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1 Purpose and significance of the study Ao-bei ductile iron has good comprehensive mechanical properties of high strength, high elongation and high impact value, and also has high bending fatigue strength and good wear resistance. In recent years, people have actively used the Austrian-bead ductile iron as a structural material, and the gear is the typical application of the Austrian-bead ductile iron. W. Using the Austrian-bead ductile iron instead of 20QMnTi to manufacture the final four-wheel tractor final transmission large gear, can reduce the gear Cost, shorten production cycle.
The retained austenite will have a strengthening effect in the work, that is, the austenite of the surface layer of the tooth has a work hardening effect, so that the surface has excellent wear resistance, because the surface is continuously strengthened during the working process, and the amount of wear on the tooth surface Irrelevant, this is not possible with surface treatments such as general carburizing and nitriding. In terms of gear structure and production process, Ao-Bei ductile iron is more suitable for making large gears, because the gears with small diameter are directly processed by the bar, the material consumption is low and the cost is low, and the larger gears are made of alloy steel or carbon steel. , it is necessary to go through the process of cutting, forging, normalizing, etc., with large material consumption, long cycle and high cost.
The key to the production of the Austrian-ball iron gear is the heat treatment technology. At present, most of the domestic and international use of traditional nitrate salt austempering to obtain the Austrian-beauty organization, although this heat treatment process is more mature, but the production equipment investment, high energy consumption, complex production process, corrosion of the workpiece, the environment is also more Large pollution, workers' health is affected. Moreover, there are two major problems in the use of the salt bath furnace, that is, the difficulty in starting the heating and the large heat loss in the furnace mouth, so that the salt furnace becomes a heating device that consumes a large amount of energy W. In addition, the nitrate salt is easily bonded to the gear teeth by the quenching treatment of the nitrate salt. On the top, it must be cleaned and thoroughly dried after cleaning, thus adding drying equipment and processes.
In foreign countries, there is a new process for austempering using high temperature oil instead of salt bath. This process overcomes some shortcomings of salt bath, but there are still some disadvantages, such as long production cycle, high cost, and long-term high temperature and good aging of oil temperature. The flash point of the oil is low, and the local flash point is more likely to cause burning and cause unnecessary loss.
In response to the above problems, the authors research and develop a new heat treatment process for austempered ductile iron that can achieve excellent performance, reduce cost and energy consumption, and is easy to operate and realize, namely, room temperature oil grading austempering.
2 room temperature oil grading austempering heat treatment process test room temperature oil grading austempering is to austenitize the workpiece after heating, quenching in a room temperature oil (cold oil) for a short time, the workpiece is quickly overcooled to above Ms point The temperature is then applied to the appropriate temperature isothermally for bainite transformation to obtain the above bainite and a certain amount of stable retained austenite-based structure. According to the process sequence, the reasonable room temperature oil grading austempering process parameters were determined, and the effects of various process parameters on the microstructure and properties of austempered ductile iron were investigated.
2.1 Effect of austenitizing temperature and time on microstructure and properties 21.1 Effect of austenitizing temperature Tests show that increasing the austenitizing temperature can increase the amount of retained austenite, the higher the austenitizing temperature, austenite The higher the carbon content in the middle, the stronger the austenite stability, and the more the austenite amount after the grading austempering, and vice versa. When the austenitizing temperature rises to 920, the heated structure is completely austenitized, the austenite is relatively uniform, and the carbon content is high, and fine upper bainite, a small amount of lower bainite and stable residue can be obtained. Austenite, this matrix structure is not only high in hardness but also has good toughness. Continue to increase the austenitizing temperature, the structure is obviously coarsened, the amount of retained austenite is large, the upper bainite needle is coarse and the distribution is sparse, the hardness is obviously reduced, and the toughness is also reduced.
The effect of 21.2 austenitizing time test shows that prolonging the austenitizing time can increase the amount of retained austenite, but it has less effect than the effect of austenitizing temperature. Therefore, under the premise of ensuring austenitization, selecting a shorter austenitizing time can improve efficiency and save energy, that is, about 5h.
2.2 Influence of staged quenching time on microstructure and properties 221 Mathematical calculation of fractional quenching time The purpose of quenching is to pre-cool the workpiece to the Ms point before the transformation, and then to perform the transformation of the Austrian-beauty structure at the moderate temperature. There is a corresponding relationship between the pre-cooling temperature of the workpiece and the grading quenching time. The longer the grading time, the lower the temperature at which the workpiece is pre-cooled, and the control grading time is essentially controlling the pre-cooling temperature of the workpiece. According to the heat transfer relationship between the test piece and the quenching medium, the classification quenching time of the sample is calculated from 13.72~25.38s.
