| Martempering in salt bath technology not only make the carburized gears with high surface hardness, good core toughness and strength, also can effectively control the distortion of gears. Martempering in salt bath technology using molten nitrate as a quenching medium, combined with surface martensitic transformation point carburizing gear quenching by setting the relevant parameters to achieve grading martensitic transformation, so that carburizing gear get excellent overall performance. However, for the martempering in salt bath technology of 17 Cr Ni Mo6 other new high-alloy low carbon steel carburizing gear, the cooling performance of salt bath medium associated with the steel hardenability and related properties of the heat treatment process is not yet fully understand. So, in order to strengthen the advantages of the martempering in salt bath technology, further promote its industrial application, the properties of martempering in salt bath technology for 17 Cr Ni Mo6 carburizing gear steel such as core hardness, surface hardness, microstructure, reference carbon content, and distortion characteristic etc were studied systematically and rapid quenching oil quenching technology were comparative analyzed in a few aspects of property in this article.1.The effects of the three main factors(water content, agitation speed and salt bath temperature) on the cooling characteristics of 55%KNO3+45%NaNO2 salt bath quenching media were studied. The results show that the cooling rate of nitrate media increases in high and low temperature with the increase of water content. The cooling rate at low temperature increases by growth of agitation speed which mainly affects the cooling rate of low temperature stage. The salt bath temperature has a little influence on cooling characteristics. Moreover, the water content has the most significant influence on the cooling capacity of salt bath quenching media.2. The effects of different cooling parameters on the hardness, microstructure and effective case depth of 20CrNi2 Mo, 17CrNiMo6 and 20 CrMnMo steel were studied. The results show that effective case depth increases with the increase of water content but there is a little influence on it with the increase of temperature. The surface hardness is not affected by moisture, salt bath temperature and core hardness with increasing water content tended to increase.3. The molten nitrate and rapid quenching oil were used as quenching media for quenching and tempering of three teeth tooth shape sample of 17CrNiMo6 carburizing gear respectively. Influence of quenching medium cooling power, module and thickness in lower section of gear root on the center hardness and microstructure and reference carbon content was analyzed in this study. The results show that the higher center hardness, the finer lath martensite and the lower reference carbon content are obtained with stronger cooling power of molten nitrate medium. The core hardness decreases with the increase of gear modulus and thickness in lower section of gear root and lath martensite of thinner thickness in lower section of gear root is more apparent by quenching under the same medium.4. Using the optimal processing parameters, 17CrNiMo6 steel C-shaped specimen was pretreated and carburized. Then, it was martempered in molten nitrate at different agitation speeds. Comprehensive analyses show that the agitation speed of 700 r/min is suitable for distortion control. Compared to the results of oil quenching, the distortion is reduced by 28.8% and 21.0%, through martempering in molten nitrate, under the agitation speeds of 300 r/min and 700 r/min respectively. This further illustrates that martempering in molten nitrate can control distortion effectively.Based on the above research, this article does some on the analysis that the cooling power of salt bath medium can be adjusted and martempering in salt bath technology can effectively control the quality of carburizing quenching gear, such as adjusting core hardness and sizing deformation. |
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