Research On The Key Technologies Of Online Quenching-Tempering For Big Diameter Bars

In the traditional production cycle, off-line heat treatment, which has the characteristics of low production efficiency and high cost, is usually adopted by the steel bars. Online quenching and tempering is one of the advanced manufacture technologies in the production of large bars. With online heat treatment technology, the large bar has better comprehensive mechanical properties, but also to save energy and improve production efficiency. At present, the online bar heat treatment technology research focus mainly on medium and small diameter bars, and there is little research about large diameter bar(?50~150 mm)for online quenching-tempering at home and abroad.In this paper, large diameter bars such as 45 steel, 40 Cr steel and 42 CrMo are taken as the research objects, and the critical theory and technical problems of large diameter bar online quenching-tempering are researched by combining experimental and numerical simulation methods. The research results provide the reference basis for the development and production of large diameter bar online quenching-tempering production line. The main research contents are as follows.The critical cooling rate of three kinds of steel bars is obtained by measuring the maximum depth of hardened layer in quenching experiments. The influence of quenching water pressure on the maximum quenching depth of different diameter of bars is studied by combining experiments and FEM simulation. The most suitable quenching water pressure is determined through the analysis and comparison. The maximum quenching depth and the longest quenching time based on the maximum depth of the bar with different diameter are calculated, which provides an important reference for the design of the online-quenching line.Tempering hardness of bars under different conditions is determined with related tempering experiments. The results show that: with the increase of tempering temperature(time), the hardness of bar section is decreased continuously, and the tempering temperature plays a decisive role in tempering hardness. The regression function between tempering hardness of bar surface with tempering temperature and tempering time is obtained. The influence of tempering process on the microstructure and mechanical properties of the bar is studied, and the results show that: The longer the heat preservation time is, the more diffuse precipitation of carbide become, the better strength and toughness of the steel bar are obtained, and the more uniform performance inside and outside along radial direction is.According to the capacity requirement, the induction heating technology is proposed to heat the temper big bars. The numerical simulation of induction heating process is done to determine appropriate tempering process. This paper analyze the temperature field of bars with different diameter in induction heating process, and the result shows that the process conditions not only meet the requirements of high-temperature tempering, but also save a lot of time than the traditional heating furnace tempering. The output of the bar with different diameters is calculated, which provides reference for the construction of the online tempering line.The temperature fields of bars with different diameters during quenching and selftempering process based on the maximum quenching time are calculated with FEM simulation, and the self-tempering possibility of different specifications are discussed. In addition, the selftempering condition of the bars with different diameter is studied based on different depth of the hardened layer.