2.2.2 Experimental study of graded quenching time It can be seen that with the increase of grading time, the hardness is increasing. The impact toughness is less than 30s and the grading time is extended with the grading time. The impact value is increasing, but the grading time continues to increase. At the time, the impact toughness is declining. It can also be seen from the metallographic structure that when the classification time is less than 5 s, the overall temperature of the sample is still high due to the short pre-cooling time, and the cooling rate in the furnace is small, resulting in insufficient test hardenability and pearlescence. The transformation of body organization. As the grading time increases, the overall temperature of the sample decreases before isothermal, which causes the cooling rate to increase significantly. The acicular structure in the product increases and refines, and the upper bainite structure begins to appear in the tissue. When the grading time is about 30 s. At the same time, we obtained the desired Aussie-beauty structure after isothermal, which has moderate hardness, good toughness and good comprehensive mechanical properties. When the grading time continues to increase, the lower bainite increases and the retained austenite decreases, so the hardness continues to rise, the toughness decreases, the grading time is too long, and the workpiece is pre-cooled below the Ms point, producing a certain amount of martensite. High hardness and low toughness. Of course, in actual production, it is also necessary to determine the classification quenching time according to the equivalent wall thickness of the workpiece.
2.3 The influence of isothermal temperature and time on the structure and properties Isothermal is the last control process of the room temperature oil classification austempering process. When the influence of the isothermal transformation temperature on the microstructure and properties is studied, the austenite ball iron Ten mechanical properties of tensile strength, elongation, impact toughness and hardness were comprehensively investigated.
Different isothermal temperatures are selected. The isothermal time is generally 90 according to the isothermal salt quenching data. However, considering the pre-cooling and quenching, there is the possibility of accelerating bainite nucleation and transformation. Therefore, isothermal 60 min and 90 min were selected in the experiment. Comparative Test.
The isothermal temperature in the range of ~380° has good comprehensive mechanical properties. The change rule is: as the isothermal temperature increases, the tensile strength and hardness decrease, and the elongation and impact values ​​increase. At around 380C, the elongation and impact values ​​no longer rise, or even slightly decrease. The isothermal time increases from 60min to 90min, the tensile strength and hardness increase, and the elongation and impact toughness decrease.
From the metallographic structure, the bainite needle in the 320C isothermal structure is fine and disordered, and the amount of retained austenite is small. This is because the isothermal temperature is lowered, the diffusion of carbon in the austenite is difficult, and the stability of the retained austenite is lowered, so that the amount is reduced. At 350C isothermal, the bainitic ferrite needles are arranged in parallel in different directions, showing typical feather-like or needle-like upper bainite. The amount of retained austenite is 3040%. At 380C isothermal, bainite strips are relatively The length is relatively thick, and the amount of retained austenite is large. This is because the isothermal temperature is high, which promotes the growth of bainite, and the carbon is easily diffused in the austenite, so that the austenite is quickly rich in carbon and tends to be stable. The metallographic structure also reflects that when the isothermal time is 60 min, the bainite transformation is almost completed, and the retained austenite has a high carbon content and becomes stable. This indicates that the graded austempering process accelerates the bainite nucleation and transformation and shortens the required isothermal time. The isothermal temperature continues between 60 and 90 min. Since the transformation has been basically completed, a small amount of unconverted bainite enters the phase transition stagnation phase, the transformation becomes extremely slow, the tissue change is not obvious, and the retained austenite is slightly reduced. However, for 380C isothermal samples, the sensitivity to isothermal time is stronger than that of 320C and 350C isothermal samples. Due to the high temperature, the phase change driving force is large [4, when the isothermal time is 90 min, the metallographic structure The medium-high carbon austenite begins to decompose into ferrite and cementite, but it is not obvious, which is consistent with the decrease of toughness and elongation.
The room temperature oil grading austempering process can obtain the fine bainite ferrite which can be obtained at low temperature treatment, and at the same time obtain the stable austenite obtained by high temperature treatment. This process also reduces sensitivity to isothermal temperatures and processing times. In this test, the suitable isothermal transition temperature is about 350C, and the isothermal time is 60~90min, and the austenite ductile iron with good comprehensive performance can be obtained.
