the hardness increase is more significant, but as the pressure increases, the hardness increases slowly and gradually becomes constant. Among the materials tested, the austempered ductile iron has the highest degree of hardening, and the lowest is 35CrMoA steel.
It can also be seen from Table 3 that the hardening rate of the austempered ductile iron with higher isothermal temperature is higher than that of the isothermal temperature, which may be related to the amount of retained austenite in the matrix. There is more retained austenite and the hardening effect is strong. The work hardening trace can also be seen from the hardness of the fracture portion of the tensile test bar and the hardness of the test portion (end). Table 4 shows the hardness comparisons of the equal quenching temperatures of 3403 and 3703, respectively.
Compared with the isothermal 3703, the hardness of the fracture part of the 3403 isothermal test bar is basically the same as that of the test bar clamping part, while the 3703 isothermal is significantly improved. It can be seen from the metallographic diagram that some retained austenite has undergone martensite transformation.
Table 3 The degree of hardening of the material under different pressures (kilogram) HRC increase rate HRC increase rate HRC increase rate HRC increase rate Base surface number and other quenching temperature (3) Fracture part clamping part 2.5 gear processing gear main The failure mode is contact fatigue damage. The R5 applied spectroscopy technique is used to analyze the contact fatigue process of the gear. The test shows that the contact fatigue failure form of the austempered ductile iron depends on the load. Under heavy load conditions (lifetime less than 5x106 times), the damage form is exfoliation; Under the loading conditions, the form of destruction is pitting. The contact fatigue strength of ductile iron is related to the isothermal temperature. When the temperature is 2503, the contact fatigue strength is about 1790MPa; the 3703 isother temperature is 1329MPa. The contact fatigue strength of 3703 isotherm and the permit of the 6110 wheel system proposed by the design department of FAW Wuxi Diesel Engine Factory. The contact stress is similar to 1333 MPa (calculated contact stress is 752~1220 MPa). The contact fatigue strength of the lower bainite gear is higher than that of the upper shell, and the bending fatigue strength is lower than that of the upper shell.
In order to reduce the dimensional deformation of the part and improve the cutting performance caused by the phase change during the isothermal treatment, the casting is subjected to graphitization annealing before processing to obtain a casting of the ferrite matrix. After the isothermal treatment of the casting, all the dimensions have changed due to the phase change and are beyond the technical requirements. Because the hardness of the lower shell is more than 40HRC, it is difficult to shave the teeth, and the keyway can not be processed. It is difficult to process the Aube gear under the processing conditions of the existing 40Cr quenched and tempered steel gear, and the precision of the gear is high, if you want to find out The deformation law of the gear after isothermal has certain difficulties in reducing its processing. In order to make full use of existing tools and mature processing technology, this topic replaces 40C-tempered steel gear with low-hardness (370 isothermal) upper bainite gear. The processing steps are: casting one annealing and one roughing one. Measuring one isothermal treatment, one cleaning, one finishing, one measurement. According to the above process, after processing the isothermal 370, due to the work hardening effect of the gear, except for the round key groove on the timing gear (K-100641-1), it is difficult to process, and the other processing steps can be carried out. The premise is that the lathe speed or the feed rate must be reduced, but it still has a large damage to the knife. To expand the test, further research is needed on the machining process of the gear.
Installed Verification Noise Reduction Test The installed test includes AK gear, AKZ gear and CK gear. The test was carried out at Jiangsu University of Science and Technology. Among several noise reduction schemes, the noise reduction effect of the Aube gear is the most obvious. Compared with the 40C-tempered steel gear, the noise of the whole machine is reduced by 1.92dB and the noise of the gear side is reduced by 5.3dB in the nine test points of the quasi-engineering method. The main reason is the elastic modulus of the austempered ductile iron (170 GN/m2). ) is lower than 40C-steel (209GN/m2), and the hardness of the Orbe gear is low, the meshing property is good at the initial running-in, the contact area is large, the surface Hertz stress is small, and the graphite in the cast iron makes it have a larger vibration than the steel. Attenuation ability. All of the above reasons are beneficial to vibration and noise reduction.
The 200-hour endurance test was carried out on an oil machine. The test required continuous operation for 200 hours under calibration conditions after 45 hours of running-in. Wear of the gear will result in an increase in the clearance between the gears, an increase in noise, and when the clearance exceeds a certain limit, the gear will be scrapped. The gap index between the gears of the factory is 0.085; 0.30mm. After 200 hours of operation of the Aube ductile iron gear, the operation is normal, the gears are dismantled according to the requirements, and the gears have no abnormal friction and wear. The gap is often in the range of 0.085; 0.17mm. Internal fluctuations, there is a margin of 0. 13mm from the limit gap, so its friction and wear performance is qualified.
In other tests, the power of the AKZ and CK machines is large, and the contact stress of the gear tooth surface is also increased accordingly. Many of their contact stresses are 1291 and 1267; 1454 MPa, which is higher than the 370 isothermal Austempered iron gear. 40C-tempered steel gears experienced abnormal wear and broken teeth during use. Therefore, the design department changed the CK gear material to 38CrMAl and widened the tooth surface to reduce the unit contact stress of the tooth surface. After the tooth surface is widened, if the Austempered ductile iron gear can be used instead of the 38CrMAl quenched and tempered steel gear, it has significant economic benefits and can reduce the management cost. Therefore, we have made a set of AKZ and CK gears for the installation test. The AKZ diesel engine was tested for 500 hours and the CK machine was tested for 200 hours.
Conclusion The isothermal temperature has a great influence on the mechanical properties of austempered ductile iron. In the isothermal temperature range tested, the hardness decreases with increasing temperature, the intensity reaches the maximum at 270, and then decreases with the increase of isothermal temperature; the elongation increases with the increase of temperature, and the highest at 370. And then decreases with increasing temperature; impact toughness increases with increasing temperature; hardness decreases with increasing temperature.
According to the test results, when the gear iron composition range is: When the austenitizing property is better. When the manganese content (0.) is high, the amount of silicon should also be appropriately increased, which is advantageous for improving the performance of the austempered ductile iron.
Compared with other materials, the work hardening effect of austempered ductile iron is more obvious, and the work hardening effect of upper shell is stronger, so it can maintain good wear resistance and high comprehensive mechanical properties.
Compared with 40Cr quenched and tempered steel gears, the austempered ductile iron gear reduces the noise of the diesel engine by 1.92dB and the gear side noise by 5.3dB. After the 45-hour running-in, the austempered ductile iron gear runs continuously for 200 hours under the calibration conditions. The gear gap has a margin of 0.13mm from the limit gap, so its wear performance is qualified.
It is feasible to replace 40Cr quenched and tempered steel gears with Ausbee ductile iron gears for diesel engines. However, the use of Aussie ductile iron gears instead of 40Cr quenched and tempered steel gears for AK machines requires further 1000-hour endurance test. At the same time, in-depth testing and research work on the cold working process is required.
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