24 Comparison of conventional nitrate salt isothermal quenching and grading austempering process 241 Comparison of hardenability test The test rod of 040, 050 was austempered in a salt bath, and the test rod of 060, 070 was austempered at room temperature. The heat-treated test bars were cut from the middle, the metallographic structure was observed, and the hardness distribution was measured to determine the influence of different processes on the hardenability.
The metallographic structure found that with the conventional austempering of nitrate salt, the 040mm test bar can be hardened, and the heart is all austenite and bainite, but the pearl of the 050 is obtained by pearlite structure. When the room temperature oil is classified and austempered, the 060 mm test bar can be hardened, and the isothermal quenching of the salt bath is increased by nearly 20 mm, but the 070 mm test bar is not hardened.
Analysis of the hardness distribution curve shows that for the 040 mm and 060 mm fully hardened test bars, the cross-sectional hardness does not change significantly along the diameter direction, while the cross-section hardness of the 050 mm and 070 mm test bars does not vary greatly along the diameter direction. Therefore, the room temperature oil grading austempering process improves the hardenability of the test piece compared with the conventional salt bath austempering process. According to the workpiece size, the alloying element can be added without adding or adding, which can ensure good structure and performance, and can reduce cost. .
242 Comparison of mechanical properties and microstructure Experimental results and mechanical properties comparison results are shown in Table 1. Table 1 Experimental process parameters and mechanical properties comparison results Process category heat treatment process mechanical properties nitrate salt austempering room temperature oil classification austempering from mechanical properties comparison found, The austempered ductile iron obtained by the isothermal quenching of room temperature oil has high strength, high hardness, and little change in elongation and impact toughness, that is, overall strength and toughness are better. From the comparison of the structure, the room temperature oil grading austempering is finer and more uniform than the salt bath austempering structure, the retained austenite distribution is finely divided, and the bainite level is raised from 2~4 to 1-2, which is equivalent to the tissue refinement nearly 1 Times.
3 Application in the final transmission gear of the tractor: The new process of producing the Austrian-bead ductile iron by the room temperature oil grading austempering was carried out to verify the final transmission large gear of the Taishan-120 small four-wheel tractor (formerly carburized steel 20CrMnTi). . The machined ductile iron gear is subjected to austempering treatment by room temperature oil classification, and is cleaned and shot peened.
3.1 Heat treatment deformation Determination of the gear diameter before and after heat treatment and the key length of the gears affecting the meshing of the gears. It is found that after the temperature quenching treatment by room temperature oil, the deformation amount is small, both are positive values ​​and within the specified tolerance zone, so when machining Tolerances must be reserved.
3.2 Application status The Taishan-120 small four-wheel tractor made by the above process was installed in the final drive large gear. After 6 months of operation, it was disassembled and inspected. There was no broken teeth and chipping on the macro, and there was no tooth surface. Crack, pitting. From the wear situation, the length of the large gear common normal is 0.08, which is still within the specified tolerance range, and the wear resistance is good.
4 Conclusions According to the spirit of the notices of the State Economic and Trade Commission, the State Administration of Taxation and the General Administration of Customs, combined with the actual situation of Gansu Province, the information report is based on the active declaration, industry review and recommendation of the enterprise, and is studied by the provincial economic and trade commission and other departments. Tianshui 213 The Electrical Equipment Co., Ltd. Technology Center was recognized as the third batch of provincial enterprise technology centers. At this point, Gansu Province Machinery Industry Group Co., Ltd. is a new process method for the production of austempered ductile iron and high-strength toughened ductile iron without salt bath without corrosion.
At room temperature oil grading austempering heat treatment, reasonable process parameters are selected to obtain austenite ductile iron with good comprehensive mechanical properties. The process also reduces the sensitivity to isothermal temperature and time.
Under the premise of ensuring the hardenability, the classification quenching time is prolonged for workpieces with large equivalent wall thickness values ​​and large structural dimensions.
(4) Compared with the salt bath austempering, the room temperature oil grading austempering process can improve the hardenability of the workpiece and reduce the amount of alloying elements.
(6) The final transmission large gear of 8.826kW small four-wheel tractor is treated by room temperature oil grading austempering. The heat treatment deformation is within the tolerance range, the performance meets the technical requirements of the tractor, and the economic benefit is remarkable.
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